Coordination and Response Notes
Cambridge IGCSE Biology: Revision notes
Key facts
- The central nervous system (CNS) is the brain and spinal cord; the peripheral nervous system (nerves) links the CNS to the body.
- Sensory neurones carry impulses to the CNS, relay neurones connect within it, and motor neurones carry impulses to effectors.
- A reflex arc is receptor, sensory neurone, relay neurone, motor neurone, effector: automatic and rapid.
- At a synapse a neurotransmitter diffuses across the gap and binds to receptors, so impulses cross in one direction only.
The nervous system
Receptors detect stimuli, the CNS processes the information and effectors produce the response.
Name both the brain and the spinal cord when you refer to the CNS. The peripheral nervous system, the nerves outside the CNS, connects the CNS to the muscles and glands. Impulses are electrical signals carried by neurones.
- 1
Receptor
Detects the stimulus
- 2
CNS
Processes the information
- 3
Effector
A muscle or gland produces the response
| Central nervous system (CNS) | Peripheral nervous system (PNS) | |
|---|---|---|
| Location | Brain and spinal cord | Nerves outside the brain and spinal cord |
| Function | Receives sensory information; coordinates responses | Transmits impulses between the CNS and the body |
Central nervous system (CNS)
- Location:
- Brain and spinal cord
- Function:
- Receives sensory information; coordinates responses
Peripheral nervous system (PNS)
- Location:
- Nerves outside the brain and spinal cord
- Function:
- Transmits impulses between the CNS and the body
Which parts make up the central nervous system?
Three neurones
Sensory neurones carry impulses to the CNS, relay neurones connect within it, and motor neurones carry impulses to effectors.
Do not mix up the direction: sensory neurones carry impulses towards the CNS and motor neurones carry them away from it. Relay neurones lie inside the CNS and connect the other two.
- 1
Receptor
detects the stimulus
- 2
Sensory neurone
carries impulses to the CNS
- 3
Relay neurone
connects within the CNS
- 4
Motor neurone
carries impulses away from the CNS
- 5
Effector
muscle or gland that responds
| Sensory | Relay | Motor | |
|---|---|---|---|
| Location | Receptor to CNS | Within the CNS | CNS to effector |
| Function | Carries impulses from receptors to the CNS | Connects sensory and motor neurones | Carries impulses to muscles or glands |
Sensory
- Location:
- Receptor to CNS
- Function:
- Carries impulses from receptors to the CNS
Relay
- Location:
- Within the CNS
- Function:
- Connects sensory and motor neurones
Motor
- Location:
- CNS to effector
- Function:
- Carries impulses to muscles or glands
Which neurone carries an impulse from the CNS to a muscle?
Reflex arc
A reflex arc is the fixed pathway receptor, sensory neurone, relay neurone, motor neurone, effector.
A reflex action is automatic and rapid, with no conscious thought. Write the sequence in order and name every part. In the knee-jerk reflex, a tap stimulates stretch receptors, a sensory neurone carries the impulse to the spinal cord, and a motor neurone makes the thigh muscle contract.
- 1
Receptor
Detects the stimulus
- 2
Sensory neurone
Impulse to the CNS
- 3
Relay neurone
Processes it in the spinal cord or brain
- 4
Motor neurone
Impulse from the CNS
- 5
Effector
Muscle contracts or gland secretes
What is the correct order of a reflex arc?
Synapses
A chemical, the neurotransmitter, diffuses across the gap and starts an impulse in the next neurone.
A synapse is a junction between two neurones with a gap between them. An impulse arrives and vesicles release neurotransmitter. It diffuses across the gap and binds to receptor proteins on the next neurone, starting a new impulse. Transmission is chemical, not electrical, and one-way, because receptors are only on the far side.
- 1
Impulse arrives
At the presynaptic axon terminal
- 2
Release
Vesicles release neurotransmitter
- 3
Diffusion
Neurotransmitter crosses the synaptic gap
- 4
Binding
It binds to receptor proteins on the next neurone
- 5
New impulse
Starts in the next neurone
Why do impulses cross a synapse in one direction only?
Try an exam question
Describe how an impulse passes across a synapse between two neurones.
[4 marks]
- [1]The impulse arrives at the end of the first neurone.
- [1]Vesicles release neurotransmitter into the gap.
- [1]The neurotransmitter diffuses across the gap.
- [1]It binds to receptor proteins on the second neurone, which starts a new impulse.
That's the notes covered.
Carry on to the next subtopic.