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Chapter 13: 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