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Intrafusal muscle fiber
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<h2 id="structure">Structure</h2> <h3>Types</h3> <p>There are two types of intrafusal muscle fibers: <a href="/facts/Nuclear_bag_fiber/uM8G32HN">nuclear bag fibers</a> and <a href="/facts/Nuclear_chain_fiber/kHNrTtlI">nuclear chain fibers</a>.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> They bear two types of sensory ending, known as annulospiral and flower-spray endings. Both ends of these fibers contract, but the central region only stretches and does not contract. </p><p>Intrafusal muscle fibers are walled off from the rest of the muscle by an outer <a href="/facts/Connective_tissue/mal3fc7k">connective tissue</a> sheath consisting of flattened <a href="/facts/Fibroblasts/n08KsFll">fibroblasts</a> and <a href="/facts/Collagen/KsyAb72K">collagen</a>.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> This sheath has a spindle or "fusiform" shape, hence the name "intrafusal".<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> </p> <h3>Innervation</h3> <p>They are innervated by <a href="/facts/Gamma_motor_neuron/gDPSWmBu">gamma motor neurons</a> and <a href="/facts/Beta_motor_neuron/UaTJIaxX">beta motor neurons</a>.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> Gamma efferents from small <a href="/facts/Multipolar_neuron/PN5BwTrM">multipolar neurons</a> from anterior gray column innervate it. These form a part of <a href="/facts/Muscle_spindle/KFgIC8Xe">neuromuscular spindles</a>. </p> <h2 id="function">Function</h2> <p>Intrafusal muscle fibers detect the amount and rate of change in muscle length.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> It is by the sensory information from gamma motor neurons and beta motor neurons that an individual is able to judge the position of their muscles.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Alpha_motor_neuron/9GMtJNtQ">Alpha motor neuron</a></li> <li><a href="/facts/Beta_motor_neuron/UaTJIaxX">Beta motor neuron</a></li> <li><a href="/facts/Extrafusal_muscle_fiber/oUnoxkwv">Extrafusal muscle fiber</a></li> <li><a href="/facts/Gamma_motor_neuron/gDPSWmBu">Gamma motor neuron</a></li> <li><a href="/facts/Type_Ia_sensory_fiber/YYCQtdbC">Type Ia sensory fiber</a></li> <li><a href="/facts/Type_II_sensory_fiber/LGhwu3lJ">Type II sensory fiber</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="http://meded.ucsd.edu/hist-img-bank/chapter_3/Slide_60_artery_vein/pages/b.2.60.2.5.htm">Histology at ucsd.edu</a> <a href="https://web.archive.org/web/20200918225045/https://meded.ucsd.edu/hist-img-bank/chapter_3/Slide_60_artery_vein/pages/b.2.60.2.5.htm">Archived</a> September 18, 2020, at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a></li> <li><a href="https://web.archive.org/web/20070224231721/http://www3.umdnj.edu/histsweb/lab6/skeletalmuscle/skmotorend.html">Histology at umdnj.edu</a></li> <li>"Chapter 1: The Muscle Spindle and the Central Nervous System". <a href="https://web.archive.org/web/20131203055643/http://www.advtherapy.net/html/book_01.pdf"><i>Neuromuscular Reeducation with Electromyometric Feedback</i></a> (PDF). Advanced Therapy Institute. Archived from <a href="http://www.advtherapy.net/html/book_01.pdf">the original</a> (PDF) on December 3, 2013. Retrieved November 30, 2013.</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Casagrand, Janet (2008) Action and Movement: Spinal Control of Motor Units and Spinal Reflexes. University of Colorado, Boulder. <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Falkel, LORI (January 1, 2007), Cooper, Cynthia (ed.), "Chapter 4 - Tissue-Specific Exercises for the Upper Extremity", Fundamentals of Hand Therapy, Saint Louis: Mosby, pp. 53–72, doi:10.1016/b0-32-303386-5/50007-2, ISBN 978-0-323-03386-2, retrieved December 21, 2020 <a href="978-0-323-03386-2" target="_blank">978-0-323-03386-2</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Casagrand, Janet (2008) Action and Movement: Spinal Control of Motor Units and Spinal Reflexes. University of Colorado, Boulder. <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Falkel, LORI (January 1, 2007), Cooper, Cynthia (ed.), "Chapter 4 - Tissue-Specific Exercises for the Upper Extremity", Fundamentals of Hand Therapy, Saint Louis: Mosby, pp. 53–72, doi:10.1016/b0-32-303386-5/50007-2, ISBN 978-0-323-03386-2, retrieved December 21, 2020 <a href="978-0-323-03386-2" target="_blank">978-0-323-03386-2</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Burke, David (January 1, 2014), "Fusimotor System", Reference Module in Biomedical Sciences, Elsevier, doi:10.1016/b978-0-12-801238-3.00249-x, ISBN 978-0-12-801238-3, retrieved December 21, 2020 <a href="978-0-12-801238-3" target="_blank">978-0-12-801238-3</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Johns, Paul (January 1, 2014), Johns, Paul (ed.), "Chapter 4 - Sensory and motor pathways", Clinical Neuroscience, Churchill Livingstone, pp. 49–59, doi:10.1016/b978-0-443-10321-6.00004-7, ISBN 978-0-443-10321-6, retrieved December 21, 2020 <a href="978-0-443-10321-6" target="_blank">978-0-443-10321-6</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Burke, David (January 1, 2014), "Fusimotor System", Reference Module in Biomedical Sciences, Elsevier, doi:10.1016/b978-0-12-801238-3.00249-x, ISBN 978-0-12-801238-3, retrieved December 21, 2020 <a href="978-0-12-801238-3" target="_blank">978-0-12-801238-3</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>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. <a href="978-1-4160-4705-6" target="_blank">978-1-4160-4705-6</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Falkel, LORI (January 1, 2007), Cooper, Cynthia (ed.), "Chapter 4 - Tissue-Specific Exercises for the Upper Extremity", Fundamentals of Hand Therapy, Saint Louis: Mosby, pp. 53–72, doi:10.1016/b0-32-303386-5/50007-2, ISBN 978-0-323-03386-2, retrieved December 21, 2020 <a href="978-0-323-03386-2" target="_blank">978-0-323-03386-2</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Burke, David (January 1, 2014), "Fusimotor System", Reference Module in Biomedical Sciences, Elsevier, doi:10.1016/b978-0-12-801238-3.00249-x, ISBN 978-0-12-801238-3, retrieved December 21, 2020 <a href="978-0-12-801238-3" target="_blank">978-0-12-801238-3</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Casagrand, Janet (2008) Action and Movement: Spinal Control of Motor Units and Spinal Reflexes. University of Colorado, Boulder. <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Johns, Paul (January 1, 2014), Johns, Paul (ed.), "Chapter 4 - Sensory and motor pathways", Clinical Neuroscience, Churchill Livingstone, pp. 49–59, doi:10.1016/b978-0-443-10321-6.00004-7, ISBN 978-0-443-10321-6, retrieved December 21, 2020 <a href="978-0-443-10321-6" target="_blank">978-0-443-10321-6</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> </ol>