Neurones, reflex arcs and synapses

Last time

Before we start: last time, in Hormones, blood glucose and body temperature, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Faster than thinking

Touch a hot pan and your hand jerks away before you even feel the pain. That is a reflex: a fast, automatic response that protects you. Today: the three types of neurone, the reflex arc, and how an impulse crosses the gap between neurones, the synapse.

The nervous system

The nervous system sends electrical impulses along nerve cells, called neurones. The central nervous system is the brain and spinal cord. It coordinates responses. The peripheral nervous system is the nerves that connect the CNS to the rest of the body.

Three neurones

Sensory neurones carry impulses from receptors to the central nervous system. Relay neurones, inside the CNS, pass impulses from sensory to motor neurones. Motor neurones carry impulses from the CNS to effectors, such as muscles or glands.

The reflex arc

Here is a reflex arc. A receptor in the skin detects the heat. The impulse travels along a sensory neurone to the spinal cord, through a relay neurone, then along a motor neurone to a muscle, which contracts.

The reflex arc in order

Learn the sequence. Stimulus, receptor, sensory neurone, relay neurone, motor neurone, effector, response. The brain is not involved in the response, which is why it is so fast.

Reflexes protect you

Pulling a hand away from a hot or sharp object. Blinking when something comes near the eye. The pupil getting smaller in bright light, which protects the retina.

Synapses

A synapse is a junction between two neurones. There is a tiny gap between them. The electrical impulse cannot jump the gap. So a chemical called a neurotransmitter carries the message across.

How a synapse works

Here are the steps in order. Each one causes the next. Synapses make sure impulses travel in one direction only, because only the first neurone has the vesicles.