Somatosensory Projection Pathways
Projection Pathways
Overview
As background, there are three main somatosensory pathways we'll address:
- The posterior column/ medial lemniscus carries input from the sensory fibers from the body, which transmits vibration and joint position sensory information along with select other large fiber modalities, including two-point discrimination.
- The anterolateral system (which, notably comprises the spinothalamic tract), which carries input from the small sensory fibers from the body, which transmit pain, itch, and thermal sensory information.
- The trigeminothalamic tract, which projects sensory information from the face (CN 5).
- All three pathways ascend the brainstem separately and then bundle within the thalamus before they project to the somatosensory cortex.
- The position of the spinothalamic tract is largely unchanged throughout its spinal cord and brainstem ascent, so start with it.
- Leg fibers synapse within the lumbar spinal cord and then decussate and ascend the anterolateral cord and brainstem to the thalamus.
- Arm fibers synapse within the cervical spinal cord, and, also decussate and ascend the anterolateral cord and brainstem to the thalamus.
- Aarm fibers synapse laterally, and the leg fibers synapse medially.
- Arm fibers ascend the brainstem medial to the leg fibers as the medial lemniscus to the thalamus and indicate with a cross that the fibers switch medial/lateral positions, opposite of their position within the spinal cord..
- This switch occurs in the internal arcuate decussation:
- The leg fibers emerge from the gracile nucleus, medially, and pass anteriorly and then laterally.
- The arm fibers emerge from the cuneate nucleus, laterally, and pass posterior to the legs and then medially.
- Thus, the arm fibers ascend the brainstem medial to the leg fibers: this is the opposite of their position within the spinal cord.
The Trigeminothalamic Tract (Simplified)
- Trigeminothalamic facial fibers decussate within the pons and ascend the brainstem, medially, to the thalamus.
Thalamocortical Projections
- Facial fibers project to the ventroposterior medial nucleus (VPM) and the body fibers (arms and legs) project to the ventroposterior lateral nucleus (VPL).
- The twisting thalamocortical sensory projections to the cerebral cortex - in their ascent, the sensory fibers again reverse their orientation: the leg fibers project medially; they terminate in the posterior paracentral gyrus.
- Arm fibers project lateral to the legs; they terminate in the upper convexity of the postcentral gyrus.
- Facial fibers project lateral to both of them; they terminate in the lower lateral postcentral gyrus.