Coordination and Response

What you'll learn

Coordination and response examines how organisms detect changes in their environment and respond appropriately to maintain survival. You'll study the nervous system (rapid, short-lived responses) and the endocrine system (slower, longer-lasting responses), including detailed structures like the eye, reflex arcs, and hormonal control mechanisms. This topic typically accounts for 10-12% of your exam marks and requires both diagram annotation and extended writing skills.

Key terms and definitions

Stimulus — a detectable change in the internal or external environment that can be detected by receptors

Receptor — a specialised cell or organ that detects a specific stimulus and converts it into an electrical impulse

Effector — a muscle or gland that produces a response to a stimulus

Synapse — the junction between two neurones where the electrical impulse is transmitted by chemical neurotransmitters

Hormone — a chemical messenger secreted by endocrine glands, transported in blood plasma to target organs where it produces a specific effect

Reflex action — a rapid, automatic response to a stimulus that does not involve conscious thought

Homeostasis — the maintenance of a constant internal environment despite external changes

Negative feedback — a control mechanism where a change triggers responses that reverse the change, returning conditions to optimal levels

Core concepts

The nervous system structure and function

The nervous system coordinates rapid responses to stimuli through electrical impulses transmitted along specialised cells called neurones.

Central Nervous System (CNS)

• Brain and spinal cord
• Processes information from receptors
• Coordinates appropriate responses

Peripheral Nervous System

• Sensory neurones: carry impulses from receptors to CNS
• Motor neurones: carry impulses from CNS to effectors
• Relay neurones: connect sensory and motor neurones within the CNS

Neurone structure:

• Cell body contains nucleus and cytoplasm
• Dendrites receive impulses from other neurones or receptors
• Axon transmits impulses away from cell body (can be over 1 metre long)
• Myelin sheath insulates axon and speeds up impulse transmission
• Axon terminals connect to other neurones or effectors

Electrical impulses travel in one direction only: receptor → sensory neurone → CNS → motor neurone → effector

Reflex arcs

Reflex actions protect the body from harm by producing automatic responses without conscious thought. This saves time in dangerous situations.

Reflex arc pathway:

1. Stimulus detected by receptor
2. Electrical impulse passes along sensory neurone
3. Impulse crosses synapse to relay neurone in CNS
4. Relay neurone passes impulse across synapse to motor neurone
5. Motor neurone carries impulse to effector
6. Effector produces response

Examples of reflex actions:

• Withdrawing hand from hot object (pain receptors)
• Pupil reflex controlling light entering eye (light receptors in retina)
• Knee-jerk reflex (stretch receptors in leg muscles)

The conscious brain receives information about the reflex after it has occurred, which is why you feel pain after pulling your hand away from something hot.

Synapses and impulse transmission

Synapses are gaps between neurones where chemical transmission occurs.

How synapses work:

1. Electrical impulse arrives at axon terminal of pre-synaptic neurone
2. Triggers release of neurotransmitter chemicals from vesicles
3. Neurotransmitters diffuse across synaptic gap (approximately 20 nm wide)
4. Neurotransmitters bind to receptor proteins on post-synaptic membrane
5. This triggers a new electrical impulse in the next neurone
6. Neurotransmitters are broken down by enzymes to prevent continuous stimulation

Functions of synapses:

• Ensure impulses travel in one direction only (neurotransmitters only on one side)
• Filter out weak stimuli (require sufficient neurotransmitter to trigger impulse)
• Allow multiple pathways for impulse transmission

The eye and accommodation

The eye is a sensory receptor organ that detects light stimuli and allows us to see.

Key structures and functions:

Structure	Function
Cornea	Transparent front layer; refracts (bends) light entering eye
Iris	Coloured ring of muscle controlling pupil size
Pupil	Central hole allowing light to enter; size controlled by iris
Lens	Transparent, elastic structure that fine-tunes focus by changing shape
Ciliary muscles	Ring of muscle that controls lens shape
Suspensory ligaments	Connect ciliary muscles to lens; transmit tension
Retina	Light-sensitive layer containing receptor cells (rods and cones)
Fovea	Region of retina with highest concentration of cone cells for detailed colour vision
Optic nerve	Carries impulses from retina to brain

Accommodation is the process of changing the lens shape to focus on objects at different distances.

Focusing on near objects:

• Ciliary muscles contract
• Suspensory ligaments slacken
• Lens becomes more curved (fatter)
• Light refracted more strongly

Focusing on distant objects:

• Ciliary muscles relax
• Suspensory ligaments pulled tight
• Lens becomes less curved (thinner)
• Light refracted less strongly

Pupil reflex (controlling light intensity):

Bright light:

• Circular muscles in iris contract
• Radial muscles relax
• Pupil constricts (becomes smaller)
• Less light enters eye, protecting retina

Dim light:

• Radial muscles contract
• Circular muscles relax
• Pupil dilates (becomes larger)
• More light enters eye for better vision

The endocrine system and hormonal control

The endocrine system coordinates slower, longer-lasting responses using chemical messengers called hormones.

Key endocrine glands and their hormones:

Gland	Hormone	Target organ	Effect
Pituitary	ADH (antidiuretic hormone)	Kidneys	Increases water reabsorption
Pancreas	Insulin	Liver, muscles	Decreases blood glucose (converts to glycogen)
Pancreas	Glucagon	Liver	Increases blood glucose (breaks down glycogen)
Adrenal glands	Adrenaline	Heart, muscles, liver	Prepares body for "fight or flight"
Testes	Testosterone	Reproductive organs	Develops male secondary sexual characteristics
Ovaries	Oestrogen	Reproductive organs, uterus	Develops female secondary sexual characteristics; controls menstrual cycle
Ovaries	Progesterone	Uterus	Maintains uterus lining during pregnancy

Comparison of nervous and endocrine systems:

Nervous system:

• Fast transmission (electrical impulses along neurones)
• Short-lived responses
• Precise target (specific muscles or glands)
• Examples: reflex actions, voluntary movements

Endocrine system:

• Slower transmission (hormones in bloodstream)
• Longer-lasting responses
• Widespread effects (hormones reach all organs via blood)
• Examples: growth, reproduction, metabolism

Blood glucose regulation

Blood glucose concentration must be maintained within narrow limits (approximately 90 mg per 100 cm³ of blood). This is controlled by negative feedback involving insulin and glucagon.

When blood glucose rises (e.g., after eating):

1. Pancreas detects high blood glucose
2. Pancreas releases insulin into bloodstream
3. Insulin travels to liver and muscle cells
4. Cells take up glucose from blood
5. Liver converts glucose to glycogen for storage
6. Blood glucose returns to normal
7. Pancreas stops releasing insulin

When blood glucose falls (e.g., during exercise):

1. Pancreas detects low blood glucose
2. Pancreas releases glucagon into bloodstream
3. Glucagon travels to liver
4. Liver converts stored glycogen to glucose
5. Glucose released into bloodstream
6. Blood glucose returns to normal
7. Pancreas stops releasing glucagon

This is negative feedback because the response (releasing insulin or glucagon) reverses the original change (high or low glucose).

Type 1 diabetes:

• Pancreas produces insufficient insulin
• Blood glucose remains too high after eating
• Can damage organs over time
• Treatment: regular insulin injections, controlled diet
• Caused by autoimmune destruction of insulin-producing cells

Adrenaline and the "fight or flight" response

Adrenaline prepares the body for rapid action in response to stress or danger.

Effects of adrenaline:

• Increases heart rate and strength of contraction (more oxygen delivered to muscles)
• Dilates pupils (improves vision)
• Increases breathing rate (more oxygen intake)
• Diverts blood from digestive system to muscles
• Converts glycogen to glucose in liver (provides energy for muscles)
• Increases mental alertness

These changes enable rapid physical activity to escape danger or confront threats.

Worked examples

Example 1: Describe the pathway of a reflex action when you touch a hot plate. (5 marks)

Mark scheme answer:

1. Heat detected by temperature receptors in skin ✓
2. Sensory neurone carries electrical impulse to spinal cord/CNS ✓
3. Relay neurone in spinal cord carries impulse across ✓
4. Motor neurone carries impulse to effector ✓
5. Muscle contracts to withdraw hand ✓

Example 2: Explain how the eye focuses on a nearby object. (4 marks)

Mark scheme answer:

1. Ciliary muscles contract ✓
2. Suspensory ligaments slacken/become loose ✓
3. Lens becomes more curved/fatter/thicker ✓
4. Light is refracted more (so it focuses on retina) ✓

Example 3: A student's blood glucose rises after eating a meal. Explain how the body returns blood glucose to normal. (6 marks)

Mark scheme answer:

1. Pancreas detects rise in blood glucose ✓
2. Pancreas secretes insulin ✓
3. Insulin travels in blood/bloodstream to liver and muscles ✓
4. Insulin causes cells to take up glucose from blood ✓
5. Liver converts glucose to glycogen ✓
6. Blood glucose decreases back to normal/this is negative feedback ✓

Common mistakes and how to avoid them

• Don't confuse sensory and motor neurones. Sensory neurones carry impulses FROM receptors TO the CNS; motor neurones carry impulses FROM the CNS TO effectors. Remember: sensory = sensing, motor = moving.

• Don't say hormones travel along nerves. Hormones are transported in blood plasma, not in neurones. Electrical impulses travel along neurones; hormones travel in blood.

• For accommodation, don't reverse the muscle actions. When focusing on near objects, ciliary muscles CONTRACT (ligaments slacken, lens fattens). Students often write "ciliary muscles relax" which is incorrect.

• Don't confuse insulin and glucagon effects. Insulin LOWERS blood glucose; glucagon RAISES blood glucose. Remember: "insulin in" (takes glucose into cells).

• In reflex arc descriptions, include the relay neurone. Many students jump from sensory to motor neurone, missing the relay neurone in the CNS which is essential for full marks.

• Don't say the iris changes size. The PUPIL changes size; the iris muscles control this change. The iris itself doesn't expand or contract as a whole.

Exam technique for "Coordination and Response"

• "Describe" vs "Explain" questions: Describe = state what happens (pathway, structures involved). Explain = give reasons why it happens (include consequences, functions). "Explain" questions require more depth and biological reasoning.

• For reflex arc questions: Always work systematically from stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response. Don't skip stages. Each stage typically earns one mark.

• Diagram annotations: When labelling the eye or neurone diagrams, ensure arrows point precisely to structures. If asked for functions, write these separately alongside each label rather than trying to incorporate them into single-word labels.

• Hormonal control questions: Structure answers chronologically and include: (1) what detects change, (2) which hormone is released, (3) how it travels, (4) target organ, (5) specific effect, (6) result. This typically maps to 5-6 marks in extended response questions.

Quick revision summary

Coordination involves detecting stimuli through receptors and producing responses via effectors. The nervous system uses electrical impulses along neurones for rapid responses including reflex actions; impulses cross synapses using neurotransmitters. The eye detects light, with the iris controlling pupil size and lens shape changing during accommodation. The endocrine system uses hormones transported in blood for slower responses. Blood glucose is regulated by insulin and glucagon through negative feedback. Adrenaline prepares the body for fight or flight responses.