Reflexes
Biology: Cambridge International A Level, topic 15.1.4
Reflexes
Reflex responses are an automatic response to a stimulus.
Quick, automatic, protective
- Reflexes are quick and automatic.
- They often help an organism respond quickly to a harmful stimulus, e.g. heat.
- The information is not processed in the brain and no decision is made about the response.
Reflex arc
- The pathway of neurones involved in a reflex response is called the reflex arc.
- There are three types of neurone involved in the reflex arc:
- Sensory.
- Relay.
- Motor.
E.g. Hand-withdrawal reflex
- When a person's hand touches a very hot object, they quickly move their arm away.
- This is an example of a reflex.
Reflex Arc
An example of a reflex arc is when a person withdraws their hand from a hot object. The steps in a reflex arc are:
Detection
- Thermoreceptors in the skin detect the heat stimulus.
Sensory neurone
- Sensory neurone carries impulse from thermoreceptors to relay neurone in the spinal cord.
Relay neurone
- Relay neurone carries impulse to motor neurone.
Motor neurone
- Motor neurone carries impulse to effectors (muscles in arm).
Response
- Arm moves away from hot object.
Test yourself on Reflexes
Which of the following are NOT neurones involved in the reflex arc?
Which of the following statements about reflexes are true?
Which of these features are NOT true of reflexes?
Steps in the Reflex Arc
What is the first step in the hand-withdrawal reflex?