Question

question 6
the threshold of excitation refers to

the point at which a neuron stops firing (i.e., stops sending an electrical impulse)
the amount of energy required to open each ion channel
the level of impulses received from other neurons at which the receiving neuron fires (i.e., sends a neural impulse)
the speed of the electrical charge that travels through the neuron

Explanation:

The threshold of excitation is the minimum level of input (from other neurons) that causes a neuron to generate an action - potential or fire a neural impulse. It is not about when a neuron stops firing, the energy for ion - channel opening, or the speed of electrical charge.

Answer:

the level of impulses received from other neurons at which the receiving neuron fires (i.e., sends a neural impulse)