The pupil constricts and dilates largely under the action of the sphincter muscle, which lines the pupil margin. It is supplied by parasympathetic fibres travelling within the third cranial nerve. The afferent stimulus is conveyed along the optic nerves and decussates at the optic chiasm and continues as the optic tract. The specific pupillomotor nerve fibres leave the optic tract without synapsing in the lateral geniculate nucleus and pass to the pretectal nucleus of the midbrain, where they synapse with interneurons. The interneurons project to both Edinger-Westphal nuclei (part of the third cranial nerve nucleus). The pupillomotor fibres then travel within the third cranial nerve to the pupil constrictor muscles of the ipsilateral eye via the ciliary ganglion (Figure 22.7).
The dilator muscle is arranged radially within the iris and responds to the sympathetic nerves conveyed in the sympathetic plexus overlying the internal carotid artery. These fibres, in turn, arise from the superior cervical ganglion. The sympathetic supply to the dilator muscle, therefore, runs a long course from the hypothalamus to the midbrain and spinal cord, and then up again from the root of the neck with the internal carotid artery.
Miosis refers to a small pupil, mydriasis to a large pupil (big word, big pupil). The pupil grows smaller with age, as does reactivity. In young children the pupils are relatively large and sometimes anxious parents bring up their children because they are concerned about this. During sleep, the pupils become small. When examining the eye with the ophthalmoscope, it is evident that the pupil constricts more vigorously when the macula is examined than when the more peripheral fundus is stimulated with the ophthalmoscope light. When an eye is totally blind, usually there is no light pupil reaction,but as a general rule,the pupils remain of equal size. It should be apparent from Figure 22.7 that the patient with cortical blindness (lesion within the occipital cortex) might have a normal pupil reaction. We must also remember that a pupil might not react to light because it is mechanically bound down to the lens by adhesions (posterior synechiae). When both maculae are damaged by senile macular degeneration, the
Pretectal nucleus Figure 22.7. The pupillary pathway. I
pupils can be slightly wider than normal and might show sluggish reactions. A relative afferent pupil defect (also known as a Marcus Gunn pupil) implies optic nerve or severe retinal disease.
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