Neural transmission across a mammalian synaptic gap is accomplished by
a) the movement of sodium and potassium ions from the presynaptic neuron into the postsynaptic neuron.
b) impulses traveling as electrical currents across the gap.
c) impulses causing the release of a chemical signal and its diffusion across the gap.
d) impulses ricocheting back and forth across the gap.
e) the movement of calcium ions from the presynaptic into the postsynaptic neuron.

C) impulses causing the release of a chemical signal and its diffusion across the gap.

Explanation:

The place where information is transmitted from one neuron to the other neuron is called gap or synapse. The gap is formed between dendrites of one neuron and axon terminals of another neuron. The presynaptic neuron transmit the signal to the postsynaptic neuron. The axon releases neurotransmitters, a chemical substances which diffuse across gap or synapse.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.