2.1. Concept of irritability and excitability
Irritability, stimulation, and stimuli
Irritability is the ability of a living structure (cells, tissues, organs) to change its vital activity under the influence of a stimulus.
Stimulation characterizes the outcome of the stimulus, i.e. a complex of metabolic, functional and structural changes that occur in the body under the action of the stimulus.
Stimuli are factors of the external and internal environment. Stimuli are subdivided into physical, chemical, biological, social, etc. categories based on their properties and into adequate and inadequate categories based on the nature of their interaction with receptors.
Cells have evolved to adapt to adequate stimuli. Cells have protein receptors to these type of stimuli. However, cells are very sensitive to inadequate stimuli; for example, several photons of light are enough for excitation of the photoreceptor.
The cell is not adapted to inadequate stimuli. For example, retinal receptors react not only to light quanta,
but also to mechanical stimulus, and electric current. However, the sensitivity to inadequate stimuli is hundreds of thousands and millions times lower.
Excitability and excitation. During differentiation of tissues over the course of evolution, in three of them irritability had the highest specific expression, called excitability.
Excitability is the ability of specialized tissues (nerves, muscles, and glands) to respond with excitation to the action of stimuli.
Excitation is a process of electrical, contractile, or secretory response of specialized tissue. During the process, a rapid fluctuation of the electrical potential of the cell membrane occurs as an essential component.
Electric responses of excitable tissues can be of two types: local (or graded) potential and impulse (propagating) potential.
Local potential types are prepotential, excitatory, and inhibitory postsynaptic potentials, and receptor potentials. They propagate with a decreasing amplitude within 1-2 mm.
Impulse potential is an actively propagated action potential, propagating without reduction of the amplitude within the excitable structure at any distance in the body.
All potentials of excitable cells are produced based on the resting membrane potential.