Pupils reacts to light, narrowing in bright light and widening in poor light - so is a reflex action.
The stimulus in the pupil reflex would be light
The constriction of pupils in response to bright light is called the pupillary light reflex. If the light is shining directly into one eye, then the pupil in that eye will constrict (a direct response), but so will the pupil in the non-illuminated eye (a consensual response).This reflex involves two cranial nerves: the optic nerve, which senses the light, and the oculomotor nerve, which constricts both pupils. It is considered involuntary since you don't think about it.
The pupil dilates when stimulated by the pinching on the nape of the neck.
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The consensual reflex of the pupil is the simultaneous constriction of the pupil in both eyes when light is shone into one eye. This reflex is controlled by the autonomic nervous system to regulate the amount of light entering the eye and protect the retina.
The difference between reflex and voluntary action is that a reflex occurs naturally in response to some sort of stimulus and voluntary action is something that is consciously carried out. An example of a reflex action is gagging and an example of voluntary action is flinching when someone is going to hit you.
When a reflex occurs in response to a specific stimulus, the brain can reinforce associated voluntary behaviors that follow the reflex. Through repetition and reinforcement, the brain can learn to anticipate and control the reflex response, turning it into a voluntary learned behavior. This process involves the formation of new neural pathways and connections that link the reflex with the voluntary behavior.
The difference between a reflex and a voluntary muscle is simply that a reflex is autonomic response to a stimulus thus making it involuntary. This in direct contradiction to a voluntary muscle movement with is done by making the conscious decision to do so.
A reflex
The oculomotor nerve (cranial nerve III) is responsible for the reflex constriction of the pupil in response to light and accommodation.
The patellar reflex is enhanced by voluntary muscle activity in the quadriceps. When you contract your quadriceps, it stretches the patellar tendon, causing a stronger reflex response when the knee is tapped.
Reflexes such as blinking and pupil reflex are centered in the brainstem. The trigeminal nerve is responsible for the blinking reflex, while the pupillary reflex is controlled by the oculomotor nerve. These reflexes help protect the eyes from harm and regulate the amount of light entering the eye.