Type II sensory fiber

Type II sensory fiber (group Aβ) is a type of sensory fiber, the second of the two main groups of touch receptors. 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.[1] 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.[2] 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.[3] 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.[3][4]

Type II sensory fiber (group Aα) is another type of sensory fiber, which participate in the sensation of body position (proprioception).[5] 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 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 Ia fiber and 2 type II fibers.[6] 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.[7] 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
  1. Olson W, Dong P, Fleming M, Luo W (May 1, 2016). "The specification and wiring of mammalian cutaneous low-threshold mechanoreceptors". Wiley Interdisciplinary Reviews: Developmental Biology. 5 (3): 389–404. doi:10.1002/wdev.229. PMC 4864430. PMID 26992078.
  2. de Moraes ER, Kushmerick C, Naves LA (October 2017). "Morphological and functional diversity of first-order somatosensory neurons". Biophysical Reviews. 9 (5): 847–856. doi:10.1007/s12551-017-0321-3. PMC 5662056. PMID 28889335.
  3. Sun W, Yang F, Wang Y, Fu H, Yang Y, Li CL, Wang XL, Lin Q, Chen J (February 2017). "Contribution of large-sized primary sensory neuronal sensitization to mechanical allodynia by upregulation of hyperpolarization-activated cyclic nucleotide gated channels via cyclooxygenase 1 cascade". Neuropharmacology. 113 (Pt A): 217–230. doi:10.1016/j.neuropharm.2016.10.012. PMID 27743933.
  4. Arcourt A, Gorham L, Dhandapani R, Prato V, Taberner FJ, Wende H, Gangadharan V, Birchmeier C, Heppenstall PA, Lechner SG (January 2017). "Touch Receptor-Derived Sensory Information Alleviates Acute Pain Signaling and Fine-Tunes Nociceptive Reflex Coordination". Neuron. 93 (1): 179–193. doi:10.1016/j.neuron.2016.11.027. PMID 27989460.
  5. Niessen MH, Veeger DH, Koppe PA, Konijnenbelt MH, van Dieën J, Janssen TW (February 2008). "Proprioception of the shoulder after stroke". Archives of Physical Medicine and Rehabilitation. 89 (2): 333–8. doi:10.1016/j.apmr.2007.08.157. PMID 18226659.
  6. Poliak S, Norovich AL, Yamagata M, Sanes JR, Jessell TM (January 2016). "Muscle-type Identity of Proprioceptors Specified by Spatially Restricted Signals from Limb Mesenchyme". Cell. 164 (3): 512–25. doi:10.1016/j.cell.2015.12.049. PMC 4733250. PMID 26824659.
  7. Mancall, Elliott L; Brock, David G, eds. (2011). "Chapter 2 - Overview of the Microstructure of the Nervous System". Gray’s Clinical Neuroanatomy: The Anatomic Basis for Clinical Neuroscience. Elsevier Saunders. pp. 29–30. ISBN 978-1-4160-4705-6.


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