A free nerve ending (FNE) or bare nerve ending, is an unspecialized,
afferent nerve fiber
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 fiber ...
sending its signal to a
sensory neuron
Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The ...
. ''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 d ...
nociceptors and are essentially used by
vertebrates
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
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 animal coverings, such as the arthropod exoskeleton, have different ...
. They penetrate the dermis and end in the
stratum granulosum
The stratum granulosum (or granular layer) is a thin layer of cells in the epidermis lying above the stratum spinosum and below the stratum corneum ( stratum lucidum on the soles and palms).James, William; Berger, Timothy; Elston, Dirk (2005) '' ...
. 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 mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, a ...
.
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
Nociception (also nocioception, from Latin ''nocere'' 'to harm or hurt') is the Somatosensory system, sensory nervous system's process of encoding Noxious stimulus, noxious stimuli. It deals with a series of events and processes required for an org ...
). Thus, different free nerve endings work as
thermoreceptors,
cutaneous mechanoreceptors and
nociceptors
A nociceptor ("pain receptor" from Latin ''nocere'' 'to harm or hurt') is a sensory neuron that responds to damaging or potentially damaging stimuli by sending "possible threat" signals to the spinal cord and the brain. The brain creates the sens ...
. In other words, they express
polymodality.
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