Skin Physiology Flashcards
5 diff types of receptors
- Free nerve endings (superficial)
- Tactile discs
- Tactile corpuscles
- Lamellar corpuscles
- Bulbous corpuscles (deep)
What is a characteristic of the receptors?
They respond likely to all types of stimuli but are sensitive to a particular type
What are free nerve endings?
- Most common receptor in the skin
- Branches into the epidermis
- Mostly unmyelinated small diameter fibres but also some small diameter myelinated fibres
– Usually small swellings at distal ends = sensory terminals
What do the sensory terminals have?
Receptors eg thermo, chemo and noci that respond to the different types of stimuli
What kind of channels present at the sensory terminals? What do free nerve endings mostly respond to?
Mostly ion channels but can be chemically activated eg when bitten, mast cells degranulate –> histamine release. Responds to this
Peritrichal endings i.e where free nerve endings wrap around the hair follicle to help with sensation
What are tactile (merkel) discs?
- Free nerve endings located in the deeper layers of the epidermis
- Associated with an epidermal merkel disc that relays info to the free nerve ending via 5HT (serotonin neurotrans)
What is a merkel disc also called
Tactile epithelial cell
Where are merkel disc mostly found?
In the fingertips with small receptive fields eg good 2 pt discrimination
What does the merkel disc distinguish between?
Helps to distinguish:
* Fine touch and light pressure
* texture, shape and edges
Another name for tactile corpuscles
Meissner corpuscles
Where are the tactile corpuscles located?
In the papillary lary of the dermis, just below the epidermis
Where are the meissner corpuscles found?
Hairless skin eg In the soles of feet, eyelids, nipples, external genitalia, lips, finger pads
Structure of a meissner corpuscle
Encapsulated
* Spiralling / branching unmyelinated sensory terminals
surrounded by modified Schwann cells (that just support but don’t myelinate the axons) and then by a thin oval of fibrous connective tissue capsule
What does deformation of the capsule do?
- Triggers entry of Na+ ions into nerve terminal i.e mechanical gating»_space; Action Potential
What does the 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 vibration (2 to 80 Hertz)
Another name for tactile corpuscles
Pacinian corpuscles
Where are the Pacinian corpuscles found?
– Scattered deep in dermis and hypodermis
Structure of a Pacinian corpuscle
– Single dendrite lying within concentric layers of collagen
fibres and specialized fibroblasts that secretes it.
– Layers separated by gelatinous interstitial fluid
– Dendrite essentially isolated from stimuli other than deep
pressure
What does deformation of the capsule do? Pacinian
Deformation of capsule opens pressure sensitive Na+ channels in
sensory axon through pressure wave through interstital fluid
* Inner layers covering axon terminal ‘relax’ quickly so APs discontinued
(rapidly adapting)
What does the tactile corpuscles sense?
– Stimulated by deep 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 eg sandpaper
Another name for Bulbous corpuscles
Ruffini’s
Where are the Bulbous corpuscles found?
Located in dermis and subcutaneous tissue
Also found in joint capsules where help signal degree of joint
rotation (proprioception)
* in fingers may have role monitoring slippage of objects
across surface skin and so modulate grip
What does deformation of the capsule do? Bulbous
Network of nerve endings intertwined with a core
of collagen fibres that are continuous with those of the
surrounding dermis. Capsule surrounds entire structure, when collagen distorted, the nerve endings are activated
What does the bulbous corpuscles sense?
- Sensitive to sustained deep pressure and stretching or
distortion of the skin
– Important for signaling continuous states of deformation of
the tissues such as heavy prolonged touch and pressure
signals
What extends from the subpap plexus?
Precapillary sphincters
What is the precap sphincters made of
Smooth muscle
What happens when the smooth muscle contracts?
The diameter reduces and the blood flow also reduces
How is smooth muscle activated?
Smooth muscle in walls of arteries and pre-capillary sphincters innervated by the sympathetic nervous system (SNS)
What does the NA act on?
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 bloodflow
Reduced SNS activation results in
Reducing SNS activation of ⍺1 receptors therefore causes
relaxation (dilation) of arteries to skin increased skin
bloodflow
Why is dilation and constriction important?
For thermoregulation and blood pressure control
What happens in an injection to the spinal cord?
Some sym nerves are affected affecting the blood vessels so NE is injected to bring BP back up
What is the core body temp?
36.5 - 37.5
What happens at v low temps?
Loss of conciousness and muscle control, Cardiac arrest and DEATH
What happens at v high temps?
Cell damage, convulsions, protein denaturation and death
1º mechanisms of heat transfer
Radiation, conduction, evaporation and convection
How does radiation cause heat loss?
As infrared rays
What are infrared rays?
Electromagnetic rays with wavelength 5 – 20 micrometers which is 10 to 30 times that of light rays
When are infrared rays radiated?
When object is not at absolute 0 temp
Loss of body heat at room temp with no clothes
60%
When is heat radiation absorbed into the body?
When the surroundings have a higher temp than the body.
What is conduction?
Transfer of heat with media that we are in direct contact in
How much % of heat is conducted to air?
15%
Does conduction stop?
Yes, when the temp of the body and medium are the same
What is convection?
When the initially warmed air is replaced by cooler air which causes conduction to keep occuring due to the gradient
Why does a small amount of convection always occur?
Due to the tendancy for
air adjacent to the skin to rise as it becomes heated
What is evaporation?
When H20 evaps from body, the heat energy used to evap it is also lost i.e 0.58ºC for eacg gm of H20
How else is H2O lost besides sweating?
Through the respiratory tract about 600-700ml every day
Why is more heat lost in H2O than in air?
Water has a specific heat thousands of times that of air and so can absorb far greater quantities of heat
Which sweat glands are important for thermoregulation?
Eccrine sweat glands
How are eccrine sweat glands activated?
By the sympathetic nervous system that is colinergic
What kind of receptors is the Ach released onto?
Muscarinic acH receptors i.e not ion channels but G protein coupled receptors (GPCR)
Other ways that eccrine glands can be stimulated
Through Adrenaline in blood affecting beta receptors –> sweating on palms, soles and axilla
What allows the hypothalamus to detect changes in body temperature?
Preoptic are of the hypo has heat and cold sensitive neurons i.e central thermoreceptors
What happens when the heat loss centre is activated?
- Decreased SNS activation of alpha1 receptors on blood vessels –> vasodilation allowing heat loss through the heat transfer ways
- Increased colinergic activity of the MAcHR –> increased sweating
- increased respiratory rate so more air passes through allowing evap on tongue
- Concious behavioural changes
Effective heat loss mechanism when the envir temp is more than the body temp
Evap
What do the central thermoreceptors do when the core body temp goes below the set point?
The heat gain centre is activated
What happens when the heat gain centre is activated?
- Vasoconstriction
- The blood going out warms the blood coming in as the arteries and veins run close to each other
- Shivering
- Non shivering thermogenesis
- Increased Thyroxine after long term exposure
What does shivering do?
- Oscillatory contraction of agonist and antagonist muscles
- ATP → ADP + Pi + movement + heat
What is 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)
When is Thyroxine increased and what does it do?
– 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
What do errector mucles do?
- 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 a hairy mammal, this
traps layer of warm air around
skin and makes you look bigger,
scarier and more formidable - not that useful for humans but
can be a good example of
physiological feed forward
How to replace fluids for 2º and 3º burns…
A lot of H2O is already lost + inflam mediators that make the capillaries even more leaky
- Rule of nines to determine how much fluid should be replaced
Head
Adult - 9%
Child - 15%
Upper limb
Adult - 9% each limb
Child - 9%
Torso (front and back)
Adult - 36%
Child - 36%
Lower limb
Adult - 18%
Child - 17%
Genitalia
1% for both
Complications of burns
- Sepsis as skin is a barrier
- Dehydration and hypothermia as the H2O is gone and skin traps heat is gone
- Hyperkaelaemia –> electrolyte imbalances as the K+ ends up in the bloodstream
- Hypermetabolism due to ^ cortisol due to stress response
- Ulcers
- Renal failure
- Respiratory dysfunction
