1.1.5.1　Nerves

There are abundant nerve fibers in the skin，which can be divided into sensory nerves and motor nerves.By contacting with the central nervous system，various stimuli can be felt，which can control the activities of target organs and complete various types of nerve reflexes.The innervation of the skin is segmental，but there is a partial overlap between adjacent segments.Nerve fibers are mostly distributed in the dermis and subcutaneous tissues.

1.Sensory nerves

The sensory nerves of the skin are extremely complex，and the rich sensory nerve endings are mainly distributed under the epidermis and around the hair follicles.Sensory nerves can be divided into nerve bodies and free nerve endings.The free nerve endings are distributed in small branches.Nerve bodies are divided into cystic bodies and non-cystic bodies（e.g.，Merkel cell-axon complex）.The cystic bodies are composed of nerve endings wrapped with connective tissue，including：

①Tactile receptors，also known as Meissner bodies，are oval.They are distributed in the dermal papillary layer，most commonly seen in the skin of the fingertips of children.

②Pain receptors is located in the epidermis with simple structure.The myelinated nerve fiber loses the nerve membrane after entering the epidermis，and branches into a mesh or small spherical shape，scattered in the gaps of epidermal cells.

③Temperature sensors are round，oval，or fusiform，with a thin layer of connective tissue envelope.After the sensory nerve endings enter the envelope，it develops into many small branches and coils into a spherical shape.Those which receive cold sensations are spherical bodies，also known as Krause balls，located in the superficial dermis；those which receive heat sensations are fusiform bodies，also known as Ruffini balls，located in the deep dermis.(https://www.daowen.com)

④Pressure receptors，also known as Pacini bodies，are in concentric circles，and their cut surfaces can have a ring-layer structure，much like an onion，so they are also called ringlayer corpuscles.Their diameters are 0.5 mm or more.They are located in the deeper part of the dermis and subcutaneous tissue.

Sensory nerves act as receptors alone or with cystic bodies to feel touch，pain，itching，temperature，and mechanical stimulation.

2.Motor nerves

The motor nerves in the skin are distributed in small branches，derived from the fibers of the sympathetic ganglia.Adrenergic nerve fibers innervate the myoepithelial cells of the erectile muscles，blood vessels，glomus，apocrine sweat glands，small glands，and sebaceous glands basement membranes，and play roles of vasoconstriction，apocrine sweat gland secretion，erector muscle contraction or myoepithelial contraction.Cholinergic nerve fibers innervate small gland secretory cells.The facial striated muscle is innervated by the facial nerve.