A free nerve ending (FNE) or bare nerve ending, is an unspecialized,
afferent nerve fiber sending its signal to a
sensory neuron. ''Afferent'' in this case means bringing information from the body's periphery toward the brain. They function as
cutaneous
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 animal coverings, such as the arthropod exoskeleton, have different ...
nociceptors and are essentially used by
vertebrates to detect noxious stimuli that often result in
pain
Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, ...
.
Structure
Free nerve endings are unencapsulated and have no complex sensory structures. They are the most common type of
nerve
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 ...
ending, and are most frequently found in 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 ...
. They penetrate the dermis and end in the
stratum granulosum. FNEs infiltrate the middle layers of the dermis and surround hair follicles.
Types
Free nerve endings have different rates of adaptation,
stimulus modalities, and
fiber types.
Rate of adaptation
Different types of FNE can be
rapidly adapting,
intermediate adapting, or
slowly adapting.
A delta type II fibers are fast-adapting while A delta type I and C fibers are slowly adapting.
[Rolf-Detlef Treede, Richard A.Meyer, Srinivasa N.Raja, James N.Campbell. Evidence for two different heat transduction mechanisms in nociceptive primary afferents innervating monkey skin. J Physiol 1995;483:747-758]
Modality
Free nerve endings can detect temperature, mechanical stimuli (touch, pressure, stretch) or danger (
nociception). Thus, different free nerve endings work as
thermoreceptors,
cutaneous mechanoreceptors and
nociceptors. In other words, they express
polymodality Stimulus modality, also called sensory modality, is one aspect of a stimulus or what is perceived after a stimulus. For example, the temperature modality is registered after heat or cold stimulate a receptor. Some sensory modalities include: light, ...
.
Fiber types
The majority of
Aδ (A delta) fibers (group III) and
C (group IV) fibers end as free nerve endings.
References
External links
*
Nociception: Transduction From the University of Utah.
*
*
' Copenhagen Medical Publishers. 1999 - 2000
*
from Dr. Daley of North Carolina Wesleyan College.
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Somatosensory system