Nervous system — CNS, PNS and impulses
Neurones carry information as electrical impulses. The system splits into the central nervous system (CNS) — the brain and spinal cord — and the peripheral nervous system (PNS) — the nerves outside the brain and spinal cord, linking the CNS to receptors and effectors. Its role is the coordination and regulation of body functions: detect a stimulus, then send impulses to an effector to produce a rapid, appropriate response.
Reflex arc — the five-structure pathway
A simple reflex arc is a fixed chain: receptor sensory neurone relay neurone motor neurone effector. A sensory neurone runs from a receptor towards the CNS; a relay neurone lies entirely within the CNS and joins sensory to motor; a motor neurone runs from the CNS out to the effector. The receptor detects the stimulus; the effector — a muscle that contracts or a gland that secretes — carries out the response.
Reflex action and sense organs
A reflex action is a means of automatically and rapidly integrating and coordinating a stimulus with the response of an effector (muscle or gland) — both elements are needed. A sense organ is a group of receptor cells responding to one specific stimulus. The five syllabus stimuli and their organs: light (eye), sound (ear), touch (skin), temperature (skin), chemicals (nose and tongue). Receptors are stimulus-specific.
Drawn from real examiner reports.
The brain is not the effector
The CNS coordinates a reflex arc; it never appears as the effector. The syllabus limits an effector to a muscle, which contracts, or a gland, which secretes a substance. Writing "brain" at the end of the pathway, or stopping at the spinal cord without naming a muscle or gland, loses the final mark. Always finish the chain with a named muscle or gland.
Flagged Jun 2022 P32 Q1 — the brain's coordinating role given as the effector
Sensory, relay and motor mixed up
Identify a neurone by what its two ends connect and which way the impulse travels, not by its shape on the page. Sensory: receptor to CNS, impulse travelling towards the CNS. Relay: entirely inside the CNS, joining sensory to motor. Motor: CNS to effector, impulse travelling away from the CNS. Guessing from diagram position is the usual cause of lost marks.
Flagged Jun 2023 P31 Q10a(ii) — neurone types not identified from a diagram
Half-definitions of reflex action
"A quick response", or "something you do without thinking", is a one-element answer. The mark scheme wants both: a reflex action automatically (no conscious decision) AND rapidly integrates and coordinates a stimulus with an effector response. Automatic alone does not establish speed; rapid alone does not establish that no conscious decision was made.
Digest cross-cutting note (all sittings) — vague single-element definitions where two linked ideas are required
Reflex arc ≠ reflex action
These two get used interchangeably; they are not the same. A reflex arc is the fixed physical pathway of structures — receptor, sensory neurone, relay neurone, motor neurone, effector. A reflex action is the automatic, rapid response that travels along it. One is a structure, the other a process, so "describe the arc" wants the chain, not the definition.
Receptor ≠ sense organ
A receptor is a single cell, or cell type, that detects one specific stimulus. A sense organ is the whole structure built from a group of receptor cells — the eye, the ear, the skin. Answering "the eye" where the question asks for the receptor, or "a light receptor" where it asks for the sense organ, does not match the mark-scheme wording.
The spinal cord belongs to the CNS
The CNS is the brain and spinal cord; the PNS is every nerve outside them. Placing the spinal cord in the PNS, or calling a nerve part of the CNS, is a common labelling error. The relay neurone is always within the CNS; sensory and motor neurones run mostly through the PNS but connect into the CNS at one end.
Write the full five-structure chain
For "describe a simple reflex arc", write every structure in the fixed order: receptor sensory neurone relay neurone motor neurone effector. Incomplete pathways — usually a missing relay neurone — drop the most marks.
Name the specific receptor and effector
Take the receptor and effector from the scenario rather than answering generically: "a temperature receptor in the skin" and "the biceps muscle", not "a receptor" and "an effector". The final structure must always be a muscle or a gland, never the brain or the spinal cord.
Trace impulse direction on diagrams
For "identify the neurone type" diagram questions, follow the direction of impulse travel and check whether each end of the neurone sits inside or outside the CNS. Judging by the drawing's shape, or by where it sits on the page, is what produces the mix-ups.
Match your points to the mark tariff
Count the marks before you write. A 5-mark reflex-arc question wants five distinct structures; a 3-mark explain wants three linked points. Offering fewer points than the tariff allows is a cross-cutting cause of lost marks across every sitting in the digest.
Define a sense organ.
A person accidentally steps on a sharp object with their bare foot and immediately lifts the foot away.
Describe the pathway of the reflex arc that produces this response, from the initial stimulus to the final response. (5 marks)
