Horse lumbrical muscle: possible structural and functional reorganization in regressive muscle

A biomechanical and evolutionary perspective on the function of the lumbrical muscle

The lumbrical muscles of the hand originate from the flexor digitorum profundus tendons and insert onto the lateral band of the extensor tendons. Owing to these movable attachments, the function of this muscle is difficult to visualize. To better determine the function of this muscle, we considered its relative anatomy, biomechanical characteristics, and evolution. With the smallest physiological cross-sectional area in the upper extremity, the lumbrical muscles have weak motor function, which is only 1/10 of the interosseous muscle. Because they are spindle rich, the lumbrical muscles play an important role in the sensory feedback of the distal interphalangeal, proximal interphalangeal, and metacarpophalangeal joints of the fingers. The first 2 lumbrical muscles have lower variation in anatomy and higher density of muscle spindles compared to the ulnar 2 lumbricals. In addition, the index and middle finger lumbrical muscles are innervated by the median nerve, which also innervates the thenar muscles of the thumb. Therefore, it is possible that the first 2 lumbricals are functionally more important than the 2 ulnar lumbricals, specifically for precision pinch movements.

Free Radical Formation after Intensive Exercise in Thoroughbred Skeletal Muscles

Although high oxygen consumption in skeletal muscle may result in severe oxidative stress, there are no direct studies that have documented free radical production in horse muscles after intensive exercise. To find a new parameter indicating the muscle adaptation state for the training of Thoroughbred horses, we examined free radical formation in the muscle by using electron paramagnetic resonance (EPR). Ten male Thoroughbred horses received conventional training for 18 weeks. Before and after the training period, all horses performed an exhaustive incremental load exercise on a 6% incline treadmill. Muscle samples of the middle gluteal muscle were taken pre-exercise and 1 min, 1 hr, and 1 day after exercise. Muscle fiber type composition was also determined in the pre-exercise samples by immunohistochemical staining with monoclonal antibody to myosin heavy chain. We measured the free radical in the muscle homogenate using EPR at room temperature, and the amount was expressed as relative EPR signal intensity. There was a significant increase in Type IIA muscle fiber composition and a decrease in Type IIX fiber composition after the training period. Before the training period, the mean value of the relative EPR signal intensity showed a significant increase over the pre-exercise value at 1 min after the exercise and an incomplete recovery at 24 hr after the exercise. While no significant changes were found in the relative EPR signal intensity after the training period. There was a significant relationship between percentages of Type IIA fiber and change rates in EPR signal intensity at 1 min after exercise. The measurement of free radicals may be useful for determining the muscle adaptation state in the training of Thoroughbred horses.

Morphological study of the Golgi tendon organ in equine superficial digital flexor tendon

The Golgi tendon organ (GTO) is an encapsulated fusiform mechanoreceptor siding in the musculo-tendinous junction of many animal species. Inhibitory function of afferent nerve fibers distributed from nearby motor units, the organ responds to active tension exerted onto the muscle. The morphological features of the equine GTO have not yet been elucidated. Additionally, there is some controversy regarding to the existence of the GTO in the equine superficial digital flexor tendon (SDFT). Therefore, immunohistochemistry and immunoelectron microscopy using alcian blue (pH 2.5) staining and the silver-enhanced colloidal gold method were carried out to determine both the location and characteristics of the GTO at the musculo-tendinous junction of the SDFT. A GTO with a fusiform structure of approximately 3 mm in length was found in the tendinous part. The lumen of the GTO was divided into compartments by septal cells. Each compartment contained collagen fibrils, nerve fibers and Schwann cells. This is the first report of the equine GTO.