Initiating Pupillary Reflexes

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Initiating pupillary reflexes. Pupils equal, round, reactive to light and accommodation is a phrase often used by clinicians when testing pupillary function. Abnormal pupillary function can indicate brainstem trauma, stroke, or drug abuse. This video will describe the mechanism for pupil constriction and dilation, list the autonomic reflex components, discuss the pupillary light reflex pathway, and demonstrate the procedure for testing the pupillary light reflex. The iris is the colored part of the eye. It contains smooth muscle that reflexively alters the amount of light that enters the eye by altering the diameter of the pupil. It contains two sets of smooth muscle: the sphincter pupillae and the dilator pupillae. In dim light, sympathetic stimulation causes contraction of the dilator pupillae muscle, resulting in pupil dilation. As pupil size increases, more light enters the eye. With near vision and in bright light, parasympathetic stimulation causes pupil constriction by contracting the sphincter pupillae. Shining light into the eye activates the pupillary light reflex. Which branch of the autonomic nervous system predominates during this reflex: the sympathetic, parasympathetic, or both branches? The sphincter pupillae muscle contracts to protect the eye from too much light. Reflexes are classified functionally into two main categories. Those that activate skeletal muscle are somatic reflexes. And those that activate smooth muscle, cardiac muscle, or a gland are autonomic reflexes, also known as visceral reflexes. Visceral reflexes have five elements. Briefly, they are a receptor, a sensory neuron, an integration center, a two-neuron chain that comprises the motor neuron, and an effector. For the pupillary light reflex, light is the stimulus. The receptors are the photoreceptors in the retina. The visceral sensory neurons are the afferent fibers of the optic nerve, cranial nerve II. The integration center is located in the midbrain. The motor neuron consists of preganglionic parasympathetic fibers that precede the ciliary ganglion, and postganglionic parasympathetic fibers. Both parasympathetic fibers are found in the oculomotor nerve, cranial nerve III. The visceral effector is the sphincter pupillae. Classify the pupillary reflex. What category of reflex is it, and which type of muscle does it activate: somatic reflex, smooth muscle; visceral reflex, skeletal muscle; or visceral reflex, smooth muscle? Somatic reflexes have skeletal muscle effectors. Visceral reflexes have cardiac muscle, smooth muscle, and glands as effectors. In order to test the pupillary light reflex, find an area where the lighting is dim. A pen light is a small flashlight used for the light source. You also need a metric ruler to measure the diameter of the pupil. First, measure the diameter of each pupil without light shining on the eyes. Stand to the left of the subject, and have the subject shield his or her right eye with a hand between the eye and the right side of the nose. Shine the light into the subject's left eye and observe the result in both the left eye and the right eye. A consensual response is when one side of the body is stimulated and the other side of the body responds. In the lab, you will observe both the pupillary reflex and the consensual reflex.