Receptors and effectors

Receptors

Receptors are groups of specialised cells. They can detect changes in the environment, which are called stimuli, and turn them into electrical impulses. Receptors are often located in the sense organs, such as the ear, eye and skin. Each organ has receptors sensitive to particular kinds of stimulus.

Different receptors

sense organs	receptors sensitive to
Skin	touch, pressure, pain and temperature
Tongue	chemicals in food
Nose	chemicals in the air
Eyes	light
Ears	sound and position of the head

The central nervous system (CNS) in humans consists of the brain and spinal cord. When a receptor is stimulated, it sends a signal along the nerve cells - neurones - to the brain. The brain then co-ordinates the response.

Effectors

An effector is any part of the body that produces the response. Here are some examples of effectors:

• a muscle contracting to move the arm
• a muscle squeezing saliva from the salivary gland
• a gland releasing a hormonehormone: chemical messengers produced in glands and carried by the blood to specific organs in the body into the blood.

Neurones

Neurones are nerve cells. They carry information as tiny electrical signals. There are three different types of neurones, each with a slightly different function.

1. Sensory neurones carry signals from receptorsreceptors: organs which recognize and respond to stimuli to the spinal cord and brain.
2. Relay neurones carry messages from one part of the CNSCNS: Central Nervous System (brain and spinal cord). to another.
3. Motor neurones carry signals from the CNS to effectorseffectors: organs which have an effect when stimulated (eg muscles or glands).

The diagram below shows a typical neurone - in this case, a motor neurone. It has tiny branches at each end and a long fibre carries the signals.

A motor neurone

Synapses

Where two neurones meet, there is a tiny gap called a synapse. Signals cross this gap using chemicals. One neurone releases the chemical into the gap. The chemical diffuses across the gap and makes the next neurone transmit an electrical signal.

Reflex actions

When a receptor is stimulated, it sends a signal to the central nervous system, where the brain co-ordinates the response. But sometimes a very quick response is needed, one that does not need the involvement of the brain. This response is known as a reflex action.

Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away from a hot flame without thinking about it. The animation below allows you to step through each stage of the reflex arc.

This is what happens:

• Receptor detects a stimulus (change in the environment).
• Sensory neurone sends signal to relay neurone.
• Motor neurone sends signal to effector.
• Effector produces a response.

The eye

The eye is a sense organ that responds to light.

Structure of the human eye

Light enters through the pupil, and is focused by the cornea and the lens onto the retina. The shape of the lens can be changed by the ciliary muscles so that the image always comes to a sharp focus at the retina.

The iris reflex

The eye needs to be able to control the amount of light entering it. In dim conditions, more light is allowed to enter so that a clear image can be formed on the retina. In bright conditions less light is allowed to enter so that the retina is not damaged.

This adjustment is done by two sets of muscles in the iris: its circular muscles contract to close up the iris, making the pupil smaller - while its radial muscles contract to open up the iris, making the pupil larger. You can see how this reflex action works in the animation.

Accommodation

The ability of the lens to change its shape to focus near and distant objects is called accommodation.

How accomodation works

You can see how this reflex action works in the animation.

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The brain - higher

The brain can be damaged by accidents or illness. When this happens, its functions can be impaired. For example, a bang on the head can cause concussion and blurred vision.

Brain tumours

A brain tumour happens when cells in the brain grow and divide uncontrollably or when cancer cells from cancer cells from other parts of the body, such as the lung or the breast, spread to the brain. The tumour damages the brain and causes it to press against the skull. This causes symptoms such as headaches, double vision, trembling and personality changes.

Stroke

A stroke happens when a part of the brain does not get enough oxygen and is damaged. This can happen if a blood vessel in the brain bursts or gets a blood clot. Unlike most other cells, brain cells cannot re-grow and permanent damage and even death can be caused.

The symptoms depend on which part of the brain was damaged. They can include:

• numbness and poor reflexes
• difficulty in walking
• loss of speech, memory or use of limbs - usually on one side of the body

Parkinson's disease

Parkinson's disease is caused by a reduced amount of dopamine, a chemical involved in transmitting nerve impulses from one neurone to another in the brain. It gradually gets worse and symptoms include:

• trembling and stiffness
• slowed movements
• difficulty speaking and swallowing

Epilepsy

Epilepsy is caused by sudden bursts of unusual activity in the brain. This makes the brain stop working properly for a short time. It can cause the person to become unconscious and their body shakes repeatedly. This type of seizure is called a tonic-clonic seizure, which used to be called a grand mal seizure.

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