Classification and Localization
Based on their location in the body, temperature-sensitive receptors are divided into three main groups:
- Superficially located receptors — situated directly within the skin layer, serving as the first line of contact with the external environment.
- Deep (tissue) receptors — located in internal tissues and blood vessel walls, monitoring the temperature of the core internal environment.
- Central receptors — located within central nervous system structures, including the hypothalamus, reticular formation of the brainstem, and the spinal cord.
Cutaneous receptors are distributed unevenly across the body surface, with individual variation in hotspot density. Interestingly, receptor density correlates with climate: residents of cold regions possess a significantly higher density of cold-sensing spots compared to warm-climate populations.
Cutaneous Thermoreceptor Characteristics
Cutaneous thermal receptors are functionally divided into cold receptors and warm receptors. Their primary anatomical distinction lies in depth. Cold structures are located closest to the surface (approx. 0.17 mm), allowing them to instantly react to drafts or contact with cool objects. Warm receptors are positioned slightly deeper — approximately at a depth of 0.3 mm.
Impulse Generation Mechanism (Steady-State)
At a comfortable, optimal temperature, receptors operate in a background mode, generating action potentials at a constant (steady-state) frequency, typically 2–5 impulses/s.
Temperature shifts alter this balance:
- Upon cooling, the activity of cold endings increases, while warm endings are inhibited.
- Upon warming, the firing frequency of warm structures increases, whereas cold receptors decrease their activity.
Analysis of steady-state activity shows that cold receptors function over a broad range from ~10 °C to 40 °C (with peak firing at 25–28 °C). Warm receptors activate around ~30 °C and function up to ~48 °C (peak activity near 42–44 °C).
An important overlap zone exists (between 30–40 °C), where both receptor types are simultaneously active. If the temperature rises above 45 °C, warm receptor activity drops sharply, which can subjectively be perceived as pain or the so-called "paradoxical cold" sensation.
Physiology During Rapid Temperature Shifts
The body's response to sudden temperature changes differs fundamentally from the steady-state mode. Rapid cooling or heating triggers a massive burst of impulse frequency, which can transiently reach 100 impulses/s.
During very rapid, aggressive stimulation (both warming and cooling), a phenomenon of nonspecific excitation occurs. At this moment, both warm and cold receptors fire simultaneously in bursts.
The biological significance of this nonspecific response is emergency defense: the nervous system immediately signals a critical threat of overheating or severe hypothermia, demanding an immediate withdrawal from the stimulus.