Disease and response · GCSE Biology
Nervous system and reflexes
Sequence a reflex arc for GCSE Biology: stimulus, receptor, sensory neurone, relay neurone, motor neurone, effector, synapse with neurotransmitter, and why the brain is not a required stop.
Homeostasis keeps conditions in a narrow range. A reflex is receptor → sensory → relay in the CNS → motor → effector. Synapses fire one way because neurotransmitter is released on one side only.
The important bits
What you need to know
- 1
Homeostasis is the regulation of the internal environment — blood glucose, temperature, water — so that enzymes and cells can function.
- 2
A receptor detects a stimulus (change in the environment). A coordinator (spinal cord or brain) processes the information. An effector (muscle or gland) produces a response.
- 3
The nervous system is fast and uses electrical impulses along neurones. The endocrine system is slower and uses hormones in the blood.
- 4
Reflex arc sequence: stimulus → receptor → sensory neurone → synapse → relay neurone in the CNS → synapse → motor neurone → effector. The response is rapid and automatic.
- 5
A synapse is a gap between neurones. An impulse arriving causes a chemical neurotransmitter to be released; it diffuses across and starts an impulse in the next neurone. That is why synapses are one-way.
- 6
Reflexes protect: withdrawing a hand from a hot object, blinking, knee-jerk. Conscious thought is too slow; the brain may become aware afterwards.
- 7
Sensory neurones carry impulses to the CNS. Motor neurones carry impulses from the CNS to effectors. Relay neurones connect them in the spinal cord or brain.
- 8
Reaction-time practical (AQA): ruler drop, independent variable such as caffeine or practice, dependent variable distance the ruler falls (converted to time), controls include ruler type, hand position, and repeats.
Quotations worth analysing
Short evidence. Real method.
“stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector”
Write it in order. Swapping sensory and motor is the classic zero. Name the synapse if marks remain.
“Neurotransmitter diffuses across the synapse and triggers an impulse in the next neurone.”
Chemical, not electrical, in the gap. One-way because the transmitter is only released from the first neurone.
“Reflexes are automatic and rapid, and they protect the body.”
Do not put the brain as a required stop in a spinal reflex. Awareness can come later.
Go deeper
How do I label a reflex-arc diagram under exam pressure?
Find the receptor in the skin or sense organ — that is the start, not the brain. Follow the neurone into the spinal cord: that is sensory (cell body often in a ganglion to the side). The short neurone entirely in the cord is the relay. The neurone leaving to a muscle is motor. The muscle is the effector. Label synapses as gaps with neurotransmitter. If the question is a blinking reflex, the coordinator may be in the brain rather than the spinal cord, but the neurone order is the same. “Nerve” is too vague; use sensory, relay, motor. Impulses are electrical along the axon; the synapse is chemical. Mixing those two is a common two-mark leak.
Go deeper
Why can a synapse only pass an impulse one way?
Vesicles of neurotransmitter sit only at the end of the first neurone. When the impulse arrives, they release the chemical into the gap. Receptors for that chemical sit only on the next neurone. The transmitter cannot be released backwards from the second cell in this design, so the pathway does not reverse. That one-way property keeps the reflex sequence in order. Drugs and toxins that block receptors or prevent release (some mark schemes mention this) stop the next impulse. For GCSE, diffusion of neurotransmitter across the gap is enough. Do not write that electricity jumps the gap. Do not write that the brain “decides” in a hot-pan reflex — the point of the arc is that it does not wait.
Go deeper
How is a reaction-time practical a fair test?
Independent variable: a factor such as caffeine, age, or practice. Dependent: how far a ruler drops before it is caught, converted to time with a look-up or the equation of motion if given. Controls: same ruler, same start height, same fingers, same sitting position, no warning, same person dropping the ruler. Repeats and a mean. Caffeine is a stimulant that can speed synaptic transmission, so you might predict a shorter catch time — but you must still control sleep and practice, because both change results. This practical measures a voluntary reaction, not a spinal reflex, so it is slower than a true reflex arc. Mention that limitation if asked to evaluate.
See the idea in action
A student steps on a pin. Pain receptors in the skin are stimulated. A sensory neurone carries an electrical impulse to the spinal cord. Neurotransmitter diffuses across a synapse to a relay neurone, then across another synapse to a motor neurone. The motor neurone stimulates leg muscles (effectors), which contract and lift the foot. The brain becomes aware afterwards. The sequence is still receptor, coordinator, effector — faster than a considered decision. Writing “the brain thinks pull your foot up” would miss the reflex definition.
Exam technique
Turn knowledge into marks
Memorise the six-part sequence and the word neurotransmitter. Label neurones by function. For homeostasis introductions, say conditions are kept in a narrow range so enzymes can work. Do not require the brain in every reflex.
Common mistakes
Do not give these marks away
- 01
Putting the brain as a required stop in a spinal reflex, or listing neurones in the wrong order.
- 02
Saying electricity jumps the synapse, or that the effector is a receptor.
- 03
Calling a voluntary ruler-catch a reflex without noting it involves conscious pathways.
Which is the correct order in a spinal reflex arc?
AEffector → motor neurone → receptor → sensory neurone
BReceptor → sensory neurone → relay neurone → motor neurone → effector
CBrain → sensory neurone → effector → receptor
DMotor neurone → synapse → receptor → brain
Show the answer
Receptor → sensory neurone → relay neurone → motor neurone → effector. The receptor starts the arc. Sensory neurones go to the CNS, relay neurones connect, motor neurones go to the effector. The brain is not required for a spinal reflex.
Quick questions
If this is the bit you searched
What is a reflex arc GCSE?
An automatic pathway: stimulus, receptor, sensory neurone, relay neurone in the CNS, motor neurone, effector. It is fast and does not wait for conscious thought.
What happens at a synapse?
An impulse causes neurotransmitter to be released. It diffuses across the gap and starts an impulse in the next neurone, so the signal travels in one direction only.
What is the difference between the nervous and endocrine systems?
Nervous: electrical impulses, fast, short-lived, neurones. Endocrine: hormones in blood, slower, often longer-lasting, glands and target organs.
What is homeostasis?
Keeping internal conditions (such as blood glucose, temperature and water) in a narrow range so cells and enzymes can work properly.