Cutaneous innervation refers to the area of the

skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...

which is supplied by a specific

cutaneous nerve A cutaneous nerve is a nerve that provides nerve supply to the skin. Human anatomy In human anatomy, cutaneous nerves are primarily responsible for providing sensory innervation to the skin. In addition to sympathetic and autonomic afferent (sens ...

Dermatome Dermatome may refer to: * Dermatome (anatomy), an area of skin that is supplied by a single pair of dorsal roots * Dermatome (embryology), the portion of the embryonic paraxial mesoderm, the somite, which gives rise to dermis * Dermatome (instrument ...

s are similar; however, a dermatome only specifies the area served by a

spinal nerve A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into th ...

. In some cases, the dermatome is less specific (when a spinal nerve is the source for more than one cutaneous nerve), and in other cases it is more specific (when a cutaneous nerve is derived from multiple spinal nerves.) Modern texts are in agreement about which areas of the skin are served by which nerves, but there are minor variations in some of the details. The borders designated by the diagrams in the 1918 edition of

Gray's Anatomy ''Gray's Anatomy'' is a reference book of human anatomy written by Henry Gray, illustrated by Henry Vandyke Carter, and first published in London in 1858. It has gone through multiple revised editions and the current edition, the 42nd (Octob ...

are similar, but not identical, to those generally accepted today.

Importance of the peripheral nervous system

The

peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system of bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside the brain ...

(PNS) is divided into the

somatic nervous system The somatic nervous system (SNS), or voluntary nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles. The somatic nervous system consists of sensory nerves carryi ...

, the

autonomic nervous system The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies viscera, internal organs, smooth muscle and glands. The autonomic nervous system is a control ...

, and the

enteric nervous system The enteric nervous system (ENS) or intrinsic nervous system is one of the main divisions of the autonomic nervous system (ANS) and consists of a mesh-like system of neurons that governs the function of the gastrointestinal tract. It is capable of ...

. However, it is the somatic nervous system, responsible for body movement and the reception of external stimuli, which allows one to understand how cutaneous innervation is made possible by the action of specific

sensory fibers A sensory nerve, or afferent nerve, is a general anatomic term for a nerve which contains predominantly somatic afferent nerve fibers. Afferent nerve fibers in a sensory nerve carry sensory information toward the central nervous system (CNS) from ...

located on the

, as well as the distinct pathways they take to the central nervous system. The skin, which is part of the

integumentary system The integumentary system is the set of organs forming the outermost layer of an animal's body. It comprises the skin and its appendages, which act as a physical barrier between the external environment and the internal environment that it serves ...

, plays an important role in the somatic nervous system because it contains a range of

nerve endings A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

that react to heat and cold, touch, pressure, vibration, and tissue injury.

Importance of the central nervous system

The

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...

(CNS) works with the peripheral nervous system in cutaneous innervation. The CNS is responsible for processing the information it receives from the

cutaneous nerves A cutaneous nerve is a nerve that provides nerve supply to the skin. Human anatomy In human anatomy, cutaneous nerves are primarily responsible for providing sensory innervation to the skin. In addition to sympathetic and autonomic afferent (sens ...

that detect a given stimulus, and then identifying the kind of sensory inputs which project to a specific region of the

primary somatosensory cortex In neuroanatomy, the primary somatosensory cortex is located in the postcentral gyrus of the brain's parietal lobe, and is part of the somatosensory system. It was initially defined from surface stimulation studies of Wilder Penfield, and paralle ...

The role of nerve endings on the surface of the skin

Groups of nerve terminals located in the different layers of the

are categorized depending on whether the skin is hairy, nonhairy, or an exposed mucous membrane.Weddell, G., and Miller, S. (1962) Cutaneous Sensibility. Annual Reviews 24:199-222

Hairy skin

The hairy parts of the body such as the forearm or the leg have two groups of nerve endings: those that end along with the

hair follicles The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between h ...

, and also with the arborizations of unmyelinated axons which are referred to as “free” nerve endings because they are served by both

myelinated Myelin is a lipid-rich material that surrounds nerve cell axons (the nervous system's "wires") to insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the axon. The myelinated axon can be l ...

and unmyelinated axons.

Nonhairy skin

Nonhairy skin (glabrous), such as the palms of hands and the soles of feet, has three types of nerve terminations. The first one,

Meissner’s corpuscles Tactile corpuscles or Meissner's corpuscles are a type of mechanoreceptor discovered by anatomy, anatomist Georg Meissner (1829–1905) and Rudolf Wagner. This corpuscle is a type of nerve ending in the skin that is responsible for sensitivity to ...

are encapsulated nerve endings attached to the

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...

in the dermal papilli that detect changes in texture and vibrations.

Merkel's discs Merkel nerve endings are mechanoreceptors, a type of sensory receptor, that are found in the basal epidermis and hair follicles. They are nerve endings and provide information on mechanical pressure, position, and deep static touch features, su ...

are arborizations of nonmyelinated axons that end in terminals on specialized

tactile Tactile may refer to: * Tactile, related to the sense of touch * Haptics (disambiguation) * Tactile (device), a text-to-braille translation device See also * Tangibility, in law * Somatosensory system, where sensations are processed * CD96 CD9 ...

cells and which detect sustained touch and pressure. Lastly, there are also “free” nerve endings which are similar in structure to those in hairy skin, though they are more numerous.

Exposed mucous membranes

The exposed

mucous membranes A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It is ...

of the lips, the anal mucous membrane, and the external genital organs form the most densely innervated parts of the body. Though there is no specific categorization, both “free” nerve endings and unencapsulated nerve endings of myelinated axons are found within the

dermis The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided i ...

of those areas. The

cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power ...

, one of the other exposed mucous membranes, contains “free” nerve endings served by nonmyelinated axons. The

conjunctiva The conjunctiva is a thin mucous membrane that lines the inside of the eyelids and covers the sclera (the white of the eye). It is composed of non-keratinized, stratified squamous epithelium with goblet cells, stratified columnar epithelium ...

contains a less dense distribution of “free” nerve endings that are served by both myelinated and unmyelinated axons.

Distribution of sensory neurons

The distribution of the sensory neurons within the skin accounts for the large and overlapping receptive fields of the skin. The size of the receptive fields in turn explains why almost any given stimulus to the human skin can potentially activate a very large number of nerve terminals. Therefore, it is more likely that a stimulus caused by the prick of a needle be detected by more than a hundred nerve endings all sharing the same receptive field, than for that same needle prick to be detected by only one nerve ending.

Types of sensory neurons

The different kinds of sensory stimuli that are picked up by sensory neurons are grouped into two categories: epicritic and protopathic. Epicritic neurons detect gentle touch such as caresses; light vibrations; the ability to recognize the shape of an object being held; and two-point discrimination, or the spacing of two points being touched simultaneously. Protopathic neurons are responsible for detecting pain, itch, tickle, and temperature. The different types of stimuli that are detected by a given receptor allow for a relative specificity between stimuli and receptor.

Pathways to the CNS

The sensory modality that is detected by the

afferent fibers Afferent nerve fibers are the axons (nerve fibers) carried by a sensory nerve that relay sensory information from sensory receptors to regions of the brain. Afferent projections ''arrive'' at a particular brain region. Efferent nerve fibers a ...

is an important factor to consider because it determines the pathway that the

dorsal root ganglion A dorsal root ganglion (or spinal ganglion; also known as a posterior root ganglion) is a cluster of neurons (a ganglion) in a dorsal root of a spinal nerve. The cell bodies of sensory neurons known as first-order neurons are located in the dorsa ...

neurons will take within the central nervous system. The sensory neurons coming from the body synapse in the dorsal horn of the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...

, bringing in information about touch sensations (epicritic), or modalities of pain (protopathic). While both types of sensory neurons must first synapse in the dorsal horn of the spinal cord, the area of the dorsal horn where they synapse is different. Their pathway to the thalamus is also different. Neurons that carry information about touch, vibration, and proprioception sensations from the lower body enter the spinal cord below spinal level T6, where they synapse in the dorsal horn to form reflex circuits, but also send axon branches through the gracile fascicle to the brainstem. Similarly, information from the upper body enters the spinal cord at level T6 and above, and ascend toward the brainstem in the

Cuneate fasciculus Cuneate means "wedge-shaped", and can apply to: * Cuneate leaf, a leaf shape * Cuneate nucleus, a part of the brainstem * Cuneate fasciculus Cuneate means "wedge-shaped", and can apply to: * Cuneate leaf The following is a list of terms which ...

. Together the gracile and cuneate form the dorsal column in the spine. Neurons that carry information about pain and temperature synapse in the dorsal horn at the anterolateral fascicles. While the neurons for

touch In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position (proprioception), and pain. It is ...

sensations ascend ipsilaterally through the

posterior column-medial lemniscus pathway Posterior may refer to: * Posterior (anatomy), the end of an organism opposite to its head ** Buttocks, as a euphemism * Posterior horn (disambiguation) * Posterior probability The posterior probability is a type of conditional probability that r ...

to the thalamus; neurons for pain and temperature ascend contralaterally to the thalamus through the

anterolateral system The spinothalamic tract is a part of the anterolateral system or the ventrolateral system, a sensory pathway to the thalamus. From the ventral posterolateral nucleus in the thalamus, sensory information is relayed upward to the somatosensory cor ...

. When both sensory pathways reach the integrating center that is the thalamus, they make their final ascent to the

somatosensory In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position (proprioception), and pain. It is ...

areas in the

postcentral gyrus In neuroanatomy, the postcentral gyrus is a prominent gyrus in the lateral parietal lobe of the human brain. It is the location of the primary somatosensory cortex, the main sensory receptive area for the somatosensory system, sense of touch. Lik ...

of the

cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting of ...

References

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External links

Overview at luc.edu
Peripheral nervous system Skin physiology