Example 1: a reaction time
A ruler falls 0.18 m before it is caught. Use t = √(2d ÷ 9.8) to find the reaction time.
- t = √(2 × 0.18 ÷ 9.8) = √0.0367.
- t = 0.19 s.
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IB DemystifiedMYP Sciences
Nervous system and homeostasis
You snatch your hand from a hot pan before you even feel the pain. On a scorching day your body sweats to keep its core within a degree or two of 37 °C. Both depend on detecting a change and responding fast.
Recommended for MYP 5 · eAssessment priority · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the nervous system
- Temperature control
- The nervous system in the real world
- Worked examples
- In the eAssessment
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- describe the structure of the nervous system and the types of neurone
- explain the reflex arc and why reflexes are fast
- explain how synapses transmit impulses, and how some drugs affect them
- explain homeostasis and negative feedback
- explain how body temperature is controlled
- evaluate decisions about heat safety and brain health
Before you start
You will use these skills. If any feel shaky, review them first.
- cells and organ systems (see Organisation)
- enzymes and temperature
- evaporation and energy transfer (see Thermal energy transfer)
Key vocabulary
- Stimulus / receptor
- A change in the environment / a cell that detects it.
- Neurone
- A nerve cell that carries electrical impulses: sensory, relay or motor.
- Synapse
- The gap between two neurones, crossed by chemical neurotransmitters.
- Effector
- A muscle or gland that produces a response.
- Homeostasis
- Keeping internal conditions within narrow limits.
- Negative feedback
- A response that reverses a change, returning a condition towards normal.
- Vasodilation / vasoconstriction
- Widening / narrowing of blood vessels supplying the skin.
Understanding the nervous system
What is it?
The nervous system detects changes and coordinates fast responses. Receptors detect stimuli; sensory neurones carry impulses to the brain and spinal cord (the CNS); motor neurones carry impulses to effectors. Homeostasis uses such systems to keep conditions like temperature steady.
Why does it work?
Impulses travel along neurones as electrical signals and cross synapses as chemicals. Reflexes use a short route through the spinal cord, so they are fast and automatic. In temperature control, the hypothalamus detects changes in blood temperature and triggers responses that reverse them: negative feedback.
How do we know?
Charles Sherrington studied reflexes in the late 1800s and named the synapse. Otto Loewi showed in 1921 that nerves release chemicals to pass on signals. Body temperature measurements during exercise and in hot or cold rooms show the feedback responses at work.
Why does it matter?
Damage to the nervous system can cause paralysis or loss of sensation. Many medicines and drugs act at synapses. Failure of temperature control in heatwaves causes heatstroke, a medical emergency.
What does it connect to?
This topic links to hormones and blood glucose control, enzymes and cells in biology; to electrical signals in physics; and to drug chemistry and public health.
Temperature control
| Too hot | Too cold |
|---|---|
| Sweating: evaporation removes energy from the skin | Shivering: muscle respiration releases heat |
| Vasodilation: more blood near the skin surface loses heat | Vasoconstriction: less blood near the surface, less heat lost |
| Hairs lie flat | Hairs stand up, trapping air |
The hypothalamus detects the temperature of the blood and coordinates these responses. Each response opposes the change, so body temperature returns towards about 37 °C: negative feedback.
The nervous system in the real world
Doctors test reflexes, such as the knee jerk, to check that the spinal cord and nerves are working. Anaesthetics used in surgery block nerve impulses so patients feel no pain. Heat action plans in cities like Karachi and Ahmedabad warn people when conditions become dangerous for the body's temperature control.
Worked examples
Example 2: negative feedback
Describe what happens when you walk from a warm room into cold air.
- Skin receptors and the hypothalamus detect the fall in temperature.
- Vasoconstriction reduces blood flow near the skin; shivering begins.
- Heat loss falls and heat production rises, so core temperature returns towards 37 °C.
In the eAssessment
Coordination questions reward precise sequences and correct terms. Expect:
- Follow pathways from stimulus to response, naming each neurone and structure.
- Explain synapses and predict the effects of drugs that act on them.
- Interpret data from reaction-time tests or exercise experiments.
- Discuss heat safety or sports injuries, using evidence carefully.
Common ways to lose marks: saying blood vessels move towards the skin; saying reflexes are decided by the brain; confusing sensory and motor neurones; and describing feedback without saying how the response reverses the change.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: modelling how sweating cools the body
Partially guided investigation · about 45 minutes · pairs
- Research question
- How does wetting a cloth wrapped round a thermometer bulb, and blowing air over it with a fan, affect the temperature fall over 10 minutes?
- Scientific background
- Sweat cools the skin because evaporating water takes energy from the surface. Evaporation is faster when air moves over the surface. A thermometer wrapped in wet cloth models skin covered in sweat.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Four thermometers or temperature probes, identical strips of cotton cloth, elastic bands, water at room temperature, a small fan, clamp stands, stopwatch.
- Method
- Wrap each thermometer bulb in an identical cloth strip: two dry and two soaked in water at room temperature.
- Place one dry and one wet thermometer in front of the fan, and the others away from any draught.
- Record each temperature every minute for 10 minutes.
- Repeat, and compare the mean temperature fall in each condition.
Safety. Keep water away from the fan and its cable, and keep fingers clear of the fan blades. Handle glass thermometers carefully.
Then evaluate: how well does this model represent real sweating skin, which is warm and has blood flowing through it?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
|---|