A thermoreceptor is a non-specialised
sense
A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of Stimulus (physiology), stimuli. (For example, in the human body, the brain which is part of the cen ...
receptor, or more accurately the receptive portion of a
sensory neuron
Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell ...
, that codes absolute and relative changes in temperature, primarily within the innocuous range. In the mammalian peripheral nervous system, warmth receptors are thought to be
unmyelinated
Myelin is a lipid-rich material that surrounds nerve cell axons (the nervous system's "wires") to insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the axon. The myelinated axon can be l ...
C-fibres (low conduction velocity), while those responding to cold have both C-fibers and thinly myelinated
A delta fiber
Group A nerve fibers are one of the three classes of nerve fiber as ''generally classified'' by Erlanger and Gasser. The other two classes are the group B nerve fibers, and the group C nerve fibers. Group A are heavily myelinated, group B ar ...
s (faster conduction velocity). The adequate stimulus for a warm receptor is warming, which results in an increase in their action potential discharge rate. Cooling results in a decrease in warm receptor discharge rate. For cold receptors their firing rate increases during cooling and decreases during warming. Some cold receptors also respond with a brief action potential discharge to high temperatures, i.e. typically above 45 °C, and this is known as a paradoxical response to heat. The mechanism responsible for this behavior has not been determined.
Location
In humans, temperature sensation enters the spinal cord along the axons of
Lissauer's tract
The posterolateral tract (fasciculus of Lissauer, Lissauer's tract, tract of Lissauer, dorsolateral fasciculus, dorsolateral tract, zone of Lissauer) is a small strand situated in relation to the tip of the posterior column close to the entrance of ...
that synapse on first order neurons in
grey matter
Grey matter is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astrocytes and oligodendrocytes), synapses, and capillaries. Grey matter is distingui ...
of the
dorsal horn, one or two vertebral levels up. The axons of these second order neurons then
decussate
Decussation is used in biological contexts to describe a crossing (due to the shape of the Roman numeral for ten, an uppercase 'X' (), ). In Latin anatomical terms, the form is used, e.g. .
Similarly, the anatomical term chiasma is named aft ...
, joining the
spinothalamic tract
The spinothalamic tract is a part of the anterolateral system or the ventrolateral system, a sensory pathway to the thalamus. From the ventral posterolateral nucleus in the thalamus, sensory information is relayed upward to the somatosensory cor ...
as they ascend to neurons in the
ventral posterolateral nucleus
The ventral posterolateral nucleus (VPL) is a nucleus of the thalamus. Together with the ventral posteromedial nucleus (VPM), ventral posterior inferior nucleus (VPI) and ventromedial posterior nucleus (VMpo), it constitutes the ventral posterior ...
of the
thalamus
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
.
In mammals, temperature receptors innervate various tissues including the skin (as
cutaeneous receptors),
cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power ...
and
urinary bladder
The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine enters ...
. Neurons from the pre-optic and hypothalamic regions of the brain that respond to small changes in temperature have also been described, providing information on core temperature. The hypothalamus is involved in
thermoregulation
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...
, the thermoreceptors allowing
feed-forward
Feedforward is the provision of context of what one wants to communicate prior to that communication. In purposeful activity, feedforward creates an expectation which the actor anticipates. When expected experience occurs, this provides confirmato ...
responses to a predicted change in core body temperature in response to changing environmental conditions.
Structure
Thermoreceptors have been classically described as having
'free' non-specialized endings; the mechanism of activation in response to temperature changes is not completely understood.
Function
![Thermoreception 2](https://upload.wikimedia.org/wikipedia/commons/3/3d/Thermoreception_2.png)
Cold-sensitive thermoreceptors give rise to the sensations of cooling, cold and freshness.
In the cornea cold receptors are thought to respond with an increase in firing rate to cooling produced by evaporation of lacrimal fluid 'tears' and thereby to elicit a blink reflex.
Other thermoreceptors will react to opposite triggers and give rise to heat and in some cases even burning sensations. This is often experienced when coming in contact with
capsaicin
Capsaicin (8-methyl-''N''-vanillyl-6-nonenamide) ( or ) is an active component of chili peppers, which are plants belonging to the genus ''Capsicum''. It is a chemical irritant for mammals, including humans, and produces a sensation of burning ...
, an active chemical commonly found in red chili peppers. When coming in contact with your tongue (or any internal surface), the capsaicin de-polarizes the nerve fibers, allowing sodium and calcium into the fibers. In order for fibers to do so, they must have a specific thermoreceptor. The thermoreceptor reacting to capsaicin and other heat producing chemicals is known as
TRPV1
The transient receptor potential cation channel subfamily V member 1 (TrpV1), also known as the capsaicin receptor and the vanilloid receptor 1, is a protein that, in humans, is encoded by the ''TRPV1'' gene. It was the first isolated member of th ...
. In response to heat, the TRPV1 receptor opens up passages that allow ions to pass through, causing the sensation of heat or burning.
TRPV1 also has a molecular cousin,
TRPM8
Transient receptor potential cation channel subfamily M (melastatin) member 8 (TRPM8), also known as the cold and menthol receptor 1 (CMR1), is a protein that in humans is encoded by the ''TRPM8'' gene. The TRPM8 channel is the primary molecular t ...
. Unlike TRPV1, TRPM8 produces cooling sensations as mentioned previously. Similar to TRPV1, TRPM8 responds to a certain chemical trigger by opening its ion pathways. In this case, the chemical trigger is often
menthol
Menthol is an organic compound, more specifically a monoterpenoid, made synthetically or obtained from the oils of corn mint, peppermint, or other mints. It is a waxy, clear or white crystalline substance, which is solid at room temperature and ...
or other cooling agents. Studies performed on mice determined that the presence of both these receptors allows for a gradient of temperature sensing. Mice lacking the TRPV1 receptor were still capable of determining areas significantly colder than on a heated platform. Mice lacking the TRPM8 receptor however, were not able to determine the difference between a warm platform and a cold platform, suggesting we rely on TRPM8 to determine cold feelings and sensations.
Distribution
Warm and cold receptors play a part in sensing innocuous environmental temperature. Temperatures likely to damage an organism are sensed by sub-categories of
nociceptor
A nociceptor ("pain receptor" from Latin ''nocere'' 'to harm or hurt') is a sensory neuron that responds to damaging or potentially damaging stimuli by sending "possible threat" signals to the spinal cord and the brain. The brain creates the sens ...
s that may respond to noxious cold, noxious heat or more than one noxious stimulus modality (i.e., they are polymodal). The nerve endings of sensory neurons that respond preferentially to cooling are found in moderate density in the skin but also occur in relatively high spatial density in the
cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power ...
,
tongue
The tongue is a muscular organ (anatomy), organ in the mouth of a typical tetrapod. It manipulates food for mastication and swallowing as part of the digestive system, digestive process, and is the primary organ of taste. The tongue's upper surfa ...
,
bladder
The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine enters ...
, and facial skin. The speculation is that lingual cold receptors deliver information that modulates the sense of taste; i.e. some foods taste good when cold, while others do not.
Mechanism of transduction
This area of research has recently received considerable attention with the identification and cloning of the
Transient Receptor Potential
Transient receptor potential channels (TRP channels) are a group of ion channels located mostly on the plasma membrane of numerous animal cell types. Most of these are grouped into two broad groups: Group 1 includes TRPC ( "C" for canonical), TRP ...
(TRP) family of proteins. The transduction of temperature in cold receptors is mediated in part by the TRPM8 channel. This channel passes a mixed inward cationic (predominantly carried by Na
+ ions although the channel is also permeable to Ca
2+) current of a magnitude that is inversely proportional to temperature. The channel is sensitive over a temperature range spanning about 10-35 °C. TRPM8 can also be activated by the binding of an extracellular ligand. Menthol can activate the TRPM8 channel in this way. Since the TRPM8 is expressed in neurons whose physiological role is to signal cooling, menthol applied to various bodily surfaces evokes a sensation of cooling. The feeling of freshness associated with the activation of cold receptors by menthol, particularly those in facial areas with axons in the
trigeminal (V) nerve, accounts for its use in numerous toiletries including toothpaste, shaving lotions, facial creams and the like.
Another molecular component of cold transduction is the temperature dependence of so-called leak channels which pass an outward current carried by potassium ions. Some leak channels derive from the family of
two-pore (2P) domain potassium channels. Amongst the various members of the 2P-domain channels, some close quite promptly at temperatures less than about 28 °C (e.g.
KCNK4
Potassium channel subfamily K member 4 is a protein that in humans is encoded by the ''KCNK4'' gene. KCNK4 protein channels are also called TRAAK channels.
Function
''KNCK4'' is a gene segment that encodes for the TRAAK (TWIK-related Arachidon ...
(TRAAK), TREK). Temperature also modulates the activity of the
Na+/K+-ATPase. The Na
+/K
+-ATPase is a
P-type pump that extrudes 3Na
+ ions in exchange for 2K
+ ions for each hydrolytic cleavage of ATP. This results in a net movement of positive charge out of the cell, i.e. a
hyperpolarizing current. The magnitude of this current is proportional to the rate of pump activity.
It has been suggested that it is the constellation of various thermally sensitive proteins together in a neuron that gives rise to a cold receptor.
This emergent property of the neuron is thought to comprise, the expression of the aforementioned proteins as well as various voltage-sensitive channels including the hyperpolarization-activated,
cyclic nucleotide-gated (HCN) channel and the rapidly activating and inactivating transient potassium channel (IK
A).
References
{{Sensation and perception
Sensory receptors