Chemical synapses make up the majority of synaptic connections in warm-blooded animals. Their primary purpose is to depolarize or hyperpolarize downstream neurons and this process is mediated by presynaptic neurotransmitters and postsynaptic receptors. These neurotransmitters bind to postsynaptic receptors causing pore opening and ion influx/outflux. The flow of ions alters the postsynaptic membrane potential increasing or decreasing the likelihood of action potential firing; a depolarizing potential can increase the probability that an action potential is triggered, while a hyperpolarizing potential can hold the neuron at a membrane potential below threshold (Fig. 1).
