You have more than 600 skeletal muscles in your body, connecting primarily to your bones via tendons. Muscles are classified according to their size, shape and fiber arrangement. They are named according to their location, function, shape, direction of their cells and the number of divisions. Muscles are highly metabolic, needing a constant supply of nutrients and oxygen. Tendons are named according to their muscles, and do not have as high a metabolic need.
Muscle Sheaths
Each muscle cell or muscle fiber is covered by a delicate connective tissue membrane called the endomysium. Muscle fibers are bundled together in fascicles. Each fascicle is wrapped with a more resilient sheath of tissue called the perimysium. Muscles such as your biceps are a collection of fascicles. Each muscle is covered by a coarse sheath of connective tissue called the epimysium. These connective tissue sheaths come together and merge with the strong fibrous cord of the muscle, the tendon.
Tendon Sheaths
Tendons have similar connective sheaths with those of muscles. Collagen molecules join to form collagen fibers or cells. A collection of collagen fibers form a fascicle and are covered by an endotendon. Groups of fascicles are wrapped by an epitendon. The tendon as a whole is covered by an paratendon.
Muscle Cells
Muscle cells are commonly referred to as muscle fibers because they do not have typical cell characteristics. Instead, early-stage muscle cells fuse together during development to form single muscle fibers. These new fibers have several nuclei. Muscle fibers have long strands of protein connected to each other. These strands are contractile units, which, when plenty of them contract, produce movement; they also cause skeletal muscle to appear striated under the microscope. Special stretch receptors called muscle spindles lie between and parallel to your muscle fibers. Muscle spindles create muscle reflexes when they sense a relaxed muscle is stretched beyond normal limits.
Tendon Cells
Tendons are made up of primarily collagen fibers. Tendons are susceptible to overuse injuries resulting in tendonitis or tendon ruptures. It takes tendon tissue much longer to heal compared to muscle tissue.
Golgi tendon organs are found at the junction between the muscle and the tendon. These receptors are stimulated when too much force is generated by your muscle. It causes your muscle to relax as a protective measure to prevent muscle tears and ruptures.
Exercise
When you want to perform barbell biceps curls, your brain sends signals to your muscles to contract. Once your muscle cells are stimulated and the Golgi tendon organ does not sense excessive force or contractions, your biceps shorten. The muscles pull on the tendons, moving your forearm closer to your arm.
References
- "Anatomy & Physiology"; Gary Thibodeau, Ph.D. and Kevin Patton, Ph.D.; 2007
- "Exercise Physiology, Energy, Nutrition & Human Performance"; William McArdle, Frank Katch and Victor Katch; 2007
