B1aL2 Receptors and Reflexes

Key Words

Effector organs - organs that respond to nerve impulses

Glands - organs that secrete hormones

Hormones - a chemical released by the body to cause changes in the body.

Motor Neurones - nerve cells that connect the brain to the muscles;

Nerves - bundles of lots of neurones

Neurones - nerve cells;

Receptor - cell that detects a stimulus;

Secrete - release a chemical.

Sensory neurone - nerve cell that caries information to the brain.

Target organs - organs that respond to hormones.

Stimulus - change in the environment

Grade E

Stimuli result from changes to the environment.

The receptors give out  electrical impulses that are carried by nerve cells (neurones) to the brain.  Nerves are bundles of nerve cells.  The nerve cells that transmit impulses from receptors are called sensory neurones.  The impulses are carried to the brain.  The brain then sends messages down motor neurones to effector organs like muscles.  This is summed up in the picture below

The common word for stimuli are the senses, which are:

• Sight;
• Hearing;
• Taste;
• Touch;
• Heat and cold;
• Pressure;
• Pain

The effectors carry out the response.  Mostly they are muscles, but may be glands.  For example, when you smell nice food, your mouth starts to water, because the salivary glands secrete saliva.

Reflexes are automatic responses to large stimuli that would otherwise damage the body.  They do not go to the brain.  Instead the sensory neurone is connected by a relay neurone to a motor neurone which activates the muscles.

If you touch a hot object, this reflex mechanism automatically makes muscles in the arm contract to remove the hand from the hot object.  It's only afterwards that the brain receives the pain message.

Grade C

Sensory Neurone                                                                                     Motor Neurone

The two pictures show the different kinds of nerve cell.  Some key points:

• In both cells, the dendrites and the axon terminals are all part of the same cell as the cell body.  The nucleus of the cell shares the same cytoplasm as the most distant dendrite.

• Surrounding the axon are myelin sheaths, which act as electrical insulation.

• The dendrites and axon terminals connect with other nerve cells.

The sensory neurones have receptors.  Each receptor only picks up one kind of sensation.  A taste receptor will not detect sound or pressure.  Overloading any receptor can cause pain.  This tells the body that there is some kind of threat and the pain will cease if the stimulus is removed.

Nerve cells are specialised animal cells.  Like all animal cells, they have:

• cell membrane;

• nucleus;

• cytoplasm.

Like all animal cells they need oxygen to get energy from glucose.

The reflex arc is shown in the diagram below:

There are three neurones involved, separated by two junctions, or synapses.  The synapses prevent unwanted messages getting across.

Grade A

In an electronic circuit, a transducer detects a change in the environment and trips the electronic circuit to respond. 

The sensory cells do much the same sort of thing.  The cells in the nose pick up chemicals in the air, and that sends of a train of electrical impulses to the brain. 

In the eye, light energy is turned into electrical impulses by the rods and the cones, to be processed by the brain into an image.

The synapse is a tight fitting junction between two nerve cells.

Chemicals called neuro-transmitters pass across the space.  The impulse releases the chemicals which cross the gap.  The receptors pick up the chemicals and set off the impulse down the next nerve cell.  There has to be a threshold level before the second cell makes an impulse.The idea is to stop spurious messages getting down the neurones.