Detection of Motor Endplates in Deep and Pennate Skeletal Muscles in-vivo using Ultrafast Ultrasound

Until now direct localization of motor endplates for any arbitrary muscle structure was reserved to cadaver studies or restricted to specific muscles in-vivo. Utilizing ultrafast ultrasound we report a versatile in-vivo method for the detection of motor endplate zones. To achieve this goal, we employ a research ultrasound system with a linear array transducer probe to record ultrasound images of electrically-stimulated medial gastrocnemius muscles. An optical flow speckle tracking algorithm is used to track the points along a single fascicle. We localize motor endplates at intersection points where mechanical contraction waves diverge bidirectionally towards the fiber ends. Contrary to common experimental methods, our proposed ultrafast ultrasound-based approach allows to localize motor endplates also in deep and pennate muscles in-vivo. These results open up new applications for ultrasound in neuromuscular research.