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What happens at the Human Neuromuscular Junction after a Traumatic Nerve Injury? A Temporal Profile of Human Motor Endplate Degradation after Brachial Plexus Injury
Justin P. Chan, BA1, Winnie A. Palispis, MD2, Jennifer Uong, BS Cand.1, Henry Hoang, BS Cand.1, Ranjan Gupta, MD2.
1University of California - Irvine, Irvine, CA, USA, 2University of California - Irvine, Orange, CA, USA.

PURPOSE: Patients with traumatic nerve injuries to the brachial plexus (BPI) have particularly poor outcomes with limited functional recovery, even after optimal surgical management. As improvements in recovery have plateaued secondary to surgical manipulations alone, adjuvant cellular and molecular therapeutic regimens are required. Yet, the appropriate time to intervene can only be determined if there is a true understanding of the process of nerve and muscular degeneration secondary to a traumatic nerve injury. While animal models have shed light on molecular changes to the muscle and motor endplate post-injury, the time course of degeneration in animal models is unlikely to be the same as in the human condition, and thus cannot provide precise information that would help inform surgical intervention and the timing for adjuvant therapy. Here, we provide novel data about the morphologic changes at the human motor endplate and ensuing degeneration at the NMJ following traumatic nerve injury.
METHODS: IRB approval was obtained so as to permit biopsies from denervated muscles in patients with BPI ranging from complete pre-ganglionic C5-T1 BPI to less severe traumatic injuries. Specimens were processed for immunohistochemistry and visualized with two-photon excitation and confocal microscopy. Motor endplates were labeled with alpha-bungarotoxin, presynaptic vesicles with synaptophysin, and axons with neurofilament. Human muscle samples from multiple timepoints after injury were analyzed along with control specimens from innervated muscles so as to create a temporal sequence of events for human motor endplate degradation following traumatic nerve injury.
RESULTS: Denervated muscle samples show distinct differences from innervated muscles (Figure 1A), including fragmentation and dispersion of acetylcholine receptors. There is also a noted decrease in NMJ volume as seen in 3D reconstruction, and a trend towards plaque endplate morphology. Moreover, comparison of denervated muscles shows signs of temporal degeneration. NMJs from early denervated muscles still show well preserved circular morphology with definite acetylcholine receptors arranged in distinct folding patterns (Figure 1B). By one year status post traumatic brachial injury, NMJs begin to present with greater fragmentation. Moreover, synaptic gutters start to fade, and asymmetry in acetylcholine receptor distribution is noted (Figure 2). Interestingly, even after one year of denervation, NMJs were able to retain their overall circular shape.
CONCLUSIONS: This study details the novel and critically important data about the sequence of events involved in human motor endplate degradation after a clearly defined traumatic nerve injury. Surprisingly, human NMJs persist and retain their structures even after the 6-month window of opportunity for meaningful functional recovery has elapsed, which may indicate a limited utility of animal models for traumatic peripheral nerve injuries. This temporal profile highlights the importance of species-specific findings and provides invaluable data that can answer important questions pertaining to the optimal timing of surgical intervention.


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