Biology

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. 

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 calledneurones, to the brain. The brain then coordinates the response.

Neurones are nerve cells that carry information as tiny electrical signals. There are three different types of neurones, each with a slightly different function:

• sensory neurones carry signals from receptors to the spinal cord and brain
• relay neurones carry messages from one part of the CNS to another 
• motor neurones carry signals from the CNS to effectors.

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.

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.

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.

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 cycle. Hormones can be used to control human fertility and have advantages and disadvantages.

The internal environment of the body is controlled by the nervous system and hormones. The maintenance of a constant internal environment is calledhomeostasis.

Water content is controlled by water loss from:

• the lungs - when we exhale
• the skin - through sweating
• passing urine - produced by the kidneys.
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