Flashcards in TBL4 - Muscle Tissue Deck (21):
What cells form the intrinsic back muscles and extrinsic muscles of the shoulder?
Mesenchymal cells of paraxial mesoderm-derived somites generate myoblasts that form the intrinsic back muscles and extrinsic muscles of the shoulder
What cells form the intrinsic shoulder muscles and the muscles of the upper limb?
Myoblasts derived from mesenchymal cells of the parietal layer of lateral plate mesoderm form the intrinsic shoulder muscles and the muscles of the upper limb
How are myotubes formed?
1) Aggregation and end-to-end fusion of myoblasts produce syncytial, multinucleated
2) Myotubes synthesize two sets of longitudinally oriented myofilaments, which align in parallel arrays (long and narrow skeletal muscle cells/fibers)
3) Myofilaments are arranged in alternating, overlapping bands, so muscle fibers have a cross-striated appearance. At birth, thick (myosin) and thin (actin) myofilaments have collected into cylindrical bundles called myofibrils, and muscle fibers can contract
as they become innervated by motor neurons
4) A single motor neuron and all the skeletal muscle fibers it innervates are known as a motor unit. Myosatellite cells, capable of division, remain closely associated with the plasma membrane, or sarcolemma, of each muscle fiber throughout life
What are the active units of myofibrils called?
Sarcomeres form subunits of the myofibrils
What is included in the I & A bands of sarcomeres? What is the function of a Z-line?
1) Each myofibril has alternating light (I, for isotropic - only contains actin) and dark (A, for anisotropic - contains both actin & myosin) bands along its length
2) Two sets of myofilaments, thick and thin, that make up each myofibril are organized into repeating units of contraction known as sarcomeres
3) Dark, transverse Z (Zwischenscheibe) bands mark the ends of each sarcomere and anchor the thin filaments. The center of the sarcomere contains the thick (myosin-containing) filaments, which form the A band; thin (actin-containing) filaments, which form the I bands, are at the ends of each sarcomere
What is the sarcoplasmic reticulum? What is the role of triads during muscle contraction?
1) The sarcotubular system is composed of two separate and distinct membrane systems of the muscle fiber, known as the sarcoplasmic reticulum (SR) and the transverse tubular system
2) The SR, similar to the smooth endoplasmic reticulum of other cells, is an elaborate, anastomosing network of tubules and cisternae that surround the myofibrils
3) At regular intervals relative to the sarcomeres, two flattened sacs of the SR, known as terminal cisternae (lateral sacs), closely associate with a central transverse tubule (T-tubule; tubular invagination of the sarcolemma) and form a muscle triad, which is the main site for excitation-contraction coupling
Chemically, how does muscle contraction occur?
1) Each skeletal muscle fiber forms a synaptic junction with the terminal end of a somatic motor axon
2) Acetylcholine is released from the axon terminal binds to the adjacent sarcolemma and induces postsynaptic action potentials that initiate muscle contraction
What are satellite cells and where do they reside in skeletal muscle?
1) Satellite cells are small, flattened mononucleated cells located along the sarcolemma of skeletal muscle fibers (between the sarcolemma of a skeletal muscle fiber and the external
2) Satellite cells serve as a population
of reserve stem cells, or resting myoblasts, either for normal postnatal growth or for repair and regeneration of damaged segments of the skeletal muscle fiber after injury. They are most abundant during early development and growth
3) Although the cells are normally quiescent in adults and their numbers and mitotic capacity decline with age, they have proliferative potential throughout life, and they increase in number in response to denervation, in mildly traumatized muscle, and in regenerating diseased muscle
How does EMG test muscle action?
1) Electromyography (EMG), the electrical stimulation of muscles. The examiner places surface electrodes over a muscle, asks the person to perform certain movements, and then amplifies and records the differences in electrical action potentials of the muscles
2) A normal resting muscle shows only a baseline activity (muscle tone), which disappears only during deep sleep, during paralysis, and when under anesthesia
3) Contracting muscles demonstrate variable peaks of phasic activity
4) EMG makes it possible to analyze the activity of an individual muscle during different movements. EMG may also be part of the treatment program for restoring the action of muscles
What is the deficiency that causes Duchenne muscular dystrophy and what are the symptoms of this disease? (add symptoms)
1) Duchenne muscular dystrophy is a genetic disorder caused by a deficiency of dystrophin, a large membrane-associated cytoskeletal protein
2) Dystrophin maintains mechanical integrity of the cell during contraction by anchoring cytoskeletal elements
3) It is the most common of a group of muscular dystrophies characterized by rapid progression of skeletal muscle degeneration occurring early in life
How does the common hereditary disorder myasthenia gravis affect the induction of postsynaptic action potentials?
1) In the acquired disorder, a distortion of the postsynaptic sarcolemmal membrane of the neuromuscular junction is accompanied by a reduction in the concentration of acetylcholine receptors
2) Antibodies are attached to the postsynaptic membrane, which makes it less sensitive to acetylcholine and leads to a reduced muscle action potential in response to a nerve impulse
Differentiate between the three types of skeletal muscle fibers
1) Type I slow-twitch fibers are highly fatigue-resistant and abundant in muscles used for low force activities like maintaining posture and leisurely walking
2) Type IIA fast-twitch fibers are moderately fatigue-resistant and predominate in muscles used for prolonged high force activities such as running a marathon
3) Type IIB fast-twitch fibers are minimally fatigue-resistant and abundant in muscles used for short high force activities such as sprinting and power lifting
What are the connective tissue sheaths that surround skeletal muscle?
1) A dense connective tissue sheath—the epimysium—surrounds the whole muscle externally
2) Connective tissue septa that make up the perimysium subdivide the muscle internally into bundles, or fascicles, each containing several muscle fibers
3) A more delicate, looser connective tissue—the endomysium—surrounds individual muscle fibers
What are tendons composed of?
Tendons are composed of dense connective tissue with closely packed Type I collagen fibers and intervening fibroblasts and capillaries
Describe tendons on their proximal and distal points (explain this please)
1) Proximally, tendons are direct continuations of the epimysium
2) Distally, tendons are continuous with the fibrous layer of the periosteum at their bony attachment sites
3) The fibrous layer of the periosteum is firmly embedded in the outer compact bone.
Why are skeletal muscles trained according to their percentages of fiber types?
1) Fiber typing can be used to determine
functional properties of a particular muscle on the basis of the distribution and percentage of fiber types
2) Physiotherapists determine how to train a muscle on the basis of knowing the function of a muscle and the percentage of its fiber types
Why are muscle-tendon junctions more susceptible to strain injuries in children?
1) Muscle injuries often occur at or near the muscle-tendon junction. Immobilization
reduces the tensile strength of the junction and predisposes it to strain injuries
Where is smooth muscle of the gut tube derived from? Where else is smooth muscle found and where is it derived from?
1) Smooth muscle of the gut tube and its derivatives is formed by mesenchymal cells of the visceral layer of lateral plate mesoderm
2) Smooth muscle, which constitutes the dermal arrector pili and resides in arteries and arterioles that supply the body wall and extremities, is derived from local mesenchymal cells
How and why is smooth muscle arranged differently than skeletal muscle?
1) Although smooth muscle can produce contractile force comparable to that of skeletal muscle, it has a much slower and more variable speed of contraction, which can be sustained for long periods. Its cells are also very efficient in terms of energy expended and show less fatigue
2) As its name implies, smooth muscle lacks visible striations or sarcomeres, in contrast to striated skeletal and cardiac muscle. By electron microscopy, the sarcoplasm of smooth muscle cells has three sets of filaments (thick, thin, & intermediate) that are oriented obliquely and longitudinally in each cell
3) Adjacent smooth muscle fibers overlap in a staggered fashion to achieve close packing
4) Dense bodies, unique to smooth muscle cells, are found in all parts of a cell, either scattered in cytoplasm or attached to the undersurface of the sarcolemma, where they link thin and intermediate filaments to the cell membrane
5) Myosin filaments associate with the actin filaments and thereby enable filament sliding during contraction of the smooth muscle fibers
6) Intermediate filaments function as a strong cablelike system that probably harnesses the force generated during contraction
How is smooth muscle activity regulated?
1) Regulation of smooth muscle activity often occurs via the autonomic nervous system, whereby axonal varicosities containing synaptic vesicles come into close contact with the sarcolemma of a smooth muscle cell
2) These varicosities along postsynaptic sympathetic fibers release norepinephrine that binds to the sarcolemma of adjacent smooth muscle fibers
3) In most areas, however, not all smooth muscle cells are innervated, and the branch of an autonomic nerve fiber supplies groups of several cells
4) Gap junctions (tiny intercellular pores) between cells allow rapid spreading of norepinephrine-induced action potentials (excitation) to spread among adjacent cells, which results in synchronous contractions of groups of smooth muscle fibers