Lecture 3 skin physiology Flashcards
Touch receptors of the skin
Free nerve endings Tactile discs Tactile (Meissner) corpuscles Lamellar (Pacinian) corpuscles Bulbous (Ruffini) corpuscles
Free nerve endings
Most common receptor in the skin
Commonly quite superficial
Mostly unmyelinated small diameter fibres (impulses are conducted slowly like the achy pain of a burn after the initial pain) but also some small diameter myelinated fibres (faster pain like the initial burn pain)
Usually small swellings at distal ends = sensory terminals. Sensory terminals have receptors that function as cation channels which causes depolarisation which therefore causes action potentials
Respond mainly to temperature (hot and cold), painful stimuli, some movement and pressure, some to itch (i.e. in response to histamine), some wrap around hair follicles (peritrichial endings) acting as light touch receptors which detect the bending of hairs (like when a mosquito lands on your skin).
Tactile (merkel) discs
Free nerve endings located in the deepest layer of the epidermis
Associated with large disk shaped epidermal (merkel) cells - communication between the tactile epithelial cell and nerve ending possibly via serotonin (5HT) (neurotransmitter, found in the brain) - Merkel cells are cells that actually senses the stimuli that is then transducer and converted into a chemical signal that then depolarises the sensory nerve terminal and that information can be transmitted to the somatosensory cortex.
Abundant in fingertips and very small receptive fields therefore they are good for two point discrimination
Sensitive to an objects physical features - texture, shape, and edges and fine touch and light pressure (because they are quite superficial)
Tactile (meissner) corpuscles
Located in papillary layer of dermis, especially in hairless skin e.g. finger pads, lips, eye lids, external genitalia, soles of feet, nipples
Encapsulated - spiralling/branching unmyelinated sensory terminals surrounded by modified Schwann cells (schwann cells here are specialised so these cells are not producing myelin) and then by a thin oval fibrous connective tissue capsule. Deformation of capsule triggers entry of Na+ ions into nerve terminal (depolarise the nerve terminal and get an action potential up to the somatosensory cortex) which leads to an action potential.
Sense…
Delicate ‘fine’ or discriminative touch - sensitive to shape and textural changes in exploratory touch such as reading a braille text. Also movement of objects over the surfaces of the skin.
Light pressure
Low frequency vibration (2 to 80 Hz)
Lamellar (pacinian) corpuscles
Scattered deep in dermis and hypodermis
Single dendrite lying within concentric layers of collagen fibres and specialised fibroblasts (central dendrite is surrounded by many layers of collagen - looks like an onion if you cut it in half)
Layers separated by gelatinous interstitial fluid
Dendrite 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 action potentials discontinued (rapidly adapting) - when they are deformed they can depolarise but then these layers can resume their normal pressure very quickly and respond to another stimulus in a very short period of time.
Stimulated by deep pressure (when first applied). Also vibration because rapidly adapting ( optimal stimulation frequency is around 250 Hz which is similar to frequency range of generated upon fingertips by textures comprising features <1 micron
Bulbous corpuscles (Ruffini’s endings)
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 (whenever you stretch the dermis you stretch the collagen which is continuous with the collagen in the receptor)
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 they help to signal degree of joint rotation (proprioception) - high density around fingernails so may have role monitoring slippage of objects across surface of skin - allowing for modulation of grip
Precapillary sphincters
Bands of smooth muscle at the start of capillary beds
If they constrict, they will reduce the blood flow into those capillaries and it will reduce the blood flow to the skin
Skin blood flow
Smooth muscle in walls of arteries and pre-capillary sphincters innervated by the sympathetic nervous system (SNS)
Noradrenaline acts on ⍺1 adrengenic receptors on this vascular smooth muscle in the skin
To reduce skin blood flow…
Sympathetic nerves release noradrenaline and to have an effect it must bind to some sort of receptor and on these smooth muscle walled blood vessels these receptors are called ⍺1 adrenergic receptors, not ion channels they are instead G protein coupled receptors (GPCRs) when activated, an ion channel doesn’t just open and close but rather a second messenger is produced which causes intracellular effects and in this case the increase in calcium which allows for more actin and myosin interactions which will cause contraction (vasoconstriction) which will decrease the amount of blood flow to the skin
GPCRs coupled to intracellular 2nd messengers - increased intracellular calcium - constriction = reduced skin blood flow
To increase skin blood flow…
Reducing sympathetic nervous system activity therefore causes relaxation (dilation) arteries to the skin and this increases blood flow to the skin
Normal human body temperature
36.5-37.5 ℃ is the normal range of body temperature
Body temperature too high…..body temperature too low
Too high - death, proteins denature, convulsions and cell damage
Too low - disorientation, loss of muscle control, loss of consciousness, cardiac arrest, death
Primary mechanisms of heat transfer
Radiation, evapouration, convection, conduction
Radiation
Causes heat loss in the form of infrared rays. Nay objects that are not at an absolute zero temperature will radiate such rays. For a person with no clothes on sitting inside at normal room temperature, about 60% of their total heat loss would be via radiation. As well as the body radiating heat in all directions, heat rays are also being radiated from the walls of rooms and other objects towards the body. Provided the temperature of the body is greater than that of the surroundings then more heat will be radiated from the body than to it. In very hot environments however the opposite may apply.
Evaporation
When water evaporates from the body surface the heat energy required to cause the water to evaporate is also lost. Even if a person is not sweating water will still evaporate without you noticing from the skin and the respiratory tract. Evapouration is particularly important in situations when the environment temperature is greater than body temperature. In these circumstances the body will GAIN heat by radiation and conduction/convection and so sweating becomes the only mechanism by which the body can rid itself of the heat.
Convection
Convection firstly involves the transfer of heat to air (or water) by conduction followed by the movement of the air (or water) away from the skin which maintains the gradient for heat loss from the body. A small amount of convection almost always occurs around the body because of the tendency for air adjacent to the skin to rise as it becomes heated. In windy conditions the layer of air immediately adjacent to the skin is replaced by new air much more rapidly than usual and so convective heat loss increases accordingly - wind chill factor
Conduction
Involves the transfer of heat to objects or media with which we are in contact. Normally a relatively small amount of hear is transferred to solid objects such as a chair but a significant amount is conducted into air. Once the temperature of the air adjacent to the skin becomes equal to the temperature of the skin then hear will cease to be lost in this manner. Therefore conduction of heat from the body to the air is self limited UNLESS the heated air moves away from the skin to be replaced by cooler air which allows heat loss to continue and this is called air convection
Heat loss to air vs water
Water can absorb far greater quantities of heat. Heat conductivity in water is very great in comparison with air. Consequently the body loses heat to water faster than to air and it is virtually impossible for the body to heat a thin layer of water next to the skin to form an ‘insulating zone’ as occurs in air
Eccrine sweat glands - how are they controlled?
Innervated by the sympathetic nervous system. Sympathetic cholinergic release ACh onto mAChRs (GPCRs)
Some eccrine sweat glands can also be stimulated by adrenaline (unrelated to temperature) in blood acting on beta receptors - ‘nervous sweating’ especially on palms and soles (and axilla to some degree)
When body temperature increases ….
Preoptic area of hypothalamus contains heat and cold sensitive neurons (central thermoreceptors)
If blood temperature goes ABOVE set point then the heat loss centre is activated.
Decrease in sympathetic nervous system activation of ⍺1 on skin blood vessels which causes vasodilation - relaxes the precapillary sphincters therefore more blood will go to the surface and provided the air temperature is less than our body temperature we will lose heat by conduction, convection and radiation
Increase in sympathetic nervous system cholinergic activation of mAChRs on sweat glands which causes sweating - evaporate water and lose the heat
Increase in respiratory rate - more air flowing across tongue which is wet and air flow up and down the trachea which is also going to increase evaporative losses such as dogs panting
Behavioural changes - feel hot then you move to somewhere cooler or get a cold drink for example
When are radiation, conduction and convection not effective heat loss mechanisms…?
Not effective when the environmental temperature is greater than body temperature
When body temperature falls…
Central thermoreceptors detect temperature BELOW ‘set point’ which activates the heat gain centre
Heat gain centre responds to low temperature in two ways:
Increased generation of body heat - shivering (shiver via contracting muscle which generates heat) and non shivering (start producing heat through hormones for example) thermogenesis
Conservation of body heat - Vasomotor centre decreases blood flow to the dermis and thereby reduces losses by radiation and convection.
Countercurrent exchange - beneficial to transfer heat from arteries to veins to conserve heat…warm blood in the arteries instead of going to the skin and loosing that heat to the environment it can be transferred to the veins that are bringing cold blood back to the core
Heat generating mechanisms
Shivering
Non-shivering thermogenesis
Increase in thyroxine
Heat generating mechanisms - shivering
Increased tone of skeletal muscle
When tone rises above critical level, shivering begins due to oscillatory contractions of agonist and antagonist muscles mediated by muscle spindles (stretch receptors)
Heat generating mechanisms -non-shivering thermogenesis
Increased sympathetic nerve activity and increase in the circulating adrenaline and noradrenaline from the adrenal medulla
Increased cellular metabolism e.g. increased glycogenolysis in the liver and muscle which provides some heat
‘uncoupling’ of oxidative phosphorylation i.e. heat produced instead of ATP (occurs in brown fat, particularly in infants - good blood supply and packed full of mitochondria)
Heat generating mechanisms - increased thyroxine
In response to an increase in 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 pili muscles
Smooth muscles innervated by the SNS (⍺ receptors)
Attach hair follicle to the upper dermis
Contraction pulls hairs upright and dimples skin therefore you get goosebumps
Also compresses sebaceous glands 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 feedforward
In cold weather, muscle contracts, makes hair more vertical
What does getting goosebumps do?
Traps air and creates an insulating area around the body
Physiological feedforward
For example, walking out on a cold day in a singlet, you shiver straight away even though your temperature has not had time to change yet and you start to produce heat by shivering before your body temperature has even changed
