Motor nucleus of trigeminal nerve

The trigeminal nerve includes just one motor nucleus, in contrast to its three sensory nuclei. The motor nucleus of trigeminal nerve is classified as a special visceral efferent type of nucleus and is located in the mid-pons. It innervates structures derived from the first branchial arch.

Within the pons, the motor nucleus of the trigeminal nerve is identifiable. This nucleus houses the cell bodies of the lower motor neurons, which form the motor root of the trigeminal nerve. These neurons travel through the mandibular division and its branches to innervate various muscles.

The motor neurons supply the muscles of mastication, including the temporalis, masseter, and the medial and lateral pterygoid muscles. Damage to the trigeminal motor nucleus or the trigeminal nerve can result in flaccid paralysis of these muscles. This condition impacts chewing and can cause jaw deviation. For instance, if the motor nucleus of trigeminal nerve on the right side or the right trigeminal nerve is impaired, the jaw will deviate towards the weaker side (i.e. the right side), indicating right-sided trigeminal nerve palsy. Conversely, if the jaw deviates to the left, it suggests a left-sided issue.

Additionally, the trigeminal motor nucleus' nerve fibers innervate the tensor tympani muscle in the middle ear, which is connected to a bony ossicle called the malleus. This muscle has a crucial function: it regulates the malleus's movement to prevent excessive vibrations in response to sound waves. If the trigeminal motor nerve is compromised, it may lead to hyperacusis, a condition where sounds appear excessively loud and jarring.

Furthermore, the trigeminal motor nerve fibers controls several other muscles, including the tensor veli palatini in the palate, the mylohyoid, and the anterior belly of the digastric muscle in the floor of the mouth.

Afferents to the motor nucleus of trigeminal nerve include:

• Corticonuclear fibers: These upper motor neurons project from both cerebral hemispheres to synapse with lower motor neuron cell bodies located in both trigeminal motor nuclei.

• Mesencephalic nucleus: The trigeminal motor nucleus receives input from the mesencephalic nucleus, forming part of the monosynaptic reflex arc.

• Additional inputs: The trigeminal motor nucleus also receives input from other structures, such as the reticular formation, the red nucleus, the tectum, and the medial longitudinal fasciculus.