Lecture 3- Skin physiology (hard) Flashcards
What are some touch receptors of the skin?
- Free nerve endings
- Tactile discs
- Tactile corpuscles
- Lamellar corpuscles
- Bulbous corpuscles
Is there a perfect receptor-function relationship?
No, there is no one receptor-one function relationship. Receptors can often respond to several different stimuli but will be most sensitive to a particular type.
What is the most common receptor in the skin?
free nerve endings
What is the structure of free nerve endings like?
- Mostly unmyelinated, small diameter fibers but also some myelinated small diameter fibers
- Usually have small swellings at distal ends (sensory terminals)
What do the sensory terminals of nerve endings do?
Have receptors that function as cation channels that result in depolarization and action potentials that travel up to the somatosensory cortex
What do nerve endings respond to?
- Detect lots of different things but mainly respond to:
- temperature (hot/cold)
- painful stimuli
- some movement and pressure
- some to itch (e.g. response to histamine)
- some wrap around hair follicles (peritrichial endings) acting as light touch receptors which detect bending hairs (e.g mosquito lands on skin)
What are tactile (Merkel) dics/where are they found?
- Free nerve endings located in the deepest layer of epidermis (stratum basale)
- Associated with large disc shaped epidermal (merkel) cells
What is the communication that occurs for tactile epithelial (Merkel) cells?
Between the cells and nerve endings possibly via serotonin (5HT)
Where are tactile (merkel) discs abundant, what do they allow?
In fingertips, have small receptive feels so good for 2 point discrimination.
What are tactile (merkel) discs sensitive to?
- an objects physical features (texture, shape and edges)
- fine touch and light pressure
Where are tactile (Messiner) corpuscles located?
-In papillary layer of dermis especially in hairless skin e.g fingerpads , lips, eyelids, external genitalia, soles of feet, nipples
What is the structure of tactile (Messiner) corpuscles receptors?
spiraling/branching unmyelinated sensory terminals, surrounded by modified Schwann cells (for support) and then by a thin oval fibrous connective tissue capsule (encapsulated)
For tactile (Messiner) corpuscles what happens when the capsule deforms?
Triggers entry of sodium ions into nerve terminal which depolarizes it causing an action potential
What do tactile (Messiner) corpuscles sense?
- delicate ‘fine’ or discriminative touch: sensitive to shape and textural changes in exploratory touch like reading braille and movements of objects over the surface of the skin
- light pressure
- low frequency vibration (2 to 80 hertz)
Where are the Lamellar (Pacinian) corpuscles located and what does this mean in terms of there function?
- scattered deep within dermis and hypodermis
- this means they are more responsive to deep pressure and vibration
What is the structure of Lamellar (Pacinian) corpuscle receptors?
- single dendrite lying within concentric layers of collagen fibers and specialized fibroblasts
- layers are separated by gelatinous interstitial fluid
- dendrite is essentially isolated from other stimuli other than deep pressure
What happens when the capsule of Lamellar (Pacinian) corpuscles deforms?
- opens pressure sensitive sodium channels in sensory axon
- inner layers covering axon terminal ‘relax’ quickly so action potentials are discontinued (they are rapidly adapting)
What are Lamellar (Pacinian) corpuscles receptors stimulated by?
- deep pressure (when first applied)
- also vibration (as rapidly adapting)
What is the optimum frequency stimulation for Lamellar (Pacinian) corpuscles, what does this mean?
around 250Hz, this is similar to the frequency range generated upon fingertips by textures comprising features less than 1 micrometer.
Where are Bulbous corpuscles (Ruffin’s endings) located?
-In dermis and subcutaneous tissue (Hypodermis)
What is the structure of Bulbous corpuscles (Ruffin’s endings) ?
- Network of nerve endings intertwined with a core of collagen fibers that are continuous with those of the surrounding dermis.
- Capsule surrounds entire structure
What are Bulbous corpuscles (Ruffin’s endings) sensitive to?
-sustained deep pressure and stretching + distortion of the skin
What are Bulbous corpuscles (Ruffin’s endings) important for?
- Important for signaling continuous states of deformation of the tissues such as heavy prolonged touch and pressure signals
- Also found in joint capsules where help signal a degree of joint rotation (proprioception)
- High density around fingernails so may have a role in monitoring slippage of objects across surface skin ( allowing modulation of grip)
In terms of skin blood flow what are smooth muscles (in walls of arteries) and pre-capillary sphincters innervated by?
The sympathetic nervous system (autonomic)
What are pre-capillary sphincters?
Branches of arteries that extend up and give rise to capillary loop
What happens in skin blood flow?
-Noradrenaline acts on α1 adrenergic receptors on the
vascular smooth muscle in the skin
– GPCRs coupled to intracellular 2nd messengers result in increased intracellular calcium ions which allows more cross bridges to form and thus constriction
-This results in reduced skin blood flow
– Reducing SNS activity therefore causes relaxation (dilation) of arteries leading to skin and therefore increased skin blood flow
What is skin blood flow important in?
Thermoregulation and blood pressure
control
What is the optimum body temp and what are consequences of getting out of this set range?
- Optimum/set point for body is at 37 degrees (36.5-37.5)
- If too high proteins denature
- If too low lose the ability to thermoregulate
What are the primary mechanisms of heat transfer?
- radiation
- evaporation
- convection
- conduction
How might we lose (or gain heat) via radiation?
- We absorb and emit radiation from our surroundings
- Generally we will lose more radiation than gain so lose heat however, in hot environments we can start to gain more than we lose so gain heat.
How might we lose (or gain heat) via evaporation? Where is this particularly important?
- Evaporate water from surface of skin to lose heat energy
- This is important in hot climates as it will be the only mechanism to lose heat (e.g. sweating)
How might we lose (or gain heat) via convection?
Warm air rises and is replaced by cold air heat lose between body and surrounding air and body can then continue via conduction.
How might we lose (or gain heat) via conduction?
- Our bodies when in contact with something cold will lose heat to it e.g. warm up the surrounding air
- In hot climates the temperature gradient can be the other way such that the environment is warmer than us so heats us up
How do eccrine sweat glands work?
- They are innervated by the sympathetic nervous system
- They are known as Sympathetic cholinergic as they release ACh onto mAChRs (a G class receptor)
What can some eccrine sweat glands be stimulated by?
By Adrenaline in blood acting on β receptors – ‘nervous sweating’ (particularly forehead and palms of hands)
What happens when body temperature increases?
If blood temp goes ABOVE ‘set point’ heat loss center is activated..
–lower SNS activation of α1 on skin blood vessels
results in vasodilation
– increase in SNS cholinergic activation of mAChRs
on sweat glands means more sweating and so heat loss via evaporative cooling
– Increase respiratory rate
– Behavioral changes (go to cooler place, take off clothes)
What does the hypothalamus contain?
Preoptic area of hypothalamus contains
heat and cold sensitive neurons (central
thermoreceptors)
When might Radiation, conduction and convection not be effective mechanisms for heat loss and why?
When environmental temp is greater than body temp temp gradient is the other way and lose too much water trying to compensate
What happens when thermoregulators detect temperature BELOW ‘set point’?
Activates heat gain center…
- heat generating mechanisms (shivering +non shivering +thyroxine)
- conservation of body heat (vasoconstriction +countercurrent exchange)
Explain vasoconstriction as a method to conserve body heat…
Vasomotor center increases blood flow to the dermis, thereby reducing loses by radiation and convection
Explain countercurrent exchange as a way to conserve body heat…
Warm blood in arteries meets with cold blood in veins so heat transferred between and not lost to outside
Explain how shivering leads to the generation of body heat…
– Increased tone of skeletal muscles
– When tone rises above critical level, shivering begins due to oscillatory contractions of agonist and antagonist muscles mediated by muscle
spindles (stretch receptors)
Explain how non-shivering thermogenesis leads to the generation of body heat…
– Increase in Sympathetic Nerve Activity and increased circulating Adrenaline/noradrenaline from adrenal medulla
– Increased cellular metabolism e.g. increased glycogenolysis in liver and muscle
– ‘uncoupling’ of oxidative phosphorylation i.e. heat produced instead of ATP (occurs in ‘brown fat’ particularly in infants)
Explain how increases thyroxine would lead to generation of body heat…
– In response to TRH and TSH
– Increases basal metabolic rate
– In adults humans may take several weeks exposure to cold before thyroid reaches new level of thyroxine secretion this is therefore a much slower method
What is the structure and function of the arrector pilus muscle?
- Smooth muscle innervated by SNS (α1 receptors)
- Attach hair follicle to upper dermis
- Contraction pulls hairs upright and dimples skin > goosebumps
- Also compresses sebaceous glans which lubricates skin
- If you are hairy this traps a layer of warm air around the skin (insulating) and also makes you look bigger +more formidable in fight or flight situations
What is the arrector pilus muscle an example of?
- Temperature regulation is mostly done by negative feedback but this is an example of physiological feedforward
- Receptors in skin inform brain and send a message to hypothalamus. The hypothalamus then takes action to prepare even though the core temperature hasn’t dropped yet.
What is a first degree burn?
-Superficial i.e. only involve the outer
layers of the epidermis
-Red/pink, dry, painful
-Usually no blisters e.g. a mild sunburn
-Skin remains a water and bacterial barrier
-Usually heals 3-10 days
-Can get pain as nerve endings become exposed/sensitive due to inflammation
What is the second degree burn? (more mild)
– Epidermis + varying amounts of dermis
– Painful, moist, red and blistered.
– Usually heal in approx 1-2 weeks
– Need good dressings (absorptive initially)
What is the second degree burn? (more severe)
-deeper
– May include whiteish, waxy looking areas
– Hair follicles, sweat glands may remain intact
– Some tactile receptors may be lost
– Usually heal in 1 month but may have some loss of
sensation and scarring
What is the third degree burn?
• Full Thickness i.e. extend into subcutaneous tissue
and may involve muscle and bone
• Varied color from waxy white through to deep
red or black
• Hard dry and leathery
• No pain in these areas as sensory nerve endings
destroyed
• If more than a few cms may require skin grafting
• Weeks to regenerate + scarring
What are some direct implications of severe burns?
- Hydration+ hypovolemic shock(not enough water in blood to be pumped-leads to fainting)
- Infection/ Sepsis
- Hypothermia
