What Are Stimuli and Their Classification
Environmental factors that act on cell membranes and elicit a response from the body are called stimuli. In normal physiology, they are conventionally divided into five main categories:
- Physical. These include mechanical impact, temperature fluctuations, electric current, and other physical phenomena.
- Chemical. These include the effects of various chemical substances—acids, alkalis, salts, as well as endogenous regulators such as hormones and neurotransmitters.
- Physicochemical. These are associated with changes in basic environmental parameters, such as osmotic pressure, ionic composition, and hydrogen ion concentration (pH).
- Biological. The effects of other living organisms, predominantly microorganisms (bacteria, viruses).
- Informational. A special class of stimuli that, in addition to their physicochemical nature, carry a specific semantic load. They are conditioned by physiological processes and social interactions (e.g., emotional states, warning cries or mating calls in animals, human speech).
Nonspecific (Trigger) Irritability
The body's response begins with changes in the structure of cell membrane components. Two main types of irritability are distinguished. The first of these is nonspecific.
Nonspecific irritability does not depend at all on the quality of the acting external agent. The primary role in forming the response is played by internal processes occurring within the living object (primarily in its membranes).
The external stimulus performs merely the function of a triggering mechanism. In physiology, this process is often compared to pulling the trigger of a gun (hence the name: trigger). Under the action of the stimulus, the molecules of the cell membrane are gradually brought to a certain state, after which the standard cell response is automatically launched.
Specific (Selective) Irritability
Specific irritability manifests primarily in response to biologically active substances (including medications). Its mechanism is significantly more complex and strictly selective.
The realization of a specific response involves several key components:
- Receptor sites. Specialized protein molecules—molecular receptors—are located in specific areas of the cell membrane. They interact with chemical substances according to 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 cell protoplasm.
- Ion channels. Selectivity also heavily depends on the physical parameters of protein ion channels in the membrane. Their diameter is such that only molecules of a strictly defined size can pass through the pores.
The result of this complex interaction is the accumulation of molecular changes in the membranes. As soon as these changes reach a critical level, the living object produces a specific response to the stimulus.
Evolutionary Development of the Property
From a phylogenetic perspective, irritability is the most primary, fundamental manifestation of any living organism's relation to environmental factors. This property is inherent in all living cells without exception.
Over the long course of evolution, tissue specialization took place. As a result, basic irritability transformed into a new, more highly organized property—excitability. In humans and higher animals, excitability is a characteristic feature of specialized tissues: nerve and muscle.
Briefly: answers to topical questions
What are the laws of stimulation of excitable tissues?
The following laws of stimulation of excitable tissues determine the stimulus parameters necessary for the generation of propagating excitation:
- All-or-none law (Bowditch's law) — subthreshold stimuli do not elicit a response, whereas threshold stimuli elicit a maximal response.
- Strength law — the greater the strength of the stimulus, the greater the response, up to a certain limit.
- Duration law — the longer the stimulation, the stronger the response of the object, up to a certain limit.
- Strength-duration law — there is an inverse relationship between the strength and the duration of the stimulating impulse: the shorter the duration of the stimulus, the greater its strength must be to elicit excitation.
- Gradient law of stimulation (accommodation) of Du Bois-Reymond — the response depends on the rate of rise of the stimulus strength.
- Law of polar action of current — excitation occurs under the cathode upon make and under the anode upon break.
- Law of physiological electrotonus — includes catelectrotonus and anelectrotonus.
How are the protein ion channels of the cell membrane structured?
Ion channels are formed by integral membrane proteins in the shape of a tube with a diameter of 0.5–0.7 nm. The structure of a typical gated specific channel includes:
- Mouth — the entrance to the channel.
- Selectivity filter — the narrow bottleneck that determines selectivity (allows an ion to pass if its diameter matches the filter diameter).
- Gate — projections of the protein molecule that regulate channel permeability.
- Gating control mechanism — ensures the opening of the gate in response to membrane potential, ligands, or membrane deformation.
In addition to gated channels, there are non-gated channels (leak channels) which are always open and lack gating mechanisms.
What are the phases of the action potential during cell excitation?
The action potential during cell excitation includes the following phases:
- Pre-spike (local response) — slow partial depolarization; the mechanism is associated with the gradual opening of pores for Na⁺ and the influx of Na⁺ into the cell.
- Spike (peak).
- Negative after-potential (after-depolarization).
- Positive after-potential (after-hyperpolarization) — a phenomenon of heightened polarization; mechanisms: increased efflux of K⁺ outwards, sometimes influx of Cl⁻ into the cell.
To memorize the 5 types of stimuli, use the phrase: "Physical Chemists Formulate Biological Information" (Physical, Chemical, Physicochemical, Biological, Informational).
Class and exam questions
What is the difference between irritability and excitability?
Irritability is the universal, primary property of all living cells to respond to the environment. Excitability is an evolutionarily advanced form of irritability that is characteristic only of specialized tissues (nerve and muscle).
What does the «lock-and-key» principle mean in membranes?
This is the principle of specific irritability, in which a chemical substance (ligand) spatially matches the structure of the protein receptor on the membrane perfectly, like a key fitting a lock.
Why is non-specific irritability called trigger-like?
Because the external stimulus acts only as a "trigger." It merely initiates internal processes within the cell, while the nature of the response itself does not depend on the quality of this external stimulus.
What else to study on this topic
- Dynamics of molecular changes during trigger irritability
- Types and structure of molecular membrane receptors
- Principles of protein ion channel function
- Informational stimuli: the influence of emotions and speech on physiology
- Differences between excitable and non-excitable tissues
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