Functional analysis of human intrafusal fiber innervation by human γ-motoneurons

Abstract

Investigation of neuromuscular deficits and diseases such as SMA, as well as for next generation prosthetics, utilizing in vitro phenotypic models would benefit from the development of a functional neuromuscular reflex arc. The neuromuscular reflex arc is the system that integrates the proprioceptive information for muscle length and activity (sensory afferent), to modify motoneuron output to achieve graded muscle contraction (actuation efferent). The sensory portion of the arc is composed of proprioceptive sensory neurons and the muscle spindle, which is embedded in the muscle tissue and composed of intrafusal fibers. The gamma motoneurons (γ-MNs) that innervate these fibers regulate the intrafusal fiber’s stretch so that they retain proper tension and sensitivity during muscle contraction or relaxation. This mechanism is in place to maintain the sensitivity of proprioception during dynamic muscle activity and to prevent muscular damage. In this study, a co-culture system was developed for innervation of intrafusal fibers by human γ-MNs and demonstrated by morphological and immunocytochemical analysis, then validated by functional electrophysiological evaluation. This human-based fusimotor model and its incorporation into the reflex arc allows for a more accurate recapitulation of neuromuscular function for applications in disease investigations, drug discovery, prosthetic design and neuropathic pain investigations.

Have We Looked in the Wrong Direction for More Than 100 Years? Delayed Onset Muscle Soreness Is, in Fact, Neural Microdamage Rather Than Muscle Damage

“…Colon et al [6] formulated a detailed description of the muscle spindle. Muscle spindles contain intrafusal muscle fibers with sensory, gamma-motoneuron (γ-MN) [7][8][9], and sympathetic innervation [10].…”

“…Motoneurons receiving afferent information can then signal intrafusal or extrafusal fibers to relax or contract in response to sensory input [7]. With the afferent sensory feedback, the reflex arc acts as an automated closed loop so that voluntary movement can be achieved accurately and properly" [6].…”

“…The peripheral ends of the intrafusal fibers slowly relax or contract under the control of γ-MNs. These γ-MNs modulate the tension, sensitivity, and length of the intrafusal fibers so muscle spindles can maintain constant sensitivity during dynamic muscle action and prevent overextension, which can lead to undue stress on the muscle as well as tendons and cause joint damage [8]" [6]. In addition to the above, there is anatomical evidence of direct sympathetic innervation in human muscle spindles as well [10].…”

“…The enhanced compression could entrap the nerve endings within the muscle spindle, and the repetition of eccentric contractions could lead to microinjury of the nerve terminals. (3) Excessively stretched muscle spindle: uncompressible fluid entraps and causes microinjury to the nerve terminals due to the superposition of compression when repetitive eccentric exercise is executed in an unaccustomed or strenuous way (drawn based on Colon's description [6]).…”

In Vitro Tissue‐Engineered Skeletal Muscle Models for Studying Muscle Physiology and Disease

“…Using primary human myoblasts, intrafusal fibers can be generated in a defined serum-free media that yields classical intrafusal fiber morphology and protein expression. [165,166] When co-cultured with proprioceptive sensory neurons derived from human neuroprogenitors, intrafusal fibers can support the formation of mechanosensory-like nerve terminal structures. Importantly, electrophysiological studies in this system demonstrated repetitive firing patterns characteristic of functional sensory innervation in vivo.…”