Body Energy Homeostasis Flashcards
What is conduction, convection, radiation and evaporation?
What is the mechanism of piloerection?
- reduces convection
• Sympathetic stimulation of the α1-adrenoreceptors triggers contraction of the arrector pili smooth muscle cells. This causes the hair shaft to change from a shallow angle, to something closer to 90°.
• This traps air close to the skin (although not very effective in humans)
• This contraction can also result in slight elevations of the skin (goose bumps)
What is the mechanism of reducing blood flow?
- reduces radiation
- hot blood is kept further away from the body surface
Apical vs non-apical skin
Apical
- Skin possesses arteriovenous anastomoses which control heat
delivery to exposed areas of skin. Skin in exposed areas of the extremities such as the toes, fingers, palms, lips, nose and ears have arteriovenous anastomoses (AVA) – these are vessels that have a thick smooth muscle cell layer which connects arterioles directly to the veins – creating a low resistance pathway for blood flow to the skin - In cool or temperate environments, a high basal sympathetic tone to the AVA keeps them constricted, limiting blood flow to the skin
- However, at higher external temperatures the sympathetic tone
decreases, leading to increased blood flow to the skin – facilitating faster heat loss
Non-apical
- controlled primarily by the release of norepinephrine from sympathetic fibres innervating vascular smooth muscle in the skin
- rostral raphae pallidus nucleus (RPA) and adjacent rostral ventrolateral medulla (RVLM) contain the sympathetic premotor neurons that are critical for the regulation of cutaneous vasomotor responses
- RPA projections activate preganglionic neurons in the
intermediolateral (IML) nucleus of the spinal cord that in turn regulate sympathetic outflow - vasomotor responses likely mediated by both direct POA / RPA projections as well as indirect pathways – possibly ventral tegmental area (VTA) and rostro- ventrolateral periaqueductal gray (PAG)
What is the hunting reaction?
Hunting reaction- In the cold, the predominant cutaneous blood flow alteration will oscillate between cold- induced vasoconstriction and vasodilatation
What is the mechanism of shivering thermogenesis?
- Rapid, repeated skeletal muscle contraction at a frequency of 8-10Hz
- Increase skeletal muscle tone.
- Shaking possibly from increased activation of γ-motor neurons. This triggers unstable activation of the muscle spindle reflex, leading to rapid activation/deactivation cycles to keep muscle length constant.
What is the mechanism of BAT?
- is found in regions surrounding the main arteries supplying the brain, heart and other major organs
- Sympathetic stimulation of BAT leads to lipolysis
• Release of fatty acids triggers uncoupling of the electron transport chain, by allowing H+ to leak through the UCP1 channel.
• The short circuiting of the electron transport chain, leads to the dissipation of the energy stored in the H+ gradient as heat. - found most prominently in the interscapular region, where it is
highly innervated by sympathetic nerves - release of norepinephrine from this sympathetic innervation induces mitochondrial leak in BAT that produces heat
- rostral raphe pallidus (RPA) is the primary site where descending signals driving BAT thermogenesis exit the brain
- exposure to cold or pyrogens activates premotor neurons in the
RPA that project to the spinal cord, many of which express either the glutamate transporter (Vglut3) or serotonin - RPA projections activate preganglionic neurons in the IML nucleus that in turn regulate sympathetic outflow and, thereby, BAT activity
What are other forms of non-shivering thermogenesis?
- Skeletal muscle can also contribute to non-shivering thermogenesis through the actions of sarcolipin (SLN).
- Sarcolipin uncouples Ca2+ transport by the SERCA pump from ATP hydrolysis – such that ATP hydrolysis occurs without Ca2+ transport.
- The free energy released from this Ca2+-independent ATP hydrolysis is dissipated as heat
What are the 2 types of sweat gland?
Apocrine sweat glands
• Few in number and large in diameter
• Ducts empty into hair follicles
• No role in temperature regulation in humans - may act as a source of
pheromones
Eccrine sweat glands
• Numerous and small in diameter
• Essential role in temperature regulation
• Acetylcholine released from sympathetic neurons stimulates increased sweating through a Ca2+-dependent stimulation of Cl-secretion from coil cells
What is the mechanism of sweating?
• controlled by the release of acetylcholine from sympathetic
innervation of peripheral sweat glands
• these sympathetic ganglia, in turn, are controlled by innervation from
preganglionic neurons located in the IML cell column of the spinal cord.
• within the brain, the pre-motor neurons for sweating that project to
the IML appear to be located in the rostral ventromedial medulla (RVMM)
• activation/stimulation of the RVMM induces sweating
• correlates with POA activation
• specific circuitry that connects the POA and RVMM is unknown
What is the mechanism of the fever host defence response?
• The first step is the immune response to foreign substances
• The second is the acute-phase response, a diffuse collection of host
reactions, including fever production, lethargy, and hyperalgesia
• Cytokines such as interleukin-1β (IL-1β) act as endogenous pyrogens
in a signaling cascade that induces peripheral (e.g., in liver) and CNS
production of prostaglandin E2 (PGE2)
• PGE2 inhibits warmth-sensitive neurons in POA (akin to the action of
stimulated skin cold thermoreceptors) and activates the thermal
effectors or heat retention (cutaneous vasoconstriction) and heat
production (brown adipose tissue thermogenesis, and shivering or
“chills”), which results in an increase in core temperature
What is the mechanism of hot flushes in menopause?
What is the neural pathways of BAT, control of blood flow to the skin, sweating and shivering thermogenesis?
What are the ascending neural pathways that transmit warm and cool signals from the periphery?
- The primary input into the 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.
Peripheral temperature sensing is mediated primarily by two classes of sensory neurons that are activated by:
• innocuous warmth (~34–42 °C)
• innocuous cold (~14–30 °C) - These neurons have cell bodies located in:
• trigeminal ganglion (for innervation of the head and face) dorsal root ganglia (for innervation of the rest of the body) - They are pseudounipolar: axons are split into two branches
• one of which innervates the skin or viscera
• one projects to the dorsal horn of the spinal cord or to the spinal trigeminal nucleus in the brainstem
What are the neural pathways in response to cooling?
What are the neural pathways in response to heating?
What is the process of thyroid hormones synthesis?
What is the response of the thyroid to traduced levels of free T3 and 4?
How do thyroid hormones increase basal metabolic rate?
How do thyroid hormones stimulate futile cycles of catabolism/anabolism?
