Synapse

A synapse is a junction between two neurons that allows communication between them. There are certain chemical messengers called neurotransmitters released by the presynaptic neuron (neuron from which the communication is to be sent) in case of chemical synapses. The presynaptic neuron transmits the signal to the postsynaptic neuron (neuron where the communication is to reach). Thus, a synapse is neural junctions that help in the transmission of chemical or electrical signals known as nerve impulses. Depending on the signal transmission via the synapse, be it chemical or electrical, the synapse is named respectively as chemical synapse or electrical synapse. A detailed description given below helps to define synapse and understand what is the function of the synapse.

Synapse

In order to understand what is a synapse in biology, it is necessary to know what is the function of neurons in general. Neurons are specialised cells that are electrically excitable and communicate with one another through chemical messengers or electrical signals. These signals are generated by external stimuli such as touch, burning sensation, etc. These signals help to protect our body and react as fast as possible for survival.

Neurons are the main cells that comprise the nervous system and are widely known as brain cells. They help to form memory, learn and react in adverse conditions. But to carry out all these functions, the neurons need to communicate with one another and they do so via specialised junctions called synapses.

Thus, a synapse is a junction that permits the transmission of signals or information between either a neuron and another neuron or between a neuron and a muscle cell. The synapse between a neuron and another neuron is called a neuronal junction and the one between a neuron and a muscle is called the neuromuscular junction. It is not necessary that neurons can form synapses with only another neuron or a muscle cell. It can form a synapse with any different target cell for communication and that target cell is known as an effector cell.

A synapse image can be described as a knoblike structure emerging from the plasma membrane of a presynaptic neuron fitting into a curve shape formed by the plasma membrane of a postsynaptic neuron or an effector cell with a small gap in between. This gap known as the synaptic cleft is approximately 0.2 microns wide. Both the presynaptic and postsynaptic sites contain large assemblies of the molecular machinery that keep the two membranes together. These molecules are also in some cases known as the synaptic adhesion molecules (SAMs) and carry out the signalling process. The synapse image is clearly outlined in the diagram below.

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Types of Synapse

A synapse can either be a chemical synapse or an electrical synapse depending upon the kind of signals it permits. It's important to understand that even though an electrically excitable neuron generates electrical impulses due to the voltage gradients across its membranes, the transmitting signal can either be chemical or electrical which in turn decides the type of synapse. The two synapses are briefly described as follows:

1. Chemical Synapse: When the electrical activity within the presynaptic neuron results in the release of chemical messengers known as neurotransmitters, then such neurotransmitters pass through a chemical synapse. The neurotransmitters released into the synaptic cleft bind to the receptors present on the postsynaptic neuronal membrane. These neurotransmitters then further may initiate a pathway of secondary messengers which can further pass the signal or inhibit it. This type of neuronal signalling is useful and essential when the signal is passed over complex and large routes.

2. Electrical Synapse: When the voltage changes in the presynaptic cell induce voltage changes in the postsynaptic cell it happens by the transmission of electrical current through the special channels known as gap junctions of an electrical synapse present on the membranes of the cells involved. The advantage of the electrical synapse is that it allows a very fast exchange of signals from one cell to another.

Signal Transmission Through the Synapse

The function of the neurons is largely owed to their cell polarity. In the case of neurons, it is the electrical polarity that allows and facilitates the transfer of electrical signals from presynaptic membranes to postsynaptic membranes or effector cell membranes. An action potential that arises by the large net flow of positively charged ions into a presynaptic neuron generates the electrical signal which is then transmitted to the postsynaptic cell. This transfer as mentioned above occurs by means of the chemical or electrical synapse. A synapse image shown below depicts the manner in which the signal is transmitted.

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Around the chemical synapse, the electrical signal leads to the release of neurotransmitters. These neurotransmitters are present inside membrane-bound vesicles known as synaptic vesicles. These vesicles under the influence of the electrical impulse drive towards the synapse and fuse with the presynaptic neuronal membrane. As the vesicles fuse with the membrane they release the neurotransmitters into the synaptic cleft which then diffuse through the cleft and bind the receptor molecules on the postsynaptic membrane. These neurotransmitters as given above initiate secondary messenger pathways and excite the signal or inhibit the signal in the postsynaptic cell. Once the neurotransmitters have passed on the information they are deactivated by enzymes present in the synaptic cleft and are taken up by the presynaptic vesicles. Hence, there are brief transmission events taking place and each one takes place only for 0.5 to 4 milliseconds. 

In the case of the electrical synapse, electrical current passes through the gap junctions. This direct communication in terms of electrical current happens to electrically charged ions that are permeated through these gap junctions. This allows for the rapid synchronisation of the nerve cells. 

Thus, from the above discussion, the synapse meaning and function is clear. Overall if one is to define synapse or briefly describe what is a synapse in Biology then one can say that a synapse is a junction that exists majorly between neurons for the transmission of electrical impulses and action potentials. But it cannot just be classified as a neuron synapse as the synapse can be between a neuron and another excitable cell such as a muscle cell known as an effector cell.