Muscle accessory mechanisms, blood supply and nerve innervation

Muscles have supporting structures, such as fascia, synovial bursae, and tendon sheaths. These structures assist muscle movement, maintain muscle position, reduce friction during movement, and provide protection. Fascia is distributed throughout the body and is divided into superficial fascia and deep fascia. Superficial fascia, also known as subcutaneous fascia, lies beneath the dermis, covering all parts of the body, and is composed of loose connective tissue. Superficial arteries, subcutaneous veins, cutaneous nerves, and lymphatic vessels run within the superficial fascia; some areas may also contain mammary glands and cutaneous muscles. The superficial fascia provides some protection to the muscles, blood vessels, and nerves located deep within it. For example, the superficial fascia of the palms and soles is relatively well-developed and can cushion pressure. The deep fascia, also known as the proper fascia, is composed of dense connective tissue and lies deep to the superficial fascia. The deep fascia has a very close relationship with muscles, layering along with the muscle structure. In the limbs, the deep fascia extends between muscle groups and attaches to bones, forming muscle septa, and together with the deep fascia surrounding the muscle groups, forms the fascial sheath. The deep fascia also surrounds blood vessels and nerves, forming a neurovascular sheath, and can provide muscle attachments or serve as muscle origins. Synovial bursae are small, closed connective tissue sacs with thin walls, filled with synovial fluid. They are mostly located at the contact points between tendons and bone surfaces to reduce friction. Some synovial bursae communicate with the joint cavity near joints. Inflammation of synovial bursae can affect local motor function of the limb. A tendon sheath is a sheath that surrounds a muscle tendon and is found in highly mobile areas such as the wrist, ankle, fingers, and toes. The tendon sheath consists of two parts: a fibrous layer and a synovial layer. The fibrous layer, also known as the tendon fibrous sheath, is the outermost layer and is a bony fibrous canal formed by the thickening of the deep fascia; it acts as a pulley and restraint for the muscle tendon. The synovial layer, also known as the tendon synovial sheath, is located within the tendon fibrous sheath and is a double-layered cylindrical sheath composed of synovium. The tendon synovial sheath is divided into a visceral layer and a parietal layer. The visceral layer surrounds the muscle tendon, and the parietal layer adheres closely to the inner surface of the tendon fibrous sheath. A small amount of synovial fluid is contained between the visceral and parietal layers, allowing the muscle tendon to slide freely within the sheath. Improper, prolonged, excessive, and rapid finger movements can lead to tendon sheath damage, causing pain and affecting tendon gliding; clinically, this is called tenosynovitis, a common and frequently occurring disease. The portion of the tendon synovial sheath that transitions from the bone surface to the two layers of synovium in the muscle tendon is called the mesentery, through which blood vessels supplying the muscle tendon pass.

Muscles are full of vitality and have a high metabolic rate, so their blood supply is very rich. Most of the major blood vessels run alongside nerves, and each muscle has its own blood supply. The blood vessels run along the intermuscular septa and interfascial spaces, with branches entering the muscle from the muscle hilum. They branch repeatedly within the muscle and eventually form a capillary network that surrounds the muscle fibers in the endothelial membrane. The capillary network then flows into venules and small veins and exits from the muscle hilum.

Based on the source, location, thickness, number of branches, and primary and secondary blood supply to muscles, muscle blood supply can be classified into four types. First, the single-branch nutrient artery type: the entire muscle is supplied by a single, relatively large artery, entering the muscle proximally, such as the tensor fasciae latae and the medial and lateral heads of the gastrocnemius muscle. Second, the double-branch nutrient artery type: the muscle is supplied by two nutrient arteries of similar diameter, such as the gluteus maximus, rectus abdominis, and rectus femoris. Third, the primary and secondary nutrient artery type: the muscle is supplied by one large nutrient artery and several smaller secondary arteries, such as the trapezius and latissimus dorsi muscles. Fourth, the segmental nutrient artery type: the muscle is supplied by several smaller arteries, branching from the arterial trunks of the limb at different planes between the muscle's origin and insertion, exhibiting a segmental distribution, such as the sartorius, tibialis anterior, and extensor digitorum longus muscles.

Muscle tendons have a relatively small blood supply, which comes from the following sources: first, longitudinal blood vessels extending from the muscle-tendon junction to the interfascicular connective tissue of the tendon; second, numerous small branches originating from interstitial blood vessels; and third, blood vessels from the bone and periosteum at the tendon insertion point.

The origin, course, and entry points of nerves into muscles are relatively constant and less variable than those of blood vessels, running alongside the main nutrient vessels of the muscle. There are two types of nerves that innervate muscles: sensory nerves and motor nerves. Sensory fibers transmit pain, temperature, and proprioceptive sensations; motor nerves primarily sense changes in muscle fiber contraction and relaxation, playing a crucial regulatory role in muscle activity. It is noteworthy that nerve fibers also have a nutritional effect on muscle fibers, releasing nutrients from their terminals to promote glycogen and protein synthesis. If a nerve is damaged, the muscle loses its innervation, glycogen synthesis slows down, protein breakdown accelerates, and the muscle gradually atrophies-a condition known as nutritional muscle atrophy, a common problem encountered in sports injuries.

Each muscle that makes up the locomotor system is a complex organ, mainly composed of two parts: the muscle belly and the tendon. The muscle belly, the main part of the muscle organ, is located in the middle and is composed of many skeletal muscle fibers connected by connective tissue, possessing contractile ability. The connective tissue surrounding the entire muscle surface is called the epimysium. The epimysium extends inward, dividing the muscle fibers into muscle bundles of different sizes, called perimysium. The perimysium further extends inward, surrounding each muscle fiber, called the endomysium. The sarcolemma is the supporting tissue of the muscle, giving it its shape. Blood vessels, lymphatic vessels, and nerves enter the muscle along with the sarcolemma, playing a crucial role in muscle metabolism and functional regulation. The tendon is located at both ends of the muscle belly, composed of dense connective tissue. In the limbs, it is often cord-like, while in the trunk, it is often thin and plate-like, also known as the aponeurosis. Tendon fibers connect the two ends of the muscle fibers through the endothelial membrane or run through the muscle belly. Tendons cannot contract, but they have strong toughness and tension and are not easily fatigued. Their fibers extend into the periosteum and bone, making the muscles firmly attached to the bone. Muscle pain often occurs during sports training, which seems to originate from within. In fact, these pains are related to fatigue and injury of the muscle belly and tendons.

To build strong muscles, you need to supplement with protein. So how does protein work on muscles? If we were as small as a cell and could freely enter the human body, we would find that muscles are made up of muscle fibers bundled together like steel cables. These cables combine to form thicker, longer groups of ropes, which contract and expand like springs when muscles exert force. Within the thickest cables are muscle fibers, nerves, blood vessels, and connective tissue. Each muscle fiber is composed of smaller myofibrils, and each myofibril is composed of two types of filamentous proteins intertwined: myosin and actin. This is the most basic unit of muscle. The massive muscles of strongmen are composed of these two incredibly small proteins. When they work together, they generate strength, and it is through these muscles that humans have gradually changed the face of the earth. As people age, the elastic fibers of the striated muscles that control bone movement are gradually replaced by connective tissue. While connective tissue is strong, it lacks elasticity, thus weakening muscles and making them unable to contract powerfully. Therefore, after the age of 30, our muscles begin to deteriorate and weaken, resulting in decreased muscle strength and slower reflexes. Maintaining a slower, more sustainable approach to muscle loss is crucial for preventing weight gain and slowing down the aging process.