SOMATOSENSORY PATHWAYS FROM THE FACE

In the last section, we covered the three modalities of sensation from the body: discriminative touch, proprioception, and pain and temperature. These same modalities need to be addressed in the face as well, but sensory input from the face does not enter the spinal cord. In fact, it all enters the brainstem via the trigeminal nerve. Just as in the spinal cord, these three modalities have different receptors, travel along different tracts, and have different targets in the brainstem. Once the pathways synapse in the brainstem, they join the pathways from the body on their way up to the thalamus.

A. The trigeminal nucleus complex:

The collection of cells in the brainstem that can be called the trigeminal nucleus is huge - it stretches from midbrain to medulla. If we could see it in a transparent brainstem, it would look something like this:

Most of the sensory fibers enter the trigeminal ganglion, regardless of which trigeminal division they are coming from. Their cell bodies, like those of all somatosensory neurons, lie outside the CNS in the ganglion, and their proximal processes enter the brainstem in the mid-pons. From there they fan out to their different targets. Each modality will be described separately below.

B. Discriminative touch:

The large diameter (Ab) fibers enter directly into the main sensory nucleus of the trigeminal (V), also called the principal nucleus. Just like the somatosensory neurons of the body, they SYNAPSE, then CROSS. The secondary afferents can then join the medial lemniscus on its way to the thalamus.

C. Pain and temperature:

The small diameter fibers carrying pain and temperature enter at mid pons, and then do something unusual - they turn down the brainstem. They travel down the pons and medulla until they reach the caudal medulla, which is where they finally synapse and cross.

The tract that the descending axons travel in is called the spinal tract of V, and the long tail of a nucleus that they finally synapse in is called the spinal nucleus of V. These names come from the fact that they actually reach as far down as the upper cervical spinal cord. The spinal nucleus of V can be divided into three regions along its length; the region closest to the mouth is called subnucleus oralis, the middle region is called subnucleus interpolaris, and the region closest to the tail is called subnucleus caudalis. The pain fibers actually synapse in subnucleus caudalis, so you may hear that term used instead of the spinal nucleus of V.

The secondary afferents from subnucleus caudalis cross to the opposite side, and join the spinothalamic tract on its way to the thalamus.

D. Proprioception:

The proprioceptive axons in the trigeminal nerve are the stretch and tendon receptors from the muscles of mastication. (Recall that all of the muscles of facial expression are controlled by the facial nerve. ) These axons coming from the face have a strange characteristic unique among primary somatosensory neurons: their cell bodies are inside the CNS. They are the only exception to the rule. Although their cells look similar to cells in the dorsal root ganglion (the cell body does not come between the distal and proximal axon processes), they are located inside the brainstem in a nucleus called the mesencephalic nucleus. The mesencephalic nucleus is essentially a dorsal root ganglion that has been pushed into the CNS, so there are no synapses within it. The fibers enter the brainstem via a small branch of the trigeminal that bypasses the trigeminal ganglion, turn up towards the mesencephalic nucleus, pass by the cell body, and leave the nucleus immediately. Most then synapse in the nearby motor nucleus where they can initiate the stretch reflexes for the muscles of mastication. The stretch reflex in the face behaves exactly like that in the body, and tapping on the tendon of the masseter (for example) will produce a twitch.

E. Motor innervation:

Motor or efferent control is not considered a sensory modality, but it is the fourth component of the extensive trigeminal complex. The motor nucleus of V lies just medial to the main sensory nucleus, and in it reside the a-motor neurons that control the muscles of mastication. The two principal muscles involved are the masseter (in your cheek) and the temporalis (over your temple), both of which tighten when you clench your teeth. The motor axons leave the mid-pons and bypass the trigeminal ganglion, and reach their targets via the mandibular division of the trigeminal nerve.

F. On to the thalamus:

The somatosensory information from the face joins that from the body and enters the thalamus with it. However, face information actually enters a different nucleus in the thalamus. Recall that information from the body enters the ventroposterior lateral nucleus (VPL). Information from the face actually enters the ventroposterior medial nucleus (VPM). The thalamocortical afferents take all of the signals, whether from VPL or VPM, to primary somatosensory cortex. Once there, it is distributed in a somatotopic (body-mapped) fashion, with the legs represented medially, at the top of the head, and the face represented laterally.

G. A review:

To test your knowledge of the sensory modalities, complete this chart.

If you can complete this table on your own, you know the somatosensory system inside and out. When you are done, click here to check your answers.

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