Coordination and Response

Living organisms must constantly respond to changes in their internal and external environments to survive. These changes are known as stimuli (singular: stimulus). The process of detecting a stimulus and carrying out an appropriate action is called coordination and response. This is vital for maintaining a stable internal environment, a state known as homeostasis. In humans, coordination is achieved by two main systems: the nervous system and the endocrine system.

### The Nervous System

The nervous system provides a rapid, short-lived response to stimuli. It is a complex network of specialised cells called neurones (or nerve cells) that transmit information as electrical signals called nerve impulses.

A typical neurone consists of a cell body containing the nucleus, extensions called dendrites that receive signals, and a long fibre called an axon that transmits the signal away from the cell body. Many axons are covered by a fatty myelin sheath, which insulates the axon and speeds up the transmission of the nerve impulse.

There are three main types of neurones:

The junction between two neurones is called a synapse. As the electrical impulse cannot cross this gap, the signal is transmitted chemically. When an impulse reaches the end of an axon, it triggers the release of chemical messengers called neurotransmitters. These diffuse across the synapse and bind to the next neurone, starting a new electrical impulse.

#### The Reflex Arc

A reflex action is an involuntary, rapid response to a stimulus that serves a protective function, such as pulling your hand away from a hot object. The pathway taken by the nerve impulses during a reflex is called a reflex arc.

The sequence is as follows:

### The Endocrine System

The endocrine system provides slower, longer-lasting control by means of chemical messengers called hormones. Hormones are produced by endocrine glands and secreted directly into the bloodstream. The blood transports them all over the body, but they only affect specific target cells or organs that have the correct receptors.

Key endocrine glands and their hormones include:

#### Homeostasis: Blood Glucose Control

Maintaining a constant blood glucose level is critical for providing cells with a steady supply of energy. This is a prime example of a negative feedback mechanism:

In summary, the nervous and endocrine systems work in a coordinated fashion. The nervous system handles immediate, rapid responses, while the endocrine system manages slower, more prolonged adjustments, both ensuring the body can effectively respond to its environment and maintain a life-sustaining internal balance.