K1v / 22B03: Briefly outline the major somatosensory pathways of the body (excluding cranial nerves)

22B03: Exam Report

Briefly outline the major somatosensory pathways of the body (excluding cranial nerves)

35% of candidates passed this question.

Many candidates struggled with this question due to poor structure and limited knowledge with incorrect facts.

Good answers were able to outline the various pathways from receptor, through the spinal cord to the higher centres with some detail of each aspect of the pathway whilst highlighting some points of difference between the pathways.

For example, information expected regarding the types of receptors involved included, vibration, pain, touch, pressure, thermoreceptors, nociceptors and free nerve endings.

Information required for the spinal nerve component would include myelinated versus unmyelinated and linked to the specific receptor, eg myelinated A alpha fibre for assessment of proprioception.

As the question specifically asked for more than one pathway those answers describing a single somatosensory pathway failed to score well.

K1v / 22B03: Briefly outline the major somatosensory pathways of the body (excluding cranial nerves)

The somatosensory pathways of the body are the neural structures which carry the perception of touch, temperature, proprioception and pain.

The major somatosensory pathways are

  1. Dorsal Column Medial Lemniscus (DCML)
  2. Anterior and lateral spinothalamic tracts (STT)
  3. Spinocerebellar tracts

DCML pathway

DCML pathway carries fine touch (tactile sensation), vibration and proprioception

  • Fine touch: sensed by Merkel cells and Meissner’s receptors, conveyed via myelinated A-b fibres
  • Vibration: sensed by Pacinian corpuscles and Ruffini endings, conveyed via myelinated A-b fibres
  • Proprioception: sensed by muscle spindles and golgi tendon organs, conveyed via myelinated A-a fibres
  • Signals from the upper limb (T6 and above) travel in the fasciculus cuneatus; signals from the lower limbs travel in the fasciculus gracilis
  • These synapse with 2nd order neurons in the medulla which decussate to the contralateral thalamus
  • 3rd order neurons run from the thalamus to the ipsilateral primary sensory cortex

The spinothalamic tracts

The spinothalamic tracts transmit crude touch, pain and temperature

  • Pain and temperature: detected by unencapsulated axon endings of sparsely myelinated A fibers (Aδ) and unmyelinated C fibers
    • Aδ fibres – ‘well defined somatic, fast’ pain, sparsely myelinated
    • C fibres – ‘dull, slow’ pain, unmyelinated
    • These fibres enter spinal cord, ascend 1-2 vertebral levels via Lissauer’s tract, and to then synapse in dorsal horn with spinal nociceptor interneurons
  • 2nd order neurons synapse with the interneurons, then decussate in the anterior commissure
    • Neospinothalamic tract (lateral STT) carrying pain and temperature sensation, runs from from Rexed lamina I to thalamus (VPL nucleus)
    • Paleospinothalamic tract (anterior STT) carrying crude touch and pressure sensation, runs from Rexed lamina II-III (i.e. substantia gelatinosa) to brainstem (e.g. PAG) and thalamus (e.g. parafascicular and centromedian nuclei)
  • 3rd order neurons ascend from the thalamus, travel through the internal capsule and terminate at the somatosensory cortex and limbic areas.

The spinocerebellar tracts

The spinocerebellar tracts consist of mostly large diameter myelinated fibres

  • carry unconscious proprioceptive information to fine tune motor movements
  • The posterior and anterior spinocerebellar tracts carry proprioceptive information from the lower limbs to the ipsilateral cerebellum
  • The cuneocerebellar tract and rostral spinocerebellar tract carry proprioceptive information from the upper limbs to the ipsilateral cerebellum.

Author: Andrew Wang