Skin Physiology Flashcards
What are free nerve endings?
They are the most common receptor in the skin, and have sensory terminals at the end of them.
The axons are mostly unmyelinated and small diameter (called C-fibres), but also some of small diameter that are myelinated (celled A-delta).
- They have receptors that can respond to various painful (nociceptive), thermal and chemical stimuli - some are cation channels, others are chemically activated. They lead to APs to CNS.
- Free nerve endings mainly respond to: temperature (for a range of temps), painful stimuli, some movement and pressure, some to itch, some wrap around hair follicles, acting as touch receptors which detect bending of hairs.
What are sensory terminals?
Small swellings at distal ends of axons
What are Tactile (Merkel) disks?
- Free nerve endings located in deepest layer of epidermis (stratum basale)
- Associated with large disc shaped epidermal cells
- Communication between tactile epithelial cell and nerve endings possibly via serotonin (serotonin being the neurotransmitter) - Abundant in fingertips and have very small receptive fields
- So good for two-point discrimination
- Also for ascertaining an objects physical features. - Sensitive to: light tough and light pressure, texture, shape and edges, low frequency vibration.
What are Tactile (Meissner) corpuscles?
- Located in papillary layer of dermis (especially in hairless skin)
- Encapsulated
- 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 - Can sense: delicate ‘fine’ or discriminative touch (sensitive to shape and textural changes in exploratory tough and movement o objects over surface of skin), light pressure, low frequency vibration.
What are Lamellar (Pacinian) corpuscles?
- Found deep in dermis and hypodermis
- Single sensory axon lying within concentric layers of collagen fibres and specialised fibroblasts
- Layers separated by gelatinous interstitial fluid
- Sensory axon terminal essentially isolated from stimuli other than deep pressure
- Deformation of capsule opens pressure sensitive Na+ channels in sensory axon
- Inner layers covering axon terminal ‘relax’ quickly so APs discontinued (rapidly adapting)
- Stimulated by deep pressure (when first applied)
- Also stimulated by vibration because rapidly adapting (optimal stimulation frequency is around 250Hz)
What are Bulbous (Ruffini) corpuscles?
- Located in dermis and subcutaneous tissue
- Network of nerve endings intertwined with a core of collagen fibres that are continuous with those of the surrounding dermis. Capsule surrounds entire structure
- Sensitive to sustained deep pressure and stretching or distortion of the skin
- Important for signalling continuous states of deformation of the tissues such as heavy prolonged touch and pressure signals
- 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 is the role of skin blood flow in temperature regulation?
- Smooth muscle in waller of arteries and pre-capillary sphincters innervated by the sympathetic nervous system (SNS), which signals through hormones when to contract or dilate.
- Noradrenaline acts on alpha-1 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 activation of alpha-1 receptors therefore causes relaxation (dilation) of arteries to skin, leading to increased skin blood flow
- Important in thermoregulation and blood pressure control
What is the importance of sympathetic ‘vasomotor tone’?
The importance is that the activation of alpha-1 receptors leads to vasoconstriction and that the reduced activation of alpha-1 receptors on vascular smooth muscle leads to vasodilation. Therefore when we undergo things like anaesthesia, we need to take into account how the sympathetic nervous system is affected and how blood pressure will react accordingly.
- Note: active vasodilation (redness when exercising) is different to this
How do eccrine sweat glands function?
They are innervated by the sympathetic nervous system and they are sympathetic cholinergenic, meaning that the neurotransmitter is ACh.
- They can also be stimulated by adrenaline in blood acting on beta receptors - ‘nervous sweating’
How does the body respond when body temperature increases?
- Preotic area of hypothalamus contains heat and cold sensitive neurons (central thermoreceptors)
- If blood temp goes above ‘set point’ heat loss centre is activated.
- SNS decreases activation of alpha-1 on skin blood vessels, leading to vasoilation
- SNS increases cholinergenic activation of mAChRs on sweat glands, leading to sweating
- increase respiratory rate, which moves air over moist surfaces, increasing evaporation
- behavioural changes
When do heat loss mechanisms such as radiation, conduction and convection work?
When the environment temperature is colder than the body temperature.
- If its the other way around, then sweating is the only cooling mechanism
How does the body respond when its temperature decreases?
- Central thermoreceptors detect temperature below ‘set point’ which activated heat gain centre.
Heat conservation mechanisms:
- Transfer heat from arteries to veins (counter current exchange) to conserve heat of blood
- Vasoconstriction so you lose less heat through radiation, conduction and convection
Heat production mechanisms:
- Shivering - contractions of muscles hydrolyses ATP which produces heat
- Non-shivering thermogenesis
- Increase in sympathetic nerve activity and increase circulating
- increased cellular metabolisms (eg. increased glycogenolysis in liver and muscle, which produces heat)
- ‘uncoupling’ of oxidative phsophorylisation. ie. heat produces instead of ATP (occurs in ‘brown fat’ particularly in infants)
- Increased thyroxine
- In response to TRH and TSH = increased basal metabolic rate (can take weeks for this to occur)
Describe how arrector pili muscles work
- Smooth muscle innervated by SNS (alpha-1 receptors)
- Attach hair follicle to upper dermis
- Contraction pulls hairs upright and dimples skin = goosebumps
- Also compresses sebaceous glands which lubricates skin
- If you are a hairy mammal, this traps a layer of warm air around skin and makes you look bigger and scarier
- Not that useful for humans but can be a good example of physiological feedforward, because body temp hasn’t get dropped yet, body is doing something in advance
What is the general rule for working out fluid replacement in the body when you have a severe burn (2nd or 3rd degree)?
Rule of 9’s. The body can be divided into sections in multiples of nines as this gives you the percentage of total body surface area involved.
What are the potential complications of severe burns?
Dehydration and hypovolemic shock
- Have lost the layer of skin that keeps water in, so you become dehydrated
Infection/sepsis
- Have lost the layer of skin that is a microbial barrier so infections can come on
Hypothermia
- As a result of losing heat from the evaporation of the water that can now escape through the wound site
