Spinal Cord (Week 1 and 2--Houser and Bisley) Flashcards
Subarachnoid space
Deep to aracnhoid meninges
Contains CSF
Contains nerve roots that compose cauda equina
Where are gray and white matter in the spinal cord?
Gray matter is the “H” in the center
White matter surrounds cord on the outside
(Note: this is opposite from the brain where gray matter is outside and white matter is inside)
Regions of the spinal cord
Gray matter: dorsal/ventral/lateral horns
White matter: columns, fasciculi
Note: dorsal = posterior; ventral = anterior
Sensory nerves with cell bodies in DRG go where?
Some go to dorsal horn, others go to dorsal column
(Also remember that sensory cells of DRG are pseudounipolar)
Motor neurons with cell bodies in ventral horn go where?
To skeletal muscle
Symp pre fibers from lateral horn (only T1-L2/3) go where?
To autonomic ganglia then to viscera
Spinal cord segments
8 cervical
12 thoracic
5 lumbar
5 sacral
1 coccygeal
Gray matter of the spinal cord–dorsal horn
“H” in middle of spinal cord
Somatosensory
Gets input from DRG cells and neurons that participate in further processing of sensory information
Subdivision of dorsal horn into nuclei or laminae (Rexed’s cyto-architectonic description)
Includes substantia gelatinosa (which has lots of cell bodies and is important for processing sensory information)
Gray matter of the spinal cord–ventral horn
Motor neurons that control skeletal muscles
Alpha motor neurons have large axons and innervate striated muscles (“final common path” of motor system)
Gamma motor neurons are smaller and innervate muscle spindle (sensory structure within skeletal muscles that contributes to muscle tone but does not directly cause contraction of skeletal muscle)
Motor neurons organized into columns or motor pools associated with certain muscle groups: proximal muscles medially in ventral horn, distal muscles laterally in ventral horn, flexor muscles dorsally in ventral horn, extensor muscles ventrally in ventral horn
Gray matter of the spinal cord–intermediate gray matter
Between dorsal and ventral horns
Contains interneurons that link sensory function to motor function
Many “polysynaptic” spinal reflexes involve interneurons in this region
Many descending pathways form contacts with neurons in this region
Note: interneurons form synaptic connections within same segment as well as in more distal segments of spinal cord
Gray matter of the spinal cord–lateral horn
Within intermediate zone of gray matter
Only in thoracic and upper lumbar segments
Mediates visceral motor function
From T1-L2 or L3, sympathetic preganglionic neurons form a column of cells (intermediolateral cell column) that occupies lateral horn –> axons exit thru ventral roots
Neurons in corresponding region of S2-4 form a sacral parasympathetic nucleus but do not form a distinct lateral horn
White matter of spinal cord
Contains columns (funiculi) in which axons ascend or descend
Dorsal columns: major sensory pathway divided into fasciculus gracilis (found at all levels and more medial) and fasciculus cuneatus (only in cervical and upper thoracic (to T6) levels; wedge-shaped)
Lateral and ventral columns: contain several motor and sensory tracts (location cannot be determined in conventional sections but has been shown experimentally)
Propriospinal system/fasciculus proprius: forms thin shell around gray matter; fibers interconnect different spinal cord levels
Why is amount of white matter greatest in cervical cord?
Everything has to synapse in the brain so there is the highest traffic of myelinated axons in the cervical region
Why are some ventral horns larger at some levels than others?
Some spinal segments innervate more muscle than others
Ex: cervical has big ventral horn because lots of muscles to supply but thoracic has small ventral horn because muscles don’t need fine detail control
Anterior median septum
Break in spinal cord between two ventral horns
Where arterial supply to spinal cord is? Anterior spinal artery comes from 2 branches off vertebral artery that come together to form anterior spinal artery?
Classification of peripheral nerve fibers
Ia: sensory; muscle spindle primary endings
Ib: sensory; golgi tendon organs
alpha: motor; efferents to extrafusal muscle fibers
gamma: motor; efferents to intrafusal muscle fibers
Do reflexes require higher brain centers to operate?
No!
“Spinal cord” reflexes!
Reflexes can be modulated by descending influences from more rostral brain regions, but do not require them
How many neurons do reflexes utilize?
One class of spinal cord reflexes uses only one synapse between sensory and motor elements (monosynaptic reflex)
However, most reflexes involve one or more interneurons within the pathway (polysynaptic reflexes)
Ways to classify axons within a peripheral nerve
Letters (for sensory and motor fibers): Group A-B are larger fibers that are myelinated and have highest conduction velocities; Group C are smallest fibers that are unmyelinated
Roman numerals (sensory fibers only): Groups I-IV
3 basic spinal cord reflexes
1) Stretch reflex
2) Golgi tendon organ reflex
3) Flexor withdrawal reflex
Major components of stretch reflex (deep tendon reflex)
1) Muscle spindle: connective tissue capsule that encloses intrafusal muscle fibers; receives sensory innervation from primary and secondary nerve (afferent?) fibers; receives motor innervation from small gamma motor neurons; is located within regular skeletal muscle and attached to muscle fibers by connective tissue; spindles in parallel with regular muscle fibers
2) Ia primary afferent fibers: have primary ending wrapped around intrafusal fiber; form excitatory connections with alpha motor neurons; send projections to dorsal nucleus of Clarke; send excitatory connections to motor neurons of synergistic muscles; send inhibitory (disynaptic) signals to motor neurons of antagonistic muscles
3) Gamma motor neurons: innervate polar ends of intrafusal muscle fibers so can influence sensitivity of muscle spindle; do not innervate regular muscle fibers directly; not contacted by primary sensory endings; innervated by other afferents from dorsal roots and by descending motor pathways that influence both alpha and gamma motor neurons
4 things the Ia fiber sends signals to
First, the Ia fiber senses stretch of the primary endings of the muscle spindle (?), then it sends signals:
1) Alpha motor neuron to same muscle
2) Dorsal nucleus of Clarke in dorsal horn (–> alpha motor neuron goes up dorsal spinocerebellar tract); also up dorsal column for proprioception
3) Alpha motor neuron to synergistic muscle
4) Inhibitory Ia nerve –> alpha motor neuron to antagonist muscles
Gamma motor neuron
Influence sensitivity of muscle spindle
Sense stretch of intrafusal fibers with sensory endings that are wrapped around intrafusal fibers
Produce contractions at polar ends of muscle spindle (this makes it easier to activate the stretch reflex, doesn’t contract the muscle itself!!)
Not contacted by primary afferent Ia fibers
Activated by descending motor pathways and other afferents from dorsal roots (cutaneous afferents)
Ensures that we can still signal length of muscle even when muscle contracted (without gamma motor neuron, would have no afferent activity coming in!)
Could contribute to hyperreflexia if over-active!
Effects of stretch and contraction on discharge rate of Ia afferent fibers from muscle spindle
Stretch of muscle = stretch of spindle = increased discharge rate of Ia fiber
Contraction of muscle = decreased stretch of spindle = decreased discharge rate of afferent Ia fiber
Makes sense because muscle spindle reflex CONTRACTS the muscle
What does the gamma motor system do during muscle contraction in the “silent” period of Ia afferents
Gamma motor system fills in continued discharge because it is activated by polar ends of spindle
Muscle spindle can respond to chanes in load by providing information to us about muscle length
Roles of the muscle spindle in motor control
Negative feedback system that monitors muscle length
1) Participate in automatic adjustments of body to maintain posture (need something operating on a lower level to help us maintain balance)
2) Compensate for changes in load during motor activity
3) Contribute to normal muscle tone
4) Contribute to sense of limb position and movement (proprioception and kinesthesia)
What do primary afferent fibers of the muscle spindle respond to?
Stretch of the regular muscle fibers
Contraction of the polar ends of the spindle (intrafusal muscle fiber)
Do gamma motor neurons contribute directly to muscle tension?
No
They exert their effects through the stretch reflex pathway
What initiates the stretch reflex?
Tendon tap stretches the muscle
You’re tapping the tendon but it’s really the muscle that is the site of action
Extrafusal vs. intrafusal muscle fibers
Extrafusal = regular muscle fibers
Intrafusal = muscle spindle = in parallel with extrafusal muscle fibers
Why does afferent activity stop after stimulating alpha motor neuron if there is NOT gamma motor neuron there?
Because as soon as muscle contracts, you obviously have no stretch of the muscle anymore so those sensory afferent wrappings from the alpha motor neuron are not stimulated
Muscle tone
Resistance you feel either actively or passively to movement of the limb
In normal muscles we have little contractions going on all the time so we can respond by holding up our arm if examiner drops it
Note: hypotonia if you let go of arm and totally falls down
Golgi tendon organ
Found near tendinous ends of skeletal muscles
Located in series with regular muscle fibers
Info from these receptors conveyed to spinal cord by Ib afferent fibers
Cell body in DRG, comes into dorsal horn, hits Ib inhibitory interneuron INHIBITS alpha motor neuron (also hits excitatory neurons to antagonist muscles!)
Gogli tendon organ reflex
Responds to muscle tension and monitors/maintains muscle force
Golgi tendon organs are sensitive to muscle contraction (and fire more when muscle contracted)
Muscle contraction stimulates golgi tendon organ afferent to inhibit muscle from contracting more and to stimulate antagonist muscles to work against original muscle
Roles of the golgi tendon organ
Provides negative feedback to regulate muscle tension
Helps maintain steady level of force
Contributes to fine adjustments in the force of contraction
Prevents muscles from generating excessive tension
What happens regarding muscle spindle and golgi tendon organ when the muscle is stretched?
Muscle spindle: fires more
Golgi tendon organ: fires only a little more
What happens regarding muscle spindle and golgi tendon organ when the muscle is contracted?
Muscle spindle: does not fire at all
Golgi tendon organ: fires a lot more
Flexion withdrawal reflex
What lets you pull your foot away after stepping on a pin before you even realize you’ve stepped on it
Mediated by somatic afferents carrying nociceptive information
Since muscles at different joints involved, this reflex involves multiple segments of the spinal cord (afferent fibers enter spinal cord and course up and down within dorsolateral fasciculus)
How does the flexion withdrawal reflex work?
Polysynaptic excitation of alpha motor neurons to approppriate muscles for withdrawal of limb from stimulus
Polysynaptic inhibition of motor neurons of antagonistic muscles
Opposite pattern on contralateral side leading to a crossed extension response
Central pattern generators
Local circuits within the spinal cord that can control complex, rhythmic patterns of movement, such as those in locomotion
Ex: dog had complete transection of spinal cord but if on treadmill, still got rhythmic movement of limbs
Major MOTOR pathways of the spinal cord
Corticospinal pathway (lateral pathway) = major motor pathway!
Rubrospinal (lateral pathway, origin at red nucleus)
Vestibulospinal (medial pathway, origin at lateral vestibular nucleus)
Medullary reticulospinal (medial pathway, origin at reticular formation of the medulla)
Pontine reticulospinal (medial pathway, origin at reticular formation of the pons)
Tectospinal (medial pathway)
Where is the location in the spinal cord of the lateral corticospinal tract?
In the lateral column of the spinal cord (right next to rubrospinal tract!)
There is no distinction to tell you exactly where it is
Where are the cell bodies of the corticospinal tract located
Pathway originates in cerebral cortex from neurons in area 4 (primary motor), 6 (supplementary motor region), 3,1,2 (primary somatosensory area), 5 (adjacent parietal area)
Many are in area 4 (primary motor area) or the precentral gyrus of the frontal lobe
Note: face and upper limb on lateral surface; lower limb on medial surface
What movements is the lateral corticospinal tract responsible for?
Voluntary motor control
Simple pathway of lateral corticospinal tract
Cortex
Cerebral peduncle
Basal pons
Pyramid
Pyramidal decussation
Lateral corticospinal tract in spinal cord
Details of pathway of lateral corticospinal tract
1) Originate in cerebral cortex from neurons in area 4, 6, 3, 1, 2, 5
2) Axons of those neurons leave cerebral cortex and converge in posterior limb of internal capsule
3) Reach midbrain, fibers are concentrated in central part of basis pedunculi (base of cerebral peduncle)
4) Reach pons, fibers diverge and descend in small bundles (located between transverse fibers of basis pontis that enter cerebellum)
5) Reach medulla, form group of fibers again in pyramid of medulla (remember, ventral surface)
6) Reach caudal medulla, fibers cross midline in decussation of pyramids and become more dorsolateral within lateral column of spinal cord
7) In spinal cord, fibers descend within lateral corticospinal tract (in lateral columns) until they reach appropriate level of spinal cord for termination. Then they move into gray matter where they form synaptic contacts with (1) alpha motoneurons of ventral horn, (2) interneurons in intermediate zone which then contact motoneurons, (3) neurons in base of dorsal horn for modulation of somatosensory information
Where else can the corticospinal fibers go?
1) Majority cross in the decussation of pyramids (85%)
2) Very small portion remain ipsilateral and descend in lateral column without crossing
3) Remaining fibers form anterior corticospinal tract and continue to be uncrossed until they reach their level of termination in the spinal cord where they cross (or bilateral termination). Note: these fibers synapse on motoneurons or interneurons that supply proximal musculature and thus are functionally related to medial pathways
Functions of lateral corticospinal tract
1) Facilitatory effects on flexor muscles and distal muscles
2) Necessary for isolated and skilled movements of digits
3) Voluntary, goal-directed or skilled movements
2 things that we consider when classifying motor disorders
1) Ability to produce desired movements (weakness or paralysis)
2) Muscle tone
Muscle tone
Normal resistance of a muscle to active or passive stretch
2 factors that influence muscle tone
1) Inherent viscoelestac properties of the muscle
2) Tension set up by contraction of a small number of skeletal muscle fibers
Note: can be influenced by alterations in local reflexes (reflex arc) and descending pathways
Clinical terms for alterations in muscle tone
Absent or decreased tone: atonia, hypotonia, flaccidity
Increased tone (spasticity or rigidity): hypertonia
Lower motor neuron signs
Result from damage of alpha motor neurons (cell bodies or axons) that innervate skeletal muscle
Paralysis or paresis
Decreased strength
Hyporeflexia
Hypotonia
Atrophy of muscles
Fibrillations and fasciculations
Upper motor neuron signs
Result from damage of multiple descending motor pathways (both excitatory and inhibitory on spinal cord circuitry)
Reflect loss of normal balance of excitatory and inhibitory inputs to motor neurons in favor of increased excitability of spinal level reflexes (must be increased excitation (facilitation) or decreased inhibition of alpha or gamma motor neurons)
Paralysis or paresis
Decreased strength
Hyperreflexia
Hypertonia (spasticity)
Minimal (disuse) atrophy
Babinski response (extenxor plantar response): toes go up instead of pointing/plantarflexion
Causes of lower motor neuron syndromes
Poliomyelitis: affects anterior horn cells themselves
Peripheral nerve injuries: interrupt axons of motor neurons
Causes of upper motor neuron syndromes
Cerebral vascular accidents (stroke)
Multiple sclerosis
Spasticity
1) Increased sensitivity of stretch reflex (hyperreflexia)
2) Increased muscle tone (hypertonia) with increased resistance to passive movement (may be greater on one side of joint than other, greatest in antigravity muscles (flexor of upper limb and extensors of lower limb) velocity dependent)
3) Clasp-knife or lengthening reaction (maybe; could be explained by golgi tendon reflex)
4) Clonus (variable)
5) Stereotyped patterns of movement (unable to “fractionate” movements at individual joints)
Fibrillations
Spontaneous contraction of single muscle fibers
Results from sensitization of single muscle fibers that are denervated and contract individually
Because single fibers contract asynchronously, fibrillations are not visually detectable (except in the tongue) but can be revealed by EMG recording
Fasciculations
Spontaneous contraction of groups of skeletal muscle fibers resulting in localized twitching which can be seen under the skin
Caused by spontaneous discharges of irritated motor neurons
Involve motor units as a whole so are visible
Most often seen in patients with chronic disease that affects the motor neurons, such as progressive muscular atrophy
Babinski sign (Extensor Plantar Response)
Dorsiflexion of the great toe instead of normal plantar flexion in response to plantar stimulation (stroking of sole of foot with blunt instrument)
Indicative of abnormalities (or incomplete development) of descending motor pathways
2 important ascending sensory pathways
Dorsal column-medial lemniscus system: fine touch, position sense
Anterolateral system: temperature, coarse touch, pain
Dorsal column-medial lemniscus system
Conveys mechanosensory information from periphery to cortex
Cutaneous mechanoreceptors for fine touch
Proprioception and kinesthesia for position
Kinesthesia vs. proprioception
Kinesthesia: awareness of body position and movement
Proprioception: sub-conscious information used in the feedback control of posture and precise movements
Where does position sense information come from?
Muscle spindles (stretch)
Golgi tendon organs (tension)
Joint receptors (extreme joint movements)
Cutaneous mechanoreceptive afferents (fine touch)
Efference copy (corollary discharge)
What is special about first, second and third order neurons in the pathway?
First order neurons are pseudounipolar
Second order neurons are where the crossing occurs in sensory pathways
Third order neurons not special
Two general primciples about motor and sensory pathways in the spinal cord
1) Most motor and sensory pathways cross the midline at some region of the CNS
2) There are topographic representations of the body at all levels of the pathways
Pathway of dorsal column-medial lemniscus system
1) Larger diameter afferents from receptors in skin, joints, muscles have cell bodies in DRG. They enter dorsal horn through medial division of dorsal root and their collaterals proceed to dorsal columns without a synapse
2) Afferents from lower limbs and lower trunk (below T6) ascend within medial column (gracile fasciculus/tract); afferents from upper trunk and upper limbs (above T6) ascend in lateral column (cuneate fasciculus/tract)
3) FIbers continue in ipsilateral dorsal column until lower medulla where they synapse on second order neurons in either gracile nucleus or cuneate nucleus
4) At caudal medulla, axons of second order neurons curve ventromedially as internal arcuate fibers and cross midline in the decussation of medial lemniscus to become contralateral medial lemniscus (on midline of ventral medulla)
[5) Note that as medial lemniscus heads rostrally, it rotates so that topographic representaton matches that in the thalamus]
6) Medial lemniscal fibers form synapses onto third order neurons in ventral posterior lateral (VPL) nucleus of the thalamus
7) Fibers from thalamus project to primary somatosensory cortex of parietal lobe (areas 3a, 3b, 1, 2) via the posterior limb of the internal capsule
Do you always have both cuneate and gracile tract?
Whenever you have cuneate tract, you will have gracile tract
Remember, cuneate tract is info from upper limbs and body (above T6) and gracile tract is from lower limbs and body (below T6)
How do fine touch and position sense from head and face get processed?
Principal (sensory) Tact of V
1) Afferents from receptors in skin of face have cell bodies in trigeminal ganglion and enter brainstem through trigeminal nerve.
2) All afferents synapse on second order neurons in principal (sensory) nucleus of trigeminal complex in mid-pons
3) Axons from second order neurons decussate in pons and join trigeminothalamic tract (which runs adjacent to medial lemniscus)
4) Axons in trigeminothalamic tract form synapses on third order neusons in ventral posterior medial (VPM) nucleus of the thalamus
5) Fibers from the thalamus project to same areas of primary somatosensory cortex via posterior limb of internal capsule
Two different locations for cell bodies of afferents from receptors from the face
1) Receptors in skin have cell bodies in trigeminal ganglion
2) Receptors in joints and muscles (for proprioception!) have cell bodies in mesencephalic nucleus of trigeminal complex (within CNS!)
How is the whole body represented in the ventral posterior (VP) complex?
VPM of thalamus: face
VPL of thalamus: body
Areas 3a, 3b, 1, 2
Area 3a: primarily proprioception input (note: closest to area 4 which is motor)
Area 3b: primarily tactile input
Area 1: primarily tactile input but receptive fields usually cover several digits
Area 2: combination of tactile and proprioception; hand configuration and stimulus shape are both important
In the somatosensory homunculus, why are the hands and lips (for example) so big?
Because there are a lot of sensory receptors so a lot of information coming in from there
Anterolateral system
Conveys pain, temperature, coarse touch information from the periphery to the cortex
Pathway of anterolateral system
1) Smaller diameter fibers from skin and deeper structures have cell bodies in DRG and enter spinal cord through dorsal roots and may ascend or descend 2-3 segments to form Lessauer’s tract (dorsolateral tract) before penetrating deeper into dorsal horn
2) The fibers synapse on second order neurons in several lamina of the dorsal horn in the spinal cord
3) Axons of second order neurons cross midline of spinal cord in anterior white commissure and assemble in spinothalamic/anterolateral tract on the contralateral side
4) Fibers of spinothalamic/anterolateral tract remain in lateral locations throughout brainstem and are eventually located dorsolateral to the medial lemniscus
5) Reach thalamus and axons synapse with third order neurons in VPL of thalamus. In addition, some axons also have synapses in other thalamic nuclei that include the intralaminar and posterior nuclei
6) Neurons in VPL project to primary somatosensory cortex
On their way through the brainstem, where else do fibers of the anterolateral system send collaterals?
1) Reticular formation (so may influence level of arousal) so referred to as spinoreticular tract
2) Terminate within brainstem rather than continuing to thalamus
3) Terminate in mesencephalon (tectum and periaqueductal gray) so referred to as spinomesencephalic tract
Differences between dorsal column and anterolateral column
Dorsal: light touch, vibration, 2 point discrimination, sense of position; decussation at caudal medulla
Anterolateral: pain, temperature, coarse touch; decussation at spinal cord
What happens if you destroy an entire half of the spinal cord?
Get deficit in touch on one side (dorsal column-medial lemniscus tract; ipsilateral) and pain on the other side (anterolateral tract; contralateral)
Pathway for pain, temperature and coarse touch from head and face in the brainstem
Spinal Tract of V
1) Afferents from face have cell bodies in trigeminal ganglion and ganglia associated with CN VII, IX, X and enter brain stem via those nerves
2) Afferents from trigeminal nerve descend in spinal trigeminal tract to medulla to synapse on second order neurons in spinal nucleus of trigeminal complex (primarily pas caudalis). Afferents from IX and X enter through medulla and synapse in spinal nucleus of trigeminal complex also
3) Axons from second order neurons decussate in caudal medulla and join anterolateral tract in brainstem
4) Fibers in anterolateral tract synapse on third order neurons in VPM of thalamus
5) Fibers from thalamus project to same areas of primary somatosensory cortex
What other brain areas receive information about pain, and what do they do with it?
Cortex: localization of pain
Sub-cortical: perception of pain
Paleospinothalamic pathways: suffering component of pain (reduced by benxodiazepines)
Spinocerebellar tracts
Both dorsal and ventral spinocerebellar tracts are located in the lateral part of the lateral columns of the spinal cord
These pathways transmit proprioceptive information (non-conscious) from muscle spindles and Golgi tendon organs and some exteroceptive information from cutaneous mechanoreceptors to the cerebellum
For both tracts there are slight differences in specific pathways for upper and lower parts of the body
Descending pathways in the modulation of pain
Multisynaptic, descending pathways from PAG in midbrain to dorsal horn of spinal cord (or pars caudalis of spinal nucleus of trigeminal complex) can decrease painful sensation by inhibiting nociceptive transmission in the dorsal horn
Two important areas of the brain: PAG and Raphe nuclei
Local and descending control of pain
1) A-alpha and A-beta fibers excite interneurons that reduce the transmission of pain information (local, use dorsal column ascending system to distract; rubbing after a sharp pain)
2) Descending fibers excite interneurons that reduce the transmission of pain information (in dorsal horn or pars caudalis–pain fibers come in but descending fibers from raphe use enkephalin to inhibit dorsal horn/pars caudalis fibers to reduce pain)
Symptoms of a patient with right-sided hemisection (destroy entire right half of spinal cord at one level)
Decreased touch and proprioception on right (ipsilateral) because damaged dorsal column-medial lemniscus tract
Decreased pain and temperature sensation on the left (contralateral) because damaged anterolateral tract
Paralysis on right side (ipsilateral) because damaged lateral corticospinal tract
(This is Brown-Sequard Syndrome)
Symptoms of a patient with lesion ventral to central canal (anterior white commissure) in cervical region
Affect upper limbs because lesion above T6
Interrupt anterolateral tract because those fibers cross at anterior white commissure
Deficit in pain and termperature sensation in a cape-like pattern over arms and shoulders
(Syringomyelia is fluid filling the spinal cord at C3, C4 that causes damage)
Symptoms of patient with complete transection at T10
Can still move upper limbs (paraplegic)
No voluntary motor control of lower limbs
No pain felt in lower limbs
Short-term would have decreased reflexes
Long-term would have increased reflexes
