1,4 This leads to relaxation of the iris sphincter and promotes mydriasis. Drops that antagonize the parasympathetic pathway block acetylcholine, a neurotransmitter of the autonomic nervous system, from reaching muscarinic receptors, which are located within the iris sphincter. Mydriatic drops work by either inhibiting the parasympathetic pathway to the iris sphincter or by promoting the sympathetic pathway to the iris dilator. Postganglionic neurons leave the ciliary ganglion to innervate the iris sphincter. The Edinger-Westphal nucleus gives rise to preganglionic fibers, which then synapse with postganglionic neurons in the ciliary ganglion. The impulses from the pretectal nucleus begin the efferent arm, which projects to the Edinger-Westphal nucleus. This stimulus travels to the optic chiasm, through the optic tract and eventually reaches the pretectal nucleus. 1,3 Pupil constriction starts when light enters the retina and activates the retinal ganglion cells-the beginning of the afferent arm-which then transmit their impulses into the optic nerve. 1,2Ĭontrarily, the parasympathetic pathway is mainly responsible for pupil miosis. The third postganglionic neuron travels to the cavernous sinus and enters the orbit through the short and long ciliary nerves, synapsing to the iris dilator.
The second neuron, which is the preganglionic neuron, exits the spinal cord, ascends through the thorax and synapses near the apex of the lung into the superior cervical ganglion. This synapse is located between the C8 and T2 vertebrae.
1,2 The first neuron begins in the hypothalamus and descends through the midbrain to synapse onto a specific area of the spinal cord, known as the ciliospinal center of Budge. The sympathetic pathway, mainly responsible for pupil mydriasis, involves a three-neuron pathway. Tropicamide has a strong mydriatic effect. 1,3 Pupil constriction starts when light enters the retina and activates the retinal ganglion cells-the beginning of the afferent arm-which then transmit their impulses into the optic nerve.
1,2 The first neuron begins in the hypothalamus and descends through the midbrain to synapse onto a specific area of the spinal cord, known as the ciliospinal center of Budge. Let’s review their function and clinical role to better understand their present uses and why some of these agents are undergoing re-evaulation for potential new ones. The drops are able to control pupil size by targeting two parts of the autonomic nervous system: the sympathetic and parasympathetic systems. Miotic and mydriatic drops work by acting on these different muscles of the iris. On the other hand, pupil dilation (mydriasis) can either be stimulated by contraction of the iris dilator or by relaxation of the iris sphincter. Pupil constriction (miosis) can either be stimulated by contraction of the iris sphincter or by relaxation of the iris dilator. O p tometrists are well-acquainted with the two opposing muscles in the iris, the sphincter and the dilator, as we witness their effects daily in clinical practice.
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