B14: The Nervous System
The nervous system and reflex actions
Nervous System
š Key Concepts
The nervous system: Enables the body to detect and respond to changes in the environment. It uses electrical impulses to carry information quickly along neurones.
Key Terms
- central nervous system (CNS)
- stimulus
- receptor
- effector
- sensory neurone
- relay neurone
- motor neurone
- synapse
- neurotransmitter
- reflex arc
- reaction time
š Structure of the Nervous System
Central Nervous System (CNS): Consists of the brain and the spinal cord. The CNS coordinates the response to stimuli ā it decides what to do and sends signals to effectors.
How information flows:
Stimulus ā Receptor ā Sensory neurone ā CNS (brain/spinal cord) ā Motor neurone ā Effector ā Response
Stimulus ā Receptor ā Sensory neurone ā CNS (brain/spinal cord) ā Motor neurone ā Effector ā Response
Receptors: Detect stimuli (changes in the environment). Examples: light receptors in the eye, sound receptors in the ear, touch receptors in the skin, chemical receptors in the nose and tongue.
Effectors: Carry out the response. Effectors can be muscles (which contract) or glands (which secrete hormones).
š Types of Neurone
| 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 |
| Location | Outside CNS (in sense organs and nerves) | Inside CNS (brain and spinal cord) | Outside CNS (in nerves) |
| Cell body position | Off to the side of the fibre | In the middle of the CNS | At the end, near the CNS |
| Axon length | Long (from receptor to CNS) | Short (within CNS) | Long (from CNS to effector) |
Example 1
A person touches a hot object. Name the type of neurone that carries the impulse from the skin to the spinal cord, and the type that carries the impulse from the spinal cord to the arm muscle.
Solution:
From skin to spinal cord: sensory neurone (carries information from receptors to the CNS).
From spinal cord to arm muscle: motor neurone (carries impulses from the CNS to the effector/muscle).
š Synapses
Synapse: The gap between two neurones where electrical impulses cannot cross directly. Instead, chemicals called neurotransmitters are released from the end of one neurone, diffuse across the gap, and bind to receptors on the next neurone, triggering a new electrical impulse.
How a synapse works:
- An electrical impulse reaches the end of the first neurone
- Neurotransmitter chemicals are released into the synaptic gap (cleft)
- Neurotransmitters diffuse across the gap
- Neurotransmitters bind to receptors on the second neurone
- A new electrical impulse is triggered in the second neurone
Example 2
Explain why the transmission of a nerve impulse across a synapse is slower than along a neurone.
Solution:
Along a neurone, the impulse travels as a fast electrical signal. At a synapse, the electrical impulse must be converted to a chemical signal (neurotransmitter), which diffuses across the synaptic gap, then triggers a new electrical impulse on the other side. This chemical diffusion is slower than electrical transmission.
š Reflex Arc
Reflex arc: The pathway taken by nerve impulses in an automatic, involuntary response (a reflex action). Reflexes are rapid and protective ā they do not involve conscious thought.
Reflex Arc Pathway:
Stimulus ā Receptor ā Sensory neurone ā Relay neurone (in CNS) ā Motor neurone ā Effector ā Response
Stimulus ā Receptor ā Sensory neurone ā Relay neurone (in CNS) ā Motor neurone ā Effector ā Response
Why reflexes are fast:
- The impulse by-passes the brain ā it goes through the spinal cord only (via a relay neurone)
- No conscious decision is needed, saving time
- This is important for protection (e.g. pulling your hand away from a hot object before you even feel the pain)
Example 3
Describe the reflex arc that causes you to pull your hand away from a hot object.
Solution:
1. Stimulus: the hot object is detected by temperature receptors in the skin
2. Receptor: temperature receptors generate an electrical impulse
3. Sensory neurone: carries the impulse to the spinal cord (CNS)
4. Relay neurone: in the spinal cord, connects the sensory and motor neurones
5. Motor neurone: carries the impulse from the spinal cord to the arm muscles
6. Effector: arm muscles contract
7. Response: hand is pulled away from the hot object
š Conscious vs Reflex Actions
| Feature | Conscious Action | Reflex Action |
|---|---|---|
| Speed | Slower (involves brain processing) | Very fast (bypasses conscious brain) |
| Pathway | Through the brain | Through spinal cord only (via relay neurone) |
| Voluntary? | Yes ā you choose to do it | No ā automatic and involuntary |
| Examples | Deciding to pick up a pen, waving | Pulling hand from heat, knee-jerk reflex, blinking |
| Advantage | Can make complex decisions | Protects the body quickly from harm |
Example 4
Explain why a reflex action is faster than a conscious action.
Solution:
In a reflex action, the impulse travels through the spinal cord via a relay neurone, bypassing the brain. In a conscious action, the impulse must travel to the brain, where the information is processed and a decision is made, before an impulse is sent to the effectors. Processing in the brain takes time, making conscious actions slower.
š Reaction Time Practical
Required practical: Investigate the effect of a factor (e.g. distraction, caffeine) on human reaction time using the ruler drop test.
Ruler drop test method:
- Person A holds a ruler vertically, with the 0 cm mark at the bottom, between Person B's thumb and forefinger
- Person A drops the ruler without warning
- Person B catches the ruler as quickly as possible
- Record the distance (in cm) that the ruler fell before being caught
- Repeat several times and calculate a mean
- To convert distance to reaction time, use: t = ā(2d/g) where g = 9.8 m/sĀ²
Example 5
In a ruler drop test, the mean drop distance is 18 cm. Calculate the reaction time. (g = 9.8 m/sĀ²)
Solution:
t = ā(2d/g)
d = 18 cm = 0.18 m
t = ā(2 Ć 0.18 / 9.8)
t = ā(0.36 / 9.8)
t = ā0.0367
t = 0.192 s (approximately 0.19 s)
ā Practice Questions
Q1: Name the three types of neurone and describe the function of each.
Q2: Describe the pathway of a reflex arc, naming each part.
Q3: Explain how an impulse crosses a synapse.
Q4: Explain why reflex actions are faster than conscious actions.
Q5: Higher In a ruler drop test, the mean drop distance is 12 cm. Calculate the reaction time. (g = 9.8 m/sĀ²)
ā Answers
- Sensory neurone ā carries impulses from receptors to the CNS. Relay neurone ā connects sensory and motor neurones within the CNS. Motor neurone ā carries impulses from the CNS to effectors (muscles or glands).
- Stimulus ā Receptor (detects stimulus) ā Sensory neurone (carries impulse to CNS) ā Relay neurone (in spinal cord, connects sensory to motor) ā Motor neurone (carries impulse from CNS) ā Effector (muscle/gland) ā Response.
- An electrical impulse reaches the end of the first neurone. Neurotransmitter chemicals are released into the synaptic cleft. The neurotransmitters diffuse across the gap and bind to receptors on the second neurone. This triggers a new electrical impulse in the second neurone.
- Reflex actions are faster because the impulse travels through the spinal cord only, via a relay neurone, by-passing the brain. Conscious actions require the impulse to travel to the brain, be processed, and a decision made, which takes additional time.
- d = 12 cm = 0.12 m. t = ā(2 Ć 0.12 / 9.8) = ā(0.24/9.8) = ā0.0245 = 0.157 s (approximately 0.16 s).
šÆ Exam Tips
- Learn the reflex arc pathway in order ā stimulus, receptor, sensory neurone, relay neurone, motor neurone, effector, response
- Remember: effectors are muscles OR glands ā not just muscles
- Synapses use neurotransmitters (chemicals), not electrical impulses ā the impulse is electrical, then chemical, then electrical again
- Reflex actions by-pass the BRAIN, not the spinal cord ā the spinal cord IS involved via the relay neurone
- In the reaction time practical, always repeat and calculate a mean to improve reliability
- When describing neurones, use the correct names: sensory, relay, motor ā don't use vague terms like "nerve cell"
- The CNS = brain + spinal cord (not the whole nervous system)
š¬ Required Practical
Measuring Reaction Time (Ruler Drop Test)
Person A holds a ruler vertically with the 0 cm mark between Person B's thumb and forefinger. Person A drops the ruler without warning and Person B catches it as quickly as possible. Record the drop distance in cm. Repeat at least 5 times and calculate a mean distance. Convert distance to reaction time using t = ā(2d/g), where g = 9.8 m/sĀ². To investigate the effect of a factor (e.g. caffeine, distraction), measure reaction time with and without the factor and compare. Control variables: same person, same hand, same ruler, same starting position, no warning before drop.
š¢ Maths Skills
Mathematical Skills
Calculating mean reaction time: Collect multiple drop distances (e.g. 15 cm, 18 cm, 14 cm, 16 cm, 17 cm), add them and divide by the number of readings. Mean = (15+18+14+16+17) / 5 = 16 cm. Convert to time: t = ā(2 Ć 0.16 / 9.8) = ā0.0327 = 0.181 s. Discard any anomalous results before calculating the mean to improve reliability.
ā ļø Common Misconceptions
Watch Out!
1. Reflexes can be controlled. Wrong: you can choose whether or not to carry out a reflex action. Correct: reflexes are automatic and involuntary ā they bypass the conscious brain to protect the body quickly.
2. Synapses speed up nerve impulses. Wrong: synapses make signals travel faster between neurones. Correct: synapses actually slow transmission because the electrical impulse must be converted to a chemical signal (neurotransmitter) that diffuses across the gap, but they enable integration and processing of signals.
āļø 6-Mark Question
Extended Answer
6 marks: Explain how a reflex arc works.
A reflex arc is the pathway taken by nerve impulses in an automatic, involuntary response. When a stimulus is detected by receptors (e.g. temperature receptors in the skin detect heat), an electrical impulse is generated. This impulse travels along the sensory neurone to the central nervous system (spinal cord). In the spinal cord, the impulse passes to a relay neurone, which connects the sensory neurone to a motor neurone. The impulse then travels along the motor neurone to an effector (a muscle or gland). The effector produces the response automatically (e.g. the muscle contracts to pull the hand away). Because the impulse bypasses the brain and goes only through the spinal cord, no conscious decision is needed, making the response very rapid and protective.
Mark scheme: 1 mark ā stimulus detected by receptors; 1 mark ā sensory neurone carries impulse to CNS; 1 mark ā relay neurone in spinal cord; 1 mark ā motor neurone carries impulse to effector; 1 mark ā effector produces response; 1 mark ā explanation of why it is fast (bypasses brain, involuntary).
š AO3: Analyse & Evaluate
Analysis and Evaluation
A student investigated the effect of caffeine on reaction time. Ten participants completed the ruler drop test before and 30 minutes after drinking a caffeinated drink. The mean drop distance before caffeine was 17.2 cm and after caffeine was 12.5 cm. A control group of ten participants drank water instead ā their mean distances were 17.0 cm and 16.8 cm. Calculate the reaction times for both groups and explain the results. Why was the control group necessary, and why was a mean used instead of individual results?
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