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Nervous System and Homeostasis

Edexcel

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Nervous System

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Nervous System – Edexcel IGCSE Biology Revision Notes

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Key Definitions and Terminology

  • **Stimulus**: A change in the environment (internal or external) that is detected by receptors.
  • **Receptor**: A specialised cell or group of cells that detects a stimulus (e.g., light receptors in the eye, temperature receptors in the skin).
  • **Effector**: A muscle or gland that carries out a response to a stimulus.
  • **Neurone (nerve cell)**: A specialised cell that transmits electrical impulses throughout the nervous system.
  • **Synapse**: The junction between two neurones, where the signal is transmitted chemically by the diffusion of neurotransmitter molecules across a small gap.
  • **Reflex arc**: The pathway taken by a nerve impulse during a rapid, automatic (involuntary) response, passing from receptor → sensory neurone → relay neurone (in the CNS) → motor neurone → effector.

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Main Concepts

1. The Role of the Nervous System

  • The nervous system allows organisms to **detect** and **respond** to changes in their environment.
  • It provides **rapid, short-lived, and specific** responses, using electrical impulses that travel along neurones.
  • The general pathway for any nervous response is:

> Stimulus → Receptor → Coordinator (CNS) → Effector → Response

2. The Central Nervous System (CNS)

  • The CNS consists of the **brain** and the **spinal cord**.
  • It acts as the **coordinator**, receiving information from receptors via sensory neurones, processing this information, and sending impulses to effectors via motor neurones.

3. Types of Neurone

  • **Sensory neurone**: Carries electrical impulses **from receptors to the CNS**. Has a long axon and a cell body located off to one side (along the pathway).
  • **Relay neurone (interneurone)**: Found **within the CNS** (brain or spinal cord). Connects sensory neurones to motor neurones.
  • **Motor neurone**: Carries electrical impulses **from the CNS to effectors** (muscles or glands). Has a long axon and a cell body at one end with many short dendrites.

4. Structure of a Neurone

  • All neurones share common features:
  • **Cell body** – contains the nucleus and cytoplasm.
  • **Axon** – a long, thin fibre that carries the electrical impulse away from the cell body.
  • **Dendrites/dendrons** – thin extensions that receive impulses from other neurones or receptors.
  • **Myelin sheath** (in some neurones) – a fatty insulating layer surrounding the axon that **speeds up the transmission** of electrical impulses.

5. Synaptic Transmission

  • When an electrical impulse reaches the end of a neurone, it **cannot cross the synapse as an electrical signal**.
  • Instead, a **chemical neurotransmitter** is released from vesicles at the end of the pre-synaptic neurone (the neurone before the synapse).
  • The neurotransmitter **diffuses across the synaptic gap (cleft)**.
  • It binds to **specific receptor molecules** on the membrane of the post-synaptic neurone (the neurone after the synapse).
  • This triggers a **new electrical impulse** in the next neurone.
  • The neurotransmitter is then broken down or reabsorbed so the synapse is ready to transmit the next impulse.
  • Synapses ensure impulses travel in **one direction only**.

6. Reflex Actions

  • A **reflex action** is a rapid, automatic (involuntary) response to a stimulus that does **not involve conscious thought**.
  • Reflexes protect the body from harm (e.g., pulling your hand away from a hot object).
  • The reflex arc is the **shortest possible nerve pathway**, which is why the response is so fast.
  • The pathway of a reflex arc:

> Stimulus → Receptor → Sensory neurone → Relay neurone (in spinal cord) → Motor neurone → Effector → Response

  • The brain is **not involved** in the decision-making for a reflex, though it may become aware of the stimulus shortly after.

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Worked Examples and Real-World Applications

Example 1: Touching a Hot Plate

  1. **Stimulus**: Heat from the hot plate.
  2. **Receptor**: Temperature (pain) receptors in the skin of the fingertips detect the heat.
  3. **Sensory neurone**: Carries the electrical impulse from the receptor to the spinal cord (CNS).
  4. **Relay neurone**: In the spinal cord, passes the impulse from the sensory neurone to the motor neurone.
  5. **Motor neurone**: Carries the impulse from the spinal cord to the effector.
  6. **Effector**: The biceps muscle in the arm (effector) **contracts**.
  7. **Response**: The hand is pulled away from the hot plate.

> This is involuntary — you pull your hand away before you consciously feel the pain.

Example 2: The Pupil Reflex

  • **Stimulus**: Bright light entering the eye.
  • **Receptor**: Light receptors in the retina detect the increase in light intensity.
  • **Coordinator**: The brain processes the information.
  • **Effector**: The **circular muscles** in the iris contract (and the radial muscles relax).
  • **Response**: The pupil **constricts** (gets smaller), reducing the amount of light entering the eye and protecting the retina from damage.

Example 3: Synaptic Transmission in Medicine

  • Many drugs and toxins affect synaptic transmission. For example:
  • Some **painkillers** work by blocking neurotransmitter receptor sites, preventing impulses from being transmitted across synapses in pain pathways.
  • **Nerve agents/poisons** (e.g., some insecticides) can prevent the breakdown of neurotransmitters, causing continuous stimulation of the post-synaptic neurone, leading to muscle spasms or paralysis.
  • This demonstrates the real-world importance of understanding how synapses function.

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Exam Technique Tips for Edexcel IGCSE

Tip 1: Use Precise Specification Language for Synapse Questions

Edexcel mark schemes are very specific about the sequence at a synapse. Always describe these steps in order:

  1. Impulse arrives at the end of the pre-synaptic neurone.
  2. **Chemical/neurotransmitter is released** (from vesicles).
  3. Neurotransmitter **diffuses across the synaptic gap/cleft**.
  4. Neurotransmitter binds to **receptor molecules** on the post-synaptic neurone.
  5. A **new electrical impulse** is triggered in the next neurone.

> ⚠️ Do not say "the impulse jumps across the gap" — this will lose you marks. The signal crosses the synapse chemically, not electrically.

Tip 2: Label and Sequence Reflex Arc Components Accurately

When asked to describe or draw a reflex arc, always state the components in the correct order and name the type of neurone at each stage. A common error is forgetting the relay neurone or placing the neurones in the wrong order. Use this mnemonic:

> S – R – M (Sensory → Relay → Motor)

Also, clearly state where each component is found (e.g., "the relay neurone is found in the spinal cord/CNS"). Edexcel often awards separate marks for identifying the location of the relay neurone.

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*These notes cover the Edexcel IGCSE Biology specification content for the Nervous System subtopic. For best results, practise past paper questions on reflex arcs, synapse diagrams, and neurone identification.*