LECTURE 3: skin physiology Flashcards
types of skin receptors
- free nerve endings
- tactile discs
- tactile corpuscles
- lamellar corpuscles
- bulbous corpuscles
what do free nerve endings respond to
- temp
- painful stimuli
- some movement and pressure
- some to itch
- some wrap around hair follicles
what are free nerve endings
most common receptor in skin
Mostly unmyelinated small diameter fibres but also some small diameter myelinated fibres
Usually have small swellings at distal ends = sensory terminals
how free nerve endings work
Sensory Terminals have receptors that function as cation channels»_space; depolarization»_space; APs
what are tactile discs
Free nerve endings located in deepest layer of epidermis
- associated with large disc shaped epidermal (merkel) cells
what do tactile discs respond to
Sensitive to an objects physical features: texture, shape and edges + Fine touch and light pressure
how to tactile discs work
Communication between the tactile epithelial cell and nerve ending possibly via serotonin (5HT)
where can you find tactile discs
Abundant in fingertips and very small receptive fields. • So good for two point discrimination
where are tactile corpuscles located
located in papillary layer of dermis, especially in hairless skin e.g. finger pads, lips, eyelids, external genitalia, soles of feet, nipples
where are lamellar corpuscles located
Scattered deep in dermis and hypodermis
what are lamellar corpuscles
Single dendrite lying within concentric layers of collagen fibres and specialised fibroblasts
Layers separated by gelatinous interstitial fluid – Dendrite essentially isolated from stimuli other than deep pressure
how lamellar corpuscles work
Deformation of capsule opens pressure sensitive Na+ channels in sensory axon: Inner layers covering axon terminal ‘relax’ quickly so APs discontinued (rapidly adapting)
what stimulates lamellar corpuscles?
Pressure (when first applied)
Also vibration because rapidly adapting: Optimal stimulation frequency is around 250Hz which is similar to frequency range of generated upon fingertips by textures comprising features < 1 μM
where are bulbous corpuscles located
in dermis and subcutaneous tissue
Also found in joint capsules where help signal degree of joint rotation (proprioception): High density around fingernails so may have role monitoring slipage of objects across surface skin – Allowing modulation of grip
what are bulbous corpuscles
Network of nerve endings intertwined with a core of collagen fibres that are continuous with those of the surrounding dermis.
Capsule surrounds entire structure
what are bulbous corpuscles sensitive to
sustained deep pressure and stretching or distortion of the skin
what are bulbous corpuscles important for
Important for signalling continuous states of deformation of the tissues such as heavy prolonged touch and pressure signals
what are encapsulated corpuscles
Spiralling / branching unmyelinated sensory terminals surrounded by modified Schwann cells and then by a thin oval fibrous connective tissue capsule
Deformation of capsule triggers entry of Na+ ions into nerve terminal»_space; Action Potential
what does deformation of encapsulated corpuscles do
triggers entry of Na+ ions into nerve terminal»_space; Action Potential
what do tactile corpuscles sense
- Delicate ‘fine’ or discriminative touch:
Sensitive to shape and textural changes in exploratory touch e.g. reading Braille text. Movement of objects over the surface of the skin - light pressure
- low frequency vibrations (2-80 Hz)
how skin blood flow can be controlled
Smooth muscle in walls of arteries and pre-capillary sphincters innervated by the sympathetic nervous system (SNS
- Noradrenaline acts on α1 adrenergic receptors on this vascular smooth muscle in the skin: GPCRs coupled to intracellular 2nd messengers»_space; increased intracellular Ca++»_space; constriction = Reduced skin blood flow
- Reducing SNS activity therefore causes relaxation (dilation) arteries to skin è Increased skin blood flow
structure and function of eccrine sweat glands and their role in thermoregulation
Innervated by the sympathetic nervous system:
Sympathetic cholinergic i.e. release ACh onto mAChRs (GPCRs)
Some eccrine sweat glands can also be stimulated by Adrenaline in blood acting on β receptors – ‘nervous sweating’ esp. on palms and soles (and axilla to some degree)
4 basic mechanisms of heat transfer
- radiation
- evaporation
- convection
- conduction
non-effective heat loss mechanisms when when environmental temp ≥ body temp.
Radiation, conduction and convection
preoptic area in hypothalamus has
heat and cold sensitive neurons (central thermo receptors)
what happens when blood temp goes ABOVE set point
heat loss centre activated
vasodilation
decreased SNS activation of alpha1 on skin blood vessels
sweating
increased SNS cholinergic activation of mAChRs on sweat glands
what happens to respiratory rate when temp is increased
increases
what factors are effected by increase in body temp
vasodilation
sweating
increased respiratory rate
heat gain centre activated by
Central Thermoreceptors detect temperature BELOW ‘set point’ = activation
how heat gain centre responds to low body temp
increased generation of body heat
conservation of body heat
heat generating mechanisms
- shivering
- non shivering thermogenesis
- increased thyroxine
how to conserve body heat
vasomotor centre decreases blood flow to the dermis, thereby reducing losses by radiation and convection
shivering
- 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
non shivering thermogenesis
- Increased 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)
increased thyroxine
- 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
arrector pilli muscles
- 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
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 day
second degree burn
- Epidermis + varying amounts of dermis
- Painful, moist, red and blistered.
- Usually heal in approx 1-2 weeks
- Need good dressings (absorptive initially)
deeper second degree burns
- 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
third degree burn
- Full Thickness i.e. extend into subcutaneous tissue and may involve muscle and bone
- Varied colour 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
head % of TBSA in adult and children
A= 9%
C=15%
upper limb % of TBSA
A = 9% each C = 9% each
trunk % of TBSA
A = 36% (front and back) C = 32% (front and back)
genitalia % of TBSA
A = 1% C = 1%
lower limb % of TBSA
A = 18% each C = 17% each
complications of severe burns for normal skin function
- dehydration and hypovolemic shock
- infection / sepsis
- hypothermia
what is normal body temp
around 37 degrees is set point
36.5-37.5
what happens if temp too low
lose ability to thermoregulate disorientation loss of muscle control loss of consciousness cardiac arrest death
what happens if body temp too high
lose ability to thermoregulate death cell damage convulsions proteins denature
