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Thermoregulation and sweating

Dr. Reem Abraham

Learning Objectives

1. Apply the functional roles of the hypothalamus, sympathetic nervous system central and peripheral thermoreceptors to the maintaining thermoregulatory homeostasis
2. Apply the functional roles of the various neurotransmitters and receptors to maintaining shell and core temperature homeostasis
3. Predict the physiological changes associated with the negative feedback in response to hypothermia, hyperthermia including exercise and exertion, and fever. At the end of this lecture, students should be able to:

The spinal cord, abdominal organs, great veins in the upper abdomen and thorax - have both heat and cold receptors which monitor core temperature Core temperature

-^ monitoring Body temperature is controlled by balancing heat production and heat loss  (^) Anterior hypothalamic- preoptic area: temperature regulating center

**Information from cutaneous and core thermoreceptors is integrated in the *anterior hypothalamic-preoptic area (AHPA) The AHPA send signals to other brain areas to mediate the appropriate homeostatic changes. Temperature regulation by the anterior hypothalamic-preoptic area

• (^) When the anterior hypothalamic preoptic area is stimulated -  sweating and vasodilation of skin blood vessels to lose heat
• (^) Anterior hypothalamic-preoptic area functions as a thermostatic body temperature control center.

Heat loss mechanisms

• (^) Radiation (60% of heat exchange) : Main heat loss mechanism; is the electromagnetic radiation (heat) transferred to bodies not in contact , including the ultraviolet light radiation from the sun, which penetrates through to the surface of the earth, and the infrared radiation from the body.
• (^) Conduction is the movement of heat to/from the body directly to objects in contact with the body. is minimal exchange
• (^) Convection is the transfer of heat to a moving gas or liquid.

• (^) Insensible water loss : A certain amount of water is vaporized at all times, from skin and lungs, even when a person is not sweating (approx. 50 mL/h in humans)
• (^) Evaporation of sweat: Heat loss by evaporation of sweat is controlled by regulating the rate of sweating
• (^) Sweating: Sweat glands are innervated by SYMPATHETIC CHOLINERGIC FIBRES that secrete Acetylcholine (via M receptors); These fibers run along with adrenergic fibers
• (^) STIMULATION OF SYMPATHETIC CHOLINERGIC FIBERS  INCREASED SWEATING
• (^) Sweat glands do not have adrenergic innervation Heat loss mechanisms

Brown fat and chemical/nonshivering thermogenesis

• (^) Sympathetic stimulation, catecholamines  rate of cellular metabolism: chemical/nonshivering thermogenesis
• (^) The degree of chemical thermogenesis is directly proportional to the amount of brown fat
• (^) Brown fat:
  • (^) more abundant in infants than adults
  • (^) extensive sympathetic innervation (norepinephrine)
  • (^) contains many mitochondria ATP production
• (^) Norepinephrine stimulates tissue expression of mitochondrial uncoupling protein (also called thermogenin) and increases thermogenesis.

• (^) Set-Point : 37.1°C (98.8°F)
• (^) At temperatures above set-point  heat loss > heat gain  decrease in body temperature to the set-point. - (^) If core temperature is above the set-point temperature, then HEAT LOSS mechanisms are activated
• (^) At temperatures below set-point  heat gain > heat loss  increase in body temperature to the set-point. Hypothalamic set-point for core body temperature control
• (^) If core temperature is below the set-point temperature, then HEAT GAIN mechanisms are activated

(Heat gain mechanisms (Heat loss mechanisms through posterior hypothalamus) through anterior hypothalamus) The reflex responses activated by cold (HEAT GAIN) are controlled from the posterior hypothalamus. The reflex responses activated by warmth(HEAT LOSS) are controlled primarily from the anterior hypothalamus Piloerection/ Horripilation: Sympathetic stimulation causes the arrector pili muscles attached to the hair follicles to contract, which brings the hairs to an upright stance (not important in humans) (Goosebumps in humans)

α and γ lower motor neurons are located in the ventral horn of the spinal cord. Hypothalamic Stimulation of Shivering Hypothalamic Stimulation of Shivering: Primary motor center in posterior hypothalamus:

• (^) Inhibited by signals from heat center in anterior hypothalamic-preoptic area.
• (^) Excited by cold signals from skin and spinal cord. Activates α and γ motor neurons innervating skeletal muscle. The skeletal muscle contracts rhythmically generating heat and body temperature increases Can raise body heat production by 4–5 fold Shivering , which involves (involuntary) rhythmic contraction of skeletal muscle , is the most potent mechanism for increasing heat production in the body. α motor neurons control extrafusal muscle fib ers γ motor neurons control intrafusal muscle

16 FEVER

• (^) Elevation of body temperature that exceeds the normal daily variation due to changes in the hypothalamic set point
• (^) Pyrogen – any substance that causes fever
• (^) Pyrogens produce fever by increasing the hypothalamic set-point temperature
• (^) Hyperpyrexia - fever of

  41.5°C (>106.7°F)

• (^) Antipyretics - drugs that Effects of changing the set-point of the hypothalamic temperature controller

Pathogenesis of fever Aspirin inhibits the cyclooxygenase enzyme, necessary for the synthesis of prostaglandins.

• (^) Pathogens release interleukin- (IL-1) from phagocytic cells. IL- then acts to increase the production of prostaglandins, ultimately raising the temperature set point in the anterior hypothalamus. Prostaglandins increase the set- point temperature (IL- 1)
• (^) The hypothalamus now “thinks” that the body temperature is too low (because the core temperature is lower than the new set-point temperature) and initiates mechanisms for generating heat—shivering, vasoconstriction, and shunting of blood

Heat exhaustion:

• (^) Excessive body responses to elevated environmental temperature.
• (^) Symptoms  headache, nausea, vomiting, heat cramps, and profuse sweating, with generally intact mental function
• (^) Normally resolves quickly when the individual is removed from the heat and rehydrated
• (^) Can progress to heatstroke if untreated.
• (^) Increased environmental temperature increased sweating  decreased ECF volume, decreased blood volume, decreased arterial pressure, and fainting. Disturbances of Temperature Regulation

Disturbances of Temperature Regulation