Lecture 16: Skeletal Muscle Tissue

Question 1

List the three types of muscle cells

Answer 1

1. Skeletal Muscle (voluntary)
2. Smooth Muscle (involuntary)
3. Cardiac Muscle

Question 2

Describe the characteristics of skeletal muscle

Answer 2

• Multinucleated syncytium
• Skeletal muscle cells (fibers) are formed during embryonic development when hundreds of individual mononucleated myoblasts join end-to-end to form a myotube.
• The myotubematures into the cylindrical myocyte with hundreds of nuclei.
• Peripheral nuclei
• Sarcomeric arrangement
• Each fiber innervated via a single motor axon
• Contraction = “all-or-none”
• Contains troponin C
• Each myocyte is about 50 to 60 μm in diameter.

Question 3

Describe Myofiber Type I

Answer 3

• Intense staining for oxidative enzymes
• Rich in NADH transferase, myoglobin, ATPase Type IIA
• Possess many mitochondria
• Primarily utilize oxidative phosphorylation
• Produce slow and continuous contractions
• Referred to as dark or red fibers

Question 4

Describe both Myofiber Type IIA, and IIB

Answer 4

• Type IIA:
• Intermediate staining for oxidative enzymes
• Utilize both aerobic and anaerobic respiration for ATP production
• Contract more rapidly than type I fibers
• Resistance to fatigue Type IIB • Light staining for oxidative enzymes
• Type IIB:
• Utilize primarily anaerobic respiration for ATP production
• Contract more rapidly than type I or type IIA fibers
• Fatigue quickly
• Referred to as white or light fibers

Question 5

Review them photomicrographs. No shortage of them this chapter.

Answer 5

Do it.

Question 6

Briefly Describe Smooth Muscle

Answer 6

• Single mononucleated cells
• No sarcomeric arrangement
• Cells innervated via ANS
• Do not respond “all-or-none”
• Cells connected via gap junctions

Question 7

Describe Cardiac Muscle Characteristics

Answer 7

• Single mononucleated cells
• Centrally positioned nuclei
• Cells often branched
• Sarcomeric arrangement
• Cells communicate via gap junctions:
• Intercalated discs
• Cells not directly innervated

Question 8

Describe the gross characteristics hierarchy of skeletal muscle

Answer 8

• A muscle is composed of many fascicles.
• A fascicle is a bundle of myofibers.
• A myofiberis a bundle of myofibrils.
• A myofibril is a linear array of sarcomeres.
• A sarcomere is made up of filaments:
• • Thin filaments = actin
• • Thick filaments = myosin
• A muscle is covered by a connective tissue sheet called the epimysium.
• A fascicle is surrounded by a connective tissue sheet called the perimysium.
• • The perimysium consists of fibroblasts and type I collagen fibers
• A myofiberis a muscle cell and is surrounded by a cell membrane.
• • The cell membrane is called the sarcolemma.

Question 9

Describe the endomysium characteristics in the skeletal muscle hierarchy

Answer 9

• The endomysium covers the muscle fiber, including the sarcolemma.
• • The endomysium consists of a basal lamina and reticular collagen fibers.
• • The basal lamina is secreted by muscle cells.
• • The basal lamina anchors muscle fibers to each other.
• • The basal lamina helps to distribute the force of contraction.
• Review Slides 22-25

Question 10

Describe the structure of a sarcomere

Answer 10

• The filaments of the sarcomere are organized in such a way as to create a repeating banding pattern when observed microscopically.
• Sarcomeres are separated from each other by transverse disks called Z bands or Z disks.
• Each sarcomere consists of a middle “A” band flanked by two “I” bands.
• The A bands consist of thick (myosin) filaments.
• The width/length of the A band is equal to the length of the bundle of myosin filaments.
• Actinfilaments interdigitate into each end of the myosin filament bundle and are attached to the Z disks at the opposite ends
• The portion of the actin filaments not within the myosin bundle form the I bands.
• The H band consists only of thick (myosin) filaments
• The H band is located in the middle of the A band.
• Review Slide 27 & 28

Question 11

Describe the sliding filament theory

Answer 11

• During a contraction, forces develop between the actin filaments and myosin filaments such that the actin filaments slide between the myosin filaments from opposite ends toward the middle of the A band.
• The width of the A band does not change during a contraction.
• The width of the I bands becomes shorter during a contraction.
• The width of the H band decreases during a contraction and the H band may disappear.
• The sarcomere shortens during a contraction as evidenced by the distance between two successive Z disks becoming shorter.
• The shortening of all the sarcomeres within the muscle fiber results in the shortening of the muscle fiber during a contraction.
• Review slides 30-32

Question 12

Describe Thick Microfilaments: Myosin

Answer 12

• Thick filaments
• Myosin is a dimeric protein with long tails and two heads at one end.
• Each head has:
• • Actin binding region
• • ATP-binding region
• • Light-chain binding region
• Two pairs of light chains:
• • Similar to calmodulin but have lost the ability to bind calcium
• • Essential light chains (Thought to contribute to stability of myosin head.)
• • Regulatory light chains (Required to maintain the stability of myosin II; Sites for phosphate binding.)
• See Slides 34

Question 13

Describe the thin myofilament: actin

Answer 13

• F actin:
• Polymer of G actin
• Each actin filament consists of two polymers wound in an alpha-helix configuration.
• Plus end inserts on Z-disk
• Actin-associated molecules:
• Troponin
• Tropomyosin:
• • Each tropomyosin sits in the groove between two actin strands of an actin filament.
• • Each tropomyosin spans 7 actin monomers.

Question 14

Describe the 3 components of troponin

Answer 14

Complex of three molecules:

• Troponin I: Inhibits binding between actin and myosin
• Troponin C: Binds calcium ions
• Troponin T: Binds to tropomyosin
• See Slide 37

Question 15

Describe the sarcoplasmic reticulum

Answer 15

• The sarcoplasmic reticulum is equivalent to the endoplasmic reticulum of cells in general.
• Enlargements of the sarcoplasmic reticulum are located next to T-tubules.
• Enlargements are referred to as cisternae.
• The cisternae sequester calcium ions.
• See Slide 39

Question 16

Describe T-Tubules

Answer 16

• T-tubules are extensions of the sarcolemma that extend down into the sarcoplasm.
• T-tubules are located at the A-I junctions.
• T-tubules are spatially related to the cisternae of the sarcoplasmic reticulum.
• T-tubules provide electromechanical coupling for myofiber contraction by transferring the action potential of the sarcolemma into calcium release from the cisternae.
• See Slide 39

Question 17

What are the 8 components of the cytoskeleton?

Answer 17

• Αβ-crystallin: Heat-shock protein that protects desmin from mechanical stress
• Dystrophin: One of the proteins that links the α-actinin/desmin complex to cytoplasmic side of sarcolemma. Anchors actin to the sarcolemma. Reinforces sarcolemma during muscle contraction
• Dystroglycan complex: Links dystrophin (intracellular) to laminin-2 (extracellular)
• α-Actinin: Attaches thin filaments to the Z line
• Nebulin: Extends from Z disc to end of actin filament. Serves as template to regulate length of actin filament
• Titin: Large fibrous protein Extends from Z-disk to middle of H-band and connects ends of thick filaments to Z-line. Provides myosin with elasticity. Centers thick filaments in the sarcomere
• Desmin intermediate filaments: Framework of desmin filaments which surrounds the Z line and extends into each sarcomere. Links myofibrils laterally and to the sarcolemma
• Plectin: Binds desmin filaments
• See Slides 44-47

Question 18

Describe Muscular Dystrophy

Answer 18

• Dystrophin, a calponin, links actin filaments to transmembrane proteins of muscle cell plasma membranes.
• The transmembrane proteins link to the extracellular matrix, which helps maintain cell stability during muscle contraction.
• Muscular dystrophy, an X-linked inherited disease, results in progressive degeneration of skeletal muscle.
• Dystrophin is either absent or abnormal in patients with Duchenne’s or Becker’s muscular dystrophy, respectively.
• See Slide 49

Question 19

Describe Satellite Cells

Answer 19

• Attach to myotubes before basal lamina is laid down
• Generally quiescent
• Function as stem cells
• Can become mitotic in times of stress:
• • HGF -> C-Met Receptor (C-Met receptor is binding site for hepatic growth factor. )
• Give rise to myogenic precursor cells:
• • Replace damaged muscle by proliferating, fusing, and differentiating into skeletal muscle fibers
• Muscle contains reserve (“stem”) cells called “satellite cells.” CD56 stains satellite-cell perinuclear regions, but does not stain differentiated myocytes
• See Slide 53

Question 20

Describe the neuromuscular spindle

Answer 20

Components:

• Extrafusal fibers:
• Intrafusal fibers:
• • Nuclear bag region is the sensory region.
• • Nuclear bag fibers
• • Nuclear chain fibers
• Alpha motor neurons:
• • To extrafusal fibers
• Gamma motor fibers:
• • To intrafusal fibers
• Primary and secondary afferent fibers
• See Slide 57