Coordination, Response & Homeostasis

The nervous system and the reflex arc (order is everything)

A reflex action is a fast, automatic (involuntary) response that protects the body. Examiners want the reflex arc in the correct sequence, and marks are awarded for the order: stimulus → receptor → sensory neurone → relay neurone (in the spinal cord/CNS) → motor neurone → effector → response.

Use precise terms: the receptor detects the stimulus, the sensory neurone carries the impulse to the CNS, the relay neurone connects within the spinal cord, the motor neurone carries the impulse to the effector (a muscle or gland), which produces the response. The gap between neurones is a synapse, where a chemical (neurotransmitter) diffuses across (Extended). A reflex is fast and does not involve conscious thought initially, which is why it protects against danger. The classic example is pulling your hand off a hot object. Missing a step or reversing the order loses sequence marks.

The eye: structure, function and the iris reflex

Know the eye's parts and functions: the cornea refracts (bends) light; the iris controls how much light enters by changing pupil size; the lens focuses light onto the retina; the retina contains light receptors; the optic nerve carries impulses to the brain.

The most-examined process is the pupil reflex. In bright light, the circular muscles of the iris contract and the radial muscles relax, making the pupil smaller to protect the retina. In dim light, the radial muscles contract and circular muscles relax, making the pupil larger to let in more light. (Extended only) Accommodation: to focus on near objects the ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes thicker/more rounded; for distant objects the reverse. Students often swap circular and radial muscles. Learn 'circular contract = constrict in bright light'.

Hormones and nervous control compared

A hormone is a chemical substance, produced by a gland, carried by the blood, which alters the activity of one or more target organs. Adrenaline (from the adrenal glands) prepares the body for action: it increases heart rate and breathing rate, raises blood glucose, and widens pupils. The 'fight or flight' response (Extended).

The comparison is a reliable exam question:

The key marks are 'travels in the blood' for hormones and 'fast but short-lived' versus 'slower but longer-lasting'. Saying hormones 'travel along nerves' is a clear error.

Homeostasis and negative feedback (Extended)

Define homeostasis as the maintenance of a constant internal environment. The mechanism is negative feedback: when a factor moves away from its set point, a change is triggered that reverses it back towards normal. The general pattern examiners want: a change is detected by a receptor, a corrective mechanism (effector) acts to return the factor to normal, then conditions are restored.

(Extended only) Blood glucose control: when glucose rises, the pancreas releases insulin, which causes the liver to convert excess glucose to glycogen, lowering glucose. When glucose falls, the pancreas releases glucagon, causing glycogen to be converted back to glucose. Type 1 diabetes is the inability to produce enough insulin. Students confuse insulin/glucagon and glucose/glycogen. Spell each precisely. Always frame the answer as 'a change triggers a response that reverses the change' to score the negative-feedback mark.

Temperature regulation and tropisms

(Extended for detail) Thermoregulation uses the skin. When too hot: vasodilation (arterioles widen so more blood flows near the skin surface, losing heat by radiation) and increased sweating (sweat evaporates, taking heat away). When too cold: vasoconstriction (arterioles narrow, reducing heat loss), shivering (muscle contraction releases heat from respiration), and hairs stand up to trap air. The classic confusion is vasodilation versus vasoconstriction. Remember 'dilation = wider = lose heat'.

For plants, tropisms are growth responses: phototropism (growth towards light) and gravitropism/geotropism (response to gravity). (Extended only) Auxin is a plant hormone made at shoot tips; it accumulates on the shaded side, causing those cells to elongate more, so the shoot bends towards the light. Saying the plant 'wants' to grow to light loses marks. Describe it as a growth response controlled by auxin. To practise the feedback and tropism answers, take a trial class.

Key terms to connect with this topic

Use these definitions inside explanations and questions rather than memorising them as isolated sentences.

• Accommodation: The process by which the eye changes the shape of the lens to focus on objects at different distances. For near objects, the lens becomes fatter; for distant objects, it becomes thinner. This is controlled by the ciliary muscles and suspensory ligaments.
• ADH: Antidiuretic hormone, produced by the pituitary gland, that controls water reabsorption in the kidneys. When blood water concentration is low, more ADH is released, making the collecting duct walls more permeable so more water is reabsorbed, producing less and more concentrated urine.
• Adrenaline: A hormone produced by the adrenal glands in response to stress or danger. Adrenaline increases heart rate, breathing rate and blood glucose concentration, preparing the body for a 'fight or flight' response by directing more oxygen and glucose to muscles.
• Auxin: A plant growth hormone (specifically indole-3-acetic acid, IAA) that controls cell elongation and is responsible for phototropism and gravitropism. Auxin accumulates on the shaded side of a shoot, causing cells there to elongate more, bending the shoot towards light.
• Cornea: The transparent front layer of the eye that refracts (bends) light entering the eye. The cornea provides most of the eye's optical power, bending light rays so they begin to focus on the retina.
• Effector: A muscle or gland that carries out a response when stimulated by a nerve impulse from a motor neurone. Muscles respond by contracting (producing movement) and glands respond by secreting substances such as hormones or enzymes.
• Endocrine gland: A ductless gland that produces hormones and secretes them directly into the blood. Examples include the pancreas (insulin, glucagon), adrenal glands (adrenaline), pituitary gland (ADH, growth hormone, FSH, LH), ovaries (oestrogen, progesterone) and testes (testosterone).
• Glucagon: A hormone produced by the alpha cells of the islets of Langerhans in the pancreas when blood glucose concentration is too low. Glucagon stimulates the liver to convert stored glycogen back into glucose and release it into the blood, raising blood glucose levels.
• Homeostasis: The maintenance of a constant internal environment in the body. For example, keeping core temperature and blood glucose concentration within narrow limits.
• Hormones: Chemical substances produced by endocrine glands and transported in the blood to target organs where they produce a specific effect. Hormones are slower-acting but longer-lasting than nerve impulses. Examples include insulin, adrenaline, oestrogen and testosterone.
• Insulin: A hormone produced by the beta cells of the islets of Langerhans in the pancreas when blood glucose concentration is too high. Insulin stimulates liver and muscle cells to take up glucose and convert it to glycogen for storage, lowering blood glucose levels.
• Iris: The coloured circular structure in the eye that controls the size of the pupil, regulating how much light enters the eye. In bright light, circular muscles in the iris contract and radial muscles relax, making the pupil smaller. In dim light, radial muscles contract and circular muscles relax, making the pupil larger.
• Lens: A transparent, flexible biconvex structure in the eye that fine-focuses light onto the retina by changing shape. For near objects, the ciliary muscles contract, suspensory ligaments slacken and the lens becomes fatter (more convex). For distant objects, ciliary muscles relax, ligaments pull tight and the lens becomes thinner.
• Motor neurone: A neurone that carries nerve impulses from the central nervous system (brain or spinal cord) to an effector, which is a muscle or gland. The motor neurone has a long axon that extends from the CNS to the effector.
• Negative feedback: A control mechanism in which a change in a condition triggers a response that reverses the change, returning the condition to its set point. The basis of homeostasis.
• Neurone: A specialised nerve cell that transmits electrical impulses rapidly around the body. Neurones have a cell body containing the nucleus, dendrites that receive impulses, and a long axon that carries the impulse away from the cell body.
• Optic nerve: A bundle of sensory neurone fibres that carries nerve impulses from the light receptors in the retina to the brain, where the impulses are interpreted as visual images.
• Receptor: A specialised cell or group of cells that detects a stimulus (a change in the environment) and converts it into a nerve impulse. Examples include light receptors in the retina, temperature receptors in the skin and chemical receptors on the tongue.
• Reflex arc: The nerve pathway of a reflex action: receptor → sensory neurone → relay neurone (in the spinal cord) → motor neurone → effector, producing a fast, automatic response that protects the body.
• Relay neurone: A short neurone found within the central nervous system (brain or spinal cord) that connects sensory neurones to motor neurones. In reflex arcs, the relay neurone transfers the impulse between the incoming sensory signal and the outgoing motor signal.
• Retina: The light-sensitive layer at the back of the eye containing photoreceptor cells (rods and cones) that detect light and convert it into nerve impulses. These impulses are sent to the brain via the optic nerve for interpretation as images.
• Sensory neurone: A neurone that carries nerve impulses from a receptor (such as a pain receptor in the skin) to the central nervous system (brain or spinal cord). It has a long dendron connecting the receptor to the cell body, and a short axon entering the CNS.
• Sweating: The secretion of sweat (a dilute salt solution) onto the skin surface by sweat glands in response to high body temperature. As sweat evaporates from the skin, it absorbs heat energy (latent heat of vaporisation), cooling the body down.
• Synapse: A tiny gap between two neurones across which nerve impulses are transmitted by chemical transmitter substances (neurotransmitters). The neurotransmitter is released from the presynaptic neurone, diffuses across the gap, and binds to receptors on the postsynaptic neurone, triggering a new impulse.
• Thermoregulation: The maintenance of a constant core body temperature despite changes in the external environment. In humans, this involves responses controlled by the hypothalamus in the brain, including vasoconstriction, vasodilation, sweating and shivering.
• Tropism: A growth response of a plant in which the direction of growth is determined by the direction of the stimulus. Phototropism responds to light, gravitropism to gravity.
• Type 1 diabetes: A condition in which the pancreas produces little or no insulin because the insulin-producing beta cells have been destroyed by the immune system. Without insulin, blood glucose levels remain dangerously high after eating. It is managed by regular insulin injections and monitoring blood glucose levels.
• Type 2 diabetes: A condition in which the body's cells become resistant to insulin, or the pancreas does not produce enough insulin, so blood glucose levels are not properly regulated. It is often linked to obesity, a high-sugar diet and lack of exercise, and is managed primarily through diet control and exercise.
• Vasoconstriction: The narrowing of arterioles near the skin surface in response to cold. This reduces blood flow close to the skin surface, decreasing heat loss by radiation from the blood to the environment and helping to maintain core body temperature.
• Vasodilation: The widening of arterioles near the skin surface in response to heat. This increases blood flow close to the skin surface, increasing heat loss by radiation from the blood to the cooler environment and helping to cool the body down.

Browse the full IGCSE Biology glossary when a related term needs checking.