With unilateral damage to the trigeminal nucleus (or nerve), there is deviation of the lower jaw toward the normal side. The lower motor neurons of the trigeminal nerve arise in the pons and pass through the petrous temporal bone in the foramen ovale adjacent to sensory trigeminal neurons and are distributed to the muscles of mastication: masseters, pterygoids, temporals, and rostral digastricus. Lesions in these nerves (or nuclei) cause true strabismus. The oculomotor nerve also supplies the levator palpebrae and pupillary constrictor muscles and the abducens nerve innervates the retractor bulbi muscle. There is extensive decussation of these tracts both in the chiasm and midbrain.Įye Position (midbrain, pons, cranial nerves III, IV, VI)įrom nuclei in the midbrain and pons, the oculomotor, trochlear, and abducens nerves exit the cranial cavity through the orbital fissure and ramify in the periorbital tissues to innervate the muscles of the eye. The afferent part of the pupillary light reflex passes via the optic nerves and optic tracts, past the thalamus, to terminate in the midbrain. Post-ganglionic neurons pass along the optic nerve to innervate the ciliary muscle and constrictor of the pupil. These neurons synapse behind the eye in the ciliary ganglion. Parasympathetic preganglionic neurons arise in the midbrain and exit the skull in the oculomotor nerve (III). Interruption of pre- or post-ganglionic sympathetic nerves to the eye causes Horner's syndrome, with miosis of the pupil, ptosis, and spontaneous sweating and vasodilatation over the side of the face. Emotional and other influences on sympathetic pupillary tone are governed by hypothalamic centers that act through upper motor neuron (UMN) tracts descending from the midbrain. After synapse in the cranial cervical ganglion adjacent to the guttural pouch, the post-ganglionic sympathetic neurons continue to the smooth muscle of the orbit and act to cause pupillary dilation. Preganglionic neurons for sympathetic supply to the head arise in the grey matter of the first four thoracic segments of the spinal cord, and subsequently course rostrally in the cervical sympathetic nerve within the vagosympathetic trunk. In the normal horse, pupil size reflects the balance of sympathetic (dilator) and parasympathetic (constrictor) influences on the smooth muscle of the iris. Pupillary Light Response, pupil size (midbrain, cranial nerves II, III) Because the diencephalon was discussed in the previous lecture under Forebrain Diseases, it will not be covered here. The hindbrain is divided into metencephalon (cerebellum) and myelencephalon (pons and medulla oblongata). The brainstem includes the diencephalon, mesencephalon (midbrain), and rhombencephalon (hindbrain).1 With the exception of the olfactory nerves (I), all cranial nerves are arrayed along the brainstem.
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