A free nerve ending (FNE) or bare nerve ending, is an unspecialized,
afferent nerve fiber
Afferent nerve fibers are axons (nerve fibers) of sensory neurons that carry sensory information from sensory receptors to the central nervous system. Many afferent projections ''arrive'' at a particular brain region.
In the peripheral nerv ...
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 receptor potentials. This process is called sensory transduc ...
. ''Afferent'' in this case means bringing information from the body's periphery toward the brain. They function as
cutaneous nociceptor
A nociceptor (; ) 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 sensation of pain to direct attention to the body part, ...
s and are essentially used by
vertebrates
Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain.
The vertebrates make up the subphylum Vertebra ...
to detect noxious stimuli that often result in
pain
Pain is a distressing feeling often caused by intense or damaging Stimulus (physiology), stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sense, sensory and emotional experience associated with, or res ...
.
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). Nerves have historically been considered the basic units of the peripheral nervous system. A nerve provides a common pathway for the Electrochemistry, electrochemical nerv ...
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) ''A ...
. 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.
Modality
Free nerve endings can detect temperature, mechanical stimuli (touch, pressure, stretch) or danger (
nociception
In physiology, nociception , also nocioception; ) 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 organism to receive a pai ...
). Thus, different free nerve endings work as
thermoreceptors,
cutaneous mechanoreceptors and
nociceptors. 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.
Classification
The term "free nerve endings" dates back to the 1890s, originally characterized by the absence of other cellular structures at their terminals. Observations that led to this characterization were limited by the relatively low resolution of
light microscopes. Advances in non-optical imaging such as
electron microscopy
An electron microscope is a microscope that uses a beam of electrons as a source of illumination. It uses electron optics that are analogous to the glass lenses of an optical light microscope to control the electron beam, for instance focusing i ...
allowed for higher resolution examination of free nerve endings, enabling the finding that they are most often bundles of axons surrounded by a
Remak bundle rather than truly unenclosed. Such discoveries have led to the proposal of more accurate terminology such as "fine nerve endings" or "non-
corpuscular afferent 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.
{{Authority control
Somatosensory system