Synaptic Transmission

When an action potential reaches the axon terminal, the electrical signal must be converted into a chemical one. The depolarization opens voltage-gated calcium channels, allowing $Ca^{2+}$ to rush into the cell down its electrochemical gradient. This influx of $Ca^{2+}$ acts as a precision trigger, signaling synaptic vesicles filled with neurotransmitters to migrate toward and fuse with the presynaptic membrane. Through a process called exocytosis, the neurotransmitters are dumped into the synaptic cleft, the microscopic space (approx. 20-50 nm) between neurons.

Once in the cleft, neurotransmitters bind to ligand-gated ion channels on the postsynaptic neuron. The effect depends on which ion channel opens. If $Na^{+}$ channels open, $Na^{+}$ enters the cell, causing a slight depolarization called an Excitatory Postsynaptic Potential (EPSP). If $K^{+}$ leaves or $Cl^{-}$ enters, the cell becomes more negative (hyperpolarized), creating an Inhibitory Postsynaptic Potential (IPSP). A single neuron may receive thousands of these signals; it only fires an action potential if the net sum reaches the threshold (usually $-55mV$) at the axon hillock.

To prevent constant stimulation, neurotransmitters must be removed quickly. There are three main 'cleanup' methods: 1. Reuptake: The presynaptic neuron uses transport proteins to 'vacuum' the neurotransmitters back inside for recycling. 2. Enzymatic Degradation: Specific enzymes (like Acetylcholinesterase) break down the neurotransmitter in the cleft. 3. Diffusion: The molecules simply drift away from the synaptic area into the surrounding extracellular fluid.