2022 Skin Anatomy Lecture

SKIN ANATOMY AND FUNCTION

Course Objectives

At the completion of this course, the
participants will be able to:

– Properly define the functional characteristics of the
three layers of skin.

– Describe the form and function of the following
somatosensory receptors: mechanoreceptors,
temperature receptors, and Nociceptors.

– Correctly identify the proper testing procedures
for determining the status of Mechanoreceptors,
thermoreceptors, and Nociceptors.

– Describe common skin conditions and
impairments that can affect the skin.

– Discuss the affect that the aging process has on
skin integrity.

Skin is our largest organ. If the skin of a typical 150-pound (68-
kilogram) adult male were stretched out flat, it would cover about 2
square yards (1.7 square meters) and weigh about 9 pounds (4
kilograms). It shields the body against heat, light, injury, and infection.
The skin also helps regulate body temperature, gathers sensory
information from the environment, stores water, fat, and vitaminD,
and plays an important role in the immune system protecting us from
disease.In addition to this, it has a remarkable regenerative quality.

Integument

The form that the skin takes will
depend upon its function.

– Color
– Thickness
– Texture
– Redundancy
– Integrity

To really understand the skin, we need to look at it from the
perspective of form and function. The form that the skin takes will
depend upon its function. For example, the color, thickness and
texture of the skin differ with each part of the body. Some areas of
the body have loose, redundant skin to allow for joint rangeof motion
whereas other parts are less redundant to provide stability (such as
our newtorso and scalp).

TYPES OF SKIN: HAIRY AND GLAROUS

Skin is composed of two basic types: thin and hairy (the most
common) and thickand hairless (associated with those parts of the
body that are used heavily and are exposed to activities that can
cause different forms of friction, such as our hands and feet).

Physiologically, the skin serves several functions. It regulates the
body’s temperature through blood flow and sweat. During exercise,
for example, theevaporation of sweat will lower body temperature. In
cooler environments, production of sweat will decrease and blood
flow increase to adjust body temperature.

The skin is a defensive barrier that
protects underlying tissues against
physical injury…

. microorganism
invasion and
dehydration….

and ultraviolet light
radiation.

Physiological Function

• Sensory Organ

The skin is a sensory
organ that contains specialized somatosensory receptorsthat detect
and locate stimuli related to temperature, touch, pressure, and pain.

Finally, it is a blood reserve that incorporates extensive networks of
blood vessels (between 8-10% of total blood flow in resting adults).
With moderate exercise, blood flow will increase in the dermis in an
effort to help dissipate heatfrom the body. In strenuous exercise,
though, blood vessels will constrict to circulate more blood to
contracting muscles.

Fingernails are a type
of modified skin.
Nails protect the
sensitive tips of
fingers and toes.
Human nails aren't
necessary for living,
but they do provide
support forthe tips of
the fingers and toes, protect them from injury, and aid in picking up
small objects. Without them, we'd have a hard time scratching an itch
or untying a knot. Nails can be an indicator of a person's general
health, and illness often affects their growth.

The skin is composed of three layers. Each of these layers varies in
form and function.

The outer layer is the epidermis.

It is a tough protective layer devoid ofa nerve and blood supply that is
structured to prevent loss of essential body fluids and to protect the
body against invasion of microorganisms that can be found in our
environment. Without this layer, we would dehydrate rapidly due to
water evaporating into the environment.

The thickness of the epidermis varies atdifferent areas of the skin.
The thinnest is found in the eyelids at just 0.05mm and the thickest is

1.5mm on the palms and soles of the feet. Although the epidermis is
quite thin, it is made up of five layers. The deepest layer is the
stratum basale whose role is to germinate new cells. This layer
contains sensitive touch receptors. At the stratum spinosum layer,
cells begin to attach themselves to one another more tightly. The
stratum granulosum begins the process of water proofing cells so that
the skin can become more water resistant.Here the cells begin to
degenerate and die. The stratum lucidum, normally foundonly in the
hands and soles of the feet, transform the cells to keratin. The
stratum corneum is the most superficial layer that consist of about 25-
30 layers ofdead cells completely filled with keratin. These cells are
continuously shed and replaced by cells from deeper strata. The
stratum corneum serves as an effective barrier against light and heat
waves, bacteria, and many chemicals from theenvironment.

The epidermis is made up of specialized cells. The keratinocyte is the
most prominent cell in the epidermis and is responsible for producing
keratin and creation of an extracellular water barrier. Melanocytes
comprise about 10% of thecells in the epidermis. These cells are
responsible for skin color. The Langerhan’s cell initiates cutaneous
contact hypersensitivity reactions that help white blood cells fight off
infection. Finally, the Merkel cell is located at the base of the
epidermis and is a very sensitive mechanoreceptor and is responsible
for identifying tactile sensations.

The thicker dermis lies below the epidermis. It also varies in
thickness depending on the location and the foundation of the skin,
0.1 mm on the eyelid and 4.0mm on the back. It contains nerve
ending, sweat glands, oil glands, and hair follicles. It also has an
extensive blood supply system. There are two layersof dermis: the
more superficial papillary layer and the deeper reticular layer. The
papillary layer has less densely packed collagen fibers and more
elastic fibers structured in an irregular network. This allows for better
movement and reboundagainst external forces. Nerve endings are
concentrated in this area of the dermis. The reticular layer is
considerably thicker and has more course elastic collagenous fibers
to provide greater stability. This layer contains the Ruffini endorgans,
blood vessels and sweat glands.

The hypodermis is a subcutaneous layer of aerolar (tissue that
lubricants and nourishes epithelial tissue) and adipose (fat) tissues.
The dermis will extend down into the hypodermis and anchor the skin
to it. The hypodermis, in turn, anchors to underlying tissues and
organs. The subcutaneous layer is composedof fat and connective
tissue and houses larger blood vessels and nerves. The
function is to regulate temperature of the skin and maintain the body’s
core temperature. The thickness of this layer varies widely between
people. It alsoacts as a shock absorber to protect interior structures.

The skin is an extensive sensory receptor for temperature, pain,
touch and pressure. The brain’s ability to discriminate between the
quality and type of stimulus is dependent on the specificity of the
receptor for the specific stimuli. For example, vision responds to the
stimuli of light, touch to pin prick, and proprioception of joint
movement or displacement. Specialized nerves can senseand
transmit heat, pain, and other noxious sensations. When these
receptors are not working properly, sensations such as numbness,
pins-and-needles, pain,tingling, or burning may be felt.

Spinal nerves have motor fibers and sensory fibers. The motor fibers
innervatecertain muscles, while the sensory fibers innervate certain
areas of skin. A skinarea innervated by the sensory fibers of a single
nerve root is known as a dermatome. Although slight variations do
exist, dermatome patterns of distribution are relatively consistent from
person to person.

When a stimulus is detected and an increase in stimulus intensity
reaches beyond the threshold for an action potential by a particular

receptor, that impulsetranslates to an increased rate of firing of the
afferent fiber, thus sending information to the central nervous system.
The receptors detect both stimulus and intensity. There are half a
dozen receptors in the skin which are consideredas somatosensory
receptors.

The most numerous sensory receptors are free nerve endings in the
papillary layer of the dermis. Networks of free nerve endings surround
most hair follicles attached to their outer root and are very sensitive.
The Meissner corpuscles are touch receptors, particularly found in
the papillary of hairless skin, such as the lips and palmar surfaces of
the fingers and toes. Merkels discs are touch receptors which are

located in the dermis layer over the palms of the hands and soles of
the feet. The Rufinni end organs respond to mechanical displacement
ofthe adjacent collagen fibers that pass through its receptor. The
Pacinian corpuscles are highly sensitive receptors located deep in the
dermis and joint capsules. Krause end bulbs, also called bulbous
capsules in the skin contain sensory nerve endings, which may be
mechanoreceptors, but which are also thought to be thermoreceptors
sensitive to cold and activated by temperatures less than 20 °C. They
occur more superficially in the skin than heat receptors and are rapidly
adapting and respond best to vibration rather than to prolonged
pressure.

Adaptation (or accommodation) is a characteristic property of sensory
receptors and describes a progressive decrease in firing frequencies
despite maintained depolarization. The frequency of action potentials
from stimulated receptors falls,although the stimulus is maintained at
constant strength.

Sensory receptors can be basically categorized into two types: slow
(incomplete)adapting and fast (completely) adapting. The slow
adapting fibers continue to fire for a prolonged period during the
maintained stimulus. The fast adapting fiber receptors will cease
firing after the initial stimulus has been applied.

Mechanoreceptors respond to pressure on the skin. Near the
surface are Merkel’s disks and Meissner’s corpuscles. Both have
small receptive fields but Meissner’s corpuscles are rapidly adapting
while Merkel's discs are slowly adapting mechanoreceptors. Deeper
in the skin are Ruffini’s end organs and Pacinian Corpuscles. Both
have large receptive fields. Ruffini's are slowly adapting and
Pacinian's are rapidly adapting mechanoreceptors.

Temperature receptors
include free nerve endings
of small unmyelinated
axons(sometimes called
hot fibers). Their firing
rate increases when skin
temperature is above 35
degrees Centigrade. Cold
fibers are free nerve
endingsof small myelinated
axons. They increase their
firing rate if skin
temperature falls below 35 degrees Centigrade and also when
temperature rises above 45 degrees Centigrade. The latter response
may account for paradoxical cold (e.g., shivering and goose bumps in
a hot bathtub).

Pain is caused by a stimuli (chemical, mechanical, thermal, or
electrical) of suchan intensity that it can produce tissue damage. Pain
is a high threshold sensation. The nociceptors (pain receptors) are

free nerve endings. Cutaneous pain may be sharp and localized, or
dull and diffuse. A painful stimulus causes atfirst a sharp pain,
followed by dull aching pain. Reflex withdrawal movements also
occur, along with an associated increase in heart rate and blood
pressure.

• Fast, sharp (pricking) pain is mediated by nociceptors innervated
by group A- delta thick myelinated nerve fibers which transmit
pain impulses at a velocity of20 meter/second.

• Slow chronic (dull, aching or burning) pain is mediated by
nociceptors innervated by C-fibers, thin unmyelinated nerve
fibers that conductpain at a low velocity of 1 meter/second.

Testing SomatosensoryReceptors

Patients are typically referred to occupational or physical therapy to
evaluate the status of their sensation. Being proficient at performing a
sensory test is certainlyimportant. However, knowing what
somatosensory receptors are responsible for which sensation will
help to define the intervention strategy. Assessing sensationinvolves
testing receptors for constant pressure, vibration and skin movement,
temperature and pain.

Constant Skin Pressure

• Found in all type of skin
• Slow Adapting Fibers

– Merkel Cell
– Rufinni End Organ

• Test:

– 2 Point Discrimination
– Deep Pressure

Receptors for constant skin pressure can be found on hairy skin,
located at the epidermis-dermis junction. On glabrous (or hairless)
skin, they are located atthe base of the skin ridge (the forming of a
fingerprint). Merkel cell translates mechanical energy into a nerve
impulse – telling the brain of the duration and magnitude of the
stimulus.
These responds to constant pressure and are slowly adapting. The
Ruffini end organ is a slowly adapting fiber. It sends neural signals in
response to experienceof pressure and constant touch. They are
thought to be responsible for the abilityto rate cutaneous pressure on
a scale of intensity. To test the status of these receptors, an
aesthesiometer would be used to measure static two point
discrimination.

Vibration and Skin Movement

• Found in both types of skin
• Quick Adapting Fibers
• No contribution to information about deep

pressure or constant touch

Receptors responding to Vibration and Skin Movement can be
found in bothhairy and glabrous skin. Unlike slowly adapting
mechanoreceptors, quick adapting fibers remain silent during skin
deformation. Receptors found in hairy skin are the typical hair follicle.
It is surrounded by a network of nerve terminals of large myelinated
axons. These rapidly adapting fibers are very sensitive to moving or
vibratory stimuli. They do not contribute to information about deep
pressure or constant touch.

Vibration and Skin Movement

• Pacinian Corpuscle

– Quickly Adapting Fiber

• Deep Pressure (constant 2PD)
• Vibrations around 250 Hz

• Meissner Corpuscle

– Quickly Adapting Fiber

• Light Touch (moving 2 PD)
• Vibrations around 40 Hz

In glabrous skin, the pacinian and
meissner corpuscles are the active
receptors. The pacinian corpuscle
is located in the deep dermis and
hypodermis. It is one ofthe body’s
largest afferent fibers and is a quickly adapting receptor. When provided
constant pressure, it produces and impulse only at the beginning and the
end of the stimulus. Pacinian corpuscles respond to deep pressure and to
vibrations around 250 Hz. The meissner corpuscle is located more
superficially in the papillary layer of the dermis. It responds to light touch
and to lower frequency vibrations of around 40 Hz. It too is a rapidly
adapting receptor. The status of these receptors can be tested using an
aesthesiometer to test for constant pressure, a tuning fork at 250 cps to
test vibratory sensation, or a moving 2 point discrimination test to measure
the status of the quickly adapting fibers.

Free Nerve Endings

• Responds to: • Routine Tests:

– Cold – Hot/Cold
– Warmth – Pin Prick
– Heat
– Pain – Soft Touch
– Itch

Free Nerve Endings come in different sizes and are the most
numerous of all receptors in the skin. They are very sensitive
mechanoreceptors that alert the central nervous system with a variety
of environmental events. Free nerve endings are found at the
epidermis-dermis junction and respond to stimuli interpreted as cold,
warmth, heat, pain, itch. Testing the status of free nerve endings can
be accomplished by performing tests of hot/cold, pin prick, and soft
touch.

Nociceptors are free nerve endings that respond to stimuli which pose
a threat. There is an important difference between nociception and
pain. Nociception isthe reception of afferent information from
specialized sensory receptors to the CNS to provide information
about tissue damage. An example of this would be your reaction
when you touch a hot surface, such as a hot stove or iron. Pain isa
subjective perception of an unpleasant sensation that originates from
a segment of the body.

This is a summary slide of what we have just discussed. For each of
the types ofreceptors listed on the left side of the table, namely
Mechanoreceptors, Thermoreceptors, and Nociceptors, you will find a
corresponding stimulus and receptor category listed.
Mechanoreceptors respond to pressure, such as hair movement, light
to deep pressure and touch. The specific nerves associated with
mechanoreceptors are free nerve endings, meissner and pacinian
corpuscles, Krause end bulbs and merckel’s disks. Thermoreceptors
detect coldand heat. The cold receptors are made up of unmyelinated
nerve fibers and the heat receptors are made up of myelinated nerve
fibers. Finally, nociceptors are free nerve ending found throughout our
bodies that can detect pain.

Things That Can Go Wrong With the Skin

Now that we have an idea of what comprises the skin, let’s take a
look at whatcan go wrong with the skin. We will start by looking at
conditions that can be seen on the surface of the skin.

Dermatitis
The term dermatitis refers to any inflammation (swelling, itching, and
redness) possibly associated with the skin. There are many types of
dermatitis, including:Atopic dermatitis or eczema. It's a common,
hereditary dermatitis that causesan itchy rash primarily on the face,
trunk, arms, and legs. It commonly developsin infancy, but can also
appear in early childhood. It may be associated with allergic diseases
such as asthma and seasonal, environmental, and food allergies.

Contact dermatitis. This occurs when the skin comes into contact
with an irritating substance or one that the person is allergic or
sensitive to. The best- known cause of contact dermatitis is poison
ivy, but there are many others, including chemicals found in laundry
detergent, cosmetics, and perfumes, andmetals like nickel plating on
jewelry, belt buckles, and the back of a snap.

Cellulitis is an infection of the skin and subcutaneous tissue that
typically occurswhen bacteria are introduced through a puncture, bite,
or other break in the skin. The area with cellulitis is usually warm,
tender, and has some redness.

• Streptococcal and staphylococcal infections are two kinds of
bacteria and arethe main causes of cellulitis and impetigo.
Certain types of these bacteria are also responsible for
distinctive rashes on the skin, including the rashes associated
with scarlet fever and toxic shock syndrome.

Skin cancer is rare in children and teens, but good sun protection
habitsestablished during these years can help prevent skin cancers
such as melanoma (which is a serious form of skin cancer that can
spread to other parts of the body) later in life, especially among fair-
skinned people who sunburneasily.

Impairment

• Peripheral Nervous System

– Disease or Injury
– Fractures or Lacerations
– Diabetes or Burns

• Central Nervous System

– Multiple Sclerosis
– Acquired Brain Injuries

Impairment can occur below the skin. Impairment to the sensory
system can occur in either the peripheral or central nervous systems.
Disease or injury of theperipheral nervous system can affect sensory
input at the receptor level, such as with diabetes or burns.
Furthermore, injuries can happen along the extremity that affects the
peripheral nerve and can cause sensory changes distally, as happens
with lacerations and some fractures. Also, sensation can be
interfered at the central nervous system by conditions such as
multiple sclerosis, acquired brain injury, and CVA. Being able to
correctly identify the source of the sensory impairment aids in the
proper intervention strategy.

Aging Process

• Brain’s ability to interpret sensory input
• Typical changes:

– decrease in acuity of many sensations
– decrease in average weight
– degeneration of neurons
– decrease in enzyme production
– depletion of the neuronal dendrites in thebrain

An important consideration to take into account is the influence that
the agingprocess has on the brain’s ability to interpret somatosensory
input. There areseveral physiological changes that normally
accompany the aging process.
Typical changes include decrease in acuity of many sensations,
decrease in average weight, degeneration of neurons and
replacement gliosis, decrease inthe number of enzymes responsible
for dopamine or norepinephrine synthesis,and depletion of the
neuronal dendrites in the brain.

Aging Process

• Physiological Changes

– a reduction of receptors that can pick upsensory stimuli
– decreased distance between the nodes ofRanvier
– degeneration of myelinated fibers in the spinalcord

Along with the physiological changes associated with age, there are
some anatomical structures that are affected as well. As noted here,
there is a reduction of receptors that can pick up sensory stimuli. The
decreased distancebetween the nodes of Ranvier will result in nerve
stimuli taking longer to get to the spinal cord. Additionally, the
degeneration of myelinated fibers in the spinal cord will slow impulses
as well. Overall, the following anatomical changes will affect the
speed in which the initial impulse translates to a perceived action by
the brain: reduction in conduction velocity of sensory nerves, loss of
sensory axons, reduction in number of Meissner’s corpuscles,
reduction in number of Pacinian corpuscles, decrease in distance
between nodes of Ranvier, and degeneration of some myelinated
fibers of the spinal cord.

Any changes in sensory input have a direct impact upon the

functional capabilities of an elder. Listed below are some common

functional changes as one ages. Having said this, you should keep in

mind that different individuals age differently, and any decrease in

physiology, anatomy, or function will be unique to each individual.

However, be cognizant that these are potential threatsto older adults.

slowed response to tactile stimuli slowed response to vibratory stimuli

decreased two-point discrimination decreased kinesthetic awareness

changes in pain perception accelerated loss of audio and visual

acuitypostural instability exaggerated body sway increased problems

with balancewide-based gait growing difficulty in recognizing body

positions in spaceworsening tendency to drop items in hand.

Changes in Functional Capabilities

• Slowed Response To Tactile Stimuli
• Slowed Response To Vibratory Stimuli
• Decreased Two-point Discrimination
• Decreased Kinesthetic Awareness
• Changes In Pain Perception
• Accelerated Loss Of Audio And Visual Acuity
• Postural Instability
• Exaggerated Body Sway
• Increased Problems With Balance
• Wide-based Gait
• Growing Difficulty In Recognizing Body Positions In Space
• Worsening Tendency To Drop Items In Hand

In Conclusion…

The purpose of this course was to review skin anatomy as it
relates to occupational and physical therapy practitioners. Our
patients, no matter the age,can present with a variety of diseases or
conditions that impact on skin. The modality that we use to treat our
patients will have to transfer through skin.
Applying a hot pack or paraffin treatment can be influenced by the
skin’s condition. The thinner skin found in older adults will require
more attention in order to decrease the possibility of burns. The
effectiveness of electrotherapy modalities can be influenced by the
condition of the skin. The dryer the skin, thegreater the impedance
and this may result in a more uncomfortable experience for the
patient. Finally, wound healing has a direct correlation with the nature

of the skin and its ability to proceed through the regenerative process.
Skin is morethan just a covering for our bodies. Better knowledge of
the properties of this largest organ of the body can help us in our
intervention strategies.