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
  • Unencapsulated – Free 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
  3. DRG neurons release another impulse via axons to spinal cord
  4. These axons synapse in grey matter of spinal cord
  5. 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

• Each tract has a different structure which we will go through separately. See ‘Sensory Tracts’

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