Sensory Neurone Pathways

Summary

  • General principles of sensory pathways
    • Sensory receptors
    • The generic sensory pathway
  • The somatosensory cortex
  • The sensory tracts
    • Spinothalamic tract
    • Dorsal column
    • Spinocerebellar tract
    • Trigeminothalamic tract

General Principles

  • There are 4 general senses (as opposed to special senses):
    1. Pain
    2. Proprioreception
    3. Temperature
    4. Touch (crude and fine)

Sensory receptors
  • General sensory receptors can be classified in 2 ways:
    1. Nerve endings: Unencapsulated vs Encapsulated 
    2. Type of receptor: Exteroreceptors vs interoreceptors vs proprioreceptors
  • Nerve endings:
    • Encapsulated – Surrounded by structural specialisation
      • Used for pressure, proprioception
      • E.g. Meissners, Pacinian, muscle spindles, GTOs
    • UnencapsulatedFree nerve ending
      • Used for temperature, pain
  • Exteroreceptors vs interoreceptors vs proprioreceptors
    • Exteroreceptors are used at body surfaces  (eyes, mouth, skin)
      • Receive changes in the external environment
    • Interoreceptors within the body
      • Receive changes in internal environment
    • Proprioreceptors are located in muscles (GTOs, Muscle spindles)
      • Receive muscle movement and position

The Sensory pathway
  • There is a standard pathway for all sensation:
    • Receptor stimulation => ganglia => either spinal cord or brainstem => thalamus => cortex/cerebellum
  • Receptor stimulation (for simplicity, we cover impulses covered by the spinal nerves rather than cranial nerves:
    1. Stimulation produces an impulse (action potential)
    2. Impulse travels along dendrons to neurones in the Dorsal Root Ganglion
      • There are equivalent ganglia for cranial nerves
    1. DRG neurons release another impulse via axons to spinal cord
    2. These axons synapse in grey matter of spinal cord
    3. A secondary neuron travels up to the thalamus, and then onwards to the primary sensory cortex
  • There are 3 neurones in general sensory pathway
    1. Primary neurone
      • Receptors for pain, temperature, crude and fine touch, conscious proprioception
      • Cell body lies in DRG or trigeminal ganglia
      • Synapses in the CNS
    2. Secondary neurone
      • Travels in different types of tracts (see separate)
      • It will usually decussate at some point (i.e. cross to the other side of the spinal cord) – this point varies depending on the type of tract
      • Synapses in the thalamus
    3. Tertiary neurone
      • Starts in thalamus
      • Synapses in primary sensory cortex

The primary sensory cortex
  • Located in the cortex
  • It is important to consider the representation of each part of the body
    • This is summarised neatly in the sensory homunculus (there is also an equivalent motor homunculus)
    • Note that there is disproportionate representation of the most sensitive areas

 

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