4. Somatosensory Pathways
Part 3 of 4

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

Main Sensory Trigeminal Pathway: Face Discriminative Touch and Proprioception

The main sensory trigeminal pathway carries and processes discriminative touch and proprioceptive information from the face (Figure 4.7). Consequently, it is the cranial homologue of the medial lemniscal pathway.

The cranial 1° main sensory trigeminal afferent neurons

• peripheral processes are located in the trigeminal (predominantly), facial, glossopharyngeal and vagus nerves.
• form mechanoreceptors in the skin, mucous membranes, muscles and joints of the face. The relationship between receptor type formed and the axon Type/Group are similar to those of the medial lemniscal 1° afferents.
• have pseudounipolar cell bodies in the cranial ganglia of the trigeminal, facial, glossopharyngeal and vagus nerves (Table II).
• send their central axons to the brain stem.
• synapse in the main sensory trigeminal nucleus (2° afferents).

The main sensory trigeminal 2° afferent axons

• decussate immediately on leaving the main sensory trigeminal nucleus.
• join the contralateral ventral trigeminal lemniscus.
• above the level of the main sensory trigeminal nucleus (i.e., the mid pons), carries information about the contralateral face (i.e., the right ventral trigeminal lemniscus carries information about the left side of the face).

The 2° main sensory trigeminal afferents in the ventral trigeminal lemniscus

• ascend to the diencephalon.
• terminate in the ventral posteromedial (VPM) nucleus of the thalamus.

The axons of the 3° main sensory trigeminal afferents (VPM neurons)

• travel in the posterior limb of the internal capsule.
• end in the postcentral gyrus of the parietal lobe.

The postcentral gyrus

• is part of the primary cortical receiving area of the somatosensory system.

The face is represented in the lower half of the postcentral gyrus (See Figure 4.4).

Figure 4.8 illustrates the course of action potentials generated in response to touching the left cheek with a wisp of cotton. A Merkel receptor in the left cheek is stimulated, and its 1° afferent generates action potentials that are conducted by the 1° afferent Ab axon, past its pseudounipolar soma, into the brain stem.

Afferent neurons in the main sensory trigeminal pathway activated by touching the left cheek 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. Figure 4.8

The 1° afferent central process conducts the action potentials into the pons where they initiate the release neurotransmitter from the 1° afferent axon terminals. The neurotransmitter is released onto 2° afferents within the main sensory trigeminal nucleus. The 2° afferent generates action potentials that are conducted along its axon, which decussates in the pons to join the ventral trigeminal lemniscus. These action potentials are conducted by the 2° afferent axon contralateral to their site of origin and contralateral to the site 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 VPM of the thalamus. The action potentials generated by the 3° VPM afferents are conducted by their axons, which travel in the posterior limb of the internal capsule, to the postcentral gyrus 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 Merkel cell. 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 Merkel cell.

There is a minor proprioceptive component for the jaw in cranial nerve V that has 1° afferent cell bodies located in the mesencephalic trigeminal nucleus. The peripheral axons of these afferents travel in the mandibular branch of the trigeminal nerve and end in the jaw muscles and joint. The central processes of most of these afferents end in the trigeminal motor nucleus that controls the muscles of the jaw. Few synapse in the main sensory trigeminal nucleus.

The neospinothalamic pathway carries and processes sharp, pricking pain and dropping temperature (cool/cold) information from the body (Figure 4.9). The pain information carried by the neospinothalamic pathway is well localized and the sensations are the short lasting “fast” or “first” pain elicited by tissue-damaging cutaneous stimuli. The neospinothalamic pathway is also characterized by somatotopic representation, which allows for accurate localization of the painful stimulus.

Recall that there are multiple spinal pathways processing pain information (see Somatosensory Systems Table I). Most of the ascending afferents of the spinal pain pathways travel with the neospinothalamic afferents in a fiber tract called the "spinothalamic tract" or "anterolateral spinothalamic tract". Elements of these other pain pathways will be mentioned below to help you understand how pain sensations may remain after damage to the neospinothalamic pathway. The pain pathways will be covered in greater detail in later chapters.

The 1° neospinothalamic afferents

• have Type Ad peripheral axons that form free nerve endings in skin, muscles and joints.
• have central processes that enter the spinal cord.
• synapse in the posterior marginal nucleus (2° afferents) of the posterior horn.

The 2° neospinothalamic afferent axons

• decussate in the spinal cord anterior white commissure.
• form the lateral part of the spinothalamic tract in the lateral funiculus.

  Note that the fibers in the lateral spinothalamic tract are contralateral to their cells of origin and contralateral to the body area they represent.

The crossed 2° neospinothalamic afferent axons

• ascend the spinal cord and brain stem as part of the spinothalamic tract.
• travel with other pain (archispinothalamic) afferents that leave the spinothalamic tract and terminate in the brain stem as:
  • spinoreticular fibers that end in the reticular formation of the brain stem.
  • spinomesencephalic fibers that end near the periaqueductal gray of the midbrain.
• travel with other pain (paleospinothalamic) afferents to the diencephalon where
  • the neospinothalamic afferents terminate in the ventral posterolateral (VPL) nucleus of the thalamus.
  • the paleospinothalamic afferents terminate in the intralaminar nuclei of the thalamus.

The spinothalamic afferent axons from the thalamus

• travel in the posterior limb of the internal capsule.
• VPL (i.e., the 3° neospinothalamics) end in the postcentral gyrus and posterior paracentral lobule of the parietal lobe.
• Intralaminar nuclei (i.e., paleospinothalamics) end in the insula and rostral cingulate gyrus.

The postcentral gyrus and posterior paracentral lobule are

• the neospinothalamic pathway termination sites.
• the primary cortical receiving areas for sharp, cutting pain information.
• not the exclusive cortical receiving area for pain information.

The insula and rostral cingulate gyrus

• are the archispinothalamic and paleospinothalamic pathways' termination sites.
• receive dull and deep pain information.
• are responsible for poorly localized, longer lasting pain sensations and add the emotional (i.e., unpleasant) features to these sensations.

Figure 4.10 illustrates the course of action potentials generated in response to a pin prick into the left foot. Free nerve endings in the left foot are stimulated by the pin prick. Action potentials are generated and conducted by the 1° afferent Ad axon, past the pseudounipolar soma, and into the spinal cord (Figure 4.10).

The action potentials enter the spinal cord via the central process of the 1° afferents to initiate the release neurotransmitter from the 1° afferent axon terminals onto 2° afferents within the posterior marginal nucleus. The 2° afferent generates action potentials that are conducted by its axon, which decussates in the anterior white commissure of the spinal cord. The crossed 2° neospinothalamic afferent axons form the lateral component of the spinothalamic tract. The action potentials conducted by the crossed 2° afferent axon are 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 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 free nerve ending. 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 a sharp pinprick.

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