Receptor Specificity
One of the primary properties of receptors is their specificity—the ability to react with maximum sensitivity to specific, so-called adequate stimuli. This sensitivity is remarkable: for example, to produce a clear sensation of a bitter taste in humans, a minuscule dose of strychnine—only 0.000005 grams—acting on the tongue receptors is sufficient.
Back in 1840, physiologist Johannes Müller formulated the law of specific nerve energies. According to this principle, the quality of the resulting sensation does not depend on the physical nature of the stimulus itself. It is determined exclusively by which sensory system was activated. In other words, the character of a sensation is an innate, genetically fixed, and immutable quality of our body's structures.
Physiological Coding
For information from a receptor to be interpreted by the brain, it must be transformed and transmitted as a neural code. Several main types of coding are distinguished:
- Labeled-line coding. This is the primary method of coding stimulus modality. The signal is transmitted along an isolated chain of strictly interconnected sensory-specific neurons extending from the receptor up to the cerebral cortex.
- Frequency coding. Transmission of stimulus intensity data occurs via changes in the mean frequency of nerve impulses. This relationship is not linear, but logarithmic, and is described by the Weber-Fechner law.
- Interval coding. The signal is encoded by temporal intervals (pauses) between impulses, forming a distinct pattern. Interestingly, this can occur even against the background of a constant average discharge frequency.
- Pattern (burst) coding. Reflects the temporal parameters of stimulation. Information is transmitted in bursts (trains) of impulses at strictly defined moments: upon stimulus onset (on), offset (off), both (on-off), or continuously throughout the entire duration of the stimulus.
- Spatial coding. Carries information about the extent and exact configuration of the applied stimulus. It is ensured by the simultaneous activity of a vast number of receptors sending impulses to the brain via multiple parallel afferent fibers.
Adaptation and Sensitization
Adaptation is the process of altering receptor sensitivity during a prolonged, continuous stimulus application. Receptors can modify their discharge pattern to adjust to the stimulus.
Based on the rate of this process, receptors are classified into:
- Rapidly adapting (phasic): generate only a brief response to the rapid onset of a stimulus.
- Slowly adapting (tonic): 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 sensory thresholds decrease and receptor excitability significantly increases.
Adaptation mechanisms are diverse and include:
- Physical or chemical shifts in the perireceptor space.
- Dynamics of intracellular physicochemical processes within the receptor itself.
- Descending regulatory influences from higher nervous centers.
An important role is played by the autonomic nervous system: specifically, the sympathetic division participates in the fine "tuning" of the receptor apparatus to the adequate stimulus.
Systemic Properties: Mobility, Reliability, and Fields
Functional Mobility This is the ability of a sensory organ to respond to stimulation by changing the number of active (currently functioning) receptors. This property reflects the organism's functional reserve. The level of mobility is not constant: it depends on environmental conditions, the current state of the organism, and central and humoral influences. The functional mobility coefficient is calculated simply: K = (Number of active receptors) / (Total number of receptors).
Receptor Reliability Uninterrupted functioning of sensory systems is ensured by two types of redundancy:
- Functional redundancy: a receptor is capable of generating additional, duplicate impulses in its response.
- Morphological redundancy: the presence of duplicate communication pathways originating 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 work in isolation; they actively interact with one another. One such interaction is lateral inhibition. Example: following a sharp mechanical tap to a skin mechanoreceptor, neighboring receptors in the field may be transiently and completely inhibited, causing them to stop responding even to threshold (excitation-level) stimuli.