Spinal Cord and Spinal Nerves Lecture Notes PDF

Spinal Cord and Spinal Nerves lecture 17: MTT, chap. 14 & 15 I. Anatomy A. spinal cord 1. located w/i vertebral canal of vertebral column 2. surrounded by CSF, meninges & vertebral bone B. meninges 1. connective tissue membranes - protect delicate brain & cord...

Spinal Cord and Spinal Nerves lecture 17: MTT, chap. 14 & 15 I. Anatomy A. spinal cord 1. located w/i vertebral canal of vertebral column 2. surrounded by CSF, meninges & vertebral bone B. meninges 1. connective tissue membranes - protect delicate brain & cord 2. dura mater a. outermost layer - dense, irregular connective tissue b. epidural space (1) between dura mater and vertabral foramen (cranium) (2) fat and connective tissue 3. arachnoid mater (middle meninges) a. delicate elastic & collagen fibers b. subdural space (1) between dura and arachnoid mater (2) contains interstitial fluid 4. pia mater a. thin, transparent layer b. adheres to spinal cord c. subarachnoid space (1) between arachnoid and pia mater (2) contains cerebrospinal fluid (CSF) 5. same meninges and spaces continue around brain C. external spinal cord anatomy 1. extends from medulla to L2 2. enlargements a. cervical - supplies upper appendages b. lumbosacral - supplies lower appendages 3. conus medullaris - tapered inferior end a cauda equina - spinal nerves arising from cord a. filum terminale - pia mater extension 4. Spinal nerves a. 31 pair of spinal nerves b. named after vertebral segment c. divisions i. cranial – C1 → C8 ii. thoracic - T1 → T12 iii. lumbar - L1 → L5 iv. sacral - S1 → S5 v. coccygeal D. internal spinal cord anatomy 1. photomicrograph a. light and dark matter (1) gray matter core (butterfly) (2) outer white matter 2. divided into left and right halves a. posterior median sulcus b. anterior median fissure 3. gray matter a. anterior and posterior horns b. gray commissure c. central canal (1) extends length of cord (2) contains CSF d. neuron cell bodies w/i spinal cord 4. white matter a. columns of myelinated motor and sensory fibers b. divided into: (1) anterior column (funiculus) (2) posterior column (funiculus) (3) lateral column (funiculus) c. contain distinct bundles of nerve fibers 5. spinal nerves a. spinal nerves connect cord to body a. spinal roots connect peripheral nerves to spinal cord b. dorsal (posterior) root (1) sensory nerves (2) dorsal root ganglion w. sensory nerve cell body c. ventral (anterior) root (1) motor neurons (2) cell bodies in cord gray matter 6. nerve plexus a. cervical - branches of C1 - C4 b. brachial - branches from C5 – T1 c. lumbar - branches from T12 – L4 d. sacral – branches from L4 - S4 7. coverings a. endoneurium surrounds nerve fiber b. perineurium surrounds fasciculus c. epineurium surrounds entire nerve 7. coverings a. endoneurium surrounds nerve fiber b. perineurium surrounds fasciculus c. epineurium surrounds entire nerve II. Spinal Cord Physiology A. introduction 1. conduct nerve impulses (white) a. receptors to brain (ascending traffic) b. brain to effectors (descending traffic) c. both routes decussate 2. center for spinal reflexes (gray) B. major ascending tracts 1. fasciculus gracilis and fasciculus cuneatus a. located in posterior column b. proprioception, fine touch, pressure & vibration c. tract decussates in medulla d. terminates in sensory cortex 2. anterior spinothalamic a. anterior column b. crude touch & pressure c. tract decussates immediately d. terminates in sensory cortex 3. lateral spinothalamic a. located in lateral column b. pain and temperature c. tract decussates immediately d. terminates in sensory cortex 4. anterior spinocerebellar tract a. located in lateral column b. proprioception c. tract decussates immediately d. decussate a second time in medulla e. terminate in cerebellum 5.posterior spinocerebellar tract a. located in lateral column b. proprioception c. do not decussate d. terminate in cerebellum C. major descending tracts 1. classes of motor activity a. somatic nervous system i. controls skeletal muscle activity ii. upper motor neuron – cell body in primary motor cortex iii. lower motor neuron – cell body in brainstem or spinal cord, axon extends to neuromuscular junction iv. Lower motor neuron excitatory b. autonomic nervous system i. controls smooth & cardiac muscle and glandular function ii. preganglionic neuron cell body in brainstem or spinal cord iii. postganglionic neuron cell body in peripheral ganglion iv. may stimulate or inhibit activity 2. pyramidal (corticospinal) a. anterior & lateral columns (corticospinal) b. precise, voluntary motor control of skeletal muscle c. upper – motor cortex, anterior horns of spinal cord d. lower – anterior horn to muscle e. decussates in brainstem 2. extrapyramidal (rubrospinal, vestibulospinal, tectospinal, reticulospinal) a. subconscious motor activity b. anterior and lateral columns c. motor tone, posture, coordination of movements d. vestibulospinal i. vestibular nucleus to spinal cord ant. horn ii. subconscious control of balance and muscle tone e. tectospinal tract i. superior and inferior colliculus to spinal cord ant. horn ii. subconscious control of eye, head and neck position f. reticulospinal i. reticular formation to ant spinal horns ii. subconscious control of reflex activity g. rubrospinal i. red nucleus to ant spinal horns ii. subconscious control of upper limb tone and movement E. motor control 1. integrated in motor cortex, basal ganglia and cerebellum 2. tremendously complex - presented within neuroscience course in clinical curricula 3. Boron and Boulpaep set a new standard (low) Chap. 16. "Circuits of the Central Nervous System" III. Spinal Organization of Motor Function A. α- motor neuron (lower motor neuron) 1. supplies skeletal muscles 2. nuclei in spinal cord anterior horn 3. large soma w. 5 – 20 dendrites 4. variable motor unit size B. control of muscle force 1. motor unit recruitment 2. temporal summation C. sensory input for motor control (proprioception) 1. muscle stretch receptors a. bundle of intrafusal fibers w/i connective tissue sheath b. distal ends connect to connective tissue of fibers c. intrafusal fiber classes (muscle fibers = extrafusal fibers) i. nuclear bag - nuclei clustered at midpoint ii. nuclear chain - single row of nuclei d. innervation i. Group Ia afferent (1) spiral endings (2) rate of stretch ii. Group II afferent (1) spray endings (2) signal muscle length iii. γ – motor neuron (1) efferent control of intrafusal fibers (2) reduce responsiveness of receptor 2. muscle spindle physiology a. linear stretch (1) Group Ia - signal during change in rate (2) Group II - signal muscle length b. tap c. phasic change d. release Grp Ia Grp II e. γ motor neurons set sensitivity i. passive stretch (1) spindle stretched (2) Grp II afferent - constant output ii. α motor stimulation (1) muscle contracts, spindle relaxes (2) Grp II afferent - decreased output iii. α & γ motor stimulation (1) Grp II afferent - constant output (2) Grp Ia afferent - phasic changes 3. Golgi tendon organ a. structure i. spray of nerve endings in tendons, connective tissue, joints ii. innervated by group Ib fibers b. function i. respond to stretch ii. high threshold D. negative feedback control of skeletal muscle activity 1. functions to match intended with actual response 2. fine tunes motor response 3. type of servo-control system IV. Spinal Reflexes A. fast, predictable, automatic responses to environmental changes B. classification 1. sensory - effector a. viscerovisceral i. sensory and effector are both visceral ii. e.g. baroreceptor b. viscerosomatic i. sensory - visceral, effector - somatic ii. e.g. abdominal cramping following appendix rupture c. somatovisceral i. sensory - somatic, effector - visceral ii. e.g. vasoconstriction following cutaneous cooling d. somatosomatic i. sensory - somatic, effector i somatic ii. e.g. knee tap reflex 2. number of neurons a. monosynaptic i. two neurons ii. knee tap b. polysynaptic i. > two neurons ii. withdrawal D. components of reflex arc 1. receptor a. specific for variety of stimuli b. pain, pressure, stretch, etc. 2. sensory neuron 3. integrating center a. monosynaptic - single synapse in gray matter b. polysynaptic - one or more association (interneuron) neurons 4. motor neuron 5. effector E. phasic stretch reflex - monosynaptic (e.g. knee jerk) 1. receptor – muscle spindles 2. sensory neuron (Grp Ia) conducts impulse to posterior horn of cord 3. motor neuron returns via anterior horn to same muscle 4. ipsilateral reflex to maintain tone and balance 5. value? 6. Jendrassik maneuver (reinforcement)  F. tonic stretch reflex (monosynaptic, polysynaptic) 1. result of slow stretch of muscle 2. receptor is group Ia & IIa fibers from muscle spindle 3. demonstrated by induced hinge movement 4. value – maintenance of posture G. tendon reflex (polysynaptic) 1. receptor – Golgi tendon organ 2. activates inhibitory association neurons in cord 3. function a. inhibits ipsilateral muscle activity b. prevents tendon damage due to excessive stretch  H. withdrawal & crossed extensor reflex (polysynaptic) 1. initiated by pain receptor 2. impulse conducted via posterior horn to cord 3. activates association neuron a. contracts ipsilateral limb flexor muscles b. withdraws limb from injury 4. association neurons initiate crossed extensor reflex for balance a. extension of contralateral muscles at same level b. allows placing weight on opposite limb 5. reflex is polysynaptic, both ipsilateral & contralateral a. significant latency & duration b. often involves several flexor grps (ipsilateral) & extensor grps (contralateral I. Clinical Value of Reflex Evaluation 1. evaluate level of anesthesia 2. evaluate degree of intervertebral disk compression 3. evaluate muscular weakness and disorders VI. Spinal Cord Transection A. stages of spinal shock 1. initial areflexia (first 24 hrs) a. all reflexes lost below spinal cord injury b. due to loss of descending excitatory input 2. denervation supersensitivity (day 1 – 3) a. hyperactive stretch and flexion reflexes b. return of polysynaptic then monosynaptic reflexes c. due to loss of descending inhibitory input 3. hyperreflexia ( 1 – 4 weeks) a. abnormally large reflexes with minimal stimulation b. axons and cell bodies sprout new dendritic connectivity B. Babinski’s sign 1. plantar stimulation results in extensor reflex 2. occurs with damage to corticospinal tract (upper motor neuron ) 3. also found in newborn infants B. decerebrate rigidity 1. transection of brainstem a. exaggerated extensor activity b. results in hyperactive stretch reflexes 2. condition reversed by transection of dorsal roots VII. Postural Reflexes A. description 1. normal reflexes assisting postural adjustments 2. receptors include vestibular apparatus, cranial proprioreceptors B. vestibuloocular reflex 1. eyes turned in opposite direction of head turning 2. maintains focus 3. nystagmus a. occurs when head movement exceeds visual field b. alternating slow and fast movements 4. vestibular placing reaction a. stimulation of macula results in forelimb extension b. preparation for landing C. locomotion 1. spinal cord maintains locomotory pattern oscillating circuits 2. tonic activity in descending pathways converted to rhythmic discharges to α-motor neurons by spinal cord activity 3. foot pad pressure facilitates alternating efferent activity