👂 Sensory neurones: in
They carry impulses from receptors in the sense organs into the brain or spinal cord.
Co-ordination and Response » The Central Nervous System and Nerve Impulses
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 2.87, 2.88
- What the central nervous system is made of
- How sense organs are linked to the central nervous system by nerves
- How impulses travel in along sensory neurones and out along motor neurones
- Why this gives rapid responses
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Unlock This CourseThe central nervous system
In the lesson Nervous and Hormonal Communication you saw that the nervous system sends electrical impulses along neurones. Now let us look at how the whole system is laid out.
The central nervous system (CNS) is made of just two parts: the brain and the spinal cord. The brain is inside the skull. The spinal cord runs down the back inside the backbone. The CNS is the co-ordinator in the pathway stimulus, receptor, co-ordinator, effector, response. It receives impulses, and it sends impulses out.
The CNS is linked to the rest of the body by nerves. Nerves reach the sense organs, such as the eyes, ears, nose, tongue and skin. Sense organs contain the receptors that detect stimuli.
Key terms:
- Central nervous system (CNS): the brain and the spinal cord.
- Nerve: a bundle of neurones that links the CNS to other parts of the body.
- Sense organ: an organ, such as the eye or skin, that contains receptors.
- Sensory neurone: a neurone that carries impulses from a receptor into the CNS.
- Motor neurone: a neurone that carries impulses from the CNS to an effector.
Impulses in and impulses out
Stimulating a receptor in a sense organ starts an electrical impulse. The impulse travels along a sensory neurone in a nerve and into the CNS. The CNS then sends impulses out along motor neurones to the effectors. The effectors are muscles and glands. Muscles respond by contracting. Glands respond by releasing a substance.
💪 Motor neurones: out
They carry impulses out of the brain or spinal cord to muscles and glands.
A handy memory trick: Sensory neurones bring Sensations in, and Motor neurones make Muscles move.
Worked example: a ringing phone
- Stimulus: the sound of your phone ringing.
- Receptors: in the ear, which is a sense organ.
- Impulses in: along sensory neurones, in a nerve, to the CNS.
- Co-ordinator: the brain.
- Impulses out: along motor neurones to the muscles in your arm.
- Response: your arm muscles contract and you pick up the phone.
Another example: a gland as the effector
The smell of hot food stimulates receptors in the nose. Impulses go to the CNS along sensory neurones. Impulses then go out along motor neurones to the salivary glands, and your mouth waters.
Why the responses are rapid
Impulses travel along neurones very quickly, and they go straight to the right place. This is why a nervous response is rapid. You can pull your hand away from something hot almost before you realise it hurts. The CNS can also receive impulses from many receptors and send impulses to many effectors, so it can co-ordinate a whole response at once.
Investigating reaction times
Reaction time is how long a response takes after a stimulus. Two suggested practicals are measuring it with a ruler drop test, and comparing human responses to different stimuli. In the ruler drop, one student drops a ruler without warning and a partner catches it. The shorter the distance the ruler falls, the faster the reaction. Repeat several times and find a mean to make the results more reliable.
Common mistakes
- Saying the nervous system is only the brain. The CNS is the brain and the spinal cord.
- Mixing up sensory and motor neurones. Sensory goes in to the CNS. Motor goes out to an effector.
- Saying the receptor is the whole eye or ear. The sense organ contains the receptors.
- Saying muscles send the impulse. Muscles and glands are effectors, so they receive the impulse and respond.
Exam-style question
(a) Name the two parts of the central nervous system. [2 marks]
(b) A cyclist hears a car horn and moves their hand to brake. Describe the path of the impulses from the ear to the muscle. [3 marks]
(c) Explain why this response is rapid. [1 mark]
Model answer
(a) Brain (1) and spinal cord (1).
(b) Impulses travel from the receptors in the ear along sensory neurones (1) to the central nervous system (1), and then out along motor neurones to the muscle in the hand (1).
(c) Impulses travel very quickly along neurones. (1)
Exam tip
In a "describe the path" question, name each stage in order and say whether the neurone is sensory or motor. Writing just "along nerves" will not earn all the marks.