Interactions Between Thermoregulation And Other Physiologic Systems
The core temperature defended by the thermoregulatory system is not a fixed value but fluctuates in response to internal and external factors. Many of these factors are unrelated to temperature per se and instead reflect interactions with other physiologic systems. One example is fever, which is the controlled increase of body temperature that occurs most commonly in response to an infection . Fever is triggered by bacterial lipids and other molecules that directly or indirectly induce the production of prostaglandin E2 by endothelial cells lining the POA . PGE2 is thought to inhibit the activity of POA neurons that function to reduce body temperature, thereby producing a regulated hyperthermia that increases the likelihood of surviving an infection.
The generation of fever
The presence of molecules associated with pathogens like bacteria and viruses is sensed by innate immune cells in the blood and lead to the production of pyrogenic intermediates like cytokines and prostaglandins that act on the preoptic area. In the preoptic area, COX2 expression in endothelial cells result in local PGE2 production, which is the dominant source of fever-inducing PGE2.
PGE2 acts through EP3 receptors expressed in the median preoptic to effect changes in body temperature. LPS â lipopolysaccharide COX2 â cyclooxygenase 2.
How The Body Regulates Heat
Understanding heatstroke, hot flashes and fever
A close look at the complex systems that keep us functioning can inspire awe. Such is the case with the body’s complicated temperature-regulating mechanism.
This intricate apparatus balances heat production with heat loss, keeping the body at a temperature just right for optimal function. This balancing act is directed automatically and seamlessly by the hypothalamus, a small portion of the brain that serves as the command center for numerous bodily functions, including the coordination of the autonomic nervous system.
Much like a thermostat regulates the temperature inside your home, the hypothalamus regulates your body temperature, responding to internal and external stimuli and making adjustments to keep the body within one or two degrees of 98.6 degrees.
Efferent Pathways From The Poa To Thermoregulatory Effectors
Thermal information received in the POA is communicated to downstream structures that control physiologic and behavioral effectors . Here we briefly outline what is known about the neural mechanisms and pathways that control each of these responses.
Descending circuits controlling thermoregulatory effectors
The CNS/PNS regions involved invarious thermoregulatory effector responses and the proposed descending pathway from the POA to motor output. Note that many of the connections in the brain that are drawn are postulated based on indirect evidence. Dashed arrows indicate that a functional connection exists, but that the anatomic pathway is unknown and may involve multiple synapses and additional brain regions. POA â preoptic area, DMH â dorsomedial hypothalamus, LH â lateral hypothalamus, PAG â periaqueductal gray, VTA â ventral tegmental area, RMR â raphe medullary region, RPA â raphe pallidus, RVLM â rostral ventrolateral medulla, RVMM â rostral ventromedial medulla, IML â interomediolateral column, SSN â superior salivary nucleus.
A. Control of physiologic responses
Skin blood flow
Evaporative heat loss
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Different Sensory Pathways Engaged In Feeling And Responding To External Temperature
Summary: Researchers shed light on thermoregulation by disabling parts of rat brains and observing their choice of a comfortable environmental temperature. The findings could provide new understanding of conditions such as heatstroke.
To maintain the body at an appropriate temperature despite changes in the environment, there are a number of physiological and behavioral responses that can be adopted, such as shivering or moving into or out of direct sunlight. Although these responses are well understood, there is still a lack of understanding of the nerve and brain pathways that control them.
Researchers at the Department of Integrative Physiology at Nagoya University Graduate School of Medicine have boosted our knowledge of sensing external temperature and responding to it to maintain body temperature, known as thermoregulation, by disabling parts of the brain in rats and then observing the animals choices of a comfortable environmental temperature. The new findings, recently published in Scientific Reports, could also help us understand conditions in which these regulatory systems go awry, such as heatstroke.
These findings show the different functions of these two sensory pathways in feeling external temperature changes and in actually responding to these changes behaviorally, Kazuhiro Nakamura says. We can now pursue a much better understanding of the circuits that control thermal comfort and how these help the temperature of the body to be maintained.
Your Brain & Nervous System
How do you remember the way to your friend’s house? Why do your eyes blink without you ever thinking about it? Where do dreams come from? Your brain is in charge of these things and a lot more.
In fact, your brain is the boss of your body. It runs the show and controls just about everything you do, even when you’re asleep. Not bad for something that looks like a big gray wrinkly sponge.
Your brain has many different parts that work together. We’re going to talk about these five parts, which are key players on the brain team:
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Pituitary Gland Controls Growth
The pituitary gland is very small only about the size of a pea! Its job is to produce and release hormones into your body. If your clothes from last year are too small, it’s because your pituitary gland released special hormones that made you grow. This gland is a big player in puberty too. This is the time when boys’ and girls’ bodies go through major changes as they slowly become men and women, all thanks to hormones released by the pituitary gland.
This little gland also plays a role with lots of other hormones, like ones that control the amount of sugars and water in your body.
Central Circuitries Mediating Thermoregulation
Downstream targets of thermosensitive POA neurons are primarily components of the sympathetic nervous system, of which the dorsomedial hypothalamus in the hypothalamus, sympathetic premotor neurons in the rostral ventromedial medulla that include the rostral raphe pallidus , and preganglionic autonomic neurons of the intermediolateral cell column in the spinal cord, are particularly important .514
Using a technique of molecular profiling of neurons in the POA activated by heat, a population of GABAergic neurons in the VMPO that coproduce PACAP and BDNF was identified, and when activated by optogenetics leads to a rapid decline in core body temperature that persists as long as the stimulation continues.511 Mechanisms for the heat loss included vasodilation and suppression of heat production from BAT, as well as behavioral responses because the animals move to colder temperatures. Mapping of these neurons not only demonstrated a direct projection to the DMH, but also the PVN, ventrolateral PAG, and structures associated with behavior , indicating that the VMPO mediates diverse autonomic and behavioral responses to heat.
K.C. Mendoza, J.D. Griffin, in, 2010
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Output And Input Pathways Of Poa Sleep Neurons
Preoptic area of the hypothalamus neurons project to multiple brain regions that are involved in various behaviors . In addition to the TMN discussed above, both anterograde and retrograde tracing have shown that POA neurons project to brainstem regions such as the laterodorsal tegmental nucleus , dorsal raphe and median raphe nucleus , LC, and ventrolateral periaqueductal gray . Retrograde tracing from these regions together with c-Fos immunohistochemistry after periods of dark exposure to enhance REM sleep showed retrogradely labeled c-Fos cells in the extended VLPO , suggesting that POA neurons projecting to LDT, DRN, and LC are involved in REM sleep regulation. Serotonergic DRN and noradrenergic LC cells are silent during REM sleep . The suppression of the DRN and LC during REM sleep is likely caused by GABA release, and one of the potential sources providing GABAergic inputs to these areas is the POA . Increasing the activity of POA neurons through local warming inhibited the DRN cells, providing further evidence for their inhibition by POA neurons . Thus, these studies suggest that increased activity of POA GABAergic neurons contributes to the suppression of DRN and LC neurons during REM sleep.
What Happens When It Gets Cold
- The sympathetic nervous system is activated which causes the peripheral blood vessels to constrict . This reduces the amount of heat lost to the surroundings.
- Blood flow is redirected to the core, in order to protect the vital organs.
- Sweat production is reduced/shut down.
- Shivering of the skeletal muscles is initiated, which generates large quantities of heat.
- Non-shivering thermogenesis is initiated in brown adipose tissue.
- Adrenaline is released, which increases heart rate , blood pressure and further constrics the peripheral blood vessels. Most importantly however, it acts on the liver to increase heat production As mentioned previously, the metabolic heat produced by the liver is crucial to the bodys ability to regulate its temperature.
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What Are The Neural Substrates Of Thermoregulatory Behavior
Thermoregulatory behavior remains the most enigmatic of the classic motivated behaviors that include eating and drinking. No forebrain region or cell type has yet been shown to be required for these responses. While the dogma has been that the POA is not involved, the fact that stimulation of specific POA cell types can drive robust heat-defensive behaviors reveals that these cells can serve as a genetic entry point into the underlying circuit . Moreover, the recent finding that lesions of the LPB, but not thalamus, block temperature selection behavior suggests that downstream targets of the LPB, such as the POA, are involved . It will be illuminating to identify these circuits and understand how they connect to the broader motivational system that drives other homeostatic behaviors.
Other Neurotransmitters Involved In Thermoregulation:
Neuropeptides can also play an important role as neurotransmitters in thermoregulation. In experimental animals, a number of neuropeptides have been shown to be involved in controlling body temperature: neurotensin produces hypothermia when injected into the brain
TRH is hypothermic in rabbits and rats, but the response varies if the injection is intraventricular naloxone does not appear to have a significant effect on body temperature somatostatin, which does not alter basal temperature, potentiates barbiturate-induced hypothermia, and inhibits the hypothermic effects of dopamine, apomorphine, and beta-endorphin.
All these peptides have shown effects on thermoregulation however, its role in maintaining body temperature and diurnal variations in fever is awaiting clarification1.
Higher, homeothermic animals tend to keep their body temperature constant, this constant is not an exact figure, there is a circadian rhythm with a temperature peak between 18 and 22 hours of the day, being minimum between 2 and 4 in the At dawn.
There are also differences between different points of the body and in some physiological states, it is known by all that Ogino studied the physiological changes due to hormonal alterations in women, relating them to temperature.
Homeothermic animals are capable of adapting to the different temperatures that exist throughout the year, and in the different areas of our planet, which they do through the acclimatization process:
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Epidemiology Including Risk Factors And Primary Prevention
The frequency of impaired thermoregulation is not known. This problem is associated with spinal cord injury above level T6 and with severe traumatic brain injury. In SCI, it occurs often during extremes of ambient temperature. In patients with TBI or brainstem stroke, it can arise in the presence of noxious stimuli, although it frequently occurs spontaneously in the absence of environmental triggers.
Sources Of Input Into The Thermoregulatory System
The primary input into thermoregulatory system comes from sensory neurons that measure the temperature of the body. Most of these sensory neurons have cell bodies located in peripheral ganglia and axons that extend out to measure the temperature of key thermoregulatory tissues . A separate set of sensory neurons are located within the brain itself and measure the temperature of the hypothalamus.
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The Biggest Part: The Cerebrum
The biggest part of the brain is the cerebrum. The cerebrum is the thinking part of the brain and it controls your voluntary muscles the ones that move when you want them to. So you need your cerebrum to dance or kick a soccer ball.
You need your cerebrum to solve math problems, figure out a video game, and draw a picture. Your memory lives in the cerebrum both short-term memory and long-term memory . The cerebrum also helps you reason, like when you figure out that you’d better do your homework now because your mom is taking you to a movie later.
The cerebrum has two halves, with one on either side of the head. Scientists think that the right half helps you think about abstract things like music, colors, and shapes. The left half is said to be more analytical, helping you with math, logic, and speech. Scientists do know for sure that the right half of the cerebrum controls the left side of your body, and the left half controls the right side.
Segregated Warm And Cold Relays In The Lateral Parabrachial Nucleus
Dorsal horn neurons send glutamatergic projections to the brain that collateralize to the thalamus and lateral parabrachial nucleus . Thermal information received in thalamus is relayed to somatosensory cortex, where it mediates the perception and discrimination of temperature . However thalamic lesions do not block behavioral or autonomic thermoregulatory responses , suggesting that the spinothalamocortical pathway is dispensable for body temperature regulation in some contexts. In contrast, lesioning or silencing of the LPB abolishes the autonomic responses to skin warming and cooling as well as temperature preference in a behavioral assay . Thus ascending input to the LPB, which in turn is relayed to the POA, is critical for the activation of thermoregulatory responses to environmental temperature.
Warm and cold-activated LPB neurons send dense glutamatergic projections to the midline POA and particularly the MnPO . This direct projection is likely to be an important pathway by which thermal information received in the LPB is transmitted to the POA, but the connectivity between specific LPB and POA cell types has not been established.
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Thermoregulatory Neurons In The Preoptic Hypothalamus
The POA is thought to be the key integratory site for thermoregulation in the brain. This is supported by many lines of evidence, including: POA lesioning or pharmacologic silencing results in animals that cannot defend their core body temperature in either a hot or cold environment , local warming of the POA causes hypothermia and heat-defensive responses that mirror the response to environmental heat , injection of the pyrogen PGE2 into the POA causes fever , the POA contains neurons that are selectively activated in vivo by warming of the skin, spinal cord, or brain , and the POA is densely connected to brain regions that receive thermal information from the periphery as well as structures that control thermoregulatory effectors.
Although the POA is critical for thermoregulation, it is also associated with many other functions, including the regulation of fluid balance, sleep, mating, and parental behaviors. These functions are likely mediated by distinct cell types, which raises the question of which cell types in the POA are specifically involved in regulation of body temperature. Recent work has begun to investigate this question by using genetic approaches for neural manipulation and recording.
What Is Thermoregulation
Thermoregulation refers to an organisms ability to maintain and regulate its internal temperature, irrespective of the surrounding thermal conditions. Organisms that have internal thermoregulatory mechanisms are called endotherms Humans, along with mammals and birds, as examples of endothermic creatures. On the other hand, creatures than rely on the external environment for heat are called ectotherms Amphibians and reptiles, like frogs, snakes and crocodiles are a few common examples of ectothermic organisms.
From an evolutionary point of view, so many organisms have such developed and advanced thermoregulatory mechanisms because it is physiologically desirable to maintain a high and stable internal temperature, in order to convert food to energy more efficiently. Nevertheless, there is a limit to this advantage, because if the body temperature gets too high, it leads to rapid depletion of energy reserves, because of the increased metabolic rate. it could even be fatal to the organism if it gets close to the upper lethal temperature.
Thermoregulation is an absolutely indispensable part of human life, without which we wouldnt be able to survive the perpetual changes in external temperature. In fact, thermoregulation is just one component homeostasis, which is a crucial aspect of any organisms survival homeostasis is the ability of an organism to maintain a state of internal equilibrium despite changes in the external environment.
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The Human Body Is An Awe
The biological systems of our planet, from single-celled creatures like bacteria all the way to complex mammals, are orders of magnitude more intricate and sophisticated, in their design and function, than the most advanced man-made technology. It is simply astounding that in a world where billions upon billions of variables are constantly in flux, life not only exists but thrives, in millions of complex forms. Each of these forms have numerous specialised adaptations that allow them to survive the harshness and unpredictability of the external environment and ensure their species continuity.
This ability of an organism to maintain its internal state relatively constant in the face of fluctuations in the external environment, is called homeostasis. Homeostasis is an umbrella term that pertains to several independent processes, aimed at maintaining different variables. Temperature is one such variable and a very important one at that.
How Is Heat Lost
The body loses heat to the surroundings through four distinct pathways – evaporation, radiation, conduction and convection.
Evaporation refers to the loss of heat through the conversion of water to vapour. Water has a very high specific heat capacity, which is to say that it takes a lot of heat energy to convert each gram of water to water vapour. When we sweat, a lot of heat energy is released in order to evaporate the sweat this huge loss of heat cools us down. Normally, about 20% of the bodys heat loss is through evaporation. During intense exercise, this number goes up to about 85%.
Radiation is the transfer of heat between objects as infrared waves. For example, we receive heat from the sun through radiation. 65% of the bodys heat loss occurs through radiation.
Conduction is the transfer of heat between two objects that are directly in contact with one another. For example, the cooling effect we experience when we jump into a swimming pool in hot weather is because of conduction. Usually, conduction accounts for not more than 2% of the bodys heat loss.
Convection is the loss of heat to the air surrounding the skin. This happens because hot air rises up and is replaced by cooler air, which them warms up and continues the cycle. Convection accounts for about 10-15% of the heat lost by the body.
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