What is thermoreception?

Introduction

Humans have thermoreceptors that can detect the flow of heat energy. These specialized sensory receptors can detect the flow of heat as a change in temperature, and convert this information into nerve impulses. Conversion into nerve impulses places the information into a form that can be processed by the central nervous system, allowing a compensating response, if required, to be initiated.

Humans and other mammals have two kinds of thermoreceptors. One type, called the warm thermoreceptor, becomes active in sending nerve impulses when the body surroundings or an object touched reaches temperatures above 30 degrees Celsius. Nerve impulses from the warm thermoreceptors increase proportionately in frequency as the temperature rises to about 43 degrees Celsius; past this temperature, impulses from the warm thermoreceptors drop proportionately in frequency until they become inactive at about 50 degrees Celsius.

The second type of thermoreceptor becomes active in generating nerve impulses at temperatures below about 43 degrees Celsius. Nerve impulses from these receptors, called cold thermoreceptors, increase proportionately as temperatures fall to about 25 degrees Celsius. Below this temperature, the frequency of nerve impulses generated by the receptors drops proportionately; as temperatures fall to about 5 to 10 degrees Celsius, activity of the cold thermoreceptors falls to zero. The activity of cold and warm thermoreceptors overlaps between temperatures of about 30 and 40 degrees Celsius. Within this range, the sensation of heat or cold results from an integration in the brain of nerve impulses generated by both cold and warm receptors.

At temperatures below about 15 degrees and above about 45 degrees Celsius, pain receptors become active and increase proportionately in activity as temperatures rise or fall beyond these levels. There is a narrow range of overlap of the limits of pain receptors and thermoreceptors, so that temperatures between about 5 and 15 degrees Celsius are felt as both cold and pain (or as “freezing cold”) and temperatures between about 43 and 50 degrees Celsius are felt as both heat and pain (or “burning hot”). Temperatures beyond the 5-degree and 50-degree limits for the thermoreceptors stimulate only the pain receptors and are felt primarily or exclusively as pain. Curiously, the cold receptors become active as pain receptors as the temperature rises above about 45 degrees Celsius. The dual activity of the cold thermoreceptors may account for the fact that freezing cold and burning heat may produce a similar sensation.

Adaptation Process

Both types of thermoreceptors adapt quickly as the temperature stabilizes. Adaptation refers to the fact that as a stimulus is maintained at a constant level, the nerve impulses generated by a receptor drop in frequency. In effect, the receptor undergoes a reduction in sensitivity if the stimulus remains constant. If the stimulus changes, the receptor again generates nerve impulses at a frequency proportional to the intensity of the stimulus. The ability of receptors to adapt makes them sensitive to a change in stimulus, which is often the factor of greatest importance to an appropriate response.

The rapid adaptation of thermoreceptors is part of common experience. In going from the outdoors into a warm room on a cold day, one immediately detects the warmer temperature and has a resultant strong sense of a temperature change. After a few minutes, one no longer notices the temperature difference, as one’s thermoreceptors adapt and reduce their generation of nerve impulses. If the temperature of the room changes by only a degree or so, however, the generation of impulses by the thermoreceptors increases again, and one becomes aware of the change.

Spatial Summation and Receptor Location

Thermoreceptors also show strong spatial summation. If only a very small region of the body is stimulated, one has difficulty discerning whether a temperature change has been experienced, or even whether the stimulus is hot or cold. As the surface area stimulated increases, impulses arriving in the brain from thermoreceptors are summed, so that perception of the change increases proportionately. If only a square centimeter of skin is stimulated by a warm or cold probe, for example, one might not be able to detect a temperature change smaller than about 1 degree Celsius. If the entire body surface is stimulated, as in total immersion in water, one becomes exquisitely sensitive to changes in temperature. Summation of information from all surface thermoreceptors may allow detection of temperature changes as small as one hundredth of one degree Celsius.

Thermoreceptors in humans are most numerous at the body surface, where they are located immediately under the skin. Each thermoreceptor can detect temperature changes over an area of about 1 millimeter in diameter. Cold thermoreceptors occur in greater numbers at the body surface than warm receptors: Depending on the body region, there may be as many as three to ten cold thermoreceptors for each warm thermoreceptor. Thermoreceptors of both types are particularly densely distributed in the skin of the tongue and the lips. In these regions, there may be as many as twenty to thirty or more thermoreceptors per square centimeter of surface. About a third as many thermoreceptors occur in the skin of the fingertips. In other parts of the body surface, only a few thermoreceptors occur per square centimeter.

Physical and Chemical Mechanisms

Although the locations of cold and heat receptors can be pinpointed on the body surface by touching the skin with a warm or cold probe, it has proved difficult to detect particular structures responsible for thermoreception. One group of cold thermoreceptors, however, has been identified as branched nerve endings that terminate near the inner surfaces of cells in the skin. Presumably, other cold thermoreceptors and the warm thermoreceptors are little more than naked nerve endings that cannot be distinguished from pain and some touch receptors, which have a similar appearance.

Little is understood about the physical and chemical mechanisms underlying thermoreception; however, it is considered likely that the reception mechanism depends on increases and decreases in chemical reaction rates in the receptor cells as the temperature rises and falls. In general, chemical reaction rates approximately double for each 10-degree increase in temperature or are halved for each 10-degree fall. Thermoreceptors probably respond to these increases or decreases in chemical reaction rates rather than directly detecting the changes in heat flow responsible for changes in temperature. The thermoreceptors responsible for detecting heat are also sensitive, to some degree, to chemicals. This explains why spices such as red peppers give the sensation of heat when placed on the tongue or rubbed into the skin. Other chemicals, such as menthol, feel cold on the tongue or skin.

Body Temperature Maintenance

Thermoreception has two primary functions in warm-blooded animals such as humans. One is detection of extreme temperatures, so that a person can respond to avoid tissue damage by burning or freezing. The second is maintenance of normal body temperature of 37 degrees Celsius.

Maintenance of body temperature, or homeostasis, involves both conscious and automated responses. At temperatures not too far above and below the range of comfort (about 22 to 24 degrees Celsius), one feels consciously warm or cool and responds by one or more voluntary methods to decrease or increase skin temperature, such as donning or removing clothing. The automated responses maintaining body temperature are complex and involve a variety of systems. Changes in internal temperature are detected by thermoreceptors in the body interior, particularly in the hypothalamus—a brain structure containing the center that detects and regulates internal body temperature. The thermoreceptors of the hypothalamus are extremely sensitive to shifts from the normal body temperature of 37 degrees Celsius. If such changes occur, the hypothalamus triggers involuntary responses that adjust body temperature.

If the internal body temperature rises above 37 degrees, sweat glands in the skin are stimulated to release their secretion, which evaporates and cools the body surface. Heat loss is also promoted by dilation of the peripheral vessels, which increases blood flow to the body surface. Blood cooled at the surface is carried to the body interior by the circulatory system, where it removes heat from internal regions and causes a drop in body temperature. In addition to these cooling mechanisms, release of thyroxin from the thyroid gland is inhibited. The resulting reduction in the concentration of this hormone in the circulation slows the rate at which body cells oxidize fuel substances and diminishes the amount of heat released by these reactions in the body.

If the internal body temperature falls below 37 degrees Celsius, a series of automated responses with opposite effects triggered. Peripheral blood vessels contract, reducing the flow of blood to the body surface. The output of thyroxin from the thyroid gland increases; the increased thyroxin concentration stimulates body cells to increase the rate at which fuel substances are oxidized to release heat within the body. Although the effect of the response in humans is not pronounced, a drop in internal temperature also stimulates contraction of small muscles at hair roots over the body. The contraction, which is felt as “goose bumps,” raises body hairs and increases the dead-air space at the surface of the body. If the drop in internal temperature becomes more extreme, shivering caused by rhythmic contractions of voluntary muscles is induced. Shivering increases body temperature through the heat released by the muscular contractions.

Role of the Hypothalamus

The hypothalamus has been identified as the region of the brain regulating body temperature through observations of the effects of injuries and electrical stimulation. Damage to the hypothalamus can inhibit such temperature-regulating responses as sweating and dilation or constriction of peripheral blood vessels. Conversely, experimental electrical stimulation of the hypothalamus can induce the regulatory responses. These observations indicate that the primary temperature-regulating center of the hypothalamus is in its anterior or preoptic region. The automated responses triggered by the hypothalamus in addition to conscious responses allow humans to maintain an almost constant body temperature in the face of a wide variety of environmental conditions. These combined automated and conscious responses allow humans to survive and remain active in a wider range of environmental conditions than any other animal.

The body temperature maintained by the thalamus is not actually set perfectly and constantly at 37 degrees. For most persons, the internal body temperature varies over a range of about 0.6 degree, with the lowest temperatures in the early morning and the highest point at about four to six in the afternoon. This daily variation in body temperature is called the circadian temperature rhythm.

Although the body temperature is normally set at 37 degrees, the set point can be adjusted upward to produce fever as a part of the body’s response to infection by invading organisms. Raising the body temperature above 37 degrees results from the same automated responses that normally raise internal temperatures—shivering, constriction of peripheral blood vessels, and an increase in the rate of metabolic reactions.

Fever

Several types of bacteria secrete substances that can directly stimulate the hypothalamus to raise its set point and induce fever. Substances of this type, capable of inducing fever, are termed pyrogens. Other substances derived through the breakdown of infecting bacteria, or from substances released through the breakdown of body tissues in disease, particularly fragments of some body proteins, can indirectly trigger the hypothalamus to raise its set point. These substances are engulfed by certain types of white blood cells, including macrophages. On engulfing the breakdown substances, the white blood cells release a powerful pyrogen called interleukin-1. This substance stimulates the secretion of a type of hormone, the prostaglandins, which in turn induces the hypothalamus to raise its temperature set point above 37 degrees. The advantage that fever provides to the body in fighting infection is unclear. Aspirin and corticosteroids are able to reduce fever by inhibiting the secretion of prostaglandins.

When the body’s ability to regulate temperature is exceeded, resulting in extreme hyperthermia or hypothermia, the results can be extremely serious. Fevers above about 41 to 42 degrees Celsius, or about 106 to 108 degrees Fahrenheit, can cause severe or fatal damage if the body temperature is not quickly lowered by treatments such as water or alcohol sponging of the skin. The high temperatures injure or kill body cells, particularly in the brain, liver, and kidneys, and cause internal bleeding. Damage to brain cells from extremely high fever is essentially irreversible and may cause permanent impairment or even death within minutes.

Hyperthermia and Hypothermia

Under some conditions, as on hot and humid days or when the body is immersed in hot water, the normal physiological reactions regulating body temperature are ineffective and body temperature may rise uncontrollably. If the air temperature rises above about 38 degrees Celsius on days in which the humidity approaches 100 percent, for example, temperature regulation by sweating and dilation of peripheral blood vessels is ineffective. Under such conditions, internal body temperature may rise to damaging levels, particularly if physical exercise is attempted. The resulting reaction, known as hyperthermia
or heat stroke, may include dizziness and abdominal distress or pain in milder cases; more severe heat stroke may produce delirium or even death. Hyperthermia differs fundamentally from fever in that the set point of the hypothalamus remains at 37 degrees. Another difference is that the circadian temperature rhythm is maintained during fever, but not in hyperthermia. In addition to high environmental heat and humidity, hyperthermia may be caused by cocaine and psychedelic drugs.

Low environmental temperatures can also exceed the body’s capacity to regulate its internal temperature. Heat loss attributable to accidental or intentional immersion in ice water, for example, induces a steady drop in internal body temperature that cannot be effectively reversed by shivering, constriction of peripheral blood vessels, or increases in chemical reaction rates. The effects of extreme cold in lowering body temperature are magnified by impairment of the regulatory function of the hypothalamus. At body temperatures below about 34 degrees Celsius, the function of the hypothalamus in temperature regulation becomes severely impaired. Shivering usually stops below 32 degrees. At internal temperatures below about 28 degrees Celsius, the temperature regulation centers of the hypothalamus cease to function entirely. Below this temperature, internal body temperature falls rapidly, breathing slows greatly or arrests, and the heart may develop an irregular beat or stop beating entirely. Death follows quickly if breathing or the heartbeat stops. Any fall of body temperature below 35 degrees is known as hypothermia.

Surgical Applications

For some surgical procedures, body temperature is deliberately reduced by administering a drug that inhibits activity of the hypothalamus. The body is then immersed in ice water or surrounded by cooling blankets until internal temperatures reach levels of 30 degrees or below. At these temperatures, the heart can be stopped temporarily without significant damage to the brain or other body tissues. Induced reduction of body temperatures in this manner is routinely used in heart surgery.

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