Final answer:
At a chemical synapse, the presynaptic cell, usually a neuron, releases neurotransmitters that travel across the synaptic cleft and bind to receptors on the postsynaptic cell, initiating a response.
Step-by-step explanation:
At a chemical synapse, the presynaptic cell may be a neuron that releases neurotransmitters into the synaptic cleft, a narrow space between the presynaptic and postsynaptic cells. When an action potential reaches the axon terminal of the presynaptic neuron, calcium channels open and calcium enters the cell, prompting synaptic vesicles filled with neurotransmitters to fuse with the cell membrane. These neurotransmitters are then released into the synaptic cleft and bind to receptors on the postsynaptic cell's membrane, initiating a response in that cell, which could be another neuron, a muscle cell, or a gland cell.
Neurotransmitters may excite or inhibit the postsynaptic cell, depending on their nature. Excitatory neurotransmitters such as acetylcholine or epinephrine increase the chance of an action potential in the postsynaptic cell, while inhibitory neurotransmitters like serotonin or GABA decrease this chance. Following the signal transmission, the neurotransmitter is either degraded or reabsorbed by the presynaptic cell to prepare for the next signal.