Innervation and Vascularization

Innervation of Skin

The dermisThe thick inner layer of the skin that contains blood vessels, nerves, and connective tissue. of the skinThe body’s largest organ, providing protection and regulation. is where the innervation is.  Recall that the epidermisThe outermost layer of the skin, made of stratified squamous epithelium. is both avascularTo be devoid of blood capillaries. Epithelial tissue is avascular, kind of like a cap of dead cells and not innervated.  The dermis provides the nutrients and gases in blood. It also provides any type of sensation or control.  Most nerves in the skin are for sensation or incoming information.  When I say incoming, I mean incoming to the brain. 

Free nerve endings in skin are abundant.  These type of nerve ending can sense temperature and pain (mostly).  These nerve ending are exactly what they sound like, just blunt ends of a nerve. I have two of them here in purple and one in yellow.  Note how one is located more superficially than the other.  This purple one here is in a dermal papilla.  Because of their placement, these two free nerve endingsUnencapsulated receptors that detect pain, temperature, and crude touch. might sense different things.  For example, the one in the dermal papilla might try to sense the temperature of the epidermal cellsThe basic structural and functional units of life.. It attempts this to understand the external environment.  The yellow, deeper free nerve ending might sense pain from a puncture wound that has breached the dermis.  Location determines function here.

The other, most abundant sensors in the dermis are collectively called mechanoreceptorsReceptors that respond to mechanical stimuli such as touch or pressure..  These are nerves with what are called encapsulated endingsSensory receptors enclosed in connective tissue capsules; include tactile and lamellar corpuscles. or endings that are structures specialized for a particular sensation.  For example, there might be pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. sensors located in the stratum basaleThe deepest layer of the epidermis, where new skin cells are formed. of the epidermis and calibrated for light touch.  Pressure sensors located more deeply, like those near the hypodermisThe layer beneath the dermis, composed of connective and adipose tissue, providing insulation and cu, are positioned to sense stronger sensations of pressure.  Other mechanoreceptors called proprioceptorsSensory receptors that monitor the position and movement of the body. can also detect the positioning of the hands to determine the next movements.  People who play instruments practice their instrument to coordinate their hands with their brains. This helps them play the music correctly.

Subpapillary Plexus

The word plexus just means network.  We are going to see many capillary plexuses. These are networks of tiny blood vessels. We will also see many nerve plexuses in this class.  It is always important for is to realize how a tissue or organ is vascularizedTo invade a tissue with blood capillaries. and how it is innervated.  How it receives nutrients and how it is controlled. 

The dermis of skin has two networks or plexuses of blood vessels.  The subpapillary plexusA network of blood capillaries that vascularize the papillary layer of the dermis of skin. Loops ext is the more superficialNear the surface of the body. of the two plexuses.  It runs just under the border of the epidermis and the dermis. It has loops that extend up into the dermal papillaeUpward extensions of the dermis that interlock with the epidermis, forming fingerprints..  Therefore, this plexus is the one supplying, via diffusionPassive movement of molecules from areas of high to low concentration., the alive cells of the epidermis.  As the epidermal cells get further and further from this plexus, they die.  This plexus can be damaged in first degree burns and is definitely damaged in second degree burns.  First and second degree burns heal. This is because the capillary plexus that is deeper remains. It supplies oxygen and nutrients to fuel the repair of the burn.

Cutaneous Plexus

The deeper of the two plexuses is called the cutaneous plexusA network of blood capillaries that vascularizes the hypodermis/dermis border of skin..  This plexus lies along the border of the reticular layerhe deeper portion of the dermis, containing collagen and elastin fibers for skin strength. of the dermis and the hypodermis.  Remember that the tops of your feet and backs of your hands have little or no hypodermis. 

Oxygen and nutrients from the cutaneous plexus diffuse superficially through the reticular layer of the dermis. They supply the cells such as fibroblasts and macrophages.  Notice how the subpapillary is connected to the cutaneous plexus. The artery in red and the vein in blue are slightly larger than the vessels of the subpapillary plexus.  Yes, this is because the subpapillary plexus is kinda like an extension of the cutaneous.  The blood in the subpapillary plexus comes from and is drained to the cutaneous plexus. 

The cutaneous plexus is definitely damaged in a 3^rd degree burn.  This is also the reason these burns are called full thickness burns. They burn all layers of skin. They remove all viable blood vessels.  Oxygen and nutrients cannot reach the burn to repair the area. Therefore, a 3^rd degree burn has little to no chance of healing.  Take a moment to consider dermal circulationThe vascularization of the dermis of skin. Two capillary plexuses vascularize the dermis: subpapilla. Think about how crucial these two plexuses are to the cells and tissues of the skin.  Blood circulation in the capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues. of these two plexuses is essential. It keeps the dermis as a living layer of skin.  It would be avascularized and dead, much like the epidermis.  The hard crunchy candy shell of many insects is somewhat analogous to this.  They don’t have skin under the shell.  They just have shell and then muscle.

Wound Healing

Would healing in the skin is remarkably similar to wound healing in bone, which we discuss in a future chapter.  With any tissue, the first step of a wound is to stop the bleeding, which is called hemostasis.  This requires that some fibrinAn insoluble protein that forms the mesh of blood clots. to precipitate out of the blood and form a blood clot to stop the bleeding. 

At the same time that hemostasis is happening, inflammation is beginning.  I know that we think of inflammation as a bad thing, but it is not.  It is a very beneficial process, but it causes redness, heat, swelling, and pain, which alarms most people.  The causes of these signsObjective clinical findings observable by a provider (e.g., edema, fever). and the last symptom are from immune cells coming to the area.  The blood vessel near the wound will dilate, allowing cells called neutrophils and macrophages to come to the area.  This causes the swelling.  The neutrophils kill any infectious invaders and the macrophages clean up the mess.  This causes the heat and the redness.  Pain is a symptom, not a sign. Signs can be seen by the medical provider. Symptoms are only reported by the patient. Pain results from the swollen area pressing on free nerve endings. These endings send pain sensations to the brain. 

Up to this point, everything has been like the response of EMTs. Just stabilize the area. This means to ensure that it’s clean and there are no invaders present.  Once the neutrophils and macrophages are done cleansing the area, this is when fibroblasts starts to come in.  These are the cells that secrete fibers for connective tissues –so these secrete collagenA structural protein in the dermis that provides strength and elasticity., elastin, and reticular fibers.  Not only will these cells start making the fibers, bit they also proliferate or multiply.  Take a look at this picture on the bottom left.  This blood clot is now just a scab on the epidermis. The dermal tissue below has turned to granulation tissue. This is a fancy name for early scar tissue.  As the fibroblasts lower in number, official scar tissue forms in the area.  This scar tissue will be remodeled over and over again over the years after the wound.  Eventually, the epidermis will turn over enough times to make the scar less evident.