Thermoreceptors

Classification and localization

Depending on their location in the body, there are three main groups of temperature-responding receptors:

1. Superficial receptors — located directly within the thickness of the skin. They are the first to contact the external environment.
2. Deep (tissue) receptors — located in internal tissues and the walls of blood vessels, monitoring the temperature of the body's internal environment.
3. Central receptors — localized in the structures of the central nervous system. These include receptors of the hypothalamus, the reticular formation of the brainstem, and the spinal cord.

Cutaneous receptors are distributed unevenly across the body surface. Each individual has unique zones of their preferential localization. Interestingly, distribution density depends on the living environment: in residents of cold-climate regions, the number of cold-sensing spots vastly exceeds the number of warm spots.

Features of cutaneous thermoreceptors

Cutaneous temperature receptors are functionally divided into cold and warm receptors. Their main anatomical difference lies in their depth. Cold structures are located as close to the surface as possible (about 0.17 mm), allowing them to instantly react to a gust of wind or touching cold objects. Warm receptors are hidden somewhat deeper — at a depth of approximately 0.3 mm.

Mechanism of impulse generation (stationary mode)

At a comfortable, optimal temperature, receptors operate in a background mode, generating action potentials at a constant (stationary) frequency, which is typically 2–5 impulses/s.

Temperature changes shift this balance:

• During cooling, the activity of cold endings increases, while warm endings are inhibited.
• During warming, the discharge frequency of warm structures increases, whereas cold receptors decrease their activity.

Analysis of stationary activity shows that cold receptors operate in a wide range from ~10 °C to 40 °C (discharge peak occurs at 25–28 °C). Warm receptors turn on at ~30 °C and operate up to ~48 °C (activity peak around 42–44 °C).

There is an important overlap zone (within 30–40 °C) where both types of receptors are simultaneously active. If the temperature rises above 45 °C, the activity of warm receptors drops sharply, which may be subjectively perceived as a pain sensation or the so-called "paradoxical cold".

Physiology during rapid temperature changes

The body's response to a sudden temperature change is fundamentally different from the stationary mode. If the skin is rapidly cooled or heated, a powerful burst-like increase in discharge frequency occurs — it can briefly reach 100 impulses/s.

With very rapid, aggressive exposure (both heating and cooling), the phenomenon of nonspecific excitation arises. At this moment, burst activity is produced simultaneously by both warm and cold receptors.

The biological significance of such a nonspecific response lies in emergency protection: the nervous system instantly signals the body about a critical threat of overheating or severe hypothermia, demanding an immediate break in contact with the stimulus.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• The phenomenon of paradoxical cold and the transition of temperature sensitivity into pain at >45 °C.
• The role of central hypothalamic thermoreceptors in maintaining homeostasis.
• Protective nonspecific receptor excitation during extreme exposures.
• The impact of climate adaptation on the distribution density of cold and warm spots.

Other topics in the "Thermoregulation" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.