Two sets of smooth muscle in the iris control the diameter of the pupil. One set of muscles, which is arranged radially (dilator pupillae), is innervated by sympathetic (norad-
renergic) fibers that arise from cells in the superior cervical ganglion. Stimulation of them causes contraction of the radial smooth muscle cells, leading to dilation of the pupil (mydriasis). The other set of smooth muscle cells in the iris (constrictor pupillae) is circular and is innervated by parasympathetic neurons arising from cells in the ciliary ganglion. Stimulation of these cholinergic neurons causes contraction of the circular smooth muscle of the iris and constriction of the pupil (miosis).
The lens, which aids in visual accommodation, is attached at its lateral edge to the ciliary body by suspensory ligaments. When the smooth muscles of the ciliary body are relaxed, the ciliary body exerts tension on the lens, causing it to flatten. Thus, the eye is accommodated for far vision. Stimulation of parasympathetic cholinergic neurons, which arise in the ciliary ganglion, causes contraction of the smooth muscle of the ciliary body; this decreases the lateral tension on the lens. Naturally elastic, the lens thickens, and the eye accommodates for near vision. Drugs that block accommodation are called cycloplegic. Since the parasympathetic system is dominant in the eye, blockade of this system by atropine or of both autonomic systems by a ganglionic blocking agent will result in pupillary dilation and a loss of accommodative capacity.
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