The Spinal Cord and Spinal Nerves

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Chapter 13
The Spinal Cord and Spinal Nerves
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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
  (excitatory) and IPSPs (inhibitory) 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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External (Gross) Anatomy of Spinal Cord

• Flattened cylinder
• Continuation of the medulla oblongata
• 42 to 45 cm (16-18 inches) long and 2cm (.75
  inch) in diameter
• Cervical enlargement
• upper limbs
• Lumbar enlargement
• lower limbs
• 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 and ventral roots of lowest spinal nerves
• Spinal segment
• area of cord from which each pair of spinal
  nerves arises

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Structures Covering and Protecting the Spinal Cord

• Vertebrae
• The vertebral column provides a bony covering of
  the spinal cord.
• Epidural space filled with fat
• The meninges are three coverings that run
  continuously around the spinal cord and brain
• Dura mater (outer layer)
• dense irregular connective tissue tube
• Subdural space filled with interstitial fluid
• Arachnoid (middle layer)
• - spider web of collagen fibers
• Subarachnoid space filled with cerebrospinal
  fluid
• Pia mater (inner layer)
• thin layer covers blood vessels
• denticulate ligiments hold in place
• Spinal cord-fixed in place
• Anchored -superiorly to the brain
• -laterally to the denticulate ligaments
• inferiorly to the coccyx

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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.
• 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.

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Spinal Cord and Spinal Nerves

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

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Internal Anatomy Gray Matter 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.
• 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.
• paired dorsal and ventral gray horns
• lateral horns only present in thoracic spinal
  cord
• gray commissure crosses the midline
• Central canal is continuous with fourth ventricle
  of brain

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Internal Anatomy White Matter of the Spinal Cord

• White matter covers gray matter
• The white matter consists of bundles of
  myelinated axons of motor and sensory 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.
• Anterior median fissure deeper than Posterior
  median sulcus
• Anterior, Lateral and Posterior White Columns
  contain axons that form ascending and descending
  tracts

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SPINAL CORD PHYSIOLOGY

• The spinal cord has two principal functions.
• The white matter tracts are pathways 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

• 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.
• Naming of tracts
• indicates position and 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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Functions of Spinal Tracts

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

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Reflexes and the Reflex Arc

• 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
• Specific nerve impulse pathway
• 5 components of reflex arc
• receptor
• sensory neuron
• integrating center
• motor neuron
• effector
• 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 (Patellar Reflex) 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 and 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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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 and muscle are protected
• Reciprocal innervation (polysynaptic)
• causes contraction of ipsilateral muscle group

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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.
• 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.

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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.

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Clinical Considerations

• Reflexes-
• diagnosis of nervous system disorders or damaged
  tissue
• Superfiscial reflexes
• withdrawal reflexes-tactile or noxious stimuli
• corneal reflex abdominal reflex gag reflex
  pharyngeal reflex.
• Deep tendon reflexes (stretch stimulus to a
  tendon)
• integrity or function of reflex arc and spinal
  cord
• patellar reflex(knee jerk) Achilles reflex
  (ankle jerk) abdominal reflex.

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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
• Knowledge of dermatomes helps 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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Connective Tissue Coverings of Spinal Nerves

• 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
• Numerous blood vessels are within the coverings

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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.
• 31 Pairs of spinal nerves
• Named and numbered by the cord level of their
  origin
• 8 pairs of cervical nerves (C1 to C8)
• 12 pairs of thoracic nerves (T1 to T12) (T2-12
  do not enter into plexuses)
• 5 pairs of lumbar nerves (L1 to L5)
• 5 pairs of sacral nerves (S1 to S5)
• 1 pair of coccygeal nerves
• Mixed sensory and motor nerves

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Branching of Spinal Nerves

• Spinal nerves formed from dorsal and ventral
  roots
• Spinal nerves branch into dorsal and ventral rami
• dorsal rami supply skin and muscles of back
• ventral rami form plexus supply anterior trunk
  and limbs
• meningeal branches supply meninges, vertebrae and
  blood vessels
• rami-communicantes branches associated with the
  autonomic nervous system

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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 and sacral regions
• No plexus (T2-T12) in thoracic region
• intercostal nerves
• innervate intercostal spaces
• T7 to T12 supply abdominal wall as well

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Disorders
DISORDERS HOMEOSTATIC IMBALANCES

• Neuritis
• inflammation of nerves
• caused by injury, vitamin deficiency or poison
• sciatica injury to the sciatic nerve and its
  branches results in sciatica, pain that extends
  from the buttock down the back of the leg