Lecture 3: SKIN PHYSIOLOGY Flashcards
What is meant by there is no perfect “one receptor - one function’ relationship?
Receptors can often respond to several different stimuli but will be most sensitive to a particular type
What are the types of receptors?
Free nerve endings, tactile (Merkel) discs, tactile (messier) corpuscles, lamellar (pacinian) corpuscles and bulbous corpuscles (Ruffini’s endings)
What is the most common receptors in skin?
Free nerve endings
What is the structure of free nerve endings?
Mostly unmyelinated small diameter fibres but also some small diameter myelinated fibres
Where are free nerve endings found?
Axon terminals branch into epidermis and detect lots of things
What do free nerve endings usually have?
Small swellings at distal ends called sensory terminals
What do sensory terminals have?
Receptors that function as cation channels»>depolarisation»>action potentials
What do free nerve endings respond to?
Mainly temperature (hot/cold), painful stimuli, some movement and pressure, some to itch (in response to histamine)
What do antihistamines do?
Block the receptors to the itch
What do some free nerve endings do?
Wrap around hair follicles (petririchial endings) acting as light touch receptors which detect bending of hairs
What are tactile (Merkel) discs?
Free nerve endings located in the deepest layer of the epidermis
What are tactile discs associated with?
Large disc shaped epithelial (Merkel) cells
How is communication between the tactile epithelial cell and nerve endings possible?
Via serotonin (5HT)
Where are tactile (Merkel) discs abundant?
In fingertips and they have very small receptive fields so good for two point discrimination
What are tactile (Merkel) discs sensitive to?
An objects physical features such as texture, shape and edges. Also fine touch and light pressure
Where are tactile (messier) corpuscles located?
In the papillary layers of the dermis between the epidermal ridges
Where are tactile (messier) corpuscles especially found?
In hairless skin (finger pads, lips, eyelids, soles of feet, external genitalia, nipples)
What is the structure of tactile (messier) corpuscles?
Encapsulated
Spiralling/branching unmyelinated sensory terminals surrounded by modified Schwann cells which don’t form myelin and then by a thin oval fibrous connective tissue capsule
What does deformation of the tactile (messier) corpuscle capsule trigger?
Entry of sodium ions into the nerve terminal»_space;> action potential
What do tactile (messier) corpuscles sense?
Delicate ‘fine’ or discriminative touch, light pressure and low frequency vibration (20-80Hz)
What is delicate ‘fine’ or discriminative touch?
Sensitive to shape and textural changes in exploratory touch and movement of objects over the surface of the skin
Where are lamellar (pacinian) corpuscles found?
Scattered deep in the dermis and hypodermis
What is the structure of lamellar (pacinian) corpuscles?
Single dendrite lying within concentric layers of collagen fibres (secreted by fibroblasts) and specialised fibroblasts
What are the layers of collagen fibres in lamellar (pacinian) corpuscles separated by?
Gelatinous interstitial fluid
What is the dendrite in lamellar (pacinian) corpuscles isolated from?
Stimuli other than deep pressure
What happens when there is deformation of the capsule in lamellar (pacinian) corpuscles?
Pressure sensitive sodium channels are opened in the sensory axon. Inner layers covering the axon terminal relax quickly so action potential is discontinued (rapidly adapting)
What are lamellar (pacinian) corpuscles stimulated by?
Deep pressure (when first applied) and also vibrating because rapidly adapting
What is optimal stimulation frequency?
Around 250 Hz which is similar to frequency range generated upon fingertips by textures comprising off <1 micrometre
Where are bulbous corpuscles (Ruffini’s endings) found?
In dermis and subcutaneous tissue
What is the structure of bulbous corpuscles (Ruffini’s endings) ?
Network of nerve endings intertwined with a core of collagen fibres that are continuous with those of the surrounding dermis. Capsule surrounds the entire structure
What are bulbous corpuscles (Ruffini’s endings) sensitive to?
Sustained deep pressure and stretching or distortion of the skin
What are bulbous corpuscles (Ruffini’s endings) important for?
Signalling continuous states of deformation of tissues such as heavy prolonged touch and pressure signals
Where are bulbous corpuscles (Ruffini’s endings) also found?
In joint capsules where they help signal degree of joint rotation (proprioreception)
Where are bulbous corpuscles (Ruffini’s endings) in high density?
Around fingernails so may have a role in monitoring slippage of objects across surface skin
What do bulbous corpuscles (Ruffini’s endings) allow?
Modulation of grip
Where do arteries supply blood to?
Skin in the subcutaneous layer
Where do branches of arteries go?
Extend into the superficial layers and give rise to capillary loops which supply blood to the epidermis
What happens after blood is supplied to the epidermis?
It drains down into the venous plexus
What is also in the walls of arteries/blood vessels?
Smooth muscle that is under the control of the sympathetic nervous system
What are precapillary sphincters?
Bands of smooth muscle at the start of the capillary beds. Contracting muscles constricts blood vessels and reduces blood flow to the upper layers of the skin
What does noradrenaline act on?
The alpha 1 adrenergic receptors on the vascular smooth muscle of the skin
What happens after the alpha 1 adrenergic receptors on the vascular smooth muscle of the skin?
G- protein coupled receptors (GPCR’s) coupled to intracellular 2nd messengers lead to increase intracellular calcium ions and therefore constriction. Reduces skin blood flow
What happens when SNS activity is reduced?
Relaxation (dilation) of arteries to the skin causing an increase in skin blood flow
What is skin blood flow important in?
Thermoregulation and blood pressure control
What is the optimal core body temperature/ set point?
36.5-37.5
What happens when core body temperature gets too high?
Proteins denature and problems occur
What are the mechanisms for heat transfer?
Radiation, conduction, convection and evaporation
What does radiation cause?
Heat loss in the form of infrared rays
What objects will radiate infrared rays?
Any objects not at absolute zero temperature
What radiates heat?
As well as the body radiating heat, heat rays are also being radiated from objects towards the body
What happens when the body temperature is greater than the temperature of the surroundings?
More heat will be radiated from the body than to it
What is conduction?
Transfer of heat to objects or media which are in contact
What is conduction mainly lost to?
A lot more to air than to objects
What happens when the temperature of the air beside the skin becomes equal to the temperature of the skin?
Heat will stop being lost bu conduction unless the heated air moves away to be replace by cooler air (air convection)
What is convection?
Transfer of heat to air (or water) by conduction followed by the movement of air (or water) away from the skin which maintains the heat gradient for heat loss from the body
Why does a small amount of convection almost always occur?
Because of the tendency of heated air surrounding the body to rise
When does convection increase?
In wind/ running
What happens when water evaporates from the body?
The heat energy required to cause the water to evaporate is also lost
What happens when not sweating?
Water is still evaporated from the skin and respiratory tract
When is evaporation important?
When the temperature of the environment is greater than body temperature because it is the only method that heat can be lost by
Heat loss to water vs air
The body loses heat to water faster than air
What are the eccrine sweat glands innervated by?
The sympathetic nervous system
What do sympathetic cholinergenic nerves do?
Release ACH onto mAChR’s (GPCR’s)
What can some eccrine sweat glands do?
Be stimulated by adrenaline in the blood acting on beta receptors - ‘nervous sweating’ especially on palms and soles
What is involved in temperature regulation?
The preoptic area of the hypothalamus contains heat and cold sensitive neurons (central thermoreceptors)
What happens when body temperature increases?
If blood goes above the set point then the heat loss centre is activated
What happens when the heat loss centre is activated?
- decreased SNS activation of alpha 1 receptors on skin blood vessels causes vasodilation
- increased SNS cholinergic activation of mAChR’s on sweat glands causes sweating
- Increased respiratory rate (evaporative heat loss increases)
- behavioural changes
What happens when body temperature falls?
When blood goes below the set point the heat gain centre is activated
What happens when the heat gain centre is activated?
Blood vessels vasoconstrictor to make less blood go to the skin and lose less heat to the environment. Also countercurrent exchange
What is countercurrent exchange?
Cold blood coming back in veins which are in close proximity with arteries is warmed up by heat transfer from the arteries before it reaches the surface
What are the heat generating mechanisms?
Shivering, non-shivering thermogenesis and increased thyroxine
What is shivering?
Increased tone of skeletal muscles
What happens when tone rises above a critical level?
Shivering begins due to oscillatory contractions of agonist and antagonist muscles mediated by muscles spindles (stretch receptors)
What happens in non-shivering thermogenesis?
Increased sympathetic nerve activity and increased circulating adrenaline/noradrenaline from the adrenal medulla
What happens to the metabolism in non-shivering thermogenesis?
Increased cellular metabolism/metabolic rate
What also happens in non-shivering thermogenesis (particularly in infants)?
uncoupling of oxidative phosphorylation - heat produced instead of ATP (occurs in brown fat, particularly in infants)
What is increased thyroxine in response to?
TRH and TSH
What does increase thyroxine do?
Increases the basal metabolic rate
How long will it take for increased thyroxine take to begin in adult humans?
It may take several weeks of cold before the thyroid reaches new levels of thyroxine secretion
What are arrector pili muscles?
Smooth muscle innervated by SNS (alpha 1 receptors)
What do arrector pili muscles attach to?
The hair follicle and the upper dermis
What happens when arrector pili muscles contract?
The hair is pulled upright and the skin is dimpled where it attaches to the dermis (goosebumps). Also compresses sebaceous glands which lubricates the skin
What happens if you are a hairy mammal?
Goosebumps can trap a warm layer of air around the skin and make you look bigger, scarier and more formidable but it isn’t that useful for humans but can be a good example of physiological feedforward
Where are first degree burns?
Superficial - only involve outer layers of the epidermis
What is the appearance of first degree burns?
Red/pink, dry, painful, usually no blisters
What is an example of first degree burns?
Mild sunburn
What happens to the skin in first degree burns?
Remains a water and bacterial barrier
How long does it take for first degree burns to heal?
Usually 3-10 days
Where are second degree burns?
The epidermis and varying amounts of the dermis
What is the appearance of second degree burns?
Painful, moist, red and blistered
How long does it take second degree burns to heal?
Approximately 1-2 weeks
What do second degree burns need?
Good dressings (absorptive initially)
What do deeper second degree burns look like?
May include whiteish, waxy looking areas
What happens to the hair follicles and sweat glands in deeper second degree burns?
The may remain intact
What may happen to receptors in deeper second degree burns?
Some tactile receptors may be lost meaning they are less painful
How long does it take for deeper second degree burns to heal?
Usually heal in 1 month but may have some loss of sensation and scarring
Where are third degree burns?
Full thickness - extend into subcutaneous tissue and may include muscle and bone
What is the appearance of third degree burns?
Varied colour from waxy white through to deep red or black. Hard, dry and leathery
What is the pain like in third degree burns?
There is no pain because sensory nerve endings are destroyed
What happens if third degree burns are more than a few centimetres?
They may require skin grafting
How long do third degree burns take to heal?
Weeks to regenerate and scarring
What must we know to find fluid replacement for burns?
The percentage of the total body surface area involved in second degree burns or more
What percentage is the adult head?
9%
What percentage are the upper limbs of adults?
9% each
What percentage is the adult trunk?
36% (front and back)
What percentage is the adult genitalia?
1%
What percentage is the adult lower limb?
18% each
What percentage is the child head?
15%
What percentage is the child trunk?
32% (front and back)
What percentage is the child upper limb?
9% each
What percentage is the child genitalia?
1%
What percentage is the child lower limb?
17% each
What are complications of severe burns which directly relate to the skin function?
Dehydration and hypovolemic shock, infection/sepsis and hypothermia
What are complications of severe burns which don’t relate to skin function?
electrolytes imbalances, hyper metabolism, gastrointestinal ulceration, renal failure and respiratory dysfunction
