OCR GCSE Biology 2025 – 400 Free Practice Questions to Pass the Exam

Transmitter substances, also known as neurotransmitters, play a critical role in the functioning of the nervous system. When an impulse reaches the end of a neuron, it triggers the release of these transmitter substances into the synaptic cleft, which is the small gap between adjacent neurons.

The correct answer highlights the mechanism by which these transmitter substances stimulate an impulse in the relay neuron. Specifically, neurotransmitters bind to receptor molecules located on the surface of the relay neuron. This binding process is essential because it causes specific ion channels in the relay neuron's membrane to open. As a result, ions flow into the neuron, leading to a change in its membrane potential. If this change is significant enough, it generates an action potential, propagating the nerve impulse along the relay neuron.

Understanding this process is crucial for comprehending how signals are communicated in the nervous system and the role of neurotransmitters in facilitating these communications. The other choices, while they may involve different aspects of neural function or signaling, do not accurately describe the specific interaction required to stimulate an impulse in the relay neuron following the reception of neurotransmitters.