Explain the process of synaptic transmission.

During synaptic transmission, an electrical impulse or action potential is relayed between two neurons via a synapse. A synapse is the space between two neurons, and acts as a point of transmission or communication. The impulse reaches the pre-synaptic terminal, the end of the axon where the pre-synaptic neuron meets the synapse. This triggers voltage-gated ion channels in the membrane to open, allowing calcium (Ca+) ions to flood into the neuron. The inside of the pre-synaptic neuron becomes more positive, due to the influx of positive ions. This causes vesicles, which are little packets of neurotransmitters bound inside a membrane, to move towards the edge of the cell. The membrane of the vesicle binds to the membrane of the pre-synaptic neuron and the neurotransmitter is released into the synapse.The neurotransmitter molecules diffuse across the synaptic gap, towards the post-synaptic neuron. Embedded in this neuron's membrane are specific receptors, which the neurotransmitter binds with. The binding triggers the opening of sodium (Na+) channels to open on the post-synaptic neuron's membrane, allowing Na+ ions to enter the cell. If enough Na+ ions enter the cell, dependent on the amount of neurotransmitters binding to their receptors, a threshold is reached. Once this is reached, the voltage-gated Na+ ion channels open fully and there is a large influx of positively charged ions into the cell. In an excitatory reaction, this causes a change in the charge of the cell called depolarisation which generates an action potential which can propagate or travel down the axon of the post-synaptic neuron. In an inhibitory reaction, hyperpolarisation occurs instead. This makes the possibility of producing an action potential in the future more unlikely or difficult by increasing the threshold.