Courseware Pathways for Eye Reflexes Dr. Kalman Rubinson Several operations allow the eyes to perform their primary function of providing meaningful pattern vision. These operations adjust to the pupil (so that the appropriate amount of light is admitted to the retina), adjust the lens (so that the image is in sharp focus on the retina), and converge the two eyes (so that the image falls on corresponding parts of the two retinas). A. The Light Reflex This reflex adjusts the diameter of the pupil in response to the amount of light falling on the retina. The pathway originates in the retina which projects, via the optic nerve and tracts, to the pretectum. This, in turn, projects bilaterally to the Edinger-Westphal nuclei, which are located in the oculomotor (III) complex of the mesencephalon. The E-W nuclei are pre-ganglionic parasympathetic nuclei which project, with the III nerve, to the ciliary ganglion which, in turn, projects to the sphincter muscle of the iris. Thus, light falling on the retina leads to activation of the pathway and constriction of the pupil. There is a less well-defined pathway from the central gray of the mesencephalon and the reticular formation to the lateral horn of the lower cervical spinal cord. These spinal sympathetic neurons project to the superior cervical ganglion which, via the internal carotid artery, projects to the dilator muscle of the iris.* This is the substrate for the opposing mechanism but, in general, the constriction pathway is the major active mechanism. Light falling on one eye will, because of the bilateral projection to the E-W nuclei, cause pupillary constriction in both eyes. The ipsilateral response is called the direct light reflex, while the response of the unilluminated eye is called the consensual light reflex. Thus, a blind eye, due to transsection of its optic nerve, will evidence a consensual reflex but not a direct one. Conversely, an eye with a severed oculomotor nerve will not respond but, when illuminated, will mediate a consensual response in the other eye. B. Accommodation This system which causes the lens to change its shape and adjust the focus of objects upon the retina. Simple optical analysis tells us that the lens must be fatter (more convex) to focus near objects and thinner (less convex) to focus distant objects. This system also begins in the retina and continues with its projection, via the LGN, to the cortex. Clearly, some cortical involvement is required in order to determine which is the object of choice and whether it is in focus. From here, signals are projected to the superior colliculus and pretectum which project to the E-W nuclei. The E-W nuclei project to the ciliary muscles via the ciliary ganglion. Activation of this system contracts the ciliary muscles and reduces tension on the fibers which suspend the lens, allowing it to round up for focusing a nearer object. The phenomenon depends upon the natural elasticity of the lens. If, as with increasing age, the lens is less elastic, then the reduction of tension will not result in more convexity nor in effective near focus. This is the basis of the phenomenon of presbyopia commonly seen in adults over 40 that requires them to acquire reading glasses. C. Convergence This is the combined operation of the two medical rectus muscles and/or the two lateral rectus muscles so that an object of interest is placed on the corresponding loci in the two retinas. Obviously, the nearer the object, the more medially converged the eyes must be. The pathway is the same as above, for accommodation, but with the projection going to the main portion of the III nerve nucleus, rather than to E-W. The oculomotor neurons project to the medial recti and, simultaneously, inhibit the abducens (VI) nerve nuclei (via the MLF) to relax the lateral recti. D. The =Near= Reflex This triad of reflexes occurs together when the individual wishes to view a close object. It consists of convergence (medially moving both eyes to foveate the near object), accommodation (to focus the near object) and pupillary constriction (to limit the illumination from the near object). The latter is required since the intensity of light falls with the square of the distance and, thus, near objects project more light into the eye than distant ones of the same inherent brightness. *The pathway for pupillary dilation, from the midbrain central grey and reticular formation to the intermediolateral spinal grey and sympathetic chain, is shared with a descending sympathetic pathway from the hypothalamus to the same target. Interruption of that pathway is associated with Hornerps Syndrome: 1. Miosis. Pupillary constriction due to inability to dilate the pupil. 2. Ptosis. Drooping of eyelid due to inability to activate the smooth muscle of the lid. (Which muscle is that?) 3. Anhydrosis. Loss of lacrimation and, if central, loss of sweating ipsilaterally, throughout the body.