Towards the front of the eye, the muscles of the iris (colored portion of the eye) control the amount of light entering the eye. Pigments in the iris give it color. The iris represents the anterior portion of the uveal tract. The iris is made up of smooth muscle fibers that adjust pupil size.
The pupil (black aperture of the eye) is not a structure at all; rather, it is an opening created by the circular iris. Its blackness is due to the lack of reflection of light from within the eye. The pupil allows light into the eye.
Control of the Iris
The iris is composed of a pigmented epithelium, the anterior muscle, and the dilator muscle. The sphincter and dilator are both smooth muscles.
The sphincter muscle contracts in response to parasympathetic stimulation, while the dilator contracts in response to sympathetic stimulation. Changes in the amount of light falling on the retina stimulate or inhibit each pathway:
In bright light, the sphincter muscle contracts while the dilator muscle relaxes, closing down the pupil and making the aperture smaller.
In dim light, the sphincter muscle relaxes while the dilator muscle contracts, opening up the pupil and making the aperture larger.
This opening and closing action controls the amount of light passing through the iris and falling on the retina to optimize illumination of the photoreceptors.
Dilation can also be caused by excess sympathetic stimulation of the dilator muscle. Numerous stimulants, including nicotine, caffeine, and amphetamines have this effect.
Nice to Know
The neurotransmitter of the parasympathetic nervous system is , which binds to acetylcholine receptors on the smooth muscle it stimulates.
The pupil may be artificially dilated or constricted by drugs which block or promote either dilator or sphincter function. Drugs that cause: