Coordination and response

In a complex organism like a mammal, millions of cells must work together. This is achieved through coordination. The body has two main coordination systems: the nervous system and the endocrine system. The nervous system uses electrical impulses travelling along specialised cells called neurones for rapid, short-lived responses, like pulling your hand away from a hot object. The endocrine system uses chemical messengers called hormones, which travel in the bloodstream to produce slower, longer-lasting responses, such as growth and development. While they work differently, both are essential for responding to changes in the internal and external environment.

A neurone, or nerve cell, is the basic unit of the nervous system. Its structure is highly adapted for its function of transmitting electrical impulses over long distances. A typical neurone has a cell body containing the nucleus, and extensions called dendrites that receive signals from other neurones. A long fibre called an axon carries the impulse away from the cell body. Many axons are covered in a fatty myelin sheath, which acts as an electrical insulator and speeds up the impulse transmission by making the impulse 'jump' between gaps. At the end of the axon are axon terminals, which pass the signal to the next neurone or to an effector, such as a muscle or gland.

A reflex action is an immediate, involuntary response to a stimulus, which serves a protective function. It happens without conscious thought. The pathway taken by the nerve impulses in a reflex is called a reflex arc. It starts when a receptor (e.g., pain receptor in the skin) detects a stimulus (e.g., a sharp pin). An electrical impulse travels along a sensory neurone to the central nervous system (CNS). In the CNS (usually the spinal cord), the impulse is passed across a synapse to a relay neurone, and then across another synapse to a motor neurone. The motor neurone carries the impulse to an effector (e.g., a muscle in the arm), which contracts to produce the response (e.g., pulling the hand away).

The central nervous system (CNS) consists of the brain and the spinal cord. It is the body's main processing and integration centre. It receives information from sense organs (via sensory neurones), processes this information, and sends out instructions to effectors (via motor neurones). Actions can be categorised as either voluntary or involuntary. Voluntary actions are the result of conscious thought and decision-making by the brain, such as deciding to pick up a book. You have a choice. Involuntary actions, like reflexes or the beating of your heart, are not under conscious control. They are automatic. The CNS coordinates both types of action to ensure the body responds appropriately to its environment.

The endocrine system is the body's second coordination system, specialising in slower, long-term control. It consists of several endocrine glands, which are 'ductless' glands that secrete chemical messengers called hormones directly into the bloodstream. The blood transports these hormones all over the body. However, a hormone only affects specific cells or organs, known as target organs. This is because the cells of the target organ have specific receptor proteins on their surface that match the shape of the hormone molecule, like a lock and key. When the hormone binds to the receptor, it triggers a response in the cell. Examples of endocrine glands include the pituitary gland, adrenal glands, and pancreas.

Homeostasis is the essential process of maintaining a constant internal environment, despite changes in external conditions. This includes keeping factors like body temperature, blood glucose levels, and water content within a narrow, stable range. The body achieves this using a control mechanism called negative feedback. A negative feedback loop works to counteract any change from the normal 'set point'. The process involves: 1) A receptor detects a change (e.g., body temperature rises). 2) The coordination centre (e.g., the brain) processes this information and organises a response. 3) An effector carries out the response to reverse the change (e.g., sweating to cool down). 4) The condition returns to normal, and the feedback loop is switched off.