3.1.3 The Reflex Arc — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

Section 1 of 9What a reflex is

A reflex action is a rapid, automatic response to a stimulus that does not involve conscious thought.

What a reflex is

Reflexes protect the body from harm and help it to function without the brain having to make a decision each time.

Examples include pulling your hand away from a hot object, blinking when something approaches the eye, the knee-jerk reflex,

and the narrowing of the pupil in bright light.

Reflexes are fast because the pathway is short and few synapses are involved. The brain is bypassed for the response itself, although you become aware of what happened a moment later because impulses also travel up the spinal cord to the brain.

Section 2 of 9The reflex arc step by step

The pathway of a reflex is called a reflex arc. Take the example of touching a hot pan.

1Stimulus

Stimulus heat is detected by a receptor in the skin of the finger.

2Sensory neurone

An impulse is sent along a sensory neurone towards the spinal cord.

3Relay neurone

At the first synapse the impulse passes to a relay neurone in the spinal cord (the coordinator).

4Motor neurone

At the second synapse the impulse passes to a motor neurone.

5Effector

The motor neurone carries the impulse to an effector, a muscle in the arm.

6Contracts

The muscle contracts, pulling the hand away. This is the response.

In a diagram of the arc the sensory neurone enters the spinal cord on the dorsal (back) side and the motor neurone leaves on the ventral (front) side.

Section 3 of 9Reflex or voluntary action

Feature	Reflex action	Voluntary action
Speed	Very fast	Slower
Conscious thought	Not involved	Involved
Coordinator	Spinal cord or unconscious brain	Conscious regions of brain
Learned?	Inborn and automatic	Often learned, can be changed
Example	Blinking	Catching a ball

Some responses that started as voluntary can become more automatic with practice, such as riding a bike, but a true reflex does not need to be learned.

Section 4 of 9Why reflexes are useful

The shortening of the reaction time can prevent serious injury.

1

The brain would take longer to process the situation and choose a response, by which time a burn or cut could be worse.

2

Because reflex responses are automatic, they do not use up the brain's capacity, leaving it free to deal with other things.

3

Reflexes such as breathing and swallowing in infants also help newborn babies to survive before they have learned anything.

4

Doctors test reflexes, for example the knee-jerk, to check that the nervous system is working normally.

The pupil reflex in the eye is an example where the effector is a muscle in the iris and the response protects the retina from damage in bright light.

Section 5 of 9Applying the idea

In exam questions the stimulus, receptor, coordinator, effector and response must be named for the situation described. For a dropped hot dish, receptor = temperature receptor in the skin. For a bright light, receptor = light receptor in the retina and effector = circular muscle of the iris. Always say the coordinator is the spinal cord for limb reflexes.

Link this topic with the structure of neurones and synapses (5.1.1), which explain why the reflex is not instant, and with reaction time (5.1.3), which is measured for voluntary responses rather than reflexes.

Section 6 of 9Making a reflex arc diagram

Exam diagrams of the reflex arc show a cross-section through the spinal cord.

1

The grey, butterfly-shaped area in the middle contains cell bodies and synapses, and the white area around it contains axons.

2

The sensory neurone enters through the dorsal root, which swells into a ganglion where the cell body lies outside the cord.

3

The relay neurone lies inside the grey matter.

4

The motor neurone leaves through the ventral root and runs to the muscle.

When labelling, draw arrows in the direction the impulse travels, and mark the two synapses as small gaps.

Making a reflex arc diagram

A common question asks which structure would be damaged if a person could feel a touch but could not move their hand: the answer is the motor neurone or its connection to the effector,

since the sensory path to the brain still works.

If instead a person could move but felt nothing, the sensory neurone would be affected.

Section 7 of 9Exam-style questions

Describe the reflex arc that occurs when a person steps on a sharp object and lifts their foot. [5]5 marks
Explain why a reflex action is faster than a voluntary action. [2]2 marks
Give two reasons why reflex actions are important. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Reflex action	A rapid automatic response that does not involve conscious thought.
Reflex arc	The pathway of neurones involved in a reflex.
Relay neurone	A neurone in the CNS linking sensory and motor neurones.
Voluntary action	A response controlled by conscious thought.
Effector	A muscle or gland that produces a response.
Spinal cord	Part of the CNS that coordinates many reflexes.
Pupil reflex	Narrowing of the pupil in bright light.

Section 9 of 9Exam technique

1

Saying the brain is the coordinator of all reflexes; limb reflexes are coordinated by the spinal cord.

2

Forgetting that the response is a muscle contraction, not the nerve.

3

Saying reflexes are learned. They are inborn and automatic.

4

Missing out the synapses when describing the pathway.

5

Describing a reflex as having no stimulus. Every reflex starts with a stimulus detected by a receptor.