The Spinal Cord Spinal Nerves and Spinal Reflexes

The Spinal Cord, Spinal Nerves, and Spinal Reflexes

The spinal cord • Gross anatomy – 3 layers of meninges – Epidural space (fat & vessels) – CSF – subarachnoid space – Terminates at L 1/2 vertebral level (conus medullaris) • Dura extends to S 2 vertebral level – Connects via filum terminale & denticulate ligaments (pia) – 31 pairs of spinal nerves (mixed) • cauda equina – Cervical & lumbar enlargements

Lumbar Tap

Spinal Cord Anatomy • Conus medullaris – terminal portion of the spinal cord • Filum terminale – fibrous extension of the pia mater; anchors the spinal cord to the coccyx • Denticulate ligaments – delicate shelves of pia mater; attach the spinal cord to the vertebrae • Spinal nerves – 31 pairs attach to the cord by paired roots – Cervical nerves are named for inferior vertebra – All other nerves are named for superior vertebra • Cervical and lumbar enlargements – sites where nerves serving the upper and lower limbs emerge • Cauda equina – collection of nerve roots at the inferior end of the vertebral canal

Cross-Sectional Anatomy of the Spinal Cord • Anterior median fissure – separates anterior funiculi • Posterior median sulcus – divides posterior funiculi

The 3 Meningeal Layers • Dura mater: – outer layer of spinal cord – subdural space: • between arachnoid mater and dura mater • Arachnoid mater: – middle meningeal layer – subarachnoid space: • between arachnoid mater and pia mater • filled with cerebrospinal fluid (CSF) • Pia mater: – inner meningeal layer

Structures of the Spinal Cord • Paired denticulate ligaments: – extend from pia mater to dura mater – stabilize side-to-side movement • Blood vessels: – along surface of spinal pia mater – within subarachnoid space

Cross-sectional anatomy • Gray matter (cell bodies, neuroglia, & unmyelinated processes) – Posterior horns (sensory, all interneurons) – Lateral horns (autonomic, T 1 -L 2) – Anterior horns (motor, cell bodies of somatic motor neurons) • Spinal roots – Ventral (somatic & autonomic motor) – Dorsal (DRG)

Cross-sectional anatomy • White matter – 3 funiculi (posterior, lateral, anterior) • Ascending, descending, transverse – Consist of “tracts” containing similarly functional axons • All tracts are paired • Most cross over (decussate) at some point • Most exhibit somatotopy (superior part of the tracts are more lateral that inferior body regions) • Most consist of a chain of 2 or 3 successive neurons

Gray Matter: Organization • • Dorsal half – sensory roots and ganglia Ventral half – motor roots Dorsal and ventral roots fuse laterally to form spinal nerves Four zones are evident within the gray matter – somatic sensory (SS), visceral sensory (VS), visceral motor (VM), and somatic motor (SM)

White Matter in the Spinal Cord • Fibers run in three directions – ascending, descending, and transversely • Divided into three funiculi (columns) – posterior, lateral, and anterior • Each funiculus contains several fiber tracts – Fiber tract names reveal their origin and destination – Fiber tracts are composed of axons with similar functions • • Pathways decussate (cross-over) Most consist of two or three neurons Most exhibit somatotopy (precise spatial relationships) Pathways are paired (one on each side of the spinal cord or brain)

White Matter: Pathway Generalizations

3 Connective Tissue Layers • Epineurium: – outer layer – dense network of collagen fibers • Perineurium: – middle layer – divides nerve into fascicles (axon bundles) • Endoneurium: – inner layer – surrounds individual axons

Peripheral Distribution of Spinal Nerves • Each spinal nerve connects to the spinal cord via two medial roots • Each root forms a series of rootlets that attach to the spinal cord • Ventral roots arise from the anterior horn and contain motor (efferent) fibers • Dorsal roots arise from sensory neurons in the dorsal root ganglion and contain sensory (afferent) fibers Figure 13– 7 a

Spinal Nerves: Rami • The short spinal nerves branch into three or four mixed, distal rami – Small dorsal ramus – to back – Larger ventral ramus – to plexuses/intercostals – Tiny meningeal branch – to meninges – Rami communicantes at the base of the ventral rami in the thoracic region – to/from ANS

Nerve Plexuses • All ventral rami except T 2 -T 12 form interlacing nerve networks called plexuses • Plexuses are found in the cervical, brachial, lumbar, and sacral regions • Each resulting branch of a plexus contains fibers from several spinal nerves • Fibers travel to the periphery via several different routes • Each muscle receives a nerve supply from more than one spinal nerve • Damage to one spinal segment cannot completely paralyze a muscle

Spinal Nerve Innervation: Back, Anterolateral Thorax, and Abdominal Wall • The back is innervated by dorsal rami via several branches • The thorax is innervated by ventral rami T 1 -T 12 as intercostal nerves • Intercostal nerves supply muscles of the ribs, anterolateral thorax, and abdominal wall

The 4 Major Plexuses of Ventral Rami 1. 2. 3. 4. Cervical plexus Brachial plexus Lumbar plexus Sacral plexus

Cervical Plexus • The cervical plexus is formed by ventral rami of C 1 -C 4 (C 5) • Most branches are cutaneous nerves of the neck, ear, back of head, and shoulders • The most important nerve of this plexus is the phrenic nerve • The phrenic nerve is the major motor and sensory nerve of the diaphragm

Brachial Plexus • Formed by C 5 -C 8 and T 1 (C 4 and T 2 may also contribute to this plexus) • It gives rise to the nerves that innervate the upper limb

Trunks and Cords of Brachial Plexus • Nerves that form brachial plexus originate from: – superior, middle, and inferior trunks – large bundles of axons from several spinal nerves – lateral, medial, and posterior cords – smaller branches that originate at trunks

Brachial Plexus: Nerves • Axillary – innervates the deltoid and teres minor • Musculocutaneous – sends fibers to the biceps brachii and brachialis • Median – branches to most of the flexor muscles of forearm • Ulnar – supplies the flexor carpi ulnaris and part of the flexor digitorum profundus • Radial – innervates essentially all extensor muscles

Lumbar Plexus • Arises from (T 12) L 1 -L 4 and innervates the thigh, abdominal wall, and psoas muscle • The major nerves are the femoral and the obturator

Sacral Plexus • Arises from L 4 -S 4 and serves the buttock, lower limb, pelvic structures, and the perineum • The major nerve is the sciatic, the longest and thickest nerve of the body • The sciatic is actually composed of two nerves: the tibial and the common fibular (peroneal) nerves

Nerve plexuses - Summary • Cervical – C 1 -C 4 – Phrenic nerve • Brachial – C 5 – T 1 (roots/trunks/divisions/cords) – Axillary, MC, median, ulnar, radial • Lumbar – L 1 -L 4 – Femoral, obturator • Sacral – L 4 -S 4 – Sciatic (common peroneal/tibial), pudendal

Dermatomes • Area of skin innervated by the cutaneous branches of a single spinal nerve. • All segments except C 1 have dermotomal distribution • UE typically from C 5 -T 1 • LE typically from L 1 -S 1 Figure 13– 8

5 Patterns of Neural Circuits in Neuronal Pools 1. Divergence: – spreads stimulation to many neurons or neuronal pools in CNS 2. Convergence: – brings input from many sources to single neuron Figure 13– 13 a

5 Patterns of Neural Circuits in Neuronal Pools 3. Serial processing: – moves information in single line 4. Parallel processing: – moves same information along several paths simultaneously Figure 13– 13 c

5 Patterns of Neural Circuits in Neuronal Pools 5. Reverberation: – – positive feedback mechanism functions until inhibited Figure 13– 13 e

Reflex activity • 5 components of a reflex arc – Receptor – Sensory neuron – Integration center (CNS) – Motor neuron – Effector

4 Classifications of Reflexes 1. By early development – Innate or Acquired 2. By type of motor response – Somatic or Visceral 3. By complexity of neural circuit – Monosynaptic or Polysynaptic 4. By site of information processing – Spinal or Cranial

Spinal Reflexes • Range in increasing order of complexity: – monosynaptic reflexes – polysynaptic reflexes – intersegmental reflex arcs: • many segments interact • produce highly variable motor response

Monosynaptic Reflexes • Have least delay between sensory input and motor output: – e. g. , stretch reflex (such as patellar reflex) • Completed in 20– 40 msec

Muscle Spindles • The receptors in stretch reflexes • Bundles of small, specialized intrafusal muscle fibers: – innervated by sensory and motor neurons • Surrounded by extrafusal muscle fibers: – which maintain tone and contract muscle

Postural Reflexes • Postural reflexes: – stretch reflexes – maintain normal upright posture • Stretched muscle responds by contracting: – automatically maintain balance

Polysynaptic Reflexes • More complicated than monosynaptic reflexes • Interneurons control more than 1 muscle group • Produce either EPSPs or IPSPs

The Tendon Reflex • Prevents skeletal muscles from: – developing too much tension – tearing or breaking tendons • Sensory receptors unlike muscle spindles or proprioceptors

Withdrawal Reflexes • Move body part away from stimulus (pain or pressure): – e. g. , flexor reflex: • pulls hand away from hot stove • Strength and extent of response: – depends on intensity and location of stimulus

Reciprocal Inhibition • For flexor reflex to work: – the stretch reflex of antagonistic (extensor) muscle must be inhibited (reciprocal inhibition) by interneurons in spinal cord

Crossed Extensor Reflexes • Occur simultaneously, coordinated with flexor reflex • e. g. , flexor reflex causes leg to pull up: – crossed extensor reflex straightens other leg – to receive body weight – maintained by reverberating circuits

Integration and Control of Spinal Reflexes • Though reflex behaviors are automatic: – processing centers in brain can facilitate or inhibit reflex motor patterns based in spinal cord • Higher centers of brain incorporate lower, reflexive motor patterns • Automatic reflexes: – can be activated by brain as needed – use few nerve impulses to control complex motor functions – walking, running, jumping

Superficial reflexes • Stroking of the skin elicits muscle contraction – Involves functional upper motor pathways as well as cord level reflex arcs • Plantar reflex (L 4 -S 2)…Babinski is normal in infants – Usually indicative of CNS damage in adults • Abdominal reflex (T 8 -T 12) – Absent with corticospinal lesion

Spinal Cord Trauma: Transection • Cross sectioning of the spinal cord at any level results in total motor and sensory loss in regions inferior to the cut • Paraplegia – transection between T 1 and L 1 • Quadriplegia – transection in the cervical region