How does the nervous system help us respond? - OCR 21st CenturyReflex arc

Reflex actions

There are three main types of neuron: sensory, motor and relay.

These different types of neurons work together in a reflex action. A reflex action is an automatic (involuntary) and rapid response to a stimulus, which minimises any damage to the body from potentially harmful conditions, such as touching something hot. Reflex actions are therefore essential to the survival of many organisms.

A reflex action follows this general sequence and does not involve the conscious part of the brain. This is why the response is so fast.

Reflex arcs

The nerve pathway followed by a reflex action is called a reflex arc. For example, a simple reflex arc happens if we accidentally touch something hot.

1. Receptor in the skin detects a stimulus (the change in temperature).
2. Sensory neuron sends electrical impulses to a relay neuron, which is located in the spinal cord of the CNS. Relay neurons connect sensory neurons to motor neurons.
3. Motor neuron sends electrical impulses to an effector.
4. Effector produces a response (muscle contracts to move hand away).

Organisms are able to modify a reflex action and overcome it, but this uses the brain and has to be learnt. For example, keeping hold of a hot object requires a nerve impulse to be sent to the motor neuron of the reflex arc to interfere with the normal reflex action to drop the object.