Briefly outline the major somatosensory pathways of the body (excluding cranial nerves)
College Answer
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.
Discussion
- Major sensory and motor pathways:
- White matter tracts, long bundles of axons, whereas the cell bodies reside in the grey matter.
- Many decussate, i.e cross midline from their origins to their destination
- Many are made up of three or more neurons
- Motor neuron pathways:
- First order neurons: motor cortex, "upper motor neurons"
- Second order neurons: grey matter of the spine, "internuncial" neurons
- Third order neurons: grey matter of the spine, "lower motor neurons"
- Sensory neuron pathways:
- First order neuron cell bodies are in the dorsal root ganglia
- Second order neurons are in the dorsal horn of the spinal cord
- Third order neurons are in the destination organ, eg. thalamus
- Main motor tracts of the spinal cord and their function:
- Posterior:
- Septomarginal fasciculus and interfascicular fasciculus: internal spinal reflex arcs
- Lateral:
- Lateral corticospinal tract: fine quick voluntary movement
- Lateral reticulospinal tract: posture, flexor movements
- Rubrospinal tract: posture, flexor movements
- Anterior:
- Anterior corticospinal tract: coarse voluntary movement
- Anterior reticulospinal tract: posture, extensor movements
- Vestibulospinal tract: posture, extensor movements
- Tectospinal tract: reflex postural movements (visual stimuli)
- Main sensory tracts of the spinal cord and their function
- Posterior:
- Dorsal column tracts: propriception, vibration, light tough
- Lateral:
- Lateral spinothalamic tract: pain and temperature
- Posterior spinocerebellar tract: tendon and joint position
- Anterior spinocerebellar tract: tendon and joint position
- Spinoolivary tract: cutaneous and proprioceptive information
- Spinotectal tract: afferent information necessary for the movement of the head in response to painful stimuli
- Anterior:
- Anterior spinothalamic tract: coarse touch and pressure