Home > Conscious > Chapter 3 > 3.1. The Structure of Synapses

 

Images

Figure 3-1. The structure of a synapse. [Source: Wikipedia]

A synapse is a structure at the junction between two neurons (Figure 3-1). In most cases, it is formed between the axon terminal of a neuron (presynaptic) and the dendrites of another neuron (postsynaptic). The contact point on dendrites is typically a raised structure called "dendritic spine" (Figure 3-2). There is a gap about 20 nanometers between the axon terminal and spine. This gap is called "synaptic cleft". The axon terminal contains synaptic vesicles which store neurotransmitters. When a nerve impulse arrives at the axon terminal, it will cause membrane depolarization to open the voltage-gated calcium channels, allowing Ca2+ ions to enter the neuron.

Ca2+ ions are the master regulator of cellular processes, because they control the activities of various enzymes. The entry of Ca2+ 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 to their receptors in the postsynaptic membrane, triggering a variety of changes in the postsynaptic neuron.

Images

Figure 3-2. Dendritic shaft and dendritic spines. [Source: Wikipedia]