6. Somatic and Visceral Sensory Systems of the Head

Revised January 25, 2021

The objectives of this chapter are to:

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

The trigeminal sensory system is the somatic sensory system of the face and mouth.

The cranial nerves mediating visceral sensation, including taste, are VII, IX and X.

(Smell is described in Chapter 14).

I. Trigeminal Sensory System

The sensory portion of the trigeminal nerve (V) (#5323) innervates the face, except the angle of the jaw, and head as far back as the vertex, and the anterior external ear (#11459). Review briefly the three major sensory divisions of the trigeminal ("triplet") nerve.  What is the peripheral dermatomal distribution of each division? In addition to innervating skin, these sensory nerves also innervate the

Cornea and conjunctiva, eyes and orbital contents

Nasal, paranasal sinus, and nasopharyngeal mucosa

Tympanic membrane

 

The organization of the conscious somatosensory system of the head is similar to the organization of the anterolateral and DCML systems.

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

A. Primary Sensory Neurons

The cell bodies (#6194) of the primary sensory neurons are in the trigeminal (semilunar, gasserian) ganglion (#52286, #7982). This ganglion is a homologue of what structures associated with the spinal cord? The centrally directed first-order sensory axons form the large sensory (major) root of V, which enters the brain stem at the midpontine level (#4152).  It tunnels 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?

(Interestingly, the cell bodies of some primary sensory neurons of V are uniquely located within the CNS, within the mesencephalic nucleus of V. These primary neurons mainly innervate muscle spindles in the jaw muscles.)

In the pons, primary sensory axons

    1. Terminate in the principal (chief) sensory nucleus of V (#6197).
    2. Turn caudally to form the spinal (descending) tract of V (#4621) (fig 6b) and end in the spinal (descending) nucleus of V.

B. Secondary Sensory Neurons

The principal sensory nucleus is at the midpontine level, at the same level as the motor nucleus of V. It is functionally equivalent to the dorsal column nuclei. It and its projections subserve fine (discriminatory) tactile sensation of the face.

The spinal nucleus of V is functionally equivalent to the dorsal horn. It and its connections subserve pain, temperature and light touch. The spinal tract and spinal nucleus both descend to the level of C2, where they merge with Lissauer's tract and the dorsal horn, respectively. In the medulla, identify the descending tract and nucleus of V (fig 6b, fig 6c, fig 6d). Observe that they have the same relative positions as Lissauer's tract and the dorsal horn (#6683) in the cord. Where are the cell bodies for the axons of the descending tract? Is this tract efferent from 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)?

Ventral trigeminothalamic tract. The secondary sensory axons arise from the principal sensory nucleus and from the spinal nucleus throughout its length and cross the midline from the mid-pons to C2. The crossed axons ascend as the ventral (anterior) trigeminothalamic tract (#10148). They are hard to see in brain stem sections. They accompany the medial lemniscus (#4245, #6178) and anterolateral system (#10130) to the thalamus, where they end in the ventral posteromedial nucleus (VPM) (fig 6e). What is its location with respect to the ventral posterolateral nucleus (VPL)?

(Dorsal trigeminothalamic tract.  Reports of this tract in humans are few and conflicting.  In monkeys, this tract arises mainly from the intraoral part of the principal sensory nucleus and projects to the ipsilateral VPM.  However, there is little or no clinical correlation for this tract in humans.)

Corneal reflex. Neurons of the spinal nucleus send axons not only to the thalamus but also to the facial nuclei (#6729) bilaterally for mediation of the corneal reflex. If it were absent, how could you tell whether the afferent limb 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? 

C.  Tertiary Sensory Neurons

VPM (#6189) lies medial to VPL. The adjacent part of VPL is the fingers/hand region.

The axons of VPM project to the primary somatosensory cortex (#4208). How do they reach this cortical area? The face, lips, tongue, and mouth are represented in the lower half of the postcentral gyrus. Branches of what cerebral artery supply this region (#12425)? What other body areas are represented in this vascular territory?

II. Visceral Sensation Mediated by Cranial Nerves

Visceral sensation is traditionally divided into general visceral sensation and special visceral sensation, which refers to taste and smell (which is covered in chapter 14).

A. General visceral sensation (IX and X)

The cranial nerves mediating general visceral sensation are the glossopharyngeal nerve (IX) and the vagus nerve (X). Their cell bodies are in the inferior ganglia of IX (the petrosal ganglion) and the X (the nodose ganglion). The glossopharyngeal and vagus nerves mediate

Via IX: from the oropharynx, posterior third of the tongue, and soft palate

Via X: from the hypopharynx, larynx, and trachea

     

Via IX afferents: Carotid sinus reflex, carotid body reflex, gag reflex

Via X afferents: Cough reflex, vomiting reflex (via gut vagal afferents)

The IX and X sensory axons for general visceral sensation enter the medulla and join the solitary tract (tractus solitarius) (#5963). This tract runs most of the length of the medulla. The tract is easily identified in fig 6f and fig 6g. The reason for its name leaps out at you. It appears solitary or isolated because it is surrounded by the solitary nucleus (nucleus of the solitary tract, nucleus solitarius) (#5962, #5964).  The fundamental plan is similar to that of the descending tract of V and descending nucleus of V: primary sensory axons enter the solitary tract, run for varying distances along it, and terminate in the nucleus solitarius. IX and X general visceral sensory axons end in the caudal (visceral) part of the solitary nucleus.

The solitary nucleus sends axons to nearby reticular formation interneurons, which mediate a number of visceral reflexes. For example, pressure receptors in the carotid sinus, innervated by cranial nerve IX, project to the solitary nucleus. They have a role in blood pressure regulation via the carotid sinus reflex.

Vomiting

An area of the reticular formation near the solitary nucleus is known as the vomiting center.  Activation of this region results in the salivary secretion and muscle contraction, both smooth and skeletal, that are characteristic of vomiting. The contraction of thoracic and abdominal skeletal 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.

B. Taste (VII, IX and X)

The taste (gustatory) system is responsible for the perception of sweet, salty, bitter, and sour. Most patients reporting a taste deficit actually have an olfactory deficit. Conversely, patients with actual taste deficits frequently do not recognize the loss. The taste system is a three-neuron pathway.

Receptors. An example of a taste bud is shown in (#6223). What innervates these visceral receptors?

Primary sensory neurons: VII, IX, AND X

The cranial nerves innervating the taste buds are the facial nerve (VII), IX and X. In the human, the known innervations for taste perception are

The cell bodies of the primary sensory neurons are located in the geniculate ganglion (#7276) and in the inferior ganglia of IX and X.  They project via the solitary tract to the rostral (gustatory) solitary nucleus.

Secondary sensory neurons: rostral (gustatory) solitary nucleus

Gustatory nerve fibers from VII, IX and X synapse in the rostral half (gustatory division) of the ipsilateral solitary nucleus. Lesions such as dorsolateral infarction of the rostral medulla (Wallenberg or lateral medullary syndrome) that affect the solitary tract and nucleus (#4383) may cause ipsilateral ageusia (absence of taste).

The axons of these neurons ascend through the ipsilateral pons (in the central tegmental tract).  Patients with pontine tegmentum damage may demonstrate ipsilateral taste deficits, or bilateral ageusia if both sides are involved.

Laterality of projection above the pons, through the midbrain to the thalamus and the cortex is unclear.  It may be predominantly bilateral (explaining why taste disorders due to stroke are relatively infrequent), contralateral or unilateral. Both contralateral and ipsilateral hypoageusia have been reported after unilateral midbrain, thalamic and cortical lesions.

Tertiary sensory neurons:  VPMpc

Axons of the gustatory division of the solitary nucleus ascend to VPMpc (the ventral posteromedial nucleus, parvicellular division) of the thalamus. VPMpc is directly medial to VPM, the thalamic somatosensory nucleus for the face and mouth.  The adjacent part of VPM is the tongue region.

Taste cortex.  The neurons of VPMpc project to the cerebral cortex. The primary taste (gustatory) cortex in humans is located in the cortex marginating the sulcus above the insula (the junction of the frontoparietal operculum and the insula).  (Previously, it was believed to be in Brodmann's area 43.) The primary cortex projects to secondary gustatory cortex in the rostral insula.

Click for the Syllabus Quiz for visceral sensation and taste.

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