Topic 14.1 · Core
The Nervous System: CNS, PNS and Neurones
The nervous system lets a mammal detect information, transmit it rapidly as electrical impulses, and coordinate an appropriate response. Almost everything else in this chapter’s nervous section — the reflex arc, synapses — sits on top of the two ideas this page covers first: how the system is organised, and which of its three neurone types is doing which job.
Central and peripheral nervous system
The nervous system has two main parts, and Cambridge wants the distinction stated precisely rather than gestured at. The central nervous system (CNS) consists of the brain and the spinal cord, and it is where information is received and coordinated. The peripheral nervous system (PNS)consists of the nerves outside the brain and spinal cord — the nerves that connect the CNS to receptors and effectors all over the body. A useful way to hold this apart under exam pressure: the CNS is where decisions and coordination happen; the PNS is what carries the impulses to and from it. A nerve, in this context, is a bundle containing many nerve fibres — it is not the same thing as a single neurone, and conflating the two is a common way to blur an otherwise correct answer.
Information itself is carried as electrical impulses along neurones, and this is worth stating explicitly in an answer rather than assumed: it is the mechanism that makes the whole system work, and it is what distinguishes nervous coordination from the chemical, blood-borne coordination covered on the hormones page. A supporting pathway worth keeping in mind for the chapter as a whole is stimulus → receptor → nervous system → effector → response, which is essentially the reflex arc in miniature, covered in full on its own page.
Sensory, relay and motor neurones
Three types of neurone matter for this syllabus, and Cambridge tests them by asking you to identify each one in an unfamiliar diagram — which means the safe approach is never to memorise one drawing, but to work out where the receptor, the CNS and the effector sit, and then follow the direction the impulse must be travelling.
- Sensory neurone — carries electrical impulses from a receptor towards the CNS.
- Relay neurone — carries impulses within the CNS, commonly linking the sensory side of a pathway to the motor side.
- Motor neurone — carries electrical impulses from the CNS to an effector.
Put together, the direction of information across all three reads as receptor → sensory neurone → relay neurone → motor neurone → effector, where the effector is the muscle or gland that actually produces the response. The exact drawn shape of a neurone in a diagram varies and is not reliable evidence on its own — what identifies a sensory neurone as sensory is that it carries impulses towards the CNS, and what identifies a motor neurone as motor is that it carries impulses away fromit. Neurones don’t choose a direction any more than a one-way pipe chooses which way water flows; the direction is fixed by the neurone’s own structure, so an answer that talks about a neurone “deciding” to fire is describing the wrong kind of process entirely.
Where this leads next
This three-neurone pathway is the backbone of the reflex arc, which adds the receptor and effector back in and gives the whole sequence its exam-required order. The neurones themselves don’t touch directly where they meet — that junction, and the chemistry that crosses it, is covered separately on the synapses page.