Stimulus and response, plant tropisms and invertebrate behaviour, receptors and effectors, the reflex arc, parts of the brain, and the effects of drug abuse.
Life Processes and DiseaseSO B 7.1SO B 7.2SO B 7.3SO B 7.4SO B 7.5SO B 7.6SO B 7.7SO B 7.8≈ 40 min🎮 3 games
Before you start
By the end of this lesson you can…
define stimulus and response; receptor and effector (SO B7.1, 7.3)
describe responses of green plants and invertebrates to stimuli (SO B7.2) and why responses aid survival (SO B7.4)
explain the relationship among receptor, CNS and effector, and a simple reflex action (SO B7.5–7.6)
describe the functions of the main regions of the brain (SO B7.7)
discuss the physiological, social and economic effects of drug abuse (SO B7.8)
6 narrated scenes. Press play, listen and read along — the points appear as they are explained. Use ‹ Prev / Next › or the scene bar to jump to any scene.
LESSON B14 · EXPLAINER
Irritability, Reflexes, the Brain and Drugs
You pull your hand off a hot pot before you even feel the pain. Your brain wasn’t in charge — so what was?
Narration uses your device’s built-in voice. No sound? Switch to “Captions only” — the captions advance on their own.
Explore in 3D
The reflex arc in 3D
Follow an impulse from receptor to effector.
Step 2 · Read
Key notes
Copy these into your notebook in your own words — that is how they stick.
Stimulus / response
Stimulus = detectable change in environment; response = reaction to it.
Receptor / effector
Receptor detects stimulus; effector (muscle or gland) produces response.
Physiological, social and economic effects — give one example of each.
EXAM TIPS & COMMON MISTAKES
Effector = RESPONDS to a stimulus (P1). Receptor = DETECTS.
Order in a reflex arc diagram: sensory neurone enters the dorsal side of the spinal cord; motor neurone leaves ventrally.
Brain damage can cause blindness with healthy eyes because the visual area of the CEREBRUM interprets impulses from the optic nerve.
Step 2 · Learn from examples
Worked examples & study resources
The kind of support a teacher gives after the lesson: worked examples to attempt first, a board illustration, key terms, common misconceptions and memory aids.
Worked example 1
The hot-iron reflex
A student accidentally touches a hot iron and pulls the hand away before feeling pain. Describe the pathway of this reflex.
Try it in your notebook first, then reveal the solution one step at a time.
Stimulus: heat. It is detected by receptors (heat/pain receptors) in the skin of the finger.
An impulse travels along a sensory neurone to the spinal cord.
In the grey matter of the spinal cord it passes across a synapse to a relay neurone, then to a motor neurone.
The motor neurone carries the impulse to the effector — the biceps muscle — which contracts and pulls the hand away (the response).
The impulse also travels up the spinal cord to the brain, so the pain is felt a fraction of a second after the hand has moved.
Answer. Heat → receptors in skin → sensory neurone → relay neurone (spinal cord) → motor neurone → effector (biceps muscle) contracts → hand withdrawn.
Examiner’s tip. Reflexes are fast and automatic because the brain is not involved in the decision — the shortest possible path is used. That protects the body from damage.
Worked example 2
A phototropism experiment
Bean seedlings were grown in a box with a slit on one side, letting light in from one direction only. A second set was grown on a slowly rotating clinostat in the same box. Predict and explain the results.
Try it in your notebook first, then reveal the solution one step at a time.
Seedlings in the box with light from one side: the shoots bend and grow towards the light — positive phototropism.
This happens because a plant growth substance (auxin) collects on the shaded side of the shoot tip, making cells there elongate more, so the shoot bends towards the light.
Seedlings on the clinostat: every side receives light equally as it turns, so the shoots grow straight up.
The clinostat is the control: it shows that the bending is caused by one-sided light.
Answer. Box seedlings bend towards the light (positive phototropism); clinostat seedlings grow straight because all sides are lit equally. Bending lets leaves capture more light for photosynthesis.
Examiner’s tip. Name the tropism precisely: photo- (light), geo- (gravity), hydro- (water); positive = towards, negative = away. Roots are positively geotropic, shoots negatively geotropic.
On the board · Voluntary actions and reflex actions
Reflex action
Voluntary action
Controlled by
Spinal cord (or lower brain)
Cerebrum
Conscious thought?
No — automatic
Yes — decided
Speed
Very fast
Slower
Learned?
Inborn (not learned)
Often learned
Examples
Knee-jerk, pupil reflex, blinking, withdrawing from heat
Writing, kicking a ball, speaking
Every reflex is automatic, fast and protective — the brain is told afterwards.
Key terms
Stimulus
A change in the environment that an organism detects.
Receptor
A cell or organ that detects a stimulus.
Effector
A muscle or gland that produces the response.
Sensory neurone
Carries impulses from receptors to the central nervous system.
Relay neurone
Links sensory and motor neurones inside the spinal cord or brain.
Motor neurone
Carries impulses from the central nervous system to effectors.
Tropism
A growth response of a plant to a directional stimulus.
Common misconceptions
✗ All reflexes are controlled by the brain.✓ Spinal reflexes pass through the spinal cord; the brain is informed after the response.
✗ Plants cannot respond to stimuli.✓ Plants respond by growth movements called tropisms, and some (e.g. Mimosa) by quick movements.
✗ Only illegal substances are drugs.✓ Alcohol, nicotine and caffeine are drugs too; misuse of legal drugs causes great harm.
Alcohol is a depressant: it slows the nervous system and reaction time. That is why Caribbean countries set legal blood-alcohol limits for drivers and use breath tests.
Try this
Test your reaction time: a partner drops a 30 cm ruler between your finger and thumb, and you catch it. Record the distance it falls. Repeat five times and take the mean.
Play & practise
Games
3 quick games on this lesson. Earn XP and badges as you play — your best score for each game is saved. Games are for practice and do not change your exam readiness.
Step 3 · Practise
Paper 1 practice — multiple choice
8 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better.
Step 4 · Apply
Paper 2 practice — structured questions
Write full answers, then reveal the mark scheme and tick the points you earned (23 marks in total). Your answers are saved on this device.
Based on CSEC P2 May/June 2024, Question 5
Question 1 [11 marks]
Stimuli and responses in humans and invertebrates.
(a) Define “stimulus” and “response”. [2]
MARK SCHEME — (a) (max 2)
(b) In a knee-jerk reflex arc, identify: A (grey matter region/relay neurone), B (sensory neurone), C (motor neurone), D (effector muscle). [4]
MARK SCHEME — (b) (max 4)
(c) Timothy had no knee-jerk response. Suggest ONE biological reason. [1]
MARK SCHEME — (c) (max 1)
(d) Millipedes gathered in Molly’s basement. Explain their response to TWO stimuli there. [4]
MARK SCHEME — (d) (max 4)
Based on CSEC P2 January 2026 (hybrid) Q2 and January 2022 Q2
Question 2 [12 marks]
The brain and reflexes.
(a) Define “reflex action” and give TWO examples. [3]
MARK SCHEME — (a) (max 3)
(b) State ONE function of the cerebrum, cerebellum and medulla. [3]
MARK SCHEME — (b) (max 3)
(c) Discuss THREE different effects of drug abuse (physiological, social or economic). [6]
MARK SCHEME — (c) (max 6)
Your progress
Lesson readiness
0% READY
Not started
Video watched (20%)0%
Multiple choice (40%)0%
Structured (40%)0%
Sources
Where this topic appears in CSEC papers
Find these in your own past-paper booklets for extra timed practice. The practice above is adapted from them.