The Reflex Arc (AQA GCSE Biology): Revision Note

Reflex arc

• An involuntary (or reflex) response does not involve the conscious part of the brain as the coordinator of the reaction

  • Awareness of a response having happened occurs after the response has been carried out

• Responses are therefore automatic and rapid – this helps to minimise damage to the body

• An example of a reflex action is as follows:

  1. The pin (the stimulus) is detected by a (pain/pressure/touch) receptor in the skin

  2. A sensory neurone sends electrical impulses to the spinal cord (the coordinator)

  3. An electrical impulse is passed to a relay neurone in the spinal cord

  4. A relay neurone connects with a motor neurone via a synsapse

  5. A motor neurone carries an impulse to a muscle in the leg (the effector)

  6. The muscle contracts and pulls the foot up and away from the sharp object (the response)

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The neurones of the reflex arc

• There are three main types of neurone in a reflex arc:

  • Sensory neurones carry impulses from sense organs to the central nervous system (CNS)

  • Relay neurones are found inside the CNS and connect sensory and motor neurones

  • Motor neurones carry impulses from the CNS to effectors (muscles or glands)

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Synapses

• Neurones never touch each other; they are separated by gaps called synapses

  • In a reflex arc there are synapses between the sensory and relay neurones, and between the relay and motor neurones

• Electrical impulses cannot jump across synapses

• Chemicals called neurotransmitters are released into the gap and diffuse across it

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Synapse function

1. The electrical impulse travels along the first neurone to the end of the cell

2. When an electrical impulse arrives at the end of the neurone, neurotransmitters are released

3. The neurotransmitters diffuse across the synaptic gap and

4. The neurotransmitters bind to the membrane of the second neurone, stimulating an electrical impulse