Skin Physiology

Question 1

What are free nerve endings in skin?

Answer 1

Mostly unmyelinated small diameter fibres but also some small diameter myelinated fibres usually small swellings at distal ends = sensory terminals

Question 2

What are sensory terminals?

Answer 2

Sensory terminals have receptors that can respond to various painful (nociceptive), thermal and chemical stimuli
Some are cation channels (e.g. TRPV1), others are chemically activated (e.g. histamine receptors)

Question 3

What does activation of sensory terminals lead to?

Answer 3

Activation leads to APs in afferent sensory axons to CNS&raquo_space; somatosensory cortex

Question 4

What do free nerve endings respond to?

Answer 4

Temperature, painful stimuli, movement, pressure and itch
They can also wrap around hair follicles acting as light touch receptors which detect bending of hairs

Question 5

How do free nerve endings detect changes?

Answer 5

Force is transmitted to free nerve endings when hair follicles wrap around them causing depolarisation which can detect change

Question 6

What are tactile (merkel) discs?

Answer 6

Free nerve endings located in the deepest layer of the epidermis
Associated with large disc shaped epidermal cells
Abundant in fingertips and very small receptive fields

Question 7

What are tactile (meissner) corpuscles?

Answer 7

Located in papillary layer of dermis (especially in hairless skin)
Encapsulated by thin oval fibrous connective capsule which covers spiralling unmyelinated sensory terminals surrounded by modified Schwann cells

Question 8

What are tactile (merkel) discs sensitive to and how do they sense?

Answer 8

Sensitive to an objects physical features (fine touch and light pressure, texture, shape and edges)
They detect stimulus and relay messages to sensory nerve terminal causing repolarisation

Question 9

What are tactile (meissner) corpuscles sensitive to and how do they sense?

Answer 9

Deformation of capsule triggers entry of Na+ ions into nerve terminal creating an AP (depolarisation)
Sensitive to delicate or discriminative touch, light pressure and low frequency vibration

Question 10

What are lamellar (pacinian) corpuscles?

Answer 10

Found deep in dermis and hypodermis, single dendrite lying within concentric layers of collagen and specialised fibroblasts
Layers separated by gelatinous interstitial fluid
Dendrite isolated from stimuli other than deep pressure

Question 11

What are lamellar (pacinian) discs sensitive to and how do they sense?

Answer 11

Deformation of capsule opens pressure sensitive Na+ channels in sensory axon - inner layers covering axon terminal relax so APs discontinued
Stimulated by deep pressure and vibration due to being rapidly adapting

Question 12

What are Bulbous corpuscles (Ruffini’s endings)?

Answer 12

Located in dermis and subcutaneous tissue
Network of nerve endings intertwined with a core of collagen fibres that are continuous with those of the surround dermis. Capsule surrounds entire structure
Also found in joint capsules which help signal degree of joint rotation

Question 13

What are Bulbous corpuscles (Ruffini’s endings) sensitive to and why is their sensitivity important?

Answer 13

Sensitive to sustained deep pressure and stretching or distortion of the skin
Important for signaling continuous states of deformation of tissues such as heavy prolonged touch and pressure signals

Question 14

Where does skin blood flow occur?

Answer 14

Smooth muscle in walls of arteries and pre-capillary sphincters innervated by the sympathetic nervous system

Question 15

How does reduced skin blood flow occur?

Answer 15

Noradrenaline acts on alpha 1 adrenergic receptors on this vascular smooth muscle in the skin
GPCRs (G protein coupled receptors) couple to intracellular 2nd messengers -> increased intracellular Ca2+ -> constriction

Question 16

What happens with SNS activation of alpha1 receptors is decreased?

Answer 16

Reducing SNS activation of alpha 1 receptors will causes relaxation (dilation) of arteries to skin resulting in increased skin bloodflow

Question 17

What is vasodilation?

Answer 17

Reduced activation of alpha1 receptors on vascular smooth muscle

Question 18

What is vasoconstriction?

Answer 18

Activation of alpha1 receptors

Question 19

What is active vasodilation?

Answer 19

During heat stress and exercise active vasodilation can occur meaning greater pressure can be accounted for simply by withdrawal of sympathetic tone

Question 20

What is normal core body temp for humans?

Answer 20

Core body temperature means temperature of blood around head, heart, trunk etc. (not in limbs) and normal ranges 36.5 - 37.5

Question 21

What are the four primary mechanisms of heat transfer?

Answer 21

Radiation
Evaporation
Convection
Conduction

Question 22

How are eccrine sweat glands stimulated?

Answer 22

By the SNS which releases ACh onto mAChRs (GPCRs) through sympathetic cholinergic
(some can be stimulated by adrenaline in blood acting on beta receptors)

Question 23

What happens when body temperature increases?

Answer 23

Preoptic area of hypothalamus contains heat and cold sensitive neurons
If blood temp goes above set point heat loss centre is activated

Question 24

What happens when heat loss centre is activated?

Answer 24

Decrease in SNS activation of alpha1 on skin blood vessels -> vasodilation
Increase in SNS cholinergic activation of mAChRs on sweat glands -> sweating
Increased respiratory rate
Behavioural changes

Question 25

How is the heat gain centre activated?

Answer 25

Central thermoreceptors detect temperature below ‘set point’ which activates the heat gain centre
Convec. conduc. and radiation are NOT effective heat loss mechanisms when environment temp > body temp

Question 26

How else can we conserve heat apart from shivering and central thermoreceptors?

Answer 26

Vasomotor center decreases blood flow in the dermis, thereby reducing losses by radiation and convection
Countercurrent exchange : warm blood from trunk -> cool blood to distal capillaries -> cool blood returns to trunk -> warm blood returns to trunk

Question 27

How does shivering gain or conserve heat?

Answer 27

Oscillatory contractions of agonist and antagonist muscle cause ATP production which produces ADP + Pi + movement + heat

Question 28

How does non-shivering gain or conserve heat?

Answer 28

Increase sympathetic nerve activity and increase circulating adrenaline/noradrenaline from adrenal medulla
Increased cellular metabolism (increased glycogenolysis in liver and muscles)
Uncoupling of oxidative phosphorylation (heat produced instead of ATP)

Question 29

How does increased thyroxine gain or conserve heat?

Answer 29

Occurs in response to TRH and TSH
Increases basal metabolic rate

Question 30

What are arrector pili muscles?

Answer 30

Smooth muscle supplied by SNS (alpha1 receptors). Attach hair follicles to upper dermis. Compresses sebaceous glans which lubricate skin
Conserves layers of heat (for animals makes look bigger ‘scarier)

Question 31

Why are second and third degree burns worse than first and how much fluid do you give a patient who has burns?

Answer 31

Second or third degree burns lose water repelling part of skin and makes capillaries leaky so needs to be rehydrated with fluid (IV)
9% head, 9% each upper limb, 36% trunk (front and back), 1% genitalia and 18% each lower limb (different for children)

Question 32

What are some complications of severe burns?

Answer 32

Dehydration and hypovolemic shock, Infection/sepsis, Hypothermia, Electrolyte imbalance, Hypermetabolism, Gastrointestinal ulceration, Renal failure, Respiratory dysfunction