Thermoreceptors Flashcards
What does the hypothalamus control? x 7
- body temperature,
- thirst,
- hunger,
- water balance,
- sexual function.
- closely connected with emotional activity
- sleep
What is external work?
energy used to contract skeletal muscles eg to move an object, or move the body in relation to the environment.
What is internal work?
1) all other biological expenditure that does not accomplish work outside the body eg contractions associated with postural maintenance or shivering
2) All energy expending activities necessary to sustain life eg Breathing, pumping blood etc.
How to calculate metabolic rate?
energy expenditure / unit of time
Units = kcal/hour (kJ/hour)
What is the basic unit of heat energy?
Calorie
What is a calorie?
The amount of heat required to raise the temperature of 1g of water by 1oC.
What conditions is the basal metabolic rate measured under?
– Awake – Relaxed – Mental Rest (>30 min) – Lying – Warm (temp 20-25ºC) – Fasting (12 hr)
There is a direct relationship between the volume of O2 utilised and the …..
Amount of heat produced
How to calculate the metabolic rate?
x L/hr x 4.8
What is food intake controlled primarily by?
Hypothalamus
What is the arcuate nucleus responsible for control of?
- Long term control of energy balance and body weight
* Short term control of food intake on a meal to meal basis
How many subunits does the arcuate nucleus have?
2
What do the 2 subunits of the arcuate nucleus release?
One - neuropeptide Y
Other - melanocortins
What is neuropeptide Y?
An appetite stimulator
What is melanocortin?
An appetite supressor
What are the hormones secreted by adipocytes called?
Adipokines
Leptin, release, function
Released from stored fat, suppressed appetite, dominant long-term regulator of energy balance and body weight.
Which adipokines is suppressed in obesity?
Adiponectin
Function of adiponectin x 4
- Promotes fatty acid oxidation by muscle
- Increases sensitivity to insulin
- Decreases body weight by increasing energy expenditure
- Anti-inflammatory actions.
What adipokine is released primarily in obesity?
Resistin
What does resistin release lead to?
Insulin resistance
What is visfatin primarily released from?
From visceral fat
Function of visfatin
Stimulates glucose uptake; binds with insulin receptor at a site distinct from the insulin-binding site
Function of tumor necrosis factor alpha (TNF-alpha) and interleukin 6 (IL-6)
Promote low-level inflammation in fat and throughout the body.
Neuropeptide source, effect
Source - arcuate nucleus of hypothalamus
Increases appetite
Melanocortins source, effect
Arcuate nucleus of hypothalamus
Decreases appetite
Leptin source, effect
Adipose tissue
Decreases appetite
Insulin source, effect
Source - endocrine pancreas
Decreases appetite
Ghrelin source, effect
Stomach
Increases appetite
Child with mutation of leptin gene before lepsin
Obeses
Core body temperature (tc)
Temperature within the inner core which consists of the abdominal and thoracic organs, CNS, skeletal muscles.
Causes of variation in core body temperature
- Diurnal variation - 1C- lowest in early morning, highest in late afternoon
- Menstrual cycle - +.5C last half
- Exercise - increased
- Age - elderly lower
Outer shell temperature
Comprised of the skin and body fat, the temperature of the outer shell is generally cooler and can vary substantially.
What can the skin temperature fluctuate between without causing damage?
Between 20°C and 40°C
What does an increase in temp do to cellular reactions?
Speeds up cellular chemical reactions
Nerve impact of temp increase
Nerve malfunction, irreversible protein denaturation
41° prob
Convulsions
43°C for body
Upper limit compatible with life
Decrease in temp on cellular reactions
Slows down cellular reactions
What does pronounced, prolonged fall in body temperature do?
Slows metabolism to fatal level
Heat input and heat output
Heat input must balance heat output to maintain stable core temperature
Heat input
– Internal heat production (Metabolism, Muscle)
– Heat gain from external environment
Heat output
Heat loss from exposed body surfaces to the external environment
Temperature control
- If core temperature ↓ heat production is increased
* If core temperature ↑ heat loss is increased, and heat production is reduced
What are the 4 mechanisms of heat exchange?
- Radiation (Heat waves)
- Conduction (Contact)
- Convection (Air)
- Evaporation (Sweat)
Radiation heat gain/loss?
Heat gain and loss
Conduction heat gain/loss?
Heat gain and loss
Convection heat gain/loss?
Heat loss
Evaporation heat gain/loss?
Heat loss
What is considered the bodies thermostat?
Hypothalamus
What detects a change in skin temperature?
Peripheral thermoreceptors in skin?
What detects a change in core temperature?
Central thermoreceptors in hypothalamus, abdominal organs etc.
NB!! What is the response to cold exposure coordinated by?
Posterior hypothalamus
To increase heat production?
- Increased muscle tone
- Shivering
- Increased voluntary exercise(behavioural)
- Nonshivering thermogenesis
To decrease heat loss
- Skin vasoconstriction
- Postural changes to reduce exposed surface area (behavioural)
- Warm clothing (behavioural)
Shivering
- Un co-ordinated contraction of skeletal muscle (10-20/sec)
- Fast response
- Little work
- Much heat (2-5 fold increase in internal heat production in minutes)
Shivering pathway
Inputs • Central thermoreceptors (tc) – hypothalamus, CNS, and abdominal organs • Peripheral thermoreceptors – Skin (tskin) Integration • Hypothalamus – Thermoregulatory Integrating Centre (TRIC) Effectors • Motor neurons • Skeletal muscle
Where does non-shivering thermogenesis occur?
Brown adipose tissue (BAT)
BAT non-shivering thermogenesis involves…
‘Uncoupling’ of mitochondrial oxidative phosphorylation to generate heat
What is non-shivering thermogenesis mediated by?
– Thyroid hormones
– Sympathetic nervous system
• Adrenaline
(Both of which increase fat metabolism)
Important of non-shivering thermogenesis in who?
-Newborns
and hibernation?
What is the response to heat exposure coordinated by?
Anterior hypothalamus
To decrease heat production
- Decreased muscle tone
- Decreased voluntary exercise (behavioural)
To increase heat loss
- Skin vasodilation
- Sweating
- Cool clothing (behavioural)
Hyperthermia in sustained exercise
- At the onset of exercise, the rate of heat production initially exceeds the rate of heat loss so the core temperature rises.
- When heat loss mechanisms are reflexly increased sufficiently to equalise the elevated heat production the core temperature stabilised slightly above the resting point for the duration of the exercise.
Vasodilation x3
- Skin is an insulator
- Skin blood flow plays a key role in temperature regulation
- Capillary loops close to surface of skin
Skin blood flow plays a key role in
temperature regulation
- 20-30 times more blood flows through the skin than is needed for skin nutrition
- In the process of thermoregulation skin blood flow can increase 6-fold
- The more blood that reaches the skin from the warm
core, the closer the skin temperature is to Ct. - Blood vessels diminish the skin’s effectiveness as an insulator by carrying heat to the surface where it can be lost.
Capillary loops close to surface of skin
- Skin arteriolar vasodilatation
- Skin arteriolar vasoconstriction
Skin arteriolar vasodilatation
Increases flow of heated blood through the skin and promotes heat loss
Skin arteriolar vasoconstriction
Reduces blood flow through the skin and decreases heat loss by keeping warm blood in the central core - Vasoconstriction caused by sympathetic stimulation which is under hypothalamic control…
What is sweating stimulated by?
Sympathetic nervous system
– ‘stress’
What controls sweating
Hypothalamus
What is involves in the thermoneutral zone?
- Ambient temperature (ta) 20-30ºC
– Skin vasomotor effects sufficient to control heat loss
Ambient temperature <20ºC
– Metabolism
– Shivering
Ambient temperature >30ºC
Sweating is dominant factor
Heat loss in sweating
By evaporation
Max sweating rate
<2 litres/hour
ta (v small a) =
Ambient temperature
Hyperthermia defn
Temperature above normal range
Causes of hyperthermia
- Exercise-induced
* Pathological
Fever
Elevated body temperature – infection or inflammation
What happens to hypothalamic thermostat at higher temperature? and how?
‘reset’ – by pyrogens released from leucocytes.
Heat exhaustion symtoms
- Excessive sweating
- Reduced blood volume
- Impaired blood flow
- BP falls - Weak/dizzy, Collapse
Treatment for heat exhaustion
• Remove from hot environment • Replace fluid (cool) – Infuse? • Rest
Heat stroke temp
41ºC or higher
Heat stroke description
- Core temp (tc) rises
* CNS malfunction
CNS malfunction in heat stroke
- Delirium
- Unconsciousness
- Thermoregulatory centre fails
• Sweating stops…
• Metabolic rate up
Treatment for heat stroke
Reduce tc - Remove from heat - Cool the person • Lavage with tepid water • Ice pack • Fan to promote sweating and evaporation
Other name for fever
Pyrexia
NB!!Mechanism of pyrexia
• Innate immune cells such as macrophages encounter bacteria or other pathogens
• Macrophages release endogenous pyrogens (EP)
– e.g. Interleukin-1b
• EP cause prostaglandin (PG) synthesis
• Act on hypothalamic thermosensitive cells
• Set-point ‘re-set’ to 38.9ºC
• Hence 37.8ºC is ‘too cold’
• Activates thermogenesis
– Shivering
• Reduces heat loss
– Vasoconstriction
What does aspirin and paracetamol block?
Prostaglandin synthesis
Hyperpyrexia defn
Fever with extreme elevation of body temperature (41.5oC or above)
Malignant hyperthermia
Drastic and uncontrolled increase in skeletal muscle oxidative metabolism, which overwhelms the body’s capacity to regulate body temperature.
Physical occurrence in malignant hyperthermia
– Muscle contraction (rigor)
– Acidosis
– Can lead to organ failure and death
Cause of malignant hyperthermia
- Adverse reaction to halothane and other gaseous anaesthetics
- Failure of muscle SR Ca2+ reuptake
Treatment for malignant hyperthermia
- Dantrolene is a muscle relaxant that works to reinstate Ca2+ uptake to SR
- Cooling
- Treat acidosis
Mild hypothermia degrees and symptoms
32ºC - Shivering, conscious
Moderate hypothermia degrees and symptoms
25ºC - impaired motor skills
Severe hypothermia degrees and symptoms
<25ºC - No shivering, impaired
consciousness/motor skills
