Trigeminal Nerve
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Cranial nerves are nerves that emerge directly from the brain, in contrast to spinal nerves which emerge from segments of the spinal cord. In humans, there are traditionally 12 pairs of cranial nerves. Only the first and the second pair emerge from the cerebrum; the remaining 10 pairs emerge from the brainstem.

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A cranial nerve nucleus is a collection of neurons (gray matter) in the brain stem that is associated with one or more cranial nerves. Axons carrying information to and from the cranial nerves form a synapse first at these nuclei. Lesions occurring at these nuclei can lead to effects resembling those seen by the severing of nerve(s) they are associated with. All the nuclei excepting that of the IV nerve supply nerves of the same side of the body.

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The cranial nerve nuclei schematically represented; dorsal view. Motor nuclei in red; sensory in blue (The olfactory and optic centers are not represented.)

Just as grey matter in the ventral (closer to front of a human) spinal cord tends to be efferent (motor) fibers, and the dorsal horn tends to contain afferent (sensory) neurons, nuclei in the brainstem are arranged in an analogous way.

    • Close to the midline are the motor efferent nuclei, such as the oculomotor         nucleus, which control skeletal muscle. Just lateral to this are the autonomic         (or visceral) efferent nuclei.
    • There is a separation, called the sulcus limitans, and lateral to this are the         sensory nuclei. Near the sulcus limitans are the visceral afferent nuclei,         namely the solitary tract nucleus.
    • More lateral, but also less posterior, are the general somatic afferent         nuclei.This is the trigeminal nucleus. Back at the dorsal surface of the brainstem,         and more lateral are the special somatic afferents, this handles such         sensation as balance.
    • Another area, not on the dorsum of the brainstem, is where the branchial         efferent nuclei reside. These formed from the branchial arches, in the embryo.         This area is a bit below the autonomic motor nuclei, and includes the nucleus         ambiguus, facial nerve nucleus, as well as the motor part of the trigeminal nerve         nucleus.

The sensory trigeminal nerve nuclei are the largest of the cranial nerve nuclei, and extend through the whole of the midbrain, pons and medulla.

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Primary terminal nuclei of the afferent (sensory) cranial nerves schematically represented; lateral view.

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Dermatome distribution of the trigeminal nerve

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Dermatome distribution of the trigeminal nerve

The mandibular nerve innervates:

    • mylohyoid muscle and anterior belly of digastric muscle
    • mucous membrane of the anterior two-thirds of the tongue
    • the inside of the cheek (the buccal mucosa)
    • teeth and mucoperiosteum of mandibular teeth
    • skin of the temporal region
    • auricula
    • lower lip, and chin
    • Muscles of mastication
    • the muscles tensor tympani and tensor veli palatini

There is also a distinct trigeminal motor nucleus that is medial to the chief sensory nucleus.

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Nuclei of origin of cranial motor nerves schematically represented; lateral view. Motor branches of the trigeminal nerve are distributed in the mandibular nerve. These fibers originate in the motor nucleus of the fifth nerve, which is located near the main trigeminal nucleus in the pons. Motor nerves are functionally quite different from sensory nerves, and their association in the peripheral branches of the mandibular nerve is more a matter of convenience than of necessity.

A little more about the brain stem. This info is from:

Brainstem - The lower extension of the brain where it connects to the spinal cord. Neurological functions located in the brainstem include those necessary for survival (breathing, digestion, heart rate, blood pressure) and for arousal (being awake and alert). Most of the cranial nerves come from the brainstem. The brainstem is the pathway for all fiber tracts passing up and down from peripheral nerves and spinal cord to the highest parts of the brain.

    • Medulla Oblongata - The medulla oblongata functions primarily as a relay station for the crossing of motor tracts between the spinal cord and the brain. It also contains the respiratory, vasomotor and cardiac centers, as well as many mechanisms for controlling reflex activities such as coughing, gagging, swallowing and vomiting.
    • Midbrain - The midbrain serves as the nerve pathway of the cerebral hemispheres and contains auditory and visual reflex centers.
    • Pons - The pons is a bridge-like structure which links different parts of the brain and serves as a relay station from the medulla to the higher cortical structures of the brain. It contains the respiratory center.

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The cerebellum (not part of the brain stem) is responsible for coordinating movement, planning, motor activities, learning and remembering of physical skills and for some cognitive abilities. Interestingly, the size of this brain region within any mammal species is a good indicator of its the physical capability.

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