Spinal Cord and Spinal Nerves

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Spinal Cord and Spinal Nerves

• Chapter 12

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Spinal Cord

• Extends from foramen magnum to second lumbar
  vertebra
• 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
  medullaris.

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• 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

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Cross Section of Spinal Cord
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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
  regions
• White matter
• Myelinated axons
• Three columns (funiculi) ventral, dorsal,
  lateral
• Each of these divided into sensory or motor
  tracts

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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
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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
  destination

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Cross section of Spinal Cord, cont.

• Commissures connections between left and right
  halves
• 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
  nerve

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Organization of Neurons in Spinal Cord and Spinal
Nerves

• 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

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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

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Dermatomal Map

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

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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)

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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
  back.
• 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.

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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
  ear
• Phrenic nerve
• From C3-C5 (cervical and brachial plexuses)
• Innervates diaphragm

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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

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Brachial Plexus Axillary Nerve

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

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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
  digitorum)
• 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
  paralysis
• Major symptom is wrist drop
• Failure of extensors of wrist and fingers to
  function
• Elbow, wrist, and fingers constantly flexed

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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
  forearm

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Brachial Plexus Ulnar Nerve

• Motor Stimulates flexor muscles in anterior
  forearm (FCU, FDP, most intrinsic muscles of
  hand)
• 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

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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,
  pronator)
• 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

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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)

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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
  thigh

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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

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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
  sheath)
• 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)

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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

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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

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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

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Coccygeal Plexus

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

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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
  information
• efferent fibers carry impulses to skeletal
  muscles
• skeletal muscles respond

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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

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Stretch Reflex
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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

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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

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Relationship of Brain and Spinal Cord Reflexes

• Sensory information goes to brain e.g., pain.
• Descending tracts convey motor impulses from
  brain to effectors.

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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
  destination

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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
  side

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White Matter Pathway Generalizations
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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-

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Dorsal Column Ascending Pathway

• Carries sensations related to discriminative
  touch, visceral pain, vibration, and
  proprioception
• 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
  decussates
• 3rd order neuron synapses with 2nd in thalamus
  and carries signal to cerebral cortex
  (postcentral gyrus)
• System is contralateral

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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
  gyrus

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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

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Spinocerebellar Pathway

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

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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
  tracts
• 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)

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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 -
  s.c.)
• 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

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The Direct (Pyramidal) System
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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
  tracts
• These motor pathways are complex and multisynaptic

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Descending Motor TractsExtrapyramidal Tracts

• Tectospinal tract (tectum of midbrain)
• reflex turning of head in response to sights and
  sounds
• 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
  signals
• Rubrospinal tracts originate in red nucleus
  of midbrain control flexor muscles (see next
  slide)

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Indirect (Extrapyramidal) System
b
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Spinal Cord Trauma and Disorders

• Severe damage to ventral root results in flaccid
  paralysis.
• 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
  activity.
• 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
  affected
• (paraplegia)

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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
  disease)
• 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
  levels
• Poliomyelitis
• Virus destroys AHMN
• Victims die from paralysis of respiratory
  muscles
• Virus enters body in feces-contaminated water
  (public swimming pools)