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The nervous system and the reflex arc
How your body detects something and responds, sometimes before you've even noticed.
The nervous system carries electrical impulses so the body can respond quickly. Receptors detect a stimulus, sensory neurons carry the signal to the central nervous system, and motor neurons carry instructions to effectors such as muscles. A reflex is a fast, automatic response that skips conscious thought.
Covers GCSE, AP and A-level. Exam boards differ slightly, so check your course specification.
Play the reflex arc
You touch something hot. Press play to follow the signal, step by step, from skin to muscle.
- 1Stimulus
- 2Receptor
- 3Sensory neuron
- 4Relay neuron
- 5Motor neuron
- 6Effector
Press play to follow the signal.
The three types of neuron
| Feature | Sensory | Relay | Motor |
|---|---|---|---|
| Carries impulses | From receptors to the CNS | Within the CNS, between sensory and motor neurons | From the CNS to effectors |
| Example | From the skin to the spinal cord | In the spinal cord | From the spinal cord to a muscle |
How the signal crosses a synapse
At a chemical synapse, a tiny gap called the synaptic cleft separates the cells. A synapse is the whole junction, not just the gap. Here is a simplified excitatory example:
- The electrical impulse reaches the end of the first neuron.
- It triggers the release of neurotransmitter chemicals.
- These diffuse across the gap and bind to receptors on the next neuron.
- If the resulting signal is strong enough to reach threshold, a new electrical impulse starts. Other neurotransmitter signals can inhibit firing.
Because neurotransmitters are only released on one side, synapses make sure impulses travel in one direction.
Reflex vs voluntary response
| Feature | Reflex | Voluntary |
|---|---|---|
| Speed | Very fast | Slower |
| Conscious thought? | No, it's automatic | Yes |
| Path | Withdrawal reflex: spinal cord; some other reflexes involve the brainstem | Involves conscious processing in the brain |
| Purpose | Protection, such as pulling your hand from heat | Deliberate actions, such as picking up a pen |
Key terms
- Stimulus
- A change in the environment that is detected.
- Receptor
- A specialised cell or sensory nerve ending that detects a stimulus.
- Effector
- A muscle or gland that carries out the response.
- CNS
- The central nervous system: the brain and spinal cord.
- Synapse
- A junction where one neuron communicates with another cell. In a chemical synapse, neurotransmitters cross the synaptic cleft.
- Neurotransmitter
- A chemical released at a synapse to pass the signal on.
Common mistakes
A withdrawal reflex does not wait for conscious thought. It is coordinated in the spinal cord, while signals also travel to the brain. Other reflexes, such as the pupil response, involve the brainstem.
Chemical synapses have a gap. Neurotransmitters cross it. Electrical synapses also exist, so not all neuronal junctions work this way.
Impulses are electrical along a neuron; chemical synapses use neurotransmitters. Mixing these up is a common lost mark.
Nervous control isn't the same as hormonal control. Nerves are fast and short-lived, hormones are slower and longer-lasting.
A worked example
This is the kind of answer SnapBio gives you when you snap a question: the answer first, then the steps.
Why can a withdrawal reflex happen before a conscious voluntary response?
How to get there
- 1
In this withdrawal reflex, the impulse is passed on in the spinal cord by relay neurons.
- 2
There is no conscious decision from the brain, and the pathway is short, with few synapses.
- 3
So the response is faster, which protects the body.
Check yourself
Which type of neuron carries impulses from the CNS to an effector?
Motor neurons carry impulses from the CNS to effectors, such as muscles.
What crosses the gap at a chemical synapse?
Neurotransmitters diffuse across the synaptic cleft and bind receptors. Their effects can encourage or inhibit an impulse in the next neuron.
A muscle contracting in a reflex is an example of a(n)…
Muscles and glands are effectors: they carry out the response.
FAQ
What is a reflex arc?
A reflex arc is the pathway carrying a rapid, automatic response. A withdrawal reflex uses receptors, sensory neurons, relay neurons, motor neurons and muscles. Some reflexes use different pathways; the knee-jerk reflex has a direct sensory-to-motor synapse.
What is the difference between nervous and hormonal control?
Nervous control uses electrical impulses along neurons, so it's very fast and short-lived. Hormonal control uses chemical messengers in the blood, so it's slower and lasts longer.
What is the central nervous system?
The brain and spinal cord. It coordinates responses. Everything else, such as sensory and motor neurons, is the peripheral nervous system.
How do signals cross the gap between neurons?
By chemicals called neurotransmitters, which diffuse across the synapse. See diffusion explained.
Sources and further reading
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