TBL4 - Muscle Tissue

Question 1

What cells form the intrinsic back muscles and extrinsic muscles of the shoulder?

Answer 1

Mesenchymal cells of paraxial mesoderm-derived somites generate myoblasts that form the intrinsic back muscles and extrinsic muscles of the shoulder

Question 2

What cells form the intrinsic shoulder muscles and the muscles of the upper limb?

Answer 2

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

Question 3

How are myotubes formed?

Answer 3

1) Aggregation and end-to-end fusion of myoblasts produce syncytial, multinucleated
myotubes
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

Question 4

What are the active units of myofibrils called?

Answer 4

Sarcomeres form subunits of the myofibrils

Question 5

What is included in the I & A bands of sarcomeres? What is the function of a Z-line?

Answer 5

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

Question 6

What is the sarcoplasmic reticulum? What is the role of triads during muscle contraction?

Answer 6

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

Question 7

Chemically, how does muscle contraction occur?

Answer 7

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

Question 8

What are satellite cells and where do they reside in skeletal muscle?

Answer 8

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
lamina)
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

Question 9

How does EMG test muscle action?

Answer 9

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

Question 10

What is the deficiency that causes Duchenne muscular dystrophy and what are the symptoms of this disease? (add symptoms)

Answer 10

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

Question 11

How does the common hereditary disorder myasthenia gravis affect the induction of postsynaptic action potentials?

Answer 11

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

Question 12

Differentiate between the three types of skeletal muscle fibers

Answer 12

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

Question 13

What are the connective tissue sheaths that surround skeletal muscle?

Answer 13

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

Question 14

What are tendons composed of?

Answer 14

Tendons are composed of dense connective tissue with closely packed Type I collagen fibers and intervening fibroblasts and capillaries

Question 15

Describe tendons on their proximal and distal points (explain this please)

Answer 15

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.

Question 16

Why are skeletal muscles trained according to their percentages of fiber types?

Answer 16

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

Question 17

Why are muscle-tendon junctions more susceptible to strain injuries in children?

Answer 17

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
2)

Question 18

Where is smooth muscle of the gut tube derived from? Where else is smooth muscle found and where is it derived from?

Answer 18

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

Question 19

How and why is smooth muscle arranged differently than skeletal muscle?

Answer 19

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

Question 20

How is smooth muscle activity regulated?

Answer 20

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

Question 21

How are smooth muscle fiber hyperplasia and hypertrophy distinguished, and how do they contribute to the pathogenesis of hypertension?

Answer 21

1) Primary pulmonary arterial hypertension (PAH) is marked by an abnormal elevation in pulmonary arterial blood pressure whereby the primary abnormality is localized to the small pulmonary arteries
2) Thickening of the tunica media in the vascular smooth muscle of pulmonary arteries may play a role in pathogenesis by hyperplasia (proliferation) and hypertrophy (increase in cell size) of smooth muscle cells accompanied by inhibition of cell apoptosis