Integumentary System including the Somatic Senses

The skin is a large organ responsible for maintaining homeostasis through temperature regulation, protection of underlying tissue, retardation of water loss, housing sensory receptors, synthesizing certain chemicals, and excreting wastes

Regulation of Body Temperature

  • The skin plays a major role in temperature regulation with the hypothalamus controlling it.
  • Active cells, such as those of the heart and skeletal muscle, produce heat.
  • Heat may be lost to the surroundings from the skin through radiation.
  • The body responds to excessive heat by dilation of dermal blood vessels and sweating.
  • The body responds to excessive cooling by constricting dermal blood vessels, inactivating sweat glands, and shivering.

Anatomy: The skin consists of an outer epidermis and a dermis, connected to underlying tissue by the subcutaneous layer (hypodermis).

Epidermis

  • The epidermis is made up of stratified squamous epithelium and lacks blood vessels.
  • The layer of reproducing cells (the stratum basale), which lies at the base of the epidermis, is well-nourished by dermal blood vessels.
  • Cells are pushed outward as new cells are formed, and become keratinized as they die.
  • Four or five layers may be seen: stratum basale, stratum spinosum, stratum granulosum, and stratum corneum are always present and the stratum lucidum is found in the thicker palms and soles.
  • The epidermis is important because it protects against water loss, mechanical injury, chemicals, and microorganisms.
  • Melanocytes, which lie deep in the epidermis and underlying dermis, produce a pigment called melanin that protects deeper cells from the sun’s ultraviolet rays.

Skin Color

  • Skin color results from a combination of genetic, environmental, and physiological factors.
  • Genetic differences in skin color result from differing amounts of melanin and in the size of melanin granules.
  • Exposure to sunlight causes darkening of skin as melanin production increases.
  • Circulation within dermal blood vessels affects skin color.

Dermis

  • The dermis binds the epidermis to underlying tissues.
  • Epidermal ridges and dermal papillae cause the border to be uneven.
  • The dermis consists of connective tissue with collagen and elastic fibers within a gel-like ground substance.
  • Dermal blood vessels carry nutrients to upper layers of skin and help to regulate temperature.
  • The dermis also contains nerve fibers, sensory fibers, hair follicles, sebaceous glands, and sweat glands.

Subcutaneous Layer

  • The subcutaneous layer (hypodermis) is composed of loose connective tissue and insulating adipose tissue.
  • It binds the skin to underlying organs and contains the blood vessels that supply the skin.

Accessory Organs of the Skin

Nails: protective coverings over the ends of fingers and toes.

  • Nails consist of stratified squamous epithelial cells overlying the nail bed, with the lunula as the most actively growing region of the nail root.
  • As new cells are produced, older ones are pushed outward and become keratinized.

Hair: can be found in nearly all regions of the skin.

  • Individual hairs develop from cells at the base of the hair follicle, an invagination of the lower epidermis that dips down into the dermis.
  • As new cells are formed, old cells are pushed outward and become keratinized, and die forming the hair shaft.
  • A bundle of smooth muscle cells, called the arrectorpili muscle, attaches to each hair follicle. These muscles cause goose bumps when cold or frightened.
  • Hair color is determined by genetics; melanin from melanocytes is responsible for most hair colors. Dark hair has eumelanin while blonde and red hair have pheomelanin.

Sebaceous Glands(oil glands) are associated with hair follicles and secrete sebum that waterproofs and moisturizes the hair shafts.

Sudoriferous glands (sweat glands)

  • Include two types:
  1. eccrine - respond to body temperature
  2. apocrine - respond to body temperature, stress, and sexual arousal
  • Modified sweat glands secrete wax in the ear canal.
  • Mammary glands, another modified type of sweat glands, secrete milk.

Skin as a Sensory Receptor: Sensory receptors detect changes in the environment and stimulate neurons to send nerve impulses to the brain.

Types of Receptors (in general - not exclusive to skin)

Five general types of receptors are recognized.

  1. Receptors sensitive to changes in chemical concentration are called chemoreceptors.
  2. Pain receptors detect tissue damage.
  3. Thermoreceptors respond to temperature differences.
  4. Mechanoreceptors respond to changes in pressure or movement.
  5. Photoreceptors (rods and cones in the retina of the eyes) respond to light energy.

Somatic Senses: Receptors associated with the skin, muscles, joints, and viscera make up the somatic senses.

Touch and Pressure Senses: Three types of receptors detect touch and pressure.

  1. Free ends of sensory nerve fibers in the epithelial tissues are associated with touch and pressure.
  2. Meissner's corpuscles are flattened connective tissue sheaths surrounding two or more nerve fibers and are abundant in hairless areas that are very sensitive to touch, like the lips.
  3. Pacinian corpuscles are large structures of connective tissue and cells that resemble an onion. They function to detect deep pressure.

Temperature Senses

  • Temperature receptors include two groups of free nerve endings: heat receptors and cold receptors which both work best within a range of temperatures.
  • Both heat and cold receptors adapt quickly.
  • Temperatures near 45o C stimulate pain receptors; temperatures below 10o C also stimulate pain receptors and produce a freezing sensation.

Sense of Pain

  • Pain receptors consist of free nerve endings that are stimulated when tissues are damaged, and adapt little, if at all.
  • Many stimuli affect pain receptors such as chemicals and oxygen deprivation.
  • Visceral pain receptors are the only receptors in the viscera (organs within a cavity like the abdominal cavity) that produce sensations.
  • Referred pain occurs because of the common nerve pathways leading from skin and internal organs.

Pain Nerve Fibers

  • Fibers conducting pain impulses away from their source are either acute pain fibers or chronic pain fibers.
  • Acute pain fibers are thin, myelinated fibers that carry impulses rapidly and cease when the stimulus stops.
  • Chronic pain fibers are thin, unmyelinated fibers that conduct impulses slowly and continue sending impulses after the stimulus stops.
  • Pain impulses are processed in the gray matter of the dorsal horn of the spinal cord.
  • Pain impulses are conducted to the thalamus, hypothalamus, and cerebral cortex.

Regulation of Pain Impulses

  • A person becomes aware of pain when impulses reach the thalamus, but the cerebral cortex judges the intensity and location of the pain.
  • Other areas of the brain regulate the flow of pain impulses from the spinal cord and can trigger the release of enkephalins and serotonin, which inhibit the release of pain impulses in the spinal cord.
  • Endorphins released in the brain provide natural pain control.

Healing of Wounds and Burns

  • Inflammation, in which blood vessels dilate and become more permeable, causing tissues to become red and swollen, is the body's normal response to injury.
  • Superficial cuts are filled in by reproducing epithelial cells.
  • Deeper cuts are closed off by clots, covered by scabs, and eventually filled in by fibroblasts, making connective tissue.
  • Blood vessels extend into the area, injured tissues are replaced, and the scab falls off.
  • Large wounds leave scars and healing may be accompanied by the formation of granulations.

Imbalances of the Skin: Infections and Allergies

  • Athlete’s Foot: itchy fungal infection of the toes
  • Boils and Carbuncles: inflammation of hair follicles and sebaceous glands
  • Cold Sores: fever blisters; small fluid filled blisters that itch an sting, caused by herpes simplex infection
  • Contact dermatitis: itching, redness, and swelling of the skin, progressing to blistering; caused by exposure of the skin to chemicals like poison ivy that provoke allergic responses
  • Impetigo: Pink, water-filled raised lesions that develop a yellow crust and eventually rupture; caused by a highly contagious staphylococcus infection
  • Psoriasis: Chronic condition characterized by reddened epidermal lesions covered with dry, silvery scales

Imbalances of the Skin: Other Skin Disorders

  • Jaundice: An abnormal yellow skin tone usually indicates a liver disorder in which excess bile pigments are absorbed into the blood, circulated throughout the body, and deposited in body tissues.
  • Decubitus ulcer: Restriction of blood supply to the skin results in cell death, and if severe or prolonged, ulcers.
  • Bruises: Reveal sites where blood has escaped from the circulation and has clotted in the tissue spaces
  • Hematoma: clotted blood mass

Burns: Tissue damage and cell death caused by heat, electricity, UV radiation or chemicals

  • When the skin is burned, two life threatening problem result

1st threat: Dehydration

  • The body loses fluids containing proteins and electrolytes
  • This can lead to a shutdown of the kidneys and circulatory shock

2nd threat: Infection

  • Leading cause of death
  • Burned skin is sterile for about 24 hours, but after that pathogens easily invade and multiply rapidly

Rule of Nines: Divides the body area into 11 areas, each accounting for 9 percent of the total body surface, plus 1% surrounding the genitals

  • Degrees of Burns
  • 1st degree: Only the epidermis is damaged; sunburn
  • 2nd degree: Involve injury to the epidermis and the upper region of the dermis; blisters appear
  • 3rd degree: Destroy the entire thickness of the skin
  • Nerve endings are destroyed so the burn is not painful
  • Regeneration is not possible
  • Skin grafting must be done to cover the underlying exposed tissues
  • Considered critical if any of the following conditions exists:
  1. Over 25 % of the body has 2nd degree burns
  2. Over 10% of the body has 3rd degree burns
  3. Any third-degree burn of the face, hands, or feet

Skin Cancer

  • Most skin tumors are benign and do not spread
  • Some are malignant (cancerous) and tend to invade other parts of the body
  • Skin cancer is the most common type of cancer in the body
  • Types of Skin Cancer
  • Basal Cell Carcinoma
  • Least malignant and most common
  • Full cure rate in 99% of patients
  • Squamous Cell Carcinoma
  • Believed to be sun-induced
  • If it is caught early and removed surgically, good chance of complete cure
  • Malignant Melanoma
  • Cancer of the melanocytes
  • Accounts for 5% of skin cancers
  • Often deadly
  • Usually appears as a spreading brown to black patch that metastasizes rapidly to surrounding lymph and blood vessels
  • ABCD Rule
  • Asymmetry: the two sides of the pigmented spot of mole do not match
  • Border Irregularity: The borders of the lesion are not smooth but exhibit indentations
  • Color: the pigmented spot contains areas of different colors
  • Diameter: the spot is larger than 6 mm in diameter (size of pencil eraser)