Coordination and response

In a complex organism like a mammal, millions of cells and many organs must work together. This is called coordination. It ensures everything happens at the right time and rate to meet the body's needs. Mammals have two coordination systems: the nervous system and the endocrine system. The nervous system provides fast, short-lived responses using electrical impulses, perfect for quick reactions. The endocrine system handles slower, longer-lasting changes, like growth, using chemical messengers called hormones that travel in the blood.

The nervous system has two main parts. The Central Nervous System (CNS) consists of the brain and spinal cord, acting as the main processing centre. The Peripheral Nervous System (PNS) is a network of nerves that connect the CNS to the rest of the body. The entire system is built from specialised cells called neurones. A typical motor neurone has a cell body with a nucleus, dendrites to receive signals, and a long axon to carry the signal away. The axon is often covered by a fatty myelin sheath, which insulates the neurone and dramatically speeds up the electrical impulse. Sensory neurones carry signals from receptors to the CNS, while motor neurones carry signals from the CNS to effectors (muscles or glands).

A reflex is an involuntary and rapid response to a stimulus. It happens without conscious thought, which makes it extremely fast and ideal for protecting the body from harm. The pathway taken by the nerve impulses in a reflex is called a reflex arc. The sequence is always the same: 1. Stimulus (e.g., a pinprick). 2. Receptor (e.g., a pain receptor in the skin) detects the stimulus. 3. Sensory neurone transmits an impulse to the CNS. 4. Relay neurone in the CNS (usually the spinal cord) passes the impulse from the sensory to the motor neurone. 5. Motor neurone transmits an impulse to an effector. 6. Effector (e.g., a muscle) carries out the response (e.g., contracting to pull the hand away).

Neurones do not physically touch each other. There is a microscopic gap between the axon terminal of one neurone and the dendrite of the next, called a synapse. Electrical impulses cannot jump this gap. Instead, the signal is transmitted chemically. When an electrical impulse arrives at the end of the first neurone (the presynaptic neurone), it triggers the release of chemicals called neurotransmitters from tiny sacs. These neurotransmitters diffuse across the synaptic gap and bind to specific receptor molecules on the membrane of the next neurone (the postsynaptic neurone). This binding triggers a new electrical impulse in the second neurone, continuing the message. The neurotransmitters are then quickly broken down or reabsorbed to prevent continuous stimulation.

The endocrine system is the body's second control system, specialising in slower, long-term regulation. It consists of several endocrine glands located around the body. These are 'ductless' glands, meaning they secrete their products directly into the bloodstream. The products are chemical messengers called hormones. Hormones travel in the blood plasma to all parts of the body, but they only affect specific 'target organs' or 'target cells'. This is because the cells of the target organ have specific receptor proteins on their surface or inside them, which have a shape complementary to the hormone molecule, like a lock and key. Cells without the correct receptor are not affected. Examples of endocrine glands include the pituitary gland, thyroid, adrenal glands, pancreas, and ovaries/testes.