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Figure 3-1. The structure of a synapse. [Source: Wikipedia]
A synapse is the contact point between an axon terminal and another neuron. In fact, they do not actually touch. There is a gap about 20 nanometers between them, called "synaptic cleft". The neuron transmitting signals is called "presynaptic neuron", and the receiving neuron is called "postsynaptic neuron". The presynaptic axon terminal contains synaptic vesicles which store neurotransmitters. When a nerve impulse reaches the axon terminal, it will cause membrane depolarization, thereby opening the voltage-gated calcium channels and subsequent entry of calcium ions into the cell (Figure 3-1).
Calcium ions are the master regulator of cellular operations, because they control the activities of many enzymes. The entry of calcium ions into the axon terminal will lead to a series of chemical reactions, resulting in the fusion of synaptic vesicles with the cell membrane and consequently the release of neurotransmitters into the synaptic cleft. These neurotransmitters may bind with their receptors in the postsynaptic membrane, triggering a variety of changes in the postsynaptic neuron.
Figure 3-2. Dendritic shaft and dendritic spines. [Source: Wikipedia]
Most synapses are formed between an axon terminal and the dendrites of another neuron. The contact point on dendrites is typically a raised structure called "dendritic spine" (Figure 3-2). Neurotransmitters may affect the chemical composition of dendritic spines and even their number, leading to changes in neuronal circuits. Examples related to memory will be given in Appendix B. The following sections describe only the basics required for discussion in later chapters. .