4.5.2.1a - The Nervous System and Reflex Arcs

From stimulus to response

A stimulus is a change in the environment. It could be a change outside the body, such as sudden bright light, or a change inside the body. AQA describes a basic control sequence like this:

stimulus -> receptor -> coordination centre -> effector -> response

In some exam questions, AQA shortens coordination centre to coordinator. Use either term carefully: it is the part that receives and processes information from receptors.

The receptor is the first biological structure in the pathway. It turns the detected change into information that can be carried through the nervous system.

In this lesson, the most important coordination centre is the CNS. It links the information coming in to the response going out.

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An effector does not decide the response. It carries it out. A muscle may contract, such as when a limb moves. A gland may secrete a hormone. The coordination centre is where information is processed so the response fits the stimulus.

Neurones and the CNS

Information in the nervous system travels along specialised cells called neurones. For this lesson, the important point is that information passes along neurones as electrical impulses.

Receptors send information along neurones to the CNS. The CNS coordinates the response of effectors. This structure is adapted for fast communication because impulses travel along neurones to link the receptor, CNS and effector in a pathway.

Keep the direction clear:

The detailed regions of the brain are not part of this lesson. Here, the brain matters only because it is part of the CNS, along with the spinal cord.

What reflex actions are

A reflex action is a rapid, automatic response to a stimulus. Reflex actions are important because they can protect the body from harm before there is time for conscious thought.

Do not write only an example if the question asks what a reflex action is. Pulling your hand away from a hot object is an example. The definition needs the ideas of automatic or involuntary, rapid, and not involving the conscious part of the brain.

The phrase does not involve the conscious part of the brain is precise. It does not mean the nervous system is absent. A reflex still uses neurones and a coordination centre, often the spinal cord.

Examples that fit this level include blinking in sudden bright light and withdrawing a hand from a hot object. Actions such as deciding to kick a ball, writing a message, or choosing to catch something are not reflex actions because they involve conscious control.

The reflex arc sequence

A reflex arc is the pathway followed by impulses during a reflex action. AQA expects you to know how each named structure relates to its function.

The main sequence is:

stimulus -> receptor -> sensory neurone -> relay neurone -> motor neurone -> effector -> response

There are synapses between neurones. A synapse is the small junction between one neurone and the next. In a reflex arc, synapses help pass the impulse from sensory neurone to relay neurone, and from relay neurone to motor neurone.