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 fibers a ...

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 cell ...

. ''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 ...

nociceptor 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 ...

s 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, ...

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 e ...

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 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 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, ...

, and fiber types.

Rate of adaptation

Different types of FNE can be

rapidly 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, ...

intermediate 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, ...

, 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 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 A thermoreceptor is a non-specialised sense receptor, or more accurately the receptive portion of a sensory neuron, that codes absolute and relative changes in temperature, primarily within the innocuous range. In the mammalian peripheral nervous s ...

cutaneous mechanoreceptors 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, ...

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 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. {{Authority control Somatosensory system