Final answer:
A fixed and dilated pupil in a head-injured patient typically indicates a problem with the oculomotor nerve or increased intracranial pressure leading to brain herniation, which is a medical emergency.
Step-by-step explanation:
A fixed and dilated pupil on a head-injured patient is often a sign of a serious condition involving the oculomotor nerve (cranial nerve III) or pressure within the skull that affects the brain structures. When the sympathetic system is activated, as in the case of trauma, norepinephrine binds to adrenergic receptors in the radial fibers of the iris smooth muscle causing pupillary dilation (mydriasis). Injuries may cause increased intracranial pressure, leading to herniation where the brain compresses against the oculomotor nerve, resulting in a fixed and dilated pupil.
This type of pupil presentation can also be triggered by administration of drugs like phenylephrine, which mimics the action of norepinephrine. However, in the context of head injury, the critical concern is increased intracranial pressure or direct trauma to the brain and oculomotor nerve. Knowledge of the pupillary light reflex is essential in assessing such conditions; a lack of response to light (direct reflex lost) may indicate damage to the optic nerve or the brain structures that control eye movement and pupil size.