neuroanatomy of spinal cord and PNS

• spinal cord

  part of CNS

  • surrounded by bones and meninges
  • extends from base of brain stem to lower back
  • protected by vertical column
  • individual vertebrae
  • meninges are between bone and spinal cord
• spinal cord divisions

  • pairs of peripheral nerves from spinal cord to periphery
  •   cervical region
  • thoracic region
  • lumber region
  • sacral region
• spinal cord peripheral nerves

  sensory and motor neurones

  sensory

  • cell body outside spinal cord in dorsal root ganglia
  • axons from periphery to spinal cord

  motor

  • cell bodies in ventral walls
  • extend out spinal cord to relevant muscles
• two sections of PNS

  somatic and autonomic
• Somatic (PNS)

  coveys all sensations except 4 special senses of head (vision, hearing, olfactory, taste)

  They do:

  • touch
  • temp.
  • Proprioception (awareness of body's location and movements)
  • pain

  causes contractions of skeletal muscles
• sensory receptors

  free nerve endings or modified endings

  • detect specific stimuli

  
• sensory transduction

  converting stimulus energy into electrical signals

  • activation of receptor = movement of ions = change in membrane potential (called receptor potential)
• mechanoreceptors

  most likely on skin

  stimuli = physical distortion (touch)

  differ in hairy and non hairy skin

  • different thresholds - deeper receptors have bigger thresholds

  free nerve endings
• Piezo channels

  turns touch into electrical signal

  • open in response to mechanical force
  • causes influx of cations = depolarisation
  • reaches threshold for action potential
• thermoreceptors

  on skin

  stimuli - temperature

  TRPV1 - hot

  TRPM8 - cold
• Nociceptors

  on skin

  stimuli - stimuli that have potential to cause tissue damage

  free nerve endings

  • mechanical nociceptors
  • thermal nociceptors
  • chemical nociceptors
• Proprioceptors: muscle tensions

  muscle spindles - AKA stretch receptors, muscle length (stretch)

  stimuli - muscle tension

  Golgi tendon organ: muscle tension

  Proprioceptors in joints: angle, direction and velocity of movement in a joint
• Receptor fields

  area receptor can detect a stimuli

  more receptors - more likely to discriminate between stimuli

  high density of smaller receptor fields = higher spatial resolution of stimulus

  can feel two points or one point

  
• somatosensory afferent neurones

  axons bring in info. from somatic sensory receptors to the spinal cord

  primary afferent neurones have cell body in dorsal root ganglia - axons project into dorsal horn of spinal cord through dorsal roots

  • A fibre - larger myelinated axons
  • C fibre - smaller diameter and unmyelinated - transmits info for pain
  • A alpha - largest
• dermatomes

  area of skin innervated by each pair of spinal nerves
• Ascending tracts

  Dorsal column-medial lemniscal pathway

  • Touch and proprioception

  Spinothalamic pathway

  • Temperature and pain
• Somatic efferent neurones:

  -          spinal cord reflexes

  -          stretch reflex

  -          Contraction of a muscle in response to its passive stretching

  -          Trigger contraction – hitting knee
• Golgi tendon reflex

  Inverse myotatic reflex

  relaxation of muscle in response to excessive force

  prevents damage from excess force
• withdrawal reflex

  -          Nociceptors

  -          Step on sharp

  -          Sensory information to spinal cord

  -          Polysynaptic – withdrawal of leg – more synaptic connections
• autonomic nervous system

  OR visceral nervous system

  • controls involuntary responses of internal organs