What cranial nerve is used for pupillary constriction?

The Third Oculomotor Nerve.

Pupil constriction is also known as the pupillary reflex or miOSis. It naturally occurs as the result of signals sent from the brain down the oculomotor nerve.

There are two main nervous systems in the body, the Central Nervous System and the Peripheral Nervous System. The Central Nervous System consists of the brain and the spinal cord. All other nerves, including the oculomotor nerve,which is made up of twelve paired cranial nerves, are part of the peripheral nervous system.

These twelve paired nerves are made of what are called efferent nerve fibers, the kind that move information away from the brain instead of towards it (afferent nerve fibers move signals toward the brain). All nerves in the peripheral system have a designation of either afferent (moving toward the brain with information) or efferent (taking signals from the brain to cause responses elsewhere in the body).

There are three different cranial nerves that affect eye movements: the third cranial nerve, the fourth and the sixth. It is the third oculomotor nervethat carries the signal from the brain to the muscle that contracts to constrict the pupil, hold the eyelid open, and also to signal most, but not all, of the movements of the eye.

There are two main triggers for the brain to send information down the oculomotor III nerve with a command to constrict the pupil:

* Bright light shining in the eye An eye is an organ that detects light. When bright light is shone on the eye, it will send a signal up to the brain by way of an afferent nerve. The brain will in turn send a signal down the third oculomotor nerve, an efferent nerve that carries signals from the brain, to the circular iris sphincter muscle (the ciliary muscle) to make it tighten. When the ciliary muscle tightens it closes the iris opening tight, like a purse string, automatically constricting the pupil to let less light in.

* Accommodation Reflex action

Those same ciliary muscles in each eye can be signaled to constrict the pupil when the brain is triggered by a specific reflex action. This reflex is called Accommodation. It occurs when the eye focuses on a near object, after a distant object and vise versa. The near object (for example, a computer screen) looks big in the field of vision, and the eye receives light from wide angles. When you focus on that near object, the pupil constricts in order to prevent bending light rays from hitting the edges of the retina which would result in a blurry image. As the pupil constricts, the lens becomes more round to allow for the diverging light rays.

Other triggers of pupil constriction are the direct action of miotic drugs and indirect action of opiates

* Actions of drugs

Certain drugs cause constriction of the pupils, such as heroin and other opiates (indirect drug action). Other drugs that also cause pupils to be "pinpoint" are called miotics (direct drug action).