The Spinal Cord

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Chapter 13

• The Spinal Cord Spinal Nerves
• Lecture Outline

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INTRODUCTION

• The spinal cord and spinal nerves mediate
  reactions to environmental changes.
• The spinal cord has several functions.
• It processes reflexes.
• It is the site for integration of EPSPs and IPSPs
  that arise locally or are triggered by nerve
  impulses from the periphery and brain.
• It is a conduction pathway for sensory and motor
  nerve impulses.
• The size of the vertebral canal varies in
  different regions of the vertebral column and
  affects spinal cord injuries.

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Chapter 13The Spinal Cord Spinal Nerves

• Together with brain forms the CNS
• Functions
• spinal cord reflexes
• integration (summation of inhibitory and
  excitatory) nerve impulses
• highway for upward and downward travel of sensory
  and motor information

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SPINAL CORD ANATOMY

• The spinal cord is protected by two connective
  tissue coverings, the meninges and vertebra, and
  a cushion of cerebrospinal fluid.
• The vertebral column provides a bony covering of
  the spinal cord (Figure 13.1b).

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Spinal Cord Protection
By the vertebral column, meninges, cerebrospinal
fluid, and vertebral ligaments.
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Meninges

• The meninges are three coverings that run
  continuously around the spinal cord and brain
  (Figures 13.1a, 14.4a).
• The outermost layer is the dura mater.
• The middle layer is the arachnoid.
• The innermost meninx is the pia mater, a thin,
  transparent connective tissue layer that adheres
  to the surface of the spinal cord and brain
• Denticulate ligaments are thickenings of the pia
  mater that suspend the spinal cord in the middle
  of its dural sheath.

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Structures Covering the Spinal Cord

• Vertebrae
• Epidural space filled with fat
• Dura mater
• dense irregular CT tube
• Subdural space filled with interstitial fluid
• Arachnoid spider web of collagen fibers
• Subarachnoid space CSF
• Pia mater
• thin layer covers BV
• denticulate ligs hold in place

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Structures Covering the Spinal Cord

• Vertebrae
• Epidural space filled with fat
• Dura mater
• dense irregular CT tube
• Subdural space filled with interstitial fluid
• Arachnoid spider web of collagen fibers
• Subarachnoid space CSF
• Pia mater
• thin layer covers BV
• denticulate ligs hold in place

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Applications

• The subarachnoid space is between the arachnoid
  mater and pia mater and contains cerebrospinal
  fluid (CSF).
• Inflammation of the meninges is known as
  meningitis.
• Removal of cerebrospinal fluid from the
  subarachnoid space is called a spinal tap (lumbar
  puncture). This procedure is used to diagnose
  pathologies and to introduce antibiotics,
  contrast media, anesthetics, and chemotherapeutic
  drugs.

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External Anatomy of the Spinal Cord

• The spinal cord begins as a continuation of the
  medulla oblongata and terminates at about the
  second lumbar vertebra in an adult (Figure 13.2).
• It contains cervical and lumbar enlargements that
  serve as points of origin for nerves to the
  extremities.
• The tapered portion of the spinal cord is the
  conus medullaris, from which arise the filum
  terminale and cauda equina.

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External Anatomy of Spinal Cord

• Flattened cylinder
• 16-18 Inches long 3/4 inch diameter
• In adult ends at L2
• In newborn ends at L4
• Growth of cord stops at age 5
• Cervical enlargement
• upper limbs
• Lumbar enlargement
• lower limbs

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Inferior End of Spinal Cord

• Conus medullaris
• cone-shaped end of spinal cord
• Filum terminale
• thread-like extension of pia mater
• stabilizes spinal cord in canal
• Caudae equinae (horses tail)
• dorsal ventral roots of lowest spinal nerves
• Spinal segment
• area of cord from which each pair of spinal
  nerves arises

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Spinal tap or Lumbar Puncture

• Technique
• long needle into subarachnoid space
• safe from L3 to L5
• Purpose
• sampling CSF for diagnosis
• injection of antibiotics, anesthetics or
  chemotherapy
• measurement of CSF pressure

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Spinal nerves

• The 31 pairs of spinal nerves are named and
  numbered according to the region and level of the
  spinal cord from which they emerge (Figure 13.2).
• 8 pairs of cervical nerves,
• 12 pairs of thoracic nerves,
• 5 pairs of lumbar nerves,
• 5 pairs of sacral nerves, and
• 1 pair of coccygeal nerves.
• Spinal nerves are the paths of communication
  between the spinal cord and most of the body.
• Roots are the two points of attachment that
  connect each spinal nerve to a segment of the
  spinal cord (Figure 13.3a).

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Spinal Cord Spinal Nerves

• Spinal nerves begin as roots
• Dorsal or posterior root is incoming sensory
  fibers
• dorsal root ganglion (swelling) cell bodies of
  sensory nerves
• Ventral or anterior root is outgoing motor fibers

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Internal Anatomy of the Spinal Cord

• The anterior median fissure and the posterior
  median sulcus penetrate the white matter of the
  spinal cord and divide it into right and left
  sides (Figure 13.3).
• The gray matter of the spinal cord is shaped like
  the letter H or a butterfly and is surround by
  white matter.
• The gray matter consists primarily of cell bodies
  of neurons and neuroglia and unmyelinated axons
  and dendrites of association and motor neurons.
• The gray commissure forms the cross bar of the
  H-shaped gray matter.
• The white matter consists of bundles of
  myelinated axons of motor and sensory neurons.

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Gray Matter of the Spinal Cord

• Gray matter is shaped like the letter H or a
  butterfly
• contains neuron cell bodies, unmyelinated axons
  dendrites
• paired dorsal and ventral gray horns
• lateral horns only present in thoracic spinal
  cord
• gray commissure crosses the midline
• Central canal is continuous with 4th ventricle of
  brain

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Internal Anatomy of the Spinal Cord

• Anterior to the gray commissure is the anterior
  white commissure, which connects the white matter
  of the right and left sides of the spinal cord.
• The gray matter is divided into horns, which
  contain cell bodies of neurons.
• The white matter is divided into columns.
• Each column contains distinct bundles of nerve
  axons that have a common origin or destination
  and carry similar information.
• These bundles are called tracts.

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White Matter of the Spinal Cord

• White matter covers gray matter
• Anterior median fissure deeper than Posterior
  median sulcus
• Anterior, Lateral and Posterior White Columns
  contain axons that form ascending descending
  tracts

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SPINAL CORD PHYSIOLOGY

• The spinal cord has two principal functions.
• The white matter tracts are highways for nerve
  impulse conduction to and from the brain.
• The gray matter receives and integrates incoming
  and outgoing information.

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Sensory and Motor Tracts

• Figure 13.4 shows the principal sensory and motor
  tracts in the spinal cord. These tracts are
  discussed in detail in Chapter 16 summarized in
  tables 16.3 and 16.4.
• Sensory (ascending) tracts conduct nerve impulses
  toward the brain.
• the lateral and anterior spinothalamic tracts and
  the posterior column tract.
• Motor (descending) tracts conduct impulses down
  the cord.
• Direct pathways include lateral and anterior
  corticospinal and corticobulbar tracts.
• Indirect pathways include rubrospinal,
  tectospinal, and vestibulospinal tracts.

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Tracts of the Spinal Cord

• Function of tracts - highways for sensory motor
  information
• sensory tracts ascend
• motor tracts descend
• Naming of tracts
• indicates position direction of signal
• example anterior spinothalamic tract
• impulses travel from spinal cord towards brain
  (thalamus)
• found in anterior part of spinal cord

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Location of Tracts inside Cord

• Motor tracts Sensory tracts
• pyramidal tract (corticospinal) ---spinothalamic
  tract
• extrapyramidal tract ---posterior column
• 
  ---spinocerebellar

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Function of Spinal Tracts

• Spinothalamic tract
• pain, temperature, deep pressure crude touch
• Posterior columns
• proprioception, discriminative touch, two-point
  discrimination, pressure and vibration
• Direct pathways (corticospinal corticobulbar)
• precise, voluntary movements
• Indirect pathways (rubrospinal, vestibulospinal)
• programming automatic movements, posture muscle
  tone, equilibrium coordination of visual
  reflexes

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Reflexes and Reflex Arcs

• The spinal cord serves as an integrating center
  for spinal reflexes. This occurs in the gray
  matter.
• A reflex is a fast, predictable, automatic
  response to changes in the environment that helps
  to maintain homeostasis.
• Reflexes may be spinal, cranial, somatic, or
  autonomic.

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Reflex Arc

• Specific nerve impulse pathway
• 5 components of reflex arc
• receptor
• sensory neuron
• integrating center
• motor neuron
• effector

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Reflex Arc

• A reflex arc is the simplest type of pathway
  pathways are specific neuronal circuits and thus
  include at least one synapse.
• Reflexes help to maintain homeostasis by
  permitting the body to make exceedingly rapid
  adjustments to homeostatic imbalances.
• Somatic spinal reflexes include the stretch
  reflex, tendon reflex, flexor (withdrawal)
  reflex, and crossed extensor reflex all exhibit
  reciprocal innervation.

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Stretch Reflex (patellar reflex)

• It operates as a feedback mechanism to control
  muscle length by causing muscle contraction.
• Prevents injury from over stretching because
  muscle contracts when it is stretched
• Monosynaptic,ipsilateral reflex arc
• Events of stretch reflex
• muscle spindle signals stretch of muscle
• motor neuron activated muscle contracts
• Brain sets muscle spindle sensitivity as it sets
  muscle tone (degree of muscle contraction at
  rest)
• Reciprocal innervation (polysynaptic-
  interneuron)
• antagonistic muscles relax as part of reflex

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Illustration of the Stretch Reflex
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Tendon Reflex

• It operates as a feedback mechanism to control
  muscle tension by causing muscle relaxation when
  muscle force becomes too extreme.
• ipsilateral polysynaptic reflex
• Golgi tendon organs are in tendon
• activated by stretching of tendon
• inhibitory neuron is stimulated (polysynaptic)
• motor neuron is hyperpolarized and muscle relaxes
• Both tendon muscle are protected
• Reciprocal innervation (polysynaptic)
• causes contraction of ipsilateral muscle group

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Illustration of Tendon Reflex
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Flexor and Crossed Extensor Reflexes

• The flexor (withdrawal) reflex is ipsilateral and
  is a protective withdrawal reflex that moves a
  limb to avoid pain (Figure 13.8).
• This reflex results in contraction of flexor
  muscles to move a limb to avoid injury or pain.
• The crossed extensor reflex, which is
  contralateral, helps to maintain balance during
  the flexor reflex.
• This is a balance-maintaining reflex that causes
  a synchronized extension of the joints of one
  limb and flexion of the joints in the opposite
  limb (Figure 13.9).

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Flexor (withdrawal) Reflex

• Step on tack (pain fibers send signal to spinal
  cord
• Interneurons branch to different spinal cord
  segments
• Motor fibers in several segments are activated
• More than one muscle group activated to lift foot
  off of tack

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Crossed Extensor Reflex

• Lifting left foot requires extension of right leg
  to maintain ones balance
• Pain signals cross to opposite spinal cord
• Contralateral extensor muscles are stimulated by
  interneurons to hold up the body weight
• Reciprocal innervation - when extensors contract
  flexors relax, etc

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Clinical Considerations

• Checking a patients reflexes may help to detect
  disorders/injury
• Plantar flexion reflex -- stroke the lateral
  margin of the sole
• normal response is curling under the toes
• abnormal response or response of children under
  18 months is called Babinski sign (upward fanning
  of toes due to incomplete myelination in child)

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SPINAL NERVES

• Spinal nerves connect the CNS to sensory
  receptors, muscles, and glands and are part of
  the peripheral nervous system.
• The 31 pairs of spinal nerves are named and
  numbered according to the region and level of the
  spinal cord from which they emerge (Figure 13.2).
• Roots of the lower lumbar, sacral, and coccygeal
  nerves are not in line with their corresponding
  vertebrae and thus form the cauda equina (Figure
  13.2).
• Spinal nerves connect to the cord via an anterior
  and a posterior root (Figure 13.3a). Since the
  posterior root contains sensory axons and the
  anterior root contains motor axons, a spinal
  nerve is a mixed nerve.

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Spinal Nerves

• 31 Pairs of spinal nerves
• Named numbered by the cord level of their
  origin
• 8 pairs of cervical nerves (C1 to C8)
• 12 pairs of thoracic nerves (T1 to T12)
• 5 pairs of lumbar nerves (L1 to L5)
• 5 pairs of sacral nerves (S1 to S5)
• 1 pair of coccygeal nerves
• Mixed sensory motor nerves

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Connective Tissue Covering of Spinal Nerves

• Spinal nerve axons are grouped within connective
  tissue sheathes (Figure 13.10).
• A fiber is a single axon within an endoneurium.
• A fascicle is a bundle of fibers within a
  perineurium.
• A nerve is a bundle of fascicles within an
  epineurium.
• Numerous blood vessels are within the coverings.

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Connective Tissue Coverings

• Endoneurium wrapping of each nerve fibers
• Perineurium surrounds group of nerve fibers
  forming a fascicle
• Epineurium covering of entire nerve
• dura mater blends into it at intervertebral
  foramen

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Endoneurium Perineurium -- Epineurium
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Branching of Spinal Nerve

• Spinal nerves formed from dorsal ventral roots
• Spinal nerves branch into dorsal ventral rami
• dorsal rami supply skin muscles of back
• ventral rami form plexus supply anterior trunk
  limbs
• meningeal branches supply meninges, vertebrae
  BV

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A Nerve Plexus

• Joining of ventral rami of spinal nerves to form
  nerve networks or plexuses
• Found in neck, arm, low back sacral regions
• No plexus in thoracic region
• intercostal nn. innervate intercostal spaces
• T7 to T12 supply abdominal wall as well

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Plexuses

• The cervical plexus supplies the skin and muscles
  of the head, neck, and upper part of the
  shoulders connects with some cranial nerves and
  supplies the diaphragm (Figure 13.12, Exhibit
  13.1).
• Damage to the spinal cord above the origin of the
  phrenic nerves (C3-C5) causes respiratory arrest.
• Breathing stops because the phrenic nerves no
  longer send impulses to the diaphragm.

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Cervical Plexus

• Ventral rami of spinal nerves (C1 to C5)
• Supplies parts of head, neck shoulders
• Phrenic nerve (C3-C5) keeps diaphragm alive
• Damage to cord above C3 causes respiratory arrest

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Phrenic Nerve
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Plexuses

• The brachial plexus constitutes the nerve supply
  for the upper extremities and a number of neck
  and shoulder muscles (Figures 13.13 and 13.14,
  Exhibit 13.2).
• A number of nerve disorders may result from
  injury to the brachial plexus.
• Erb-Duchene palsy or waiters tip palsy
• Klumphes palsy
• wrist drop
• carpal tunnel syndrome
• claw hand
• winged scapula

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Brachial Plexus

• Ventral rami from C5 to T1
• Supplies shoulder upper limb
• Passes superior to 1st rib under clavicle
• Axillary n. deltoid teres m.
• Musculocutaneous n. elbow flexors
• Radial n. shoulder elbow extensors
• Median ulnar nn. flexors of wrist hand

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Branches off Brachial Plexus
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Plexuses

• The lumbar plexus supplies the anterolateral
  abdominal wall, external genitals, and part of
  the lower extremities (Figure 13.15, Exhibit
  13.3).
• The largest nerve arising from the lumbar plexus
  is the femoral nerve.
• Injury to the femoral nerve is indicated by an
  inability to extend the leg and by loss of
  sensation in the skin over the anteromedial
  aspect of the thigh.
• Obturator nerve injury is a common complication
  of childbirth and results in paralysis of the
  adductor muscles of the leg and loss of sensation
  over the medial aspect of the thigh.

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Lumbar Plexus

• Ventral rami of L1 to L4
• Supplies abdominal wall, external genitals
  anterior/medial thigh
• Injury to femoral nerve causes inability to
  extend leg loss of sensation in thigh
• Injury to obturator nerve causes paralysis of
  thigh adductors

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Branches of Lumbar Plexus

• Notice Femoral and Obturator nerves
• Found anterior and medial to hip joint

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Plexuses

• The sacral plexus supplies the buttocks,
  perineum, and part of the lower extremities
  (Figure 13.16, Exhibit 13.4).
• The largest nerve arising from the sacral plexus
  (and the largest nerve in the body) is the
  sciatic nerve.
• Injury to the sciatic nerve (common peroneal
  portion) and its branches results in sciatica,
  pain that extends from the buttock down the back
  of the leg.
• Sciatic nerve injury can occur due to a herniated
  (slipped) disc, dislocated hip, osteoarthritis of
  the lumbosacral spine, pressure from the uterus
  during pregnancy, or an improperly administered
  gluteal injection.

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Sacral Plexus

• Ventral rami of L4-L5 S1-S4
• Anterior to the sacrum
• Supplies buttocks, perineum part of lower limb
• Sciatic nerve L4 to S3 supplies post thigh
  all below knee
• Peroneal nerve injury produces foot drop or
  numbness
• Tibial nerve injury produces calcaneovalgus (loss
  of function on anterior leg dorsum of foot)

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Branches of Sacral Plexus

• Notice Sciatic nerve origins

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Sciatic Nerve Branches

• Notice Common Peroneal nerve and Tibial nerve
  behind the knee
• Notice Sciatica pain extends from the buttock
  down the leg to the foot

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Dermatomes

• The skin over the entire body is supplied by
  spinal nerves that carry somatic sensory nerves
  impulses into the spinal cord.
• All spinal nerves except C1 innervate specific,
  constant segments of the skin the skin segments
  are called dermatomes (Figure 13.17).
• Knowledge of dermatomes helps a physician to
  determine which segment of the spinal cord or
  which spinal nerve is malfunctioning.
• Skin on face supplied by Cranial Nerve V

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Dermatomes

• Damaged regions of the spinal cord can be
  distinguished by patterns of numbness over a
  dermatome region
• Infusing local anesthetics or cutting roots must
  be done over 3 adjacent spinal nerves.
• Spinal cord transection
• injury that severs the cord loss of sensation
  motor control below the injury

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Disorders

• Neuritis
• inflammation of nerves
• caused by injury, vitamin deficiency or poison
• Shingles
• infection of peripheral nerve by chicken pox
  virus
• causes pain, skin discoloration, line of skin
  blisters
• Poliomyelitis
• viral infection causing motor neuron death and
  possible death from cardiac failure or
  respiratory arrest

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Clinical Correlations

• Erb-Duchene palsy
• waiters tip position
• fall on shoulder
• Radial nerve injury
• improper deltoid
• injectionor tight cast
• wrist drop
• Median nerve injury
• numb palm fingers inability to pronate flex
  fingers
• Ulnar nerve injury (clawhand)
• inability to adduct/abduct fingers, atrophy of
  interosseus
• Long thoracic nerve injury (winged scapula)
• paralysis of serratus anterior, cant abduct
  above horizontal

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• end