Assistant
Introduction
By Gregory R. Waryasz, MD
Muscle tissue is arranged in parallel to allow for contraction.
Skeletal muscle is striated under the light microscope. Skeletal muscle cells are long cylindrical myocytes referred to as myofibers. There is an external lamina surrounding each fiber. The size of a myofibril is approx 10 to 100 microns in diameter. There are many nuclei present in the muscle cells. Under the light microscope the skeletal muscle cells when stained with hematoxylin and eosin show a stippled appearance due to myofilaments.
Sacromeres are the basic unit of contraction for skeletal muscles. A sarcomere spans from one Z line to the next and is composed of one A band separating two I bands. The bands refer to different shading and appearance of components of the sarcomere under the microscope. I bands are thin filaments. A bands and thick and thin filaments. H bands are only thick filaments.
Myofilaments are proteins in the muscle cell responsible for muscle cell contraction. Types include thin myofilaments and thick myofilaments. Thin myofilaments are 6 to 8nmdiameter and 1.0um long, This type of myofilament is made up of the protein actin. Thick myofilaments are approximately 15nm in diameter and 1.5um long. This type of myofilament is made of the protein myosin II. Groups of myofilaments make up myofibrils. Myofibrils in groups are called muscle fibers. The endomysium is the level of tissue that surrounds each muscle fiber, this layer contains small blood vessels and nerves. The muscle fibers are bound together by connective tissue known as the perimysium to form muscle fascicles. The muscle fascicles are surrounded by the perimysium. Fascicles typically have a specific function. Within the perimysium are larger blood vessels and nerves. Collections of fascicles are bound together by the epimysium which contains larger neurovascular structures. Skeletal muscle is highly vascularized by microcirculation.
The junction between muscle and bone or other structures is known as a tendon. The tendon is made up of connective tissue from the endomysium, perimysium, and epimysium.
There are 3 basic types of skeletal muscle; type I, type IIa, and type IIb. Type 1 skeletal muscle is slow oxidative meaning that it is responsible for activities that require higher endurance. Athletically these fibers are found in larger amounts in distance runners. Type IIa skeletal muscle is fast oxidative glycolytic. This type of muscle is fast twitch fatigue-resistant motor units that are found in higher percentages in sprinters. Type IIb skeletal muscle is fast glycolytic. This type of muscle is fast-twitch fatigue-prone motor units that are found in higher concentrations in weight lifters and very short distance sprinters.
Muscle contraction occurs by the sliding filament mechanism. This refers to how a cell shortens when the thin filaments move on the stationary thick filaments. During contraction, the sarcomere shortens, the Z lines are pulled closer together, the H bands become more narrow and disappear, the I bands shorten, the A bands remain the same, and the myofibrils and muscles shorten. The energy for contraction is provided by adenosine triphosphate (ATP). The stimulus for contraction is derived from neurologic stimulation through a motor endplate’s neurmomuscular junction. Acetylcholine is the neurotransmitter that depolarizes the cell membrane or sarcolemma. Depolarization leads to calcium release that allows calcium to bind to myosin. Once contraction is done, the muscles relax and calcium is reuptaked into the sarcoplasmic reticulum to be ready for the next initiation of contraction.
References
Lieberman J (ed), AAOS Comprehensive Orthopaedic Review, American Academy of Orthopaedic Surgeons, 2008.
Moore K, Dalley A (eds), Clinically Oriented Anatomy (5th edition), Lippincott Williams & Wilkins, Philadelphia, PA, 2006.
Ross MH, Pawlina W, Histology: A Text and Atlas with Correlated Cell and Molecular Biology, Lippincott Williams & Wilkins, Philadelphia, PA, 2006.