WHAT IS CELLULAR EXCITABILITY? - BIOLOGY - 2025

The excitability is a property of cells that allows them to respond to stimulation by rapid changes in membrane potential. These are produced by the flow of ions across the plasma membrane.

The term "cellular excitability" is commonly associated with the cells that make up the nervous system, called neurons. However, there is recent evidence that shows excitability in astrocytes, thanks to changes in the cytosol in terms of calcium ion concentrations.

Source: pixabay.com

Thanks to the active transport and the permeability of biological membranes, they have a bioelectric potential. This characteristic is what defines the electrical excitability of cells.

Historical perspective

The first models that claimed to integrate the role of ions and the generation of electrical signals in the body argued that neurons were similar to a tube through which substances ran that inflated or deflated muscle tissue.

In 1662, Descartes used principles of hydraulics to describe a potential model of the functioning of the nervous system. Later, with the contributions of Galvani, it was concluded that electricity was able to excite muscles, producing contractions.

Alessandro Volta was opposed to these ideas, arguing that the presence of electricity was not due to fabrics, but to the metals that Galvani used in his experiment. For Volta, electricity had to be applied to muscle, and his testimony managed to convince the academics of the time.

It took many years to prove Galvini's theory, where muscles were the source of electricity. In 1849, the creation of a device with a sensitivity necessary to quantify the generation of electrical currents in muscles and nerves was achieved.

Excitable cells

Traditionally, an excitable cell is defined as an entity capable of propagating an action potential, followed by a mechanism - either chemical or electrical - of stimulation. Several types of cells are excitable, mainly neurons and muscle cells.

Excitability is more of a general term, interpreted as the ability or ability to regulate the movement of ions across the cell membrane without the need to propagate an action potential.

What makes a cell excitable?

The ability of a cell to achieve conduction of electrical signals is achieved by combining characteristic properties of the cell membrane and the presence of fluids with high salt concentrations and of various ions in the cellular environment.

Cell membranes are made up of two layers of lipids, which act as a selective barrier to the entry of different molecules into the cell. Among these molecules are ions.

Inside the membranes are embedded molecules that function as regulators of the passage of molecules. Ions have pumps and protein channels that mediate entry and exit to the cellular environment.

The pumps are responsible for the selective movement of the ions, establishing and maintaining a concentration gradient appropriate to the physiological state of the cell.

The result of the presence of unbalanced charges on both sides of the membrane is called an ion gradient and results in a membrane potential - which is quantified in volts.

The main ions involved in the electrochemical gradient of the membranes of neurons are sodium (Na ⁺), potassium (K ⁺), calcium (Ca ²⁺) and chlorine (Cl ^-).

Excitability in neurons

What are neurons?

Neurons are nerve cells, which are responsible for processing and transmitting chemical and electrical signals.

They make connections between them, called synapses. Structurally they have a cell body, a long process called the axon, and short processes that start from the soma called dendrites.

Neural excitability

The electrical properties of neurons, including pumps, form the "heart" of neuronal excitability. This translates into the ability to develop nerve conduction and communication between cells.

In other words, a neuron is "excitable" thanks to its property of changing its electrical potential and transmitting it.

Neurons are cells with several particular characteristics. The first is that they are polarized. In other words, there is an imbalance between the repetition of the charges, if we compare the exterior and the interior of the cell.

The variation of this potential over time is called the action potential. Not just any stimulus is capable of provoking neural activity, it needs to have a “minimum amount” that exceeds a limit called the excitation threshold - following the all or nothing rule.

If the threshold is reached, the potential response takes place. Next, the neuron experiences a period where it is not excitable, such as a refractory period.

This has a certain duration, and passes to hyperpolarization, where it is partially excitable. In this case, you need a more powerful stimulus than the previous one.

Excitability in astrocytes

What are astrocytes?

Astrocytes are numerous cells derived from the neuroectodermal lineage. Also called astroglia, as they are the most numerous glial cells. They participate in a large number of functions related to the nervous system.

The name of this type of cell derives from its starry appearance. They are directly associated with neurons and the rest of the body, establishing a boundary between the nervous system and the rest of the body, by means of interval junctions.

Astrocytic excitability

Historically, astrocytes were thought to function simply as a support stage for neurons, the latter being the ones that have the only leading role in orchestrating nerve reactions. Thanks to new evidence, this perspective has been reformulated.

These glial cells are in an intimate relationship with many of the brain's functions, and how the brain responds to activity. In addition to participating in the modulation of these events.

Thus, there is excitability in astrocytes, which is based on variations of the calcium ion in the cytosol of the cell in question.

In this way, astrocytes can activate their glutamatergic receptors and respond to the signals emitted by neurons that are located in a nearby region.

References

1. Chicharro, JL, & Vaquero, AF (2006). Exercise physiology. Panamerican Medical Ed.
2. Cuenca, EM (2006). Fundamentals of physiology. Editorial Paraninfo.
3. Parpura, V., & Verkhratsky, A. (2012). The astrocyte excitability brief: from receptors to gliotransmission. Neurochemistry international, 61 (4), 610-621.
4. Price, DJ, Jarman, AP, Mason, JO, & Kind, PC (2017). Building brains: an introduction to neural development. John Wiley & Sons.
5. Schulz, DJ, Baines, RA, Hempel, CM, Li, L., Liss, B., & Misonou, H. (2006). Cellular excitability and the regulation of functional neuronal identity: from gene expression to neuromodulation. Journal of Neuroscience, 26 (41) 10362-10367.