Final answer:
Neurotransmitters are the chemicals released by synaptic vesicles during synaptic transmission, allowing neurons to communicate through the synaptic cleft by binding to the receptors on the postsynaptic neuron.
Step-by-step explanation:
The chemical released by the synaptic vesicles of neurons during synaptic transmission is called neurotransmitter. During the transmission of a nerve impulse, when an action potential reaches the axon terminal of the presynaptic neuron, voltage-gated Ca²⁺ channels open. The influx of Ca²⁺ triggers the synaptic vesicles to release their neurotransmitter contents into the synaptic cleft. These molecules then bind to receptors on the postsynaptic neuron, potentially leading to a depolarization or hyperpolarization of the postsynaptic membrane. Neurotransmitters like acetylcholine or dopamine are essential for this communication between neurons.