6. Somatic and Visceral Sensory Systems of the Head

The objectives of this chapter are to:

  1. Study the organization of the trigeminal sensory system, the somatic sensory system of the head.
  2. Study the organization of the visceral sensory system of the head, including taste sensation.

The trigeminal system is the somatic sensory system of the head. The sensory portion of the trigeminal cranial nerve (#5323) innervates the face, except the angle of the jaw, and head as far back as the vertex (#11459). The visceral receptors of the head and neck are innervated by CN VII, IX, and X. Taste receptors are specialized visceral receptors. The primary somatic and visceral sensory neurons project to different brain stem nuclei.

In the spinal cord, primary somatic sensory neurons and primary visceral sensory neurons converge on the same secondary sensory neurons in the dorsal horn. Information from abdominal viscera is transmitted through the anterolateral system.

I. The Trigeminal System

The organization of the somatic sensory system of the head is similar to the organization of the anterolateral and DCML systems.

  1. Primary sensory neurons have cell bodies in a peripheral ganglion and send central axonal processes into the CNS.
  2. Secondary sensory neurons have cell bodies in the CNS. Their axons terminate in a specific sensory nucleus of the thalamus.
  3. Tertiary sensory neurons have cell bodies in a specific sensory nucleus of the thalamus and send axons to the primary somatosensory cortex.

The secondary sensory neurons not only project to thalamus but also make reflex connections in other brain stem nuclei. Some of these will be considered in this chapter.

A. The Primary Sensory Neurons

Review briefly the three major sensory divisions of V. What is the peripheral distribution of each division? In addition to innervating skin, these sensory nerves also innervate the oral mucosa and the teeth and their supporting tissues. The cell bodies (#6194) of these primary sensory axons are located in the Gasserian or trigeminal ganglion (#52286, #7982). This ganglion is a homologue of what structures associated with the spinal cord? The centrally directed first-order sensory axons tunnel through the middle cerebellar peduncle (#5638) to enter the lateral pontine tegmentum (fig 6a). If a tumor (#5825) produces sufficient pressure on the middle cerebellar peduncle, these sensory axons will degenerate. What will result? Adjacent cranial nerves may also be affected. Which cranial nerves (#10531)?

In the pons, the primary sensory axons terminate either in the principal sensory nucleus of V ( or chief sensory nucleus) (#6197) or turn caudally to form the descending tract of V (or spinal tract of V) (#4621) (fig 6b).

B. The Secondary Sensory Neurons

The principal sensory nucleus is functionally equivalent to the dorsal column nuclei. It and its projections subserve fine (discriminatory) tactile sensation of the face. The spinal or descending nucleus of V is functionally equivalent to the dorsal horn. It and its connections subserve pain and thermal sensibility of the face.

In the medulla, identify the descending tract and nucleus of V (fig 6b, fig 6c, fig 6d ). Observe that they have the same relative position in the brain stem that the dorsal horn has in the cord (fig 6d) or spinomedullary junction (#6690). Where are the cell bodies for the axons of the descending tract? Is this tract efferent or afferent to the descending nucleus? Note also that the tract and nucleus are located near the anterolateral system fibers (#6508). Branches of what artery supply this area of the medulla (#5284)?

Neurons of the descending nucleus not only send axons to thalamus but also project axons to different cranial nerve motor nuclei, e.g. XII (#6319), VII (#6729), IX, and X (#6239) for the mediation of several reflexes, including the corneal reflex. Diagram this reflex. If it were absent, how could you tell whether the afferent or efferent limb is involved? If the afferent limb is interrupted, how could you tell whether the difficulty is located along the peripheral nerve or within the brain stem?

The corneal reflex can be studied under various conditions. If you would like to go to these animated pathways now, click at the end of this line: corneal reflex.

Because secondary sensory axons from the trigeminal sensory nuclei do not form discrete fasciculi, they are hard to visualize in brain stem sections. Secondary sensory axons from the descending nucleus (#6208) cross the midline and join the medial lemniscus. These are called the ventral (anterior) trigeminothalamic axons (#10148). They accompany the medial lemniscus (#4245, #6178) and anterolateral system (#10130) to the thalamus, where they end in the medial part of the ventral posterior nucleus (fig 6e). This portion of the nucleus is called the ventral posteromedial nucleus (VPM). What is its location with respect to VPL?

Not all secondary sensory axons from the principal or chief sensory nucleus cross the midline to join the ventral trigeminothalamic tract. Some of them are uncrossed or ipsilateral axons that form the dorsal trigeminothalamic tract (#10242), which is located in the dorsolateral quadrant of the tegmentum (it is not easily identified). This tract also ends in VPM (#6189). A consequence of this is that the acutely sensitive perioral region is represented bilaterally in VPM. Are all portions of the face represented equally in VPM?

The axons of VPM project to what cortical area (#4208)? How do they reach this cortical area? The face is represented on the lateral surface of the postcentral gyrus. Branches of what cerebral artery supply this region (#12425)? What other body areas are represented in this vascular territory?

Click for the Pathway Quiz
Click for an animation of the facial muscles

II. Nucleus and Tractus Solitarius

Sensory information from the viscera in the head, throat, thoracic, and abdominal cavities are processed in the solitary complex. For example, the solitary complex receives input from stretch receptors in the lungs innervated by cranial nerve X that function in respiratory reflexes. Pressure receptors in the arterial system innervated by cranial nerve IX also project to the solitary complex. They have a role in blood pressure regulation. The solitary system also processes information from the taste receptors innervated by cranial nerves VII, IX, and X (#9962).

An example of a taste bud is shown in (#6223). What innervates these visceral receptors? Where are the cell bodies for these sensory axons (#7276)? What and where is the geniculate ganglion (#5449)?

The primary sensory axons for visceral sensation enter the medulla and join the tractus solitarius (#6228). What cranial nerves contribute axons to this tract (#4300)? The tract is easily identified in (fig 6f) and (fig 6g). The reason for its name leaps out at you. This tract runs most of the length of the medulla and is surrounded entirely by the nucleus solitarius, a group of secondary sensory neurons (#4383). The structural relations and projections of the solitary complex are not well worked out and, as Churchill summarized Russia, they seem to be an "enigma wrapped in mystery." However, the fundamental plan is similar to that of the descending tract of V and descending nucleus of V: primary sensory neurons enter the solitary tract, run for varying distances along it, and terminate in the nucleus solitarius. Some of the secondary sensory axons project directly to the thalamus. They end in the ventromedial corner of VPM. Unlike the other secondary sensory axons that have been studied, these are ipsilateral, i.e., the left nucleus solitarius projects to the left VPM.

Secondary sensory neurons of the solitary nucleus also send axons to cranial nerve motor nuclei and the surrounding reticular formation. These connections are important in several reflex actions, including gagging and vomiting. The area of the reticular formation near the solitary nucleus (#10060) is known as the vomiting center. Electrical stimulation of this region results in the posture, glandular secretion (salivary), and muscle contraction -- both smooth and striated -- that are characteristic of vomiting. The contraction of thoracic and abdominal striated muscles depends on the stimulation of ventral horn cells at the appropriate spinal cord levels. How is the nucleus solitarius connected with these? What are the natural stimuli that usually provoke vomiting?

Vomiting can also be induced by the ingestion of certain chemicals, such as apomorphine, that are called emetic substances. These substances do not act directly on the nucleus solitarius. Instead, they excite neurons in a chemosensitive area of the medulla known as the area postrema (#5308 arrow). The blood brain barrier is leaky in this area. The area postrema is conveniently located in the floor of ventricle IV adjacent to the nucleus solitarius (#6236) and sends axons to it. By means of this connection, stimulation of the area postrema triggers vomiting.

Now is the time to think about and diagram the neuroanatomy that is relevant to sneezing and gagging as well as vomiting. Many of the interneurons for components of these reflexes are located in the reticular formation.

Click for the Syllabus Quiz
Click for the Movie From: "Parallel Pathways, A Tour Through the CNS". Edward Allen Neilson, W. Curtis Wise, Henry F. Martin. Department of Physiology, The Medical University of South Carolina. Charlston, South Carolina.

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