The nervous system allows the body to respond to changes in the environment in a process usually coordinated by the brain. Reflex actions are extra-rapid responses to stimuli; this process also involves the nervous system but bypasses the brain.

Most animal cells have a nucleus, cytoplasm and cell membrane. Light receptors have these cell components too.
The human central nervous system (CNS) consists of the brain and spinal cord. When a receptor is stimulated it sends a signal along the nerve cells, also called neurones, to the brain. The brain then coordinates the response.

When a receptor is stimulated it sends a signal to the central nervous system, where the brain coordinates the response, but sometimes a very quick response is needed, one that does not involve the brain: this is a reflex action.

This is what happens:
receptor detects a stimulus - a change in the environment
sensory neurone sends impulses to relay neurone
motor neurone sends impulses to effector
effector produces a response.