Flashcards in Lecture 16: Skeletal Muscle Tissue Deck (20)
List the three types of muscle cells
1. Skeletal Muscle (voluntary)
2. Smooth Muscle (involuntary)
3. Cardiac Muscle
Describe the characteristics of skeletal muscle
- 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.
Describe Myofiber Type I
- 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
Describe both Myofiber Type IIA, and IIB
* 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
Review them photomicrographs. No shortage of them this chapter.
Briefly Describe Smooth Muscle
- Single mononucleated cells
- No sarcomeric arrangement
- Cells innervated via ANS
- Do not respond “all-or-none”
- Cells connected via gap junctions
Describe Cardiac Muscle Characteristics
- Single mononucleated cells
- Centrally positioned nuclei
- Cells often branched
- Sarcomeric arrangement
- Cells communicate via gap junctions:
- Intercalated discs
- Cells not directly innervated
Describe the gross characteristics hierarchy of skeletal muscle
- 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.
Describe the endomysium characteristics in the skeletal muscle hierarchy
- 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
Describe the structure of a sarcomere
- 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
Describe the sliding filament theory
- 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
Describe Thick Microfilaments: Myosin
- 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
Describe the thin myofilament: 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:
-- Each tropomyosin sits in the groove between two actin strands of an actin filament.
-- Each tropomyosin spans 7 actin monomers.
Describe the 3 components of troponin
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
Describe the sarcoplasmic reticulum
- 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
- 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
What are the 8 components of the cytoskeleton?
* 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
Describe Muscular Dystrophy
- 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
Describe Satellite Cells
- 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