PHYSIOLOGY DEPARTMENT
The Spinal
Cord, Spinal Nerves,
and Spinal Reflexes
AKBAROVA SAIDA
Peripheral and Central
Nervous System
PNS
sensory
motor
Lesions
CNS
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
External Spinal Cord
Conus medullaris
Filum terminale
Cauda equina
Dura mater
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
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 13–7a
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
The 4 Major Plexuses of Ventral Rami
1.
2.
3.
4.
Cervical plexus
Brachial plexus
Lumbar plexus
Sacral plexus
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
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 13–8
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 13–13a
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 13–13c
5 Patterns of Neural Circuits in Neuronal Pools
5. Reverberation:
–
–
positive feedback mechanism
functions until inhibited
Figure 13–13e
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
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