Functional Neuroanatomy & The Exam Flashcards
What is the resting membrane potential for a large nerve fiber?
-90 millivolts
What happens when you have a calcium deficit?
Causes Na+ channels to become activated with little increases in membrane potential, leading to a very excitable nerve (ie. Tetany)
What is the threshold for stimulation of an action potential?
-65 mV
What 3 things result in the inhibition of excitability?
High extracellular Ca (decr membrane permeability to Na), local anesthetics (Na channel blockers), when AP strength to excitability threshold = <1
What are the different cells that form myelin for the CNS and PNS, and what is the clinical significance of this?
CNS: oligodendroglial cell processes; PNS: rolled up Schwann cell membrane; different populations of cells > get different cancer manifestations
The amount of Ca inflow from voltage-gated Ca channels on the presynaptic membrane after an AP depolarizes it is directly related to what?
Transmitter release
What are the two types of ion channels located on the postsynaptic neuron?
- Cation channels (allow Na ions to enter, excitatory)
- anion channels (allow Cl ions to enter, inhibitory)
What is occurring during excitation of a postsynaptic receptor?
- Opening Na channels
- decr conduction thru Cl/K channels (both raise intracellular membrane potential towards zero)
- internal metabolic changes to excite cell activity
What is occurring during inhibition of a postsynaptic receptor?
Opening of Cl channels, incr K out of neuron (both make the intracellular membrane potential more negative), activation of receptor enzymes to inhibit cellular activity
How do small-molecule neurotransmitters differ from neuropeptides and what are some examples of each?
Small molecule NTs are rapidly acting, while neuropeptides are slow acting or they are growth factors that have central and long-acting effects
- Small: ACh, NE, Epi, GABA, Glycine, Glutamate
- Neuropeptides: ACTH, GH, Insulin, Glucagon, Angiotensin
Describe acetylcholine
- Acetyl coenzyme A + choline
- transported into vesicles
- released into cleft
- rapidly split into acetate/choline by cholinesterase in cleft
- choline actively recycled
- usually excitatory (inhibitory in some PS nerve endings ie. Vagus)
What is a condition in which the patient has adequate ACh, but doesn’t have enough receptors for it to bind to?
Myasthenia gravis
Describe norepinephrine
Synthesized in the adrenergic nerve terminal (tyrosine>dopa>dopamine), transported into vesicles> dopamine to NE
Removal: active reuptake into adrenergic endings, diffusion away, enzymatic destruction (ie. monoamine oxidase)
An excitatory postsynaptic potential does what to elicit an action potential?
Incr Na permeability, neutralizing the RMP, and requires a discharge of many terminal at once or in sequence (spatial vs. temporal summation)
Inhibitory postsynaptic potentials do what to prevent the generation of an action potential?
Open Cl channels > Cl in and/or K out, incr negativity in the cells (hyperpolarization)
What occurs with presynaptic inhibition?
Release of inhibitory substance (GABA) onto presynaptic fibrils > cancels effect of sodium influx; occurs in many sensory pathways to minimize sideways spread
What is the effect of fatigue on synaptic transmission?
Decrease in discharge of postsynaptic neuron
What is the effect of pH on synaptic transmission?
Alkalosis increases excitability, acidosis depresses it
What are the effects of caffeine, theophylline and theobromine on synaptic transmission?
Reduce threshold for excitation
What are the effects of strychnine on synaptic transmission?
Inhibits glycine (inhibitory NT) in spinal cord, causing excitation/tetany
What are the effects of anesthetics on synaptic transmission?
Anesthetics increase threshold for excitation, decreasing transmission
Describe the process of skeletal muscle excitation
Mostly Na enters the muscle fiber, creating a local positive potential in the muscle fiber (the end plate potential)
When does fatigue occur at neuromuscular junctions?
Stimulation of the nerve >100x/second for minutes depletes ACh vesicles, so impulses fail to pass, resulting in fatigue
What 3 drugs stimulate the muscle fiber by ACh-like action and are long-lasting because they are not broken down by cholinesterase?
Methacholine, carbachol, nicotine > can result in long-lasting toxicities
What are 4 drugs that stimulate the NMJ by inactivating acetylcholinesterase?
Neostigmine, physostigmine, diisopropyl, fluorophosphate > cause muscle spasm
What type of drugs block transmission at the NMJ?
Curariform drugs; e.g. D-tubocurarine blocks ACh on the receptor, preventing AP
Describe the process going on during skeletal muscle contraction
AP travels along a motor nerve to the NMJ > nerve secretes ACh > opens ACh-gated channels > Na diffuses in, intimating AP > AP travels along muscle membrane, T tubules > AP depolarizes muscle, causing SR to release Ca > Ca intimates actin and myosin sliding > Ca pumped back into SR, stopping contraction
What is the function of transverse or T-tubules in skeletal muscle?
Skeletal muscle fibers are too large for surface AP to cause current flow deep in the muscle, so T tubules penetrate all the way through, acting as internal extensions of the muscle membrane
Ca is pumped back into the SR and bound by what molecule?
Calsequestrin
What are some characteristics of smooth muscle structure?
Can be multi-unit or unitary, have dense bodies, not Z-discs, have slow myosin head cycling (less ATPase activity and lower energy requirements), and have calmodulin, but no troponin
Describe smooth muscle action potentials
APs are typically a spike, but those with plateaus account for prolonged contraction, and they have many voltage-gated Ca and few Na channels
Describe the concept of slow wave potentials in smooth muscle
Some smooth muscle is self-excitatory, and APs can be generated by stretch/can get contraction without AP in small fibers
All autonomic preganglionic neurons are what type?
Cholinergic; ACh is excitatory to all postganglion neurons
Most parasympathetic postganglionic neurons are what, compared to what most sympathetic postganglionic neurons are?
PS: cholinergic; S: adrenergic
Where would you find muscarinic receptors vs. nicotinic receptors?
M: on all effector cells stimulated by postganglionic cholinergic neurons of S/PS systems
N: in autonomic ganglia at synapses b/t pre- and postganglionic neurons of S/PS systems; NMJs
Describe a reflex arc
polysynaptic w/ multiple inter neurons more common
- receptor endings of primary afferent axon > cell body in DRG > synapses on efferent neuron in ventral horn > motoneuron axon passes out into spinal nerve to effector organ
Describe alpha/skeletomotor neurons?
Innervate oridinary (extramural) skeletal muscle fibers; large diameter myelinated axon > fast conduction
Describe fusimotor (gamma) neurons
Innervate intrafusal muscle fibers (part of the muscle spindle), and are thin myelinated axon, so slower conduction
Describe intrafusal muscle fibers
- No actin/myosin centrally
- fusimotor (gamma) neurons innervate end-portions
- detects stretch
- rate of firing related to degree of stretch
What is the difference between alpha, gamma, and interneurons?
- Alpha - innervate sk mm, single fiber + all mm fibers it innervates = motor unit
- Gamma = transit impulses thru smaller nerves, intrafusal mm fibers to control mm tone
- Interneurons = present in spinal cord gray matter, renshaw cells - lateral inhibition of motor neurons
Define lower motor neuron
The efferent neuron of the PNS that connects the CNS with the muscle to be innervated
What nerves constitute general somatic efferent (GSEs)
- Striated skeletal muscle
- all spinal nerves
- CN II, IV, V, VI, VII, IX, X, XI, XII
What nerves constitute general visceral efferent (GVEs)
- Innervate smooth and cardiac mm and glands
- sympathetic: all spinal & splanchnic nerves
- PS: sacral spinal nerves, CN III, VII, IX, X, XI
What sections of the brain does the forebrain include?
- Cerebrum (telencephalon):
- cerebral cortex (frontal, piriform, parietal, temporal, occipital lobes)
- hippocampus
- basal nuclei
- Diencephalon: thalamus, hypothalamus
What does the midbrain (mesencephalon) contain?
CN III, CN IV, rostral and caudal colliculi
What will you find within the rostral fossa and caudal fossas, and what structure lies between these 2 fossas?
Rostral: cerebrum, dienceophalon
Caudal: cerebellum, brainstem, midbrain
- separated by the tentorium cerebellum
What is the difference between upper and lower motor neurons?
UMN: starts in the brain and goes down spinal cord to synapse on motor neuron
LMN: comes from SC and goes to the periphery
What are LMN signs?
- Loss of reflexes
- loss of tone
- paresis to paralysis (motor deficit); weak and floppy
What are upper motor neuron signs?
- normal to increased reflexes
- increase in tone
- paresis to paralysis
- weak and stiff
What is the purpose of the ascending reticular activating system?
Directs all the sensory info coming into your body towards the brain and SC, which is relayed through the thalamus and projected constantly up to the cortex to stimulate and keep you awake (where caffeine works)
What tracts will you find in the dorsal vs. the ventral and lateral columns of the spinal cord?
Dorsal = sensory
Ventral/lateral = motor
How does the information regulating conscious proprioception make it to the brain?
- Travels from distal receptors up the peripheral nerve, then up the SC via the dorsal funiculi.
- These tracts are ipsilateral until they decussate at the level of the pons before radiating to the cortex
How do postural responses differ from reflexes?
Responses not reflexes b/c must travel to the brain, be interpreted, then acted upon with the resultant physical movement
Why are CP deficits often the first abnormalities seen with disease of the spinal cord?
B/c most pathways consistent of large myelinated fibers with very fast transmission times that are most often situated on the periphery of the SC, making them susceptible to the first effects of compressive lesions
If a compressive lesion affecting CP is located caudal to or below the rostral brainstem (b/t pons and medulla), what side would you expect the deficit to be on?
The same side as the lesion
How do you test for pain perception?
- use a strong instrument (ie hemostats or Carmalts) to squeeze the bone of the digit
- a positive response is a conscious response (turning toward stimulus, trying to bite)
*with a severed SC, the leg can still pull back b/c of withdrawal reflex
What is the difference between paresis and plegia?
- Paresis is a motor deficit or weakness
- an affected limb cannot support weight well and may not be able to move well
- Plegia is if there is no voluntary movement at all
Describe the complex controls of gait
- Is controlled from midbrain and brainstem descending pathways
- a cerebrocortical lesion may affect gait, but will not abolish it (plegia)
What is ataxia and what are the 3 types of it?
Lack of coordination
- cerebellar - dysmetria w/ movements that are too long/too short (e.g. goosestepping)
- vestibular - often involves leaning/listing to one side, as well as crossing over of feet at walk
- proprioceptive - involves scuffing and hypometria
What are some indications for a neuro exam?
- Seizures
- behavior change
- circling, paresis
- ataxia
- pain
- lameness
- trauma
What are the 6 components of a neuro exam?
- Mentation
- gait and posture
- cranial nerves
- postural reaction
- segmental reflexes
- palpation and range of motion
What is the difference between the various mentation states?
- Inappropriate: Disoriented, confused
- Obtunded: dull, lethargic, less wakeful or responsive
- Stuporous: responsive to noxious stimuli
- Comatose: unresponsive to noxious stimuli
- Vegetative: lacks awareness but brainstem ok
- Brain dead: coma, apneic, no reflexes, flat EEG
The personality component of mentation is controlled by what part of the brain?
The limbic system
What sensory tract activates Purkinje cells and what side does it detect?
Spinocerebellar; ipsilateral
Which sensory tract registers pain?
Spinothalamic; bilateral sensation
Which sensory tracts detect pelvic limb sensation? Thoracic limb sensation?
PL: fasciculus gracilis
TL: fasciculus cuneatus
*Contralateral sensation
What do you look at when you’re checking a patient’s posture?
Head: tilt, turn, resting or intention tremors, head held low, neck guarding
Body: kyphosis, lordosis, scoliosis, torticollis, laterally recumbent
What is the difference between decerebrate, decerebellate, and Schiff-Sherrington?
Decerebrate: comatose, midbrain lesion, rigid extension in all limbs
Decerebellate: acute cerebellar lesions, extended TLs, flexed PLs (“star gazing)
SS: severe, acute T3-L3 lesions, not prognostic
List the 12 cranial nerves
I: Olfactory
II: Optic
III: Oculomotor
IV: Trochlear
V: Trigeminal
VI: Abducent
VII: Facial
VIII: Vestibulocochlear
IX: Glossopharyngeal
X: Vagus
XI: Accessory
XII: Hypoglossal
CN I is linked to what parts of the brain?
limbic system and rhinencephalon
What nerves does the PLR test?
II & III
What nerves does the menace reflex test?
II and VII
What nerves does the dazzle reflex test?
II and VII, sub-cortical reflex
What muscles does the oculomotor nerve (CN III) innervate?
dorsal, medial, ventral rectus, ventral oblique, and levator palpebrae of superior eyelid; PS motor fibers for pupillary constriction
What does the trochlear nerve (CN IV) control?
motor pathway to the dorsal oblique m.
What are the different branches of the trigeminal nerve (CN V), where do they come out of, and what do they innervate?
Sensory branches:
- Ophthalmic n. (V1) - orbital fissue
- Maxillary n. (V2) - round foramen
Motor branches:
- Mandibular n. (V3) - oval foramen
- Muscles of mastication
What does the abducent nerve (CN VI) innervate? What would a lesion to this nerve cause?
motor to the lateral rectus and retractor bulbi mm; lateral strabismus with inability to retract the globe
What does the facial nerve (CN VII) innervate?
Motor: muscles of facial expression
Sensory:
- taste
- palate
- rostral 2/3 of tongue
- inner surface of the pinna
What are the branches of the vestibulocochlear nerve (CN VIII) and what do they innervate? What are some signs of vestibular disease?
Vestibular: sensory for orientation of the head with respect to gravity
- physiological/pathological nystagmus (doll’s eye)
- strabismus
- head tilt
- ataxia
Cochlear: sensory for hearing, difficult to test, BAER
What do CN IX, X, and XI control?
- Nucleus ambiguus (IX, X)
- sternocleidomastoid and trapezius mm.
- gag reflex
- larynx
How do you test CN XII?
tongue strength, movement, and position
- look for asymmetrical tongue
What cranial nerves are you testing for with the palpebral reflex?
- Medial canthus: afferent - V1, efferent - VII
- Lateral canthus: afferent - V2, efferent - VII
*lateral ALWAYS weaker
What cranial nerves are you testing for when you’re checking for physiologic nystagmus?
Afferent: CN VIII
Efferent: CN III, IV, VI
- also checking strabismus, positional nystagmus
What nerves are you testing with the gag reflex test?
Afferent: CN IX, X
Efferent: IX
What CN are you testing when checking tongue function?
CN XII
What postural reactions do you test during the neuro exam?
- placing/knuckling (CP)
- hopping
- hemistanding/hemiwalking
- wheelbarrow
- visual and tactile placing
- extensor postural thrust
What are you testing with the thoracic limb withdrawal segmental reflex?
- biceps and musculocutaneous n.
- C6-C8
- Triceps and radial n.
- C8 -T2
What segmental reflex are you testing with the cutaneous trunci test?
- lateral thoracic n. and C8-T1
What segmental reflex are you testing with the patellar reflex test?
- femoral n. and L4-L6
- Gastroc/cranial tibial n.
What segmental reflex are you testing with the pelvic limb withdrawal test?
- Mostly sciatic n. and L6-S1
What do you check for with the perineal reflex?
tail tuck and anal sphincter contraction
T or F: you can sever the spinal cord cranial or caudal to the cell bodies involved in a segmental reflex (intumescence) and the reflex will still be intact
True; NOT voluntary motor function!
