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The Spinal Cord, Spinal Nerves, and Spinal Reflexes


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Title: The Spinal Cord, Spinal Nerves, and Spinal Reflexes

The Spinal Cord, Spinal Nerves, and Spinal
The spinal cord
  • Gross anatomy
  • 3 layers of meninges
  • Epidural space (fat vessels)
  • CSF subarachnoid space
  • Terminates at L1/2 vertebral level (conus
  • 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
  • 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
  • Posterior median sulcus divides posterior

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
  • Most consist of a chain of 2 or 3 successive

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
  • Fiber tracts are composed of axons with similar
  • Pathways decussate (cross-over)
  • Most consist of two or three neurons
  • Most exhibit somatotopy (precise spatial
  • 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 137a
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
  • 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 plexus
  2. Brachial plexus
  3. Lumbar plexus
  4. 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

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)

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

  • Area of skin innervated by the cutaneous branches
    of a single spinal nerve.
  • All segments except C1 have dermotomal
  • UE typically from C5-T1
  • LE typically from L1-S1

Figure 138
5 Patterns of Neural Circuits in Neuronal Pools
  • Divergence
  • spreads stimulation to many neurons or neuronal
    pools in CNS
  • Convergence
  • brings input from many sources to single neuron

Figure 1313a
5 Patterns of Neural Circuits in Neuronal Pools
  • Serial processing
  • moves information in single line
  • Parallel processing
  • moves same information along several paths

Figure 1313c
5 Patterns of Neural Circuits in Neuronal Pools
  • Reverberation
  • positive feedback mechanism
  • functions until inhibited

Figure 1313e
Reflex activity
  • 5 components of a reflex arc
  • Receptor
  • Sensory neuron
  • Integration center (CNS)
  • Motor neuron
  • Effector

4 Classifications of Reflexes
  • By early development
  • Innate or Acquired
  • By type of motor response
  • Somatic or Visceral
  • By complexity of neural circuit
  • Monosynaptic or Polysynaptic
  • 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
  • e.g., stretch reflex (such as patellar reflex)
  • Completed in 2040 msec

Muscle Spindles
  • The receptors in stretch reflexes
  • Bundles of small, specialized intrafusal muscle
  • 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

Withdrawal Reflexes
  • Move body part away from stimulus (pain or
  • 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
  • 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
  • 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
  • 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
  • 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