8/23 Spinal Cord & Spinal Nerves - Glendinning Flashcards
spinal nerves
spinal cord
31 pairs of spinal nerves → form part of peripheral nervous system
C1-8
T1-12
L1-5
S1-5
coccygeal
spinal cord ends approx L1 at conus medullaris
below conus medullaris, find spinal nerves arranged in cauda equina within lumbar cistern (aka inside dural sac)
- injury below L1? cauda equina lesion
relationship between spinal nerves and vertebral column
*cervical
C1-C7 exit ABOVE vertebrae
C8 exits below C7 vertebra
all other spinal nerves exit below vertebrae
spinal nerve components and location in spinal cord
dorsal root → dorsal horn : primary sensory afferents
ventral horn → ventral root : “lower” or alpha motor neuron (efferent) to sk muscle
lateral horn (T1-L3) → ventral root
- lateral horn contains sympathetic preganglionics in intermediolateral nucleus (intermediate nucleus)
parasympathetic nervous system
pregang neurons
arise from cranial nerves (III, VII, IX, X)
also from S2-S4 intermediate zone (intermediomedial nucleus)
distribution of white matter and grey matter at diff levels of spinal cord
more white matter at the top, more grey matter at the bottom
why?
sensory info is coming in at successively higher levels of teh spinal cord
motor is giving off as it goes down
???
dermatomes
myotomes
spinal nerves innervate the skin in an orderly, rostral-caudal arrangement
- touching a segment of skin stimulates/tests a specific dermatome → a specific spinal nerve and spinal cord segment
myotomes are the set of muscles innervated by a single spinal nerve
radiculopathy
typical sx
damage to a spinal nerve
most common: herniated disc
also seen: osteophytes, spinal stenosis or foraminal stenosis (hypertrophy of ligamentum flavum or facet)
typical sx:
- burning, tingling pain that radiates from back along dermatome
- lancinating or stabbing pain (pay attn to LOCATION)
- numbness (anesthesia, analgesia)
- worsening of symptoms with caughing, sneezing, straining
- muscle weakness
- **T1 radiculopathy can cause Horner’s synrome
-
interrupts sympathetic pathway to eye
- constricted pupil (miosis)
- anhidrosis (decr sweating of skin of face)
- ptosis (drooping) eyelid
-
interrupts sympathetic pathway to eye
external features of the spinal cord
- dorsal median sulcul
- dorsal intermediate sulcus
- dorsalateral sulcus
- ventral median fissure → LARGE, containing anterior spinal artery
blood supply of spinal cord
anterior spinal artery → 1 in ventral median fissure
- supplies anterior 2/3 of spinal cord
posterior spinal artery → 2 in posterolateral sulci
- supplies posterior 1/3 of spinal cord
vasocorona
- series of branches from anterior and posterior spinal arteries that form a crown around the cord
anterior and posterior radicular arteries arise from segmental arteries at each spinal level to supply roots and ganglia
- Artery of Adamkiewicz : unusually large ant radicular artery arising from left of T9-L1 → supplies most of lumbar and sacral spinal cord
anterior and posterior spinal medullary arteries arise at intermittent levels (from radicular arteries?) to augment blood supply
radicular arteries supplying the spinal cord
associated clinical issues
cervical radicular artery
thoracic radicular arteries
great radicular artery of Adamkiewicz
clinical issues:
- T4-T9 watershed area → underperfusion
- fracture dislocations of vertebra → interfere with blood supply
- arterial disease obstructing great radicular artery
- occlusion of aorta during surgery → ischemia
lamina and functional organization of grey matter
spinal cord grey matter is divided into lamina based on cytoarchitecture
10 lamina
I-VI dorsal horn
- sensory processing
VII intermediate zone
- sympathetic pregang neurons in intermediolateral cells column (T1-L3)
- parasympathetic neurons in intermediomedial cell colum (S2-S4)
VIII-IX ventral horn
- motor neurons and interneurons
X grey matter surrounding central canal
nuclei of spinal cord grey matter and functions
substantia gelatinosa : tip of dorsal horn - pain processing
nucleus proprius : processing of touch
intermediolateral nucleus :
Clarke’s nucleus : processing of proprioception
spinal cord white matter
TRACTS (made up of axons)
long pathways to and from cortex/brainstem
- interruption of long pathways is why spinal cord injuries lead to major loss of fx BELOW LEVEL OF LESION
short pathways to and from SC segments
3 sections:
- dorsal funiculus
- lateral funiculus
- ventral funiculus
stretch reflex
feedback control of motor neurons
- stimulus: stretch
- response: contraction
ex. load increases → biceps/brachioradialis stretch → reflex contraction of biceps/brachioradialis & simultaneous inhibition of triceps
clinical stretch reflex : quadriceps muscle
muscle stretch receptor excited → Ia afferent makes excitatory synapse onto…
- quadriceps motor neurons → muscle contraction
- inhibitory interneuron → inhibits hambstring (flexor) motor neurons
location and grading of stretch reflexes
0+ absent
1+ trace
2+ normal
3+ brisk
4+ non sustained clonus
5+ sustained clonus
*1+ 2+, 3+ are generally considered normal UNLESS asymmetry/diff between upper and lower limbs
locations:
- L3-4 patella
- C5-6 biceps
- C5-6 brachioradialis
- C7-8 triceps
- S1 Achilles
muscle spindles
arranged in parallel with skeletal muscle fibers
proprioceptors : provide information about body position/movement
- comprised of inftrafusal muscle fibers within a connective tissue capsule
Group IA afferent: respond to rapid stretch (dynamic)
Group II afferent: respond to sustained stretch (tonic)
muscle spindle modulation by gamma motor neurons
gamma motor neurons only innervate muscle spindle intrafusal fibers
- DONT cause skeletal muscle fibers to contract
- instead, firing results in increased excitability of the muscle spindle
why do you need gamma motor neurons?
- when muscle is contracted/shortened, intrafusal spindle fiber is slack
- gamma fires to get rid of that slack so that the muscle can still be responsive
- maintain the prioprioceptor feedback of a muscle no matter what the length of the muscle is
therefore…stretch PLUS increased gamma efferent discharge = robust contraction
*extrafusal fiber aka skeletal muscle
normally, alpha and gamma motor neurons are coactivated
- alpha innervate sk muscle
- gamma innervate intrafusal
- gamma activity increases during skilled movements and motor learning
why is the stretch reflex used clinically?
absent or decreased? pathology to afferent, efferents, or spinal cord connection
increased? pathology above spinal cord segment
muscle tone
resting tension in a muscle produced by muscle elasticity
- contributes to postural control, ability to store energy when muscle is stretched (ex. walking, balancing)
hypotonia when spinal nerves are damaged
hypertonia when you get supraspinal lesions because stretch reflexes are increased
Ib inhibitory reflex
stimulus: muscle tension
circuit: Golgi-tendon-organ → Ib afferent → Ib inhibitory interneuron → motor neuron to homonymous muscle
(also hits excitatory interneuron → motor neuron to antagonist muscles)
Golgi tendon organ responds to CONTRACTION (not stretch)
why?
used to think it was for protection BUT turns out mostly active during small movements → helps us achieve more fine, coordinated movements
flexor withdrawal reflex
feedback control to remove a limb from a painful stimulus
ex. if you step on a tack…
- stimulated leg flexes to withdraw
- opposite leg extends to support
central pattern generators
used to generate circuits for walking
spinal shock
spinal cord injury → immediate flaccid paralysis
- loss of all motor and autonomic fx below lesion
- flaccid paralysis
- bowel and bladder paralysis
- loss of vasomotor tone (hypotension)
- usually 1-6 weeks
mechanism?
loss of descending facilitation that keeps the spinal cord circuits in a continual state of activation/readiness
