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MOTOR, CHIEF SENSORY AND MESENCEPHALIC NUCLEI OF THE TRIGEMINAL

MOTOR V

Axons arising from neurons in motor V constitute the bulk of the motor root of the trigeminal and supply the muscles of mastication (masseter, temporalis and lateral and medial pterygoid muscles), as well as the tensor tympani, tensor veli palatini, anterior belly of digastric and the mylohyoid muscles.

Lesions involving motor V or the motor fibers in the trigeminal nerve result in ATROPHY of the muscles listed above ipsilateral to the lesion. Since the pterygoids OPEN the jaw in concert with a downward and opposing inward motion, when they are weak on one side the inward vector of the opposing pterygoid is unopposed. Thus, the jaw deviates toward the WEAK SIDE (i.e., lesion side) upon opening.







You Med 1s are lucky that corticobulbar input to motor V (like that to nucleus ambiguus) is BILATERAL. Therefore lesions of this system (i.e., in motor cortex) DO NOT cause obvious defects of jaw function. (Remember, lesions of the corticobulbar input to the hypoglossal nucleus and motor VII DO cause observable deficits, and you should know them COLD!)

CHIEF SENSORY V

Discriminatory touch, as well as conscious proprioception and vibration information from the face do not “use” the spinal tract and nucleus V. Instead, they pass into the CHIEF SENSORY NUCLEUS (or principal sensory V). This nucleus lies LATERAL to motor V (Motor = Medial) in the pons at the level of the entering (and exiting) C.N. V fibers. Axons of cells in the chief sensory nucleus V cross and pass rostrally in the trigeminothalamic tract (TTT; along with the pain and temperature fibers from the caudal spinal nucleus V) to the ventral posteromedial (VPM) nucleus of the thalamus.





MESENCEPHALIC NUCLEUS V

This nucleus consists of a slender strand of cells extending from the chief sensory nucleus through the midbrain. The nucleus is located beneath the lateral edge of the floor of the fourth ventricle in the pons and in the lateral region of the periaqueductal grey matter in the midbrain. These cells are incorporated into the neuraxis, rather than being in the cerebrospinal ganglia (i.e., the trigeminal ganglion, the geniculate ganglion, etc.). Most of the peripheral processes of mesencephalic V neurons occupy the motor root of the trigeminal nerve and are distributed to muscle spindles in the muscles of mastication. The central processes of mesencephalic V neurons terminate within motor nucleus V. This connection establishes the stretch reflex originating in the muscle spindles of the masticatory muscles, together with a reflex for the control of the force of the bite. The mesencephalic nucleus V may be thought of as being similar to Clarke’s column and the accessory cuneate nucleus (i.e., unconscious proprioception).



Damage to the C.N. V results in problems with the CORNEAL REFLEX. This reflex involves C.N. V (on the sensory side) and C.N. VII (on the motor side). Stimulation of the cornea of one eye results in the closing of BOTH eyelids. The pathways involved include touch fibers in the ophthalmic division of C.N. V (innervate cornea) that pass into the chief sensory nucleus V. Some pain fibers probably pass into spinal tract V, but let’s concentrate on the touch fibers to the CHIEF. Cells in the CHIEF project BILATERALLY to motor VII (we cannot identify these axons in our sections). The result is closing of BOTH eyelids upon stimulation of ONE cornea.

The response on the side that is stimulated is called the DIRECT corneal reflex while that on the opposite side is called the CONSENSUAL corneal reflex (analogous to the gag reflex). Go over the diagram and think about the results of lesions of the various components.



REMEMBER:

a) Pain and temperature ----- Spinal tract & nucleus V
b) 2-pt. discrimination, ----- Chief sensory nucleus V
vibration, conscious
proprioception
c) unconscious propriocep- ----- Mesencephalic nucleus V
from muscle spindles
d) Motor innervation of ----- Motor nucleus V
muscles of mastication







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