Nervous vs hormonal control
Nervous control sends an electrical impulse along a neurone: fast, short-lived, aimed at one precise effector. Hormonal (chemical) control sends a hormone through the blood: slower, longer-lasting, able to act on several target organs at once. Full marks need BOTH halves of each pair — faster/slower, shorter/longer, one effector/many targets — never an isolated fact about one system.
CNS = brain + spinal cord
The CNS is the brain AND the spinal cord — naming only the brain is incomplete. The brain coordinates conscious, voluntary responses; the spinal cord relays impulses to and from the brain and organises reflex actions without it. Peripheral nerves lie outside the CNS and carry impulses both ways, giving the pathway: stimulus → receptor → peripheral nerve → CNS → peripheral nerve → effector → response.
Reflex arc — five parts in fixed order
A reflex arc runs receptor → sensory neurone → relay neurone (inside the CNS) → motor neurone → effector, the effector being a muscle that contracts or a gland that secretes. A reflex action is what that pathway produces: automatic (involuntary) and rapid, with no involvement of the conscious part of the brain. Both elements are required — "a fast response" alone is not the definition. That speed is exactly what makes reflexes protective.
Drawn from real examiner reports.
Effector ≠ the stimulus
The effector is the structure that RESPONDS — the muscle that contracts or the gland that secretes. The stimulus is whatever triggered the receptor: the pin, the hot object, the hammer tapping the knee. Examiners repeatedly saw "the hammer" offered as the effector. Name the muscle or gland, never the object that caused the reflex.
Flagged s23 Papers 31/32 Q8
Spinal cord ≠ relay neurone
The spinal cord is an organ of the CNS; the relay neurone is one cell running inside it. A labelled block of cord tissue on a reflex-arc diagram must be called "spinal cord", not "sensory neurone" or "relay neurone" — examiners record exactly this substitution. The neurones are the wiring; the cord is the housing they run through.
Flagged s23 Papers 31/32 Q8
The relay neurone is the middle one
The relay neurone sits between the sensory and the motor neurone, inside the CNS — never first, never last. Candidates who fail to identify it on a diagram usually place it at one end of the pathway. Anchor it: sensory carries the impulse IN, motor carries it OUT, relay is the connection made inside the spinal cord.
Flagged s23 Paper 12 Q26 · w22 Paper 32 Q9b
Synapse ≠ synapsis (meiosis)
(Extended) A synapse is the tiny gap between two neurones where a neurotransmitter carries the signal across. Synapsis is the pairing of homologous chromosomes in meiosis — a different process in a different topic. The two words differ by one letter, and only "synapse" is credited here. Write it deliberately every time.
Flagged s23 Paper 31 Q8c
Receptor proteins are post-synaptic
(Extended) Receptor proteins for the neurotransmitter sit on the post-synaptic membrane — the receiving (dendrite) end of the NEXT neurone. Asked to mark them on a diagram, many candidates put the cross along the axon or at the wrong end of the neurone. The transmitting side releases; the receiving membrane binds.
Flagged w22 Paper 43 Q1b ii
It binds proteins, not "the neurone"
(Extended) The neurotransmitter diffuses across the synaptic gap and binds to specific receptor proteins on the post-synaptic membrane. Writing that it "binds to the next neurone" drops the mark: the named structure is what earns credit, and it is also what explains why transmission can only run one way across a synapse.
Flagged s23 Paper 22 Q24
"Nervous is fast" is half an answer
A compare question needs both systems in the same sentence, in comparative language. "Nervous is fast" and "hormones travel in the blood" are true statements that compare nothing. Write instead: nervous control is FASTER but SHORTER-LASTING than hormonal control, which is SLOWER, LONGER-LASTING and reaches MORE target organs.
Flagged w22 Paper 43 Q1c
Walk the arc in order
Answer reflex-arc questions along the pathway, never out of sequence: stimulus (the trigger, not a labelled part) → receptor detects → sensory neurone in → relay neurone inside the CNS → motor neurone out → effector contracts or secretes.
Pair every comparison
Take one feature at a time and write both halves: speed (faster/slower), duration (shorter-lived/longer-lasting), target (one precise effector / several target organs). Three paired sentences answer a three-mark "compare" question cleanly.
(Extended) Five steps across a synapse
Impulse reaches the pre-synaptic neurone → neurotransmitter released into the gap → diffuses across → binds to receptor proteins on the post-synaptic membrane → new impulse starts in the next neurone. Diffusion and the receptor proteins are the steps most often missed.
Say why a reflex beats a decision
"Explain why the response is faster" wants the shortcut named: the impulse is coordinated by the relay neurone in the spinal cord and does not go to the conscious part of the brain. Pain registered afterwards is not part of the arc.
Nervous control uses electrical impulses carried along neurones. It is fast, produces a short-lived response, and typically acts on a specific, precise effector.
Chemical (hormonal) control uses hormones — chemical messengers carried in the blood plasma. It is slower, produces a longer-lasting response, and can act on multiple target organs at once.
| Feature | Nervous control | Chemical (hormonal) control |
|---|---|---|
| Signal type | Electrical impulse (neurone) | Hormone (blood) |
| Speed | Fast | Slow |
| Duration | Short-lived | Long-lasting |
| Target | Specific, single effector | Often several target organs |
The nervous system is made up of:
List the five components of a reflex arc, in order.
(a) State what is meant by a reflex action. [2]
(b) Name the two parts that make up the central nervous system (CNS). [2]
(c) State ONE difference between the central nervous system and peripheral nerves. [1]
