Nerves, Spinal cord, and Pathways Flashcards
Spinal Cord
Organization
-
31 segments:
- 8 cervical
- 12 thoracic
- 5 lumbar
- 5 sacral
- 1 coccygeal
- Each sement provides a pair spinal nerves
- Segments C1 ⇒ C7: spinal nerves exit above vertebrae
- Spinal nerve C8 exits between vertebrae C7 and T1
- At and below T1: spinal nerves exits below vertebrae
-
Cervical enlargement: C4 ⇒ T1
- Innervates upper limbs
-
Lumbosacral enlargement: L2 ⇒ S3
- Innervates lower limbs
-
Spinal cord ends at conus medullaris (L1-L2)
- Cauda equina nerve roots below this continue to the lumbar cisterna to exit.
Spinal Meninges
Pia mater, arachnoid, and dura mater.
- Subarachnoid space filled with CSF.
- Epidural space filled with fatty tissue and venous plexus.
-
Denticulate ligaments anchor spinal cord laterally to dura.
- Are extensions of pia mater and arachnoid.
-
Filum terminale provides vertical support to spinal cord.
- Becomes coccygeal ligament.
-
Lumbar cisterna extends to end of dural sac at S2.
- Lumbar punctures are done between L3 & L4 in adults.
Spinal Cord
Blood Supply
- Spinal cord predominantly supplied by:
-
2 posterior spinal arteries
- Supplies dorsal 1/3 of cord
- I.E. dorsal columns
- Supplies dorsal 1/3 of cord
-
1 anterior spinal artery
- Supplies the anterior 2/3 of the cord
-
2 posterior spinal arteries
-
Segmental arteries enter the spinal cord at each level
- Give rise to the ventral & dorsal radicular arteries
- Mostly supply nerve roots and vascular plexus surrounding the spinal cord
- 6-10 radicular arteries are larger ⇒ great radicular artery of Adamkiewicz
- Supplies most of the lumbosacral cord
- These anatomose with posterior and anterior spinal arteries forming the vasocorona
- Give rise to the ventral & dorsal radicular arteries
-
Vulnerable zone exists in midthoracic region (T4-T8)
- Area of relatively decreased perfusion
- Susceptible to infarction during surgeries
-
Spinal vein distribution similar to spinal arteries
-
Epidural veins (Batson’s plexus) do not contain valves
- Acts as potential pathway for spread of infection and cancer
-
Epidural veins (Batson’s plexus) do not contain valves
Spinal Cord
Grey & White Matter
Distribution
Butterfly shaped core of grey matter surrounded by white matter.
-
White matter
- contains ascending and descending fiber tracts
- organized into dorsal, lateral and ventral funiculi
-
Grey matter
- predominantly neurons
- divided into dorsal horn, intermediate zone, and ventral horn
- Sensory fibers from spinal nerves enter via dorsal root
-
Motor fibers exit spinal cord via ventral root
- Joins the spinal nerves
Spinal Nerves
- Protected by 3 layers of connective tissue:
- epineurium
- perineurium
- endoneurium
- Each spinal nerve innervates a specific dermatome / myotome
- Carries both sensory and motor fibers
Sensory Fibers
Cell bodies in dorsal root ganglia.
Neurons pseudounipolar.
Enter spinal cord via dorsal root.
Two types of sensory fibers:
-
Somatic fibers:
-
epicritic information
- fine touch sensation
- proprioception
-
protopathic information
- pain
- temperature sensation
-
epicritic information
-
Visceral fibers:
- visceral sensation from glands and viscera
Motor Fibers
Two types of motor fibers:
-
Somatic motor fibers:
- carry motor commands to skeletal muscles
- somatic motor neuron cell bodies in ventral horn
- exit via ventral root
-
Visceral motor fibers:
- carry motor commands to glands and viscera
-
visceral neuron cell bodies in the intermediolateral nucleus (IML) of lateral horn
- T1-L2 and S2-S4
- exit via ventral root
Cervical
Dermatomes
The area of the skin supplied by a single spinal nerve.
- C2 ⇒ back of the head
- C6 ⇒ thumb, index, and lateral forearm
- C7 ⇒ middle finger
- C8 ⇒ ring and small finger
Thoracic
Dermatomes
The area of the skin supplied by a single spinal nerve.
- T4 or T5 ⇒ nipple
- T10 ⇒ umbilicus
Lumbar and Sacral
Dermatomes
The area of the skin supplied by a single spinal nerve.
- L1 ⇒ inguinal ligament
- L5 ⇒ big toe
- S1 ⇒ small toe, lateral foot, heel
- S4-S5 ⇒ perianal region
Myotomes
The area of a muscle supplied by a single spinal nerve.
Muscles are usually innervated by several spinal nerves.
C5 and C6* ⇒ biceps
C6, C7*, C8 ⇒ triceps
L2, L3, L4* ⇒ quadriceps
L5, S1*, S2 ⇒ gastrocnemius
* = provides major innervation
Touch Receptor
Parameters
- Duration
- Intensity
- Modality
- Localization
Fine (Discriminative) Touch
Receptors
Epicritic Information
Allows localization and sense of detailed features
Faciliated by 5 types of tactile mechanoceptors:
-
Meissner’s corpuscle
- detects stroking, fluttering
-
Hair
- detects light stroking
-
Pacinian corpuscle
- detects vibration
-
Merkel disk
- detects pressure, texture
-
Ruffini ending
- detects skin stretch
Fine Touch
Transduction
Transduction is the process of transforming the energy content of a stimuli into coded bioelectrical signals.
- Mechanoreceptor membrane mechanically coupled to ion channels via cytoskeleton bridges
- Deformation of receptor membrane by a mechanical force opens ion channels allowing sodium enters cell
- Intensity of the stimulus is converted into an amplitude modulated local receptor potential
- Receptor potential converted into a frequency modulated signal of action potentials
- APs propagate via axons to CNS.
Fine Touch
Fibers
Each tactile mechanoceptor innervated by a type A-𝛽 fiber.
- Large, myelinated, and fast
- Enters via medial division fibers of dorsal root
- Send collaterals into:
-
dorsal column
- ascending sensory pathway within dorsal funiculus
- relays fine touch and proprioception to somatosensory cortex
-
dorsal horn
- local projections into spinal cord
- used locally for sensory integration and reflex control
-
dorsal column
Touch Receptor
Adaptation
Tactile mechanoceptors can be divided into:
-
Fast adapting receptors ⇒ responds mostly to changes
- Includes:
- Meissner’s corpuscle
- Hair
-
Pacinian corpuscle
- Best at detecting vibrations
- Generally detects only the onset and/or offset of stimulation
- Adapts rapidly
- Includes:
-
Slowly adapting receptors ⇒ responds as long as the stimulus is present
- Includes:
-
Merkel disk
- Best at extracting texture information
- Ruffini endings
-
Merkel disk
- Active during the whole duration of the stimulation
- Slowly adapting
- Includes:
Response properties dependent on amount of encapsulation.
Proprioceptors
Epicritic information
Mechanoceptors that provide information about muscles.
Brain uses this information to compute where body parts are in space (proprioception).
Two most important proprioceptors:
-
Muscle spindles
- Made of 2 types of intrafusal fibers:
- nuclear bag fibers
- nuclear chain fibers
- Placed in parallel to extrafusal muscle fibers
- Made of 2 types of intrafusal fibers:
-
Golgi tendon organs
- Placed in series with extrafusal muscle fibers
Proprioceptor
Innervation
-
Muscle spindles
- Innervated by 2 types of sensory fibers
-
type Ia fibers
- monitors muscle length
- monitors rate of change of muscle length
-
type II fibers
- monitors only muscle length
-
type Ia fibers
- * Also innervated efferently by 𝛾-motor neurons
- Innervated by 2 types of sensory fibers
-
Golgi tendon organs
- Innervated by type Ib fibers
- monitors the tension generated by the muscle
- Innervated by type Ib fibers
Cell bodies of these primary sensory neurons located in the DRG.
Fibers large, myelinated, and fast.
Enter spinal cord via the medial division fibers of dorsal root.
Pain and Temperature
Sensation
Protopathic Information.
- Pain and temperature receptors present on free nerve endings.
- Nerve endings express protein receptors
- Transduce thermal and nociceptive stimuli into electrical signaling
- Primary sensory neurons in the DRG.
- Two types of nerve fibers are involved:
-
A-𝛿 fibers
- lightly myelinated and small
- relatively fast conducting
- provide cold and sharp pain sensation
-
C fibers
- smaller non-myelinated fibers
- slowly conducting
- provide warmth and dull pain sensation
-
A-𝛿 fibers
- Fibers enter spinal cord via lateral division fibers of dorsal root
- Terminate in the dorsal horn
Nociceptive Pain
(Physiological)
- Involves direct stimulation of nociceptors
- Serves protective biological function
- Acts as a warning of on-going tissue damage
Neuropathic Pain
(Intractable)
- Results from injury to PNS or CNS causing changes in circuit sensitivity and CNS connections
- Serves no protective biological function
- Often associated with:
- paresthesia
- numbness
- allodynia ⇒ pain from normally non-painful stimuli
- hyperalgesia ⇒ exagerated pain from painful stimuli
- Can be continuous, episodic, stabbing, burning, shooting, aching, pins and needles, or electric-shock like
Referred Pain
- Pain from visceral organs perceived to be somatic in origin
- as if it originated from the skin or outer body
- Ex. heart attack produces sensation of pain in superior thoracic wall and medial aspect of left arm
- May be due to convergence of somatic and visceral info onto the same somatic dorsal horn neurons
Visceral
Sensory Input
-
Visceral sensory neurons monitor stimuli within visceral organs
- stretch
- temperature
- chemical changes
- irritation
- Most receptors are on free nerve endings
- Information carried by A-𝛿 and C fibers
- Cells bodies located in the DRG
- Fibers enter spinal cord via dorsal root
Visceral
Motor Output
Two neuron pathway used to carry information from CNS to peripheral organs.
-
First neuron (preganglionic)
- preganglionic sympathetic neurons located in lateral horn of spinal cord at T1-L2 (+/- 1)
-
preganglionic parasympathetic neurons located
- brainstem
- intermediate zone of spinal cord at S2-S4
- carried by B type fibers
- slightly myelinated
- exit via ventral root
- synapses with postganglionic neurons
-
Second neuron (postganglionic)
- both PNS and SNS postganglionic neurons located in ganglia outside of the spinal cord
- carry motor commands to target visceral organs
- utilizes slow unmyelinated C fibers
Somatic Motor
Output
- Two types of motor neurons contribute to majority of output:
-
𝛼-motor neurons
- large, myelinated, and fast conducting axons
- synapse on extrafusal muscle fibers
- produce tension when contracted
-
𝛾-motor neurons
- smaller, myelinated, and fast conducting axons
- synapse on muscle spindles
- regulate muscle spindle excitability
-
𝛽-motor neurons
- poorly characterized in humans
- believed to innervate both intrafusal & extrafusal fibers
-
𝛼-motor neurons
- Motoneurons located in ventral horn of spinal cord
- Axons exit via ventral root
Deep Tendon Reflexes
Clinical Application
DTRs used clinically to assess sensory and motor functions.
Reflex arc consists of a sensory and a motor component.
-
Sensory side:
- stretch of tendon detected by muscle spindle
- activation of Ia fiber
-
Motor side:
- Ia afferent fiber synapses in ventral horn on 𝛼-motor neuron which innervates the stretched muscle
- excitation of the 𝛼-motor neuron results in muscle contraction
Hyporeflexia or areflexia seen if sensory or motor branch of reflex arc compromised (spinal nerve or peripheral nerve lesions).
Hyperreflexia seen if a upper motor neuron lesion compromises descending spinal cord regulation of the pathway.
Sensory Exam
-
Pain ⇒ safety pin & Light touch ⇒ brush
- patient with eyes closed
- stimulated alternatively with needle and brush
- report if there is a difference between sensations
-
Vibration ⇒ 128 Hz tuning fork
- tested on large toes or fingers
- report when vibration stops
-
Two-point discrimination ⇒ pair of calipers
- alternate between touching patient with one or both points
- find the minimal seperation that can be distinguished between two points
-
Position sense
- examiner manually moves patient’s toe or finger
- ask the patient to report direction of movement
Motor Exam
- Motor strength
- strength of each muscle compared to contralateral conunterpart
- rated on a scale of 0-5/5
- 0/5 = no contraction, 5/5 = normal strength
- Deep tendon reflexes
- reflex hammer used to stretch the muscle and tendon
- reflexes rated on a scale of 0-5+
- 0 = absent reflex, 2+ = normal, 5+ sustained clonus
Spinal Cord
Morphology Overview
-
White matter
- Dorsal funiculus
- Lateral funiculus
- Ventral funiculus
-
Grey matter
- Dorsal horn
- Intermediate zone
- Ventral Horn
-
Small lateral horn starts at T5 and above
- contains intermediolateral nucleus
-
Grooves
- dorsal mediun sulcus
- dorsal intermediate sulcus starting at T5 and above
- dorsal lateral sulcus
- corresponds to dorsal root entry
- ventral median fissue
- Ventral/anterior white commissure
Dorsal Funiculus
Contains the dorsal columns
Consists of 2 large fasciculi of myelinated fibers.
Carries sensory information about fine touch, proprioception, and vibratory sense.
-
Fasciculus gracilis (FG)
- carries ipsilateral information from lower body
- only fasciculus at T6 and below
-
Fasciculus cuneatus (FC)
- carries ipsilateral information from upper body
- present from T5 to C1
Lateral Funiculus
Contains 3 motor tracts:
-
Lateral corticospinal tract (LCST)
- fibers originate from the contralateral motor cortices
- voluntary control of distal motor neurons
- suppression of infantile reflex
-
Medullary reticulospinal tract (MRST)
- originates from the medullary reticular formation
- inhibits extensor and facilitates flexor motor neurons
- opposite action of the PRST
-
Rubrospinal tract (RST)
- originates from the red nucleus of the midbrain
- facilitates upper limb flexors
- tract present only at cervical and upper thoracic levels
Contains 3 sensory tracts:
The anterolateral system
-
Spinothalamic tract (STT)
- carries contralateral pain and temperature sensation
-
Dorsal spinocerebellar tract (DSCT)
- carries uncouscious proprioception from lower body
- present only at and above L2-L3
-
Ventral spinocerebellar tract (VSCT)
- reports to cerebellum about spinal motor output
Ventral Funiculus
Contains 6 motor tracts:
-
Pontine reticulospinal tract (PRST)
- originates from the pontine reticular formation
- faciliates extensors and inhibits flexors
- opposite action of the MRST
-
Ventral corticospinal tract (VCST)
- voluntary control of axial motor neurons
-
Lateral vestibulospinal tract (LVST)
- originates from the lateral vestibular nucleus
- facilitates extensor antigravity muscles
-
Medial longitudinal fasciculus (MLF)
- originates from the medial vestibular nucleus
- vestibular
- eye movements
- balance function
-
Medial vestibulospinal tract (MVST)
- controls head and neck position in relation to gravity
- runs within the descending MLF tract
- only present at cervical and upper thoracic levels
-
Tectospinal tract (TST)
- controls head and neck movements for reflex of orientation towards visual, auditory, and somatosensory stimuli
- only present at cervical and upper thoracic levels
Lissauer’s Tract
(Dorsolateral tract of Lissauer’s)
Allows collaterals of entering pain and temperature fibers to ascend and descend 1-3 segment before terminating in grey matter.
Associated with the anterolateral system.
Found near the entrance of the dorsal root.
Propiospinal Tract
(Faciculus proprius)
Contains short and long fibers interconnecting spinal segments to each other.
Anterior White Commissure
Contains the crossing fibers of:
Spinothalamic tract (STT)
Ventral spinocerebellar tract (VSCT)
Ventral corticospinal tract (VCST)
White Matter Diagram
From Notes
Sacral Level
White Matter
Only fasciculus gracilis present → dorsal column is carrying only information from the legs.
Small amount of white matter relative to grey matter.
Large ventral horn → indicates section is from the l_umbosacral enlargement (L2 → S3)_
Lumbar Level
White Matter
Only fasciculus gracilis present → dorsal column is carrying information only from the legs.
Increase in proportion of white matter to grey matter compared to sacral level.
Large ventral horn → indicates this section comes from the lumbosacral enlargement (L2 → S3).
Thoracic Level
White Matter
Below T5
Only faciculus gracilis present → dorsal column carrying information only from the legs and lower trunk.
Higher proportion of white matter relative to grey matter.
Lateral horn present.
Small ventral horn → indicating there are no limbs to innervate at this level.
Low Cervical Level
White Matter
Both fasciculus gracilis and fasciculus cuneatus present → dorsal column contains information from both legs and arms.
Dorsal intermediate sulcus present between FG and FC.
Greater amount of white matter than at lumbar level.
Large ventral horn → indicates this section comes from the cervical enlargement (C4 → T1)
High Cervical Level
White Matter
Both fasciculus gracilis and fasciculus cuneatus present → dorsal column carrying information from legs and arms.
Dorsal intermediate sulcus present.
Lissauer’s tract replaced by the spinal tract of CN V.
Large amount of white matter.
Small ventral horn → no limbs to innervate.
This section is just below the decussation of the pyramids.
Gray Matter
- Divided into 3 parts:
- Dorsal horn
- Intermediate zone
- Ventral horn
- Divided into 10 laminae → Rexed’s lamina I → X
-
Dorsal root fibers enter spinal cord
- medial division
- lateral division
- Lissauer’s tract utilized by l_ateral division fibers_
Dorsal Horn
Grey matter containing Rexed’s laminae I → VI
-
Pain processing
- Lamina I → posterior marginal nucleus
- Lamina II → substantia gelatinosa
- Lamina V
-
Sensory integration
- Lamina III-IV → nucleus proprius
- Lamina V-VI
Intermediate Zone
Grey matter containing Rexed’s lamina VII.
Only present at T1 → L2 (+/- 1) are:
-
Clarke’s nucleus (nucleus dorsalis of Clarke)
- Carries info about unconscious proprioception from the legs
- Neurons are at the origin of the dorsal spinocerebellar tract (DSCT)
-
Intermediolateral cell column (IML)
- Found within the lateral horn
- Neurons are preganglionic sympathetic neurons
Ventral Horn
Grey matter containing Rexed’s Laminae VIII and IX.
Laminae IX contains 𝛼 and 𝛾 motor neurons.
Axons exit via the ventral root.
Sacral Level
Gray Matter
Prominent substantia gelatinosa (RL II) and nucleus proprius (RL III-IV).
Large ventral horn → section is at the lumbosacral enlargement (L2 → S3).
Contains a medial and lateral motor neuron pool responsible for innervation of the legs.
Lumbar Level
Gray Matter
Decreased size of substantia gelatinosa (RL II) and nucleus proprius (RL III-IV) in dorsal horn compared to sacral level.
Clarke’s nucleus begins to appear in the intermediate zone.
Large ventral horn → indicating segment is from the lumbosacral enlargement (L2→S3).
Contains both medial and lateral motor neuron pools for innervation of the legs.
Thoracic Level
Gray Matter
Below T5.
Minimal presence of the substantia gelatinosa (RL II) and nucleus proprius (RL III-IV) in dorsal horn.
Clarke’s nucleus prominent within intermediate zone.
Lateral horn present.
Small ventral horn → only medial motor neuron pool present.
No limbs to innervate.
Low Cervical Level
Gray Matter
Slightly increased presence of substantia gelatinosa (RL II) and nucleus proprius (RL III-IV) in dorsal horn compared to thoracic level.
Large ventral horn → indicates segment at level of cervical enlargement (C4 → T1).
Both medial and lateral motor neuron pools for innervation of the arms.
High Cervical Level
Gray Matter
Dorsal horn replaced by spinal nucleus of CN V.
Lissauer’s tract replaced by spinal tract of CN V.
Together relay pain and temperature from the face.
Small ventral horn → only medial motor neuron pool.
No limbs to innervate.
Spinal accessory nucleus present which appears similar to a lateral horn.
Contains motor neurons innervating trapezius and sternocleidomastoid muscles.
This section is at spinal cord level ~ C1 just below the pyramidal decussation.
Differentiating Spinal Cord Segments
- Proportion of grey matter to white matter greatest at sacral level.
-
Ventral horn enlarged at the lumbosacral (L2⇒S3) and cervical enlargements (C4⇒T1)
- Differentiate between the two using presence of the dorsal intermediate septum.
-
Thoracic levels characterized by
- a small ventral horn
- presence of a lateral horn (intermediolateral nucleus)
- Clarke’s nucleus should be present at all thoracic levels
- if section is thoracic below T5, dorsal intermediate septum will be absent
- if section is T5 and above, dorsal intermediate septum will be present
-
Thoracic and high cervical appear similar
- no Clarke’s nucleus or intermediolateral nucleus at high cervical levels
- If Clarke’s nucleus can be identified in a lumbar section, then section is probably between L1 ⇒ L3
Classification of Fibers
Classified according to function and diameter.
Schemes utilize either
Roman numerals (function) or letters (diameter)
Medial Division Fibers
DRG
Ia, Ib, II, and A-𝛽 fibers.
- Large, myelinated, fast
- Carry epicritic information
- proprioception
- fine touch
- vibratory sense
- Project into the dorsal column
- Fasciculus gracilis and cuneatus
- Project on laminae III-IX of the spinal cord
- integrates with other sensory modalities
- participates in reflex circuits
Lateral Division Fibers
DRG
A-𝛿 and C fibers
- A-delta fibers are slow, small, and slightly myelinated
- C fibers are slow, small, and unmyelinated
- Carry protopathic information
- pain
- temperature
- __Enters first in Lissauer’s tract
- Sends ascending and decending collaterals
- Can be used by local circuits to organize complex reflexes i.e. flexion/withdrawal
- Project predominantly to laminae I, II, and V of dorsal horn in spinal cord
Dorsal Columns
Organization & Somatotopy
- Types Ia, IIb, II, and A-𝛽 fibers enter via the dorsal root medial division fibers.
- Form dorsal columns → brainstem dorsal column nuclei
- Fasciculus gracilis (FG) → nucleus gracilis
- Fasciculus cuneatus (FC) → nucleus cuneatus
-
Somatotopy organized with cervical fibers closest to grey matter.
-
Gracile fasciculus runs medially
- Contains sacral, lumbar, and thoracic fibers (up to T6)
-
Cuneate fasciculus runs laterally
- Contains upper thoracic (T5 → T1) and cervical fibers
-
Gracile fasciculus runs medially
Dorsal Columns
Lesion
- Unilateral lesion of dorsal column results in ipsilateral loss of fine touch and proprioception at and below the level of the lesion
- If lesion isolated to fasciculus gracilis → only info from lower body is lost.
Spinothalamic Tract (STT)
Organization & Somatotopy
- Types A-𝛿 and C fibers enter through dorsal root lateral division fibers.
- Joins Lissauer’s tract
- ascending/descending collaterals reaching 1-3 segments rostrally and caudally
- Most fibers enter spinal cord at level of entry and +/- 1
- Joins Lissauer’s tract
- Synapses on second neurons in dorsal horn laminae I, II, and V.
-
Laminae I and V neurons at the origin of STT.
- Axons cross midline in the ventral white commissure.
- Forms the contralateral STT.
- Destination of STT fibers in the ventral posterior lateral nucleus (VPL) of thalamus.
- Somatotopy of STT with cervical fibers closes to the gray matter.
Spinothalamic Tract (STT)
Lesion
Lesion of STT results in contralateral loss of pain and temperature sensation starting 1 segment below the lesion.
Due to redundancy provided by Lissauer’s tract.
Lateral Corticospinal Tract (LCST)
Organization & Somatotopy
- LCST formed from pyramidal tract fibers which crossed within the pyramidal decussation (~85%)
- Fibers originated in the contralateral cerebral cortex
- 15% which do not cross form the ventral corticospinal tract (VCST)
- main target is the medial (axial) motor neuron pool in ventral horn
- Destination:
- intermediate zone
-
ventral horn of spinal cord
- main target is the lateral motor neuron pool
- Functions:
- voluntary control of distal skeletal muscles
- suppression of infantile reflexes
- Somatotopy of LCST with cervical fibers closest to gray matter
Lateral Corticospinal Tract (LCST)
Lesion
- Unilateral lesion of LCST @ cervical level
- Loss of motor control of ipsilateral arm and leg
- Lesion which affects only the lateral part of LCST tract (L & S fibers only)
- May spare motor control of ipsilateral arm
- Only impacts motor control of ipsilateral leg
Lower Motor Neurons
Somatotopy
LMN axons synapse on skeletal muscles.
Located in ventral horn of spinal cord.
- MN innervating axial and proximal muscles situated more medially ⇒ medial motor neuron pool.
- MN innervating distal muscles situated more laterally ⇒ lateral motor neuron pool.
- Flexor MN situated more dorsally.
- Extensor MN situated more ventrally.
Both medial and lateral motor neuron pools present at the sacrolumbar enlargement (L2 → S3) and cervical enlargement (C4 → T1).
VCST controls axial muscles.
LCST controls distal muscles.
Lateral Lesion
of
Ventral Horn
Lesion of the lateral portion of ventral horn destroys the lateral motor neuron pool.
Produces motor deficit in the ipsilateral distal muscles that receive innervation from this segment.
In such a case, control of axial-proximal muscles is not affect.
Deep Tendon Reflex
Spinal Cord Path
A stretched muscle elicits a reflex contraction to oppose the stretch.
Important for the constant automatic corrects with movement and posture.
- Tapping tendon stretches the muscle activating annulospinal endings within the muscle spindle.
- Ia afferent fibers enter spinal cord via medial division of dorsal root.
- Send collaterals to ventral horn that:
- synapse directly onto 𝛼-MNs causing contraction of homonymous muscles.
- excite motor neurons supplying agonist (synergistic muscles) via Ia excitatory interneurons
- inhibit antagonist muscles via Ia inhibitory interneurons
Inverse Myotatic Reflex Circuit
Spinal Cord Path
GTO’s detect an increase in muscle tension and mediate an inhibition of the motor neurons producing the tension.
Protective mechanism to relax an active muscle before tendon tension becomes too high.
- Contraction of muscle activates GTO.
- Ib afferents enter via medial division of dorsal root.
- Sends collaterals into dorsal and intermediate horn.
- Laminae V, VI, and VII
- Synapse onto Ib inhibitory interneurons
- Inhibition of 𝛼-motor neurons in lamina IX innervating homologous muscle
- Autogenic inhibition
Flexion Extension Reflex Circuit
Spinal Cord Path
Stimulation of cutaneous receptors causes flexion of an entire limb in response to noxious stimuli.
- Noxious tactile stimulus activates free nerve ending nociceptors on A-𝛿 and C fibers.
- A-𝛿 and C fibers enter spinal cord via lateral division of dorsal root.
- Pathway must spread over several spinal segments:
- A-𝛿 and C fibers send branches into Lissauer’s tract to reach +/- 1 segments
- A-𝛿 and C fibers branches also synapse on interneurons in spinal gray which have processes extending over several segments
-
Axons of interneurons:
- activate MNs innervating flexor muscles of ipsilateral extremity
- inhibit MNs innervating ipsilateral extensor muscles
- Through crossed connections, flexor reflex accompanied by adaptive extension of contralateral leg to support the body.
Neural Circuits
Nervous system organized to send information in two kinds of circuits:
-
Reflex circuits:
- shorter and faster
- results in involuntary reactions
-
Relay circuits:
- longer distances
- will often include cortical processing
- results in awareness of sensation or voluntary control of movement
Cortical Spinal Tracts
(CST)
Control of voluntary movements.
&
Suppression of innate reflexes.
[Cortex]
Pyramidal neurons (lamina V of cerebral cortex) mostly in precentral gyrus but also other cortical areas
⇒ corona radiata
⇒ posterior limb of internal capsule
[Midbrain]
Middle 1/3 portion of cerebral peduncle
(Crus cerebri and Basis pedunculi)
[Pons]
Pyramidal tract in basal pons
(Longitudinal pontine fibers)
[Open Medulla]
Pyramid
[Closed Medulla]
85% of fibers cross at the Pyramidal decussation
15% of fibers remain on ipsolateral side
[Spinal Cord - starting at cervical]
Decussated fibers form the Lateral Cortical Spinal Tract (LCST)
Cervical fibers closer to gray matter
Controls distal motor neurons for fine movements
Remainder of fibers form the Ventral Cortical Spinal Tract (VCST)
Cross at segmental level
Influences bilateral neurons controlling axial muscles
Descending Regulation
- Descending pathways from the brain continuously modulate transmission in spinal circuits.
- Damage to these pathways results in removal of inhibitory influence
- LMN brought closer to threshold for AP
- Increases background activity of 𝛼-MN
- 𝛾-MN overactivity increases muscle spindle sensitivity
- LMN brought closer to threshold for AP
- Results in a strong faciliation of transmission from Ia efferent fibers → 𝛼-MNs
- Leads to hypertonia and hyperreflexia
- Treat with baclophen to presynaptically inhibit Ia fibers
- Drug injected into the spinal cord
- Binds to GABA-B receptors
- Decreases influx of Ca2+ into presynaptic terminal of Ia fibers
- Reduces the amount of neurotransmitter released into synapse
- Also increases K+ conductance in postsynaptic neuron causing hyperpolarization
Upper Motorneuron Signs
(UMN)
Voluntary control over LMN is lost resulting in lack of inhibitory influence by regulatory descending tracts on local reflex circuits.
- Presents with:
- Spastic paralysis
- Hyperrefelxia
- Hypertonicity
- Babinski reflex
- Diffuse atrophy
UMN syndrome typically involves the corticospinal tract and reticulospinal tracts plus other descending pathways (corticobulbuar and corticoreticular).
Lesion confined to the LCST or its neurons rare & presents with Babinski reflex but otherwise shows LMN signs.
Dorsal Columns System
(DC)
Carries afferent epicritic sensation.
Fine touch, vibration sense, proprioception.
Type A𝛼 and A𝛽 (or Ia, Ib, and II)
[Lumbar Spinal Cord]
Primary neuron enters via medial division of dorsal root ganglion (DRG).
Carries lower limb information.
Axons form the gracile fasciculus.
[Cervical Spinal Cord]
Primary neuron enters via medial division of dorsal root ganglion (DRG).
Carries upper limb information.
Axons form the lateral cuneate fasciculus at or above T5.
Somatotopy: cervical fibers located laterally next to gray matter
[Medulla]
Gracile and cuneate fasciculi run ipsolaterally up to medulla.
Axons terminate on secondary neurons in gracile and cuneate nuclei.
Second neurons cross midline as internal arcuate fibers.
Somatotopy: lower limb info ventral; upper limb info dorsal
[Pons and Midbrain]
Axons form the medial lemniscus (ML).
Somatotopy: lower limb info lateral; upper limb info medial
[Thalamus]
Medial lemniscus terminates in the ventral posterior lateral (VPL) nucleus.
Third neurons in VPL thalamus.
Somatotopy: lower limb info lateral; upper limb info medial
[Cortex]
VPL axons terminate in the postcentral gyrus (primary somatosensory cortex)
Somatotopy: lower limb info medial; upper limb info lateral
Facial
Epicritic Pathway
Fine touch and proprioception transmitted via the trigeminal nerve (CN V).
[Pons]
Enters at the mid-pons.
Majority of fibers crosses midline.
Travels medially within longitudinal pontine fibers.
Some fibers travel via Dorsal trigeminal tract.
[Midbrain]
Anterior position within medial leminiscus.
Some fibers travel via Dorsal trigeminal tract.
[Thalamus]
Project into the ventral posterior medial (VPM) thalamus.
[Cortex]
Project to lateral somatotopy of post-central gyrus.
Spinothalamic Tract
(STT)
Part of the anterolateral system along with spinoreticular and spinotectal fibers.
Carries protopathic sensation from contralateral side of the body.
Pain, temperature, and crude touch.
Type A-𝛿 and C fibers.
[Spinal cord]
Primary neurons in dorsal root ganglion (DRG)
Axons enter via lateral division of DRG
Sends collaterals into Lissauer’s tract
(~ 1 level rostrally and caudally)
Axons terminate in dorsal horn (lamina I, II, and V)
Second neurons in Lamina I and V
Axons decussate immediately in ventral white commissure
Axons form the spinothalamic tract (STT)
Stomatotopy: cervical fibers located ventrally next to gray matter
[Pons, Medulla, Midbrain]
Axons ascend as the STT carrying information from contralateral side of the body.
Somatotopy: cervical ⇒ sacral goes ventral ⇒ dorsal
Medulla: STT is lateral/dorsal to inferior olive
Pons/Midbrain: STT lateral to medial lemniscus
[Thalamus]
STT axons terminate at the ventral posterior lateral nucleus (VPL)
Third neurons in the VPL thalamus.
Somatotopy: lower limb lateral, upper limb medial
[Cortex]
VPL axons terminate in the postcentral gyrus
Somatotopy: lower limb medial, upper limb lateral
Facial
Protopathic Pathway
Pain and temperature information carried via
Trigeminal nerve (CN V)
[Pons]
Enters spinal cord at the mid-pons.
Descends down to the caudal medulla and crosses midline.
Ascends via the Spinothalamic tract (STT).
[Midbrain]
Anterior position within the STT
[Thalamus]
Fibers terminate within the ventral posterior medial (VPM) thalamus.
Third neuron originates in the VPM.
[Cortex]
VPM fibers terminate laterally within the postcentral gyrus.
Amyotrophic Lateral Sclerosis
(ALS or Lou Gehrig’s Disease)
Degenerative disease of unknown etiology affecting both upper and lower motor neurons.
- As disease progresses, upper limb LMN die before those innervating lower limbs
- LMN signs in arms
- UMN signs in legs
- Terminal stage results in total flaccid paralysis involving all voluntary motor systems with exception of eye movements
- Eventually leads to respiratory failure and death
Neurosyphilis
Tabes dorsalis seen in late stages of neurosyphilis.
- Spinal cord DRG neurons compromised, especially in lumbosacral region
- Results in degeneration of axons forming the dorsal columns
- Tabetic patients have:
- sensory losses in BLE
- sensory ataxia
- high-stepping tabetic gait
- incontinence
- Romberg’s test used to assess deficits
- Diagnosis:
- blood tests for treponemes
- CSF with lympocyte predominant meningitis
- Treat with IV penicillin G
Syringomyelia
Fluid-filled cylindric enlargement of the central canal.
- Secondary to:
- spinal cord tumors
- congenital abnormalities
- trauma
- Usually develops at lower cervical or upper thoracic levels
- As syrinx enlarges, it first compresses fibers crossing ventral white commissure
- Segmental loss of pain and temperature sensation
- As syrinx enlarges rostrally and laterally, other structures compromised ⇒ ventral horn and lateral funiculus
- LMN signs in affected segments
- UMN signs below the syrinx
Poliomyelitis
Viral disease kills ventral horn motor neurons.
LMN symptoms bilaterally in myotomes affected by polio virus.
Usually isolated to the lower extremities.
Plasticity and mixed motor units can result in spastic movements and muscle failure.
Multiple Sclerosis
Autoimmune inflammatory disorder affecting CNS.
- Like caused by T-cells reactive against oligodendrogial myelin of CNS
- PNS myelin spared
- Demyelination causes dysfunctional conduction in large myelinated fibers
- dorsal columns
- LCST
- Localized plaques of demyelination/inflammation in CNS can be episodic
- Eventually forms sclerotic glial scars
- Spinal cord associated symptoms include:
- bladder dysfunction
- b/l deficit in fine touch and proprioception
- b/l UMN symptoms
- optic neuritis
- cerebellar ataxia
- vertigo
- vomiting
- nystagmus
- trigeminal neuralgia
- transient pain
- fatigue
Anterior Spinal Artery
Stroke
-
Anterior spinal artery supplies anterior 2/3 of spinal cord
- Lateral and ventral funiculus affected
- Stroke may present with:
- bilateral loss of pain and temperature
- bilateral LMN symptoms at the lesion
- bilateral UMN symptoms below the lesion
- Fine touch and proprioception preserved because dorsal columns supplied by posterior spinal arteries
Ankle Jerk
DTR
Tests S1
Mediates plantar flexion (Achilles reflex)
Gastrocnemius muscle
Biceps Jerk
DTR
Tests C5→C6
Mediates flexion of the elbow
Biceps muscle
Forearm Jerk
DTR
Tests C5→C6
Slight elbow flexion and wrist extension
Brachioradialis muscle
Triceps Jerk
DTR
Tests C7→C8
Mediates elbow extension
Triceps muscle
Ciliospinal Center of Budge
- Located within the intermediolateral cell column at T1.
- (Between C8→T2)
- Contains preganglionic sympathetic neurons
- Axons synapse on postganglionic neurons in superior cervical ganglion
- Controls the dilator muscle of the pupillae
Phrenic nucleus
- Group of neurons in ventral horn at C3→C5
- Axons form phrenic nerves
- Contracts diaphragm during inspiration
Onuf’s nucleus
- In ventral horn from S1→S3
- Contains motor neurons that form pudendal nerve
- Innervates striated muscles of rectum and external urethral sphincter
- Involved in control of micturition and defecatory continence
Sacral Parasympathetic
Nucleus
- within intermediate zone between S2→S4
- contains preganglionic parasympathetic neurons
- origin of pelvic splanchnic nerve
- controls pelvic viscera
Vitamin B12 Deficiency
Subacute Combined Degeneration
- Due to poor diet or malabsorption of Vit B12 (pernicious anemia)
- Produces demyelination in dorsal and lateral funiculi and peripheral nerves
- Symptoms include:
- muscle weaknes
- diffuse paresthesias
- loss of position and vibratory sense
- ataxia
- spacticity
- bowel and bladder problems
- elevated serum homocysteine and methylmalonic acid
