Final answer:
The afferent limb of the pupillary reflex transmits sensory information from the retina to the brain via the optic nerve. It does not directly control the constriction or dilation of the pupil; such actions are part of the efferent limb involving the parasympathetic and sympathetic nervous systems.
Step-by-step explanation:
The functions of the afferent limb of the pupillary reflex primarily include transmitting sensory information to the brain. When light hits the retina, photosensitive cells trigger a signal that is carried through the optic nerve to the pretectal nucleus in the brain. This sensory input is the afferent limb of the reflex, signaling that environmental light levels have changed which necessitates an adjustment in pupil size.
The activation of the retina by light does not itself control pupil constriction or dilation; this control is part of the reflex's efferent limb. Upon receiving the signal from the afferent limb, the efferent limb, which involves the parasympathetic nervous system, activates to constrict the pupil through the oculomotor nerve, postganglionic fibers, and ciliary ganglion affecting the circular fibers of the iris. Conversely, under low light conditions, the sympathetic nervous system acts through different pathways to dilate the pupil, allowing more light to enter the eye.