Specificity of the receptor apparatus
One of the main properties of receptors is their specificity — the ability to react with maximum intensity to specific, so-called adequate stimuli. High sensitivity is striking: for example, for a person to experience a distinct bitter taste, it is sufficient to expose the tongue's receptors to an infinitesimal dose of strychnine — a mere 0.000005 grams.
As early as 1840, physiologist J. Müller formulated the law of specific nerve energies. According to this rule, the quality of the resulting sensation is completely independent of the physical nature of the stimulus itself. It is determined exclusively by which sensory system was activated. In other words, the nature of the sensation is an innate, genetically fixed, and immutable quality of our body's structures.
Physiological coding
For information from a receptor to be understood by the brain, it must be transformed and transmitted in the conventional form of a neural code. There are several main types of coding:
- Labeled line code. This is the main method of encoding the sensory modality of a stimulus. The signal is transmitted along an isolated chain of strictly connected sensory-specific neurons running from the receptor itself all the way to the cerebral cortex.
- Frequency code. Transmission of data on stimulus quality occurs via changes in the average frequency of nerve impulse firing. This relationship is not linear, but logarithmic, and is described by the Weber—Fechner law.
- Interval code. The signal is encoded by time intervals (pauses) between impulses, forming a specific pattern. Interestingly, this can occur even against the background of a constant average discharge generation frequency.
- Mode code. Reflects the temporal parameters of exposure. Information is transmitted in bursts (salvos) of impulses at strictly defined moments: upon stimulus onset (on), offset (off), at both of these moments (on-off), or continuously throughout the entire duration of the stimulus.
- Spatial code. Conveys information about the extent and precise configuration of the acting stimulus. It is ensured by the simultaneous operation of a vast number of receptors that send impulses to the brain along multiple parallel afferent fibers.
Adaptation and sensitization
Adaptation is the process of changing receptor sensitivity that occurs against the background of ongoing, continuous exposure to a stimulus. Receptors are able to change the pattern of their impulse activity, adjusting to the stimulus.
Depending on the rate of development of this process, receptors are divided into:
- Rapidly adapting: generate only a short response to a rapid stimulus onset.
- Slowly adapting: continue to respond to the stimulus with a train of impulses throughout its entire duration.
A special form of the adaptation process is sensitization — a state in which perception thresholds decrease and receptor excitability increases significantly.
Mechanisms of adaptation are quite diverse and include:
- Physical or chemical shifts in the perireceptor space.
- The dynamics of intracellular physicochemical processes within the receptor itself.
- Descending regulatory influences from higher nervous centers.
The autonomic nervous system plays an important role: in particular, the sympathetic division participates in the fine "tuning" of the receptor apparatus to the action of an adequate stimulus.
Systemic properties: mobility, reliability, and fields
Functional mobility This is the ability of a sense organ to respond to irritation by changing the number of active (currently functioning) receptors. This property reflects the body's functional reserve. The level of mobility is not constant: it depends on environmental conditions, the current state of the body, and central and humoral influences. The functional mobility coefficient is calculated simply: K = (Number of active receptors) / (Total number of receptors).
Receptor reliability Uninterrupted operation of sensory systems is ensured by two types of redundancy:
- Functional redundancy: the receptor is able to generate additional, duplicate impulses in its response.
- Morphological redundancy: the presence of duplicate communication channels running from neighboring receptors.
Functional receptive fields A receptive field is defined as the entire set of receptors that activate one specific sensory neuron. Elements within such a field do not function in isolation; they actively interact with one another. One variant of this interaction is lateral inhibition. Example: following a sharp mechanical impact in the area of a cutaneous mechanoreceptor, adjacent receptors of the field may be transiently completely inhibited, causing them to stop responding even to threshold (excitation-sufficient) stimuli.
Briefly: answers to topical questions
Which receptors are classified as rapidly adapting (phasic) and slowly adapting (tonic)?
Depending on the rate of adaptation, receptors are classified into rapidly and slowly adapting.
- Rapidly adapting: Pacinian corpuscles, Meissner corpuscles, and the hair follicle apparatus (a rapid stimulus evokes a short response).
- Slowly adapting: Golgi tendon organs, muscle spindles (proprioceptors), Merkel discs, and free nerve endings (respond with a train of impulses throughout the entire duration of the stimulus).
How are receptors classified by the mechanism of excitation generation (primary- and secondary-sensory)?
According to their working mechanisms, receptors are divided into primary sensory and secondary sensory.
- Primary sensory receptors — characterized by the direct transition of irritation into excitation. The receptor potential arises directly in the encapsulated ending of a nerve fiber (e.g., a Pacinian corpuscle).
- Secondary sensory receptors — characterized by the presence of a specialized intermediary cell. The receptor potential arises in the receptor cell, which then releases a mediator that induces a generator potential in the sensory fiber (e.g., hair cells of the organ of Corti).
What is the mechanism of lateral inhibition in receptive fields?
The mechanism of lateral inhibition consists in blocking the conduction of excitation along pathways running parallel to the central activating pathway.
Upon activation of afferent pathways, intersegmental inhibitory neurons inhibit a group of neurons; this inhibition is initiated by the excitation of a neighboring group of neurons. In receptive fields, field elements can interact with each other, for example, through lateral inhibition. Following a mechanical tap in the area of a skin mechanoreceptor, the receptors of the field are completely inhibited for a short time and do not respond to threshold stimuli.
The physiological significance of lateral inhibition is to ensure discriminative sensitivity: the ability to distinguish several separate points within a receptive field and to enhance the contrast of perception.
Class and exam questions
What is the essence of the law of specific nerve energies?
The essence of the law formulated by J. Müller is that the quality of our sensation does not depend on the nature of the stimulus itself. It is determined solely by the "specific energy" inherent in the sensory system itself.
What is labeled-line coding?
This is the main type of coding for the sensory modality of a stimulus. It represents the transmission of a signal along a strict, isolated chain of connected sensory-specific neurons — from the receptor to the cerebral cortex.
What does the functional mobility coefficient indicate?
It shows the proportion of the engaged functional reserve of the sense organ and is calculated as the ratio of the number of actively functioning receptors to the total number of receptors in a given area.
How does the sympathetic nervous system affect receptors?
The sympathetic division of the autonomic nervous system exerts regulatory influences, participating in "tuning" receptor sensitivity to the action of an adequate stimulus.
What else to study on this topic
- Mathematical justification of the Weber–Fechner law and logarithmic relationship.
- Detailed mechanisms of lateral inhibition in receptive fields.
- Differences in ionic mechanisms of rapidly and slowly adapting receptors.
- Effect of humoral factors on the functional mobility of receptors.
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