The main characteristic of our nervous system is its ability to transmit information from one cell to another. This intercellular communication takes place in several ways, and one of them is through the electrical synapses, small cracks that allow the passage of the electrical current .

Although these types of synapses are more typical of invertebrate and lower vertebrate animals, they have also been observed in some areas of the nervous system of mammals, including humans.

In recent years, electrical synapses have lost their importance in favour of chemical synapses, which are more numerous and complex. In this article we will see how these electrical synapses are and what characterizes them.

How are the electrical synapses?

The transfer of information between neurons occurs at the level of a specialized junction known as a synapse. In this synaptic space, neurons communicate and use mainly two pathways: 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 pre- and postsynaptic neurons are linked by a "gap" junction, or communicating junction, through which electrical current flows from one cell to another and directly .

These gap joint channels have a low resistance (or high conductance), that is, the passage of electrical current, whether positively or negatively charged ions, flows from the presynaptic to the postsynaptic neuron generating either depolarization or hyperpolarization.

Hyperpolarization and depolarization

At rest, a neuron has a rest 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, hyperpolarization occurs when the membrane potential becomes more negative at a particular point on the neuronal membrane, while 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, communication between neurons through electrical synapses differs substantially from that which occurs at chemical synapses . The main difference is 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 intercellular communication at such a high speed allows the simultaneous functional coupling (a synchronization) of neuron networks that are linked by electrical synapses.

Another difference between electrical and chemical synapses lies in their regulation . The latter must follow a complex process of multiple steps, subject to numerous control points, which finally lead to the release and binding of the neurotransmitter with the receptor. This contrasts with the simplicity of electrical synapses, where intercellular channels allow the bidirectional flow of ions and small molecules in almost any situation.

Advantages of electric 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 two-way transmission of the action potentials . Chemistry, however, can only communicate in a unidirectional way.

Coordination capacity

In the electrical synapses a synchronization in the neuronal activity is generated, what makes that the nervous cells can coordinate between them .

Speed

As far as the communication speed is concerned, it is faster at the electrical synapses, because the action potentials travel through the ion channel without having to release any chemicals .

Disadvantages

Electrical synapses also have disadvantages over 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 ability to modulate signals that their chemical counterparts do.

Properties of this type of synapse

Most of the intercellular channels that form the electrical synapses are voltage-dependent ; that is, their conductance (or, conversely, their resistance to the passage of electrical current) varies according to the difference in potential on both sides of the membranes that form the junction.

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

It also happens that most of the communication channels are closed in response to a decrease in the intracellular pH or due to an increase in cytoplasmic calcium (many of the metabolic processes in the cell take place in the cytoplasm).

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

Neural Connectivity

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

This connectivity between different nerve centers originates during embryonic development and is refined as it grows and develops. The basic wiring in different vertebrate animals shows a general similarity, a reflection of the 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 and these serve as a reference for knowing how to position itself and place itself within the neuronal network.

Neuronal connectivity studies have also shown that there is usually a predictable correspondence between the position of neurons in the source center and that of their axons in the target center, and it is possible to establish accurate topographical maps of the connection between the two areas.

Bibliographic references:

  • Waxman, S. (2012). Clinical neuroanatomy. Padova: Piccin.