Reflexes

Section 1 of 7What is a reflex action?

Involuntary response

• Involuntary response A reflex is a response that does not need the conscious part of the brain to coordinate it.
• You only become aware of what happened after the response has already taken place.

Key features

• Key features Reflexes are automatic, rapid and protective, because a quick response can prevent injury.

Why speed matters

• Why speed matters The short pathway, with few neurones, means the response takes only a fraction of a second.

Section 2 of 7Examples of reflexes

Reflex	What happens	Protective value
Withdrawal	Limb muscles contract and pull the limb away from heat or a sharp object	Reduces tissue damage
Blinking	Eyelid muscles close the eye in response to dust, an allergen or bright light	Protects the eye
Pupil reflex (bright light)	Muscles make the pupil smaller	Protects the retina from damage
Pupil reflex (dim light)	Muscles make the pupil larger	Lets more light enter so you can see

Section 3 of 7Parts of a reflex arc (HT only)

Reflex arc

Stimulus and receptor

• Stimulus and receptor A stimulus is a change in the surroundings. Receptor cells in sense organs detect it.

Sensory neurone

• Sensory neurone Carries impulses from the receptor to the central nervous system (CNS).

Coordinator

• Coordinator A relay neurone in the CNS, usually the spinal cord, passes impulses on without using the conscious brain.

Motor neurone

• Motor neurone Carries impulses from the CNS to the effector.

Effector

• Effector A muscle or gland that produces the response.

Section 4 of 7The pain-withdrawal reflex (HT only)

1

A receptor in the skin of the foot detects the sharp pin as a pain stimulus.

2

A sensory neurone carries electrical impulses to the spinal cord.

3

The impulse passes to a relay neurone in the spinal cord.

4

The relay neurone passes the impulse across a synapse to a motor neurone.

5

The motor neurone carries the impulse to a leg muscle, the effector.

6

The muscle contracts and pulls the foot away from the pin, all within a fraction of a second.

Section 5 of 7Direction and exceptions (HT only)

1Receptor

• Receptor The impulse always starts at the receptor.

2CNS

• CNS It then travels along the sensory neurone to the spinal cord.

3Effector

• Effector It leaves along the motor neurone to the muscle or gland.

4Eye reflexes

• Eye reflexes In blinking and the pupil reflex the coordinator is in the brain, but still not the conscious part.

Section 6 of 7Key terms

Term	Meaning
Reflex action	An automatic, rapid, protective response that does not involve conscious thought.
Receptor	A cell that detects a stimulus.
Effector	A muscle or gland that carries out a response.
Synapse	A gap between two neurones where the signal passes from one to the next.
Relay neurone	A neurone in the CNS that links a sensory neurone to a motor neurone.

Section 7 of 7Exam technique

1

Saying that the brain decides the reflex response. The conscious brain is not involved, so the response is automatic.

2

Reversing sensory and motor neurones. Sensory goes towards the CNS; motor goes away from it.

3

Describing the pupil reflex as dilating in bright light. The pupil gets smaller in bright light.

4

Leaving out the synapse when labelling a reflex arc diagram.