Type II sensory fiber
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Type II sensory fiber (group Aβ) is a type of sensory fiber, the second of the two main groups of stretch receptors. They are non-adapting, meaning that even when there is no change in muscle length, they keep responding to stimuli. In the body, Type II fibers are the second most highly myelinated fibers. Type II sensory neurons are pseudounipolar and their reside in ganglia either in the dorsal horn or the brainstem.
The muscle's instantaneous length, or position, is directly proportional to their firing rate. This information would indicate the position of one's leg once it has stopped moving. They do not respond to rate of length changes as do the Ia fibers.
Type II fibers connect to nuclear chain fibers 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 I fiber and 2 type II fibers. These connections, referred to as "flower spray endings" due to their appearance, embed into the poles (ends) of the fibre. It is thought that the relative position of the equatorial regions of the spray when stretched determines the action potential output.
Type II fibers assist in the transmission of somatosensory information as well as nociceptive information. In normal physiological conditions they transmit tactile touch, the responses of different type II fibers to these stimuli can be subdivided based on their adaptation properties, traditionally into rapidly adapting (RA) or slowly adapting (SA) neurons. Type II RA neurone endings can take the form of Meissner's corpuscles, Pacinian corpuscles, or Lanceolate Endings, whereas type II SA neurone endings are Merkel cell-neurite complexes or Ruffini endings. 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 which is produced by COX1, and type II fibers with free nerve endings are likely to be the subdivision of fibers that carry out this function.
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