Type II sensory fiber (group Aβ) is a
type of sensory fiber
An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, th ...
, the second of the two main groups of touch
receptors
Receptor may refer to:
*Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse
*Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a n ...
. The responses of different type Aβ fibers to these stimuli can be subdivided based on their adaptation properties, traditionally into rapidly adapting (RA) or slowly adapting (SA) neurons. Type II sensory fibers are slowly-adapting (SA), meaning that even when there is no change in touch, they keep respond to stimuli and fire action potentials. In the body, Type II sensory fibers belong to pseudounipolar neurons. The most notable example are neurons with
Merkel cell-neurite complexes on their dendrites (sense static touch) and
Ruffini endings (sense stretch on the skin and over-extension inside joints). Under pathological conditions they may become hyper-excitable leading to stimuli that would usually elicit sensations of tactile touch causing pain.
These changes are in part induced by
PGE2
Prostaglandin E2 (PGE2), also known as dinoprostone, is a naturally occurring prostaglandin with oxytocic properties that is used as a medication. Dinoprostone is used in labor induction, bleeding after delivery, termination of pregnancy, and ...
which is produced by
COX1
Cytochrome c oxidase I (COX1) also known as mitochondrially encoded cytochrome c oxidase I (MT-CO1) is a protein that in humans is encoded by the ''MT-CO1'' gene. In other eukaryotes, the gene is called ''COX1'', ''CO1'', or ''COI''. Cytochro ...
, and type II fibers with
free nerve ending
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 cu ...
s are likely to be the subdivision of fibers that carry out this function.
Type II sensory fiber (group Aα) is another type of sensory fiber, which participate in the sensation of body position (proprioception). In each muscle, we have 10-100 tiny muscle-like pockets called muscle spindles. The type II fibers (aka secondary fibers) connect to
nuclear chain fiber
A nuclear chain fiber is a specialized sensory organ contained within a muscle. Nuclear chain fibers are intrafusal fibers that, along with nuclear bag fibers, make up the muscle spindle responsible for the detection of changes in muscle len ...
s and static nuclear bag fibers in muscle spindles, but not to dynamic nuclear bag fibers. The typical innervation to muscle spindles consists of one type Ia fiber and 2 type II fibers. The type Ia fiber has "anulospiral" endings around the middle parts of the intrafusal fibers compared to type II fibers that have "flower spray" endings which may be spray shaped or annular, spreading in narrow bands on both sides of the chain or bag fiber.
It is thought that the Ia fibers signal the degree of change in muscle movement, and the type II fibers signal the length of the muscle (which is later used for forming the perception of the body in space).
References
Sensory systems
{{biology-stub