The Human Nervous System » The Nervous System and Reflexes

What the nervous system does

Touch something very hot and your hand jerks away before you even think about it. Hear a car horn and you turn your head. In both cases your body has noticed a change and reacted to it. That is the job of the nervous system.

The nervous system enables humans to react to their surroundings and to coordinate their behaviour. It works by sending very fast messages around the body.

Key terms:

• Nervous system: the system that lets you react to your surroundings and coordinate your behaviour.
• Neurone: a nerve cell that carries information as electrical impulses.
• Electrical impulse: the signal that passes along a neurone.
• Central nervous system (CNS): the brain and spinal cord.

Information from receptors passes along neurones as electrical impulses to the CNS. The CNS then coordinates the response of effectors.

From stimulus to response

Remember, a stimulus is a change in the surroundings and a receptor detects it. Every nervous reaction follows the same chain of five steps.

The coordinator is the CNS, the brain and spinal cord. It receives the impulses from the receptors and decides what should happen. It then sends impulses to the effectors.

How the nervous system is adapted to its function

You need to be able to explain how the structure of the nervous system suits what it does.

• Receptors detect changes, so the CNS is told about the surroundings.
• Neurones carry information as electrical impulses. Impulses travel very quickly, so messages arrive in a tiny fraction of a second.
• The CNS is one central place that receives all the information and coordinates the response, so the body acts in a sensible, organised way.
• Effectors are muscles and glands that carry out the response.

Sensory and motor neurones can be very long, so an impulse can travel from your toe to your spinal cord without being passed on many times. Relay neurones are short and stay inside the CNS.

The reflex arc

Some responses do not involve thinking. These are reflex actions. The pathway an impulse takes in a reflex is called a reflex arc.

Key terms:

• Reflex action: an automatic and rapid response that does not involve the conscious part of the brain.
• Reflex arc: the pathway of neurones that carries the impulse in a reflex action.
• Sensory neurone: carries impulses from a receptor to the CNS.
• Relay neurone: carries impulses within the CNS, from the sensory neurone to the motor neurone.
• Motor neurone: carries impulses from the CNS to an effector.
• Synapse: the tiny gap where two neurones meet.

Here is the reflex arc for pulling your hand away from a hot pan:

1. Receptors in your skin detect the heat (the stimulus).
2. A sensory neurone carries impulses from the receptor to the spinal cord.
3. At a synapse the signal passes to a relay neurone in the spinal cord.
4. At a second synapse the signal passes to a motor neurone.
5. The motor neurone carries impulses to a muscle in your arm, the effector.
6. The muscle contracts and your hand is pulled away (the response).

The signal goes to the spinal cord, not to the conscious part of the brain, so the reaction is very quick.

How each structure relates to its function

Why reflexes are important

Reflex actions are automatic and rapid. They do not involve the conscious part of the brain, so you do not have to decide to do them. That makes them important in two ways.

• Protection: pulling away from something hot or sharp quickly limits the damage.
• Speed: the short pathway, with no thinking time, means the response is very fast.

Using data about the nervous system

You may be asked to extract and interpret data from graphs, charts and tables. You may also need to translate information about reaction times between numerical and graphical forms.

Reaction time is the time between a stimulus and the response. Here is a table of one student's results.

• Attempt 1: 0.25 s
• Attempt 2: 0.20 s
• Attempt 3: 0.21 s
• Attempt 4: 0.22 s

Now compare two responses. A reflex takes 0.05 s and a response that you think about takes 0.25 s. The reflex is 0.25 ÷ 0.05 = 5 times quicker.

How do you plan a reaction time investigation? That is the next lesson, Required Practical: Reaction Time.

Exam-style question

A person touches a hot plate and pulls their hand away.

(a) Name the type of response shown. [1 mark]

(b) Put these in the correct order: motor neurone, receptor, relay neurone, effector, sensory neurone. [2 marks]

(c) Explain why this response is important. [2 marks]