lecture 3 - skin physiology Flashcards
What is the most common type of receptor in the skin?
Free nerve endings
Are the majority of free nerve endings in the skin myelinated or unmyelinated?
Majority unmyelinated with small diameter
What is the name for a small swelling at the distal end of a free nerve ending?
Sensory terminal
What is the sensory terminal of a free nerve ending?
A swelling at the distal end that acts as a receptor with cation channels
How do free nerve endings generate nerve signals?
They have cation channels that when opened lead to a local depolarisation and subsequent action potential.
What stimuli do free nerve endings predominantly respond to?
Temperature, pain, some movement/pressure, itch,
How do free nerve endings detect itchiness?
They responded to histamines
How do free nerve endings detect light touch from hairs bending?
Nerve endings wrap around hair follicles.
What is the name for a free nerve ending that wraps around a hair follicle to detect bending of the hair?
Peritrichial ending
What are the 5 types of touch receptors on the skin?
Free nerve endings, Tactile (Merkel) discs, tactile (Meissen) corpuscles, lamella (pacinian) corpuscles. bulbous corpuscles (Ruffini’s endings)
What is the alternative name for tactile discs?
Merkel discs
Where are tactile/merkel discs found?
Free nerve endings located in deepest layer of the epidermis
What are merkel cells?
Large disc shaped epidermal cells involved in sensory perception of tactile discs
How do merkel cells communicate?
Via serotonin (5HT) neurotransmitters
Where are tactile/merkel discs most abundant, and how does this reflect their function?
In fingertips and small receptive fields, as they are good for 2 point discrimination.
What are tactile/merkel discs sensitive to?
An object’s physical features: texture, shape, edges. Fine touch and light pressure
What is the alternative name for tactile corpuscles?
Meissner corpuscles
Where are tactile/meissner corpuscles located?
In the papillary layer of the dermis, especially in hairless skin (finger pads, lips, eyelids, external genitalia, soles of feet, nipples)
What is the structure of a tactile/meissner corpuscle?
Spiralling/branching unmyelinated sensory terminals surrounded by modified schwann cells then by a thin oval capsule of fibrous connective tissue.
Are the sensory terminals of tactile/meissner corpuscles myelinated or unmyelinated?
Unmyelinated - but they are surrounded by modified schwann cells
How are action potentials generated by tactile/meissner corpuscles?
Deformation of the capsule triggers entry of Na+ into a nerve terminal, triggering an action potential.
What type of sensation are tactile/meissner corpuscles used for?
Fine, discriminative touch. Sensitive to shape and texture. used for exploratory touch, e.g. braille. Detect light pressure and low frequency vibration (2-80 Hz)
What frequency range of vibration can be detected by tactile/meissner corpuscles?
2 to 80 hertz
What is the alternative term for lamellar corpuscles?
Pacinian corpuscles
Where are lamellar/pacinian corpuscles located?
Scattered deep in the dermis and hypodermis
What is the structure of a lamellar/pacinian corpuscle?
Single dendrite lying within concentric layers of collagen fibres and specialised fibroblasts, with layers separated by gelatinous interstitial fluid
What are the layers of collagen and fibroblasts of a lamellar/pacinian corpuscle separated by?
Gelatinous interstitial fluid
Why are lamellar/pacinian corpuscles only activated by deep pressure?
They are located deep in the skin, and the dendrites is isolated from light pressure by the collagen capsule.
How are lamellar/pacinian corpuscles rapidly adapting?
Inner layers of the capsule that cover the axon terminal ‘relax’ quickly so that Action Potenials are discontinued.
What is the optimal stimulation frequency of a lamellar/pacinian corpuscle?
approximately 250Hz.
The optimal stimulation frequency (250Hz) of lamellar/pacinian corpuscles is generated by fingertip interaction with features of what size?
less than 1 micrometer
What is the alternative name for bulbous corpuscles?
Ruffini’s endings
Where are bulbous corpuscles located?
The dermis and subcutaneous tissue (hypodermis)
What is the structure of bulbous corpuscles?
Network of nerve endings that are continuous with those of the surrounding dermis. Surrounded by collagen capsule that transmits distortion
What stimulation are bulbous corpuscles sensitive to?
Sustained deep pressure, stretching/distortion of the skin - prolonged touch
What type of touch receptor is found within the joint capsule and is responsible for proprioception of joint rotation?
Bulbous corpuscles/Ruffini’s endings
What is the role of bulbous corpuscles in joint capsules?
Help in proprioception - signal the degree at which a joint has rotated
Why is there a high density of bulbous corpuscles around the fingernails?
Monitoring of slippage of objects and modulation of grip.
How is blood flow to the skin mechanically regulated?
Smooth muscles in walls of arteries and pre-capillary sphincters are contracted/relaxed via innervation by the sympathetic nervous system
What neurotransmitter is involved with triggering vascular smooth muscle contractions in the skin to regulate blood flow?
Noradrenaline, which acts on a1 (alpha 1) adrenergic receptors on the vascular smooth muscle in the skin
What receptors does noradrenaline act on to produce contraction of vascular smooth muscle in the skin?
a1 (alpha 1) adrenergic receptors
What is the process of vascular smooth muscle contraction, after a neurotransmitter signal have been received?
GPCRs (G-protein coupled receptors) are coupled to intracellular 2nd messengers, leading to an increase in intracellualr Ca2+, thereby facilitating contraction/vasoconstriction
How is vasodilation facilitated by the skin, in terms of negative feedback?
Reduced sympathetic nervous system activity reduces contraction of vascular smooth muscle in the skin, increasing skin blood flow.
What are the 2 key factors influenced by skin blood flow?
Thermoregulation, blood pressure
What are the 4 heat transfer mechanisms?
Radiation, evaporation, convection, conduction
What is conduction, in terms of body temperature regulation?
The transfer of heat between a hot/cold object and the skin
What is evaporation, in terms of body temperature regulation?
Transfer of heat from skin to water, allowing it to evaporate. e.g. sweating
What is convection, in terms of body temperature regulation?
Transfer of heat from skin to the surrounding air. The warmed air rises away from the skin and is replaced by cool air which is in turn warmed.
What is radiation, in terms of body temperature regulation?
Transfer of heat to or from the body via infrared radiation
How are eccrine sweat glands predominantly controlled?
They are innervated by the sympathetic nervous system
What type of synapses do eccrine sweat glands receive?
Sympathetic cholinergic
How do sympathetic cholinergic synapses trigger eccrine sweat glands?
Acetylcholine (ACh) is released onto mACHRs (muscarinic acetylcholine receptors)/GPCRs in the cell membrane.
How are eccrine sweat glands triggered hormonally?
Some glands can be stimulated by adrenaline in the blood, which acts on beta receptors causing ‘nervous sweating’
What causes ‘nervous sweating’?
Adrenaline in blood acting on beta receptors on eccrine sweat glands
Where does ‘nervous sweating’ usually occur on the body?
Palms and soles, axilla (armpit)
What is the axilla?
The armpit
What part of the brain detects body temperature?
The pre optic area of the hypothalamus, which contains central thermoreceptors
What are mAChRs?
muscarinic acetylcholine receptors
What are GPCRs?
G protein coupled receptors
What are central thermoreceptors?
Heat and cold sensitive neurons found in the pre optic area of the hypothalamus
How is the body’s heat loss centre activated?
If the hypothalamus detects the blood temperature exceeding the set point
When the heat loss centre is activated, what happens to the skin blood flow?
Less SNS activation of alpha 1 receptors on skin blood vessels leads to vasodilation
When the heat loss centre is activated, what happens to sweat production?
Increased SNS cholinergic activation of mAChRs on eccrine sweat glands.
What happens to respiratory rate when the heat loss centre is activated?
Increased respiratory rate
What mechanisms are activated by the heat loss centre (4)?
Vasodilation, sweating, increased respiratory rate, behavioural changes
What activates the heat gain centre?
Central thermoreceptors detect blood temperature going below set point, and activate the hypothalamus
What are key heat generation mechanisms (3)?
Shivering, non-shivering thermogenesis, increased thyroxine release
How is shivering initiated?
Skeletal muscle tone is increases until it rises above a critical level, causing oscillatory contractions of agonist and antagonist muscles mediated by muscle spindles/stretch receptors
What is non-shivering thermogenesis?
Increased sympathetic nerve activity, releasing noradrenaline/adrenaline from the adrenal medulla to increase cellular metabolism
What cellular processes does non-shivering thermogenesis induce?
Increased cellular metabolism - e.g. glyocgenolyis in liver and muscle.
What hormones increase cellular metabolism in non-shivering thermogenesis?
Adrenaline and noradrenaline
What is the role of thyroxine in heat generation?
Increased thyroxine/T4 is released as a result of TRH and TSH release. This increases the basal metabolic rate.
What is thyroxine?
T4 thyroid hormone.
What is the long term heat generation mechanism?
Thyroxine release - increases metabolic rate after lengthy exposure to cold .
What are the arrector pili muscles activated by?
They are smooth muscle, activated by SNS at alpha 1 receptors.
What do the arrector pili muscles attach?
Hair follicle to upper dermis
How do arrector pili muscles affect sebaceous glands?
They compress them, allowing them to release sebum to lubricate the skin
How are the arrector pili muscles involved in thermoregulation?
They causes goosebumps which allow the hairs to trap a layer of warm air around the skin, preventing heat loss.
What does fluid replacement in burn victims depend on?
The percentage of total body surface that is involved.
What rule is used in burn fluid replacement?
‘Rule of 9’s’ (in adults)
In burn fluid replacement, what percentage of the body is given by the head?
9%
In burn fluid replacement, what percentage of the body is given by the upper limbs?
9% each
In burn fluid replacement, what percentage of the body is given by the trunk (front and back)?
36%
In burn fluid replacement, what percentage of the body is given by the genitalia?
1%
In burn fluid replacement, what percentage of the body is given by the lower limbs?
18% each
What are the complications of severe burns that are directly related to skin function?
Dehydration/hypovolemic shock, infection/sepsis, hypothermia
What complications of severe burns are not directly related to skin function?
Electrolyte imbalances, hypermetabolism, gastrointestinal ulceration, renal failure, respiratory dysfunction
What is an example of an electrolyte imbalance caused by severe burns?
Hyperkalaemia (high K+ conc.)
