Spinal Cord (Chapter 12)

• Spinal cord


  Extends from foramen magnum to second lumbar vertebra
• Segments of the spinal cord

  • Cervical
  • Thoracic
  • Lumbar
  • Sacral
• Spinal cord


  • Gives rise to 31 pairs of spinal nerves
  • 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
• Meninges


  Connective tissue membranes surrounding spinal cord and brain
• Meninges


  • Dura mater
  • Arachnoid mater
  • Pia mater
• Epidural space


  Location for epidural anesthesia
• Subdural space


  Contains serous fluid
• Subarachnoid space


  Contains CSF and blood vessels within web-like strands of arachnoid tissue
• Spinal cord cross section

  • Anterior median fissure and posterior median sulcus: deep clefts partially separating left and right halves
• White matter


  • Myelinated axons forming tracts
  • Three columns (funiculi): ventral, dorsal, lateral
  • Each of these divided into tracts (fasciculi; pathways)
• Gray matter


  Neuron, cell, cell bodies, dendrites, axons
• Gray matter horns


  • Posterior (dorsal)
  • Anterior (ventral)
  • Lateral (associated with ANS)
• Commissures


  Connections between left and right halves
• Roots


  • Spinal nerves arise as rootlets then combine to form roots
  • Dorsal root and ventral root
  • The two roots merge laterally to form a spinal nerve
  • Dorsal root has a ganglion
• Dorsal root ganglion


  Collections of cell bodies of unipolar sensory neurons forming dorsal roots
• Motor neuron cell bodies

  • In anterior and lateral horns of spinal cord gray matter
  • Multipolar somatic motor neurons in anterior (motor) horn
  • Autonomic neurons in lateral horn
• Reflex


  Automatic response to a stimulus that occurs without conscious thought
• Reflex arc


  Neural pathway that controls a reflex
• Components of a reflex arc

  • Sensory receptor
  • Sensory neuron
  • Interneuron
  • Motor neuron
  • Effector organ
• Monosynaptic reflexes


  No interneurons
• Polysynaptic reflexes


  Multiple synapses with interneurons
• Stretch reflex


  • Muscles contract in response to a stretching force applied to them
  • Muscle spindle: specialized muscle cells that respond to stretch
  • Gamma motor neurons control sensitivity of muscle spindle
  • Sensory neurons innervate the noncontractile centers of the muscle spindle cells
  • Sensory neurons synapse with alpha motor neurons which cause the muscle to contract
• Golgi tendon reflex


  • Golgi tendon organ: Encapsulated nerve endings that have at their ends numerous terminal branches with small swellings associated with bundles of collagen fibers in tendon
  • Prevents contracting muscles from applying excessive tension to tendons
  • Produces sudden relaxation of the muscles
• Withdrawal reflex


  • Function is to remove a body limb or other part from a painful stimulus
  • Reciprocal innervation: causes relaxation of extensor muscle when flexor muscle contracts
  • Crossed extensor reflex: when a withdrawal reflex is initiated in one lower limb, the crossed extensor reflex causes extension of opposite lower limb
• EPSP


  Excitatory post-synaptic potential: the preparation of the post-synaptic membrane to fire an action potential
• IPSP


  Inhibitory post-synaptic potential: an electrical charge that builds up in the post-synaptic membrane inhibiting the firing of an action potential
• Spinal nerves


  • Consist of axon bundles, Schwann cells, and connective tissue
  • Endoneurium: surrounds individual neurons
  • Perineurium: surrounds axon groups to form fascicles
  • Epineurium: surrounds the entire nerve
• Organization of spinal nerves

  • 31 pairs of spinal nerves
  • First pair exit vertebral column between skull and atlas
  • Last four pairs exit via the sacral foramina
  • Others exit through intervertebral foramina
  • Eight pairs cervical, twelve pairs thoracic, five pairs lumbar, five pairs sacral, one pair coccygeal
• KNOW FIGURE 12.14 pp 451
• Dermatomal map


  Skin area supplied with sensory innervation by spinal nerves
• KNOW FIGURE 12.15 pp452
• Branches of spinal nerves

  • Dorsal Ramus: innervate deep muscles of the trunk
  • Ventral Ramus: form intercostal nerves, cervical plexus, brachial plexus, lumbar plexus, sacral plexus, coccygeal plexus
  • Communicating Rami: communicate with sympathetic chain of ganglia
• Cervical plexus


  • Innervates superficial neck structures, skin of neck, posterior portion of head
  • Ansa cervicalis: loop between C1 and C3
  • Phrenic nerve: from C3 to C5, innervates diaphragm
• KNOW FIGURE 12.17 pp454
• Brachial plexus branches


  • Axillary
  • Radial
  • Musculocutaneous
  • Ulnar
  • Median
  • Smaller nerves such as pectoral, long thoracic, thoracodorsal, subscapular, suprascapular
• Lumbar and sacral plexuses

  • Obturator
  • Femoral
  • Tibial
  • Common fibular (peroneal)
• Sciatic (ischiadic) nerve


  Tibial and common fibular nerve together
• Coccygeal plexus


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