AQA GCSE Reflexes & Synapses (Biology)
Reflexes
A reflex action is a rapid, automatic response to a stimulus.
It does not involve the conscious part of the brain
Reflexes are most commonly processed in the spinal cord.
A reflex action will help to protect us from danger.
Examples of reflexes:
Pulling your hand away from a hot object
Knee-jerk reflex
Reflex Arc
The reflex arc is the pathway of neurones that the impulse travels along.
Below is a flow chart to show the pathway from a stimulus to a response using a reflex arc.
Reflex arc pathway
Pathway of a reflex action
Stimulus
– A change in the environment (e.g. heat from a hot object such as a candle flame)
Receptor
– Detects the stimulus (e.g. pain receptors in the skin)
Sensory neurone
– Carries electrical impulses from the receptor to the spinal cord
Relay neurone (spinal cord)
– Connects sensory neurone to motor neurone
– Allows the response to happen without conscious thought
Motor neurone
– Carries impulses from the spinal cord to the effector
Effector
– A muscle or gland. In this case a muscle will contract to move the lower arm away from the candle.
Response
– Muscle contracts or gland releases a substance (e.g. hand pulls away)
Synapse
A synapse is the gap between two neurones.
A synapse occurs between:
1.Sensory and relay neurone
2.Sensory and motor neurone
3. Relay and motor neurone
It is where a signal is passed from one neurone to another.
How a synapse works
1.An electrical impulse arrives at the end of a neurone
2.This causes chemicals (neurotransmitters) to be released
3.Neurotransmitters diffuse across the synaptic gap
4. Neurotransmitter binds to receptors on the next neurone
5. This triggers a new electrical impulse in the next neurone
See the diagram below.
Why synapses are important
Make transmission one-way because the neurotransmitter (chemical) is produced in 1st neurone and receptors are on 2nd neurone
Allow control of responses
Cause a slight delay, which helps prevent over-reaction
Are involved in reflexes and brain processing
Synapses in reflexes (link to reflex arc)
In a reflex arc, synapses occur:
Between sensory neurone → relay neurone
Between relay neurone → motor neurone
These synapses are usually in the spinal cord
Practice Questions
1.Describe what a reflex is
2.State what is meant by the term a reflex arc
3. Describe how a synapse works.
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