Peripheral Division and Trigeminal Ganglion
Information gathering from facial structures begins at the periphery. Sensory innervation is provided by branches of the trigeminal nerve (N. trigeminus, cranial nerve V). Information travels to the trigeminal ganglion (ganglion trigeminale), which houses the cell bodies of the first pseudounipolar (sensory) neurons.
The main branches of the nerve are distributed as follows:
- N. ophthalmicus (ophthalmic nerve, V1 branch) — exclusively sensory in function.
- N. maxillaris (maxillary nerve, V2 branch) — also entirely sensory.
- N. mandibularis (mandibular nerve, V3 branch) — a mixed nerve carrying sensory fibers alongside a motor component to innervate the muscles of mastication.
Nuclei of the Trigeminal Nerve
From the trigeminal ganglion, fibers project to the brainstem structures, housing the nuclei of the trigeminal nerve (nuclei nervi trigemini). There are three sensory (afferent) nuclei and one motor (efferent) nucleus.
Sensory Nuclei:
- Mesencephalic nucleus (nucl. mesencephalicus n. trigemini). Located in the midbrain (mesencephalon). It mediates proprioception—deep muscle sense from the periodontium and masticatory muscles.
- Main (or pontine) sensory nucleus (nucl. principalis n. trigemini). Located in the pons. It processes tactile sensation, including discriminative touch and pressure.
- Spinal trigeminal nucleus (nucl. spinalis n. trigemini). Extensive in length, it spans the pons, medulla oblongata, and extends into the upper cervical segments of the spinal cord (medulla spinalis). Descending fibers (tractus spinalis n. trigemini) terminate here. It mediates pain and thermal sensations.
Motor Nucleus:
- Motor nucleus (nucl. motorius n. trigemini) is located in the pons, medial to the chief sensory nucleus. It supplies the muscles of mastication. Axons from this nucleus exit the brainstem via the motor root and join the N. mandibularis.
Central Pathways (Trigeminal Lemniscus)
The first-order neurons terminate in the brainstem nuclei, where second-order neurons originate. The axons of these second-order neurons decussate—crossing to the opposite side of the brainstem—to form ascending pathways to the thalamus known as the trigeminal lemniscus.
Two primary tracts are recognized:
- Anterior trigeminothalamic tract (tractus trigeminothalamicus anterior) — primarily transmits pain and temperature impulses from the spinal trigeminal nucleus.
- Posterior trigeminothalamic tract (tractus trigeminothalamicus posterior) — conveys tactile and proprioceptive impulses from the chief and mesencephalic nuclei.
Subcortical and Cortical Centers
Third-order sensory neurons are located in the thalamic nuclei (nucll. thalami), where impulses arriving via the brainstem pathways are relayed.
Axons of the third-order neurons project through the internal capsule to the cerebral cortex (cortex cerebri). The ultimate destination for processing all somatosensory information from the head and face is the postcentral gyrus (gyrus postcentralis).