Coordination and Control

• One sentence that prevents four of the six. "Circular closes the pupil in bright light; ciliary contracts for near objects; insulin lowers high glucose; arterioles, not capillaries, change width." Write it at the top of your rough working before you start the paper.
• The direction test. If you can only remember one thing, remember what the names mean. Sensory comes from "sense" — it brings the sensation in. Motor comes from "motion" — it sends the instruction to move out. Relay means to pass something on — it sits in the middle and hands the signal over. Get the direction right and the diagram cue usually falls into place behind it.
• The hook that survives exam pressure. Picture the word circular as a drawstring on a bag: pull it and the opening closes. Picture radial as the spokes of a wheel: pull outwards on the rim of a hole and the hole opens. Bright light closes the drawstring; dim light pulls the spokes.
• Hot and cold are mirror images. Every response in this section is the reverse of one in the previous section: more sweat / less sweat, dilate / constrict, hairs flat / hairs raised. Learn one column properly and you get the other by reversing it — with one exception. Shivering has no opposite. There is no "anti-shivering" when you are hot; that response exists on the cold side only.
• Two words, one letter apart, opposite jobs. Insulin puts glucose into cells and into store, so it lowers blood glucose. Glucagon makes stored glucose gone from the liver and into the blood, so it raises blood glucose. If you remember nothing else from this section, remember which direction each one pushes.
• Say it out loud, twice. "High glucose, insulin, glucose into glycogen, concentration falls." "Low glucose, glucagon, glycogen into glucose, concentration rises." Two sentences, four seconds. They are worth more marks per second of revision than anything else in this chapter.
• The five-second check before you hand the paper in. Scan your answers for four words: circular, ciliary, insulin, arteriole. Circular closes the pupil in bright light. Ciliary contracts for near objects. Insulin lowers a high glucose concentration. Arterioles, not capillaries, change width. Four checks, four of the most commonly reversed statements in the whole topic.
• Circular closes the pupil, in bright light. Ciliary contracts for near objects. Insulin lowers a high blood glucose concentration. Arterioles, never capillaries, change their width.

• State that the mammalian nervous system consists of the central nervous system (brain and spinal cord) and the peripheral nervous system (the nerves outside them).
• State that electrical impulses travel along neurones.
• Identify sensory, relay and motor neurones from a simplified diagram, and state what each one carries and in which direction.
• Define a reflex action as a rapid and automatic response to a stimulus, and explain why reflexes are protective.
• Describe a complete reflex arc: stimulus, receptor, sensory neurone, relay neurone in the CNS, motor neurone, effector, response.
• Define a synapse as a junction between two neurones, and identify vesicles, the synaptic gap, neurotransmitter and receptor proteins.
• Describe the five events of synaptic transmission in the correct order.
• Explain why an impulse can only travel one way across a synapse.
• State that sense organs are groups of receptor cells that respond to particular stimuli: light, sound, touch, temperature and chemicals.
• Identify the cornea, iris, pupil, lens, ciliary muscles, suspensory ligaments, retina, fovea, optic nerve and blind spot on a labelled longitudinal section of the eye.
• State the function of each of those structures.
• Explain the pupil reflex in bright and in dim light, naming the circular and radial iris muscles and stating that they act antagonistically.
• Explain accommodation for near and for distant objects, in terms of the ciliary muscles, the suspensory ligaments and the shape of the lens.
• Distinguish the pupil reflex from accommodation, and state that accommodation does not change pupil size.
• Define a hormone as a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.
• State the source and role of adrenaline, insulin, glucagon, FSH, LH, testosterone, oestrogen and progesterone.
• Describe the effects of adrenaline in a demanding situation, including increased heart rate and increased blood glucose concentration.
• Compare nervous and hormonal control by signal, route, speed, duration and targeting.
• Define homeostasis as the maintenance of a constant internal environment, and explain that "constant" means within narrow limits.
• Describe negative feedback as a loop in which the response opposes the deviation that caused it.
• Identify hairs, hair erector muscles, sweat glands, receptors, sensory neurones, blood vessels and fatty tissue on a diagram of the skin.
• Explain how the body responds when it is too hot: sweating and evaporation, vasodilation, and hairs lying flat.
• Explain how the body responds when it is too cold: reduced sweating, vasoconstriction, shivering, hairs raised, and insulation by fatty tissue.
• Explain that vasodilation and vasoconstriction happen in the arterioles supplying the surface capillaries, not in the capillaries themselves.
• Explain the control of blood glucose concentration by the pancreas and the liver, using insulin and glucagon.
• State the signs of Type 1 diabetes and its treatment by the administration of insulin.

Why a body needs both. Pulling your hand off a hot pan must happen in milliseconds and must stop immediately afterwards — that is a job only the nervous system can do. Keeping your blood glucose in range for hours after a meal needs a signal that reaches the liver and every muscle at once and stays switched on — that is a job only a hormone can do. The two systems are not competing; they cover different kinds of problem.

Effector

A muscle or a gland that carries out a response after receiving information from a coordinator. A muscle responds by contracting; a gland responds by secreting a substance. No other type of structure counts as an effector in the Cambridge O Level Biology syllabus.

Coordination

The process by which an organism detects a change in its internal or external environment and produces an appropriate response to it. In mammals the pathway always runs stimulus, receptor, coordinator, effector, response, and is carried out either by electrical impulses along neurones or by hormones transported in the blood.

Peripheral Nervous System

All the nerves outside the brain and the spinal cord. The peripheral nervous system carries impulses from receptors towards the central nervous system along sensory neurones, and from the central nervous system out to effectors along motor neurones.

Synapse

A junction between two neurones. The two neurones do not touch: a narrow synaptic gap separates them. An electrical impulse arriving at the first neurone causes vesicles to release a neurotransmitter, which diffuses across the gap and binds to complementary receptor proteins on the next neurone, stimulating a new electrical impulse there.

Reflex Action

A rapid and automatic response to a stimulus. A reflex action begins without a conscious decision being made, which is why it can protect the body from damage faster than a deliberate movement could. The brain is usually informed shortly afterwards, but it does not have to act before the response begins.

Reflex Arc

The pathway of neurones along which the impulses of a reflex action travel. The complete sequence is stimulus, receptor, sensory neurone, relay neurone in the central nervous system, motor neurone, effector, response. It is the shortest route between detecting a stimulus and responding to it.

Retina

The light-sensitive layer lining the inside of the back of the eye. It contains light receptor cells, some of which are sensitive to different colours. Light focused onto the retina generates electrical impulses that travel along the optic nerve to the brain.

Sense Organ

A group of receptor cells that responds to a particular stimulus, such as light, sound, touch, temperature or chemicals. A sense organ detects the stimulus and generates electrical impulses that travel along sensory neurones to the central nervous system, where they are interpreted.

Hormone

A chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs. Hormones travel in the blood plasma and reach the whole body, but only cells carrying complementary receptors respond to them. Hormonal responses are generally slower to begin and longer-lasting than nervous ones.

Vasodilation

The widening of the arterioles that supply the capillary networks near the skin surface. More warm blood then flows close to the surface, so more thermal energy is transferred to the surroundings and body temperature falls towards the set point. Capillaries themselves cannot widen, because their walls are one cell thick and contain no muscle.

Hypothalamus

A region of the brain that monitors the temperature of the blood flowing through it and coordinates the responses that correct a deviation from the set point. It acts together with temperature receptors in the skin, which detect changes in the temperature of the surroundings.

Neurotransmitter

A chemical stored in vesicles at the end of a neurone and released into the synaptic gap when an impulse arrives. It diffuses across the gap and binds to complementary receptor proteins on the next neurone, where it stimulates a new electrical impulse. It acts only across the width of one synapse, unlike a hormone, which is carried around the body in the blood.

Central Nervous System

The brain and the spinal cord together. The central nervous system receives information from receptors by way of sensory neurones, processes it, and coordinates the response that is sent out along motor neurones. It is one of the two divisions of the mammalian nervous system, the other being the peripheral nervous system.

Antagonistic Muscles

A pair of muscles that produce opposite effects, so that when one contracts the other relaxes. The circular and radial muscles of the iris are an antagonistic pair: contraction of the circular muscles makes the pupil smaller, and contraction of the radial muscles makes it larger.

Sensory Neurone

A neurone that carries electrical impulses from a receptor to the central nervous system. In a simplified diagram it is recognised by a cell body lying part-way along the fibre, on a short side branch, rather than at either end.

Relay Neurone

A neurone found within the central nervous system that connects a sensory neurone to a motor neurone. It is short and carries many connections, and in a simplified diagram it is recognised by its small size and the number of short branches leaving the cell body.

Endocrine Gland

A gland that secretes hormones directly into the blood, rather than through a duct. The endocrine glands required in this chapter are the pituitary gland, the adrenal glands, the pancreas, and the ovaries or testes.

Target Organ

An organ whose activity is altered by a particular hormone. A hormone is carried in the blood to every organ, but only cells carrying receptors complementary to that hormone respond, so the response is specific even though the delivery is not.

Motor Neurone

A neurone that carries electrical impulses from the central nervous system to an effector, which is a muscle or a gland. In a simplified diagram it is recognised by a cell body at one end, carrying many dendrites, with a long fibre leading away to the effector.

Receptor

A cell, or group of cells, that detects a stimulus and generates an electrical impulse in response. Receptors are selective: each type responds only to a particular kind of stimulus, so light receptors cannot detect sound and temperature receptors cannot detect light.

Hair Erector Muscle

A small muscle attached to a hair follicle in the skin. When it contracts, the hair is pulled upright and a layer of air is trapped next to the skin, reducing heat loss. When it relaxes, the hair lies flatter and less insulating air is trapped. The effect is far more important in furry mammals than in humans.

Adrenaline

A hormone secreted by the adrenal glands in situations such as fear or vigorous activity. It prepares the body for action by increasing the heart rate and increasing the blood glucose concentration, so that more glucose and more oxygen are delivered to the muscles for respiration.

Accommodation

The change in shape of the lens that allows the eye to focus a sharp image of a near or a distant object on the retina. For a near object the ciliary muscles contract, the suspensory ligaments slacken, the lens becomes thicker and refracts light more. For a distant object the ciliary muscles relax, the suspensory ligaments tighten, the lens becomes thinner and refracts light less. Accommodation does not change the size of the pupil.

Lens

A transparent, flexible, biconvex structure behind the pupil that refracts light and focuses it onto the retina. Its shape is changed by the ciliary muscles acting through the suspensory ligaments, which is how the eye focuses on near and distant objects.

Pupil Reflex

A reflex action in which the size of the pupil changes in response to light intensity. In bright light the circular muscles of the iris contract and the radial muscles relax, making the pupil smaller so that less light reaches the retina. In dim light the radial muscles contract and the circular muscles relax, making the pupil larger so that more light enters.

Vasoconstriction

The narrowing of the arterioles that supply the capillary networks near the skin surface. Less blood then flows close to the surface, so less thermal energy is transferred to the surroundings and body temperature rises towards the set point. Blood flow to the skin is reduced, not stopped.

Homeostasis

The maintenance of a constant internal environment. Conditions such as body temperature and blood glucose concentration are held within narrow limits around a set point rather than at a fixed unchanging value, because enzyme-controlled reactions in cells work properly only across a narrow range of conditions.

Negative Feedback

A control mechanism in which a change in a variable produces a response that opposes that change, returning the variable towards its set point. As the variable returns, the corrective response is reduced. The word negative refers to opposing the deviation, not to any harmful effect.

Shivering

Rapid repeated contraction and relaxation of skeletal muscles when the body is too cold. The increased muscle activity increases the rate of respiration in those muscles, and more energy is transferred to the surroundings as heat, raising body temperature towards the set point.

Type 1 Diabetes

A condition in which the pancreas does not produce enough insulin. Without sufficient insulin the blood glucose concentration becomes abnormally high after a meal and remains high, and glucose appears in the urine. It is treated by the administration of insulin, under professional medical guidance.

Insulin

A hormone secreted by the pancreas when the blood glucose concentration rises above the set point. It causes body cells to take up more glucose from the blood and causes the liver to convert glucose into glycogen for storage, so the blood glucose concentration falls back towards the set point.

Glucagon

A hormone secreted by the pancreas when the blood glucose concentration falls below the set point. It causes the liver to convert stored glycogen back into glucose and to release that glucose into the blood, so the blood glucose concentration rises towards the set point.

Blood Glucose Concentration

The concentration of glucose dissolved in the blood plasma. It is controlled by negative feedback: the pancreas detects a change and releases insulin when it is too high or glucagon when it is too low, and the liver acts as the main effector by converting glucose to glycogen or glycogen back to glucose.

• Two marks. "You sweat and your blood vessels get bigger so you cool down." Sweating is there and vasodilation is roughly there, but nothing is linked: it does not say the sweat must evaporate, it does not say which vessels widen, and "cool down" is a restatement of the question rather than a mechanism.
• Nerve or neurone? These are not synonyms and the exam distinguishes them. A neurone is a single nerve cell. A nerve is a bundle containing the fibres of many neurones, wrapped together in connective tissue. So the peripheral nervous system is made of nerves, and each nerve contains many neurones. Writing "a nerve carries an impulse to the CNS" is loose; "a sensory neurone carries an impulse to the CNS" is precise, and precise is what gets the mark.
• A wiring point students invent. Not every pathway contains all three neurones, and the syllabus does not claim it does. What you must be able to describe is the reflex arc, which does contain all three. Do not write that a sensory neurone connects directly to a muscle: sensory neurones end in the CNS, and the instruction to a muscle always leaves along a motor neurone.
• Does the brain know? Yes — just not first. While the reflex is being carried out in the spinal cord, impulses also travel up to the brain, which is why you become aware of the pain a moment later. The examinable point is the ordering: the brain is informed, it does not decide. Writing "the brain is not involved at all" is as wrong as writing "the brain decides to move the hand"; the accurate statement is that the response begins without waiting for the brain.
• Write it carefully. "The eye sees the object" will not earn a mark on a question about receptors. "Light receptors in the retina detect the light and impulses are sent along the optic nerve to the brain, where they are interpreted" earns several.
• Two things the pupil is not. It is not a muscle, and it is not a black disc. It is a hole. The blackness you see is simply the inside of the eye, which is dark. Because it is a hole, it cannot contract or dilate by itself — the iris muscles around it change its size. Write "the pupil becomes smaller", never "the pupil contracts".
• Three things a hormone is not. It is not an enzyme — it is not a catalyst and it is not used to speed up a specific reaction. It is not an electrical impulse — it is a chemical, and it travels in blood, not along a neurone. And it is not a neurotransmitter — that acts across a single synapse, not around the whole body.
• Do not write "always". The syllabus says hormonal responses are generally slower and often longer-lasting, and that wording is deliberate. An answer claiming every hormonal response is slower than every nervous one is making a stronger claim than the biology supports. Keep the hedge: "generally", "usually", "often".
• The overshoot mistake. Do not write that the response continues at full strength until the variable is exactly at the set point and then stops dead. The corrective response is reduced gradually as the deviation shrinks, which is why the variable settles into a narrow band rather than swinging wildly past the set point in the other direction.
• Do not make the skin do the hypothalamus's job. Skin receptors detect the temperature of the environment at the surface, not the temperature of the core. If a question asks how the body detects that its own temperature has risen, the answer must include the hypothalamus monitoring the blood.
• Do not let the temperature fall for ever. These responses do not run until the body is cold. As the temperature returns towards the set point, the deviation being detected gets smaller and the corrective response is reduced — sweating slows, the arterioles return towards their normal diameter. That is negative feedback, and mentioning it turns a good answer into a complete one.
• Glycogen, glucagon, glucose. Three words, and in an exam under pressure they blur. Glucose is the sugar dissolved in the blood. Glycogen is the storage carbohydrate made of many glucose molecules, held in the liver. Glucagon is the hormone. Write each one out slowly the first time you use it in an answer.
• Direction check. Insulin goes with high and pushes down. Glucose to glycogen. Out of the blood, into store. If your answer has insulin doing anything that raises blood glucose, you have written glucagon's job under insulin's name.
• Do not stop at "the liver converts glycogen to glucose". That is only half the response. The glucose is no use to anyone while it is still inside a liver cell — the liver must also release it into the blood. Answers that omit the release step routinely drop a mark.

• Coordination behaves differently on the two papers, and knowing which behaviour you are facing changes how you write.
• Distractors here are almost always reversals. Four options, two of which are the correct mechanism with one word swapped: circular for radial, insulin for glucagon, contract for relax. Read the whole option before choosing, and check the direction of every arrow in a given diagram before you look at the answers.
• Expect one labelling question (eye, skin, neurone or reflex arc), one sequencing question worth four to six marks, and one data question on a blood-glucose or temperature graph. The sequencing question is where the marks concentrate and where they are most often thrown away.
• State wants a fact, no reason. Describe wants what happens, in order. Explain wants what happens and why — every "because" is a mark. Compare wants both sides of each point in the same sentence, not two separate lists.
• Mechanism questions in this topic are marked by counting linked steps, and a step only counts if it is connected to the one before it. Compare these two answers to "Explain how the body responds when its temperature rises above the set point" (4 marks).
• Two marks. "You sweat and your blood vessels get bigger so you cool down." Sweating is there and vasodilation is roughly there, but nothing is linked: it does not say the sweat must evaporate, it does not say which vessels widen, and "cool down" is a restatement of the question rather than a mechanism.