The Human Nervous System

The human nervous system is divided into two parts:

1. Central nervous system (CNS).

• The brain and spinal cord.
• This is the coordination centre — it processes information from receptors and decides on responses.

2. Peripheral nervous system.

• All the nerves that branch out from the CNS to every part of the body.
• Carries information FROM receptors INTO the CNS, and FROM the CNS OUT to effectors.

Why have a CNS?

• Centralising the processing means decisions can be made based on lots of inputs at once (sight + smell + memory all feeding the brain).
• The brain can then send out coordinated, well-timed signals to effectors.

Speed of communication. Nervous impulses are electrical and travel along neurones at very high speeds — much faster than hormones. That's why the nervous system handles emergency responses (pulling away from a hot pan), while hormones handle slower adjustments (digestion, blood glucose regulation).

Functions of the nervous system:

1. Allows the body to respond to its environment (light, sound, pain, temperature).
2. Coordinates the body's behaviour and movement.
3. Carries out reflexes to protect the body from damage.
4. Underpins consciousness, memory and decision-making (the brain — covered in 4.5.2.3).

A neurone (nerve cell) is the basic unit of the nervous system. It is a long, thin cell adapted to carry electrical impulses quickly over long distances.

Three types you must learn:

Neurone type	Carries impulses FROM	Carries impulses TO	Location
Sensory neurone	Receptor	CNS	Peripheral nerves
Relay neurone	Sensory neurone	Motor neurone	Inside the CNS (brain and spinal cord)
Motor neurone	CNS	Effector (muscle or gland)	Peripheral nerves

Synapses. Where two neurones meet there is a tiny gap called a synapse. The electrical impulse can't jump across — instead:

1. The impulse arrives at the end of one neurone.
2. The neurone releases chemical neurotransmitters into the synapse gap.
3. The neurotransmitters diffuse across the gap.
4. They bind to receptors on the next neurone.
5. This triggers a NEW electrical impulse in the next neurone.

Why synapses matter.

• They allow connection between many neurones (so the CNS can integrate lots of info).
• They are one-way (chemicals are released on one side only) — so signals travel in the right direction.
• They are the SLOWEST step in a nerve pathway (diffusion across the synapse).

A reflex is an automatic and rapid response to a stimulus that does NOT involve conscious thought. Examples:

• Pulling your hand off a hot plate.
• Blinking when something flies near your eye.
• The knee-jerk reflex when the patellar tendon is tapped.
• The pupil narrowing when you walk into bright light.

Why are reflexes important?

• They protect the body from damage (you move BEFORE you've even thought about it).
• They are fast because the signal skips the conscious brain.

The reflex arc — the pathway the signal follows. Learn this in order:

Stimulus → Receptor → Sensory neurone → Relay neurone (in spinal cord) → Motor neurone → Effector → Response

Worked example: touching a hot plate.

1. Stimulus — heat from the plate.
2. Receptor — thermoreceptors in the skin of your finger.
3. Sensory neurone — carries impulse FROM the receptor TO the spinal cord.
4. Relay neurone — in the spinal cord; passes impulse from sensory to motor neurone WITHOUT going to the conscious brain.
5. Motor neurone — carries impulse FROM the spinal cord TO the muscle in your arm.
6. Effector — the bicep muscle.
7. Response — bicep contracts → arm pulled away.

You probably feel the pain a fraction of a second LATER because the impulse also travels (more slowly) up to the conscious brain — but the arm has already moved by then.

Voluntary action. You DECIDE to do it. The conscious brain is involved. Example: deciding to pick up a cup of tea.

Reflex action. Happens AUTOMATICALLY. The conscious brain is NOT needed for the response itself. Example: dropping the cup if it suddenly becomes too hot.

Feature	Reflex	Voluntary
Conscious brain involved?	NO (signal goes to spinal cord and back)	YES
Speed	Very fast	Slower (more steps)
Purpose	Protection from damage	Choice and behaviour
Learnable?	Many are innate; some can be learned	All learned
Example	Hand off hot plate, blinking	Picking up a pen, walking

Why isn't every action a reflex? Because reflexes are inflexible. You couldn't have a thoughtful conversation, plan a meal, or play a sport if every response was hardwired and automatic. The brain trades a tiny delay for huge flexibility.

Reaction time is the time taken between a stimulus (something detected by your sensory receptors — a flash of light, a sound, a falling ruler) and your response to it (pressing a button, catching the ruler).

Why is reaction time studied?

• It reflects how quickly your nervous system can process information.
• It is important in everyday life: driving (braking when a child runs into the road), sport (returning a serve, dodging a tackle), and safety in workplaces using machinery.
• It is a quantitative way to investigate the nervous system in class without any specialist equipment.

Typical values.

• Healthy young adult reaction time for a SIMPLE visual stimulus = roughly 0.20–0.25 seconds.
• Reaction time is SHORTER for sound (~0.15 s) than for sight (~0.20 s) because the auditory pathway is slightly shorter than the visual one.
• It is LONGER when a CHOICE is involved (e.g. pressing one of two buttons depending on the colour shown) because the brain has to decide.

The ruler-drop test is the standard GCSE method for measuring reaction time. It works because the ruler accelerates downwards due to gravity at a constant rate (about 9.8 m/s²), so the distance it falls is a direct measure of the time before it was caught.

Equipment.

• A 30 cm or 1 m ruler.
• A test subject and a partner.
• A table edge to rest the forearm on.

Method.

1. The subject sits with their arm resting on a table so the hand hangs over the edge. Thumb and forefinger are positioned around the 0 cm mark of the ruler, NOT touching it.
2. The partner holds the ruler vertically, with the 0 cm mark level with the subject's fingers.
3. WITHOUT warning, the partner drops the ruler. The subject catches it as soon as they see it move.
4. Record the distance the ruler fell — the mark at which the subject's fingers caught it (in cm).
5. Repeat at least three times. Calculate the mean distance.
6. Convert distance to time using a conversion table or the equation $t = \sqrt{2s/g}$ where $s$ is distance (in metres) and $g$ = 9.81 m/s².

Sample conversion (using $t = \sqrt{2s/g}$):

Distance fallen (cm)	Reaction time (s)
5	0.10
10	0.14
15	0.18
20	0.20
25	0.23
30	0.25

Control variables (very common exam question):

• Same hand each time (handedness affects results).
• Same height the ruler is held (so it always falls from the same start).
• No warning given (don't say 'now!').
• Same person dropping the ruler (consistency).
• Subject not looking at the dropper's hand (so they react to the ruler movement, not anticipation).

Repeats and mean. Always repeat (≥3 times) and take the mean to reduce the effect of random variation.

Reaction time is not fixed. It varies between people and even within the same person depending on conditions.

Factors that DECREASE (speed up) reaction time:

• Caffeine — a stimulant in coffee, tea and energy drinks. Studies and class experiments show a small but measurable improvement (~5–10% faster).
• Practice — repeated trials get faster as you anticipate the timing (though this isn't testing pure reflex any more).
• Being alert and well-rested.

Factors that INCREASE (slow down) reaction time:

• Alcohol — a depressant; slows neural transmission, hence drink-drive laws and the importance of reaction time when driving.
• Tiredness / fatigue — the brain processes information more slowly.
• Distractions — phone use, music, conversation while driving can double reaction time.
• Drugs that act as depressants (sedatives, painkillers).
• Cold — chilled hands react more slowly.

Age effects.

• Children's reaction times are slower than young adults' because their nervous systems are still developing and they have less practice.
• Reaction times are FASTEST in young adults (~18–35 years).
• They become SLOWER with age beyond about 50 — nerve conduction speeds drop slightly and processing in the brain slows.

A typical class investigation: measure reaction time before and after drinking a cup of coffee, or before and after a 20-minute period of distraction (e.g. listening to loud music). The expected result: caffeine slightly improves, distraction worsens.

Ethics and safety. GCSE classes do NOT test alcohol effects on students. You may use secondary data from published studies. Always discuss informed consent if testing classmates.

Anomalies. A single very long distance might mean the subject was distracted or guessed wrongly — you can usually identify it as an outlier and exclude it (note that you have done so).

Calculating a mean. Add up all valid trials and divide by the number of trials. Don't round too early — keep extra decimal places until the end.

Comparing two conditions (e.g. before vs after caffeine):

1. Test each condition multiple times (e.g. 5 trials).
2. Calculate the mean for each.
3. Compare the means: which is smaller (= faster reaction)?
4. State whether the change is large enough to be meaningful, considering the variation between trials.

Fair test rules:

• Change ONE variable at a time (the independent variable, e.g. presence of caffeine).
• Keep all other variables constant (control variables: same hand, same person, same ruler, same dropper, same time of day if possible).
• Repeat enough times to spot any random variation.

Common pitfall in exam questions: evaluating whether results are reliable / valid. Reliable = consistent on repeat (small spread of values). Valid = the test actually measures what it claims to measure (the ruler test measures simple visual reaction time, not, say, fitness).

The human brain is a delicate organ inside the skull, made of billions of neurones. AQA GCSE focuses on three regions with very different jobs:

1. Cerebrum (cerebral cortex).

• The largest part, the wrinkly outer surface that you picture when you think 'brain'.
• Divided into a left and a right hemisphere.
• Functions: consciousness, intelligence, memory, language, vision, hearing, voluntary movement — basically all 'higher-order' thinking and behaviour.

2. Cerebellum.

• At the back and slightly lower part of the brain, beneath the cerebrum.
• Smaller, also wrinkled.
• Function: coordination of muscle movements and balance. Damage to the cerebellum makes a person uncoordinated and unsteady (sometimes described as 'drunk-like').

3. Medulla.

• At the base of the brain, sitting at the top of the spinal cord (part of the brain stem).
• Function: controls unconscious activities — heart rate, breathing rate, blood pressure. You'll recognise this from the homeostasis loop: when CO₂ in your blood rises during exercise, it is the medulla that detects it and increases breathing rate.

Scientists have mapped which regions of the brain do what by using three main approaches.

1. Studying patients with brain damage.

• When a region of the brain is damaged by injury, stroke or surgery, doctors can observe which abilities the patient loses.
• Famous historical example: Phineas Gage, a railway worker whose frontal lobe was destroyed by an iron rod — his personality changed, suggesting that region controlled personality.
• Strength: real data from real people.
• Limit: every brain injury is different; only one (uncontrolled) case at a time.

2. Electrical stimulation of the brain.

• During brain surgery (often while the patient is awake under local anaesthetic), a surgeon can apply a small electrical current to a part of the brain and see what the patient experiences or does (e.g. moving a limb, recalling a memory, smelling something).
• Strength: precise — pinpoints which spot does what.
• Limit: can only be done during surgery (rare and risky), so opportunities are limited.

3. MRI scans (Magnetic Resonance Imaging).

• Non-invasive imaging technique that uses powerful magnets and radio waves to produce detailed 3D pictures of soft tissues inside the body — including the brain.
• A variant, functional MRI (fMRI), can show which areas of the brain are MORE active when the subject performs different tasks (e.g. reading, listening to music).
• Strength: non-invasive, no surgery needed, gives detailed images and real-time function (fMRI).
• Limit: expensive equipment; subjects must lie very still; doesn't show individual neurones, only regions.

Used together, these methods have built up the modern map of brain function: language in Broca's area on the left cerebrum, visual processing in the occipital lobe, motor control in the motor cortex, and so on.

The AQA exam may ask you to evaluate why studying or treating the brain is harder than for many other organs. The main reasons:

1. The brain is delicate.

• Made of soft tissue protected by the skull. Any surgery is risky — a slip can cause permanent damage.
• Even bleeding inside the skull (a haemorrhage) can press on the brain and kill cells.

2. The brain is extremely complex.

• ~86 billion neurones with trillions of connections. Functions overlap and are spread across regions.
• One region (e.g. the cerebellum) carries out many sub-tasks; one task (e.g. speaking) uses many regions.

3. The brain is inaccessible.

• Encased in the skull. Cannot just be 'opened up' for examination without significant risk.
• Brain tissue cannot easily be biopsied (sampled) without damaging the surrounding tissue.

4. The blood-brain barrier.

• A specialised barrier of cells in the brain's blood vessels stops most substances passing from the blood into brain tissue.
• It protects the brain from toxins and pathogens, but it also stops most drugs from reaching the brain — making chemical treatments harder to deliver.

5. Damaged neurones don't regenerate (in most cases).

• Once neurones die (from a stroke, head injury, or disease like Alzheimer's), they generally don't grow back. Lost function may be permanent.

Specific examples of difficult conditions:

• Brain tumours — hard to remove without damaging surrounding healthy tissue.
• Strokes — region starved of oxygen dies quickly; surviving neurones can sometimes take over function over months/years of rehab.
• Parkinson's, Alzheimer's — neurodegenerative diseases where cells progressively die; no cure, only symptom management.

Ethics in brain research.

• Many techniques (especially invasive ones) cannot be tested on healthy volunteers because of risk.
• Animal models are useful but raise their own ethical questions.
• Trials of new drugs are slow and tightly regulated.

The eye is a complex sense organ. Light enters at the front, is focused by the cornea and lens onto the retina at the back, and the optic nerve carries impulses to the brain.

Parts to learn (AQA list):

• Sclera — the tough, white outer layer that protects the eye and gives it shape.
• Cornea — the transparent front of the eye. It refracts (bends) light as it enters, doing most of the focusing.
• Iris — the coloured ring of muscle. Controls how much light enters by adjusting the size of the pupil.
• Pupil — the hole in the middle of the iris through which light enters.
• Lens — a transparent, flexible structure behind the pupil. Fine-tunes the focus of light onto the retina by changing shape.
• Ciliary muscles — a ring of muscle around the lens. Contracting/relaxing changes lens shape.
• Suspensory ligaments — tiny tough fibres that connect the ciliary muscles to the lens.
• Retina — the light-sensitive layer at the back of the eye. Contains rods (low light, black-and-white) and cones (colour, bright light) — the receptor cells that turn light into electrical impulses.
• Optic nerve — bundle of sensory neurones that carries impulses from the retina to the brain (where they are interpreted as images).

The iris has two sets of muscles arranged around the pupil:

• Circular muscles (run around the pupil in a ring).
• Radial muscles (run outwards from the pupil like wheel spokes).

Bright light:

• Circular muscles contract, radial muscles relax.
• Pupil becomes small — less light enters — protects the retina from damage.

Dim light:

• Circular muscles relax, radial muscles contract.
• Pupil becomes large — more light enters — improves vision in low light.

This is a reflex — automatic, fast, doesn't involve conscious thought. Sensory neurones in the retina detect light level → relay neurone in the brain stem → motor neurones → muscles of the iris.

Accommodation is the process of the eye changing the shape of the lens to focus light from objects at different distances onto the retina.

Distant object (more than 6 m away):

• Light rays arriving at the eye are nearly parallel — only need a little bending.
• Ciliary muscles RELAX.
• Suspensory ligaments are pulled tight.
• Lens is stretched thin (less curved).
• Refracts light only slightly — focuses parallel rays on the retina.

Near object (closer than 6 m):

• Light rays from the object are diverging — need to be bent more.
• Ciliary muscles CONTRACT.
• Suspensory ligaments become slack (loose).
• Lens bulges thicker (more curved) under its own elasticity.
• Refracts light strongly — focuses diverging rays on the retina.

A common confusion: when the ciliary muscle CONTRACTS, the ring of muscle gets SMALLER, which makes the suspensory ligaments SLACK (not tight!), allowing the elastic lens to become more rounded.

Myopia (short sight).

• The person can see near objects clearly but distant objects look blurry.
• Cause: the lens is too curved, OR the eyeball is too long.
• Light from distant objects is focused in front of the retina, so by the time it reaches the retina, the image is blurred.
• Correction: wear a concave (diverging) lens in spectacles or contact lenses. The diverging lens spreads light out slightly before it enters the eye, so the focal point falls correctly on the retina.

Hyperopia (long sight).

• The person can see distant objects clearly but near objects look blurry.
• Cause: the lens is too flat (e.g. age-related loss of elasticity), OR the eyeball is too short.
• Light from near objects would focus behind the retina — the retina sees a blurred image.
• Correction: wear a convex (converging) lens. The converging lens bends light inwards before it enters the eye, so the focal point falls on the retina.

Other technologies (just for awareness):

• Hard and soft contact lenses — same correction, sit on the cornea.
• Laser eye surgery — reshapes the cornea so it refracts light correctly.
• Replacement lens surgery — the natural lens is replaced with an artificial one.

The human body keeps its core temperature very close to 37 °C. This is no accident: it is the optimum temperature for the enzymes that catalyse virtually every chemical reaction in our cells (digestion, respiration, DNA replication, neurotransmitter synthesis…).

If body temperature rises too high (above ~40 °C):

• Enzymes start to denature — the active site changes shape and stops working.
• Respiration and other reactions slow or stop.
• Cells die. Untreated, heatstroke is fatal.

If body temperature falls too low (below ~35 °C):

• Enzyme-controlled reactions become too slow because particles have less kinetic energy.
• Respiration is impaired. Less heat is produced.
• The person becomes drowsy, then unconscious (hypothermia).

So the body must keep its temperature just right — a small range around 37 °C — even when the surroundings are very hot or very cold. This is thermoregulation, a key part of homeostasis.

Body temperature is monitored by the thermoregulatory centre in the brain (located in a part called the hypothalamus — you don't need to name it for AQA, but it's there).

The thermoregulatory centre uses two sources of information:

1. Receptors in the brain itself.

• These detect the temperature of the blood flowing through the brain.
• Because blood circulates everywhere, its temperature is a good proxy for core body temperature.

2. Temperature receptors in the skin.

• These detect the temperature of the skin and surroundings.
• Information is sent as electrical impulses along sensory neurones to the thermoregulatory centre.

By combining both signals, the centre can tell:

• Whether the body itself is overheating or cooling (from blood), AND
• Whether the environment is challenging (from skin) — so it can react early, before core temperature is dangerously off.

When the centre detects a change, it sends impulses along motor neurones to effectors in the skin and muscles. The effectors carry out the response that brings temperature back to normal.

TOO HOT — body needs to LOSE heat.

• Sweating. Sweat glands release sweat onto the skin surface. The water in sweat evaporates, taking thermal energy from the skin and cooling the body.
• Vasodilation of skin blood vessels. The arterioles supplying capillaries near the skin surface widen. More warm blood flows close to the skin surface, so more heat is lost by radiation. (Hair muscles also relax so hairs lie flat — less insulating air layer.)

TOO COLD — body needs to GENERATE/RETAIN heat.

• Shivering. Skeletal muscles contract and relax rapidly. Muscle contraction needs respiration, which releases heat — warming the body.
• Vasoconstriction of skin blood vessels. Arterioles supplying skin capillaries narrow. Less warm blood flows near the surface, so less heat is lost.
• Hair erection. Tiny erector muscles at the base of each hair contract, making the hairs stand up. This traps an insulating layer of air next to the skin (in humans, the effect is small — 'goose-bumps' — but in furry mammals it's significant).

Key common error: describe sweating as 'water leaves the skin' — that's not enough. You must say evaporation of water/sweat removes thermal energy. Without evaporation (e.g. in humid air), sweat just sits on the skin and cooling doesn't work — which is why humid heat is so dangerous.

Body temperature control is a classic negative feedback mechanism. The loop:

1. Set point = 37 °C.
2. Receptors (in the skin AND brain) detect any change away from 37 °C.
3. Coordinator = thermoregulatory centre in the brain. Decides which effectors to activate.
4. Effectors (sweat glands, skin arterioles, skeletal muscles, hair muscles) carry out the response.
5. Response brings the temperature BACK TOWARDS 37 °C.
6. Receptors detect that the temperature is normal again — the response is switched off.

The word negative means the response opposes the change. Too hot → response cools. Too cold → response warms.

This same pattern (receptor → coordinator → effector → response opposes change) appears in many other AQA homeostasis topics: blood glucose control, water and ion balance, even the menstrual cycle.