Muscle Microanatomy Quiz

Skeletal Muscle Structure

• Thick and thin myofilaments give skeletal myocytes a striated appearance, with A bands (dark) composed of actin and myosin, and I bands (light) composed of actin
• I bands are bisected by the Z-line, which actin filaments are anchored to
• One sarcomere is located from one Z-line to the next Z-line
• Actin and myosin are contractile proteins, while tropomyosin is a regulatory protein
• Myofilaments can only be seen with an electron microscope
• Individual myocytes are surrounded by reticular fibers forming the endomysium
• The sarcomere is the contractile or functional unit of the myocyte

Skeletal Muscle Contraction

• Motion is mediated by muscle cells and is based on the conversion of chemical energy (ATP) into mechanical energy
• Myofibrils are composed of repeating assemblies of thick and thin filaments constituting dark A and pale I segments
• During muscle contractions, the actin filament slides over the myosin filament, resulting in a shortening of the I band
• Actin and myosin are held in position in the myofibril by other proteins such as desmin, tropomyosin, and troponin
• The summation of all sarcomeres shortening produces contractions of a muscle cell
• The sliding filament model: step 1: each sarcomere shortens, while the myofilament length is constant; step 2: the I band shortens and almost disappears; step 3: the thin (actin) filaments slide past the thick (myosin) filaments

Skeletal Muscle Development

• Myotubes are multinucleated tubes formed due to mesenchymal cells (myoblasts) aligning and fusing together
• Myotubes differentiate, forming functional myofilaments, and the nuclei are displaced against the plasma membrane
• Satellite cells are mesenchymal stem cells that function in muscle repair
• Satellite cells are positioned between the basal lamina and sarcolemma of the muscle cell
• They retain mitotic potential, which allows them to accomplish some repair
• Fibroblasts also form connective tissue (scars) as part of the repair process

Skeletal Muscle Organization

• The epimysium (fascia) is a layer of dense irregular connective tissue that surrounds the muscles
• Multiple bundles of fascicles make up a whole muscle
• Perimysium is a dense connective tissue that surrounds each fascicle
• Each fascicle is made up of many muscle cells
• Myofibers contain cylindrical bundles of myofibrils
• Myofibrils contain many smaller bundles of myofilaments
• Myofibrils are the contractile element within skeletal muscle cells, as they are composed of actin (thin) and myosin (thick) myofilaments

Skeletal Muscle Fiber Types

• Type 1 (slow) fibers compose red muscle, which have a slow twitch, use aerobic metabolism, and are fatigue resistant
• Type 2 fibers compose white muscle that uses anaerobic metabolism and can be further divided into type 2A and 2B
• Type 2A (intermediate) uses mixed oxidative-glycolytic metabolism and is slow fatiguing
• Type 2B (fast) is fast contracting, fast fatiguing, and uses glycolytic metabolism

Cardiac Muscle

• Cardiac muscle cells are arranged into fibers, with each cell only containing one central nucleus
• Cardiac cells are cross-striated and has intercalated discs that contain gap junctions and desmosomes
• Intercalated discs attach cardiac muscle cells to each other, providing strength and the ability to function as a syncytium
• Cardiac muscle cells have a sarcoplasmic reticulum, mitochondria, and the mitochondria make up 20% of the cellular volume

Smooth Muscle

• Smooth muscle cells have a fusiform spindle shape, and they are surrounded by a basal lamina and reticular fibers
• Smooth muscle cells have a single central nucleus
• Smooth muscle cells have parasympathetic and sympathetic innervation, causing them to have involuntary contractions
• Smooth muscle cells can act as a single unit or multi-unit
• Single-unit smooth muscle cells are found in visceral organs and have sparse innervation, yet cells communicate via multiple gap junctions
• Multi-unit smooth muscle cells are found in the iris of eyes and have precise contractions
• Smooth muscle functions include peristalsis, vascular dynamics, propulsion, and secretion

Contractile Non-Muscle Cells

• Myoepithelial cells have an ectodermal origin and contain actin and myosin
• They can be stimulated by hormones such as those produced by the mammary gland
• They are basket-like shaped and are known as "basket cells"
• Myoepithelial cells are located in salivary, mammary, and lacrimal glands
• They are enclosed in clusters of glandular cells
• Myofibroblasts are from the mesoderm and are involved in wound contraction