Mesencephalic nucleus of trigeminal nerve

The mesencephalic nucleus of trigeminal nerve (a.k.a. the mesencephalic trigeminal nucleus) is one of its three sensory nuclei, alongside the main sensory nucleus and the spinal trigeminal nucleus. This nucleus is located within the periaqueductal gray matter surrounding the cerebral aqueduct in the midbrain. It is composed of unipolar neuronal cell bodies that convey sensory proprioceptive information from the facial region, including the muscles of mastication, the tongue, facial muscles, and extraocular muscles.

These afferent neurons bypass the trigeminal semilunar ganglion, meaning they transmit proprioceptive information without synapsing there. Instead, this information travels through the sensory root of the trigeminal nerve directly to the mesencephalic nucleus. As such, the mesencephalic nucleus of trigeminal nerve is unique for housing the cell bodies of primary afferent neurons, which are generally located in ganglia of peripheral nerves rather than within the central nervous system.

The proprioceptive information processed by these unipolar neurons in the mesencephalic nucleus is then transmitted to the ventral posteromedial (VPM) nucleus of the thalamus through the trigeminothalamic tract or trigeminal lemniscus. Additionally, this information is relayed to the trigeminal motor nucleus, forming part of the monosynaptic reflex arc responsible for the jaw jerk reflex.