![]() ![]() 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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