A presynaptic neuron is a type of neuron that sends signals to another neuron, called a postsynaptic neuron, across a synapse. In a typical synapse, the presynaptic neuron releases neurotransmitters, such as dopamine, serotonin, or acetylcholine, into the synaptic cleft, which is the small gap between the presynaptic and postsynaptic neurons. The neurotransmitters then bind to receptors on the postsynaptic neuron, triggering an electrical signal that is transmitted along the postsynaptic neuron.

Presynaptic neurons are responsible for initiating and modulating neurotransmission in the brain and are crucial for the normal functioning of the nervous system. They can be affected by various factors such as drugs, stress, and disease, which can alter the release of neurotransmitters and lead to changes in neural signaling and behavior.