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How does the iris help regulate the amount of light reaching the retina?
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Photoreception
The mammalian eye has a cornea and a lens and functions as a dioptric system, in which light rays are refracted to focus on the retina.
The iris regulates the amount of light reaching the retina by controlling the size of the pupil, which acts as an aperture for the eye. This process is managed by two sheets of smooth muscle within the iris that work in opposition.12
- Constriction: In bright light, the circular sphincter muscle shortens, constricting the pupil to limit the amount of light entering the eye. This response, known as the light reflex, helps protect the retina and increases the depth of focus.12
- Dilation: In low-light conditions, the radially oriented dilator muscle shortens, expanding the pupil to allow more light to enter. This dilation can also be triggered by strong psychical stimuli, such as fear or pain.12
Changes in pupil size typically involve reciprocal innervation, where the activation of one muscle is accompanied by the inhibition of the other.1