4. Somatosensory Pathways
Part 2 of 4

Patrick Dougherty, Ph.D.
(Content provided by Chiyeko Tsuchitani, Ph.D.

Somatotopic Organization

Somatosensory neurons are topographically (i.e., spatially) organized so that adjacent neurons represent neighboring regions of the body or face (Figure 4.4). This organization is preserved by a precise point-to-point somatotopic pattern of connections from the spinal cord and brain stem to the thalamus and cortex. Consequently, within each somatosensory pathway there is a complete map (spatial representation) of the body or face in each of the somatosensory nuclei, tracts, and cortex. Additional information on somatotopic organization is presented in Chapter 5 of Section II.

The sensory information processed by the somatosensory systems travels along different anatomical pathways depending on the information carried. For example, the posterior column-medial lemniscal pathway carries discriminative touch and proprioceptive information from the body, and the main sensory trigeminal pathway carries this information from the face. Whereas, the spinothalamic pathways carry crude touch, pain and temperature information from the body, and the spinal trigeminal pathway carries this information from the face.

The posterior (dorsal) column - medial lemniscal pathway (i.e., the medial lemniscal pathway) carries and processes discriminative touch and proprioceptive information from the body (Figure 4.5). It is important to keep in mind that within the medial lemniscal pathway, the afferents carrying discriminative touch information are kept separate from those carrying proprioceptive information up to the level of the cerebral cortex.

Figure 4.5

The medial lemniscal pathway. Press to view the course of the pathway. Click on the structure labels to view their locations in the sections. Click on the label "Cuneate fasciculus" or "Gracile fasciculus" to view the somatotopic organization of the posterior funiculus and the blood supply provided by the posterior spinal artery. Click on the label "Medial lemniscus" to view its somatotopic organization and the blood supply provided by the paramedian branches of the basilar artery.

The peripheral axons of the 1° afferents are myelinated, large or medium diameter axons. Each axon travels via a posterior root, spinal nerve and peripheral nerve to skin, muscle or joint- where it forms or innervates a somatosensory receptor.

The 1° medial lemniscal afferent peripheral process that end in the

• skin, are Ab axons that branch to innervate hair follicles or Merkel’s cells or form Meissner, Pacinian or Ruffini corpuscles.
• joints, are Ab axons that branch to form encapsulated endings similar to the Ruffini and Pacinian corpuscles and Golgi tendon organs.
• muscle, are Group I and II axons that branch to terminate in muscle spindles (Ia and II axons) or Golgi tendon organs (Ib axons).

The 1° medial lemniscal afferent central axons

• join a posterior root, enters the spinal cord, and ascends to the brain stem in the posterior column of the spinal cord (Figure 4.5).
• of coccygeal to mid-thoracic posterior roots (i.e., up to T7) ascend the spinal cord in the ipsilateral gracile fasciculus.
• of the upper thoracic (level T6 and above) and cervical roots collect in the ipsilateral cuneate fasciculus.
• of the gracile and cuneate fasciculi are collectively called the posterior funiculus or posterior column.
• ascends the spinal cord in the posterior funiculus up to the medulla without synapsing or decussating (i.e., without crossing the midline to the contralateral half of the spinal cord).

In the medulla,

• the 1° afferents in the gracile fasciculus synapse in the gracile nucleus
• the 1° afferents in the cuneate fasciculus synapse in the cuneate nucleus.
• the axons of the gracile and cuneate nuclei (2° afferents) pass anteriorly and decussate to form the medial lemniscus, contralateral to their cells of origin.
• above the level of the gracile and cuneate nuclei, each half of the body is represented contralaterally (e.g., left half of body in right medial lemniscus) within the medial lemniscal pathway.

The 2° medial lemniscal afferents

• ascend the brain stem in the medial lemniscus to the diencephalon.
• terminate in the ventral posterolateral (VPL) nucleus of the thalamus.
• carrying cutaneous information terminate in the core of the VPL.
• carrying proprioceptive information terminate in the surrounding shell of the VPL.

The axons of the VPL 3° afferent neurons

• travel in the posterior limb of the internal capsule.
• terminate in the postcentral gyrus and posterior paracentral lobule of the parietal lobe.

The postcentral gyrus and posterior paracentral lobule

• are called the primary somatosensory cortex.
• are the primary cortical receiving areas of the somatosensory system.

  The lower part of the body (foot and leg) are represented in the posterior paracentral lobule, whereas the upper body (chest, arm, and hand) are represented in the upper postcentral gyrus (See Figure 4.4).

Figure 4.6

Afferent neurons in the medial lemniscal pathway activated by touching the left foot with a wisp of cotton. Press to animate. The flash of light at each synapse represents the release of neurotransmitter by the presynaptic axon terminal.

The action potentials ascend the spinal cord via the central process of the 1° afferent in the fasciculus gracilis of the posterior column until they reach the medulla. In the medulla, the action potentials initiate the release of neurotransmitter from the 1° afferent axon terminals onto 2° afferents within the gracile nucleus. The 2° afferent generates action potentials that are conducted by its axons, which decussate to form the medial lemniscus. These action potentials are conducted by the 2° afferent axon contralateral to their site of origin and contralateral to the foot where the stimulus was applied. The action potentials ascend to the thalamus where they initiate the release of neurotransmitter from the 2° afferent axon terminals. They release neurotransmitters onto the 3° afferents in the core of the VPL of the thalamus. The action potentials generated by the 3° VPL afferents are conducted by their axons, which travel in the posterior limb of the internal capsule, to the posterior paracentral lobule of the parietal cortex. These action potentials initiate the release of neurotransmitter from the 3° afferent axon terminals onto cortical neurons and initiate the higher-order processing of the stimulus information generated by the Meissner corpuscle. The point-to-point connections within the pathway provide the basis for a somatotopic map that is used to locate the area of contact with the stimulus and for modality specific information used to identify the stimulus as tactile and from a Meissner corpuscle.

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