B24: The Nervous System
The nervous system structure, sensory relay and motor neurones, synapses, the reflex arc, and the required practical on reaction time for GCSE Biology.
Key Definitions
- Central nervous system (CNS) – the brain and spinal cord; processes information and coordinates responses.
- Sensory neurone – a nerve cell that carries impulses from receptors to the CNS.
- Relay neurone – a nerve cell within the CNS that connects sensory and motor neurones.
- Motor neurone – a nerve cell that carries impulses from the CNS to effectors (muscles or glands).
- Synapse – the gap between two neurones where electrical impulses are transmitted chemically via neurotransmitters.
- Reflex arc – the pathway taken by nerve impulses in an automatic, unconscious reflex action.
Structure of the Nervous System
The nervous system allows the body to detect and respond rapidly to changes in the environment. It consists of two main parts:
- Central nervous system (CNS) – made up of the brain and spinal cord
- Peripheral nervous system (PNS) – all the nerves that connect the CNS to the rest of the body
Types of Neurones
| Feature | Sensory Neurone | Relay Neurone | Motor Neurone |
|---|---|---|---|
| Function | Carries impulses from receptors to the CNS | Connects sensory and motor neurones in the CNS | Carries impulses from the CNS to effectors |
| Cell body position | Off to the side of the axon | In the CNS | At the end of the neurone, in the CNS |
| Axon length | Long | Short | Long |
| Myelin sheath | Yes (insulates and speeds up impulse) | Usually no | Yes (insulates and speeds up impulse) |
Key Structures in Neurones
- Dendrites – branched extensions that receive impulses from other neurones or receptors.
- Cell body – contains the nucleus and controls the neurone's activities.
- Axon – the long fibre along which electrical impulses travel.
- Myelin sheath – a fatty layer that insulates the axon and speeds up the transmission of electrical impulses.
Synapses and Neurotransmitters
A synapse is the tiny gap between two neurones. Electrical impulses cannot cross this gap directly. Instead:
- An electrical impulse reaches the end of the first neurone (the presynaptic neurone).
- This triggers the release of neurotransmitters (chemical messengers) into the synaptic cleft.
- The neurotransmitters diffuse across the gap and bind to receptors on the second neurone (the postsynaptic neurone).
- This triggers a new electrical impulse in the second neurone.
- Neurotransmitters are then broken down or reabsorbed to prevent repeated stimulation.
Worked Example 1 – Transmission Across a Synapse
Question: Describe how an electrical impulse passes from one neurone to the next across a synapse.
Solution: When an electrical impulse reaches the end of the presynaptic neurone, it triggers the release of neurotransmitter molecules from vesicles. These neurotransmitters diffuse across the synaptic cleft (the gap between the neurones) and bind to receptor molecules on the membrane of the postsynaptic neurone. This binding triggers a new electrical impulse in the second neurone, which then travels along its axon. The neurotransmitters are then broken down or reabsorbed to prevent repeated stimulation.
The Reflex Arc
A reflex is a rapid, automatic, unconscious response to a stimulus that does not involve conscious thought. Reflexes protect the body from harm. The pathway taken is called the reflex arc:
Reflex Arc Pathway
Stimulus → Receptor → Sensory neurone → Relay neurone (in CNS) → Motor neurone → Effector → Response
Worked Example 2 – The Reflex Arc for a Hot Object
Question: You touch a hot pan and pull your hand away before you consciously feel the heat. Describe the reflex arc involved.
Solution:
- Stimulus: The hot pan is detected by thermoreceptors and pain receptors in the skin.
- Receptor: The receptors convert the stimulus into electrical impulses.
- Sensory neurone: Carries the impulse from the skin to the spinal cord (CNS).
- Relay neurone: In the spinal cord, the sensory neurone synapses with a relay neurone, which synapses with a motor neurone.
- Motor neurone: Carries the impulse from the spinal cord to the effector muscles in the arm.
- Effector: The arm muscles contract.
- Response: The hand is pulled away from the hot pan.
The impulse also travels to the brain, but this takes longer – the reflex action has already occurred before you consciously feel the heat.
Conscious Actions vs Reflex Actions
| Feature | Conscious (Voluntary) Action | Reflex Action |
|---|---|---|
| Speed | Slower (involves the brain) | Very fast (bypasses conscious brain) |
| Control | Under conscious control | Automatic, unconscious |
| Pathway | Receptor → sensory neurone → brain → motor neurone → effector | Receptor → sensory neurone → relay neurone (spinal cord) → motor neurone → effector |
| Decision-making | Involves conscious thought | No conscious thought required |
| Example | Deciding to pick up a cup | Pulling hand away from a flame |
Worked Example 3 – Conscious vs Reflex Action
Question: A person steps on a sharp stone. Their leg pulls up before they feel the pain. Then they decide to sit down to examine their foot. Explain the difference between these two responses.
Solution: The first response (pulling the leg up) is a reflex action. It is automatic and unconscious. The impulse travels via the reflex arc through the spinal cord only. The brain is not involved in the decision, making it very fast. The second response (deciding to sit down) is a conscious (voluntary) action. Information about the pain is sent to the brain, where a conscious decision is made. This is slower because it involves the brain processing information and making a decision before sending impulses to the muscles.
Required Practical: Reaction Time
Investigating the Effect of a Factor on Reaction Time
- Aim: To investigate whether a factor (e.g. caffeine, distraction) affects reaction time.
- Method: A partner holds a ruler vertically with the 0 cm mark between your thumb and forefinger. Without warning, they drop the ruler. You catch it as quickly as possible and record the distance (in cm) that the ruler fell. Repeat several times and calculate a mean. Then repeat the test with the factor being investigated (e.g. after drinking a caffeinated drink, or while distracted).
- Variables: Independent = presence of the factor (e.g. caffeine vs no caffeine). Dependent = distance the ruler falls (which can be converted to reaction time). Control = same person, same hand, same ruler, same starting position.
- Conversion: Use the equation d = 1/2 gt2 (where g = 9.8 m/s2) to convert drop distance to reaction time: t = sqrt(2d/g).
Worked Example 4 – Reaction Time Calculation
Question: In a reaction time test, a student catches the ruler after it falls 18 cm. Calculate their reaction time. (g = 9.8 m/s2)
Solution:
d = 18 cm = 0.18 m
t = sqrt(2d / g) = sqrt(2 x 0.18 / 9.8) = sqrt(0.36 / 9.8) = sqrt(0.0367) = 0.192 s
The reaction time is approximately 0.19 seconds.
Worked Example 5 – Improving the Reaction Time Practical
Question: A student wants to investigate whether caffeine affects reaction time. Describe how they should carry out this investigation and what variables they should control.
Solution:
Method: Measure baseline reaction time using the ruler drop test (5 repeats, calculate mean). Give the participant a caffeinated drink. Wait 20 minutes for the caffeine to take effect. Measure reaction time again (5 repeats, calculate mean). Compare the two mean reaction times.
Control variables: Same person, same hand, same ruler, same starting position, same time of day, same environment (no distractions). The person dropping the ruler should not give any visual cues.
Improving reliability: Use a larger sample of people, repeat on different days, use a computer-based reaction time test for greater precision.
Practice Questions
Foundation
Q1. Name the three types of neurone and state the function of each.
Show Answer
Sensory neurone – carries impulses from receptors to the CNS. Relay neurone – connects sensory and motor neurones within the CNS. Motor neurone – carries impulses from the CNS to effectors (muscles or glands).
Foundation
Q2. Describe the pathway of a reflex arc, naming each component in order.
Show Answer
Stimulus → Receptor → Sensory neurone → Relay neurone (in CNS) → Motor neurone → Effector → Response
Higher
Q3. Explain how an electrical impulse is transmitted across a synapse.
Show Answer
When an electrical impulse reaches the end of the presynaptic neurone, it triggers vesicles to release neurotransmitter molecules into the synaptic cleft. These diffuse across the gap and bind to receptors on the postsynaptic neurone. This triggers a new electrical impulse in the second neurone. Neurotransmitters are then broken down or reabsorbed to prevent continued stimulation.
Foundation
Q4. Explain why reflex actions are faster than conscious actions.
Show Answer
Reflex actions are faster because the impulse travels through the spinal cord (via the relay neurone) rather than going to the brain for conscious processing. The brain takes longer to process information and make a decision, whereas the reflex arc is a short, direct pathway that bypasses conscious thought.
Higher
Q5. In a ruler drop test, a student catches the ruler after it falls 12 cm. Calculate their reaction time. (g = 9.8 m/s2)
Show Answer
d = 12 cm = 0.12 m
t = sqrt(2d / g) = sqrt(2 x 0.12 / 9.8) = sqrt(0.24 / 9.8) = sqrt(0.0245) = 0.157 s
The reaction time is approximately 0.16 seconds.
Exam Tips
- Learn the reflex arc sequence in order. Use the mnemonic: Stimulus, Receptor, Sensory, Relay, Motor, Effector, Response.
- When describing synapses, always mention neurotransmitters as the chemical that crosses the gap – do not say the electrical impulse jumps across.
- For reaction time practicals, always mention repeating and calculating a mean to improve reliability.
- Remember: reflexes are unconscious and rapid; voluntary actions are conscious and slower.
- Be clear about the difference between the CNS (brain + spinal cord) and the peripheral nervous system (all other nerves).
🔬 Required Practical
Measuring Reaction Time Using the Ruler Drop Test
Hold a ruler vertically with the 0 cm mark between the participant's thumb and forefinger. Drop the ruler without warning; the participant catches it as fast as possible. Record the distance fallen (cm) and convert to reaction time using t = √(2d/g) where g = 9.8 m/s². Repeat at least 5 times and calculate a mean. Compare conditions (e.g. with/without caffeine). Control variables: same person, same hand, same ruler, same starting position, no visual cues from the dropper.
🔢 Maths Skills
Mathematical Skills
Calculating mean reaction time: add all the distances (or times) from your repeats and divide by the number of repeats. Remove any anomalous results before calculating the mean. Convert drop distance (d in metres) to reaction time using t = √(2d/g). For example, if the mean drop distance is 15 cm = 0.15 m: t = √(2 × 0.15 / 9.8) = √(0.0306) = 0.175 s.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Reflexes can be consciously controlled. Correct: Reflexes are automatic and unconscious — the impulse passes through the spinal cord, not the conscious brain.
2. Wrong: Synapses slow impulses unnecessarily. Correct: Synapses enable integration of signals — they allow summation, inhibition, and communication between neurones, which is essential for coordinated responses.
✍️ 6-Mark Question
Extended Answer
6 marks: Explain how a reflex arc works and why reflexes are important.
A reflex arc is the pathway taken by nerve impulses in an automatic, unconscious response. A stimulus (e.g. a hot object) is detected by receptors in the skin. The receptor generates an electrical impulse that travels along a sensory neurone to the spinal cord. In the CNS, the sensory neurone synapses with a relay neurone, which synapses with a motor neurone. Neurotransmitters cross each synapse to transmit the signal. The motor neurone carries the impulse to an effector (a muscle), which contracts to move the hand away. Reflexes are important because they are very fast (the impulse bypasses the brain for conscious processing) and they protect the body from harm before pain is even felt. The brain receives information about the event afterwards, but the response has already occurred.
Mark scheme: 1 mark for stimulus and receptor; 1 mark for sensory neurone to CNS; 1 mark for relay then motor neurone via synapses; 1 mark for effector response; 1 mark for speed/unconscious nature; 1 mark for importance in protection.
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A student investigated the effect of caffeine on reaction time. Results: without caffeine, mean drop distance = 14.2 cm; with caffeine (after 200 mg), mean drop distance = 19.5 cm. (a) Calculate the reaction time for each condition. (b) What conclusion can you draw? (c) Evaluate the method — suggest one improvement and explain why the improvement makes the data more valid.
Answers: (a) Without: t = √(2×0.142/9.8) = 0.170 s; With: t = √(2×0.195/9.8) = 0.199 s. (b) Caffeine appears to increase reaction time (slower response), which contradicts the common expectation — consider whether the sample size was adequate. (c) Improvement: use a computer-based reaction time test for greater precision and to eliminate human error (e.g. the dropper giving visual cues). More participants would also improve reliability.
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