Final answer:
To increase neurotransmitter release, the presynaptic cell must send action potentials with higher frequency. The all-or-none law does not vary the potential's strength, but more frequent action potentials result in more neurotransmitter release due to repetitive Ca²⁺ entry and vesicle fusion.
Step-by-step explanation:
To increase the amount of neurotransmitter released onto a postsynaptic cell, the presynaptic cell would have to send action potentials more frequently (option C). When an action potential reaches the axon terminals, voltage-gated Ca²⁺ channels open, allowing calcium ions to enter the neuron's synaptic end bulb. This influx of Ca²⁺ is crucial for the release of neurotransmitters, as it facilitates the fusion of neurotransmitter vesicles with the presynaptic membrane, eventually leading to exocytosis of the neurotransmitter into the synaptic cleft.
The all-or-none law indicates that an action potential occurs fully or not at all; the strength or amplitude of the action potential does not change once it is triggered. However, the frequency with which these action potentials are sent can vary, affecting the amount of neurotransmitter released. Shortening the time between action potentials (increasing their frequency) can cause more neurotransmitter to be released because each action potential triggers the entry of Ca²⁺ and the subsequent release of neurotransmitter.