The Reflex Arc (Cambridge International IGCSE Biology)

Spec point — 14.1.3 · Describe a simple reflex arc in terms of: receptor,sensory neurone, relay neurone, motor neuron and effector Describe a reflex action as a means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors (muscles and glands) Describe a synapse as a junction between two neurons

The reflex arc

Voluntary responses

• A voluntary response is a response where a conscious decision is made to carry out a particular action

• Voluntary responses start in the brain

  • An example is reaching out to pick up a cup of coffee

Reflex responses

• An involuntary (or reflex) response does not involve the brain as the coordinator of the reaction
• A person is not aware they have completed a reflex response until after the response has been carried it out
• Reflexes are automatic and rapid responses to a stimulus; such as touching something sharp or hot
• As it does not involve the brain, a reflex response is quicker than any other type of nervous response
• This helps to minimise the damage to the body

A reflex arc

• An example of a reflex arc is:

1. The pin (the stimulus) is detected by a pain/pressure/touch receptor in the skin.
2. The sensory neurone sends electrical impulses to the spinal cord (the coordinator).
3. The electrical impulse is passed on to a relay neurone in the spinal cord.
4. The relay neurone connects to a motor neurone, transmitting the impulse onward.
5. The motor neurone carries the 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).

The reflex pathway

Synapses

• Where two neurones meet or join, they do so at a junction called a synapse
• Synapses are important because they allow the nervous system to function as a connected network of neurones
• Impulses can pass from one neurone to another across a synapse, allowing them to be directed along the appropriate route, e.g. to the correct part of the brain

Exam Hint Reflex actions are:

1. Automatic
2. Fast
3. Protective

A common exam question is to be asked to draw arrows on the neurones in the reflex arc diagram to show the direction of movement of the impulse. Make sure you read questions carefully – not all questions have a line underneath them to write an answer, so they can be easy to miss!