How are signals transferred across a synapse?

The function of the synapse is to transfer electric activity from one cell to another. At chemical synapses, the presynaptic neuron is separated from the postsynaptic neuron by a synaptic cleft. Transmission of chemical signals across the synapse is initiated when an action potential arrives at the axon terminal of the presynaptic neuron. The change in membrane potential leads to the opening of voltage-gated calcium channels in the presynaptic membrane. The presynaptic neuron contains sphere-shaped vesicles containing neurotransmitter which fuse with the presynaptic membrane once the calcium ion concentration is high enough. This allows vesicles to release the neurotransmitters via exocytosis into the synaptic cleft. The post synaptic membrane has many specific receptors to which only one type of neurotransmitter can bind to. Binding of the neurotransmitter to its receptors on the postsynaptic membrane opens up ligand-gated sodium channels. These allow an influx of sodium ions. If the depolarization of the postsynaptic membrane reaches threshold potential, an action potential is generated in the postsynaptic neuron. The mechanisms by which neurotransmitters are removed from the synaptic cleft include: diffusion, reuptake into nerve terminals or surrounding glial cells by specific transporter proteins in the presynaptic membrane and degradation by enzymes.