Thermoregulation

Question 1

What is normal body temperature and the acceptable range?

Answer 1

• normal: 36.7 degrees C (98.06 degrees F)
• range: 36.3-37.1 degrees C

Question 2

What is the general control system of body heat?

Answer 2

controlled variable (e.g. core temp) is measured by

>

sensor (thermometer)

>

controller (establishes set point)

>

effector (e.g. furnace, AC) which changes

>

controlled variable

Question 3

How does temperature vary based upon body part?

Answer 3

• core is the controlled variable (maintained across range of environmental temps)
• oral temp is closest to core temp
• hands and feet vary depending on environmental temp

Question 4

How does body temp vary over a 24 hour period?

Answer 4

• varies ~1-2 degrees over 24 hr period
• temp is lowest in the mornings
• highest in the evening
• circadian rhythm infuences body temp

Question 5

What is the purpose of maintaining a certain body temp?

Answer 5

• temp affects enzyme activity (w/ every 10 degrees of temp change), will change cellular function for better or worse (usually worse)
• environmental temp is a threat to temp since humans are always generating heat, heat loss is an issue

Question 6

Where are thermoreceptors located and what is their specific function in each of these locations?

Answer 6

(thermoreceptors are the “sensors” in the thermoregulation pathway)

• brain (pre-optic and superoptic region of hypothalamus, core): neuron cell bodies sensitive to changes in temp, 3x as many warm sensitive, relay info to other areas of hypothalamus
• viscera (gut, core): senses core temp and threats to maintenance (food ingested that may change body temp, relays info to hypothalamus)
• skin (cutaneous thermoreceptors): often bimodal (e.g. temp and touch), may be warm or cold sensitive (10x cold sensitive), tell us about environmental conditions

(all of the above relay info to hypothalamus)

Question 7

In terms of the thermoregulation, what structure is the controller (establishes set point)?

Answer 7

• hypothalamus is the “controller” in the thermoreg pathway
• has connections to control hormonal, autonomic, and behavioral changes that are part of thermoreg

Question 8

How does hypothalamus control body temp?

Answer 8

1) determines set point
2) receives info about current temp
3) decides what to do and generates response that it relays to parts of brain that execute action
- anterior hypothalamus: responds to heat and controls heat loss behaviors
- posterior hypothalamus: reponds to cooling and controls heat production behaviors
- with sleep, temp decreases in set point
- with exercise, temp increase in set point (as high as 40 degrees C)

Question 9

What is the effector (variable that causes changes to core temp) within the thermoreg system?

Answer 9

heat loss/heat production

Question 10

What are the effectors for heat production and how is this process modulated?

Answer 10

• effectors: ANS (sympathetic), endocrine (thyroxin, epi), muscular activity (voluntary and involuntary), non-shivering thermogenesis (increase in metabolism not due to muscles, aka brown fat)
• modulated by control from the posterior hypothalamus when the body is abnormally cold
• muscular activity: shivering (dorsomedial posterior hypothalamus, increases motorneuron excitation) and increase in voluntary activity (via cortex, e.g. running/jumping)
• non-shivering thermogenesis: strong hormonal influence (sympathetic innervation of adipose tissue, thyroxin increases metabolic rate which cold is stim for TRH release; and epinephrine), increase in food intake (leads to increase in metabolism), and brown adipose tissue (adrenergic innervation for initiation)

Question 11

What is the importance of brown adipose tissue in humans?

Answer 11

• low efficiency hydrolysis of ATP via uncoupling proteins leads to more heat production than otherwise
• brown fat is innervated by sympathetic fibers, also activated by circulating epinephrine
• importance: critical in infants due to their heat loss from high surface area; was originally thought to be non-functioning in adults, but recent info suggests white fat expresses right uncoupling proteins and can be activated by sympathetic innervation

Question 12

How is heat loss modulated?

Answer 12

• modulated by the anterior hypothalamus when the body is abnormally hot
• evaporative heat loss: energy lost as water that evaporates, two types are insensible (respiratory) and sweating (controlled)
• convection: movement of molecules away from contact (air heating and rising)
• conduction: transfer of heat between objects in physical contact w/ another
• radiation: infared radiation transferring heat between 2 objects not in physical contact (example of babies in incubator that are double lined)

(convection, conduction, and radiation controlled through wearing clothes, feed forward behavior that prevents change in temp)

• blood to skin: determines how much heat moves from blood to external environment (blood moves away from skin (pale) when cold and blood moves toward skin (flushing) when hot)

Question 13

What is the general structure of sweat glands and how are they innervated?

Answer 13

• sweat glands have lots of nearby blood vessels, a coiled region that starts sweat production, and a duct leading to skin
• sweat glands are innervated by sympathetic cholinergic neurons (Ach is NT) and binds to muscarinic receptor
• sweating is a form of evaporative heat loss that is not dependent on air temp

Question 14

How is sweat produced?

Answer 14

• initially serum is filtrated from blood of plasma, ions (Na, Cl) and proteins
• serum moves to the coiled part of sweat gland
• as fluid travels up to the skin surface, water and Na+ are reabsorbed (back to the blood) in the duct of the sweat gland
• low flow rate (don’t need a lot of sweat): sweat is concentrated (little water due to reabsorption) and high Na+ (couldn’t follow water)
• high flow rate (need a lot of sweat): sweat is dilute due to lots of water (no time to reabsorb) and w/ acclimation low Na+ due to aldosterone action (increases temp = increases Na+ reabsorbed)

Question 15

What is a fever defined as and what are its implications?

Answer 15

• fever: “controlled” increase in body temp
• implications: set point increases as body temp is doing what hypothalamus directs it to do

Question 16

How is a fever generated and what physiological processes result in the “stages” of a fever?

Answer 16

• fever production: pathogen secretes endotoxins and immune cells activated by pathogen release cytokines, both of which activate > prostaglandin E2 > (via EP3 receptor?) > increases hypothalamic set point for temp (T body < T set point)
• shivers/chills: body subjectively reporting that is cold b/c T body is decreased below T set point, thus body is instructed to increase heat production (shivering/chills, gives individual feeling of being cold)
• fever breaks: pathogen is gone and no longer producing endotoxins, thus the immune system is not activated > hypothalamic set point returns to normal (T body > T set point)
• sweating/lethargy: body subjectively reporting that it is hot b/c T body is elevated above T set point, thus body is instructed to partake in heat loss behaviors (apathy, lethargy, sweating, anorexia)

Question 17

Compare fever and hyperthermia:

Answer 17

• core temp is increased in both
• set point is increased in fever, but normal in hyperthermia
• active thermoregulatory response is heat production in fever and heat loss in hyperthermia
• signs present: fever (pallor, shivering, behavioral changes a/w being cold), hyperthermia (flushed skin, sweating, behavioral changes a/w being hot)
• subjective perception: fever (paitent will describe being cold), hyperthermia (patient describes being hot)
• cause: fever (infectious agent producing inflammatory mediators acting in brain causing increased set point), hyperthermia (increased heat production/decreased ability to eliminate heat, aka dehydration)

Question 18

How is autonomic control of body temp regulated?

Answer 18

• major hypothalamic nucleus: paraventricular nuclei
• outflow occurs via dorsal longitudinal fasciculus (DLF)
• outputs: PAG, parabrachial nucleus, mesencephalic raphe nucleus, locus ceruleus, dorsal motor nucleus of vagus, nucleus ambiguous, parasympathetic/sympathetic neurons in SC (bolded structures are how hypothalamus regulates autonomic response)
• DLF also provides inputs from locus ceruleus, parabrachial nuclei, and PAG to the paraventricular nucleus
• medial forebrain bundle: provides substantial inputs to hypothalamus, esp from nucleus tractus solitaries (visceral sensory), out flow is to same nuclei as DLF
• mammillotegmental tract: smaller than DLF or MFB, mammillary bodies of hypothalamus to autonomic nuclei