Neuroanatomy Flashcards
Thalamic blood supply
Primarily PCA + branches (some anterior supply laterally (anterior choroidal->LGN)
Paramedian
Tuberothalamic (PCOM branch)
Inferolateral
Posterior choroidal
Anterior thalamic syndrome
Perseverations, superimposition of unrelated information, apathy, amnesia, emotional facial paresis
Paramedian thalamic syndrome
Disinhibition, loss of self activation, and amnesia, vertical gaze paresis - thalamic dementia
Inferolateral thalamic syndrome
Ataxia, hypesthesia, executive dysfunction (pure sensory, sensiromotor, Dejerine-Roussy)
Posterior thalamic
Hypoesthesia, homonymous horizontal sectoranopsia, aphasia, neglect
Field cuts in LGN
Anterior choroidal
Lateral choroidal form posterior choroidal of posterior circulation
Anterior thalamic nuclei
Anterior, lateral dorsal
Medial thalamic nuclei
Medial dorsal
Lateral thalamic nuclei
VA, VL, VPM/VPL, LGN, MGN, pulvinar
Intralaminar thalamic nuclei
Midline thalamic nuclei - adjacent to third ventricle
Reticular
Thin layer surround thalamus
Thalamic relay nuclei: specific vs non-specific vs intralaminar vs reticular
Specific: unique functional and anatomic correlate to projection (e.g., retina,anterior visual pathway->LGN->occiptal cortex)
Non-specific: pulvinar receives input from multiple sensory modalities (SS, vision) and project to parietal, temporal and occipital association areas
Diffuse relay nuclei with other deep structures (BG, brainstem)
Reticular nucleus - inhibitory intrathalamic, reciprocal projections
What generates sleep spindles
Reticular nucleus
What special sense bypasses the thalamus?
Olfactory
Thalamic nuclei involved in sensory processing
Thalamic nuclei involved in motor control
Thalamic nuclei involved in oriented towards behaviorally relevant stimuli
Thalamic nuclei involved in emotional processing
Intralaminar nuclei
What causes thalamic dementia?
Infarct of paramedian artery or other lesion of DM nucleus
Bilateral thalamic infarcts from single occlusive event?
Artery of Percheron
Which thalamic nuclei does not project to the cortex?
Reticular nucleus - gates stimuli, generates sleep spindles
Which thalamic nuclei does not project to the cortex?
Reticular nucleus - gates stimuli, generates sleep spindles
STN
The only excitatory nuclei from the basal ganglia
Basal ganglia prefrontal circuit
Basal ganglia limbic circuits
BG hypodensities and calcifications are common in what condition?
MELAS
Where do you see bilateral putamen necrosis?
Methanol poisoning
Sxs from BG lesions 2/2 perinatal ischemia may be delayed for…?
7-14 years
BG are prominently affected in what condition?
Leigh’s syndrome
STN lesions result in…?
Contralateral ballismus
Spinal nerve
Formed by the junction of the anterior and posterior roots at the level of the neural foramen (which is just beyond the dorsal root ganglion).
– The anterior roots contain lower motor neuron axons from the anterior horn cells
– The posterior roots contain axons from the sensory cell bodies in the dorsal root ganglion
Spinal Root / Vertebrae Relationships above L1/L2
C1-C7 roots exit through the neural foramina above the corresponding vertebrae
– 1st cervical nerve exits the spinal canal between the Atlas and the occiput and does not project to the skin
The 8th cervical nerve exits the spinal canal between C7 and T1 vertebrae.
– There is no C8 vertebra
All other spinal nerves exit through the neural foramina beneath the vertebrae of their same number
– From T1-L1/2, directly off the spinal cord
– Below L1/2, from the cauda equina
Spinal nerve roots exiting below L1/L2
Below L1/2 in the cauda equina, the nerve roots are organized to exit the spinal canal.
– The nerve root exiting below its vertebrae is quite lateral.
– The nerve root that will be exiting below the next caudal vertebrae is central lateral
– The remaining nerve roots are cental.
Neural foraminal stenosis and disc herniation effects above L1/L2
Neural foraminal stenosis affects the nerve root exiting at that level.
Disc herniation above the conus medullaris affects the nerve root exiting at that level.
Above the C7 vertebrae, the inferior vertebrae for either neural foraminal stenosis or disc herniation correlates with the nerve root affected
- The C5 nerve root is impinged by C4/5 neural foraminal stenosis or disc herniation.
The C8 nerve root is impinged by C7/T1 neural foraminal stenosis or disc herniation.
From T1/2-L1/2, the superior vertebrae for either neural foraminal stenosis or disc herniation correlates with the nerve root affected
- The T5 nerve root is impinged by T5/6 neural foraminal stenosis or disc herniation.
Neural foraminal stenosis and disc herniation effects above L1/L2
Neural foraminal stenosis affects the nerve root exiting at that level.
Typical centro-lateral disc herniation at the level of the cauda equina affects the nerve root exiting 1 vertebral segment below that level.
Far lateral disc herniation at the level of the cauda equina affects the nerve root exiting at that level.
The L5 nerve root is impinged by an L4/5 centro-lateral disc herniation, L5/S1 far lateral disc herniation or L5/S1 neural foraminal stenosis
Radiculopathies
Classically present with neck or low back pain that radiates in the distribution of a nerve root
– Sensory symptoms are often vague because the dermatomes overlap. Focal severe sensory loss argues against a radiculopathy.
– Weakness of the muscles that receive innervation from that nerve root are usually mild to moderate in severity. Complete muscle paralysis is uncommon because the myotomes overlap.
Structural: Herniated discs, Spondylosis, or Mass lesions (metastases, abcess) - common in C5-C7, L5, S1
Infiltration: Carcinoma, Lymphoma, Sarcoid
Infection: Lyme, VZV, CMV, HSV
Infarction: Vasculitis, DM
Demyelination: Early GBS
Spinal nerve sympathetic component
The second order sympathetic neurons arise from the intermediolateral column from T1- L2.
Each spinal nerve from T1-L2 contains sympathetic axons that leave the spinal nerve in a white communicating ramus to enter the sympathetic chain.
– Sympathetic axons synapse at that level or ascend and descend in the sympathetic chain.
All spinal nerves receive third order sympathetic neurons from the sympatheic chain via a gray communicating ramus.
Spinal nerve sensory and motor
Each spinal nerve divides into a
Ventral primary ramus which innervates
-Plexus of a limb: anterior segment sensory receptors, limb / anterior trunk muscles
Dorsal primary ramus which innervates: posterior segment sensory receptors, paraspinal muscles (involvement of paraspinal muscles on EMG is specific but not sensitive for either a spinal nerve (radiculopathy) or anterior horn cell disease)
Conus vs cauda equina lesions
Spinal segments and nerves
There are 31 spinal segments and nerves
– 8 Cervical
– 12 Thoracic
– 5 Lumbar
– 5 Sacral
– 1 Coccygeal
Vertebral bodies
There are 33 vertebral bodies
– 7 Cervical
– 12 Thoracic
– 5 Lumbar
– 5 Sacral
– 4 Coccygeal
Spinal cord vs vertebral segments
Spinal Cord vs. Vertebral Body Levels
C1 cord at C1 Vertebral Body
C8 cord at C7 Vertebral Body
T1 cord at T1 Vertebral Body
T12 cord at T8 Vertebral Body
L1-L5 between T9 and T11 Vertebral Bodies
S1-S5 between T12 and L2 Vertebral Bodies
Spinal cord and vertebral column development
In the 1st Trimester, the spinal cord is the same length as the vertebral column
During fetal development and early childhood, the vertebral column grows while the spinal cord does not grow at same rate
– The spinal cord “migrates” upward
– In the Newborn, the spinal cord ends at L3 – In the Adult, the spinal cord end at L1/2.
Spinal cord tracts
Desending tracts
Appendicular Movement
Lateral Corticospinal tract
Rubrospinal tract
Lateral Vestibulospinal tract
Corticospinal tract decussation at the medulla
80-90% of axons cross in the medullary pyramids prior to forming the lateral corticospinal tract
– Axons to the upper extremity are more medial and anterior in the pyramid and cross rostral to the legs
– 2% of axons do not cross and enter the lateral corticospinal tract
8% of axons do not cross and enter the anterior corticospinal tract
Hemiplegia Cruciata
A lesion in the rostral medial medullary pyramid effecting the upper extremity axons after they have crossed and the lower extremity axons before they cross.
– This results in arm weakness ipsilateral to the lesion and contralateral leg weakness.
Lateral corticospinal cord in the spinal cord
Lateral Corticospinal tract
Descends posteriorly in the lateral compartment of the spinal cord.
– The arm fibers are medial and the leg fibers are lateral.
Synapses on the lower motor neurons in the anterior horn.
– Flexor muscles are medial in Rexed lamina IX and extensor muscles are lateral
Any lesion in the spinal cord, produces ipsilateral weakness.
Rubrospinal tract
Another appendicular movement tract
– Originates from the red nucleus in the midbrain.
– Decussates in the midbrain and descends just ventral to the lateral corticospinal tract in the lateral compartment
– Terminates in the cervical spinal cord
– Partially mediates upper limb flexion
Lateral Vestibulospinal tract
Another appendicular movement tract
– Originates from the lateral vestibular nucleus
– Descends ipsilaterally in the ventral compartment
– Synapses on interneurons that decussate at that level to innervate bilateral medial anterior horn cells that control balance and stimulates appendicular extension
– Partially mediates arm and leg extension
Ventral Corticospinal tract:
Descending axial tract
– Remains ipsilateral in the ventral compartment
– Stimulates axial movement
Tectospinal tract
Descending axial tract
– Originates from the deeper layers of the superior
colliclus.
– Decussate in the dorsal tegmentum and descend in the medial dorsal portion of the ventral compartment of the brainstem.
– Terminates in the cervical spine
– Mediates movement of the neck and upper trunk (probably in coordination with eye movements.
Medial Vestibulospinal tract
Descending axial tract
– Originates from the medial vestibular nucleus and descends bilaterally.
– Also mediates movement of the neck and upper trunk (probably in coordination with eye movements.
Pontine and Medullary Reticulospinal tracts
Descending axial tract
– Receives input from premotor cortex
– Originate in the pontine and medullary reticular formation, respectively
– Descends ipsilaterally in the vental compartment.
– Mediate automatic movement (e.g. maintain posture).
Anterolateral tracts - arms and legs
The lateral spinothalamic tract originates from sensory receptors for temperature and pain. Leg fibers are lateral and arm fibers are medial.
DCML - arms and legs
Arms lateral in cuneate fascilus and legs medial in cuneate gracilis
Spinocerebellar tracts - arms and legs
Dorsal spinocerebellar tract
– Large diameter axons from the legs and trunk synapse on Clarke’s nucleus (C8-L2) in the medial intermediate zone of the ventral horn.
– The axons from Clarke’s nucleus ascend ipsilaterally in the lateral column (lateral to the corticospinal tract).
Cuneospinocerebeller tract (The dorsal spinocerebellar tract for the arms) – Large diameter axons from the arms ascend to the medulla in the cuneatus fassicle of the dorsal columns but bypass the cuneatus nucleus to synapse on the accessory cuneate nucleus.
In the medulla, these axons enter the cerebellum via the inferior cerebellar peduncle.
Spinocerebellar tracts - spinal border cells
Ventral spinocerebellar tract
– Originates from spinal border cells in the thoracic and lumbar ventral horn
– These axons decussate in the ventral commisure and ascend the spinal cord in the lateral column (lateral to the ALS).
Rostral spinocerebellar tract (the Ventral spinocerebellar tract for the arms)
– Originates from spinal border cells in the cervical ventral horn. Also decussate in the ventral commisure to join the ventral spinocerebellar tract.
These axons enter the cerebellum via the bilateral superior cerebellar peduncles. Some fibers decussate again, while others do not.
Blood supply to spinal cord
The spinal cord is supplied by a single anterior spinal artery and a pair of posterior spinal arteries.
– These arteries arise from the vertebral arteries.
The anterior spinal artery supplies the anterior 2/3of the cord, which includes the anterior horn, ALS, and corticospinal tracts.
The posterior spinal arteries supply the dorsal columns.
The spinal arteries narrow in the thoracic cord (may even be noncontiguous).
As a result, the spinal arteries can be divided into 3 longitudinal segments based on their blood supply
– C1-T2: supplied by radicular arteries from the vertebral and ascending cervical arteries
– T3-T7: spinalarteriesfromT3-T7aresuppliedby radicular arteries from intercostal arteries
– T8-conus: suppliedby radicular arteries from the artery of Adamkiewicz
Blood flow to these segments is reconstituted by radiculomedullary arteries.
The radiculomedullary arteries originate from radicular arteries.
Radicular arteries
Radicular arteries originate from segmental arteries, which include the ascending cervical, intercostal, lumbar, and sacral arteries.
There are thirty-one pairs of radicular arteries, each passing through the neural foramina to supply each spinal nerve, the vertebral body and the dura via a small dural branch.
Only 6 to 10 radicular arteries have radiculomedullary branches
– Their exact number and anatomic location is quite variable.
Spinal AVMs
Oculosympathetics
Aniscoria
Physiologic (same in dark/light)
Small pupil (greater in dark) - Horner’s
Large pupil (greater in light) - CN III
None in PURE afferent disease! This has APD
Testing for Horner’s - confirming the dx
Topical cocaine is used to confirm the clinical diagnosis of ocular sympathetic denervation, or Horner Syndrome (HS). Cocaine blocks re-uptake of norepinephrine (NE) by sympathetic nerve terminals in the iris dilator muscle, transiently increasing its concentration in the synaptic junction. Norepinephrine activates alpha1 receptors in the iris dilator to cause pupil dilation. In HS, cocaine fails to dilate the affected pupil as much as the unaffected pupil, but its indirect action makes it a weak dilator, and the test can give equivocal results. Cocaine is also a controlled substance and therefore difficult to obtain. A practical and reliable alternative to cocaine is apraclonidine, an ocular hypotensive agent that has a weak direct action on alpha1 receptors and therefore minimal to no clinical effect on the pupils of normal eyes. Patients with HS have denervation supersensitivity of the alpha1 receptors in the iris stroma of the affected eye, making the pupil dilator responsive to apraclonidine. In patients with HS, reversal of anisocoria occurs after bilateral instillation of apraclonidine 1% or 0.5%.
Cocaine
Urine drug test for cocaine will be positive for a few days after testing (5)
Apraclonidine
Denervation must be present long enough for receptor upregulation to have occurred (14) Positive tests have been noted within hours of a carotid dissection but the onset of denervation sensitivity are variable (15) False negatives can occur in the setting of acute Horner syndrome or in long-standing cases if strict “reversal of anisocoria” criteria used (16, 17) Apraclonidine has limited use in pediatric Horner syndrome due to the risk of CNS and respiratory depression (18)
https://www.ophthalmologyreview.org/articles/horner-syndrome-pharmacologic-diagnosis
Horner’s localization
HYDROXYAMPHETAMINE
Hydroxyamphetamine remains a useful tool for localization of the lesion once a diagnosis of Horner syndrome has been confirmed (20). However, it is limited by accessibility and some considerations detailed below. Since it’s still tested (and important to understand from a mechanistic and historical perspective), you still need to know how it works and what it does.
Mechanism of action: increases the release of norepinephrine from the presynaptic neuron (21). In intact presynaptic (3rd order, postganglionic) neurons, this results in pupil dilation; if this neuron is not intact, the pupil does not dilate.
Note anisocoria (which pupil is small, which pupil is larger)
Instill 1 drop of hydroxyamphetamine (1%) in each eye
Wait 45-60 minutes
Re-evaluate anisocoria
Results:
In patients with normal pupils, there is a symmetric 2 mm dilation of each pupil (anisocoria remains) (22).
In patients with Horner syndrome, the reaction is based on whether or not there is an intact 3rd-order (postganglionic) neuron (23):
Both pupils dilate: intact 3rd-order neuron (localizes to 1st- or 2nd-order neuron)
Only non-Horner pupil dilates: not intact 3rd-order neuron (localizes to 3rd-order neuron)
https://www.ophthalmologyreview.org/articles/horner-syndrome-pharmacologic-diagnosis
Tonic pupil
Adie tonic’s pupil denotes a pupil with parasympathetic denervation that constricts poorly to light but reacts better to accommodation (near response)
Light-near dissociation causes
Pupils in coma
Metabolic - small, reactive
Midbrain - mid position, fixed
Pons - pinpoint
Third nerve (uncal) - dilated, fixed
Pupils in coma
Metabolic - small, reactive
Midbrain - mid position, fixed
Pons - pinpoint
Third nerve (uncal) - dilated, fixed
CN IV
Innervates superior oblique - acts to depress, best in ADduction
Inferior oblique
CN III
Action elevation, best in ADduction
Actions of extraocular muscles
Aberrant regeneration of the third nerve
Lid/gaze dyskinesis: retraction on downgaze, adduction
Pupil/gaze dyskinesis: constriction on downgaze, convergence/adduction
Indicates trauma, compressive etiology
Aberrant regeneration of the third nerve
Lid/gaze dyskinesis: retraction on downgaze, adduction
Pupil/gaze dyskinesis: constriction on downgaze, convergence/adduction
Indicates trauma, compressive etiology
Fourth nerve palsy signs
If nerve - ipsilateral eye, if nucleus/fascicle before decussation- contralateral eye
Bincoular, vertical/oblique diplopia, worse in contralateral and down gaze
Fourth nerve palsy often presents with a head tilt away from the affected eye.
With ipsilateral head tilt, the medial utricle is excited.
The medial utricle projects to the contralateral trochlear and oculomotor nucleus through the MLF.
With ipsilateral head tilt, the ipsilateral eye elevates (sup rectus) and intorts (sup rectus and sup oblique) while the contralateral eye depresses (inf rectus) and extorts (inf rectus and inf oblique)
The ocular counterrolling reflex (top) causes a compensatory cyclorotation of both eyes to maintain the subjective visual vertical. On left head tilt, the right eye infraducts and excyclotorts and the left eye supraducts and incyclotorts relative to the position of the head and true vertical. Head velocity signals are encoded by the semicircular canals. The eyes maintain this position tonically because of otolithic inputs from the utricle and saccule on the side of the lower ear. In a right superior oblique palsy (middle right), the right eye is extorted because of the lack of intorsion from the paretic superior oblique. Increased activity to the other intorter—the right superior rectus—causes a hypertropia that worsens when more intorsion is demanded by tilting the head to the right (middle left). A left ocular tilt reaction caused by abnormalities in the vertical vestibulo-ocular reflex projections from the left vestibular system causes a left hypotropia with bilateral torsion in the direction of head tilt (lower right). For each eye, the line through the cornea represents the torsional vertical axis.
Sixth neve palsy
Binocular, horizontal diplopia
Increased in ipsilateral gaze
Worse at distance
Skew deviation
Vertical misalignment
Hypertropia
Midbrain - ipsilateral
Medulla - contralateral
Skew deviation
Vertical misalignment
Hypertropia
Midbrain - ipsilateral
Medulla - contralateral
Downbeat, periodic alternating nystagmus
Localization: cervicomedullary junction,
Downbeat - Clonazepam, lioresal, gabapentin
Periodic alternative - lioresal, phenytoin
Upbeat
Localization: cerebellum, pontomesencephalic, pontomedullary
Convergence retraction
Localization: dorsal midbrain
Brun’s
Localization: cerebellopontine angle
Seesaw
Parasellar>>midbrain
Treatment: Lioresal, clonazepam
Oculopalatal myoclonus
Treatment: Valproic acid, gabapentin
Superior oblique myokymia
Treatment: Carbamazepine, gabapentin
Oculomasticatory myorhythmia
Treatment: CTX
Dx and tx of benign positional vertigo
https://geekymedics.com/dix-hallpike-and-epley-manoeuvres-osce-guide/
Ear down/torsional direction tells you which ear is affected
If upbeat - right PC, if downbeat right AC
Opsoclonus
Features and Causes of Opsoclonus
Dramatic involuntary conjugate multidirectional saccades (saccadomania)
Differs from flutter (no vertical component)
Causes:
- Paraneoplastic (neuroblastoma, anti-Ri, anti-Hu antibodies)
- Postinfectious, encephalitis
- May be benign in neonates
CRAO
Inherited retionpahties - leukodystrophies (Tay-Sachs, Niemann-Pick)
Cherry red spot
Inherited retionpahties - VHL
AD
Retinal angiomas, cerebellar hemangioblastoma
Inherited retionpahties - Kearns-Sayre syndrome
Mitochondrial disorder
Retinal degeneration, chronic progressive external ophthalmoplegia, cardiac conduction defects
Inherited retionpahties - Tuberous sclerosis
Autosomal dominant
Retinal hamartoma, epilepsy, adenoma sebaceum, renal angiomyolipoma, cardiac rhabdomyoma, ungual fibroma
Inherited retionpahties - SCA type 7
AD
Retinal degeneration and progressive ataxia
Acute papilledema
Chronic papilledema with hemorrhages resolved and “champagne cork gloss”
Pseudopapilledema
IIH
Foster Kennedy Syndrome
Optic nerve swelling and contralateral optic nerve pallor Intracranial mass (i.e., subfrontal meningioma) causing compressive optic neuropathy and papilledema
Optic neuritis
Nonarteric ischemic optic neuropathy
Nonarteric ischemic optic neuropathy
Ischemic optic neuropathy
Inflammatory optic neuropathy
Temporal arteritis
Compressive optic neuropathy
Optic neuropathy and neoplasms
Leber’s hereditary optic neuropathy
Mitochondrial, maternal inheritance (11778)
9:1 male predominance
Age 20-30
Painless sequential visual loss
Hyperemia, mild swelling, telangiectasias
Leber’s hereditary optic neuropathy
Mitochondrial, maternal inheritance (11778)
9:1 male predominance
Age 20-30
Painless sequential visual loss
Hyperemia, mild swelling, telangiectasias
Kjer’s dominant optic atrophy
Temporal excavation
Childhood presentation, insidious onset, then stable course
OPA gene mutation
Toxic/nutritional optic neuropathies
Painless, insidious, symmetric
Centrocecal scotoma
Causes: Tobacco-alcohol, B12, ethambutol, amiodarone
Optic nerve hypoplasia
Surrounding visible sclera
Evaluate for: Septo-optic dysplasia (DeMorsier’s syndrome)
– Absent septum pellucidum – Endocrine abnormalities – Cortical heterotopia
Superior disc hypoplasia associated with maternal diabetes
Chiasmal field deficits
Junctional scotoma
Inferior nasal fibers carries visual inferior from superior temporal visual field of contralateral optic nerve
Visual fields in LGN - anterior choroidal vs posterior choroidal
Optic radiations
Optic radiations
Occipital field defects
Macular sparing
The favored explanation for why the center visual field is preserved after large hemispheric lesions is that the macular regions of the cortex have a double vascular supply from the middle cerebral artery (MCA) and the posterior cerebral artery (PCA). If there is damage to one vascular pathway, like in the case of a MCA or PCA stroke, there is still another blood supply that the macular portions of the visual cortex can rely on. Vision in the center of the visual field is then preserved whereas vision in peripheral areas is lost due to the resulting infarct.
Cortical layers
I – molecular (aka tangential) layer. Abuts pia. Consists of horizontal cell axons and dendrites from pyramidal cells in other layers. NO CELL BODIES
II - external granular layer. Granule cell dendrites from molecular layer and axons to deeper layers.
III - external pyramidal (suprastriate) layer. 2 sublayers of pyramidal cells 1) superficial-medium cells (ipsilateral) 2) deeper-large cells (contralateral). dendrites reach to layer I, and axons to other cortical areas.
IV - internal granular layer (external band of Baillarger). stellate cells which mediate between inputs from other areas to pyramidal dendrites and axons, and from pyramidal dendrites and axons. Often divided into two sublayers IVa and IVb. External band of Baillarger is dense horizontal plexus of myelinated fibres in this layer.
V - internal pyramidal layer. pyramidal cells intermingled with granule and Martinotti cells. The dendrites of the large-sized pyramidal cells reach to layer I, the dendrites of the small-sized pyramidal cells reach only to layer IV, or stay within layer V.
VI – polymorphic layer (aka multiform or fusiform layer). Consists of spindle cells with axons perpendicular to the cortical surface. Larger ones send dendrites to layer I and smaller ones to layer IV.
White matter cortical connections
Projection fibers to and from the cortex - internal/external capsule
Commissural Fibers between hemispheres - anterior and posterior commissure and corpus callosum
Association Fibers between cortical areas within a hemisphere - arcuate fascilius, U fibers
Nucleus accumbens
Seen where the caudate and putamen are not divided by the anterior limb of the internal capsule
Septal nuclei and Nucleus Basalis of Meynert
Amygdala
Hippocampal formation
Papez circuit
Developmental of ventricle system
Ventricular System
Begins as a hole or space in the middle of the developing nervous system that runs from the most rostral end (lamina terminalis) down through the spinal cord
As the brain vesicles grow, the ventricular system grows and expands along with it.
BBB
Regulated interface between the peripheral circulation and the CNS
Components include endotheilial tight junctions, basal lamina, astrocyte processes
Tight junction is an intricate complex of transmembrane (junctional adhesion molecule-1, occludin, and claudins) and cytoplasmic (zonula occludens-1 and -2, cingulin) proteins linked to the actin cytoskeleton
A small number of regions in the brain (circumventricular organs, pineal gland) do not have a blood–brain barrier
Generally permeable to smaller, lipophillic molecules
Impermeable to most macromolecules, microorganisms
Specific activated transport for glucose and amino acids
Parasympathetic nervous system
Cell bodies located in the brain and sacral (S2-4) spinal cord
– Axons exit via cranial or sacral nerves and synapse with neurons located in a ganglion that is integrally associated with the target organ/viscera (Long pre-synaptic and short post-synaptic)
– Sacral paraspympathetic innervation includes bladder, ureter, kidneys, rectum, and colon beyond the left colonic flexure
Parasympathetic nervous system
Cell bodies located in the brain and sacral (S2-4) spinal cord
– Axons exit via cranial or sacral nerves and synapse with neurons located in a ganglion that is integrally associated with the target organ/viscera (Long pre-synaptic and short post-synaptic)
– Sacral paraspympathetic innervation includes bladder, ureter, kidneys, rectum, and colon beyond the left colonic flexure
Sympathetic nervous system
The second order sympathetic neurons arise from the intermediolateral column from T1-L2.
Each spinal nerve from T1-L2 contains sympathetic axons, which can either
– Leave the spinal nerve in a white communicating ramus to enter the sympathetic chain to either synapse at that level or ascend and descend in the sympathetic chain.
– Continue on to synapse in collateral ganglia (celiac, superior mesenteric, and inferior mesenteric ganglia) near their target.
All spinal nerves receive third order sympathetic neurons from the sympatheic chain via a gray communicating ramus.
CERVICAL spinal nerves do not have white rami, but they do have grey rami
Recall that there are eight cervical nerves -> eight sets of grey rami.
The cervical ganglia have undergone fusions
– superior cervical ganglion joins to C1-4
– middle cervical ganglion connects to C5 and C6
– inferior cervical ganglion connects to C7 and C8
Spinal nerves below L2 do not have white rami, but they do have grey rami
All presynaptic fibers for ganglia L3-5 and S1-4 enter the lateral chain via white rami of L1,L2.
Unlike with the cervical ganglia, there has been no fusion between any of the lumbosacral series
The coccygeal ganglia merge into a single midline ganglion.
Urination
Peripheral bladder sensation
Stretch receptors in the detrusor muscle of the bladder are activated with increased urinary volume.
Sensory afferents stimulate efferent parasympathetic bladder contraction (Detrusor spinal reflex)
– This reflex is inhibited by descending input from the pons
Central bladder sensation
Bladder fullness sensed by sympathetic (T12-L2) and parasympathic (S2-S4) afferents
Projection to periaquaductal gray (PAG)
PAG projects to the pontine micturation center (PMC, medial pons) and an area of lateral pons that maintains continence
Central control (from frontal cortex, hypothalamus)
Sympathetic bladder fibers
Exit T10-L2
Many different pathways to the bladder (several ganglia of pelvic plexus)
Excites involuntary sphincter muscles via alpha1-adrenergic receptors**
Inhibits detrusor muscle via beta-adrenergic receptors**
Maintains urinary retention (continence)
Parasympathetic bladder fibers
Exit S2-S4
Activate the detrusor muscle of the bladder via M3 muscarinic receptors**
Causes urination
The pontine micturation center activates the sacral parasympathetic system and inhibits the sympathetic system
Sacral parasymapthetic fibers activate the detrusor muscle of the bladder via M3 muscarinic receptors**
Inhibition of sympathetic system results in reduced involuntary sphincter contraction
Clinical Correlates: Bladder
Injury to descending inhibition from the pons (usually in the spinal cord) results in decreased inhibition of the detrusor spinal reflex.
– As a result, small increases in urinary volume that activate stretch receptors in the detrusor muscle result in reflexive parasympathetic bladder contraction (spastic bladder)
– This is treated with antagonists to normal parasympathetic bladder contraction (anti-muscarinics oxybutynin, tolterodine)
Lesions to the sacral cord or cauda equina cause decreased parasympathetic bladder contraction (flaccid bladder and overflow incontinence), which is treated with catheterization
Functional divisions of the cerebellum
Functional Divisions
Vestibulocerebellum: flocculus and nodulus
– Modulates gaze holding
Spinocerebellum (somatotopically organized): vermis and intermediate hemispheres
– The vermis modulates movement of the axial muscles. The dorsal vermis modulates saccadic accuracy
– The Intermediate hemispheres modulate limb movement.
Cerebrocerebellum: lateral hemispheres
– Plans movement and modulates fine movements.
Microanatomy of the cerebellum
The cerebellar circuit is a 3 or 4 neuron arc.
Mossy fibers enter the cerebellum to synapse and excite granular cell neurons, which excite Purkinje cell neurons, which inhibit deep cerebellar nuclei
Climbing fibers from the inferior olive enter the cerebellum to synapse directly and excite Purkinje cell neurons, which inhibit deep cerebellar nuclei
Cerebellar interneurons modulate this arc.
Modulating Interneurons of the Cerebellum
Stellate and Basket cells inhibit Purkinje cells – Basket cells have a stronger effect because they
synapse closer to the cell body.
Golgi cells inhibit granular cells
In total, all intrinsic cerebellar neurons are inhibitory except the granular cells!
Deep Cerebellar Nuclei and Projections
Vestibulocerebellum: flocculus and nodulus
– Medial vestibular nucleus (displaced cerebellar nucleus)
Spinocerebellum: vermis
– Fastigial nucleus, which projects to the reticular formation and lateral vestibular nucleus (involuntary axial tracts)
Spinocerebellum: intermediate hemispheres
– Interposed nucleus (globose and the emboliform nuclei), which projects to the red nucleus and VL of the thalamus (involuntary and voluntary appendicular tracts)
Cerebrocerebellum
– Dentate nucleus, which projects to the red nucleus and VL of the thalamus (involuntary and voluntary appendicular tracts)
Guillain-Mollaret Triangle
CL dentate
Cerebellar peduncles
Large bundles of axons entering and exiting the cerebellum.
Superior: Efferent greater than Afferent
–Receives Ventral Spinocerebellar and Rostral Spinocerebellar tracts for Spinocerebellum
– Projects output from dentate and interposed nuclei to red nucleus and ventral lateral nucleus of the thalamus
Middle: Purely Afferent
– Receives Pontine relay axons from Cortex to Cerebrocerebullum
Inferior (Restiform): Mixed Afferent and Efferent
– Receives Dorsal Spinocerebellar and Cuneospinocerebellar tracts for Spinocerebellum (Vestibular nuclei projections for Vestibulocerebellum\_
– ReceivesInferiorOlive(climbingfibers)foralldivisions
– Projects output from the fastigial nucleus to the reticular formation and lateral vestibular nucleus and flocculus/nodulus purkinje cells to the medial vestibular nucelus
Lesion of the flocculus
Gaze-evoked nystagmus
Lesion of the nodulus
Periodic alternating nystagmus
Lesion of the triangle of Mollaret
Oculopalatal Tremor
Lesion of the vermis
Truncal ataxia and tremor
Lesion of the intermediate hemispheres
Ipsilateral limb ataxia
Sensory CN
General (Somatic): Receives sensory innervation from muscle (proprioception) and skin (pressure, vibration, temperature, pain, itch).
– The 5th cranial (trigeminal) nerve primarily serves this role.
Visceral: Receives sensory innervation from organs (10th CN) and taste receptors (7th, 9th, 10th).
Special: Receives sensory innervation from cochlea (8th CN) and vestibular apparatus (8th CN).
Motor CN
General (Somatic): Innervates skeletal muscle of the eyes (3rd, 4th, 6th), and tongue (12th)
Visceral(Autonomic): Parasympathetic innervation to the pupil/lens (3rd), lacrimal gland (7th), salivary gland (7th), parotid gland (9th), and heart/lungs/2/3 GI tract (10th)
Branchial: Innervates skeletal muscles derived from a particular embryologic structure (pharyngeal arches). (5th, 7th, 9th, 10th,11th).
4-4-4 rule of brainstem
There are 4 cranial nerves in the medulla (cranial nerves 9 to 12)
There are 4 cranial nerves in the pons (cranial nerves 5 to 8)
There are 4 bumps in the midbrain. These are the colliculi.
– At the level of superior colliculus leaves the third nerve.
– At the level of the inferior colliculus leaves the fourth nerve.
Ventral to dorsal localization
- *Pure sensory cranial nerves (5 and 8) and the cerebellar peduncles are dorsal in the brainstem.**
- *Medial longitudinal fasciculus is dorsal**
- *Corticospinal tract is ventral**
Motor cranial nerve involvement can be challenging to localize in the ventral to dorsal plane.
Most of the motor cranial nerve nuclei are dorsal and medial but their axons travel ventrally to exit, except for the 4th cranial nerve.
A 4th nerve lesion within the brainstem must be dorsal.
Otherwise, unless you can differentiate a motor cranial nerve nucleus lesion from a fascicular lesion, you do not want to rely on motor cranial nerve involvement to differentiate dorsal from ventral.
Midline structures
Localized in the transverse plane by the long tracts that are affected.
Medial: 4 “M” structures
Motor (corticospinal tract)
Medial lemniscus - DCML
Motor nuclei (3, 4, 6, and 12) - CN deficit
Medial longitudinal fasciculus - INO
With the exception of the caudal medulla for proprioception/vibration (prior to decussation of DCML as it ascends), the lesion will be contralateral to the hemiparesis or hemisensory deficit.
With the exception of the trochlear nucleus, the lesion will be ipsilateral to the INO or cranial nerve deficit
Lateral structures
Localized in the transverse plane by the long tracts that are affected.
Lateral: 4 “S” structures
Spinothalamic tract
Spinocerebellum (cerebellar peduncles).
Spinal nucleus of 5
Sympathetic system (descending first order axons)
With the exception of the medulla, the lesion will be contralateral to the facial or hemisensory deficit.
With the exception of the brachium conjunctivum (superior cerebellar peduncle), the lesion will be ipsilateral to the Horner’s syndrome or ataxia.
Aniscoria
Pathologic Anisocoria is always an Efferent Problem
Midbrain syndromes
CN III: nerve vs. nucleus
Ptosis and superior rectus are bilateral in lesions of the nucleus
CN IV palsy
Innervates the superior oblique that intorts the eye and depresses the adducted eye.
Superior oblique dysfunction results in an elevated eye (hypertropia) in primary gaze, which increases with eye adduction.
The axons from the trochlear nucleus decussate to form the contralateral trochlear nerve
A nuclear lesion causes a contralateral hypertropia (vs nerve, which causes ipsilateral)
Foville syndrome
Foville Syndrome:
Infarction of the dorsal pontine tegmentum involving:
6th nerve nucleus: Ipsilateral horizontal gaze palsy (“nuclear” 6th nerve palsy)
7th nerve branchial nucleus (or fascicle): ipsilateral facial weakness with forehead involvement.
+medial lemniscus and corticospinal tracts ->contralateral hemisensory loss, hemiparesis
CN VI: Lesion of peripheral nerve vs nucleus
Peripheral nerve - atrophy of ipsilateral LR, ipsilateral eye deviated medially, diplopia
Nucleus - inability to move moves both eyes past midline to look ipsilateral to the lesion, atrophy of ipsilateral LR, but not contralateral medical rectus
Horizontal eye movement dysfunction (VI)
Nuclear VI vs 1 and 1/2 syndrome
Nuclear VI affects both the ipsilateral lateral rectus muscle and the contralateral MLF
1 and ½ syndrome affects the ipsilateral abducens nucleus or PPRF (thereby affecting the ipsilateral later rectus and contralateral MLF) and the ipsilateral MLF
Pontine syndromes involving the 6th nerve
Cavernouos sinus
ICA
III, IV, VI
V1, V2
Cavernous sinus vs superior orbital fissure vs orbital apex
Lesions involving 3rd, 4th, 6th nerves and V1 (2)
Trochlear head tilt
Occurs contralateral to the affected eye
With ipsilateral head tilt, the medial utricle is excited.
The medial utricle projects to the contralateral trochlear and oculomotor nucleus through the MLF.
With ipsilateral head tilt, the ipsilateral eye elevates (sup rectus) and intorts (sup rectus and sup oblique) while the contralateral eye depresses (inf rectus) and extorts (inf rectus and inf oblique)
The ocular counterrolling reflex (top) causes a compensatory cyclorotation of both eyes to maintain the subjective visual vertical. On left head tilt, the right eye infraducts and excyclotorts and the left eye supraducts and incyclotorts relative to the position of the head and true vertical. Head velocity signals are encoded by the semicircular canals. The eyes maintain this position tonically because of otolithic inputs from the utricle and saccule on the side of the lower ear. In a right superior oblique palsy (middle right), the right eye is extorted because of the lack of intorsion from the paretic superior oblique. Increased activity to the other intorter—the right superior rectus—causes a hypertropia that worsens when more intorsion is demanded by tilting the head to the right (middle left). A left ocular tilt reaction caused by abnormalities in the vertical vestibulo-ocular reflex projections from the left vestibular system causes a left hypotropia with bilateral torsion in the direction of head tilt (lower right). For each eye, the line through the cornea represents the torsional vertical axis.
Skew deviation
Low-Low: Disruption of the medial utricle fibers prior to decussation in the pontomedullary junction, results in an imbalance of tonic signal from the utricles simulating a contralateral head tilt.
The ipsilateral eye is depressed and extorted (“hypotropia”)
High-High: Disruption of the medial utricle fibers after decussation as they ascend in the MLF, also results in an imbalance of tonic signal from the utricles.
However, since the fibers have already crossed, this simulates an ipsilateral head tilt.
The ipsilateral eye is elevated and intorted (hypertropia)
With ipsilateral head tilt, the medial utricle is excited.
The medial utricle projects to the contralateral trochlear and oculomotor nucleus through the MLF.
With ipsilateral head tilt, the ipsilateral eye elevates (sup rectus) and intorts (sup rectus and sup oblique) while the contralateral eye depresses (inf rectus) and extorts (inf rectus and inf oblique)
The ocular counterrolling reflex (top) causes a compensatory cyclorotation of both eyes to maintain the subjective visual vertical. On left head tilt, the right eye infraducts and excyclotorts and the left eye supraducts and incyclotorts relative to the position of the head and true vertical. Head velocity signals are encoded by the semicircular canals. The eyes maintain this position tonically because of otolithic inputs from the utricle and saccule on the side of the lower ear. In a right superior oblique palsy (middle right), the right eye is extorted because of the lack of intorsion from the paretic superior oblique. Increased activity to the other intorter—the right superior rectus—causes a hypertropia that worsens when more intorsion is demanded by tilting the head to the right (middle left). A left ocular tilt reaction caused by abnormalities in the vertical vestibulo-ocular reflex projections from the left vestibular system causes a left hypotropia with bilateral torsion in the direction of head tilt (lower right). For each eye, the line through the cornea represents the torsional vertical axis.
Brainstem lesions and CN deficits - laterality
Localized in the vertical plane by the cranial nerve nuclei or cranial nerve axons that are affected.
With the exception of the 4th cranial nerve, the lesion is ipsilateral to the cranial nerve deficit
Brachial plexus - what you need to know
Remember To Drink Cold Beer
Roots Trunks Divisions Cord Branches
MARMU
Musculocutanoeus Axillary Radial Median Ulnar
ULNAR nerves from the posterior cord
Upper subscapularis nerve Lower subscapularis nerve Nerve to lastissimus dorsi Axillary Radial
Good video https://www.youtube.com/watch?v=UlDFSlRBeCE&t=1089s
Long thoracic nerve
Off the C5-C7 nerve roots
Innervates serratus anterior
Suprascapular nerve
Off the upper trunk from C5-C6
Innervates supraspinatus and infraspinatus - first 15 degrees of arm abduction
Thoracodorsal nerve
AKA nerve to lasttismus dorsi
Off the posterior cord, C6-C8
Innervates lattismus dorsi
Medial cutaneous nerves
Off the medial cord
C8-T1
Musculocutaneous nerve
Sensation to lateral forearm
Responsible for
- Biceps: arm flexion
What cannot you not distinguish clinically
Middle and lower trunk injuries CANNOT be distinguished from root injuries clinically!!!
To distinguish middle and lower trunk from root injuries, you need NCS/EMG.
Lumbar plexus injuries CANNOT be distinguished from L2-4 root injuries clinically!!!
To distinguish lumbar plexus from L2-4 root injuries, you need NCS/EMG.
Axillary
Sensation to posterior/lateral surface of deltoid
Deltoid - arm abduction
Radial nerve
Posterior interosseous nerve (superficial branch) - cutaneous innervation of lateral aspect of dorsal hand
Triceps - arm extension (at the axilla or more proximal only)
Brachioradialis - arm extension when half pronated
Extensor carpi radialis longus - wrist extension and medial deviation
Supinator - supination
Posterior interosseous nerve (deep branch) - wrist, finger, and thumb extension
Median nerve
Sensation over the radial aspect of palm of the hand and thumb and finger tips - distinction between via forearm vs via CT
Pronator teres - forearm pronation
Flexor carpi radialis - wrist flexion
Anterior interosseous muscles - inability to make OK sign (flexor digitorum profundus 2&3, flexor pollicis longus)
Via carpal tunnel - finger flexion, thumb abduction (abductor pollicis brevis)
Ulnar nerve
Sensation over medial aspect of hand - palmar +/- dorsal, depending on where the compression is
Flexor carpi ulnaris - wrist lateral flexion
First dorsal interosseus) - finger abduction/adduction
Flexor digitorum profundus 4&5 - 4th and 5th digit flexion
Lumbar plexus
L1-L4 (part with obturator)
I (twice) Got Lunch On Friday
Posterior - lateral femoral cutaneous and femoral, rest are anterior
Sacral plexus
L4 (part without obturator)-S4
Each branch will come from three nerve roots and if you put them in order the branches of the lumbrosacral plexus will start with one nerve root lower than the preceding branch
Lateral femoral cutaneous
Sensation to lateral aspect of thigh
Implicated in meralgia parasthetica
Obturator nerve
Sensation as below - small aspect of medial thigh
Adduction brevis, longus, magnus - thigh adduction
Femoral nerve
Sensation - medial aspect of thigh and lower leg
Hip flexion - femoral above the inguinal canal, not below; innervated by L1-L3 (psoas) and L2-L3 (illiaus) from branches off the femoral above the inguinal canal
Quadriceps vastus, rectus femoris - knee extension
Gluteal nerves
Superior gluteal nerve (L5 primarily, but also L4, S1) innervates gluteus minimus, medius muscles, tensor fasciae latae, performs hip abduction and internal rotation
Inferior gluteal nerve (S1-S2, but also L5 minimally) – gluteus maximus, performs Hip extension
Sciatic nerve
L4-S3 (L5-S2 tibial and common perineal (other sources say L4-S3))
Sensation to lateral lower leg - involving tibial and perineal territories
Semimembranosus, semitendinosus - hip extension
Tibial nerve
Part of the sciatic nerve nerve (L5-S2 (other sources say L4-S3 like sciatic))
Sensation to lateral aspect of the back of the lower leg and dorsum of the foot
Ankle and toe plantarflexion (calf): S1
Soleus
Gastrocnemius
Plantaris
Popliteus
Flexor digitorum longus
Flexor hallucis longus
Foot Inversion: Tibialis posterior L5
Peroneal
Common peroneal = fibular (L5-S2 (other sources say L4-S3 like sciatic) , predominantly L5)
Sensation to the lateral aspect of the front of the lower leg
Biceps femoris short head
Superficial peroneal
Peroneus longus and brevis - foot eversion
Deep peroneal
Extensor hallucis + digitorum longus - toe dorsiflexion
Tibialis anterior - ankle dorsiflexion
LS plexus
L4 (part of L4 = part wo obturator) to S4
Upper arm weakness
Lower arm weakness
Hand weakness
Thigh weakness
Foot drop
Can’t stand on toes
EOMI
Primary and secondary action
Superior oblique: Depression/intorsion - CN IV
Inferior oblique: Elevation/extorsion
Superior rectus: Elevation/intorsion
Inferior rectus: Depression/extorsion
Only act in the horizontal plane no secondary action
Medial rectus: Adduction
Lateral rectus: Abduction - CN VI
Others all CN III
Eyelid innnervation
Upper and lower eyelids open and close 2/2 CN VII innervation of the orbicularis oculi
Upper eyelid opening by levator palpebrae superioris 2/2 CN III.
Upper eyelid opening by Muller’s muscle (arises from undersurface of lelevator palpebrae) 2/2 sympathetic innervation -> 1-2 mm of upper eyelid elevation
Superior and inferior tarsal muscles contribute to slight upper eyelid elevation and lower eyelid depression respectively, also 2/2 sympathetic innervation
- Due to the sympathetic innervation of the eyelid muscles, slight overelevation of the eyelid may be seen in high sympathetic states (such as fear), and subtle ptosis may be seen in low sympathetic states (such as fatigue).
- Slight overdepression of the lower eyelid may be seen in high sympathetic states and subtle elevation of lower eyelid in low sympathetic states
- In normal patients, the upper eyelid should cover the superior 1 to 1.5 mm of the limbus (junction of the sclera with the cornea), and the lower eyelid should lie at the inferior limbus.
Horner’s syndrome - clinical
Horner’s syndrome is characterized by:
1. Ptosis of the upper eyelid (due to impaired superior tarsal and Müller’s muscles, which normally contribute to upper eyelid elevation).
2. Slight elevation of the lower eyelid (due to impaired inferior tarsal muscle function, which normally contributes to lower eyelid depression).
3. Pupillary miosis (impaired pupillodilator function).
4. Facial anhidrosis (if dissection or other lesion extends proximal to the region of the carotid bifurcation, because sweating fibers travel primarily with the ECA and would not
be involved in an ICA dissection).
5. Enophthalmos (appearance of enophthalmos from decrease in
palpebral fissure).
Horner’s syndrome - sympathetic pathway
3 neuron pathway
1) 1st order: central - originate in the posterior hypothalamus and descend through brainstem to the first synapse located in the lower cervical and upper thoracic spinal cord (C8 to T2, aka clilospinal center of Budge)
2) Second order neurons exist spinal cord, travel near apex of lung, under subclavian artery, and ascend the neck and synapse in the superior cervical ganglion, near the bifurcation of the carotid artery at the level of the angle of the mandilbe
3) The third order neurons travel with the carotid artery - vasomotor and sweat fibers branch off at the superior cervical ganglion near the level of the carotid bifurcation and travel to the face with the ECA. The oclumosympathetic fibers continue with the ICA through the cavernous sinus to the orbit, where they travel with V1 division of the trigeminal nerve to their destinations.
Differentiation and localization in Horner’s syndrome
Differentiation between causes of Horner’s syndrome can be difficult and depends on the location along the pathway.
- In general, a lesion to the first-order neurons (central neurons) will be associated to brainstem or other focal neurologic findings from a central lesion.
- A second-order (preganglionic) lesion is often associated with lesions of the neck, mediastinum, or lung apex.
- A third-order (postganglionic) lesion is often associated with pain or headache, caused by conditions such as a skull base tumor, or carotid dissection.
Cocaine 4% or 10% eye drops are sometimes used for confirmation of a Horner’s syndrome. Cocaine blocks the reuptake of norepinephrine released at the neuromuscular junction of the iris dilator muscle, allowing more local availability of norepinephrine. Following instillation of cocaine, the sympathetically denervated eye will not respond and the anisocoria will become more pronounced. (The Horner’s pupil will not change, but the unaffected pupil will become more dilated).
Hydroxyamphetamine 1% eye drops will differentiate between a lesion affecting the first- or second- order neurons from a third-order neuron. There is no pharmacologic test to distinguish between a first-and second-order lesion. Hydroxyamphetamine causes release of stored norepinephrine in the third-order neurons. Following instillation, if the Horner’s pupil dilates, the lesion is either involving the first- or second-order neurons. If the Horner’s pupil does not dilate, there is a third-order neuron lesion.
Complete pupil-sparing oculomotor nerve palsy
Most often caused by ischemia to the oculomotor nerve. This is frequently associated with diabetes, especially in the setting of other vascular risk factors.
The pupillomotor fibers travel along the peripheral aspects of the oculomotor nerve, whereas the somatic fibers to the muscles innervated by the oculomotor nerve travel centrally. The terminal branches of the arterial supply to the nerve are most affected by microvascular changes from diabetes and other risk factors as the vessels decrease in diameter from the periphery of the nerve to the central regions. Therefore, the supply to the periphery of the nerve (where the pupillomotor fibers reside) is spared, whereas the central fibers are affected. Compressive lesions (such as posterior communicating artery aneurysms) typically affect the peripheral pupillomotor fibers, leading to pupil dilatation with poor response to light (although rarely there may be some pupil sparing).
Nuclei of CN III
At the level of the superior colliculus in the dorsal midbrain, there are paired and separate oculomotor subnuclei for the inferior rectus, medial rectus, and inferior oblique—all providing ipsilateral innervation. HERE nuclei for SR contralateral, per Ray bilaterally
The paired midline Edinger–Westphal subnuclei provides parasympathetic innervation to the iris sphincters and ciliary muscles. There is also a midline subnucleus providing innervation to both levator palpebrae superioris muscles. Therefore, a lesion to this single midline nucleus can cause bilateral ptosis, but it would be rare to affect only this nucleus without affecting nearby structures, and other clinical findings are expected to be present.
Pupillary reflex
Afferent neurons beginning in retinal ganglion cells (carrying signals from light stimulation) travel through the optic nerve to the optic chiasm where decussation occurs. Nasal retinal fibers (carrying information from temporal fields) decussate at the chiasm and travel in the contralateral optic tract. Temporal retinal fibers (carrying information from nasal fields) travel ipsilaterally in the optic tract. In the optic tracts, some neurons project to the ipsilateral lateral geniculate body (for vision) and a few leave the optic tract, ipsilaterally enter the brachium of the superior colliculus, and synapse in the ipsilateral pretectal nuclei (for pupillary response).
Therefore, each pretectal nucleus receives light input from the contralateral visual hemifield.
From each pretectal nucleus, the afferent signals travel via interneurons, connecting ipsilaterally and contralaterally in the Edinger– Westphal nuclei, respectively, completing the afferent arm.
From the Edinger–Westphal nucleus, efferent preganglionic parasympathetic fibers travel concurrently through the bilateral oculomotor nerves to the ciliary ganglia, which innervate the iris sphincter muscles and the ciliary muscles, resulting in pupillary constriction and ciliary muscle activation that leads to accommodation (for near vision) with increased curvature of the lens.
Trochlear nerve pathway
Of note, the trochlear nerve fibers decussate just before they exit dorsally at the level of the inferior colliculi of the midbrain. Therefore, motor neurons from each trochlear nucleus innervate the superior oblique muscle contralateral from its nucleus After exiting, the trochlear nerve curves ventrally around the cerebral peduncle and passes between the posterior cerebral and superior cerebellar arteries, lateral to the oculomotor nerve. Although it is the smallest nerve, the trochlear nerve has the longest intracranial course due to this dorsal exit, making it more prone to injury, as seen in this patient. The trochlear nerve innervates the superior oblique muscle, which allows for depression and intorsion of the eye, especially when the eye is adducted.
Lesions of the trochlear nerve can either involve the nucleus or the nerve, but both virtually present with similar symptoms. The only difference is that a unilateral trochlear nuclear lesion affects the contralateral nerve and superior oblique muscle, while a fascicular lesion (after the decussatino
0 affects the ipsilateral nerve and muscle
Trochlear nerve palsy
Patients with trochlear nerve palsies may complain of vertical diplopia and/or tilting of objects (torsional diplopia).
Because of loss of intorsion and depression from the superior oblique muscle, the affected eye is usually extorted and elevated due to unopposed action of its antagonist, the inferior oblique.
Objects viewed in primary position or downgaze may appear double (classically, when going down a flight of stairs).
Symptoms of diplopia often improve with head tilting to the contralateral side of the affected eye, and the patient adapts to this primary head position to avoid the diplopia.
MC aneurysm location for CN III palsy
PComm
But can also be seen in aneurysms of the basilar tip, PCA, SCA, Pcomm
Uncal herniation and CN III
Uncal herniation also is a classic cause of third nerve palsy, although the patient is often comatose by the time this would occur.
Pathways that control horizontal eye movement
The PPRF is also known as the conjugate gaze center for horizontal eye movements.
The PPRF receives contralateral cortical input. Normally, on horizontal eye movement initiated by the contralateral premotor frontal cortex, the PPRF activates the ipsilateral abducens nerve nucleus and, thus, the ipsilateral lateral rectus muscle.
From the activated ipsilateral abducens nerve nucleus, fibers cross the midline, enter the contralateral MLF, and activate the contralateral medial rectus subnucleus of the oculomotor complex and, thus, the contralateral medial rectus muscle. The end result is a finely coordinated gaze deviation to one side, with abduction of one eye and adduction of the other.
INO
The INO is named for the side of limited adduction.
Results from a lesion in the MLF, ipsilateral to the impaired adducting eye, as it runs through the pons or midbrain tegmentum. Patients may complain of horizontal diplopia on lateral gaze, which is not usually present in primary gaze. The classic findings include impaired adduction on lateral gaze (the side of the affected MLF), with nystagmus in the contralateral abducting eye. Slowing of the adducting eye may be a sign of a partial INO, as can be detected on optokinetic nystagmus testing.
A bilateral INO, due to bilateral MLF lesions, will cause exotropia of both eyes and is known as “wall-eyed bilateral INO” (WEBINO)
Which nerve is mostly likely to be affected with increased ICP
The abducens nerve (cranial nerve VI) is prone to a stretching injury, especially as it passes over the petrous ridge, and is the most likely nerve to be involved with elevated intracranial pressure. An abducens nerve palsy due to elevated intracranial pressure is often bilateral and is termed a “false localizing sign” because this long cranial nerve could be affected anywhere along its path, and does not necessarily reflect a specific central lesion. The action of the abducens nerve is purely abduction of the eye due to its innervation of the lateral rectus muscle.
Adie’s tonic pupil
It is thought to result from a lesion in the postganglionic parasympathetic pathway to either t_he ciliary ganglion or the short ciliary nerves_ and is most often attributed to viral etiology, although evidence is lacking.
Acutely, there is unilateral mydriasis and the pupil does not constrict to light or accommodation because the iris sphincter and ciliary muscle are paralyzed. Sectoral palsy of part of the iris sphincter may be involved, and is considered the earliest and most specific feature. Patients often complain of photophobia, visual blurring, and ache in the orbit.
Within a few days to weeks, denervation supersensitivity to cholinergic agonists develops and this is most often tested with low-concentration pilocarpine 0.125%, in which the tonic pupil will constrict but the normal pupil is unaffected by the low concentration.
Eventually, slow, sustained constriction to accommodation and slow redilation after near constriction occur, and the baseline pupil decreases slightly in size (in ambient light), whereas the other features remain. In general, the chronic stage is characterized by the pupillary light reflex rarely improving, whereas the accommodation reflex does improve, although it often remains slower (tonic). This is termed “light-near dissociation.” It is sometimes associated with diminished or absent deep tendon reflexes and this is referred to as “Holmes–Adie syndrome,” or Adie’s syndrome.
Argyll Robertson pupils
Classically associated with neurosyphilis, bilateral irregular mitosis with little or no constriction to light but constriction to accommodation without a tonic (slower) response
RAPD
This is done by shining the light in the first eye for 3 seconds. In a normal response, the pupil of the eye being illuminated reacts briskly and constricts fully to the light, as does the pupil of the other eye (consensual reflex). Then, the light should be moved quickly to shine in the other eye for 3 seconds. Changes in the pupil should be noted, whether the pupil being illuminated stays the same size, constricts further, or gets bigger. In the absence of a RAPD, both pupils should again constrict to the light shone in the opposite eye as well. When the light is shone into an eye with a RAPD, the pupils of both eyes will constrict, but not completely. This is explained by a defect in the afferent pathway in this eye. When the light is then moved to stimulate the normal eye, both pupils will constrict further since the afferent pathway of this eye is not impaired. Then, when the light is moved back to shine into the abnormal eye again, both pupils will get larger due to the afferent defect in the pathway of that eye.
Optic neuritis
Develops over hours to days and is associated with symptoms of reduced color perception (especially red → red desaturation), reduced visual acuity, visual loss, eye pain and photopsias.
⅓ patients have papilla’s with hyperemeia and swelling of the disc, blurring of disc margins, and distended veins. The rest of the cases have only retrobulbar involvement and therefore have a normal fundoscopic exam.
Treatment: IV Solumedrol followed by oral prednisone taper
Fundoscopic exam - papilledema
Early finding in papilledema is loss of spontaneous venous pulsations, although the absence of spontaneous venous pulsations can also be a normal variant. Disc margin splinter hemorrhages may be seen early also. Eventually, the disc becomes elevated, the cup is lost, and disc margins become indistinct. Blood vessels appear buried as they course the disc. Engorgement of retinal veins lead to a hyperemic disc. As the edema progresses, the optic nerve head appears enlarged and may be associated with flame hemorrhages and cotton wool spots, as a result of nerve fiber infarction.
Anterior ischemic optic neuropathy (vs PION)
AION is considered to be the most common optic nerve disorder in patients older than age 50. It can also affect the retrobulbar optic nerve in isolation, in which case it is termed posterior ischemic optic neuropathy (diagnosis of exclusion).
AION is a result of ischemic insult to the optic nerve head. Clinically, it presents with acute, unilateral, usually painless visual loss, although 10% of patients may have pain that can be confused with optic neuritis. Fundoscopic examination shows optic disc edema (unless retrobulbar), hyperemia with splinter hemorrhages, and crowded and cupless disc.
The painless vision loss is one key feature in differentiating AION from optic neuritis, which is often associated with painful eye movements.
Fundoscopic exam in AION vs GCA
In contrast to giant cell arteritis (GCA), the optic disc edema in AION is more often hyperemic rather than pallid, as would be more common in GCA.
Visual pathways
In contrast to giant cell arteritis (GCA), the optic disc edema in AION is more often hyperemic rather than pallid, as would be more common in GCA.
Optic atrophy, signs of chronic optic neuritis - persistent visual loss, color desaturation (especially red), and possibly a persistent relative afferent pupillary defect. In optic atrophy the disc appears shrunken and pale, especially in the temporal half, and this pallor extends beyond the margins of the disc.
Bell’s palsy - clinical features
Relatively abrupt onset of unilateral facial paralysis, which often includes difficulty closing the eye, drooping eyebrow, mouth droop with loss of nasolabial fold, loss of taste sensation on the anterior two-thirds of the tongue (in distribution of facial nerve), decreased tearing, and hyperacusis. Patients may complain of discomfort behind or around the ear prior to symptom onset. There may also be a history of recent upper respiratory infection. It is important to differentiate between a peripheral and central (upper motor neuron) lesion. Sparing of the forehead muscles suggests a central lesion because of bilateral cortical supply to the facial subnuclei innervating the forehead, as opposed to unilateral cortical supply to the facial subnucleus innervating the lower face (below the eye). However, a lesion to the facial nerve nucleus itself in the pons can lead to complete facial paralysis (of both the upper and lower face). Bell’s palsy should classically involve only the facial nerve, although additional cranial nerve involvement has been infrequently reported, including the trigeminal, glossopharyngeal, and hypoglossal nerves. Some studies have reported ipsilateral facial sensory impairment suggesting trigeminal neuropathy, although this sensation has often been attributed to abnormal perception on the basis of “droopy” facial muscles.
For patients with new onset Bell’s palsy, steroids can be very effective and should be offered to increase the probability of recovery of facial nerve function (2 Class I studies, Level A). The addition of antiviral agents does not significantly increase the probability of facial functional recovery, but a modest benefit cannot be excluded. Due to the possibility of a modest benefit, patients might be offered antivirals (in addition to steroids) (Level C), particularly in more severe cases of facial paralysis or those with possible zoster sine herpete.
Artificial tears and eye patches should also be used for eye protection when needed. Nerve stimulation and surgical decompression are not routinely recommended on the basis of current evidence.
Imaging in Bell’s palsy
Imaging should be considered if there is slow progression beyond 3 weeks, if the physical signs are atypical, or if there is no improvement at 6 months. f imaging is pursued, an MRI with and without gadolinium is optimal. Electrodiagnostic studies may be considered in patients with clinically complete lesions for prognostic purposes if they do not improve.
“Crocodile tears”
Results when misdirected regenerating facial nerve axons originally supplying the submandibular and sublingual salivary glands, innervate the lacrimal gland through the greater petrosal nerve. This anomalous innervation results in abnormal unilateral lacrimation when eating. I_n addition, some axons from the motor neurons to the labial muscles involved in smiling may regenerate and misdirect to the orbicularis oculi, which results in closure of the eye on smiling_. This phenomenon is termed synkinesis. The reverse may also occur and result in twitching of the mouth on blinking.
Dix-Hallpike
With the patient sitting, the neck is extended and turned to one side. The patient is then rapidly brought back to a supine position, so that the head hangs over the edge of the bed. This position is kept until 30 seconds have passed if no nystagmus occurs. The patient is then returned to a sitting position and observed for another 30 seconds for nystagmus. Then the maneuver is repeated with the head turned to the other side.
This maneuver is most useful for diagnosing posterior canal BPPV (the most common form), and the nystagmus is usually characterized by beating upward and torsionally. After it stops and the patient is sitting again, the nystagmus may occur in the opposite direction (reversal).
Besides posterior BPPV, there are three other types of BPPV, including anterior canal, horizontal canal, and pure torsional BPPV.
Anterior canal BPPV (superior canal BPPV) has similar provoking factors as posterior canal BPPV, but t_he nystagmus is downbeat and torsional._
For both posterior and anterior SCC BPPV, the direction of the torsional component of the nystagmus will always be toward the affected ear in the initial provoking position.
Horizontal canal BPPV is provoked by turning the head while lying down and sometimes by turning it in the upright position, but not by getting in or out of bed or extending the neck. Therefore, the nystagmus is elicited by a lateral head turn in the supine position, rather than with the head extended over the edge of the bed, and is characterized by horizontal nystagmus beating toward the floor after turning the affected ear down.
The nystagmus lasts less than 1 minute, pauses for a few seconds, and then a reversal of the nystagmus is seen. Pure torsional nystagmus may mimic a central lesion, and results from canalithiasis, simultaneously involving both the anterior and posterior canals, though is less common. This form of BPPV tends to persist longer than other forms of BPPV.
Dix-Hallpike
With the patient sitting, the neck is extended and turned to one side. The patient is then rapidly brought back to a supine position, so that the head hangs over the edge of the bed. This position is kept until 30 seconds have passed if no nystagmus occurs. The patient is then returned to a sitting position and observed for another 30 seconds for nystagmus. Then the maneuver is repeated with the head turned to the other side.
This maneuver is most useful for diagnosing posterior canal BPPV (the most common form), and the nystagmus is usually characterized by beating upward and torsionally. After it stops and the patient is sitting again, the nystagmus may occur in the opposite direction (reversal).
Besides posterior BPPV, there are three other types of BPPV, including anterior canal, horizontal canal, and pure torsional BPPV.
Anterior canal BPPV (superior canal BPPV) has similar provoking factors as posterior canal BPPV, but t_he nystagmus is downbeat and torsional._
For both posterior and anterior SCC BPPV, the direction of the torsional component of the nystagmus will always be toward the affected ear in the initial provoking position.
Horizontal canal BPPV is provoked by turning the head while lying down and sometimes by turning it in the upright position, but not by getting in or out of bed or extending the neck. Therefore, the nystagmus is elicited by a lateral head turn in the supine position, rather than with the head extended over the edge of the bed, and is characterized by horizontal nystagmus beating toward the floor after turning the affected ear down.
The nystagmus lasts less than 1 minute, pauses for a few seconds, and then a reversal of the nystagmus is seen. Pure torsional nystagmus may mimic a central lesion, and results from canalithiasis, simultaneously involving both the anterior and posterior canals, though is less common. This form of BPPV tends to persist longer than other forms of BPPV.
Central nystagmus
Has the following characteristics: nonfatiguing, absent latency (onset of nystagmus immediately after provocative maneuver), not suppressed by visual fixation, duration of nystagmus is greater than 1 minute, and may occur in any direction.
Although purely torsional or vertical nystagmus is classically central in origin, pure torsional BPPV may mimic central nystagmus.
Central vertigo is usually subjectively less severe than peripheral vertigo, but gait impairment, falls, and unsteadiness are much more pronounced and other neurologic signs often coexist. Hearing changes and tinnitus are usually absent.
Peripheral nystagmus
Peripheral nystagmus is characterized by fatigability with repetition, latency typically of 2 to 20 seconds, suppression by visual fixation, duration of nystagmus less than 1 minute, unidirectional, and usually horizontal, occasionally with a torsional component. Walking is typically preserved, although unilateral instability may exist. Hearing changes and tinnitus are more common with peripheral lesions.
Alexander’s law
Alexander’s law refers to gaze-evoked nystagmus that occurs after an acute unilateral vestibular loss. It was first described in 1912 and has three elements to explain how the vestibulo-ocular reflex responds to an acute vestibular insult. The first element says that spontaneous nystagmus after an acute vestibular impairment has the fast phase directed toward the healthy ear. The direction of the nystagmus, by convention, is named for the fast phase, so the spontaneous nystagmus is directed toward the healthy ear. The second element says nystagmus is greatest when gaze is directed toward the healthy ear, is attenuated at central gaze and may be absent when gaze is directed toward the impaired ear. The third element says that spontaneous nystagmus with central gaze is augmented when vision is denied.
The amplitude of nystagmus increases with gaze toward the side of the fast phase (toward the unaffected ear and away from the affected ear), and this is known as Alexander’s law.
What type of BPPV is Epley’s maneuver best for
The Epley maneuver is most efficacious for posterior canal repositioning, whereas anterior and horizontal canal repositioning often require different maneuvers.
Vestibular sensory organs and VOR
Otolothic organs:
Saccule and utricle = expansions of the membranous labyrinth, within each: macula - layer of hair cells overlain by heavy gelatinous otolithic membrane covered by calcium carbonate particles → bending of the hair cells and a subsequent change in neuronal activation, Detects linear and vertical motions of the head relative to gravity
3 semicircular canals = oriented at right angles to each other, are tubes of membranous labyrinth extending from each utricle that contain endolymph, each canal dilates at the base→ampulla, with sensory hair cells embedded in a gelatinous cap: cupola and DOES NOT CONTAIN otoconia; during head rotation, inertia causes the endolymph to lag behind and push on the cupola bending the hair cells and causing neuronal activation, sensitive to angular movements of the head
Information regarding head movement transmitted to the ocular motor nuclei allowing for keeping line of sight stable via excitatory and inhibitory projection
Cold caloric testing
Cold caloric testing is helpful to assess brainstem integrity (which helps define whether brain death is present or not) and this is a passive way to evaluate the VOR. It should be done using cold water at 30°C and by bringing the head of the bed to 30 degrees from the horizontal position in order to bring the horizontal canals into a more vertical plane for optimal testing.
The temperature difference between the body and the infused water creates a convective current in the endolymph of the nearby horizontal semicircular canal. Warm and cold water would produce currents in opposite directions and therefore a horizontal nystagmus in opposite directions. With cold water infusion, the endolymph falls within the semicircular canal, decreasing the rate of vestibular afferent firing and both eyes then slowly deviate toward the ipsilateral ear. Therefore, if cold water is infused into the left ear, the following will occur; excitatory signals are sent to the left lateral rectus and right medial rectus, as well as inhibitory signals to the left medial rectus and right lateral rectus. This results in tonic deviation of the eyes to the left. In a healthy person with normal functioning cortex, following a latency of about 20 seconds, nystagmus appears and may persist up to 2 minutes. The fast phase of nystagmus reflects the cortical correcting response and is directed away from the side of the cold water stimulus. If the cortical circuits are impaired (e.g., comatose state, as in this patient), the nystagmus will be suppressed and not present, and only the tonic deviation will be evident (with intact brainstem).
Trigeminal nerve innervation
Innervates the anterior belly of the digastric (posterior: facial nerve),mylohyoid, lateral pterygoid (also medial), tensor veli palatine, tensor tympani
Also masseter, deep temporal
Facial nerve innervation
Stapedius, posterior belly of the digastric, stylohyoid, frontalis, occipitalis, orbicularis oculi, corrugated supercilii, procerus, buccinator, orbicularis oris, nasalis, levator labii superioris, alaeque nasi, zygomaticus major and minor, levator anguli oris, mentalis, depressor anguli oris, depressor labii inferioris, risorius, and platysma.
What muscle is innervated by the glossopharyngeal nerve? What gland?
The stylopharyngeus muscle.
Parotid gland
Facial nerve
The facial nerve (cranial nerve VII) is a mixed nerve, containing motor fibers to the facial muscles, parasympathetic fibers to the lacrimal, submandibular, and sublingual salivary glands, special sensory afferent fibers for taste from the anterior two-thirds of the tongue, and somatic sensory afferents from the external auditory canal and pinna.
Course of the facial nerve and lesions
A lesion anywhere from the facial nerve nucleus to the distal branches can cause facial weakness in a peripheral distribution. Use whether lacrimination, hyperacusis, taste involved to localize further.
Two roots arise from the pontomedullary junction and merge to form the facial nerve.
- One of these roots provides motor innervation to the facial muscles.
- The second root is a mixed visceral nerve carrying parasympathetic fibers and is called the nervus intermedius. The preganglionic cell bodies of the parasympathetics are scattered in the pontine tegmentum, which are called the superior salivatory nuclei (SSN), and their fibers travel in the nervus intermedius.
The facial nerve courses laterally through the cerebellopontine angle with the vestibulocochlear nerve to the internal auditory meatus leading to the facial, or fallopian, canal.
The facial canal is located in the petrous part of the temporal bone and consists of labyrinthine, tympanic, and mastoid segments.
- Within the labyrinthine segment, the facial nerve bends sharply backward. At this genu, there is a swelling that forms the geniculate ganglion. This ganglion contains nerve cell bodies of taste axons from the tongue and somatic sensory axons from the external ear, auditory meatus, and external surface of the tympanic membrane.
- The parasympathetic greater petrosal nerve arises from the geniculate ganglion and is the first branch of the facial nerve. The greater petrosal nerve leaves the geniculate ganglion anteriorly, enters the middle cranial fossa extradurally, and enters the foramen lacerum en route to the pterygopalatine (sphenopalatine) ganglion. From the pterygopalatine ganglion, postganglionic fibers travel with branches of the maxillary portion of the trigeminal nerve (V2) to supply the lacrimal and mucosal glands of the nasal and oral cavities.
After the geniculate ganglion region and the branch of the greater petrosal nerve, the facial nerve axons then pass backward and downward toward the stylomastoid foramen.
- The next branch as the facial nerve passes downward is the nerve to the stapedius, prior to exit from the stylomastoid foramen. The stapedius muscle dampens the oscillations of the ossicles of the middle ear. Impairment of the stapedius nerve and muscle will cause hyperacusis, in which sounds are much louder.
- After the branch of the stapedius nerve and just before the exit from the stylomastoid foramen, the facial nerve gives off the third branch, the chorda tympani nerve. The chorda tympani nerve passes near the tympanic membrane, where it is separated from the middle ear cavity by a mucus membrane. It continues anteriorly and joins the lingual nerve of V3 where it carries general sensory afferents for the anterior two-thirds of the tongue. The chorda tympani contains secretomotor fibers to sublingual and submandibular glands, as well as visceral afferent fibers for taste. The cell bodies of the gustatory neurons lie in the geniculate ganglion and travel via the nervus intermedius back to the nucleus tractus solitarius (gustatory nucleus) (think solitary tastes here). Therefore, the nervus intermedius carries efferents from the superior salivatory nucleus and taste afferents to the nucleus tractus solitarius. It is important to remember that the parotid glands are innervated by the glossopharyngeal nerve, whereas all other glands in the head and face are innervated by the facial nerve.
The facial nerve then exits at the stylomastoid foramen, turns anterolaterally, and travels through the parotid gland. After the facial nerve exits the stylomastoid foramen, it gives off different branches to the various facial muscles.
:eft facial nerve nuclei→geniculate ganglion→stapedius→chorda tympnai→ stylomastoid foramen (just facial muscles)
Greater petrosal comes off at geniculate ganglion: lesion after geniculate ganglion spares lacrimal and mucosal glands of the nasal and oral cavities.
Stapedius comes off at stapedius: lesion after this spares hyperacusis
Chorda tympani comes off at chorda tympani: lesion after this spares taste
Nucleus solitarius →taste
Superior salivary nucleus → parasympathetics to head and neck
Nucleus tract solitarius
Involved with both taste and baroreceptor reflexes
Rostral → taste and receives taste afferents from the facial nerve: anterior ⅔ of the tongue, glossopharyngeal nerve: posterior ⅓ of the tongue, and the vagus nerve: tongue, epiglottis, pharynx
The caudal part of this nucleus is involved in the baroreceptor reflexes. Baroreceptors in the wall of the carotid sinus are stimulated by increased blood pressure and the glossopharyngeal afferents travel to the caudal nucleus tractus solitarius. As a result, interneurons stimulate the dorsal motor nucleus of the vagus nerve, leading to activation of parasympathetic vagal efferents projecting to the heart and causing slowing of the heart rate.
Nucleus ambiguus
The nucleus ambiguus is the central nucleus responsible for innervation of the muscles of the larynx and pharynx, innervated by the glossopharyngeal and vagus nerves (with some laryngeal muscle innervation contributed by the spinal accessory nerve).
Superior salivatory and inferior salivatory nuclei
The superior salivatory nucleus is the source of parasympathetic innervation to the head and neck. The inferior salivatory nucleus innervates the parotid gland via the glossopharyngeal nerve
Trigeminal nerve
The trigeminal nerve carries sensory information from the face, and supplies the sensory and motor innervation to the muscles of mastication. The nerve emerges from the pons in the midlateral surface. The trigeminal ganglion (gasserian or semilunar ganglion) is a sensory ganglion localized in the floor of the middle cranial fossa within a depression known as Meckel’s or trigeminal cave. Three primary divisions emerge from the gasserian ganglion (not the sphenopalatine ganglion, which is discussed below): the ophthalmic (V1), maxillary (V2), and mandibular (V3).
Key: gasserian ganglion
The ophthalmic division (V1) leaves the gasserian ganglion and exits the cranium through the cavernous sinus and the superior orbital fissure en route to the orbit. It branches into the tentorial, frontal, lacrimal, and nasociliary nerves. It mediates the afferent limb of the corneal reflex while the efferent limb is provided by the facial nerve. The V1 division supplies sensation to the skin of the nose, upper eyelid, forehead, and scalp (as far back as lambdoidal suture); upper half of cornea, conjunctiva, and iris, mucus membranes of frontal, sphenoidal, and ethmoidal sinuses, upper nasal cavity and septum, and lacrimal canals; and dura mater of the anterior cranial fossa, falx cerebri, and tentorium cerebelli.
The maxillary division (V2) leaves the gasserian ganglion, travels through the cavernous sinus, exits the cranium through the foramen rotundum, enters the sphenopalatine fossa, and then enters the orbit through the inferior orbital fissure. Branches include the zygomatic, infraorbital, superior alveolar, and palatine nerves. The V2 division supplies sensation to the lower eyelid, lateral nose, upper lip and cheek, lower half of cornea, conjunctiva, and iris; mucus membranes of maxillary sinus, lower nasal cavity, hard and soft palates, and upper gum; teeth of the upper jaw; and dura mater of the middle cranial fossa.
The mandibular division (V3) leaves the gasserian ganglion, exit the cranium through the foramen ovale, travels in the infratemporal fossa, and branches into the buccal, lingual, inferior alveolar, and auriculotemporal nerves. The V3 division does not travel through the cavernous sinus and is therefore spared in cavernous sinus thrombosis. Besides the muscles of mastication, V3 supplies sensation to skin of the lower lip, lower jaw, chin, tympanic membrane, auditory meatus, upper ear; mucus membranes of floor of the mouth, lower gums, anterior two-thirds of the tongue (not taste, which is facial nerve), and t_eeth of lower jaw;_ and dura mater of the posterior cranial fossa (although most of posterior fossa innervation arises from upper cervical nerves).
V1/V2→cavernous sinus, V1→superior orbital fissure, V2→foramen rotundum
V3→foramen ovale
Cavernous sinus
The cavernous sinus contains the ICA (siphon), postganglionic sympathetic fibers, and cranial nerve VI on the medial wall (adjacent to the sphenoid sinus), whereas cranial nerves III, IV, V1, and V2 are found along the lateral wall. The cavernous sinus receives blood from the middle cerebral vein and drains into the jugular vein (via the inferior petrosal sinus) and into the transverse sinus (via the superior petrosal sinus). The two cavernous sinuses are connected by intercavernous sinuses that lie anterior and posterior to the hypophysis forming a venous circle around it.
Sphenopalatine ganglion (pterygopalatine ganglion
The sphenopalatine ganglion (pterygopalatine ganglion) is a parasympathetic ganglion found in the pterygopalatine fossa. It is the largest of four parasympathetic ganglia of the head and neck, along with the submandibular ganglion, otic ganglion, and ciliary ganglion. The sphenopalatine ganglion is associated with the branches of the trigeminal nerve. It supplies the lacrimal glands, paranasal sinuses, glands of the mucosa of the nasal cavity and pharynx, the gingiva, and the mucus membrane and glands of the hard palate.
Also associated with branches of the facial nerve
Hypoglossal nerve
Most of the corticobulbar projections to the hypoglossal nuclei are bilateral, although there is one exception.
The cortical neurons that drive the genioglossus muscles project only to the contralateral hypoglossal nucleus. There is one genioglossus muscle on each side of the tongue and they pull the tongue anterior and medial. Therefore, if tongue deviation is due to an upper motor neuron lesion affecting the genioglossus projections, tongue deviation will be contralateral. A lower motor neuron lesion causes ipsilateral tongue deviation.
The hypoglossal nerve provides innervation to all intrinsic tongue muscles and three (genioglossus, styloglossus, and hyoglossus) of the four extrinsic tongue muscles, with the fourth (palatoglossus) being innervated by the vagus nerve.
Accessory nerve
Activation of the accessory nerve causes ipsilateral head tilt and contralateral head rotation. The accessory nerve innervates the sternocleidomastoid and the trapezius muscle on each side.
The action of each sternocleidomastoid (SCM) is to pull the mastoid process toward the clavicle, resulting in contralateral head rotation and turning of chin to the contralateral side (ipsilateral head tilt).
Each SCM is innervated by the ipsilateral motor cortex.
*Each trapezius is innervated by the contralateral motor cortex.
Gag reflex
The gag reflex is mediated by the nucleus ambiguus. The afferent limb is by the glossopharyngeal nerve and the efferent limb is by the vagus nerve.
The vagus nerve exits the cranium through the jugular foramen with the glossopharyngeal and spinal accessory nerves. Through fibers originating from the nucleus ambiguus, the vagus innervates palatal, pharyngeal, and laryngeal muscles. A vagus nerve lesion will cause impaired swallowing (likely the cause of this patient’s aspiration pneumonia), hoarse voice, and flattening and lowering of the palate, which causes the uvula to point toward the contralateral side.
The vagus supplies parasympathetic innervation to the heart, lungs, gastrointestinal (GI) tract, and trachea. Parasympathetic neurons of the vagus are located in the dorsal motor nucleus of the vagus (which supplies the GI tract, liver, pancreas, and respiratory tract) and medial part of the nucleus ambiguus (which supplies the cardiac plexus). The vagus nerve also supplies sensation to the base of the tongue, epiglottis, and pharynx.
V2 skull foramina
Foramen rotunda
Carotid artery skull foramina
Foramen lacerum
The internal carotid artery passes through the carotid canal and runs along the superior surface of the foramen lacerum but does not travel through it.
CN V1 skull foramina
Superior orbital fissure
CN III skull foramina
Superior orbital fissure
What passes through the foramen spinous?
Middle meningeal artery
CN V3 skull foramina
Foramen ovale
Glossopharyngeal nerve skull foramina
Jugular foramen
Vagus nerve skull foramina
Jugular foramen
What passes through the internal acoustic meatus
CN VIII
CN 1 skull foramina
The olfactory nerve bundles pass through the cribriform plate foramina.
CN II skull foramina
Cranial nerve II (optic nerve) passes through the optic canal along with the ophthalmic artery.
Orbital fissure
Cranial nerves III (oculomotor nerve), IV (trochlear nerve), V1 (trigeminal nerve, 1st division; ophthalmicnerve), and VI (abducens nerve) all pass through the superior orbital fissure.
CN V2 skull foramina
Cranial nerve V2 (trigeminal nerve, 2nd division; maxillary nerve) passes through the foramen rotundum.
CN V3 skull foramina
Foramen ovale
Skull foramina for CN V
An easy way to remember the sequence of which foramen each of the three trigeminal nerve branches pass is the term “Standing Room Only” (Superior orbital fissure, foramen Rotundum, foramen Ovale) for V1, V2, V3, respectively.
Internal acoustic meatus
Cranial nerves VII (facial nerve) and VIII (vestibulocochlear nerve) pass through the internal acoustic meatus.
Jugular foramen
Cranial nerves IX (glossopharyngeal nerve), X (vagus nerve), and XI (spinal accessory nerve) pass through the jugular foramen.
Hypoglossal canal
Cranial nerve XII (hypoglossal nerve) passes through the hypoglossal canal.
Foramen magnum
The medulla oblongata, vertebral arteries, and meninges pass through the foramen magnum.
Skull foramina and contents
Central nystagmus
Nonfatiguing, absent latency, not suppressed by visual fixation, duration > 1 minute, any direction but purely vertical/torsional is classic (though pure torsional BPPV may mimic)
Central vertigo
Subjectively less severe vertigo than peripheral, more prominent gait impairment, other neurological signs coexist, absent hearing changes and tinnitus
Peripheral nystagmus
Fatigable, latency present, suppression by visual fixation, duration of nystagmus is less than 1 minute, direction is unidirectional and usually horizontal with a torsional component
Peripheral vertigo
Subjectively more severe, walking typically preserved, hearing changes and tinnitus common
Taste anterior ⅔ of the tongue
Facial nerve
Tactile sensation to anterior ⅔ of the tongue
Trigeminal nerve
Taste posterior ⅓ of the tongue
Glossopharyngeal nervfe
Superior salivary nucleus
Parasympathetic source to head and neck
Motor nuclei to pharyngeal and laryngeal muscles
Nucleus ambiguuus
Provides innervation to the parotid gland of the glossopharyngeal nerve
Inferior salivary nucleus
Nuclei for taste
Rostral nucleus solitarius
What is exceptional about CN IV’s nucleus/nerve?
It is the only cranial nerve that decussates, but it decussates early so it may cause contralateral vs ipsilateral dysfunction of SO depending on where the lesions relative to decussation.
In other cases (e.g., tongue muscle innervation), there may be decussation along the tract but the cranial nerve is not the one that decussates
Nuclei for baroreceptor reflex
Caudal nucleus solitaires
Corneal reflex
Afferent: Trigeminal nerve (split between V1 and V2)
Efferent: Vagus nerve
Gag reflex
Afferent: Glossopharyngeal nerve
Efferent: Vagus nerve
Pupil sparing CN III palsy
Diabetic pupil/diabetic cranial nerve palsy
Ptosis, miss, anhidrosis (when proximal to carotid bifurcation)
Horner’s syndrome
Fourth nerve palsy
Contralateral head tilt
Argyll Roberston pupil
Neurosyphillis; accommodation reflex present, pupillary reflex absent
Marcus Gunn pupil
Afferent pupillary defect: no response to direct light, but response to consensual light in contralateral eye present
“Down and out pupil”
Third nerve palsy
Painful vision loss
Optic neuritis
Painless vision loss
Anterior ischemic optic neuropathy
Cocaine 4% or 10% eye drops
Confirmation of Horner’s pupil: no change in size of Horner’s pupil; unaffected side dilates
Hydroxyamphetamine 1% eye drops
Horner’s pupil dilates: first or second-order neuron Horner’s pupil
Horner’s pupil does not dilate: Third-order Horner’s neuron
Carpal tunnel
The carpal tunnel is bounded dorsally and laterally by the carpal bones, and the transverse carpal ligament forms the palmar border. The structures that pass through the carpal tunnel include the median nerve most superficially (on the palmar aspect), flexor pollicis longus tendon, four tendons of the flexor digitorum superficialis, and four tendons of the flexor digitorum profundus.
Brachial plexus key points
The upper extremity receives innervation from the C5 to T1 nerve roots. In the intervertebral foramina, the motor and sensory roots join to form a spinal nerve, which then branches into ventral and dorsal rami before exiting the foramina. The ventral (anterior) rami of these nerve roots join to form a plexus of nerves known as the brachial plexus
The point where the C5 and C6 nerve roots meet is called Erb’s point.
The cords are named according to their relationship to the axillary artery.
As discussed and shown in Figure 9.5, both the dorsal scapular nerve and the long thoracic nerve arise directly from the ventral (anterior) rami of the nerve roots.
Median nerve
The median nerve is derived from the lateral and medial cords. The median nerve runs down the midline of the arm and crosses over the brachial artery to lie just medial to it as it passes under the bicipital aponeurosis in the antecubital fossa. In the forearm, the median nerve innervates pronator teres, flexor carpi radialis, and flexor digitorum superficialis. In the forearm, the median nerve gives off the anterior interosseous nerve that innervates flexor digitorum profundus to the second and third digits, flexor pollicis longus, and pronator quadratus.
Before entering the carpal tunnel, the median nerve gives off the palmar cutaneous sensory nerve, a pure sensory nerve. The median nerve then passes through the carpal tunnel (discussed in question 14) and gives off the thenar motor branch, which innervates abductor pollicis brevis and opponens pollicis. The median nerve also innervates the first and second lumbricals.
The lumbrical muscles of the hand flex the fingers at the metacarpophalangeal (MCP) joints, and extend them at the interphalangeal (IP) joints.
The median nerve provides sensory innervations to the lateral (radial) two-thirds of the palm and the distal dorsal aspect of the first to third digits and the distal lateral (radial) half of the fourth digit through the palmar cutaneous nerve and through digital branches.
The median nerve is prone to injury with supracondylar fractures. Median nerve palsy can also occur due to entrapment in ligaments or between muscles (
Radial, medial, ulnar in broad strokes
Radial - extensor, supination, flexion of the wrist radially and ulnarly (ulnarly is posterior interosseous), includes upper arm
Medial - flexor, pronation, flexion of the wrist radially, only forearm and hand
Ulnar - intrinsic hand muscles, flexion of the wrist ulnarly, only forearm and hand
Lumbrosacral plexus
Consists of contributions from both the lumbar and sacral plexi connected via the lumbosacral trunk
Lumbar plexus: T12-L4, 3 minor nerves: iliohypogastric and ilioinguinal (from L1, some contributions from T12), genitofemoral nerve (from L1 and L2); 3 major nerves: obturator, femoral (from L2-L4, obturator-anterior, femoral-posterior), lateral femoral cutaneous (L2 and L3)
Lumbosacral trunk: L4-L5
Sacral plexus: L4-LS4 with contributions from LS trunk (L4-L5) → superior gluteal nerve (L4-S1: gluteus medius, minimus, and tensor fasciae latae: hip aduction*), inferior gluteal nerve (L5-S2, gluteus maximus, hip extension), posterior cutaneous nerve of the thigh ( S1-S3, sensory cutaneous innervation to the lower buttock and posterior thigh). pudendal nerve (S2, S3, and S4, sensory innervation to the perineal region and perianal region through the inferior rectal nerve, perineal nerve, and dorsal nerve of the penis or clitoris)
Sciatic: L4-S3 anterior portion=tibial, L4-S2 posterior portion=common peroneal
*hese muscles contribute to thigh abduction, with the tensor fasciae latae acting as the main abductor when the hip is flexed, and the gluteus medius and minimus acting as the main abductors when the hip is extended.
Sciatic nerve: the largest nerve of the lumbosacral plexus and the largest nerve in the body.
Sciatic nerve
The sciatic nerve originates from the L4, L5, S1, S2, and S3 roots. This nerve is the largest nerve in the body, and gives off two initial branches: the superior and inferior gluteal nerves. The superior gluteal nerve innervates the gluteus medius, minimus, and tensor fasciae latae. The inferior gluteal nerve innervates the gluteus maximus (Table 9.2).
The sciatic nerve is composed of two different nerves running together: the tibial nerve medially and the common peroneal nerve laterally. Both are in the same sheath, but remain separated throughout their course with no intercrossing fibers. The common peroneal nerve is more prone to injuries, because it is smaller, more lateral, and has less supportive tissue.
- In the thigh, the tibial division innervates the adductor magnus: adduction, semimembranosus+ semitendinosus: knee flexion, and long head of the biceps femoris: knee flexion. The short head of the biceps femoris: knee flexion is supplied by the common peroneal division. The tibial nerve then continues in the posterior aspect of the leg and gives innervation to the gastrocnemius, soleus, and tibialis posterior.
- The tibial nerve gives off the sural nerve that provides sensory innervation to the lateral aspect of the leg and foot.
- The peroneal nerve continues, and after the popliteal fossa, it passes behind the fibular head and divides into the superficial and deep peroneal nerves. The superficial peroneal nerve gives off branches to the peroneus longus and brevis, which permit foot eversion. The deep peroneal nerve supplies the tibialis anterior, extensor hallucis, extensor digitorum longus and brevis, and peroneus tertius.
- The superficial perineal nerve innervates the skin in the lower two-thirds of the lateral aspect of the leg and dorsum of the foot.
- _A lesion in the deep peroneal nerve produces foot drop with inability to dorsiflex the foot without impairing eversion of the foo_t. Preservation of foot inversion distinguishes peroneal neuropathy from L5 radiculopathy, in which the tibialis posterior muscle (innervated by the tibial nerve) is involved, impairing foot inversion.
- The sensory territory of the deep peroneal nerve is the web space between the first and second toes.
Femoral nerve
The femoral nerve is a large nerve that originates from the posterior divisions of L2, L3, and L4
- Travels through the psoas major muscle which it innervates, then through the iliacus muscle which it also innervates. After this course, the femoral nerve passes under the inguinal canal into the femoral triangle, located lateral to the femoral artery.
- It then divides into several terminal branches. There are three cutaneous branches: (1) the medial femoral cutaneous, (2) the intermediate femoral cutaneous, and (3) the saphenous nerve. These branches carry sensory information from the anteromedial thigh, medial leg, medial malleolus, and arch of the foot.
- The motor branches provide innervation to the quadriceps (rectus femoris, vastus lateralis, medialis, and intermedius), sartorius, and pectineus (Table 9.3).
- The patellar reflex is carried through the femoral nerve.
Femoral nerve injury will manifest as
- Weakness in hip flexion and knee extension, loss of the patellar reflex, and sensory findings in the anteromedial thigh and medial leg.
- The femoral nerve can be injured in the retroperitoneal or intrapelvic space, or at the inguinal ligament.
- Clinically, the distinction between injury at these sites can be made by detection of weakness on hip flexion, reflecting psoas compromise (before the inguinal canal), and electrophysiologically by the presence of fibrillations in the iliacus muscle. Both these muscles are innervated proximal to the inguinal ligament, and their compromise will suggest an intrapelvic injury rather than an inguinal injury.
- At the inguinal region, the femoral nerve can be damaged by inguinal masses or hematomas, during hip surgery or perineal surgeries, especially associated with prolonged lithotomy position, such as in this case.
It is important to distinguish femoral nerve injury from L2-L3-L4 radiculopathy and lumbar plexopathy. The presence of impairment of other nerves will suggest these possible diagnoses.
- For example, adductor weakness suggests involvement of the obturator nerve, which can occur in L2-L3-L4 radiculopathy or a lumbar plexopathy.
- Also, the presence of w_eakness in the distal lower extremity muscles will imply injury to other nerves, excluding a selective femoral nerve injury_.
- Abnormal SNAPs do not correlate with a radiculopathy from an intraspinal canal lesion, because SNAPs will be normal in these lesions
Can SNAPS be normal in radiculopathy even when there is a sensory component?
YES
Reflexes
Only muscle from common femoral nerve
Biceps femoris short head
Foot inversion
Tibial
Tibialis posterior (anterior = deep peroneal, ankle/toe dorsiflexion)
Foot eversion
Superficial peroneal to peroneal longus, brevis
Foot eversion
Anterior and medial thigh sensation
Femoral nerve
Tibial nerve - detailed
The tibial nerve is a division of the sciatic nerve, and at the level of the thigh, it provides innervation to the semimembranosus, semitendinosus, and long head of the biceps femoris (see Fig. 9.8) →knee flexion.
Proximal to the popliteal fossa, the tibial division of the sciatic nerve separates from the peroneal division and gives off the sural nerve, which provides sensory innervation to the lateral (posterior) aspect of the leg and foot. The tibial nerve then continues down the leg innervating the gastrocnemius + soleus: ankle plantar flexion, tibialis posterior →foot inversion, flexor digitorum longus, and flexor hallucis longus→toe plantar flexion
At the medial ankle, the tibial nerve passes under the flexor retinaculum through the tarsal tunnel and gives three terminal branches - calcaneal: pure sensory and innervates the heel, medial planter: innervates the abductor hallucis, flexor digitorum brevis, and flexor hallucis brevis, as well as the skin of the medial sole, and lateral plantar: innervates the abductor digiti quinti pedis, flexor digiti quinti pedis, adductor hallucis, and interossei, as well as the skin of the lateral sole.
An entrapment neuropathy of the tibial nerve at the level of the tarsal tunnel will not produce weakness on plantarflexion. This entrapment neuropathy may manifest with burning pain in the plantar region, worse with standing and walking, with sensory deficits in the sole and sometimes atrophy in this area. Sensation in the dorsum: top of the foot is normal, as well as the ankle reflex.
Motor functions of LS
Superior gluteal - hip aBduction
Inferior gluteal - gluteus maximus → thigh extension
Obturator - hip aDduction
Femoral - hip flexion and knee extension
Sciatic/tibial - hip extension, ankle inversion, ankle/toe plantarflexion
Sciatic/peroneal - ankle eversion, ankle/toe dorsiflexion
Ulnar nerve*
The ulnar nerve is a continuation of the medial cord. The medial cord gives a contribution to the median nerve and then gives off two branches: (1) the medial brachial cutaneous nerve and (2) medial antebrachial cutaneous nerve, which provide sensory innervation to the medial half of the arm and forearm, respectively. It then continues as the ulnar nerve. The ulnar nerve predominantly carries C8 and T1 fibers. If the upper extremity were to be divided into upper arm (above the elbow), forearm (elbow to wrist), and hand (below the wrist), the ulnar nerve does not innervate any muscles in the upper arm.
At the elbow, it emerges from the triceps and enters the postcondylar groove, a bony canal between the medial epicondyle and the olecranon of the ulna. This is where the ulnar nerve is most susceptible to injury (see questions 41 and 45). The ulnar nerve then travels down the forearm, where it gives branches to flexor carpi ulnaris and then flexor digitorum profundus to the fourth and fifth digits. It gives off two sensory branches: (1) the dorsal ulnar cutaneous nerve and (2) palmar ulnar cutaneous nerve. Proximal to the wrist it is joined by the ulnar artery.
The ulnar nerve then enters the hand via Guyon’s canal, which is bounded by the carpal bones dorsally and laterally, the transverse carpal ligament medially, and the palmar carpal ligament ventrally (the ulnar nerve does not pass through the carpal tunnel). The ulnar nerve then bifurcates into a deep motor branch and a superficial sensory branch distal to Guyon’s tunnel. The deep motor branch innervates hypothenar eminence muscles: abductor digiti minimi, flexor digiti minimi, and opponens digiti minimi. The ulnar nerve provides motor innervation to many of the intrinsic hand muscles, which are largely involved in fine finger movements: the fourth and fifth lumbricals, and the dorsal and palmar interossei. It also innervates two thenar muscles: (1) adductor pollicis and (2) flexor pollicis brevis.
The ulnar nerve provides sensory innervation to the hypothenar eminence, the palmar and dorsal medial portion of the hand, fifth digit, and half of the fourth digit.
Ulnar nerve sensation
With lesions at or distal to the wrist, sensation over the hypothenar eminence is spared because the palmar cutaneous branch arises proximal to Guyon’s canal.
Lumbrosacral radiculopathy
Lumbosacral radiculopathy is commonly caused by disc herniation or degenerative spine changes. S1 radiculopathy commonly manifests as pain radiating from the buttock down the posterior thigh, posterior leg, and lateral foot, with sensory impairment in this dermatomal region, especially the lateral foot and fifth toe. The most prominent weakness is plantarflexion and toe flexion, and the ankle deep tendon reflex will be reduced or absent: can’t stand on toes. Muscles involved in S1 radiculopathies include the abductor hallucis, abductor digiti quinti pedis, soleus, medial and lateral gastrocnemius, extensor digitorum brevis, biceps femoris (long and short head), and gluteus maximus. Muscles partially innervated by S1 that may also be affected are the tibialis posterior, flexor digitorum brevis, gluteus medius, and tensor fasciae latae. Although the SNAPs should be normal in S1 radiculopathies, the H- reflex is commonly reduced or absent.
Ulnar neuropathy at or above the elbow
Significant loss of fine motor coordination due to weakness of the third and fourth lumbricals and the palmar and dorsal interossei.
Flexor pollicis brevis is innervated by both the median and ulnar nerve, so even with complete ulnar palsy some thumb abduction can still be achieved by the part of the muscle innervated by the median nerve.
When asked to make a fist, the hand assumes the appearance of a claw, with the fourth and fifth digits hyperextended at the metacarpophalangeal joint and partially flexed at the interphalangeal joint. This occurs because the third and fourth lumbricals as well as the interossei and flexor digiti minimi are weak, and there is unopposed action of the radial nerve–innervated muscles (Table 9.6), causing hyperextension at the metacarpophalangeal joints.
Other signs seen with ulnar nerve palsy include (1) Wartenberg’s sign, or fifth digit abduction at rest due to paralysis of the third palmar interossei with unopposed action of extensor digiti minimi and extensor digitorum communis (radial nerve–innervated muscles), and (2) Froment’s sign, whereby during attempted forceful adduction of the thumb, as with an attempt to hold a piece of paper between the thumb and the index finger, thumb flexion occurs. This occurs because the adductor pollicis is weak, and thumb flexion (by the intact flexor pollicis longus) is the compensatory action.
Ulnar nerve compression at the wrist, at Guyon’s canal, may occur in bicycle riders or others who frequently place pressure at the medial wrist area. Claw hand, Wartenberg’s sign, and Froment’s sign can all be seen with ulnar neuropathy at Guyon’s canal.
Involvement of flexor carpi ulnaris and flexor digitorum profundus (which receive motor branches from the ulnar nerve in the forearm) indicate that the lesion is proximal to the wrist. In addition, with lesions at or distal to the wrist, sensation over the hypothenar eminence is spared because the palmar cutaneous branch arises proximal to Guyon’s canal. With a lesion at the wrist, CMAP amplitudes would be abnormally low with stimulation at the wrist, and a reduction in CMAP amplitude would not occur with more proximal stimulation.
FCU - wrist flexion in an ulnar direction
FDP - Flexion at DIP of fourth and fifth digits
Radial nerve*
The radial nerve is a continuation of the posterior cord. It carries C5, C6, C7, and C8 fibers (C5, C6: BR, C7, C8 Triceps for reflexes).
- The first branch of the radial nerve is the posterior cutaneous nerve to the arm.
- In the arm, the radial nerve gives branches to long, medial, and lateral heads of the triceps muscle (it is the sole innervation of the triceps) and then travels along the spiral groove (see Table 9.6 for root innervations and action of the muscles innervated by the radial nerve).
- It then gives off the posterior cutaneous nerve, which runs with the radial nerve in the spiral groove of the humerus.
- In the spiral groove, the radial nerve gives off the lateral cutaneous nerve to the arm.
- Distal to the spiral groove, it gives branches to the brachioradialis (a forearm flexor), and extensor carpi radialis longus and brevis.
Distal to the elbow, it bifurcates into the posterior interosseous and superficial sensory radial nerves. The posterior interosseous nerve innervates extensor carpi ulnaris, extensor digitorum communis, extensor digiti minimi, abductor pollicis longus, extensor pollicis longus and brevis, and extensor indices.
The radial nerve provides sensory innervation to the posterior arm through the posterior cutaneous nerve to the arm, to the lateral arm via the lateral cutaneous nerve to the arm, and to the posterior forearm through the posterior cutaneous nerve to the forearm. The superficial sensory radial nerve provides sensory innervation to the dorsolateral half of the hand, the proximal two- thirds of the thumb (including the lateral thumb, over the anatomic snuffbox), and the second and third digits (discussed in question 66). Remember that the distal dorsal aspects of the second and third digits are innervated by the median nerve.
Common peroneal injury at the fibular head
The common peroneal nerve is a division of the sciatic nerve, giving off the lateral cutaneous nerve of the calf before turning around the fibular head and passing through the fibular tunnel (see Fig. 9.9). This nerve then divides into superficial and deep peroneal nerves. This nerve then divides into superficial and deep peroneal nerves. The superficial peroneal nerve innervates the peroneus longus and brevis, as well as the skin in the lower two- thirds of the lateral aspect of the leg and dorsum of the foot. The deep peroneal innervates the tibialis anterior, extensor hallucis, extensor digitorum longus and brevis, and peroneus tertius. The sensory territory of the deep peroneal nerve is the web space between the first and second toes.
Foot drop can be seen with common or deep peroneal lesions, as well as with sciatic nerve lesions, L5 radiculopathies, and plexopathies. Localization is dependent on the distribution of motor abnormalities seen throughout the leg, especially of muscles not innervated by the deep peroneal nerve. In this case, the patient has abnormalities in both deep (tibialis anterior, extensor hallucis, and extensor digitorum brevis) and superficial peroneal nerve– innervated muscles (peroneus longus). Clinically, foot dorsiflexion weakness is attributed to the deep peroneal nerve–innervated muscles, whereas foot eversion weakness is attributed to superficial peroneal nerve–innervated muscles. Therefore, presence of weak dorsiflexion and eversion points toward a common peroneal nerve injury (as opposed to isolated superficial or deep peroneal nerve injuries) like in this patient. The fact that the short head of the biceps femoris is spared suggests that the lesion is distal to this level.
Femoral nerve injury in the intrapelvic region
Femoral nerve injury in the intrapelvic region can be caused by pelvic surgery, pelvic masses, or retroperitoneal hematomas. The history suggests a retroperitoneal hematoma, which can compress the femoral nerve in the intrapelvic region. This patient has pain with hip flexion and knee extension weakness, as well as absent patellar reflex. There are also sensory findings in the femoral nerve and saphenous nerve distribution. The presence of fibrillations in the iliacus and psoas muscles suggests an intrapelvic injury rather than an inguinal injury, given that these muscles are innervated by the femoral nerve in the intrapelvic region and prior to its course through the inguinal region.
Involvement of the psoas distinguishes intrapelvic injury from injury at the inguinal ligament (discussed also in question 29). The fact that thigh adductors and other muscle groups are spared suggests that this is not a plexopathy or radiculopathy as only the femoral nerve–innervated muscles are involved. The abnormal SNAPs point away from the diagnosis of a radiculopathy.
Posterior interosseous nerve palsy vs supinator syndrome
Can occur as a diabetic mononeuropathy, but can also result from posterior interosseous nerve compression (such as due to lipomas or nerve sheath tumors) or can be seen in Parsonage–Turner syndrome (discussed in question 52).
The posterior interosseous nerve is a pure motor nerve. With a posterior interosseous nerve palsy, there is not an obvious wrist drop because radial nerve branches to extensor carpi radialis longus and brevis originate proximal to the posterior interosseous nerve (lesion is distal to there branching off); there is however weakness of wrist extension in an ulnar direction.
Strength of more proximal radial nerve–innervated muscles excludes a radial neuropathy at the spiral groove or elbow, and intact superficial sensory radial nerve responses on NCS further support this. Intact triceps reflex and normal strength of forearm extension exclude a C7 radiculopathy.
Supinator syndrome causes a painful posterior interosseous nerve palsy due to compression or irritation of this nerve as it passes through the supinator muscle. Absence of pain on forced supination makes this uncommon disorder less likely.
L2-L3-L4 radiculopathy
Radiculopathies involving the upper lumbar roots are more difficult to assess and less common than those involving the lower lumbosacral roots. L2- L3 radiculopathy manifests with pain in the hip and groin radiating down the anterior and medial thigh. If there is an L4 radiculopathy, the pain may also radiate down to the medial leg. L2-L3-L4 radiculopathy may affect hip flexion and knee extension, as well as ankle dorsiflexion due to involvement of the L4 root, which partially innervates the tibialis anterior along with L5. The patellar reflex is reduced or absent in L2-L3-L4 root lesions.
The iliacus muscle is innervated by L2-L3 roots. The quadriceps femoris is innervated by L3-L4 roots, and the tibialis anterior is innervated by L4-L5 roots.
L2-L3-L4 radiculopathies should be distinguished from a lumbar plexopathy and femoral neuropathy. The differences can be difficult to detect clinically, but may be further clarified with EMG. In radiculopathies, the SNAPs are normal, whereas in plexopathies, they are abnormal. Paraspinal fibrillations are seen in radiculopathies, but not in plexopathies. In femoral neuropathy, the manifestations should be restricted to the distribution and muscles supplied by this nerve. In this case, it should also be considered in the differential diagnosis. However femoral nerve injury does not involve the tibialis anterior muscle, which is an L4, peroneal nerve–innervated muscle.
Median nerve palsy at the level of the antecubital palsy
Brachial artery injury often occurs concomitantly, given its proximity to the median nerve in this area. The sensory fibers of the median nerve are derived mainly from C6 and C7 through the lateral cord.
With complete median nerve palsy, on attempt to make a fist, the first digit does not flex, the second digit partially flexes, and the fourth and fifth digits flex normally, assuming a hand position similar to that used in religious blessings, termed the Benedictine sign.
Ischemic monomania
Ischemic monomelia, as can occur during placement of arteriovenous shunts for dialysis, is painful and causes circumferential sensory loss in multiple nerve distributions
Anterior interosseous nerve syndrome
The anterior interosseous nerve is a pure motor branch of the median nerve and innervates flexor digitorum profundus to the second and third digits, flexor pollicis longus, and pronator quadratus (Table 9.1). Weakness of these muscles, in the absence of sensory loss, suggests isolated involvement of the anterior interosseous nerve, as can occur with trauma, fracture, or in neuralgic amyotrophy (Parsonage–Turner syndrome; see question 52). Patients complain of weakness in grasping objects with their thumb and index finger, and on attempt to make an “okay sign,” the distal phalanges are unable to flex, and instead, the fingertip pulps touch.
Pronator teres syndrome
Pronator teres syndrome results from compression of the median nerve as it passes between the two heads of pronator teres. It is uncommon, but occurs in people who perform repetitive forceful pronation and may be associated with medial epicondylitis, or “golfer’s elbow.” Symptoms include gradual onset of a deep ache in the forearm that may worsen with pronation and weakness in median nerve–innervated muscles. Because branches from the median nerve that innervate the pronator teres arise proximal to this muscle (before the nerve passes under this muscle), pronator teres strength is intact in this syndrome. Apparently normal strength in pronator teres excludes a complete median nerve palsy at the elbow. Weakness of muscles not innervated by the anterior interosseous nerve (such as finger flexors and thumb abductors) indicates that this is not isolated anterior interosseous nerve syndrome. Weakness of flexor carpi radialis, which is predominantly C6 and C7 in innervation (see Table 9.1), indicates that this is not a medial cord lesion.
Namely wrist flexion would be spared in only anterior interosseous lesion
Median nerve*
Radial neuropathy at the axilla, spiral groove, and
Proximal to spiral groove: Loss of sensation over the posterior arm (due to involvement of the posterior cutaneous nerve of the arm, the most proximal branch of the radial nerve), as well as weakness of the triceps (which receives innervation from the radial nerve proximal to the spiral groove), indicates that the lesion is proximal to the spiral groove. Proximal radial nerve lesions are not common, but can occur with repetitive pressure to the axilla, as occurs with prolonged crutch use, or with prolonged pressure otherwise on the axilla, as occurs in “Saturday night palsy,” when a person sleeps with his or her arm hung over the back of a chair. Intact arm abduction, mediated in large part by axillary nerve–innervated muscles, excludes a posterior cord lesion. With posterior cord lesions, muscles innervated by both the axillary and radial nerves are weak.
At the spiral groove. Intact strength of the triceps and intact sensation on the posterior aspect of the arm support that the lesion is distal to origin of the posterior cutaneous nerve to the arm and branches to triceps, excluding a proximal radial nerve lesion or a posterior cord lesion; intact triceps strength and normal triceps deep tendon reflex are evidence that this is not a C7 radiculopathy. Reduced sensation over the lateral aspect of the arm suggests that the lesion is more proximal than the elbow, as the lateral cutaneous nerve to the arm arises from the radial nerve within the spiral groove. The spiral groove is the most common location for radial nerve injury, and a common mechanism is humeral fracture.
Injury to spinal accessory nerve
Can occur with radical neck dissection - because in the neck it is interwoven among several lymph nodes
The spinal accessory nerve carries fibers from the lower medulla as well as C1 to C4. It exits the skull basethrough the jugular foramen and innervates the sternocleidomastoid, which turns the head contralaterally, as well as the trapezius. The trapezius is involved in shoulder shrug along with assisting in elevation of the scapula and assisting the deltoid in arm abduction beyond 90 degrees.
Jugular foramen syndrome, or Vernet’s syndrome
Results from compressive lesions of the foramen, such as metastases or schwannomas, and there would be evidence of involvement of the vagus and glossopharyngeal nerves in addition to the spinal accessory nerve
Internal acoustic foramen is VII/VIII
Jugular foramen syndrome, or Vernet’s syndrome
Results from compressive lesions of the foramen, such as metastases or schwannomas, and there would be evidence of involvement of the vagus and glossopharyngeal nerves in addition to the spinal accessory nerve
Wartenberg’s syndrome
This type of neuropathy, also called Wartenberg’s syndrome, can result from compression or irritation of this nerve due to tight handcuffs or watches, venipuncture, or surgery. Rarely, compression may occur due to pinching of the nerve between the brachioradialis and extensor carpi radialis longus tendons as occurs with repetitive pronation. Symptoms include dysesthesias and numbness over the dorsolateral aspect of the hand. There is no motor weakness, as the superficial sensory radial nerve is a pure sensory nerve. Treatment is conservative, and includes avoidance of pressure to the nerve and medications for neuropathic pain (such as amitriptyline, pregabalin, or gabapentin) if necessary.
Musculocutaneous neuropathy
Such neuropathies in isolation are rare, but can occur with anterior shoulder dislocations and other types of trauma. The musculocutaneous nerve is a continuation of the lateral cord and carries predominantly C5 and C6 fibers (but also C7).
- The musculocutaneous nerve innervates the coracobrachialis muscle that assists the deltoid in anterior flexion of the arm at the shoulder and stabilizes the humerus during forearm flexion.
- The musculocutaneous nerve then innervates the brachialis muscle and the biceps brachii, which flex the forearm at the elbow. The brachioradialis, innervated by the radial nerve, also contributes to forearm flexion.
- The biceps brachii also supinates the forearm, and is the main forearm supinator when the forearm is flexed.
- Supinator takes the role otherwise
The musculocutaneous nerve provides sensory innervation to the lateral half of the forearm via the lateral antebrachial cutaneous nerve, but this nerve does not provide any sensation below the wrist. The latter point, along with intact strength of the brachioradialis in the case (as evidence by stronger forearm supination with the forearm extended), is evidence that this is not a C6 radiculopathy. Absence of palpable contraction of the biceps during attempted forearm flexion is evidence that this is not biceps tendon rupture.
Axillary nerve
Axillary neuropathies occur with fractures at the surgical neck of the humerus and with anterior shoulder dislocations.
The axillary nerve is a continuation of the posterior cord, and carries predominantly C5 and C6 fibers. The axillary nerve innervates the deltoid muscle, which is the main arm abductor, particularly between 30 and 90 degrees (supraspinatus innervated by the suprascapular nerve from the upper trunk significantly contributes to arm abduction in the first 30 degrees of abduction and the trapezius contributes to greater than 90 degrees). The deltoid has three heads: (1) the anterior head, which is involved arm flexion (in front of the body), assisted by serratus anterior, (2) the lateral head, which along with the anterior head is mainly involved in arm abduction to the side and slightly anteriorly, and (3) the posterior head, which is involved in posterior movement of the abducted arm. The axillary nerve also innervates teres minor, which externally rotates the arm along with infraspinatus. The axillary nerve provides sensory innervation to the upper lateral arm through the upper lateral brachial
Dorsal scapular nerve
Arises from the C5 nerve root, innervates the rhomboids and elevator scapulae
With chronic denervation of the rhomboids, intrascapular wasting can occur.
C5 and the phrenic nerve
It is important to note that C5 has a significant contribution to the phrenic nerve, which also receives contributions from C3 and C4. A proximal lesion to C5 can lead to weakness of the ipsilateral diaphragm.
Suprascapular nerve entrapment
From upper trunk via C5-C6, innervates supraspinatus and infraspinatus.
The clinical picture is one of poorly localizable shoulder pain, and weakness of the supraspinatus, which abducts the arm, particularly during the first 30 degrees of abduction, and infraspinatus, which externally rotates the shoulder when the elbow is flexed and fixed at the patient’s side.
Injury to long thoracic nerve and serrates anterior
The long thoracic nerve arises from the C5, C6, and C7 roots and innervates serratus anterior, which acts to abduct the scapula. Injury to it causes winging of the scapula, which is most evident when the arms are extended and pressure is applied anteriorly (as if doing a push-up on a wall).
What causes scapular winging?
Injury to serratus anterior and injury to rhomboids
Thoracodorsal injury
The thoracodorsal nerve arises from the posterior cord and innervates the latissimus dorsi, which acts to adduct the arm. The other two branches of the posterior cord are also involved in arm abduction (as well as internal rotation): (1) the upper subscapular nerve, which innervates the subscapularis muscle, and (2) the lower subscapular nerve, which innervates teres major as well as a portion of the subscapularis muscle.
Thoracoabdominal polyradiculopathy
This patient has a thoracoabdominal polyradiculopathy, which is seen in patients with long-standing diabetes, and presents with pain and dysesthesias, patchy sensory and motor changes in thoracic and abdominal nerve root territories, usually unilateral but may be bilateral. It is commonly confused with intra-abdominal processes and extensive gastrointestinal workups are often undertaken before the diagnosis is made. The pathology is not known but thought to be an ischemic radiculopathy. EMG of the abdominal wall and paraspinals may assist with the diagnosis, showing fibrillations in the involved muscles in one or more adjacent myotomes. Recovery is protracted, and may occur spontaneously or with the treatment of the diabetes.
Neurogenic thoracic outlet syndrome
The signs and symptoms result from compression on the C8 and T1 nerve roots. The brachial plexus passes through the scalene triangle, which is formed by the anterior scalene, middle scalene, and first rib. An anomalous fibrous band between the scalene muscles, a cervical rib, or an elongated C7 transverse process can lead to neural compression or irritation, resulting in neurogenic thoracic outlet syndrome. There is weakness of intrinsic hand muscles and sensory loss in a C8 and T1 distribution; the pattern of weakness distinguishes this disorder from the other disorders listed, which would lead to weakness in muscle groups innervated by the single nerves list_ed. Arm abduction and external rotation can precipitate symptoms and reduce the radial pulse. Sensory loss in both C8 and T1 dermatomes distinguishes thoracic outlet syndrome from isolated radiculopathy (the most medial aspect of the ventral forearm is innervated by C8, whereas the T1 nerve root innervates an area of the forearm lateral to that)._ Involvement of the medial antebrachial cutaneous nerve makes thoracic outlet syndrome more likely than ulnar neuropathy at the elbow.
*In this case, there is weakness in thumb abduction and thumb flexion and weakness of fingers in a median nerve pattern
Injury to the lower trunk
A lower trunk lesion can occur due to stretching, as occurs with excessive arm abduction such as when grabbing onto something during falling, motor vehicle accidents, or less commonly with birth injury. Lower trunk lesions lead to weakness in ulnar and median nerve– innervated muscles (see Table 9.1), leading to weakness of intrinsic hand muscles and sensory loss on the medial forearm and hand. Sensory loss occurs in a C8 and T1 distribution; the medial arm,
Upper trunk or Erb’s palsy
This type of injury is called Erb’s palsy and commonly occurs when the shoulder is forcefully pulled down while the neck is flexed in the opposite direction, as can occur with birth injury. Other mechanisms of injury include falls and motorcycle accidents. The posture held by the patient depicted in question 84, “waiter’s tip” position, is a classic example of Erb’s palsy. Arm adduction and internal rotation result from unopposed action of the pectoralis major; the clavicular head of the pectoralis major is innervated by the lateral pectoral nerve that arises from the lateral cord and acts to adduct and internally rotate the arm. Forearm extension results from unopposed action of the triceps and forearm pronation from unopposed action of pronator teres; both of these muscles are predominantly C7 innervated, and normal strength in these muscles as well as a normal triceps deep tendon reflex excludes a C7 root lesion and a middle trunk lesion (because the middle trunk carries only C7 fibers).
L5 radiculopathy
L5 radiculopathy may manifest as pain from the buttock radiating down the lateral thigh, anterolateral leg, and dorsum of the foot, with sensory impairment in this dermatomal region extending to the big toe. The weakness is prominent on toe extension and ankle dorsiflexion, as well as inversion and eversion of the foot. The only reflex found to be abnormal is the hamstring reflex, which is not routinely checked. As foot drop is a frequent manifestation that may be seen with L5 root lesion and common peroneal neuropathy, one important means of distinguishing the two is NCS. Superficial peroneal SNAPs are abnormal in common peroneal nerve lesions, but normal in L5 radiculopathies. Another key diagnostic feature is to detect abnormalities in L5-innervated muscles that are not innervated by the peroneal nerve, such as the tibialis posterior and flexor digitorum longus, both of which are innervated by the tibial nerve. The L5 nerve root provides innervation to the tensor fasciae latae, gluteus medius, semitendinosus and semimembranosus, tibialis anterior, extensor hallucis, peroneus longus, extensor digitorum brevis, tibialis posterior, and flexor digitorum longus muscles.
Muscles involved in S1 radiculopathies include the abductor hallucis, abductor digiti quinti pedis, soleus, medial and lateral gastrocnemius, extensor digitorum brevis, biceps femoris (long and short head), and gluteus maximus. Muscles partially innervated by S1 that may also be affected are the tibialis posterior, flexor digitorum brevis, gluteus medius, and tensor fasciae latae. Although the SNAPs should be normal in S1 radiculopathies, the H- reflex is commonly reduced or absent.
Foot drop is L5, unable to stand on toes is S1
Superior gluteal nerve (L5) innervated gluteus minimus, medius muscles, tensor fasciae latae•Performs hip abduction and internal rotation
Inferior gluteal nerve (S1-S2) – gluteus maximus•Performs Hip extension
- C - B: Levator palpebrae sends fibers bilaterally whereas the superior rectus sends fibers contralaterally
