Ch 13

• Nervous System Organization


  • CNS = brain and spinal cord
  • PNS = all other neural tissue

  
• Structures in the PNS


  • Ganglia = collection of somas together in one place
  • Nerves = bundles of axons

  
• Structures in the CNS


  • Center = collection of somas with a common function
  • Nucleus = a center with a visible boundary
  • Neural cortex = gray matter (somas) covering the brain
  • Tracts = bundles of axons with common origins, destinations and functions
  • Columns/funiculi = large tracts in the spinal cord
  • Pathways = centers and tracts that link the brain with the body

  
• Sensory pathways


  Receptor ! CNS
• Motor pathways


  CNS ! effector
• Spinal Cord


  • 45cm (18") from brain to L2
  • Inside vertebral canal (stacked vertebral foramen)
  • Surrounded by CT: Spinal Meninges
  • Support and protect spinal cord

  
• Spinal roots exit vertebral canal through intervertebral foramen

  
• Dorsal and ventral roots combine to form spinal nerve

  
• Spinal Nerves


  • 31 pair
  • Exit via intervertebral or sacral foramen
  • Name for location of exit on spine, beginning between skull and C1

  
• Cord and column grow together until age 4; after column continues but cord does not: roots "stretch" to reach foramen
• Adult: cord ends at L1-L2
• "Stretched" spinal roots after L2 = cauda equina

  
• Lumbar puncture


  Spinal tap, at L3-L4, draw CSF from subarachnoid space
• Intervertebral foramen maintained by intervertebral discs between vertebrae

  
• Herniated disc


  Nucleus pulposus ruptures through anulus fibrosus, compresses nerves in intervertebral foramen and/or spinal cord in vertebral canal
• Slipped disc


  Intervertebral disc distorted or displaced, causes pressure
• Nerve structure


  • Axons repair if cut if follow original path
  • Severed nerves do not usually repair: axons do not line up correctly
  • Spinal nerves branch off cord near to what they innervate
  • Cervical and lumbar enlargements of cord house cell bodies of motor neurons for muscles of appendages
• Dermatome


  Region of skin surface innervated by one pair spinal nerves

  
• Most spinal nerves do not go directly to target: axons from multiple nerves intermingle in a nerve plexus

  
• Shingles hides in a specific nerve, comes out in corresponding area of body
• Paralysis


  Loss of motor function: disorder of ventral root or anterior gray horn
• Paresthesias


  Sensory loss: disorder of dorsal root or posterior gray horn
• Complete transection results in loss of both motor and sensory below injury
• Paraplegia


  Sever between T1 and L4, loss of lower limb function
• Quadriplegia


  Sever in cervical, loss of all limb function (above C5 can kill)
• Organization of Neural Pathways


  • 10 million sensory neurons (receptor to CNS)
  • 500 thousand motor neurons (CNS to effector)
  • 20 billion interneurons (coordinate sensory and motor)
  • Interneurons organized into neuronal pools = functional groups with limited input sources (sensory) and output locations (motor)
  • Spread of info organized into neural circuits
  • 5 neural circuits
• Reflexes


  • Rapid automatic response to specific stimuli
  • Used to maintain homeostasis
  • Simple reflex = sensory perception in, motor response out
  • Simple reflexes can be grouped together for complex actions
• Reflex arc


  • Single reflex
  • Reflex arcs = negative feedback: action opposes stimulus as form of defense, fast response, but not always coordinated
• Reflex Classification


  • Superficial somatic reflex = stimuli originate at skin or mucous membrane
  • Stretch reflex = stimuli from overstretched tendon
  • Response delayed by each synapse but capable of more complex output
• Common spinal reflexes


  • Patellar Reflex
  • Withdrawal reflexes
• Patellar Reflex


  • Monosynaptic stretch reflex
  • Carried on type A fibers
  • Sudden stretch of patellar ligament activates muscle spindles ! signal quadriceps group to contract
• Muscle spindle


  • Constantly signal CNS
  • Relaxed = signal less
  • Stretched = signal more ! threshold, trigger reflex arc
  • Prevent overstretching of muscles and tendons
  • Aid in maintaining upright position
• Withdrawal reflexes


  • Complex polysynaptic spinal reflex
  • Consists of three parts: Flexor reflex = flex to withdraw, Reciprocal inhibition = inhibit extensors, Crossed extensor reflex = maintain balance
• Reflexes automatic but can be impacted by higher brain centers: fine tune or combine reflexes, take cues from reflex for coordinated voluntary movements, facilitate or inhibit reflexes
• Reflexes serve as diagnostic tool to assess health and function of spinal cord and brain