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 L1/2 vertebral level

(conus medullaris)• Dura extends to S2 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, T1-L2)– 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

Figure 13–7a

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

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 T2-T12 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 T1-T12 as intercostal nerves

• Intercostal nerves supply muscles of the ribs, anterolateral thorax, and abdominal wall

The 4 Major Plexuses of Ventral Rami

1. Cervical plexus2. Brachial plexus3. Lumbar plexus4. Sacral plexus

Cervical Plexus

• The cervical plexus is formed by ventral rami of C1-C4 (C5)

• 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 C5-C8 and T1 (C4 and T2 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 (T12) L1-L4 and innervates the thigh, abdominal wall, and psoas muscle

• The major nerves are the femoral and the obturator

Sacral Plexus• Arises from L4-S4 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 – C1-C4– Phrenic nerve

• Brachial – C5 – T1 (roots/trunks/divisions/cords)– Axillary, MC, median, ulnar, radial

• Lumbar – L1-L4– Femoral, obturator

• Sacral – L4-S4– Sciatic (common peroneal/tibial), pudendal

Figure 13–8

Dermatomes

• Area of skin innervated by the cutaneous branches of a single spinal nerve.

• All segments except C1 have dermotomal distribution

• UE typically from C5-T1• LE typically from L1-S1

Figure 13–13a

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 13–13c

3. Serial processing:– moves information

in single line

4. Parallel processing:– moves same

information along several paths simultaneously

5 Patterns of Neural Circuits in Neuronal Pools

Figure 13–13e

5. Reverberation:– positive feedback mechanism– functions until inhibited

5 Patterns of Neural Circuits in Neuronal Pools

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 (L4-S2)…Babinski is normal in infants– Usually indicative of CNS damage in adults

• Abdominal reflex (T8-T12)– 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 T1 and L1

• Quadriplegia – transection in the cervical region