Biology · The Nervous System
Sensory Receptors and Neurones
Your body is always collecting information and acting on it. This lesson follows a message from a sensory receptor, along sensory, relay and motor neurones, to an effector, and shows how the structure of a neurone suits its job.
Learning Objectives
- 1Describe the path taken by information from a stimulus to a response.
- 2Explain the function of sensory receptors, and name the receptors that detect different stimuli.
- 3Describe the structure of a neurone, including the dendron, axon and myelin sheath, and explain how it is adapted to carry electrical impulses.
- 4Compare the functions of sensory, relay and motor neurones.
Retrieval practice
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1
What is a specialised cell?
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A cell with a special structure that suits it to doing one particular job.
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2
What do we call a group of similar cells working together?
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A tissue.
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3
What is a cell membrane?
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The outer layer of a cell that controls which substances enter and leave it.
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4
Why does a muscle cell contain many mitochondria?
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Mitochondria release the energy for contraction by respiration.
Detecting and responding
A change in your surroundings, such as a loud noise, a sudden bright light or a hot object, is called a stimulus. To survive, an animal has to detect stimuli and respond to them quickly. In humans this job is done by the nervous system. It carries information as tiny electrical impulses along nerve cells called neurones, so messages travel from one part of the body to another in a fraction of a second. The brain and the spinal cord together make up the central nervous system (CNS), which receives the information and decides what to do.
Key words for the nervous system
Every nervous response uses the same set of parts, so learn what each one does.
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Stimulus
A change in the environment, inside or outside the body, that can be detected, such as light, sound, heat, pressure or a chemical.
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Receptor
A cell or group of cells that detects a stimulus and turns it into an electrical impulse.
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Central nervous system
The brain and spinal cord. It receives impulses from receptors, coordinates the response and sends out impulses to effectors.
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Effector
A muscle or a gland that carries out the response. Muscles contract and glands secrete chemicals such as hormones or enzymes.
From stimulus to response
Every response, from blinking to answering a question, follows the same route.
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1
Stimulus
A change in the surroundings is detected.
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2
Receptor
The sensory receptor converts the stimulus into an electrical impulse.
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3
Sensory neurone
The impulse travels along a sensory neurone towards the central nervous system.
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4
Central nervous system
The brain or spinal cord coordinates the response, using relay neurones.
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5
Motor neurone
The impulse is sent along a motor neurone away from the central nervous system.
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6
Effector
A muscle or a gland receives the impulse.
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7
Response
The muscle contracts or the gland secretes a substance.
The path of a nervous message
Information flows in one direction, from receptor to effector.
Sensory receptors
Each type of receptor responds to one kind of stimulus and is found in a particular organ.
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Light receptors
Rod and cone cells in the retina of the eye detect light and send impulses to the brain along the optic nerve.
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Sound and balance receptors
Receptor cells in the ear detect sound vibrations and changes in the position of the head.
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Touch, pressure, temperature and pain receptors
These are found in the skin. Each type responds only to its own stimulus, so a cold receptor is not triggered by pressure.
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Chemical receptors
Receptors in the nose detect chemicals in the air (smell) and receptors on the tongue detect chemicals in food (taste).
The three types of neurone
Neurones are specialised cells. The information passes through three kinds of them in turn.
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Sensory neurone
Carries impulses from a receptor to the central nervous system. A long dendron brings the impulse to the cell body, which lies to one side, and a short axon takes it into the CNS.
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Relay neurone
Lies completely inside the CNS (the brain or the spinal cord) and passes impulses from sensory neurones to motor neurones. It is short with many fine branches, so it can connect to several neurones.
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Motor neurone
Carries impulses from the CNS to an effector. Its cell body is in the CNS, and a very long axon runs out of the spinal cord all the way to a muscle or a gland.
A motor neurone
The long axon carries the impulse away from the cell body towards the muscle.
How a neurone is adapted to its job
The structure of a neurone is directly related to its function, which is a common exam question.
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Long axon or dendron
Neurones can be up to a metre long. This lets one cell carry an impulse over a long distance, for example from the spinal cord to the foot.
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Myelin sheath
A fatty layer around the axon or dendron. It insulates the neurone and makes the impulse travel much faster, up to about 100 metres per second.
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Branched ends
Dendrites at the start and axon terminals at the end connect with many other neurones, so the information can be shared and spread.
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Carries electrical impulses
The membrane of a neurone can carry an electrical signal, so the information moves along it extremely quickly.
Sensory and motor neurones
Sensory neurone
- Carries impulses from a receptor to the CNS
- Cell body to one side of the long dendron and axon
- Starts at a receptor in an organ such as the skin or eye
- Ends in the spinal cord or brain, at a relay neurone
Motor neurone
- Carries impulses from the CNS to an effector
- Cell body at the start of the neurone, inside the CNS
- Starts at a relay neurone in the spinal cord or brain
- Ends at a muscle or a gland, which carries out the response
Case study
Multiple sclerosis
In multiple sclerosis (MS) the body's own immune system attacks the myelin sheath that covers neurones in the brain and spinal cord. Without proper insulation, impulses travel more slowly or become scrambled and lost. People with MS can have blurred vision, muscle weakness, loss of balance and fatigue. It shows how important the myelin sheath is to the speed of messages in the nervous system.
Summary and exam focus
- A stimulus is a change in the environment, which a receptor detects and converts into an electrical impulse.
- Impulses travel along sensory neurones to the CNS, through relay neurones, and along motor neurones to an effector.
- Effectors are muscles, which contract, or glands, which secrete.
- A neurone has a cell body, a dendron or dendrites, an axon and, in many cases, a myelin sheath.
- The myelin sheath insulates the neurone and speeds up the impulse.
Exam focus
Describe how a sensory neurone, a relay neurone and a motor neurone work together when a person touches something sharp, and explain how the structure of a neurone helps it to do its job. (6 marks) (6 marks)
Put the pathway in order using the key words receptor, sensory neurone, relay neurone, motor neurone and effector. Then link a named structure, such as the myelin sheath or the long axon, to a function, such as speeding up impulses or covering long distances.
Key terms
The words this lesson expects you to use. Each one is linked from the first place it appears above.
- Stimulus
- A change in the environment that a receptor can detect.
- Receptor
- A cell that detects a stimulus and turns it into an electrical impulse.
- Neurone
- A nerve cell, specialised to carry electrical impulses around the body.
- Central nervous system
- The brain and the spinal cord.
- Sensory neurone
- A neurone that carries impulses from a receptor to the central nervous system.
- Relay neurone
- A neurone inside the central nervous system that connects sensory neurones to motor neurones.
- Motor neurone
- A neurone that carries impulses from the central nervous system to an effector.
- Effector
- A muscle or a gland that responds to the impulses sent by the nervous system.
- Dendron
- A long extension of a neurone that carries an impulse towards the cell body.
- Axon
- A long extension of a neurone that carries the impulse away from the cell body.
- Myelin sheath
- A fatty layer around an axon or dendron that insulates it and speeds up the impulse.
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