Chemical Synaptic Transmission
Introduction to Chemical Synapses
In the past few years of studies, chemical synapses, a group of biological connectors, have shown a promising role in mediating nervous system signal transmission. In general, different signals can be sent by specific synapses to both neuron cells and non-neuron cells, like muscles. A variety of neurons can form information delivery systems in the central nervous system with the help of chemical synapses. Moreover, pilot studies have demonstrated that chemical synapses are critical for nervous system connectivity, body control, as well as biological information computing. A neuron usually can release specific neurotransmitters into the synaptic cleft linked to another neuron. Recent reports have revealed that the neurotransmitters in synaptic vesicles can be released into the synaptic cleft by exocytosis. These neurotransmitters will be bind with their receptors and trigger several potential mechanisms, such as enzyme degradation, to mediate the function of chemical synapses.
Fig.1 The modality of chemical synaptic transmission. (Pereda, 2014)
Chemical Synaptic Transmission
Nowadays, the mechanism of the transmission at chemical synapses has been extensively studied. Normally, when the action potential reaches the end of the presynaptic neuron, the transmission process on the chemical synapse begins. Action potential reaching a certain level can change the membrane potential, which leads to the open of presynaptic membrane voltage-gated calcium channel and a rapid flow of Ca²⁺ into the presynaptic terminals. Furthermore, the high concentration of Ca²⁺ in the presynaptic is essential to the fusion between synaptic vesicles and the plasma membrane of presynaptic neurons. This Ca²⁺-dependent fusion can lead to the release of many neurotransmitters into the synaptic cleft.
In recent years, many studies have shown that receptors can bind neurotransmitter molecules in two common ways. First, the receptor can directly open the presynaptic membrane voltage-gated channel to alter the local transmembrane potential. The second way is based on the regulation of chemical messengers in postsynaptic neurons. However, recent researches have suggested that different changes in the membrane potential (transmembrane voltage) can be observed when neurotransmitters bind to receptors on the receiving cell, some of which can activate their action potential while some can not. As a result, two main postsynaptic potentials, excitatory and inhibitory potentials, have been identified to make the neuron close to the discharge threshold or keep it away from the discharge threshold.
Steps for Neurotransmitter Release
- A transmitter is synthesized and stored in synaptic vesicles
- An action potential reaches the presynaptic terminal
- The opening of voltage-gated calcium channel
- An increase in the calcium concentration
- The production of calcium-sensitive proteins
- Retrieval of vesicular membrane from the plasma membrane
- The release of neurotransmitter contents into the synaptic cleft
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- Pereda, A. E.; et al. Electrical synapses and their functional interactions with chemical synapses. Nature Reviews Neuroscience. 2014, 15(4): 250-263.