The ability of some organic structures to respond to various types of stimuli (electrical, chemical, mechanical, light, etc.) with characteristic changes in their physical state is called cellular excitability. Generally, the excitability phenomena follow the "all or nothing" law, and there is an excitability threshold below which the substrate is not excitable. In contrast, for more intense stimuli than the limiting ones, the response remains unchanged, and it does not increase proportionally to the intensity of the stimulus. Muscle tissue and nervous tissue are excitable structures. The response to excitatory stimuli consists of the onset of action potentials that propagate along with the tissues, and they are transmitted from one tissue to the other tissue. Muscle tissue, on the other hand, responds to excitatory stimuli by contracting.
At the basis of the excitation phenomena, there is a chemical-physical mechanism, which involves electrical modifications and electrolytic exchanges at the cell membrane level. The excitation of a biological substrate determines the increase in energy consumption and cellular metabolism; repeated stimuli can cause the exhaustion of cellular energy reserves and the accumulation of waste materials (catabolites) in the cell. This condition is accompanied by a transitory state of inexcitability, called "fatigue inexcitability," which should not be confused with the phenomenon of refractoriness. It is the complete blocking or reduction of excitability that occurs physiologically immediately after the stimulus. Due to too intense or prolonged stimuli, some receptors can reduce their excitability by increasing the stimulation threshold; this phenomenon is called adaptation.
- 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 through the plasma membrane.
- The term "cellular excitability" is commonly associated with the cells that make up the nervous system, called neurons.
- Due to the active transport and permeability of the biological membranes, they have a bioelectric potential. This characteristic is what defines the electrical excitability of the cells.
The first models that were intended 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 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. Subsequently, with the contributions of Galvani, it was concluded that electricity was able to excite the muscles, producing contractions.
Alessandro Volta was opposed to these ideas, arguing that the presence of electricity was not due to the tissues, but to the metals that Galvani used in his experiment. For Volta, electricity had to be applied to the muscle, and his testimony managed to convince the researchers of that time.
Many years had to go by to prove Galvini's theory, where muscles were the source of electricity. In 1849, the creation of a device with a sensitivity needed to quantify the generation of electrical currents in muscles and nerves was achieved.
Traditionally, an excitable cell is defined as an entity capable of propagating action potential, followed by a mechanism - whether chemical or electrical stimulation. Several types of cells are excitable, mainly neurons and muscle cells. Excitability is more a general term, interpreted as the ability to regulate the movement of ions through the cell membrane without the need to propagate an action potential.
What makes a cell excitable?
The ability of a cell to achieve the 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 several ions in the cellular environment. The cell membranes are formed by two layers of lipids, which act as a selective barrier to the entry of different molecules into the cell. Among these molecules are the ions.
Inside the membranes are embedded molecules that function as regulators of the passage of molecules. The ions have pumps and protein channels that mediate the entry and exit into the cellular environment. The pumps are responsible for the selective movement of the ions, establishing and maintaining a concentration gradient appropriate for the physiological state of the cell. The result of the presence of unbalanced loads on both sides of the membrane is called an ionic gradient and results in a membrane potential - which is quantified in volts.
The main ions involved in the electrochemical gradient of neuron membranes 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 signals of the chemical and electrical type. They establish connections between them, called synapses. Structurally they have a cellular body, a long extension called an axon and short extensions that start from the soma called dendrites.
The electrical properties of neurons, including pumps, make up the "heart" of their excitability. This translates into the ability to develop nerve conduction and communication between cells.
In other words, a neuron is "excitable" due 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. That is, there is an imbalance between the repetition of the charges if we compare the exterior and interior of the cell. The variation of this potential over time is called the action potential. Not any stimulus is capable of provoking neural activity; it is necessary that it has 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, as a refractory period. This has certain duration and goes on to hyperpolarization, where it is partially excitable. In this case, you need a more potent stimulus than the previous one.
Excitability in astrocytes
What are astrocytes?
Astrocytes are numerous cells derived from the neuroectodermal lineage. Also called astroglia, for being 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 organism and establishing a boundary between the nervous system and the rest of the organism with interval junctions.
Historically, it was thought that astrocytes functioned simply as support scenarios for neurons, the latter having the only leading role in orchestrating nerve reactions. Due to new evidence, this perspective has been reformulated. These glial cells are in an intimate relationship related to many of the brain's functions and how it responds to activity. In addition to participating in the modulation of these events, there is excitability in astrocytes, which is based on the variations of the calcium ion. In this way, astrocytes can activate their glutamatergic receptors.
Author: Vicki Lezama