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Spinal Cord and Spinal Nerves


Connected to 31 pairs of spinal nerves. All are mixed nerves; I.e., contain ... Carries sensations related to discriminative touch, visceral pain, vibration, ... – PowerPoint PPT presentation

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Title: Spinal Cord and Spinal Nerves

Spinal Cord and Spinal Nerves
  • Chapter 12

Spinal Cord
  • Extends from foramen magnum to second lumbar
  • Segmented
  • Cervical
  • Thoracic
  • Lumbar
  • Sacral
  • Connected to 31 pairs of spinal nerves
  • All are mixed nerves I.e., contain both sensory
    and motor fibers
  • Not uniform in diameter throughout length
  • Cervical enlargement supplies upper limbs
  • Lumbar enlargement supplies lower limbs
  • Conus medullaris tapered inferior end.
  • Cauda equina origins of spinal nerves extending
    inferiorly from lumbosacral enlargement and conus

  • Spinal Meninges
  • Dura mater outermost layer continuous with
    epineurium of the spinal nerves
  • No firm connections to vertebrae
  • Epidural space external to the dura anesthesia
    injected here in sc. Contains blood vessels,
    areolar connective tissue and fat.
  • Arachnoid mater delicate net-work of collagen
    and elastic fibers
  • Subarachnoid space between pia and arachnoid
  • CSF and blood vessels within web-like strands of
    arachnoid tissue
  • Fluid functions as a shock absorber
  • Pia mater thin layer of elastic and collagen
    fibers bound tightly to surface of brain and
    spinal cord
  • Denticulate ligaments extend from pia through
    arachnoid to dura prevent lateral movement
  • Forms the filum terminale, which anchors spinal
    cord to coccyx and the denticulate ligaments that
    attach the spinal cord to the dura mater

Cross Section of Spinal Cord
Cross Section of Spinal Cord
  • Anterior median fissure and posterior median
    sulcus deep clefts partially separating left and
    right halves
  • Gray matter contains neuron cell bodies,
    dendrites, axons
  • Divided into
  • Posterior (dorsal) horns
  • Anterior (ventral) horns
  • Lateral horns (found only in thoracic and lumbar
  • White matter
  • Myelinated axons
  • Three columns (funiculi) ventral, dorsal,
  • Each of these divided into sensory or motor

Organization of Gray Matter
1. Posterior gray horns - contain somatic and
visceral sensory cells bodies (SS and VS
in diagram) 2. Anterior horns - contain somatic
motor cell bodies (SM) 3. Lateral horns - located
ONLY in thoracic and lumbar regions contain
visceral motor cell bodies
Organization of White Matter
  • Divided into three funiculi (columns)
    posterior, lateral, and anterior
  • Each column contains several fiber tracts
    (bundles of axons)
  • All axons with a tract relay the same information
    in the same direction
  • Ascending tracts - carry sensory information
    toward the brain
  • Descending tracts - carry motor commands to
    spinal cord
  • Fiber tract names reveal their origin and

Cross section of Spinal Cord, cont.
  • Commissures connections between left and right
  • Gray with central canal in the center
  • White
  • Roots spinal nerves arise as rootlets then
    combine to form roots
  • Dorsal (posterior) root has a ganglion
  • Ventral (anterior)-no ganglion
  • Two roots merge laterally and form the spinal

Organization of Neurons in Spinal Cord and Spinal
  • Dorsal root ganglion collections of cell bodies
    of unipolar sensory neurons forming dorsal roots.
  • Motor neuron cell bodies are in anterior and
    lateral horns of spinal cord gray matter.
  • No ganglion formed
  • Multipolar somatic motor neurons in anterior
    (motor) horn
  • Autonomic neurons in lateral horn
  • Axons of motor neurons form ventral roots and
    pass into spinal nerves

Spinal Nerves
  • Thirty-one pairs of spinal nerves
  • First pair exit vertebral column between skull
    and atlas (C1)
  • Last four pair exit via the sacral foramina
  • Others exit through intervertebral foramina
  • Eight pair cervical, twelve pair thoracic, five
    pair lumbar, five pair sacral, one pair coccygeal

Dermatomal Map
  • Spinal nerves indicated by capital letter and
  • Dermatomal map skin area supplied with sensory
    innervation by spinal nerves

Spinal Nerves
  • Medially, give rise to the roots that attach the
    nerve to the s.c.
  • Laterally, give rise to the rami that innervate
    the dorsal and ventral regions of the body
  • Dorsal ramus
  • Contains both sensory and motor neurons that
    innervate the dorsal regions of the body
  • Ventral ramus
  • Contains both sensory and motor neurons that
    innervate the ventral regions of the body
  • Braid together to form plexuses (plexi)

Branches of Spinal Nerves
  • Dorsal Ramus innervate deep muscles of the trunk
    responsible for movements of the vertebral column
    and the C.T. and skin near the midline of the
  • Ventral Ramus what they innervate depends upon
    which part of the spinal cord is considered.
  • Thoracic region form intercostal nerves that
    innervate the intercostal muscles and the skin
    over the thorax
  • Remaining spinal nerve ventral rami form five
    plexuses (intermingling of nerves).
  • Ventral rami of C1-C4 cervical plexus
  • Ventral rami of C5-T1 brachial plexus
  • Ventral rami of L1-L4 lumbar plexus
  • Ventral rami of L4-S4 sacral plexus
  • Ventral rami of S4 and S5 coccygeal plexus
  • Communicating Rami communicate with sympathetic
    chain of ganglia.

Cervical Plexus
  • Formed by ventral rami of spinal nerves C1-C4
  • Motor Innervates muscles of the neck (SCM,
    trapezius), laryngeal muscles
  • Sensory Skin of upper chest, shoulder, neck, and
  • Phrenic nerve
  • From C3-C5 (cervical and brachial plexuses)
  • Innervates diaphragm

Brachial Plexus
  • Formed by ventral rami of spinal nerves C5-T1
  • Five ventral rami form three trunks that separate
    into six divisions then form cords that give rise
    to (RTDC)
  • Branches/nerves
  • Axillary
  • Radial
  • Musculocutaneous
  • Ulnar
  • Median

Brachial Plexus Axillary Nerve
  • Motor Innervates deltoid and teres minor
  • Deltoid - Abducts arm
  • Teres - Laterally rotate arm
  • Sensory from skin of the lateral shoulder

Brachial Plexus Radial Nerve
  • Motor Stimulates extensor muscles of arm,
    forearm, and hand
  • Triceps, supinator, brachioradialis, ECR, ECU,
    and some extensors of the fingers (extensor
  • Cause extension movements at elbow and wrist,
    thumb movements
  • Sensory From skin on posterior surface of arm
    and forearm, lateral 2/3 of dorsum of hand
  • Damage due to compression results in crutch
  • Major symptom is wrist drop
  • Failure of extensors of wrist and fingers to
  • Elbow, wrist, and fingers constantly flexed

Brachial PlexusMusculocutaneous Nerve
  • Motor Stimulates flexors in anterior arm
    (biceps brachii, brachialis)
  • Causes flexion movements at shoulder and elbow
  • Sensory From skin along lateral surface of

Brachial Plexus Ulnar Nerve
  • Motor Stimulates flexor muscles in anterior
    forearm (FCU, FDP, most intrinsic muscles of
  • Results in wrist and finger flexion
  • Sensory From skin on medial surface of hand,
    little finger, and medial surface of ring finger
  • Most easily damaged
  • Hitting the funny bone excites it

Brachial Plexus Median Nerve
  • Motor Innervates all but one of the flexors of
    the wrist and fingers, and thenar muscles at base
    of thumb (Palmaris longus, FCR, FDS, FPL,
  • Causes flexion of the wrist and fingers and thumb
  • Sensory From skin of anterolateral 2/3 palm,
    thumb, index and middle fingers lateral surface
    of ring finger
  • Damaged in carpal tunnel and suicide attempts

Lumbosacral Plexus
  • Lumbar plexus formed by ventral rami of spinal
    nerves L1-L4
  • Sacral plexus formed by ventral rami of L4-S4
  • Usually considered together because of their
    close relationship
  • Four major nerves exit and enter lower limb
  • Obturator
  • Femoral
  • Tibial
  • Common fibular (peroneal)

Lumbar PlexusObturator Nerve
  • Motor Innervates adductor group and gracilis
  • Causes adduction of the thigh and knee (gracilis)
  • Sensory From skin of the superior medial side of

Lumbar Plexus Femoral Nerve
  • Motor
  • Innervates anterior muscles of thigh
  • Quadriceps group and sartorius
  • Cause extension of the knee
  • Innervates flexors and adductors of hip
    Pectineus and Iliopsoas
  • Cause flexion of the hip
  • Sensory From skin of the anterior and lateral
    thigh medial surface of leg and foot

Sacral Plexus Sciatic Nerve
  • Thickest and longest nerve of the body
  • Innervates posterior thigh and entire lower leg
  • Composed of 2 nerves (tibial n. and common
    fibular nerve (or peroneal nerve) in a common
  • Leaves pelvis via greater sciatic notch
  • Courses deep to gluteus and enters posterior
    thigh just medial to the hip joint
  • The 2 divisions diverge just above the knee.
  • Sciatic Nerve Injury
  • May be due to fall, disc herniation, impro-per
    administration of injection into buttock
  • When transected, leg is nearly useless
  • Leg cannot be flexed (hamstrings paralyzed)
  • All foot and ankle movement is lost
  • Foot drops into plantar flexion (footdrop)

Tibial Nerve
  • Tibial
  • Innervates muscles of
  • Posterior thigh -hamstrings
  • (knee flexors, hip extensors)
  • Posterior leg(gastrocnemius, - plantar
    flexors FDL, FHL - toe flexors)
  • Branches in foot to form medial and lateral
    plantar nerves
  • Tibial nerve injury, paralyzed calf
  • muscles
  • - cannot plantar flex footshuffling
  • gait develops

Common Fibular (Peroneal) Nerve
  • Common Fibular Nerve
  • Innervates anterior and lateral muscles of the
    leg and foot
  • Extensors that dorsiflex the foot- Tibialis
    anterior, EDL, EHL)
  • Sensory From skin of the lateral and anterior
    leg and dorsum of the foot.
  • Common fibular nerve is susceptible to injury
    because of its superficial location at the head
    and neck of the fibula.
  • Tight leg cast, remaining too long in a
    side-lying position on a firm mattress may
    compress this nerve and result in
  • footdrop

Other Nerves of the Lumbosacral Plexus
  • Nerves that innervate the skin of the suprapubic
    area, external genitalia, superior medial thigh,
    posterior thigh
  • Iliohypogastric nerve - Muscles of abdominal wall
    and pubic region
  • Genitofemoral nerve - Skin of scrotum (males) and
    labia (females) inferior abdominal muscles
  • Pudendal nerve - innervates muscles and skin of
    the perineum (region encompasssing external
    genitalia and anus) external anal sphincter
    mediates erection, and is involved in voluntary
    control of urination the shameful nerve

Coccygeal Plexus
  • S4-S5 coccygeal nerve
  • Muscles of pelvic floor
  • Sensory information from skin over coccyx

Nature of Somatic Reflexes
  • Quick, involuntary, stereotyped reactions of
    glands or muscle to sensory stimulation
  • automatic responses to sensory input that occur
    without our intent or often even our awareness
  • Functions by means of a somatic reflex arc
  • stimulation of somatic receptors
  • afferent fibers carry signal to dorsal horn of
    spinal cord
  • one or more interneurons integrate the
  • efferent fibers carry impulses to skeletal
  • skeletal muscles respond

The Muscle Spindle
  • Sense organ (proprioceptor) that monitors length
    of muscle and how fast muscles change in length
  • Composed of intrafusal muscle fibers, afferent
    fibers and gamma motorneurons

Stretch Reflex
Flexor Withdrawal Reflexes
  • Occurs during withdrawal of foot from pain
  • Polysynaptic reflex arc
  • Neural circuitry in spinal cord controls sequence
    and duration of muscle contractions

Crossed Extensor Reflexes
  • Maintains balance by extending other leg
  • Intersegmental reflex extends up and down the
    spinal cord
  • Contralateral reflex arcs explained by pain at
    one foot causes muscle contraction in other leg

Relationship of Brain and Spinal Cord Reflexes
  • Sensory information goes to brain e.g., pain.
  • Descending tracts convey motor impulses from
    brain to effectors.

White Matter in the Spinal Cord
  • Divided into three funiculi (columns)
    posterior, lateral, and anterior
  • Each column (funiculus) contains several
  • Fiber tracts are either
  • Ascending (sensory)
  • Descending (motor)
  • Fiber tract names often reveal their origin and

White Matter Pathway Generalizations
  • Tracts may decussate (cross-over)
  • Most consist of two or three neurons
  • Pathways are paired (one on each side of the
    spinal cord or brain)
  • Contralateral means origin and destination are on
    opposite sides while ipsilateral means on same

White Matter Pathway Generalizations
Ascending Tracts
  • Carry sensory signals up to the spinal cord
  • Typically uses 3 neurons
  • 1st order neuron - detects stimulus and carries
    it to spinal cord
  • 2nd order neuron - within s.c. continues to the
    thalamus (the sensory relay station)
  • 3rd order neuron - carries signal from thalamus
    to sensory region of cerebral cortex
  • Most have names with prefix spino-

Dorsal Column Ascending Pathway
  • Carries sensations related to discriminative
    touch, visceral pain, vibration, and
  • 1st order neuron - detects stimulus
  • Fasciculus gracilis
  • Carries sensation from below T6
  • Fasciculus cuneatus
  • Carries sensation from T6 or higher
  • 2nd order neuron synapses with 1st in medulla and
  • 3rd order neuron synapses with 2nd in thalamus
    and carries signal to cerebral cortex
    (postcentral gyrus)
  • System is contralateral

Spinothalamic Pathway
  • Carries sensations of pain, pressure,
    temperature, light touch, tickle and itch
  • Located in the anterior and lateral columns
  • Decussation of the second order neuron occurs in
    spinal cord
  • Third order neurons arise in thalamus and
    continue to cerebral cortex of the postcentral

Spinoreticular Tract
  • Pain signals from tissue injury
  • Decussate in spinal cord and ascend with
    spinothalamic fibers
  • End in reticular formation (medulla and pons)
  • 3rd and 4th order neurons continue to thalamus
    and cerebral cortex

Spinocerebellar Pathway
  • 1st order neurons originate in muscles and
  • 2nd order neurons ascend in ipsilateral lateral
  • Terminate in cerebellum (a large motor control
    are of the brain)
  • Transmit proprioceptive signals from limbs and

Descending (Motor) Pathways
  • Descending tracts deliver efferent impulses from
    the brain to the spinal cord, and are divided
    into two groups
  • Direct pathways equivalent to the pyramidal
  • Indirect pathways, essentially all others
  • Motor pathways involve two neurons
  • Upper motor neuron (UMN)
  • Begins with soma in cerebral cortex or brainstem
  • Its axon terminates ON the LMN in anterior horn
  • Lower motor neuron (LMN)
  • Soma in anterior horn axon leads to muscle
  • aka anterior horn motor neuron (also, final
    common pathway)

The Direct (Pyramidal) System
  • Direct pathways originate with the pyramidal
    neurons in the precentral gyri (aka, primary
    motor area).
  • Pyramidal neuron is the UMN it forms the
    corticospinal tract (cortico cortex spinal -
  • UMN synapses in the anterior horn with LMN
  • LMN (anterior horn motor neurons) activates
    skeletal muscles
  • The direct pathway regulates fast and fine
    (skilled) movements
  • Lateral corticospinal tracts UMN decussates in
    pyramids of medulla
  • Anterior corticospinal tracts UMN decussates at
    the spinal cord level

The Direct (Pyramidal) System
Indirect (Extrapyramidal) System
  • Upper motor neuron (UMN) originates in nuclei
    deep in cerebrum (not in cerebral cortex) .e.,
    in brain stem,
  • UMN does not pass through the pyramids
  • LMN is an anterior horn motor neuron
  • This system includes the rubrospinal,
    vestibulospinal, reticulospinal, and tectospinal
  • These motor pathways are complex and multisynaptic

Descending Motor TractsExtrapyramidal Tracts
  • Tectospinal tract (tectum of midbrain)
  • reflex turning of head in response to sights and
  • Reticulospinal tract (reticular formation)
  • controls limb movements important to maintain
    posture and balance
  • Vestibulospinal tract (brainstem nuclei)
  • postural muscle activity in response to inner ear
  • Rubrospinal tracts originate in red nucleus
    of midbrain control flexor muscles (see next

Indirect (Extrapyramidal) System
Spinal Cord Trauma and Disorders
  • Severe damage to ventral root results in flaccid
  • Skeletal muscles cannot move either voluntarily
    or involuntarily
  • Without stimulation, muscles atrophy.
  • When only UMN of primary motor cortex is
    damaged, spastic paraly-
  • sis occurs.
  • Spinal motor neurons remain intact, muscles
    continue to be
  • stimulated irregularly by spinal reflex
  • Muscles remain healthy longer but their
    movements are no longer
  • subject to voluntary control.
  • Muscles commonly become permanently shortened.
  • Transection (cross sectioning) at any level
    results in total motor and
  • sensory loss in body regions inferior to site
    of damage.
  • If injury in cervical region, all four limbs
    affected (quadriplegia)
  • If injury between T1 and L1, only lower limbs
  • (paraplegia)

Spinal Cord Trauma and Disorders
  • Spinal shock - transient period of functional
    loss that follows the injury
  • Results in immediate depression of all reflex
    activity caudal to lesion.
  • Bowel and bladder reflexes stop, blood pressure
    falls, and all muscles
  • (somatic and visceral) below the injury are
    paralyzed and insensitive.
  • Neural function usually returns within a few
    hours following injury
  • If function does not resume within 48 hrs,
    paralysis is permanent.
  • Amyotrophic Lateral Sclerosis (aka, Lou Gehrigs
  • Progressive destruction of anteror horn motor
    neurons and fibers of the
  • pyramidal tracts.
  • Lose ability to speak, swallow, breathe.
  • Death within 5 yrs
  • Cause unknown (90) others have high glutamate
  • Poliomyelitis
  • Virus destroys AHMN
  • Victims die from paralysis of respiratory
  • Virus enters body in feces-contaminated water
    (public swimming pools)
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