The Nervous System and Reflexes

Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.

The reason this matters is enzymes. Enzymes control nearly every chemical reaction in your cells, and they only work efficiently within a narrow range of temperature and pH. If internal conditions drift too far, enzymes stop working properly and cells fail. Homeostasis keeps conditions in the range where enzymes — and therefore all cell functions — work best.

In the human body, the three conditions you must know are controlled by automatic systems:

Condition controlled	Why it must be kept stable
Blood glucose concentration	Cells need a steady fuel supply for respiration
Body temperature	Enzymes work best around 37 °C; too hot denatures them
Water levels	Cells are damaged if too much water enters or leaves them

Homeostasis maintains the optimum conditions for enzyme action and for all the functions of cells.

These automatic control systems respond using either nervous responses (electrical impulses) or chemical responses (hormones) — or both.

Every automatic control system in the body — whether it controls glucose, temperature or water — is built from the same three components. Learn these three words and what each does, because the same structure appears in every homeostasis question.

Component	What it is	What it does
Receptor	Cells that detect stimuli	Detect a stimulus — a change in the environment
Coordination centre	Brain, spinal cord or pancreas	Receive and process information from receptors
Effector	A muscle or a gland	Bring about a response that restores the optimum level

A stimulus is any change the body can detect — a rise in temperature, a bright light, a touch. The receptor detects it, the coordination centre decides what to do, and the effector carries out the response. Muscles respond by contracting; glands respond by secreting a substance such as a hormone.

The general pathway for any nervous response is:

Three named coordination centres are on the AQA spec: the brain, the spinal cord and the pancreas. The pancreas is the coordination centre for controlling blood glucose.

The nervous system lets humans react to their surroundings and coordinate their behaviour. It works using electrical impulses carried along cells called neurones (nerve cells).

The central nervous system (CNS) is made of just two structures:

• the brain
• the spinal cord

Information from receptors travels as electrical impulses along neurones to the CNS. The CNS processes that information and then coordinates the response of the effectors. The effectors are either muscles that contract or glands that secrete hormones.

There are three types of neurone you need to know:

Neurone	Carries impulses from → to
Sensory neurone	Receptor → CNS
Relay neurone	Found inside the CNS; links sensory to motor neurone
Motor neurone	CNS → effector

The CNS is only the brain and the spinal cord. The neurones running to and from it (sensory and motor neurones) are part of the wider nervous system, not part of the CNS itself.

This neurone-based control is fast and precise — exactly what the body needs to respond to sudden changes in its surroundings.

Neurones do not physically touch. Where one neurone meets the next there is a tiny gap called a synapse. The electrical impulse cannot jump across this gap on its own, so the signal is passed across chemically.

Here is what happens at a synapse, in order:

1. An electrical impulse arrives at the end of the first neurone.
2. This triggers the release of a chemical (a neurotransmitter) into the synapse (the gap).
3. The chemical diffuses across the gap.
4. The chemical binds to the membrane of the next neurone.
5. This starts a new electrical impulse in the next neurone.

So the signal travels electrically along a neurone, but chemically across a synapse. The chemical that carries the signal across is called a neurotransmitter.

At a synapse the signal is carried by a chemical that diffuses across the gap. A common exam error is to say the electrical impulse jumps the gap — it does not. The impulse stops; a chemical crosses; a new impulse begins.

(Extra context — not required by AQA 8461: the named chemical at many synapses is acetylcholine. AQA only asks for "a chemical" or "a neurotransmitter", so naming it is optional.)

A reflex action is an automatic and rapid response to a stimulus. Reflexes are protective — they happen before you have time to think, which is what makes them so fast.

The critical point for the exam: reflex actions are automatic and rapid, and they do not involve the conscious part of the brain. The signal travels through the spinal cord (or unconscious parts of the brain) and back out, so a response happens without you having to decide on it.

The pathway a reflex takes is called a reflex arc:

Structure	Role in the reflex arc
Receptor	Detects the stimulus
Sensory neurone	Carries impulse to the CNS
Synapse	Passes the signal chemically between neurones
Relay neurone	Connects sensory to motor neurone inside the CNS
Motor neurone	Carries impulse from the CNS to the effector
Effector	Muscle or gland that produces the response

The relay neurone sits inside the CNS (the spinal cord or unconscious brain). Because the impulse is processed there rather than in the conscious brain, the response is faster and you cannot choose to stop it.

Trace a real reflex step by step. You touch a sharp pin and your hand pulls away before you even register the pain. Naming each structure in order is exactly what AQA asks for.

1. Stimulus — the sharp point of the pin (and the pain/pressure).
2. Receptor — pain receptors in the skin of your finger detect it.
3. Sensory neurone — carries the electrical impulse to the spinal cord.
4. Synapse — a chemical diffuses across the gap to the relay neurone.
5. Relay neurone — passes the impulse across the spinal cord (the CNS).
6. Synapse — a chemical crosses to the motor neurone.
7. Motor neurone — carries the impulse out to the muscle.
8. Effector — the biceps muscle in your arm.
9. Response — the muscle contracts and your hand pulls away.

Notice that the conscious brain is not part of this chain. The decision to pull away is never made consciously — the spinal cord handles it. You feel the pain a fraction of a second later, after your hand has already moved, because that pain signal travels separately up to the brain.

This is why reflexes are protective: removing the hand from the pin happens faster than a thought could, which limits the damage done.

Required practical 7 asks you to plan and carry out an investigation into the effect of a factor on human reaction time. The standard method uses the ruler-drop test.

Method:

1. The person being tested sits with their forearm resting on the edge of a table, hand over the edge, thumb and forefinger open.
2. A second person holds a 30 cm ruler vertically so the 0 cm mark is between the open thumb and finger.
3. Without warning, the holder releases the ruler. The catcher closes their fingers to catch it as fast as they can.
4. Record the distance (in cm) on the ruler at the point where it was caught.
5. Repeat several times and calculate a mean distance. A shorter distance caught means a faster reaction time.

The ruler falls further the longer it takes to react, so the catch distance is a measure of reaction time. You can read reaction time from the distance using $t=\sqrt{\frac{2d}{g}}$, where $d$ is the distance fallen and $g\approx 9.8{\text{m/s}}^2$ — but for the practical, the catch distance itself is usually the recorded measure.

Designing a fair test — to investigate one factor (for example, whether caffeine or background noise changes reaction time):

• Change only the one factor you are testing (the independent variable).
• Keep everything else the same: same ruler, same hand, same starting position, same person.
• Take repeats and use the mean to reduce the effect of random error.

A reliable result needs a control: measure the person's reaction time before the factor is applied, then again after, and compare the means.

1. Saying a reflex involves the conscious brain

Reflex actions do not involve the conscious part of the brain — that is what makes them automatic and fast. The signal is processed in the spinal cord (or unconscious brain) via the relay neurone. Writing that you "decide" to move loses the mark.

2. Saying the electrical impulse jumps across the synapse

At a synapse the impulse stops and a chemical (neurotransmitter) diffuses across the gap, triggering a new impulse in the next neurone. The electrical impulse itself does not cross.

3. Putting the relay neurone outside the CNS

The relay neurone is inside the CNS — in the spinal cord (or unconscious brain). Sensory and motor neurones run to and from the CNS, but the relay neurone is within it.

4. Mixing up the neurone order

The order is always receptor → sensory neurone → relay neurone → motor neurone → effector. Sensory carries impulses in to the CNS; motor carries them out to the effector.

5. Confusing receptor, coordination centre and effector

Term	Example	Job
Receptor	Pain receptor in skin	Detects the stimulus
Coordination centre	Spinal cord, brain, pancreas	Processes the information
Effector	Muscle or gland	Carries out the response

6. Forgetting effectors are muscles OR glands

An effector is a muscle that contracts or a gland that secretes. Many students only write "muscle" and miss that glands are effectors too.