![]() The sensory component of the facial nerve contains fibers from the tongue (taste) and middle ear.īalance and Equilibrium (medulla, cranial nerve VIII), hearing (medulla, cranial nerve VIII, forebrain) The facial nerve also contributes parasympathetic neurons to lacrimal glands. Saliva often drools from the affected side of the mouth, and the horse has difficulty prehending food, especially grain. With involvement of the nucleus or proximal nerve, there is drooping of the ear and lip, ptosis, collapse of the nostril, and the muzzle is pulled toward the normal side. The facial nerve is distributed to the muscles of facial expression including those of the ear, eyelid, nose, and lips, and the caudal belly of the digastric muscle. The nerve courses through the facial canal in the petrous temporal bone adjacent to the middle ear and emerges through the stylomyastoid foramen. The facial nerves arise from nuclei in the rostral medulla and exit the calvarium with CN VIII via the internal acoustic meatus. Bilateral severe involvement of the trigeminal nuclei (or nerves) causes a dropped jaw, weak jaw tone, slight tongue protrusion, and inability to prehend or chew feed.įacial Expression and Movement (medulla, cranial nerve VII) By 2 weeks after injury, there is obvious muscular atrophy. 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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