Cambridge O Level Biology 5090: Coordination and control (5090)

This study guide teaches Topic 14, Coordination and control, for Cambridge O Level Biology 5090. It is aligned to the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), and covers every learning outcome in sections 14.1 to 14.6: the nervous system, sense organs, hormones, homeostasis, temperature control and blood glucose control. The 5090 syllabus is not tiered, so every candidate studies all of it. This topic lists no set practical.

Use it with the revision notes and the practice questions. The course hub lists every topic and the printable checklist lets you tick off outcomes. Why body temperature matters links back to Enzymes, and the liver’s other jobs are in Excretion. To find weak spots fast, try the free 10-minute diagnostics.

What this topic covers

Section	What you must be able to do
14.1 Mammalian nervous system	Describe the CNS and PNS; identify sensory, relay and motor neurones; describe reflex arcs and reflex actions; describe synapse structure and events; state that synapses make impulses travel one way
14.2 Mammalian sense organs	Describe sense organs; identify and give functions of parts of the eye; explain the pupil reflex and accommodation
14.3 Mammalian hormones	Define a hormone; name five glands and their hormones; explain the role of adrenaline; compare nervous and hormonal control
14.4 Homeostasis	Define homeostasis; explain negative feedback with a set point
14.5 Temperature control	Identify parts of the skin; describe insulation and the roles of the hypothalamus and skin receptors; explain sweating, shivering, hair erector muscles, vasodilation and vasoconstriction
14.6 Blood glucose control	Explain why blood glucose is controlled; describe the roles of liver, pancreas, insulin and glucagon; describe signs and treatment of Type 1 diabetes

14.1 The mammalian nervous system

The nervous system (the brain, spinal cord and nerves) coordinates and regulates body functions. It has two parts:

• Central nervous system (CNS): the brain and the spinal cord.
• Peripheral nervous system (PNS): the nerves outside the brain and spinal cord.

Electrical impulses travel along neurones (nerve cells). A nerve is a bundle of many neurones.

Three types of neurone

Neurone	Carries impulses	How to identify it on a diagram
Sensory	From a receptor to the CNS	Long fibre on both sides of the cell body; the cell body sits on a short side branch, outside the spinal cord
Relay	Within the CNS, from sensory to motor neurone	Short, with many branches, drawn entirely inside the spinal cord or brain
Motor	From the CNS to an effector	Cell body at one end, inside the CNS, with many short branches; one long fibre runs to a muscle or gland

Reflex arcs and reflex actions

A reflex action is a rapid and automatic response to a stimulus. The pathway is a reflex arc:

stimulus → receptor → sensory neurone → relay neurone (in CNS) → motor neurone → effector → response

The effector is a muscle (which contracts) or a gland (which secretes).

Worked example. Describe the reflex arc when your hand touches a hot pan.

1. Temperature receptors in the skin of the hand detect the heat (the stimulus).
2. They start impulses in a sensory neurone, which carries them to the spinal cord.
3. In the spinal cord, a relay neurone passes the impulses to a motor neurone.
4. The motor neurone carries impulses to the effector, a muscle in the arm.
5. The muscle contracts and the hand is pulled away, quickly and automatically.

Synapses

A synapse is a junction between two neurones. Its structure:

  neurone 1 ending                         neurone 2
 ┌──────────────────┐                  ┌──────────────
 │ vesicles (o o o) │   synaptic gap   │ receptor proteins
 │ of neuro-        │ ←── (tiny) ──→   │ on the membrane
 │ transmitter      │                  │
 └──────────────────┘                  └──────────────
        impulse arrives  ──────────────►  new impulse

Events at a synapse, in order:

1. An impulse arrives and stimulates the release of neurotransmitter molecules from vesicles into the synaptic gap.
2. The neurotransmitter molecules diffuse across the gap and bind with receptor proteins on the next neurone.
3. An impulse is stimulated in the next neurone.

Synapses ensure that impulses travel in one direction only. Vesicles are only on one side of the gap and receptor proteins are only on the other, so the signal cannot pass backwards.

14.2 Mammalian sense organs

A sense organ is a group of receptor cells responding to a specific stimulus: light (eyes), sound (ears), touch and temperature (skin), and chemicals (nose and tongue).

Parts of the eye and their functions

Part	Function
Cornea	Transparent front layer; refracts light
Iris	Coloured ring of muscle; controls how much light enters the pupil
Pupil	The hole in the centre of the iris that light passes through
Lens	Focuses light onto the retina
Ciliary muscles and suspensory ligaments	Control the shape of the lens
Retina	Contains light receptors, some sensitive to light of different colours
Fovea	Part of the retina with the greatest density of light receptors
Optic nerve	Carries impulses to the brain
Blind spot	Where the optic nerve leaves the eye; it has no light receptors

On a diagram, the ciliary muscles form a ring around the lens, the suspensory ligaments join them to the lens, the fovea is opposite the lens on the retina, and the blind spot is where the optic nerve joins.

The pupil reflex

The iris has two sets of muscles that work antagonistically (one contracts while the other relaxes):

Light intensity	Circular muscles	Radial muscles	Pupil
Bright	Contract	Relax	Becomes smaller (constricts), so less light enters and the retina is protected
Dim	Relax	Contract	Becomes wider (dilates), so more light enters and you can see

This is a reflex: light receptors in the retina are the receptors, and the iris muscles are the effectors.

Accommodation

Accommodation is changing the shape of the lens to focus on near or distant objects.

	Near object	Distant object
Ciliary muscles	Contract	Relax
Suspensory ligaments	Slacken (less tension)	Pulled tight (more tension)
Lens shape	Fatter, more convex	Thinner, less convex
Refraction of light	More	Less

Worked example. You look up from a book to a bird in a distant tree. Explain how your eye focuses on the bird.

1. The ciliary muscles relax.
2. The suspensory ligaments are pulled tight, so tension in them increases.
3. They pull the lens into a thinner shape.
4. Light is refracted less, so it is focused on the retina.

14.3 Mammalian hormones

A hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

Endocrine gland	Position (on a diagram)	Hormone(s)
Pituitary gland	Base of the brain	Follicle-stimulating hormone (FSH) and luteinising hormone (LH)
Adrenal glands	On top of the kidneys	Adrenaline
Pancreas	Below the stomach	Insulin and glucagon
Testes	Scrotum (male)	Testosterone
Ovaries	Lower abdomen (female)	Oestrogen and progesterone

Adrenaline

The adrenal glands release adrenaline in frightening, stressful or exciting situations, such as being chased by a dog, waiting to sit an exam or the start of a race. It:

• increases the blood glucose concentration, so more glucose is available for respiration in muscles;
• increases the heart rate, so glucose and oxygen reach the muscles faster.

More respiration releases more energy for muscle contraction.

Nervous and hormonal control compared

	Nervous control	Hormonal control
Speed of action	Fast	Slower
Duration of effect	Short	Longer lasting

14.4 Homeostasis

Homeostasis is the maintenance of a constant internal environment, such as body temperature and blood glucose concentration.

Negative feedback works around a set point (the normal value):

 value rises above set point → detected → response brings value DOWN
 value falls below set point → detected → response brings value UP

The change itself switches on a response that reverses it, so the value stays close to the set point.

14.5 Temperature control

The skin

On a diagram of the skin identify: hairs; hair erector muscles, small muscles attached to each hair; coiled sweat glands with ducts to the surface; receptors linked to sensory neurones; blood vessels; and fatty tissue, the bottom layer.

Insulation

Mammals keep a constant internal temperature, around 37 °C in humans. Insulation reduces heat loss to cold surroundings. The fatty tissue under the skin is a poor conductor of heat, and a layer of air trapped by hairs or fur insulates too.

The hypothalamus and skin receptors

Temperature receptors in the skin detect changes in the temperature outside and send impulses along sensory neurones to the brain. The hypothalamus in the brain acts as the control centre: it monitors the temperature of the blood flowing through it, compares it with the set point and coordinates the responses below.

The responses

Response	When	How it helps
Sweating	Too hot	Sweat glands secrete sweat; water evaporates from the skin surface, transferring heat away from the body
Shivering	Too cold	Rapid, involuntary muscle contractions; the extra respiration releases heat
Hair erector muscles contract	Too cold	Hairs stand up and trap a thicker layer of air, which insulates
Vasodilation	Too hot	Arterioles supplying skin surface capillaries widen; more blood flows near the surface, so more heat is lost by radiation
Vasoconstriction	Too cold	Arterioles supplying skin surface capillaries narrow; less blood flows near the surface, so less heat is lost

Worked example. A swimmer gets out of a cold lake. Explain two ways her body reduces heat loss.

1. Skin receptors detect the cold and the hypothalamus detects a fall in blood temperature below the set point.
2. Vasoconstriction: arterioles supplying the skin surface capillaries narrow, so less warm blood flows near the skin surface and less heat is lost by radiation.
3. Hair erector muscles contract: hairs stand up and trap a layer of air, which insulates the skin.

Shivering produces heat; it does not reduce heat loss.

14.6 Blood glucose control

Why control is needed

Cells need a steady supply of glucose for respiration. Too little and cells, especially brain cells, cannot release enough energy. Too much glucose in the blood lowers its water potential, so water moves out of cells by osmosis.

The liver and the pancreas

The pancreas detects the blood glucose concentration and releases the right hormone. The liver is the target organ that stores or releases glucose.

Blood glucose	Pancreas releases	Effect on the liver	Result
Too high (after a meal)	Insulin	Converts glucose to glycogen and stores it	Blood glucose falls
Too low (exercise, fasting)	Glucagon	Converts glycogen to glucose and releases it	Blood glucose rises

This is negative feedback around a set point.

Worked example. The table gives one person’s blood glucose concentration after a meal eaten at 0 minutes.

Time / min	0	30	60	90	120
Blood glucose / mg per 100 cm³	85	135	115	95	85

Calculate the percentage increase from 0 to 30 minutes and explain the change after 30 minutes.

increase = 135 − 85 = 50 mg per 100 cm³
percentage increase = 50 ÷ 85 × 100 = 58.8 % ≈ 59 %

Explanation: glucose from digested food is absorbed, so the concentration rises above the set point. The pancreas detects this and releases more insulin. Insulin makes the liver convert glucose into glycogen, so the concentration falls back to 85 by 120 minutes.

Type 1 diabetes

In Type 1 diabetes the pancreas does not produce enough insulin. The signs you need are:

• increased blood glucose concentration;
• glucose in the urine.

The treatment is the administration of insulin, usually by injection.

Common errors

• Writing that neurotransmitter “travels as an impulse” across the gap; it diffuses.
• Saying the ciliary muscles relax for near vision. They contract, and the ligaments slacken.
• Mixing up circular and radial muscles in the pupil reflex.
• Saying blood vessels “move closer to the skin”. Arterioles widen or narrow; nothing moves.
• Saying sweat cools you by being produced. Cooling happens when water evaporates.
• Writing “glucagon” when you mean “glycogen”. Glucagon is a hormone; glycogen is the stored carbohydrate.
• Saying a hormone travels along nerves. Hormones are carried in the blood.

Next, condense this into the revision notes and test yourself with the practice questions.

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), Topic 14: Coordination and control (14.1 Mammalian nervous system; 14.2 Mammalian sense organs; 14.3 Mammalian hormones; 14.4 Homeostasis; 14.5 Temperature control; 14.6 Blood glucose control), published by Cambridge Assessment International Education.