This is a remarkable core. It consists of the cell bodies belonging to the nerve fibers that convey proprioception from the masticatory muscles and wholly or partly from the mimic muscles and the muscles of the tongue, i.e. the nucleus contains proprioceptive primary-afferent nerve cell bodies. At all other sites in the nervous system, the primary-afferent cell bodies are found outside the CNS and usually in spinal or cranial nerve ganglia.
The nucleus extends as a thin strand of cells from the upper part of the pons up to the upper part of the mesencephalon. The peripheral nerve fibers, after they together with the rest of the trigeminal nerve have penetrated the pons, form the ascending mesencephala trigeminal pathway whose cell bodies thus form the mesencephalic nucleus. From the nucleus emanates a descending axon bundle that mainly forms synaptic contacts in the sensory main nucleus and to some extent in the motor trigeminal nucleus.