Ch 1 · Nervous Control in Humans · IGCSE Biology 0610
Topic 1 of 5 in Coordination and Response — IGCSE Biology 0610 — 58 lessons.
Introduction
Touch a hot pan and your hand is moving away before you feel the pain. That speed comes from the nervous system, a network of specialised cells carrying electrical impulses. This lesson describes how the nervous system is organised, how a reflex arc produces a fast automatic response and, for Extended candidates, how impulses cross the gaps between neurones and why some drugs act there.
Key ideas — Organisation of the nervous system
The human nervous system has two parts:
🧑🏫 Teacher's explanation
Imagine your body is a massive, complex city. The central nervous system (CNS) is the City Hall and the main control tower: it is where all the important decisions are made, where information is processed, and where the 'plan of action' is decided. The peripheral nervous system (PNS) is the network of roads, railways, and fibre-optic cables that connect every house (receptor) and factory (effector) to that central control tower. Without the roads, the City Hall cannot talk to the rest of the city; without the City Hall, the roads have no destination.
📖 Definitions
🔍 Worked example — Easy
A student places their hand on a hot pan and immediately pulls it away. Identify the receptor, the effector, and the type of response. Explain why this is classified as a reflex action.
🔍 Worked example — Standard
Describe the difference between the central and peripheral nervous systems in terms of their composition and function. Why is the division into these two parts biologically advantageous?
🔍 Worked example — Exam-style
Explain how the structure of a nerve differs from a single neurone. If a drug blocks the transmission of impulses at the synapse between a sensory neurone and a relay neurone in the spinal cord, what would be the effect on a reflex action? Justify your answer.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(b) Explain how a reflex action allows for a rapid response to a harmful stimulus without the involvement of conscious thought. (c) State one difference between a neurone and a nerve. [8 marks]
Key ideas — Neurones and the reflex arc
Three types of neurone appear in a reflex arc. You must recognise them from diagrams:
🧑🏫 Teacher's explanation
Imagine a high-speed highway system where traffic (nerve impulses) must move from a sensor to an actuator. In a reflex arc, this traffic bypasses the 'city centre' (the brain) and uses a local 'junction' (the spinal cord) to make a split-second decision. This allows your hand to pull away from a hot stove before you even feel pain, preventing injury.
📖 Definitions
🔍 Worked example — Easy
Identify the three types of neurones in a standard reflex arc diagram and state the specific structural feature that distinguishes the sensory neurone from the motor neurone.
🔍 Worked example — Standard
Explain why a reflex action is faster than a voluntary action involving the brain, referencing the pathway length and the role of the spinal cord.
🔍 Worked example — Exam-style
Describe the events occurring at a synapse during the transmission of an impulse from a sensory neurone to a relay neurone, and explain why this ensures the impulse travels in one direction only.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Identify the structures labelled A, B, and C in Figure 1. (b) Explain how the structure of the synapse ensures that the impulse travels in one direction only. (c) Explain why the reflex action is faster than a voluntary action involving the brain. [8 marks]
Key ideas — What a reflex action is
A reflex action is a means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors (muscles and glands). Two words carry the marks: automatic (involuntary, no conscious decision) and rapid (only a few neurones and synapses are involved, so there is little delay).
🧑🏫 Teacher's explanation
Imagine you touch a hot pan. Your hand pulls away before you even feel pain. This is a reflex action. It is a pre-programmed safety routine that runs in the background, like a smoke alarm that sounds before you notice the smoke. The key is that it is automatic and rapid.
📖 Definitions
🔍 Worked example — Easy
Identify the five components of a simple reflex arc in the order they appear, starting from the stimulus.
🔍 Worked example — Standard
Explain why a reflex action is faster than a voluntary response. Include the role of the spinal cord and the number of synapses.
🔍 Worked example — Exam-style
Describe the events occurring at a synapse during the transmission of an impulse from a sensory neurone to a relay neurone.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Synapses
🧑🏫 Teacher's explanation
Think of a synapse like a single-lane bridge over a river. The nerve impulse is like a car driving along a road. When the car reaches the river (the synaptic gap), it cannot jump across. Instead, it must unload its passengers (neurotransmitters) into a boat. These passengers float across the water and get picked up by another car on the other side. Because the 'passengers' can only be unloaded from one side and picked up on the other, the traffic can only ever move in one direction.
📖 Definitions
🔍 Worked example — Easy
Identify the sequence of events that occurs when an impulse reaches the end of a presynaptic neurone.
🔍 Worked example — Standard
Explain why impulses can only travel in one direction across a synapse.
🔍 Worked example — Exam-style
A drug is known to inhibit the enzyme that breaks down acetylcholine (a neurotransmitter) in the synaptic gap. Predict and explain the effect of this drug on nerve impulse transmission.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Describe how this drug would affect the transmission of an impulse across the synapse. [2] (b) Explain why this drug might lead to over-stimulation of the postsynaptic neurone. [2] [4 marks]
Key ideas — Drugs and synapses
🧑🏫 Teacher's explanation
Think of a synapse like a gap between two islands. To get a message from one island to the other, you can't just jump; you have to send a boat (a neurotransmitter) across the water. The boat must dock at a very specific pier (a receptor) that matches its shape. If the pier is busy or the boat is the wrong shape, the message stops.
📖 Definitions
🔍 Worked example — Easy
A neurotransmitter is released into the synaptic cleft. If the concentration of this neurotransmitter in the cleft increases from 2.0 × 10^-6 mol/L to 8.0 × 10^-6 mol/L due to a drug, calculate the percentage increase in concentration.
🔍 Worked example — Standard
Explain how a drug that inhibits the enzyme responsible for breaking down neurotransmitters in the synapse would affect the transmission of nerve impulses.
🔍 Worked example — Exam-style
Heroin is a drug that mimics the shape of natural pain-relieving neurotransmitters. Describe the effect of heroin on the synapse and explain why this leads to a feeling of well-being.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Worked example
🔍 Worked example — Easy
Identify the five components of a simple reflex arc in the correct order, starting from the stimulus.
🔍 Worked example — Standard
Explain why a reflex action is faster than a voluntary action involving the brain.
🔍 Worked example — Exam-style
A student draws a reflex arc but places the relay neurone in the brain. Explain why this is incorrect and how it affects speed.
⚠️ Common errors
Common mistakes
Practice
Answers