Coordination and response
The Nervous System: Structure and Function
The nervous system is the body's rapid communication network. It allows organisms to detect changes in their environment (stimuli) and respond appropriately. This coordination is essential for survival, such as pulling a hand away from a hot surface.
The system is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
| Component | Description |
|---|---|
| Central Nervous System (CNS) | Consists of the brain and the spinal cord. It acts as the processing centre, integrating information and coordinating responses. |
| Peripheral Nervous System (PNS) | Consists of all the nerves outside the brain and spinal cord. These nerves carry electrical impulses between the CNS and the rest of the body (receptors and effectors). |
Why this matters: The CNS processes information, while the PNS acts as the wiring that connects the 'computer' (CNS) to the sensors (receptors) and motors (effectors).
Neurones
A neurone is a specialized cell that transmits electrical impulses. These impulses travel along the neurone as an electrochemical signal.
There are three main types of neurones, each with distinct structures adapted for their function:
| Type | Key Structural Features (for diagram identification) |
|---|---|
| Sensory Neurone | Has a cell body located on a short stalk branching off the main axon (dorsal root ganglion). This cell body is positioned between the receptor and the CNS. It has one long dendrite and one long axon. |
| Relay Neurone | Located entirely within the CNS. It has many short dendrites at one end and a shorter axon at the other, distinguishing it from the long, unbranched axons of sensory/motor neurones. |
| Motor Neurone | Has a cell body located at one end, with many short dendrites leading into it and a single long axon extending away from the cell body towards the effector. |
Note on Impulse Travel: Electrical impulses always travel in one direction: Receptor → Sensory Neurone → Relay Neurone → Motor Neurone → Effector.
The Reflex Arc
A reflex action is an automatic, rapid response to a stimulus that does not involve conscious thought. It protects the body from harm.
The pathway taken by the impulse is called the reflex arc. It involves five key components:
| Component | Role in Reflex Arc |
|---|---|
| Detects the stimulus (e.g., heat, pain). |
| Transmits the impulse from the receptor to the CNS. |
| Located in the CNS. Connects the sensory neurone to the motor neurone. |
| Transmits the impulse from the CNS to the effector. |
| The muscle or gland that produces the response (e.g., muscle contracts). |
Why reflexes are rapid: The impulse does not travel to the brain for conscious processing. It bypasses higher brain centres, allowing for an immediate protective reaction.
Synapse
A synapse is the junction (gap) between two neurones. It is not a physical connection but a small space called the synaptic gap (or synaptic cleft).
Because electrical impulses cannot jump across the gap, chemical messengers are used to transmit the signal.
Transmission at the Synapse
The transmission of an impulse across a synapse involves specific structural components and chemical events:
| Structure/Event | Description |
|---|---|
| Vesicles | Small sacs located at the end of the axon of the first neurone (presynaptic neurone). They contain neurotransmitter molecules. |
| Release | When an impulse reaches the end of the first neurone, it stimulates the vesicles to release neurotransmitters into the synaptic gap. |
| Diffusion | The neurotransmitter molecules diffuse (move from high to low concentration) across the synaptic gap to the second neurone. |
| Receptor Proteins | Located on the membrane of the second neurone (postsynaptic neurone). They are complementary in shape to the neurotransmitter molecules. |
| Binding & Impulse | Neurotransmitters bind with the receptor proteins, which stimulates an electrical impulse in the second neurone. |
Directionality: Synapses ensure impulses travel in one direction only. This is because only the first neurone has vesicles containing neurotransmitters, and only the second neurone has the complementary receptor proteins.
⚠︎ Confusing Neurone Types in Diagrams
Error: Students often confuse sensory and motor neurones because both have long axons. They may also incorrectly describe relay neurones as having symmetric branching.
Correct Understanding:
- Sensory Neurone: Look for the cell body on a stalk off to the side of the axon, located between the receptor and CNS.
- Motor Neurone: Look for the cell body at one end with many short dendrites entering it, and a long axon leaving it.
- Relay Neurone: Look for a neurone entirely within the CNS with many short dendrites at one end, not a single long unbranched axon like the others.
Describing Synaptic Transmission
When to use: When asked to describe how an impulse is passed across a synapse (typically 3-5 marks).
Why examiners accept this: Examiners look for specific keywords related to the chemical nature of the transmission. You must mention neurotransmitters, vesicles, diffusion, and receptor proteins.
Example Answer: 'An impulse stimulates the release of neurotransmitter molecules from vesicles in the first neurone. These molecules diffuse across the synaptic gap. They bind with receptor proteins on the second neurone, stimulating an impulse.'
Explaining One-Way Transmission
When to use: When asked why impulses only travel in one direction across a synapse.
Why examiners accept this: The key is the asymmetry of the structures. You must explain that the components are located on specific sides of the gap.
Example Answer: 'Impulses travel in one direction because only the first neurone has vesicles containing neurotransmitters, and only the second neurone has complementary receptor proteins.'
Past Paper Style Questions
Q:
State two components of the central nervous system.
A:
Brain and spinal cord.
Q:
Identify the type of neurone that carries impulses from the CNS to an effector.
A:
Motor neurone.
Q:
Describe the role of a reflex action.
A:
To automatically and rapidly integrate stimuli with responses (protecting the body).
Q:
Explain how the structure of a synapse ensures impulses travel in one direction only.
A:
Only the first neurone has vesicles containing neurotransmitters. Only the second neurone has complementary receptor proteins.
Q:
Describe the events that occur at a synapse when an impulse arrives.
A:
- Impulse stimulates release of neurotransmitter from vesicles. 2. Neurotransmitter diffuses across synaptic gap. 3. Neurotransmitter binds to receptor proteins on next neurone. 4. Impulse stimulated in next neurone.