By the end of this lesson
- Identify the components of the CNS.
- Describe links between sense organs and the CNS.
- Distinguish sensory, relay and motor pathways.
BIG QUESTION
How does information from distant sense organs reach a central decision-making system?
Peripheral nerves connect receptors and effectors to the brain and spinal cord.
The CNS integrates information
The central nervous systemThe brain and spinal cord, which coordinate responses to information from receptors. consists of the brain and spinal cord. It receives information, integrates inputs and coordinates outputs.
Different brain regions perform specialised functions, while the spinal cord also coordinates many reflexes.
Sense organs connect through sensory nerves
Receptor cells in sense organs convert stimuli into electrical impulses. sensory neuroneA neurone that carries electrical impulses from a receptor to the central nervous system. carry these impulses toward the CNS through peripheral nerves.
A nerve is a bundle of many neurone fibres, not one very long cell.
Outputs reach effectors
Within the CNS, relay neuroneA neurone in the central nervous system that passes impulses from a sensory neurone to a motor neurone. connect pathways. motor neuroneA neurone that carries impulses from the central nervous system to an effector. carry impulses away from the CNS to muscles or glands.
The route enables both conscious responses and automatic reflexes.
Neurones in a response pathway
Direction is defined relative to the CNS.
| Neurone | Impulse direction/location | Role |
|---|---|---|
| Sensory neurone | Receptor → CNS. | Carries detected information inward. |
| Relay neurone | Within CNS. | Links and processes pathways. |
| Motor neurone | CNS → effector. | Carries output to muscle/gland. |
| Peripheral nerve | Between organs and CNS. | Bundle containing many fibres. |
Language that earns precision
- central nervous system
- Brain and spinal cord.
- peripheral nerve
- Bundle of neurone fibres outside the CNS.
- sensory neurone
- Neurone carrying impulses from receptors toward the CNS.
- motor neurone
- Neurone carrying impulses from the CNS to an effector.
Common misconceptions
Claim: The CNS includes all nerves in the body.
Correction: It includes only brain and spinal cord; other nerves are peripheral.
Claim: A nerve is one neurone.
Correction: A nerve contains bundles of many neurone fibres.
Claim: Every sensory signal must reach conscious brain regions before action.
Correction: Spinal reflexes can produce rapid action before conscious awareness.
Match the answer to the command word
The CNS is brain plus spinal cord.
Receptor → sensory neurone → CNS/relay → motor neurone → effector.
Recall, apply, then justify
Describe how a sense organ is linked to an effector through the nervous system.
Hint
Use the three neurone roles.
Suggested marking points
- A receptor in the sense organ detects a stimulus.
- A sensory neurone carries impulses to the CNS.
- Relay neurones connect/process the signal within the CNS.
- A motor neurone carries impulses from the CNS.
- A muscle or gland acts as the effector.
Distinguish the CNS from a peripheral nerve.
Hint
Components and location.
Suggested marking points
- The CNS is the brain and spinal cord.
- A peripheral nerve is a bundle of neurone fibres connecting organs to the CNS.
Explain how spinal-cord damage can interrupt responses even when receptors, muscles and the brain remain functional.
Planning hint
The communication route is broken.
What a strong design includes
- Receptors can still detect stimuli.
- Sensory impulses may fail to pass the damaged spinal region to the brain.
- Motor commands may fail to pass from brain to lower motor neurones.
- Effectors below the injury may not receive appropriate signals.
- Some local spinal reflexes may remain depending on injury level.
- Outcome depends on which pathways are damaged.