For the brain to function correctly, it is necessary for the neurons to communicate with each other. These functional interactions between neurons are called synapses. But how does this interconnection occur? How many types of synapses are there?
Apparently, two main modes of synaptic transmission are recognized: the electrical synapse and the chemical synapse. In general, synaptic communication usually occurs between the termination of the axon (the longest part) of the sending nerve cell and the cellular soma of the receiving neuron.
However, contrary to what we may think, the synapse does not occur by direct contact. The neurons are separated from each other by a small groove: the synaptic or intersynaptic space. The two main types of synapses are explained below. Both are interneuronal connections, but each type has its own characteristics. Let’s see how each one develops.
Types of synapses: the chemical synapse
In the chemical synapse, the information is transmitted by neurotransmitters. That is why it is called chemistry; the neurotransmitters would be in charge of transmitting the message.
In addition, these synapses are not symmetric, but asymmetric. This means that they do not occur exactly the same from one neuron to another. They are also unidirectional: the postsynaptic neuron, the one that receives the synapse, can not transmit information to the presynaptic neuron, which sends the synapse.
The chemical synapse has other specific characteristics. For example, it shows a high plasticity. That is, the synapses that have been most active will transmit the information more easily. Thus, this plasticity allows adaptation to changes in the environment. Our nervous system is intelligent and the communication of those routes that we use frequently prevails.
This type of synapse has the advantage of being able to modulate impulse transmission. But how do you get it? This is because it has the ability to vary:
- The neurotransmitter.
- The firing frequency.
- The intensity of the impulse.
The chemical transmission between neurons is produced by neurotransmitters that can be modified. Thus, the transmission of the chemical synapse occurs in the following manner.
Chemical synapse process
- First, the neurotransmitter is synthesized and stored in vesicles.
- Second, an action potential invades the presynaptic membrane.
- Then, the depolarization of the presynaptic terminal causes the opening of voltage-dependent calcium channels.
- Then there is an influx of calcium through the channels.
- This calcium causes the vesicles to fuse to the presynaptic membrane.
With this, the neurotransmitter is released into the synaptic cleft via exocytosis.
- The neurotransmitter binds to receptors in the postsynaptic membrane.
- The opening or closing of postsynaptic channels occurs afterwards.
- Then, the postsynaptic current causes excitatory or inhibitory postsynaptic potentials that change the excitability of the postsynaptic cell.
- Finally, there is a recovery of the vesicular membrane of the plasma membrane.
The electrical synapse
At the electrical synapses, information is transmitted through local currents. In addition, there is no synaptic delay (the time it takes for the synaptic connection to take place).
This type of synapse has some opposite characteristics to chemical synapses. Thus, they are symmetric, bidirectional and have low plasticity. The latter implies that information is always transmitted in the same way. Thus, when an action potential occurs in a neuron, it replicates in the next neuron.
Do these two types of synapses coexist?
It is now known that electrical synapses and chemical synapses coexist in most organisms and in brain structures. However, we are still knowing details of the properties and distribution of these two modes of transmission.
It seems that most of the research efforts have focused on exploring how the chemical synapse works. Thus, much less is known about electrical synapses. In fact, as we have explained before, it has been thought that electrical synapses were typical of invertebrates and cold-blooded vertebrates. However, now a lot of data indicate that the electrical synapses are widely distributed in the mammals’ brain.
To conclude, it seems that both synapses, the chemical and the electrical, cooperate and interact widely. In addition, it seems that the speed of the electrical synapse can be combined with the plasticity of chemical transmission, allowing decision making or giving different responses to the same stimulus at different times.