How do we feel temperature?

How do we feel temperature?

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Recently I watched a Youtube Video by Veritasium

He said we are actually feeling the rate of heat transfer, not the temperature.

But how do we feel temperature? How is feeling temperature different from feeling heat and heat transfer?

Why Does Being in the Heat Make Us Feel Tired?

If you're out and about on a sweltering day, it probably won't be long before you start to feel tired and sluggish. But why does being out in the heat bring on feelings of drowsiness?

The reason for this lethargy is simple: Your body is working hard to keep you cool, and this extra labor makes you feel tired, said Dr. Michele Casey, the regional medical director at Duke Health in North Carolina.

"Your body, especially in the sun, has to work hard to maintain a consistent, normal, internal temperature," Casey told Live Science. [What Would Happen If You Fell Into a Volcano?]

On a hot day, your body makes several adjustments to maintain its temperature. For instance, it dilates your blood vessels, a process known as vasodilation, which allows more blood to flow near the skin's surface. This allows warm blood to cool off, releasing heat as it travels near the skin, Casey said.

This increased blood flow near the skin explains why some people look redder when they're feeling hot, according to the BBC.

In addition to vasodilation, the body secretes sweat onto the skin. This sweat then cools the skin as it evaporates, Casey said. But in order to do this extra work, your heart rate increases, as does your metabolic rate (the number of calories your body needs to function), she said.

"All that work&mdashincreasing your heart rate, your metabolic rate&mdasheventually makes you feel tired or sleepy," Casey said.

Furthermore, most people spend their lives slightly dehydrated. Being hot and sweaty only worsens that dehydration, and a symptom of dehydration is fatigue, she noted.

Getting skin damage from the sun can also heighten dehydration. When the sun's rays beam down on your skin, it can cause pigmentation changes, wrinkles and burns. "These chemical changes actually cause fatigue," Casey said. "That's because your body is working to repair the damage."

Sunburns impair your body's ability to regulate its temperature, she said. What's more, when you sunburn, your body diverts fluid from the rest of the body toward the burn in an attempt to heal the skin. This diversion means you have less fluid overall for sweating, which can lead to more dehydration and fatigue, Casey said.

Links Between Temperature and Sleep

We are taught that the ideal human body temperature is 98.6 Fahrenheit. However, this is merely an average. Most people are a degree or two warmer or cooler than this. In addition, all of us experience body temperature variations throughout our day.

Just before we wake up, our bodies turn on the thermostat. Our average body temperature begins to rise, reaching a few degrees higher than normal by early afternoon. From here, we slowly cool down as we approach bedtime, then continue to cool down as we sleep. Just before dawn, we begin the gradual warming process once again in preparation for another busy day.

There are several ways our internal clocks control temperature. When we are in REM sleep, our bodies stop heating effectively, leaving us to absorb the temperature around us. This cooler temperature is not a reason to bundle up several studies have found that we actually sleep better when we are cold. In addition, heating up your body appears to signal to your internal clocks that it is time to awake and be active.

However, these changes in temperature are slight — representing just one to three degrees over several hours. If you find that you often feel cold when you are tired, there is a good chance you are waiting too long to go to sleep. Even if you are keeping super-human hours, your body temperature continues to drop as late night approaches.

Sunburn sensitises our heat-detecting channel, lowering the threshold at which we feel pain

The best understood of them is called TRPV1, and it responds to extreme heat. TRPV1 isn't typically activated until a stimulus reaches 42C (107.6F), which both humans and mice typically regard as painfully hot. Once your skin reaches that threshold the channel becomes activated, which in turn activates the entire nerve, and a signal gets transmitted to the brain with a simple message: ouch!

"For cold, in principle, the same mechanisms apply," Grandl explains, except the protein in question is called TRPM8, and instead of reacting to extreme cold, this channel instead activates upon exposure to cool, but not painfully cold, temperatures.

That leaves TRPA1, which is perhaps the least understood of this class of proteins. While researchers have found that it becomes activated in response to extremely cold stimuli, it isn't clear whether it's actually involved in the work of detection itself.

Stick your finger in a candle flame, and TRPV1 will kick in (Credit: iStock)

Together these three proteins – TRPV1, TRPM8, and TRPA1 – enable the skin to detect a range of temperatures and the body to respond accordingly. And because they're nociceptors, these proteins' job is to help you avoid certain temperatures rather than to seek them out. Mice with defective versions of the TRPM8 receptor, for example, no longer avoid cool temperatures. That means that mice – and us, probably – don't actively seek pleasant temperatures. Instead, they actively avoid both cold and extreme heat, which explains why they seem to prefer warm, balmy environments.

While researchers have defined the thermal boundaries at which these TRP receptors become active, that doesn't mean that they can't be modulated. After all, a lukewarm shower can feel excruciatingly hot if you've got a sunburn. "It's been shown that this is specifically because the inflammation in the skin sensitises the TRPV1 channel," says Grandl, lowering the threshold at which these nerves communicate the sensation of pain to the brain.

To measure the ambient air temperature, all you need is a thermometer and to follow these simple rules. Don't and you'll risk getting a "bad" temperature reading.

  • Keep the thermometer out of direct sunlight. If the sun is shining on your thermometer, it's going to record the heat from the sun, and not the ambient heat in the air. For this reason, always be careful to place thermometers in the shade.
  • Don't place your thermometer too low near the ground or too high above it. Too low, and it will pick up excess heat from the ground. Too high and it will cool from winds. A height of around five feet above ground works best.
  • Place the thermometer in an open, well-ventilated area. This keeps the air circulating freely around it, which means it will represent the temperature of the surrounding environment.
  • Keep the thermometer covered. Shielding it from the sun, rain, snow, and frost provides a standardized environment.
  • Place it over a natural (grassy or dirt) surface. Concrete, pavement, and stone attract and store heat, which they can then radiate towards your thermometer giving it a higher temperature reading than the actual environment.

What happens to your body when its gets cold

When HMS Beagle docked at the southern tip of Tierra del Fuego, Charles Darwin remarked on the capacity of the locals to deal with cold:

A woman, who was suckling a recently born child, came one day alongside the vessel and remained there out of mere curiosity, whilst the sleet fell and thawed on her naked bosom, and on the skin of her naked baby.

Japanese pearl divers dive for long periods in cold water without the comfort of wetsuits, whereas many of us whimper as the waters of the relatively warm Pacific or Indian Oceans reach our midriff.

Why is there such variation in our reaction to cold?

The perception of cold begins when nerves in the skin send impulses to the brain about skin temperature. These impulses respond not only to the temperature of the skin, but also to the rate of change in skin temperature.

So we feel much colder jumping into cold water, when skin temperature drops rapidly, than after we have stayed there for a while, when our skin temperature is low but constant.

The burst of nerve impulses generated by falling skin temperature provides early warning of an event likely to cause body core temperature (the temperature of the internal organs) to fall. If unchecked, a fall in body core temperature can result in lethal hypothermia.

The perception of cold begins when nerves in the skin send impulses to the brain about skin temperature. Viewminder/Flickr, CC BY-NC-ND

In healthy people, physiological systems prevent hypothermia from occurring. Impulses from the skin arrive at the hypothalamus, a brain area responsible for controlling the internal environment of the body, which generates instructions in the nervous system that prevent a drop in body core temperature.

Nervous impulses sent to muscles generate extra metabolic heat through shivering. Blood vessels that would otherwise transport warm blood from the internal organs to the cold skin, where the blood would lose heat, constrict, constraining most blood, and its heat, to the internal organs.

Impulses arriving at the cerebral cortex, the part of the brain where reasoning occurs, generate information about how cold we feel. These combine with impulses arriving from the limbic system, responsible for our emotional state, to determine how miserably cold we feel. These feelings motivate us to perform certain behaviours, such as curling up or putting on more clothes, and to complain.

Feeling cold is not the same as being cold. Jumping into a cool swimming pool feels cold, but it can cause body core temperature to rise because of the warm blood retained in the core. Body temperature can stay elevated for up to an hour.

Many of us also have felt cold at the beginning of a fever, when the body core temperature starts to rise. During a fever, the nerve circuits that control body temperature are reset to a higher level, so the body responds as if it is cold until its temperature stabilises around that higher level.

While fever indicates a problem, is there anything wrong with feeling excessively cold rather than actually being cold?

Some of us have the misfortune to suffer from Raynaud’s phenomenon, a condition in which the blood flow is too low to keep the fingers and toes warm.

Feeling excessively cold during pregnancy, when the foetus acts as a small furnace, may be a symptom of low thyroid hormone activity, needing hormone supplementation.

But some healthy people can feel colder than do others in the same environment. Women often report that they feel colder than men in the same environment. This is probably because they have a lower skin temperature, a consequence of more subcutaneous fat and the hormone oestrogen.

Feeling cold is not the same as being cold. Sam Einhorn/Flickr, CC BY-NC-SA

Some of us may inherit feeling excessively cold. A study of twins found that the prevalence of the feeling of cold hands and feet is highly heritable, implying a genetic basis for exaggerated temperature perception.

Some of us also may feel cold simply because of how others close to us look, a phenomenon called “cold contagion”. In one study, healthy volunteers felt colder if they were shown videos of actors pretending to be cold than if the actors pretended to be warm. The temperature of the volunteers’ hands dropped as the blood vessels to their hands constricted, even though they were not in a cold environment.

Most of us who are healthy but claim to feel excessively cold, however, have only ourselves to blame. Unlike Darwin’s Fuegians, we have habituated ourselves to feeling comfortably warm. In the developed world we rarely expose ourselves to cold, letting expensive clothing protect us from outdoor cold and letting power companies warm our living and working spaces.

Allowing power companies to do the work that our metabolism used to do when we experienced cold may actually contribute to obesity. We’d probably all be much better off if we spent more time being cold.

Duncan Mitchell, Honorary Professorial Research Fellow at the University of the Witwatersrand, Johannesburg Adjunct Professor in the School of Anatomy, Physiology and Human Biology, University of Western Australia Andrea Fuller, Professor, School of Physiology Director, Brain Function Research Group , University of the Witwatersrand, and Shane Maloney, Professor and Head of School, Anatomy Physiology and Human Biology, University of Western Australia

This article was originally published on The Conversation. Read the original article.

Integration of Signals from Mechanoreceptors

The many types of somatosensory receptors work together to ensure our ability to process the complexity of stimuli that are transmitted.

Learning Objectives

Describe how the density of mechanoreceptors affects the receptive field

Key Takeaways

Key Points

  • The various types of receptors, nociceptors, mechanoreceptors (both small and large), thermoreceptors, chemoreceptors, and proprioreceptors, work together to ensure that complex stimuli are transmitted properly to the brain for processing.
  • The distribution of mechanoreceptors within the body can affect how stimuli are perceived this is dependent on the size of the receptive field and whether single or multiple sensory receptors are activated.
  • A large receptive field allows for detection of stimuli over a wide area, but can result in less precise detection a small receptive field allows for detection of stimuli over a small area, which results in more precise detection.
  • The two-point discrimination test can be used to determine the density of receptors within various locations by measuring whether a two-point stimulus (such as thumb tacks) is detected as one or two points.

Key Terms

  • mechanoreceptor: any receptor that provides an organism with information about mechanical changes in its environment, such as movement, tension and pressure

Integration of Signals from Mechanoreceptors

The configuration of the different types of receptors working in concert in the human skin results in a very refined sense of touch. The nociceptive receptors (those that detect pain) are located near the surface. Small, finely-calibrated mechanoreceptors (Merkel’s disks and Meissner’s corpuscles) are located in the upper layers and can precisely localize even gentle touch. The large mechanoreceptors (Pacinian corpuscles and Ruffini endings) are located in the lower layers and respond to deeper touch. Consider that the deep pressure that reaches those deeper receptors would not need to be finely localized. Both the upper and lower layers of the skin hold rapidly- and slowly-adapting receptors. Both primary somatosensory cortex and secondary cortical areas are responsible for processing the complex picture of stimuli transmitted from the interplay of mechanoreceptors.

Sensory receptor structure: Structure of four different types of sensory receptors found within the sensory system.

Density of Mechanoreceptors

In the somatosensory system, receptive fields are regions of the skin or of internal organs. During the transmission of sensory information from these fields, the signals must be conveyed to the nervous system. The mechanoreceptors are activated, the signal is conveyed, and then processed. Some types of mechanoreceptors have large receptive fields, while others have smaller ones. Large receptive fields allow the cell to detect changes over a wider area, but lead to a less-precise perception. Touch receptors are denser in glabrous skin (the type found on human fingertips and lips, for example), which is typically more sensitive and is thicker than hairy skin (4 to 5 mm versus 2 to 3 mm). Thus, the fingers, which require the ability to detect fine detail, have many, densely-packed (up to 500 per cubic cm) mechanoreceptors with small receptive fields (around 10 square mm), while the back and legs, for example, have fewer receptors with large receptive fields. Receptors with large receptive fields usually have a “hot spot”: an area within the receptive field (usually in the center, directly over the receptor) where stimulation produces the most intense response. Tactile-sense-related cortical neurons have receptive fields on the skin that can be modified by experience or by injury to sensory nerves, resulting in changes in the field’s size and position. In general, these neurons have relatively large receptive fields (much larger than those of dorsal root ganglion cells). However, the neurons are able to discriminate fine detail due to patterns of excitation and inhibition relative to the field, which leads to spatial resolution.

The relative density of pressure receptors in different locations on the body can be demonstrated experimentally using a two-point discrimination test. In this demonstration, two sharp points, such as two thumbtacks, are brought into contact with the subject’s skin (though not hard enough to cause pain or break the skin). The subject reports if they feel one point or two points. If the two points are felt as one point, it can be inferred that the two points are both in the receptive field of a single sensory receptor. If two points are felt as two separate points, each is in the receptive field of two separate sensory receptors. The points could then be moved closer and re-tested until the subject reports feeling only one point. The size of the receptive field of a single receptor could be estimated from that distance.

Etiology of Fever

Many disorders can cause fever. They are broadly categorized as

Inflammatory (including rheumatic, nonrheumatic, and drug-related)

The cause of an acute (ie, duration ≤ 4 days) fever in adults is highly likely to be infectious. When patients present with fever due to a noninfectious cause, the fever is almost always chronic or recurrent. Also, an isolated, acute febrile event in patients with a known inflammatory or neoplastic disorder is still most likely to be infectious. In healthy people, an acute febrile event is unlikely to be the initial manifestation of a chronic illness.

Infectious causes

Virtually all infectious illnesses can cause fever. But overall, the most likely causes are

Upper and lower respiratory tract infections

Most acute respiratory tract and gastrointestinal infections are viral.

Specific patient and external factors also influence which causes are most likely.

Patient factors include health status, age, occupation, and risk factors (eg, hospitalization, recent invasive procedures, presence of IV or urinary catheters, use of mechanical ventilation).

External factors are those that expose patients to specific diseases—eg, through infected contacts, local outbreaks, disease vectors (eg, mosquitoes, ticks), a common vehicle (eg, food, water), or geographic location (eg, residence in or recent travel to an endemic area).

Some causes appear to predominate based on these factors (see Table: Some Causes of Acute Fever).

Some Causes of Acute Fever

Upper or lower respiratory tract infection

Urinary tract infection (particularly in patients with an indwelling catheter)

Surgical site infection (postoperatively)

Rickettsial infections (eg, African tick typhus, Mediterranean spotted fever)

Multidrug resistant bacteria

Mosquitoes: Arboviral encephalitis

Bacteria: Infection due to encapsulated organisms (eg, pneumococci, meningococci), Staphylococcus aureus, gram-negative bacteria (eg, Pseudomonas aeruginosa), Nocardia species, or Mycobacteria species

Fungi: Infection due to Candida, Aspergillus, Histoplasma, or Coccidioides species microsporidia, Pneumocystis jirovecii or fungi that cause mucormycosis

Parasites: Infection due to Toxoplasma gondii, Strongyloides stercoralis, Cryptosporidium species, or Cystoisospora (previously Isospora belli)

Drugs that can increase heat production

Drugs that can trigger fever

The Science of Sweat

From nerves to exercise, this is sweat's cool factor.

Sweat gets a bad rap. We blame it for stink. We accuse it of staining clothes and ruining white tees, and we react in disgust when it appears. But this ill will toward perspiration is misguided. Truth is, we need sweat &mdash we just also need the right antiperspirant like Degree Men Black + White Deodorant to fight it and protect our clothing from white marks and yellow discolorations.

When your body starts to heat up, whether it's because of exercise, work, or outside temperature, your brain reacts by releasing sweat from the more than 2.5 million eccrine glands spread out across nearly all of your body, pouring liquid through pores to lower your body temperature. But when sweat simply drips off you and hits the floor, it can't lower your body temperature. To reap the cooling effect of sweat, though, that salty liquid must evaporate off the skin and turn into a gas, says William Byrnes, a sweat expert at the University of Colorado.

Cooling sweat isn't the only type of sweat. Humans also have apocrine glands, primarily in the armpit and groin. These glands also act as scent glands&mdashin animals, musky sweat can help attract both males and females, Byrnes says. The milky fluid from this sweat contains more nutrients, which makes it more attractive to the bacteria Staphylococcus hominis that largely resides in the armpit and groin. When these bacteria and sweat interact, we get body odor.

Like eccrine glands, these glands get activated during exercise, but apocrine glands also come alive when we get emotional, nervous, or excited. That means the most smell-inducing (and white tee-ruining) activities may not be running up a hill or playing basketball, but rather going in for a first kiss or giving an all-staff presentation.

Sweat isn't triggered by heart rate or movement, but by receptors in the hypothalamus area of the brain. Individuals living in hot and humid environments will adapt to the weather, just as people aerobically trained will sweat more and sooner. The body welcomes "adaptions that help us with heat regulation," Byrnes explains.

Because the stink of sweat doesn't come from the odorless, colorless liquid the body produces in an effort to cool the skin, but rather from contact with bacteria present on the body, we have to combat it from all angles. With this bacteria concentrated in the armpit, applying a powerful deodorant to that specific area, instead of rubbing it all over, proves the most effective way to combat B.O.

The most smell-inducing activities are less running up a hill and more going in for a first kiss or giving an all-staff presentation.

To take odor protection one step further, researchers the world over continue to study how to limit this bacteria, and companies work to mask the smell with pleasant fragrances. Degree, for example, uses tiny capsules of scent meant to break down throughout a day for freshness.

But smell definitely isn't the only problem with sweat. The yellowing of our shirts, another unpleasant side effect, is again caused not by sweat alone but by acidic chemicals in some antiperspirants as they react with sweat. Thankfully, deodorants like Degree Men Black + White Deodorant don't cause those reactions, preventing staining on your white shirts. And not only will Degree not cause those yellow stains, it'll also save your darker clothing from the unattractive white marks left by some other brands.

The next time you want to curse your perspiration, just remember: Sweat offers the body-altering benefit of keeping us from overheating, while all these ill effects like stink and pit stains come from bacteria. The moral? Leave sweat alone and instead blame bacteria&mdashand get yourself a great deodorant to fight it.