Electrical synapses: how they are and how they function in the nervous system

Beyond chemical synapses, there are also electrical synapses.

The main characteristic of our nervous system is its capacity to transmit information from one cell to another. This intercellular communication occurs in several ways, and one of them is via electrical synapses, small clefts that allow the passage of electrical current..

Although this type of synapse is more typical of invertebrate and lower vertebrate animals, it has also been observed in some areas of the nervous system of mammals, including humans.

In recent years, electrical synapses have lost prominence in favor of chemical synapses, which are more numerous and complex. In this article we will see what these electrical synapses are like and what characterizes them.

What are electrical synapses like?

The transfer of information between neurons occurs at a specialized junction known as synapse. In this synaptic space, neurons communicate and mainly use two ways: the chemical synapse, when the transmission of information occurs by releasing substances or neurotransmitters, and the electrical synapse.

In the electrical synapse, the membranes of the pre- and postsynaptic neurons are joined by a gap junction, or communicating junction, through which electrical current flows directly from one cell to the other..

These gap junction channels have a low resistance (or high conductance), i.e. the passage of electric current, either positively or negatively charged ions, flows from the presynaptic neuron to the postsynaptic neuron generating either depolarization or hyperpolarization.

Hyperpolarization and depolarization

At rest, a neuron has a resting potential (potential across the membrane) of -60 to -70 millivolts. This implies that the inside of the cell is negatively charged relative to the outside..

At an electrical synapse, a hyperpolarization occurs when the membrane potential becomes more negative at a particular point on the neuronal membrane, whereas depolarization occurs when the membrane potential becomes less negative (or more positive).

Both hyperpolarization and depolarization occur when ion channels (proteins that allow specific ions to pass through the cell membrane) in the membrane open or close, altering the ability of certain types of ions to enter or leave the cell.

Differences with chemical synapses

From a functional point of view, the communication between neurons through electrical synapses differs substantially from that occurring at chemical synapses.. The main difference is the speed: in the latter, there is a synaptic delay from the time the action potential reaches the presynaptic terminal until the neurotransmitter is released, whereas in electrical synapses the delay is practically non-existent.

This high-speed intercellular communication allows simultaneous functional coupling (a synchronization) of networks of neurons that are linked by electrical synapses.

Another difference between electrical synapses and chemical synapses lies in their regulation.. The latter must follow a complex multi-step process, subject to numerous checkpoints, leading ultimately to the release and binding of the neurotransmitter to the receptor. All this contrasts with the simplicity of electrical synapses, where intercellular channels allow bidirectional flow of ions and small molecules in almost any situation.

Advantages of electrical vs. chemical synapses

Electrical synapses are the most common in less complex vertebrate animals and in some areas of the mammalian brain.. They are faster than chemical synapses but less plastic. Nevertheless, this type of synapse has several very remarkable advantages:

Bidirectionality

The electrical synapse has a bidirectional transmission of action potentials.. Chemical synapses, however, can only communicate unidirectionally.

Coordination capacity

At electrical synapses, a synchronization of neuronal activity is generated, This enables the nerve cells to coordinate with each other..

Speed

As for the speed of communication, it is faster at electrical synapses, because action potentials travel through the ion channel without having to release any chemicals. travel through the ion channel without having to release any chemical substances..

Disadvantages

Electrical synapses also have disadvantages compared to chemical synapses. Primarily, they cannot convert an excitatory signal from one neuron into an inhibitory signal in another. That is, they lack the flexibility, versatility and capacity to modulate signals that their chemical counterparts do possess.

Properties of this type of synapse

Most of the intercellular channels that form electrical synapses are voltage dependent. are voltage-dependentIn other words, their conductance (or, conversely, their resistance to the passage of electric current) varies as a function of the potential difference on both sides of the membranes forming the junction.

In some junctions, in fact, this voltage sensitivity of the channels makes it possible to conduct depolarizing currents in only one direction (known as synapses). (known as rectifying electrical synapses).

It also happens that most of the communication channels close in response to a decrease in intracellular pH or due to an elevation of cytoplasmic calcium (many of the cell's metabolic processes occur in the cytoplasm).

It has been suggested that these properties have a protective role in uncoupling injured cells from other cells, since in the former there are significant increases in calcium and cytoplasmic protons that could affect adjacent cells if they were to pass through the communicating channels.

Neuronal connectivity

Numerous investigations have shown that neurons are not anarchically connected to each other, but rather that the relationships between different nerve centers follow patterns that transcend a given animal species, being characteristic of the animal group..

This connectivity between different nerve centers originates during embryonic development and is refined as the animal grows and develops. The basic wiring in different vertebrate animals shows a general resemblance, a reflection of patterns of gene expression inherited from common ancestors.

During the differentiation of a neuron, its axon grows guided by the chemical characteristics of the structures it encounters along its path, and these serve as a reference to know how to position and place itself within the neuronal network.

Studies of neuronal connectivity have also shown that there is usually a predictable correspondence between the position of neurons in the center of origin and that of their axons in the center of destination, making it possible to establish precise topographic maps of the connection between the two areas.

Bibliographic references:

• Waxman, S. (2012). Neuroanatomia clinica. Padova: Piccin.