Irisreflexe

Irisreflexe, also known as the pupillary light reflex, is an involuntary reflex that controls the diameter of the pupil in response to light. It is a critical component of the visual system, ensuring that the amount of light entering the eye is regulated to optimize vision and protect the retina.

The reflex is mediated by two muscles within the iris: the sphincter pupillae and the dilator pupillae. When exposed to bright light, the sphincter pupillae muscle constricts, causing the pupil to become smaller (miosis). Conversely, in dim light, the dilator pupillae muscle contracts, making the pupil larger (mydriasis).

The afferent pathway of the reflex begins with photoreceptor cells in the retina, which detect the light intensity. Signals are then transmitted via the optic nerve to the pretectal nucleus in the midbrain. From there, signals are relayed to the Edinger-Westphal nucleus, which sends parasympathetic fibers through the oculomotor nerve (cranial nerve III) to the iris. These fibers innervate the sphincter pupillae muscle, causing it to constrict.

The efferent pathway for dilation involves sympathetic nerve fibers originating from the hypothalamus, descending through the brainstem and spinal cord, and then traveling to the dilator pupillae muscle.

Testing the iris reflex involves shining a light into each eye and observing the pupil's constriction. A direct response occurs when the pupil of the illuminated eye constricts, while a consensual response occurs when the pupil of the opposite eye also constricts. Abnormalities in the iris reflex can indicate neurological damage or other medical conditions affecting the cranial nerves or autonomic nervous system.