Synaptic Transmission

The Chemical Bridge

Neurons rarely touch. Across the microscopic synapse (or synaptic cleft), the electrical action potential cannot jump directly. It must switch to a chemical message.

When an impulse reaches the axon terminal, vesicles release neurotransmitters into the cleft. These chemicals drift across and bind to receptor sites on the receiving neuron like a key in a lock.

Common pitfall: Picturing the electrical impulse as jumping the gap itself. It does not. The signal converts to chemical across the synapse, then back to electrical.

$$\text{net signal} = \text{excitatory} - \text{inhibitory input}$$

Resetting and Drugs

Once delivered, the message must be cleared so the synapse can reset. The primary mechanism is reuptake, where the sending neuron reabsorbs leftover neurotransmitters for future use.

Drugs exploit this exact machinery to alter behavior and mood:

For example, many common antidepressants act as reuptake inhibitors. By blocking serotonin reuptake, they leave more of the neurotransmitter active in the synaptic cleft.