LECTURE 3: skin physiology

Question 1

types of skin receptors

Answer 1

1. free nerve endings
2. tactile discs
3. tactile corpuscles
4. lamellar corpuscles
5. bulbous corpuscles

Question 2

what do free nerve endings respond to

Answer 2

1. temp
2. painful stimuli
3. some movement and pressure
4. some to itch
5. some wrap around hair follicles

Question 3

what are free nerve endings

Answer 3

most common receptor in skin

Mostly unmyelinated small diameter fibres but also some small diameter myelinated fibres

Usually have small swellings at distal ends = sensory terminals

Question 4

how free nerve endings work

Answer 4

Sensory Terminals have receptors that function as cation channels&raquo_space; depolarization&raquo_space; APs

Question 5

what are tactile discs

Answer 5

Free nerve endings located in deepest layer of epidermis

- associated with large disc shaped epidermal (merkel) cells

Question 6

what do tactile discs respond to

Answer 6

Sensitive to an objects physical features: texture, shape and edges + Fine touch and light pressure

Question 7

how to tactile discs work

Answer 7

Communication between the tactile epithelial cell and nerve ending possibly via serotonin (5HT)

Question 8

where can you find tactile discs

Answer 8

Abundant in fingertips and very small receptive fields. • So good for two point discrimination

Question 9

where are tactile corpuscles located

Answer 9

located in papillary layer of dermis, especially in hairless skin e.g. finger pads, lips, eyelids, external genitalia, soles of feet, nipples

Question 10

where are lamellar corpuscles located

Answer 10

Scattered deep in dermis and hypodermis

Question 11

what are lamellar corpuscles

Answer 11

Single dendrite lying within concentric layers of collagen fibres and specialised fibroblasts

Layers separated by gelatinous interstitial fluid – Dendrite essentially isolated from stimuli other than deep pressure

Question 12

how lamellar corpuscles work

Answer 12

Deformation of capsule opens pressure sensitive Na+ channels in sensory axon: Inner layers covering axon terminal ‘relax’ quickly so APs discontinued (rapidly adapting)

Question 13

what stimulates lamellar corpuscles?

Answer 13

Pressure (when first applied)

Also vibration because rapidly adapting: Optimal stimulation frequency is around 250Hz which is similar to frequency range of generated upon fingertips by textures comprising features < 1 μM

Question 14

where are bulbous corpuscles located

Answer 14

in dermis and subcutaneous tissue

Also found in joint capsules where help signal degree of joint rotation (proprioception): High density around fingernails so may have role monitoring slipage of objects across surface skin – Allowing modulation of grip

Question 15

what are bulbous corpuscles

Answer 15

Network of nerve endings intertwined with a core of collagen fibres that are continuous with those of the surrounding dermis.

Capsule surrounds entire structure

Question 16

what are bulbous corpuscles sensitive to

Answer 16

sustained deep pressure and stretching or distortion of the skin

Question 17

what are bulbous corpuscles important for

Answer 17

Important for signalling continuous states of deformation of the tissues such as heavy prolonged touch and pressure signals

Question 18

what are encapsulated corpuscles

Answer 18

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&raquo_space; Action Potential

Question 19

what does deformation of encapsulated corpuscles do

Answer 19

triggers entry of Na+ ions into nerve terminal&raquo_space; Action Potential

Question 20

what do tactile corpuscles sense

Answer 20

• Delicate ‘fine’ or discriminative touch:
  Sensitive to shape and textural changes in exploratory touch e.g. reading Braille text. Movement of objects over the surface of the skin
• light pressure
• low frequency vibrations (2-80 Hz)

Question 21

how skin blood flow can be controlled

Answer 21

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

• Noradrenaline acts on α1 adrenergic receptors on this vascular smooth muscle in the skin: GPCRs coupled to intracellular 2nd messengers&raquo_space; increased intracellular Ca++&raquo_space; constriction = Reduced skin blood flow
• Reducing SNS activity therefore causes relaxation (dilation) arteries to skin è Increased skin blood flow

Question 22

structure and function of eccrine sweat glands and their role in thermoregulation

Answer 22

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 β receptors – ‘nervous sweating’ esp. on palms and soles (and axilla to some degree)

Question 23

4 basic mechanisms of heat transfer

Answer 23

1. radiation
2. evaporation
3. convection
4. conduction

Question 24

non-effective heat loss mechanisms when when environmental temp ≥ body temp.

Answer 24

Radiation, conduction and convection

Question 25

preoptic area in hypothalamus has

Answer 25

heat and cold sensitive neurons (central thermo receptors)

Question 26

what happens when blood temp goes ABOVE set point

Answer 26

heat loss centre activated

Question 27

vasodilation

Answer 27

decreased SNS activation of alpha1 on skin blood vessels

Question 28

sweating

Answer 28

increased SNS cholinergic activation of mAChRs on sweat glands

Question 29

what happens to respiratory rate when temp is increased

Answer 29

increases

Question 30

what factors are effected by increase in body temp

Answer 30

vasodilation sweating increased respiratory rate

Question 31

heat gain centre activated by

Answer 31

Central Thermoreceptors detect temperature BELOW ‘set point’ = activation

Question 32

how heat gain centre responds to low body temp

Answer 32

increased generation of body heat | conservation of body heat

Question 33

heat generating mechanisms

Answer 33

1. shivering 2. non shivering thermogenesis 3. increased thyroxine

Question 34

how to conserve body heat

Answer 34

vasomotor centre decreases blood flow to the dermis, thereby reducing losses by radiation and convection

Question 35

shivering

Answer 35

- Increased tone of skeletal muscles - When tone rises above critical level, shivering begins due to oscillatory contractions of agonist and antagonist muscles mediated by muscle spindles (stretch receptors

Question 36

non shivering thermogenesis

Answer 36

- 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 oxidative phosphorylation i.e. heat produced instead of ATP (occurs in ‘brown fat’ particularly in infants)

Question 37

increased thyroxine

Answer 37

- In response to 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

Question 38

arrector pilli muscles

Answer 38

- Smooth muscle innervated by SNS (α1 receptors) - Attach hair follicle to upper dermis - Contraction pulls hairs upright and dimples skin > goosebumps - Also compresses sebaceous glans which lubricates skin

Question 39

first degree burn

Answer 39

- Superficial i.e only involve the outer layers of the epidermis - Red/pink, dry, painful - Usually no blisters e.g. a mild sunburn - Skin remains a water and bacterial barrier - Usually heals 3-10 day

Question 40

second degree burn

Answer 40

- Epidermis + varying amounts of dermis - Painful, moist, red and blistered. - Usually heal in approx 1-2 weeks - Need good dressings (absorptive initially)

Question 41

deeper second degree burns

Answer 41

- May include whiteish, waxy looking areas - Hair follicles, sweat glands may remain intact - Some tactile receptors may be lost - Usually heal in 1 month but may have some loss of sensation and scarring

Question 42

third degree burn

Answer 42

- Full Thickness i.e. extend into subcutaneous tissue and may involve muscle and bone - Varied colour from waxy white through to deep red or black - Hard dry and leathery - No pain in these areas as sensory nerve endings destroyed - If more than a few cms may require skin grafting - Weeks to regenerate + scarring

Question 43

head % of TBSA in adult and children

Answer 43

A= 9% | C=15%

Question 44

upper limb % of TBSA

Answer 44

``` A = 9% each C = 9% each ```

Question 45

trunk % of TBSA

Answer 45

``` A = 36% (front and back) C = 32% (front and back) ```

Question 46

genitalia % of TBSA

Answer 46

``` A = 1% C = 1% ```

Question 47

lower limb % of TBSA

Answer 47

``` A = 18% each C = 17% each ```

Question 48

complications of severe burns for normal skin function

Answer 48

1. dehydration and hypovolemic shock 2. infection / sepsis 3. hypothermia

Question 49

what is normal body temp

Answer 49

around 37 degrees is set point | 36.5-37.5

Question 50

what happens if temp too low

Answer 50

``` lose ability to thermoregulate disorientation loss of muscle control loss of consciousness cardiac arrest death ```

Question 51

what happens if body temp too high

Answer 51

``` lose ability to thermoregulate death cell damage convulsions proteins denature ```