Lecture Three - Skin Nurophysillogy Flashcards
Touch receptors of the skin
No perfect one _____ - one____ relationship. Receptors can often _____ __ several _____ _____ but will be most ____ to a ____ type.
There is no erpfect one receptor - one function relationship. Receptors can often respond to several different stimuli but will be most sensitive to a particular type
What is the most common receptor in skin?
Free nerve endings
Free nerve endings - myelinated / unmylineated ??
mostly unmyleinated small diameter fibres but also some small diameter myelinated fibres
What are the small swellings usually present at the distal ends of free nerve endings ?
Sensory terminals
Sensory thermals
Small swellings usually present at distal ends of free nerve endings
Sensory terminals have…
Receptors that can respond to various painful (nociceptice), thermal and chemical stimuli
Types of receptors on sensory terminals of free nerve endings and what happens once activated
- some are cation channels (e.g TRPV1 receptors)
- others chemically activated (e.g H1 receptors)
Activation leads to APS in afferent sensory axons to CNS»_space; somatosensory cortex
Example of receptors with cation channels
TRPV1 receptors
Example of receptors that are chemically activated
H1 receptors
What do free nerve endings respond to?
- temperature
- painful stimuli
- some movement and pressure
- some to itch (e.g in response to histamine)
- some wrap around hair follicles (peritrichial endings) acting as light touch receptors which detect blinding of hairs
What are tactile (Merkel) discs
Free nerve endings located in deepest layer of epidermis
What are tactile discs associated with?
Large disc shaped epidermal (Merkel) cells
Communication between the tactile epithelial cell and nerve ending can possible occur via…
Serotonin (5HT)
What does serotonin (5HT) allow?
Possible communication between the tactile epithelial cell and nerve ending
Where are tactile receptors very abundant
Fingertips
Size of receptive fields of tactile disks on fingertips
Very small - good two point discrimination
What are tactile discs sensitive to?
Sensitive to an objects physical features
- fine touch and light pressure
- texture, shape and edges
Where are tactile corpuscles located
- in the papillary layer of the dermis
- especially in hairless skin e.g: finger pads, lips, eyelids, external genitalia, soles of feet, nipples
Are tactile corpuscles encapsulated ?
Yep
Structure of the capsule of tactile corpuscles
- spiralling / branching unmyelinated sensory terminals surrounded by modified Schwann cells and then by a thin oval fibrous connective tissue capsule
In tactile corpuscles what causes an AP?
- deformation of capsule triggers entry of Na+ ions into nerve terminal —-> AP
Tactile corpuscles sensations
- delicate ‘fine’ or discriminative touch
- sensation to shape or textural changes in exploratory touch e.g reading braille text
- movement of objects over the surface of the skin
- light pressure
- low frequency vibration (2-80Hz)
Where are lamellar (pacinian) corpuscles located?
- scattered deep in dermis and hypodermis
Structure of lamellar corpuscles
- single dedrite lying within concentric layers of collagen fibres and specialised fibroblasts
- layers are separated by gelatinous interstitial fluid
Lamellar corpuscles and stimuli
Dendrite essentiallty isolated from stimuli other then Deep pressure
What does deformation of lamellar corpuscle capsule do?
Deformation of capsule opens pressure sensitive NA+ channels in sensory axon
- inner layers covering the axon terminal ‘relax’ quickly so APS discontinued (rapidly adapting)
Lamellar corpuscles stimulation
- stimulated by deep pressure (when first applied)
- also by vibration?? Because rapidly adapting
- optimal stimulation frequency is around 250Hz which is similar to frequency range of generated upon fingertips by textures compotisng features <1microM
Bulbous corpuscles (Ruffin is endings) location
- located in the dermis and subcutaneous tissue
Structure of bulbous corpuscles
- network of nerve endings intertwines with a core of collagen fibres that are continuous with those of the surrounding dermis.
- capsule surrounds entire structure
Bulbous corpuscles sensations
- 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
Which receptor is located in joint capsules and fingles and why?
Bulbous corpuscles are found in joint capsules where they help signal degree of joint rotation (propriocetpion)
Also found in finger where they may have to role of monitoring slippage of objects across surface skin and so modulate grip
Smooth muscles in walls of arteries and pre-capillary sphincters are innervated by…
… the sympathetic nervous system
Noradrenaline acts on…
A1 adrenergic receptors on the vascular smooth muscle in the skin
- GPCRs couples to intracellular 2nd messages —-> increases intracellular Ca++ ——> constriction ——> reduced skin blood flow
- reducing SNS activation of a1 receptors therefore causes relaxation (dilation) of arteries to skin ——> increased skin blood flow
Skin blood flow is important in…
Thermoregulation and blood pressure control
Importance of sympathetic ‘vasomotor tone’
- reduced activation of a1 receptors on vascular smooth muscle ——> vascodilation
- activation of a1 receptors —-> vasoconstriction
- during heat stress and exercise there is also ‘active vascodilation’ i.e grater then can be accounted for simply by withdrawal of sympathetic tone
Body temperature regulation - normal range
Normal range = 36.5-37.5
What happens if body temp too high
40 ish - disoriented, proteins and enzymes denature
What happens if body temp too low
35 - get confused and lose the ability to thermoregulate
Primary mechanisms of heat transfer
- radiation
- evaporation
- convection
- conduction
Eccrine sweat glands are…
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 B receptors - ‘nervous sweating’ esp. on palms and soles (and axilla to some degree)
When body temperature increases
- preoptic area of hypothalamus contains heat and cole sensitive neurons (central thermoreceptors)
- if blood temp goes ABOVE ‘set point’ heat loss centre is activated
- decrease SNA activation of a1 on skin blood vessels —-> vasodilation
- increase SNS cholinergic activation of mAChRs on sweat glands ——> sweating
- increases respiratory rate
- behavioural changes
Temperature drops
- central thermoreceptors detect temperatures BELOW ‘set point’ which activates the heat gain centre
Responses to high body temperature co-orditanted be the heat loss centre
- behavioural changes
- vascodilation and shunting of the blood to skin surface
- sweat production
- respiratory heatloss
Responses to low body temperature co-ordinated by the heat-gain centre
- the heat-gain center responds to low body temperature in two ways:
INCREASED GENERATION OF BODY HEAT: - no shivering thermogenesis
- shivering thermogenesis
CONSERVATION OF BODY HEAT: - vasomotor centre decreases blood flow the dermis, thereby reducing losses by radiation and convection
- countercurrent exchange (as arteries are parallel to veins - they heat up cold blood coming in)
Heat generating mechanisms - shivering
- oscillatory contractions of agonist and antagonist muscle
- ATP —> ADP + Pi +movement + heat
Heat generating mechanisms - 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 exidative phosphorylation i.e heat produces instead of ATP occurs (occurs in ‘brown fat’ particularly in infants
Heat generating mechanisms - increases thyroxine
- in response to TRH and TSH
- increases basal metabolic rates (in adults humans may take several weeks exposure to cold before thyroid reaches new level of thyroxine secretion)
What kind of muscle are arrector pili?
Smooth
What are the smooth muscle of the arrector pili innervated by?
The SNS (a1 receptors)
What do the arrector pili muscles attach what to what?
Attach hair follicle to upper dermis
Contraction of the arrector pili muscle ..
Pulls hair upright and dimples skin —-> goose bumps
- also lubricates sebaceous glands which lubricates skin
For hairy mammals arrector pili muscles…
Traps a 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 work out fluid replacement - the total body surface area involved is needed
How to calculate the volume of lactated ringers to be infused
Potential complications of severe burns help remind us of normal skin function
- dehydration and hypovolemic shock
- infection / sepsis
- hypothermia
Severe burns can also cause dysfunction in other systems :
- electrolyte imbalances ( e.g hyperkalaemia increases K+)
- hypermetabolism
- gastrointestinal ulceration
- renal failure
- respiratory dysfunction
What receptor is very common in fingers
Tactile discs
