Synapse Types: Neural Communication

For the brain to function properly, neurons need 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?

Two major modes of synaptic transmission appear to be recognized : the electrical synapse and the chemical synapse. Synaptic communication generally occurs between the axon termination (the longest part) of the sending nerve cell and the cell soma of the receiving neuron.

However, contrary to what we may think, the synapse does not occur by direct contact. 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

At the chemical synapse, information is transmitted by neurotransmitters. That is why it is called chemistry; neurotransmitters would be responsible for transmitting the message.

Furthermore, these synapses are not symmetric, but asymmetric. This means that they are not produced exactly the same from one neuron to another. They are also unidirectional: the postsynaptic neuron, the one that receives the synapse, cannot transmit information to the presynaptic neuron, the one that sends the synapse.

The chemical synapse has other specific characteristics. For example, it shows high plasticity. That is, the synapses that have been more 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 ways that we use frequently prevails.

This type of synapse has the advantage of being able to modulate the transmission of the impulse. But how do you do it? This is because it has the ability to vary:

• The neurotransmitter.
• The firing frequency.
• The intensity of the impulse.

In summary, chemical transmission between neurons occurs through neurotransmitters that can be modified. Thus, the transmission of the chemical synapse occurs as follows.

Chemical synapse process

• First, the neurotransmitter is synthesized and stored in vesicles.
• Second, an action potential invades the presynaptic membrane.
• Subsequently, the depolarization of the presynaptic terminal causes the opening of voltage-gated calcium channels.
• This is followed by an influx of calcium through the channels.
• This calcium causes the vesicles to fuse to the presynaptic membrane.
• With this, the neurotransmitter is released to the synaptic cleft via exocytosis.

• The neurotransmitter binds to receptors on the postsynaptic membrane.
• The opening or closing of the postsynaptic channels then occurs .
• Then, the current excitatory postsynaptic potential cause inhibitory postsynaptic or changing the excitability of the postsynaptic cell.
• Finally, there is a recovery of the vesicular membrane from the plasma membrane.

The electrical synapse

At electrical synapses, information is transmitted through local currents. Also, there is no synaptic lag (time it takes for synaptic connection to occur).

These types of synapses have some characteristics opposite to chemical synapses. Thus, they are symmetric, bidirectional and have low plasticity. The latter implies that the information is always transmitted in the same way. Thus, when an action potential occurs in one 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 learning details of the properties and distribution of these two modes of transmission (1).

Apparently, most research efforts have focused on exploring how the chemical synapse works. Thus, much less is known about electrical synapses. In fact, as we have discussed before, electrical synapses have been thought to be typical of cold-blooded invertebrates and vertebrates. However, now a large body of data indicates that electrical synapses are widely distributed in the mammalian brain (2).

To conclude, it appears that both synapses, chemical and electrical, cooperate and interact extensively. Furthermore, 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.