The nervous system and its effectorsSpec 8.2, 8.10
In short
The mammalian nervous system consists of the central nervous system, the brain and spinal cord, and the peripheral nervous system, which is all the neurones outside them. A receptor detects a stimulus, sensory neurones carry impulses to the CNS, and motor neurones then make an effector respond.
The mammalian nervous system has two parts. The central nervous system (CNS) is the brain and spinal cord. The peripheral nervous system (PNS) is all the neurones outside the CNS: the cranial and spinal nerves that connect the CNS to receptors and effectors. The PNS contains sensory neurones, which bring information into the CNS, and motor neurones, which carry instructions out.
| Part | Made up of | Role |
|---|---|---|
| Central nervous system (CNS) | Brain and spinal cord | Receives and processes information, then sends out impulses; the coordinator |
| Peripheral nervous system (PNS) | Sensory and motor neurones in nerves outside the brain and spinal cord | Links receptors to the CNS and the CNS to effectors |
Nervous systems allow an organism to respond to a stimulus, a change in the internal or external environment. The response follows a fixed sequence:
- A receptor detects the stimulus and converts its energy into electrical energy, producing a nerve impulse.
- A sensory neurone carries the impulse to the CNS.
- The CNS acts as the coordinator: relay neurones pass the impulse on and, in the brain, the information is processed.
- A motor neurone carries impulses from the CNS to an effector.
- The effector responds: a muscle contracts or a gland secretes.
The effector is the muscle or gland, not the motor neurone. The spinal cord is part of the CNS, but the nerves leaving it belong to the PNS.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026