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

What initiates a new electrical impulse in a neuron?

1. The release of neurotransmitters

2. The contraction of muscle cells

3. The response to external stimuli

4. The activity of sensory organs

The correct answer is: The release of neurotransmitters

The initiation of a new electrical impulse in a neuron primarily occurs in response to a change in membrane potential triggered by the binding of neurotransmitters to receptors on the neuron's surface. When neurotransmitters are released from the presynaptic neuron into the synaptic cleft, they bind specifically to receptors on the postsynaptic neuron, leading to either depolarization or hyperpolarization of the postsynaptic cell. If the binding of neurotransmitters leads to sufficient depolarization that surpasses the threshold potential, it causes the opening of voltage-gated ion channels. This results in an influx of positively charged sodium ions, generating an action potential, which is the electrical impulse that propagates along the neuron. This process illustrates the key role neurotransmitters play in signaling within the nervous system, acting as the catalyst for propagating communication between neurons. In contrast, while the contraction of muscle cells, the response to external stimuli, and the activity of sensory organs are important for overall functionality and response mechanisms within the body, they do not directly initiate a new electrical impulse in a neuron. Instead, these processes may involve the transmission of electrical impulses initiated by neurotransmitter activity.