Year 9 Science | Victorian Curriculum 2.0
Nervous & endocrine systems, homeostasis
Topic 02 | Biological sciences | Practice

What you will learn

Worked example 0 Real-world example: getting cold on a winter morning

You step outside on a 4∘C4^{\circ}\text{C} morning wearing only a t-shirt. Your core body temperature is 37∘C37^{\circ}\text{C}. Trace the response.

  1. Temperature sensors in skin detect cold; they send nerve impulses to the hypothalamus in the brain.
  2. The hypothalamus compares the signal with the set point (37∘C37^{\circ}\text{C}) and finds the body is cooling.
  3. It triggers responses: blood vessels in skin constrict (less heat lost), muscles shiver (heat from contractions), and the adrenal gland releases adrenaline (raises metabolic rate).
  4. Core temperature returns to 37∘C37^{\circ}\text{C}; sensors stop firing, responses ease off.

Key idea: a change triggers a response that reverses it. This is negative feedback.

1. Neurons and nerve impulses

A neuron (nerve cell) has three main regions.

The nerve impulse is an electrical signal — a brief change in voltage across the cell membrane — that travels along the axon.

dendritescell bodymyelin sheathaxonterminalsdirection of impulse
A typical motor neuron. Signals enter through dendrites, are integrated in the cell body, and travel along the myelinated axon to the axon terminals.

2. The synapse

Neurons do not touch each other. Where one neuron meets the next there is a tiny gap called the synapse.

  1. An impulse reaches the axon terminal.
  2. Chemicals called neurotransmitters are released from vesicles into the gap.
  3. They bind to receptors on the next neuron’s dendrites, triggering a new impulse.
  4. Enzymes break the neurotransmitter down so the next signal can be sent clearly.

Common neurotransmitters: acetylcholine (muscles), dopamine (reward, movement), serotonin (mood), noradrenaline (alertness).

Worked example 1 Reflex arc

You touch a hot stove and pull your hand away before you consciously feel pain. Trace the path.

  1. Heat receptors in the skin send impulses along a sensory neuron to the spinal cord.
  2. In the spinal cord, a relay neuron passes the signal directly to a motor neuron (bypassing the brain).
  3. The motor neuron triggers arm muscles to contract, pulling the hand away.
  4. A second pathway reaches the brain slightly later — that is when you feel the pain.

Key idea: reflexes are fast because they skip the brain. Survival value: minimise tissue damage.

3. The endocrine system

Hormones are chemical messengers released by endocrine glands into the bloodstream. They travel to target cells that have matching receptors.

GlandHormoneMain effect
Pituitary (in brain)growth hormone, ADHgrowth; water retention
Thyroidthyroxineraises metabolic rate
Pancreasinsulin, glucagonlowers / raises blood glucose
Adrenaladrenaline, cortisolfight-or-flight response; stress
Ovariesoestrogen, progesteronefemale development and cycle
Testestestosteronemale development

Hormones are slower than nerves but reach every cell via blood. They can last minutes, hours, or days.

4. Homeostasis and negative feedback

Homeostasis is the maintenance of a stable internal environment despite changes outside. Key examples: core temperature, blood glucose, water balance, blood pH.

A negative feedback loop has four parts.

  1. Stimulus: a change from the set point.
  2. Receptor: detects the change.
  3. Control centre: compares the signal with the set point (often the hypothalamus).
  4. Effector: carries out a response that reverses the change.
Stimulus(change from set point)Receptor + centreEffector responsenegative feedback (reverses change)
Generic negative feedback loop. The response opposes the original change, returning the variable toward the set point.
Worked example 2 Blood glucose regulation

After a meal, blood glucose rises above 5.55.5 mmol/L.

  1. The pancreas detects the rise.
  2. Beta cells release insulin into the blood.
  3. Insulin binds to liver, muscle, and fat cells, which take up glucose (stored as glycogen or fat).
  4. Blood glucose falls back to the set point; insulin release slows.

If glucose later falls below the set point (e.g. between meals):

  1. Alpha cells in the pancreas release glucagon.
  2. The liver breaks down glycogen, releasing glucose back into the blood.

Key idea: two hormones, insulin and glucagon, push glucose in opposite directions around the set point.

Worked example 3 Thermoregulation

On a hot day your core temperature rises above 37∘C37^{\circ}\text{C}.

  1. Skin and hypothalamus detect the rise.
  2. Blood vessels in skin dilate (vasodilation) — more blood near the surface; more heat radiates away.
  3. Sweat glands release sweat — evaporation cools the skin.
  4. Behaviour: you seek shade, drink water.
  5. Temperature returns to 37∘C37^{\circ}\text{C}.

Key idea: homeostasis usually combines physiological responses (automatic) and behavioural responses (conscious).

Practice: Year 9

Fluency

Structure and terminology

    1. Label the three main parts of a neuron and describe the function of each.
    2. Define synapse and explain how signals cross it.
    3. Name two endocrine glands and the main hormone released by each.
    4. What is meant by homeostasis? Give two examples of variables it regulates.
    5. State the four components of a negative feedback loop.
    6. Contrast the nervous and endocrine systems under the headings: speed, duration, message type.
Reasoning

Apply the ideas

    1. Explain why a reflex action does not require conscious thought and why this is useful.
    2. Insulin is given as an injection rather than a tablet. Suggest why (think about what happens to proteins in the stomach).
    3. A person with an overactive thyroid has high thyroxine. Predict two effects on the body.
    4. When frightened, heart rate rises, pupils dilate, and blood is redirected to muscles. Which hormone causes this, and from where is it released?
    5. Describe how the body returns blood glucose to normal after eating a sugary snack.
Problem solving

Feedback loops

    1. Draw a labelled flowchart of the negative feedback loop that cools the body when it overheats. Include: stimulus, receptor, control centre, effector, response, set point.
    2. A patient with type 1 diabetes produces no insulin. Predict what happens to their blood glucose after a meal, and why injected insulin helps.
    3. Shivering generates heat. Explain how this is a response in a negative feedback loop, naming the stimulus and the effector.
    4. Compare the time scales of (a) catching a ball (nervous), (b) puberty (endocrine). Estimate how many orders of magnitude apart they are in seconds.

Challenge

Reasoning

Harder reasoning

    1. During childbirth, oxytocin release causes contractions, which trigger more oxytocin release. Identify this as positive or negative feedback and explain why it must eventually stop.
    2. Adrenaline can act in under a second even though it is a hormone. Propose one reason why it is faster than most hormones (hint: where it is released, and how close the adrenal glands are to blood vessels).
    3. A person drinks 2 L of water in 10 minutes. Describe how the kidneys and hormone ADH restore normal blood water content. Identify the stimulus, receptor, effector, and response.
    4. Some medications block the enzyme that breaks down acetylcholine in synapses. Predict the effect on muscle activity, and explain why an overdose could be dangerous.
Year 9 Science study companion | Practice