Nervous Coordination and Reflexes

Detecting changes and coordinating responses

The nervous system enables humans to react to their surroundings and coordinate their behaviour. It connects the structures that detect changes with the structures that act on them.

A stimulus is a change that can be detected, such as pressure from a sharp object touching the skin. Receptors are specialised cells or groups of cells that detect stimuli. Information from receptors travels as electrical impulses along nerve cells called neurones.

The central nervous system (CNS) consists of the brain and spinal cord. It acts as the coordinator: it receives information and coordinates a response by sending impulses to an effector. Effectors are muscles or glands. A muscle responds by contracting; a gland can respond by secreting a hormone.

The overall sequence is:

Stimulus → receptor → coordinator → effector → response

These stages have different jobs. The receptor detects the change, the coordinator links that information to an appropriate action, and the effector carries out the action.

Neurones connect receptors, the CNS and effectors

There are three important types of neurone. Sensory neurones take impulses from receptors to the spinal cord and brain. Relay neurones, inside the CNS, pass impulses from one neurone to another. Motor neurones carry impulses from the CNS to .

Their arrangement allows information to travel into the CNS and instructions to travel out to the appropriate muscles or glands. For example, a sensory neurone can bring information from the skin to the spinal cord, while a motor neurone carries an impulse from the spinal cord to a muscle.

Neurones have a cell body containing the nucleus and long extensions that carry information. A long axon allows an impulse to travel over a considerable distance along one cell. This reduces the number of transfers needed between cells. Many axons also have a fatty myelin sheath, which insulates them and speeds up impulse transmission. These features help the nervous system coordinate responses quickly, even when receptors and effectors are far apart.

A withdrawal reflex protects the body

A reflex action is automatic and rapid: it happens without a conscious decision. Its pathway is called a reflex arc. The pain withdrawal reflex helps minimise injury by moving a body part away from something harmful.

If a foot touches a sharp pin, receptors in the skin detect the stimulus. An electrical impulse travels along a sensory neurone to the spinal cord. There, the signal passes to a relay neurone and then to a motor neurone. The motor neurone carries an impulse to a muscle in the leg. The muscle contracts, pulling the foot away from the pin.

In this example, the spinal cord is the coordinator, the muscle is the effector and withdrawal of the foot is the response. The pathway does not wait for conscious processing in the brain, so the response is rapid. Information can also reach the brain, making the person aware of what has happened, but conscious awareness is not needed to begin the withdrawal.

Synapses transfer signals between neurones

The tiny gap where two neurones meet is called a synapse. An electrical impulse cannot simply cross this gap. Instead, chemicals transmit the signal.

When an electrical impulse reaches the end of the first neurone, it causes chemicals called neurotransmitters to be released. They diffuse across the gap and bind to the membrane of the next neurone, stimulating a new electrical impulse in it.

The signal is therefore electrical along each neurone, but chemical across the synapse. In the withdrawal reflex, this transfer occurs between the sensory and relay neurones, and again between the relay and motor neurones.

Coordination pathway

Stimulus → receptor → coordinator → effector → response

• CNS: brain and spinal cord.
• Sensory neurone: receptor → CNS.