OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

Why do nerve impulses only travel in one direction across the synapse?

1. Due to the size of the synapse

2. Because receptor molecules are only on the relay neuron

3. Because transmitter molecules only come from the relay neuron

4. Due to mucus lining the synapse

The correct answer is: Because receptor molecules are only on the relay neuron

The reason nerve impulses travel in one direction across the synapse is predominantly because receptor molecules are specifically located on the postsynaptic neuron, not on the presynaptic neuron. When an action potential reaches the end of the presynaptic neuron, it triggers the release of neurotransmitter molecules into the synaptic cleft. These neurotransmitters then bind to the receptor sites on the membrane of the postsynaptic neuron. Since the presynaptic neuron produces and releases neurotransmitters, and the postsynaptic neuron has the corresponding receptors to detect and respond to these signals, this structural arrangement ensures that the transmission only flows one way— from the presynaptic neuron to the postsynaptic neuron. As a result, this directional flow is essential for the proper functioning of neural communication and helps maintain the integrity of signal transmission across neurons. Other options do not accurately capture this mechanism, as the size of the synapse and the presence of mucus are not relevant to the directionality of impulse transmission.