8.2—Nervous system control of effectors

The basic pathway is stimulus → receptor → sensory neurone → CNS → motor neurone → effector. The CNS integrates information and the effector produces a response through muscle contraction or gland secretion.

Bright light activates retinal receptors; impulses reach the CNS and motor neurones stimulate circular iris muscles so the pupil constricts. In low light, radial muscles contract and the pupil dilates.

Antagonistic effectors allow opposite outcomes, while a rapid neural pathway limits exposure to harmful conditions.

A receptor detects a change but does not itself perform the response. Keep sensory input, CNS coordination and effector action distinct.