Main Model
Anterior : Krause's corpuscle
Sensory Receptors of the Skin
Sensory receptors are specialized neurons and epithelial-like cells that receive and convert a physical
stimulus into an electrical signal transmitted to the
central nervous system.
There are three general categories of sensory
receptors:
1. Exteroceptors, that provide information about
the external environment.
2. Proprioceptors, located in muscles (such as
the muscle spindle), tendons, and joint capsules and
provide information about the position and movement of the body.
3. Interoceptors, that provide sensory information
from the internal organs of the body.
The classification of sensory receptors of the skin
is based on the type of stimulus:
1. Mechanoreceptors respond to mechanical
deformation of the tissue or the receptor itself (for example, stretch, vibration, pressure, and touch).
There are four primary mechanoreceptors in human skin:
1. Merkel disk.
2. Meissner corpuscle.
3. Ruffini ending.
4. Pacinian corpuscle.
The first two are located at the epidermal-dermal
junction; the other two are located in the deep dermis
and hypodermis.
Meissner corpuscle, Ruffini ending, pacinian
corpuscle, and Krause end bulb are encapsulated receptors. Krause end bulb is a thermoreceptor found
only in specialized regions.
The nerve ending of the Merkel disk mechanoreceptor discriminates fine touch and forms a flattened
discoid structure attached to the Merkel cell. Merkel
cells are found in the stratum basale of the epidermis
of the fingertips and lips.
The Meissner corpuscle, or tactile corpuscle, is
found in the upper dermis, bulging into the epidermis. They are found primarily in the
fingertips and eyelids. This receptor is well suited for the detection of shape and texture during active touch.
The Ruffini ending, or bulbous corpuscle, is
located in the deep dermis. It detects skin stretch and deformations within joints. They also provide
feedback when gripping objects and controlling finger
position and movement (for example, when we use
the computer keyboard).
The pacinian corpuscle is found in the deep dermis and hypodermis. It responds to stimuli of deep,
transient pressure and high-frequency vibration. They
are found in the bone periosteum, joint capsules,
pancreas, breast, and genitals.
2. Thermoreceptors respond to a temperature
stimulus, either warmth or cold. The Krause end bulb is encapsulated but is not a mechanoreceptor.
It is a thermoreceptor that detects cold. Krause end
bulbs are found in the conjunctiva of the eye, in the
mucosa of the lips and tongue, and in the epineurium
of nerves. They are also found in the penis and the
clitoris (hence, the name of genital corpuscle).
3. Nociceptors respond to pain stimuli. The
simplest form of a pain detector are the free nerve endings. They derive from the dermal nerve plexus,
supplied by cutaneous branches of the spinal nerves.
Sensory nerve fibers extending toward the skin surface, shed their myelin sheaths before branching as
naked axons between collagen fibers, forming dermal
nerve endings, or within the epidermis, as epidermal
nerve endings.
The very sensitive peritrichial nerve endings are
wrapped around the hair follicle just under the sebaceous glands. The myelinated portions of the nerve
endings form a palisade of naked terminals along the external root sheath of the hair follicle, surrounded by
circumferential terminals. Peritrichial nerve endings
are stimulated when the hairs bend.
The perception of pain is associated to acute inflammation, one of the classical responses to tissue injury. Injured cells release chemical mediators, including substance P, acting on local blood vessels and
nerve endings. Substance P triggers the degranulation
of mast cells, histamine in particular, that enhances vascular dilation and plasma leakage, thereby causing edema in the injury surrounding area.
Hyperemia accounts for the triple response of
Lewis when a line is made on the skin with a pointed
object: flush (capillary dilation), flare (redness spreading because of arteriolar dilation), and wheal (localized edema). The triple response develops between 1
to 3 minutes.
In summary, the nociceptive receptors (the pain
detectors) are found near the skin surface. Merkel
disks and Meissner corpuscles, fine mechanoreceptor, are located at the epidermal-dermal junction so
they can detect gentle touch. Pacinian corpuscles and
Ruffini endings, the large encapsulated mechanoreceptors, are found in the deep dermis and hypodermis
and respond to transient deeper touch.