In one breath
Sensation from the face, the oral cavity and the head reaches the brain through the trigeminal nerve, whose first-order cell bodies sit in the semilunar (gasserian) ganglion. Their central axons split and end in two brainstem nuclei: spinal V, fed by small nociceptive fibres, which is the trigeminal counterpart of the spinothalamic system, and main sensory V, fed by large touch fibres, which is its counterpart of the dorsal column nuclei. Second-order fibres climb as the trigeminal lemniscus beside the medial lemniscus to the VPL and PO nuclei of the opposite thalamus, and on to a very large face area in the sensory cortex. Compression of the nerve roots produces trigeminal neuralgia: brief stabbing facial pain set off by a harmless touch to a trigger zone.
Builds on: Pain: concepts, receptors and fibers · Pain pathways and perception · Leads to: Thalamus: nuclei, functions and thalamic syndrome · Somatosensory cortex · Sensory lesions and sensory function tests
Why the face has its own pathway
- A map of somatic sensation over the whole body is incomplete until trigeminal input is added to it, because the face and head do not report through the spinal cord at all.
- The trigeminal nerve serves the face, the oral cavity and the head.
- Its route differs from the pain pathway of the rest of the body. Signals from the trunk and limbs make their first synapse in the dorsal horn of the spinal cord (Afferent fibers, dorsal root and spinal laminae); trigeminal signals make theirs in brainstem nuclei, and reach the cord’s equivalent structures only by descending inside the brainstem.
- Because the whole first relay is packed into the brainstem and can be reached surgically, the trigeminal system is the standard model for studying facial pain and how to abolish it (Endogenous pain control and gate control theory).
The three divisions
The nerve divides into three, and each division takes one horizontal band of the face.
| Division | Band of face |
|---|---|
| Ophthalmic (V1) | Upper: forehead and eye |
| Maxillary (V2) | Middle: cheek and upper lip |
| Mandibular (V3) | Lower: jaw and lower lip |
- Worth knowing, though not in your pages: your book gives this only as a labelled figure, so the detail is standard anatomy. V1 also takes the scalp back to the vertex, the upper eyelid, the cornea and the bridge of the nose; V2 the lower eyelid, the upper teeth, the palate and the nasal mucosa; V3 the lower teeth, the chin, the front two-thirds of the tongue for touch, and the temporomandibular joint. The angle of the jaw is the classic exception: it is supplied by cervical nerves, not by V3.
First-order neurons
- They are pseudobipolar cells, the term your book uses, with their cell bodies in the semilunar or gasserian ganglion — the trigeminal ganglion.
- The central (proximal) axon of each cell divides into an ascending and a descending branch, so that one afferent can reach both brainstem nuclei.
- Those two nuclei are spinal V, the spinal nucleus, and main sensory V, the main sensory nucleus.
- Worth knowing, though not in your pages: these ganglion cells are usually called pseudounipolar, since the single process from the cell body splits in a T. The descending branches together make the spinal (descending) tract of the trigeminal nerve, which runs down the brainstem as far as the upper cervical cord — which is why a lesion of the medulla takes pain from one side of the face and the opposite side of the body (Sensory lesions and sensory function tests).
Spinal V
The caudal of the two nuclei, lying lowest, with three subdivisions stacked one above the other.
| Subdivision (top down) | What it receives |
|---|---|
| Oral | Nociception, mainly from the oral cavity |
| Interpolar | Touch and nociception from teeth, gums, mouth lining |
| Caudal | Nociception from the face, in quantity |
- The nucleus as a whole takes mostly nociceptive input, carried by small-diameter fibres.
- That is why spinal V is regarded as the spinothalamic pathway of the trigeminal system: same kind of fibre, same kind of sensation, same slow crossed route to the thalamus (Ascending tracts: dorsal column and anterolateral system).
Applied: cutting the caudal subdivision
Chronic facial pain that no drug will control, as in trigeminal neuralgia, can be relieved by a surgical lesion of the caudal subdivision of spinal V. It works because that subdivision is where the bulk of the facial nociceptive afferents end, and because it lies low enough in the brainstem to be reached without sacrificing touch, which relays higher up in main sensory V.
Caudal nucleus, caudal subdivision
Two different “caudal”s. Spinal V as a whole is the caudal nucleus, the lower of the two. Within it, the caudal subdivision is the lowest of three. The pain operation is aimed at the subdivision, not at the whole nucleus. And note the layout: the nociceptive face input is at the bottom, the oral input at the top.
Main sensory V
- It lies rostrally, above spinal V.
- Its input comes from low-threshold mechanoreceptors of the face and teeth, delivered by large-diameter fibres, and reaches it on the same side.
- Touch, large fibres, one relay nucleus: main sensory V is therefore the trigeminal homologue of the dorsal column nuclei (Ascending tracts: dorsal column and anterolateral system).
Spinal V versus main sensory V
| Feature | Spinal V | Main sensory V |
|---|---|---|
| Position | Caudal | Rostral |
| Fibre size | Small | Large |
| Sensation | Mainly pain | Touch |
| Homologue | Spinothalamic | Dorsal column nuclei |
Trigeminal lemniscus and the thalamus
- Second-order fibres leave both nuclei as the trigeminal lemniscus, which travels up alongside the medial lemniscus.
- The projection is mainly to the contralateral thalamus, ending in its specific relay nuclei (Thalamus: nuclei, functions and thalamic syndrome).
| Thalamic nucleus | Fed by |
|---|---|
| VPL | Main sensory V, and rostral subdivisions of spinal V |
| PO (posterior) | Caudal subdivisions of spinal V |
- Read the split by its two ends. VPL takes the touch nucleus together with the subdivisions that serve the mouth; PO takes the caudal subdivision, which carries the bulk of the facial nociception.
- Your book’s thalamus chapter fills in why that matters: PO is the thalamus’s pain nucleus, it answers many modalities at once, and unlike VPL it has no somatotopic map (Thalamus: nuclei, functions and thalamic syndrome).
VPL or VPM for the face
Your book routes the face to VPL, and that is the answer it will mark. Most other texts, and your book’s own thalamus chapter, put facial sensation in VPM (ventral posteromedial), keeping VPL for the trunk and limbs — which fits the somatotopy of VP, where the face sits medial to the arm and the leg (Thalamus: nuclei, functions and thalamic syndrome). Worth knowing, though not in your pages: if a question asks for the thalamic relay of the trigeminal nerve on its own, write VPM.
Cortical representation
- VPL projects to the ventrolateral part of the primary somatosensory cortex (SI), which is its lower, lateral end.
- The face occupies an enormous stretch of SI, out of all proportion to its size, because the density of innervation there is so high. The mouth and tongue areas are large for the same reason.
- Trigeminal input from the thalamus reaches both SI and SII (Somatosensory cortex).
Applied physiology
Trigeminal neuralgia
- The pain is severe, stabbing and acute, and each attack is brief.
- It is set off by an innocuous stimulus — touch that should not hurt at all — applied to a trigger zone.
- The trigger zone is a very small and sharply limited patch, and it may sit on the nose, cheek, lip, teeth, oral mucosa or scalp.
- The trigger zone need not lie in the area where the pain is felt: touching the lip can throw the pain into the cheek.
- Cause: in most patients the trigeminal nerve roots have been compressed. Compression irritates the nerve or makes it degenerate, and the damaged nerve then fires episodic bursts of severe pain.
- Treatment: analgesics first. If they do not control it, consider surgical release of the compression, or a lesion of the trigeminal ganglion.
Applied: what the trigger zone tells you
Pain from a stimulus that should not hurt is allodynia, and that is exactly what the trigger zone produces. Worth knowing, though not in your pages: the usual explanation is that damage at the root lets large touch fibres drive the nociceptive neurons they would normally help inhibit, so touch prises the gate open instead of closing it (Endogenous pain control and gate control theory).
- Worth knowing, though not in your pages: the condition is also called tic douloureux; V2 and V3 are affected far more often than V1; the compressing vessel at the root is usually a loop of the superior cerebellar artery; the drug of choice is carbamazepine, not an ordinary analgesic; and the operation for the compression is microvascular decompression. Trigeminal neuralgia in a young adult should raise the question of multiple sclerosis.
Not a facial palsy
Trigeminal neuralgia is a sensory disorder of the fifth nerve: pain, with no weakness. Facial weakness is the seventh nerve. The only motor territory of the trigeminal nerve is the muscles of mastication, so a trigeminal motor lesion shows up as a weak, deviating jaw, never as a drooping face.
Draw it: the trigeminal pathways
Draw the brainstem as a tall block. On the right side, outside it, draw a small oval for the gasserian (trigeminal) ganglion, with three roots leaving it for the face labelled V1, V2, V3. From the ganglion, run one fibre into the brainstem and split it into a short ascending branch and a long descending branch. The ascending branch ends in an upper oval, the main sensory nucleus (large fibres, touch). The descending branch runs down into a long lower column, the spinal nucleus, which you divide with two lines into oral, interpolar and caudal from top to bottom, and tail off into the upper cervical cord. Now take second-order axons from both nuclei, cross them to the left side, and run them upwards as one bundle labelled trigeminal lemniscus, drawn hugging the medial lemniscus. End the bundle in two thalamic ovals: VPL (arrows from main sensory V and from oral plus interpolar) and PO (arrow from caudal). From VPL, draw the last fibre to the ventrolateral end of SI, and mark a wide face-and-mouth strip there. Add a short branch to SII. Finally put a star on the caudal subdivision and label it site of lesion for intractable facial pain.
Exam-answer skeleton: "Trigeminal pathway" (short note)
- What it carries and why it is separate: face, oral cavity and head, with the first relay in the brainstem instead of the spinal dorsal horn.
- First-order neurons: pseudobipolar cells of the gasserian ganglion; the central axon splits into ascending and descending branches.
- Spinal V: three subdivisions (oral, interpolar, caudal), small nociceptive fibres, so it is the spinothalamic system of the face.
- Main sensory V: large fibres from low-threshold mechanoreceptors, ipsilateral, so it is the dorsal column nucleus of the face.
- Second-order fibres cross and ascend as the trigeminal lemniscus with the medial lemniscus.
- Thalamus: VPL from main sensory V and rostral spinal V; PO from caudal spinal V.
- Cortex: ventrolateral SI, with a huge face and mouth area, and SII as well. Add the labelled diagram here.
- One line of applied physiology: trigeminal neuralgia, and the lesion of the caudal subdivision that relieves it.