Biology The Nervous System

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  • Created by: Zhraa
  • Created on: 14-04-14 16:53

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.


Receptors are groups of specialised cells that can detect changes in the environment called stimuli. Receptors are often located in the sense organs, such as the ear, eye and skin. Each organ has receptors sensitive to particular kinds of stimulus. This table shows some receptors and the stimuli they detect.

Receptors and stimuli

Receptors found in                       Receptors sensitive to

Eyes                                           light

Ears                                            sound and position of the head

Tongue                                        chemicals in food

Nose                                           chemicals in the air

Skin                                             touch, pressure, pain and temperature


Most animal cells have a nucleuscytoplasm 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.

Reflex actions

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.


In a simple reflex action:

stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector

An effector is any part of the body that produces the response. Here are someexamples of effectors:

  • a muscle contracting
  • a gland releasing (secreting) a hormone or other chemical.

Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away from a hot flame without thinking about it. This animation allows you to step through each stage of the reflex arc.

This is what happens:

  1. receptor detects a stimulus - a change in the environment
  2. sensory neurone sends impulses to relay neurone
  3. motor neurone sends impulses to effector
  4. effector produces a response.

Control in the human body

Hormones are chemical substances that regulate processes in the body. Hormones are secreted by glands and travel to their target organs in the bloodstream. Several hormones are involved in the female menstrual


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