Mesencephalic nucleus of trigeminal nerve

Anatomy

The MNTN is located in the brainstem, more specifically (sources vary) spanning the length of the midbrain****/in the caudal midbrain and rostral pons. It is situated (sources vary) near/within**** the periaqueductal gray, lateral to the cerebral aqueduct.****

The mesencephalic nucleus is the only structure in the central nervous system to contain the cell bodies of first order sensory neurons.**** The mesencephalic nucleus can thus be considered functionally as a primary sensory ganglion embedded within the brainstem, making it neuroanatomically unique.

Microanatomy

Unlike many nuclei within the central nervous system (CNS), the mesencephalic nucleus contains no chemical synapses, neurons instead being electrically coupled. Neurons of this nucleus are pseudounipolar, receiving proprioceptive afferent information from the mandible and sending efferent projections to the trigeminal motor nucleus to mediate the monosynaptic jaw jerk reflex.

Development

The pseudounipolar neurons in the mesencephalic nucleus are embryologically derived from the neural crest. However, instead of joining the trigeminal ganglion, the neurons migrate into the brainstem. The MNTN neurons directly receive sensory information, akin to neurons in the dorsal root ganglia.

Function

The MNTN is involved in reflex proprioception of the periodontium and of the muscles of mastication in the jaw that functions to prevent biting down hard enough to lose a tooth. To subserve this reflex protective function, mechanoreceptive nerves in the periodontal ligament sense tooth movement and project to the mesencephalic nucleus. Likewise, afferent fibers from muscle spindles, the sensory organs of skeletal muscle, are stimulated by the stretch of hard contraction of jaw muscles. The temporomandibular joint receptors and the Golgi tendon organs of the jaw muscles do not project to the mesencephalic nucleus. The mesencephalic nucleus is one of four trigeminal nerve nuclei, three sensory and one motor. The other two sensory nuclei are the chief sensory nucleus mediating conscious facial touch and the spinal trigeminal nucleus, mediating pain and temperature in the head, and is of importance in headache. The trigeminal motor nucleus innervates the muscles of mastication, mylohyoid, anterior belly of digastric, tensor veli palatini, and tensor tympani.

Clinical significance

Clinically, because of its reflex function, the mesencephalic nucleus can be tested with the jaw jerk reflex. Because of its function in oral proprioception, lesions of the trigeminal mesencephalic nucleus cause effects on feeding. The mesencephalic nucleus can be thought of simply as the "nucleus that keeps your teeth in" by preventing one from biting down hard enough to lose a tooth on foods containing e.g. bone, cherry seeds, apricot stones etc.

References

References

1. (2022). "Neuroanatomy, Trigeminal Nucleus". StatPearls Publishing.
2. Sinnatamby, Chummy S.. (2011). "Last's Anatomy".
3. (1973). "Gray's Anatomy". Longman, Harlow.
4. (1971). "Electrotonic coupling between neurones in the rat mesencephalic nucleus". Journal of Physiology.
5. (1988). "Human jaw muscle motor behaviour. II. Reflex and receptor mechanisms". Swedish Dental Journal.
6. (2002). "Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus". Progress in Neurobiology.
7. (2012). "Onecut factors control development of the Locus Coeruleus and of the mesencephalic trigeminal nucleus". Molecular and Cellular Neuroscience.
8. (1989). "Physiological and morphological characteristics of periodontal mesencephalic trigeminal neurons in the cat--intra-axonal staining with HRP". Brain Research.
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10. (1972). "A functional analysis of the components of the mesencephalic nucleus of the fifth nerve in the cat". Journal of Physiology.
11. (2013). "The effects of bilateral lesions of the mesencephalic trigeminal sensory nucleus on nocturnal feeding and related behaviors in mice". Life Sciences.