Body energy homeostasis

Question 1

What is the regulation limits for core vs peripheral temperature?

Answer 1

Core (major thoracic/abdo organs +brain) - tightly regulated so brain, heart etc not affected - between 36.6-37.4 C - two thirds of body heat content
Peripheral (skin, limbs) - vary widely - between 30-32 C usually - one third of body heat content

Question 2

What can increase body heat?

Answer 2

Basal metabolism
Skeletal muscle activity
Metabolic factors
Thyroid disease
Medications
Convulsions
Fever
Environmental heat

Question 3

What are the methods of heat transfer with the environment?

Answer 3

Conduction - via direct contact, significant in water
Convection - via air circulation, varies with wind velocity
Radiation - via electro-magnetic waves, bidirectional
Evaporation - via conversion of liquid to gas, dominant mechanism of heat loss in hot environments

Question 4

What is the central thermoregulatory network made up of?

Answer 4

Afferent neurons - peripheral and central thermoreceptors
Thermal afferent pathways within CNS
Thermoregulatory integration centre in preoptic anterior hypothalamus
Efferent pathways providing autonomic and somatomotor inputs to effectors
Thermal effectors that control heat transfer between body and environment - control heat production

Question 5

What does the central thermoregulatory network regulate?

Answer 5

Passive poikilothermic system - circulatory system and physic laws determine heat transfer with environment

Question 6

Acclimatisation

Answer 6

Occurs with repeated exposure
E.g. in heat:
Increased sweat volume and decreased Na conc
Increased plasma volume and cardiac stroke volume

Question 7

TRP channel roles (4)

Answer 7

Thermosensitive receptors - allow transduction of changes in body temp
TRPM8 on cutaneous afferent neurons = skin cold receptors in dorsal root and trigeminal ganglia
TRPV3 and TRPV4 in keratinocytes release paracrine agents responsible for skin warmth sensation
TRPM2 expressing neurons in pre optic hypothalamus act as central warm sensitive neurons

Question 8

What are the ascending neural pathways of temperature signals from the periphery?

Answer 8

Primary input: sensory neurons with cell bodies in peripheral (trigeminal and dorsal root) ganglia
Axons split - extend to skin/viscera and dorsal horn of spinal cord or spinal trigeminal nucleus
Two classes: activated by innocuous warmth and innocuous cold
Integrates into preoptic area of hypothalamus (POA)

Question 9

What are the afferent pathways of central temperature sensation?

Answer 9

Separate sensory neurons in brain itself - measure temp of hypothalamus
Integrates into the preoptic area of hypothalamus (POA)

Question 10

What are the afferent inputs into the POA?

Answer 10

Skin temperature
Pyrogenic mediator
Brain temperature
Spinal cord temperature
Visceral temperature

Question 11

What are the sympathetic and somatic outputs from the POA?

Answer 11

BAT thermogenesis
Cutaneous vasomotion
Tachycardia
Shivering thermogenesis

Question 12

What is the response to cold which reduces convection?

Answer 12

Piloerection
Sympathetic stimulation of the a1-adrenoreceptors > triggers contraction of arrector pili smooth muscles > increases angle hair shaft sits at
Traps air to skin
Elevation of skin aswell

Question 13

What response to the cold reduces radiation (evaporation/convection)?

Answer 13

Reducing blood flow to the skin
Reduces radiation from the blood and cools the skin, so reduces evaporation and cooling

Question 14

What are the different input routes that leads to reduced blood flow to the skin

Answer 14

• Extracellular environment > skin thermoreceptors > anterior hypothalamus > (sympathetic outflow) vasoconstriction of cutaneous arterioles
• Cold core body temperature > anterior hypothalamus > vasoconstriction of cutaneous arterioles
• Cold extracellular environment > (increased affinity of adrenoreceptors) vasoconstriction of cutaneous arterioles

Question 15

What is the difference between apical and non-apical skin?

Answer 15

Apical skin: more arteriovenous anastomosis - thick smooth muscle layer, connects arterioles to veins, create low resistance for blood flow to skin e.g. palmar surfaces of hands and feet, and face
Non-apical skin: few AV shunt vessels, blood flow is primarily nutritive in function - over most of the body surface

Question 16

How does AV anastomoses in apical skin respond to hot vs cold external temperatures?

Answer 16

Cool external temp:
High basal sympathetic tone to the AVA keeps them constricted, limiting blood flow to skin
Higher external temp:
sympathetic tone decreases, increases blood flow to the skin > faster heat loss

Question 17

How is the blood flow to the skin controlled?

Answer 17

Release of norepinephrine from sympathetic fibres innervating vascular smooth muscle in skin
RPA nucleus and RVLM in the medulla contain sympathetic premotor neurons
These have projections which activate preganglionic neurons in the IML nucleus in spinal cord which regulate sympathetic outflow > vasoconstriction
Vasomotor responses mediated by: direct POA/RPA projections or indirect VTA and PAG

Question 18

What is the hunting reaction?

Answer 18

In cold external environments, the predominant cutaneous blood flow alteration oscillates between cold-induced vasoconstriction and vasodilation
Vasodilation is in response to ischaemia = reactive hyperaemia
- Anaerobic metabolites trigger upstream arteriolar vasodilation

Question 19

What is Raynaud’s syndrome/phenomenon?

Answer 19

Exaggerated vascular response to cold temperature or emotional stress
Basal NOx production by the endothelium keeps vascular smooth muscle cells relaxed
Reduced NO production by endothelium would cause a tendency for exaggerated vasoconstrictive responses
= Sudden onset of digits, sharply demarcated color changes

Question 20

What relevance does BMR have in thermoregulation?

Answer 20

In a thermoneutral environment, heat loss to the environment is matched by the heat production created by basal metabolic rate (BMR).
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