The Five Senses and Beyond: The Encyclopedia of Perception - Jennifer L. Hellier 2017

Thermoreceptors

Thermoreceptors are a class of receptors responsible for thermal sensation, which stems from the relationship between the amount of heat produced by the organism’s metabolism and the amount of heat expelled to the environment by that organism. These specialized cells are activated by temperature changes. A signal is then sent to the nervous system via an action potential. Different temperature thresholds trigger different classes of thermoreceptors to fire action potentials (Schepers & Ringkamp, 2009). An action potential is a signal relayed from the thermoreceptor down the axons and to the central nervous system, which will then respond with an action accordingly. These receptors are important for an organism’s survival because they are crucial for maintaining homeostatic temperature.

Temperature Test

Temperature for living organisms is essential to maintain health and homeostasis (physiological balance within the body). Animals, and particularly humans, have the ability to sense heat and cold from the environment around them, which allows them to respond in an appropriate manner. Furthermore, the brain helps maintain the animal’s core temperature so that the body can protect itself during extreme temperature situations. In animals, temperature and pain sensations travel together in the nervous system. Specifically, these sensations are mediated by the same fiber types within the peripheral nervous system, travel in the spinal cord within the same pathway, and are perceived within the somatosensory cortex (postcentral gyrus of the brain). Regulation of body temperature is mediated by the hypothalamus, a deep brain structure located below the thalamus.

Materials:

Volunteer

Tuning fork (can be purchased on Amazon.com) or other metallic instrument

Thermometer

Cold water or ice

Cup

Directions:

Skin temperature test: Use a tuning fork or other metallic instrument to test a person’s ability to feel a cold sensation on the skin. Never use heat to test temperature as it can be too hot and burn the person. Place the tuning fork in a cup of cold or ice water for a few minutes. The metal is a good conductor of temperature and will cool down quickly. Ask the person to close their eyes and tell you when they feel a change in temperature on the skin. Place the cooled tuning fork gently on their right forearm and ask them if they feel a cold sensation. Move the cooled instrument to the person’s upper right arm and ask them if they feel a cold sensation. Repeat this same test with the remaining extremities (left arm and both legs).

Core temperature test: This test is what most people think of when talking about testing temperature and is generally taken at each health care visit. To take a person’s temperature, use the thermometer as recommended by the manufacturer. Today, the most common thermometers are the instant-read type and can be used in the ear canal (tympanic) or along the forehead (temporal) to measure temperature. Normal readings are around 98.6°F (37°C), while temperatures above 100°F (37.8°C) are considered to be a fever. If you do not have a thermometer, you can use the back of your hand or your lips to test for a fever on the person’s forehead or the back of his or her neck. Note that the back of the hand and lips are more sensitive to temperature than the palm of the hand. Determine if the person’s forehead or neck is warmer than normal. Do not feel the person’s hands or feet as often the extremities will feel cool when there is a fever.

Jennifer L. Hellier

Function of Thermoreceptors

Thermoreceptors essentially function as the thermostat of a living organism. Most organisms lose their enzymatic function if they are exposed to temperatures outside their homeostatic temperature range, which can have lethal effects for the organism. Early research suggests that some organisms such as humans and primates have the ability to perform thermal adaptation after prolonged exposure to nonhomeostatic temperatures (McCleskey, 1997). Thermal adaptation suggests that thermoreceptors are still firing action potentials, but they do so in a decreased intensity because they are desensitized to their stimuli—the threshold temperature. The organism at this point will experience diminishing thermosensation and feel that it is less hot or less cold. Although the autonomous nervous system can adjust internal temperatures to a certain extent, thermal adaptation essentially silences the temperature alarms of the body. It does not necessarily mean the organism is out of danger in extreme conditions.

Anatomy and Physiology

Thermoreceptors are divided into two categories: peripheral thermoreceptors and central thermoreceptors. Peripheral thermoreceptors are located on the skin and mucous membranes. These are the first detectors of temperature changes outside (external environment) of the organism. Peripheral receptors are divided into two classes: cold receptors and hot receptors. Different temperature thresholds trigger them. In humans, the homeostatic temperature is 37°C. Cold receptors will fire action potentials when the skin is exposed to a temperature of 25°C (Schepers & Ringkamp, 2009). As temperatures continue dropping and/or as exposure to cold is prolonged, cold receptors will eventually stop firing. Hot receptors will fire an action potential when the skin is exposed to 30°C or warmer. This is technically still below homeostatic internal temperature, but hot receptors are thought to fire action potentials not only to sense hot, but also warm temperatures (McCleskey, 1997).

A different set of receptors called thermal nociceptors are activated when temperatures are so extreme that they begin to cause pain. These receptors alert the body to immediate danger. These receptors are technical pain receptors. Current research indicates that the thresholds for thermal nociceptors are far beyond those of hot or cold thermoreceptors. In these extreme cases, hot and cold thermoreceptors will no longer actively fire action potentials.

All information captured by thermoreceptors is then quickly relayed to the central thermoreceptors. Central thermoreceptors are mainly located in the anterior hypothalamus. The central thermoreceptors are responsible for responding by activating the autonomic nervous system (sympathetic nervous system) as well as communicating with the cerebral cortex. The cerebral cortex will activate a voluntary response that involves the organism consciously making a choice to react to the situation, such as to put on a jacket if the person is cold. The autonomic nervous system’s response to a dropping core temperature can include vasoconstriction of the cutaneous blood vessels, neurotransmitter production such as epinephrine, and also production of hormones such as thyroid-stimulating hormone. The response to rising core temperature includes vasodilation of the cutaneous blood vessels and acetylcholine production, which activates sweat glands.

Disease and Disorders

Since thermoreceptors are the primary messengers of thermosensation, defects in these structures will inhibit the signaling pathway. This will prevent the organism from being able to feel changing temperatures. A genetic mutation in the coding region of these receptors may cause a genetic defect in which the organism can no longer sense hot or cold. Trauma and burns can also cause nerve damage and impair thermoreceptor function.

Melissa Tjandra

See also: Free Nerve Endings; Nociception; Nociceptors; Sensory Receptors; Thermal Sense

Further Reading

McCleskey, Edwin W. (1997). Thermoreceptors: Recent heat in thermosensation. Current Biology, 7(11), R679—R681.

Schepers, Raf J., & Matthias Ringkamp. (2009).Thermoreceptors and thermosensitive afferents. Neuroscience and Biobehavioral Reviews, 33(3), 205—211. Retrieved from http://www.sciencedirect.com/science/article/pii/S0149763408001206?np=y