Which one of the following statements about neurotransmission is correct? 关于神经传递，下列哪一项陈述是正确的？单项选择题

Although there are exceptions, neurotransmitters typically have relatively low affinity for, and only transiently activate, their receptors. 虽然也有例外，但神经递质通常对其受体的亲和力较低，并且只能短暂激活受体。

A neuron will release either an excitatory or an inhibitory neurotransmitter depending on whether it is excited or inhibited by it’s synaptic inputs. 神经元会根据其突触输入是兴奋还是抑制，释放兴奋性神经递质还是抑制性神经递质。

Chemical neurotransmission requires the secretion of calcium at synapses. 化学神经传递需要突触处分泌钙离子。

All neurotransmitter receptors are large proteins; all neurotransmitters are amino acids. 所有神经递质受体都是大型蛋白质；所有神经递质都是氨基酸。