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Simple Reflex Arc (Edexcel GCSE Biology: Combined Science)
Revision Note
Simple Reflex Arc
- A reflex response (also known as an involuntary 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 and aids survival
- Pain-withdrawal, blinking and coughing are all examples of reflex responses that help us to avoid serious injury, such as damage to the eye or choking
The reflex arc
- A reflex arc is the pathway of a reflex response (specifically, the pathway taken by electrical impulses as they travel along neurones)
- An example of a reflex response is the pain-withdrawal reflex that occurs when someone steps on a pin. The reflex arc for this response is outlined below
- The pin (the stimulus) is detected by a (pain/pressure/touch) receptor in the skin on the person's foot
- A sensory neurone sends electrical impulses to the spinal cord (the coordinator)
- An electrical impulse is passed to a relay neurone in the spinal cord (part of the CNS)
- A relay neurone synapses with a motor neurone
- A motor neurone carries an impulse to a muscle in the leg (the effector)
- When stimulated by the motor neurone, the muscle will contract and pull the foot up and away from the sharp object (the response)
- This all occurs within a fraction of a second
The reflex arc pathway (in this case for a pain-withdrawal reflex). Reflex actions are automatic and rapid; they do not involve the conscious part of the brain
The neurones of the reflex arc
- There are three main types of neurone in a reflex arc: sensory, relay and motor
- Sensory neurones carry impulses from sense organs to the CNS (brain or spinal cord)
- 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)
You need to be able to recognise the three types of neurone in a reflex arc
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