Classification of Stimuli
Environmental factors that act on cell membranes and elicit a response from the organism are called stimuli. In normal physiology, they are generally divided into five main categories:
- Physical. These include mechanical action, temperature fluctuations, electric current, and other physical phenomena.
- Chemical. These include the effects of various chemicals (acids, alkalis, salts) as well as endogenous regulators such as hormones and neurotransmitters.
- Physicochemical. Associated with changes in basic environmental parameters, such as osmotic pressure, ionic composition, and hydrogen ion concentration (pH).
- Biological. The impact of other living organisms, predominantly microorganisms (bacteria, viruses).
- Informational. A special class of stimuli that, in addition to their physicochemical nature, carry specific semantic meaning. They are driven by physiological processes and social interactions (e.g., emotional states, distress calls in animals, human speech).
Nonspecific (Trigger) Irritability
The organism's response begins with structural alterations in cell membrane components. There are two main types of irritability, the first being nonspecific.
Nonspecific irritability is completely independent of the quality of the acting external agent. Internal processes occurring within the living object (primarily in its membranes) play the main role in forming the response.
The external stimulus performs only a triggering function. This process in physiology is often compared to pulling the trigger of a gun (trigger). Under the influence of the stimulus, the molecules of the cell membrane are gradually brought to a certain state, after which a standard cellular response is automatically launched.
Specific (Selective) Irritability
Specific irritability manifests primarily in response to biologically active substances (including pharmacological agents). Its mechanism is significantly more complex and strictly selective.
The implementation of a specific response includes several key components:
- Receptor zones. Specialized protein molecules—molecular receptors—are located on specific areas of the cell membrane. They interact with chemical substances based on a strict spatial lock-and-key principle.
- Ligands. These are chemical substances capable of specifically binding to a receptor. The formation of the receptor-ligand complex triggers a cascade of biochemical reactions within the membrane and the cell protoplasm.
- Ion channels. Selectivity also heavily depends on the physical parameters of protein ion channels in the membrane. Their pore diameter allows only molecules of a strictly defined size to pass through.
The outcome of this complex interaction is the accumulation of molecular changes in the membranes. Once these changes reach a critical threshold, the living object produces a specific response to the stimulus.
Evolutionary Development of the Property
From a phylogenetic perspective, irritability is the most primitive, fundamental manifestation of any living creature's interaction with environmental factors. This property is inherent in all living cells without exception.
Over the long course of evolution, tissue specialization occurred. As a result, basic irritability transformed into a new, highly organized property known as excitability. In humans and higher animals, excitability is a characteristic feature of specialized tissues: nervous and muscle tissue.