Coordination and Response
The nervous system allows organisms to detect changes in their surroundings (stimuli) and coordinate rapid responses. While conscious actions go through the brain, **reflex actions** are automatic, fast, and protective, bypassing conscious brain mapping.
Step-by-Step Reflex Arc Pathway
You must learn the exact sequence of neurones and events in a standard reflex arc (e.g. pulling your hand away from a hot stove):
Stimulus & Receptor
A receptor cell detects a change in the environment (e.g. temperature change in the skin) and generates an electrical impulse.
Sensory Neurone
The electrical impulse travels along the sensory neurone from the receptor to the Central Nervous System (spinal cord).
Synapse & Relay Neurone
In the spinal cord, the impulse reaches a **synapse** (junction). Chemical neurotransmitters diffuse across the gap to trigger an impulse in the relay neurone.
Motor Neurone
The impulse travels from the relay neurone, across another synapse, and along the motor neurone to the effector.
Effector & Response
The effector (a muscle contracting or a gland secreting hormone) carries out the rapid response (e.g. muscle contracts to pull the arm back).
Worked Exam Example
Worked GCSE Example
Question: Describe the steps that occur when an impulse arrives at a synapse to cross to the next neurone.
Solution:
- The electrical impulse arrives at the pre-synaptic neurone membrane.
- This triggers the release of chemical **neurotransmitters** from vesicles.
- The neurotransmitters **diffuse** across the microscopic synaptic cleft (gap) down a concentration gradient.
- The neurotransmitters bind to specific **receptor** proteins on the membrane of the post-synaptic neurone.
- This binding triggers a new electrical impulse in the second neurone.
Frequently Asked Questions
The Central Nervous System in humans consists exclusively of the **brain** and the **spinal cord**. It is connected to the rest of the body by peripheral nerves.
Neurotransmitter vesicles are only present in the pre-synaptic neurone, and receptors are only located on the post-synaptic neurone membrane, ensuring unidirectional signal flow.
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