APK 3110C Lecture Notes - Lecture 8: Multinucleate, Energy, Hypothalamus
6 May 2018
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APK 3110C
Week 8
Chapter 18
• Skeletal muscle cells have multiple nuclei
• Cardiac muscle cells have a single central nucleus
• Skeletal muscles are voluntary
• Cardiac muscle is involuntary
• Skeletal muscle fiber types – Type 1, Type 2a and 2x
• Cardiac muscle fiber types – only 1 Fiber type
Skeletal muscle composition
• 75% of muscle mass is water
• 20% of muscle mass is protein
o titin – largest and most abundant protein (10% of muscle mass)
o myosin (60% of muscle protein)
o actin and tropomyosin
• 5%
o salts, high-energy phosphates, lactate, calcium, etc.
• 0.7 grams of myoglobin per 100 grams of muscle tissue
• Endomysium – connective tissue around muscle fiber
• Perimysium – surrounds bundle of up to 150 fibers
• Epimysium – fibrous connective tissue that surrounds the entire muscle
• Tendons – connect ends of muscle to periosteum (bones covering)
Muscle Structure
• Sarcolemma – thin elastic membrane surrounding contents of muscle fibers
o Beneath endomysium
o Has plasma membrane and basement membrane
▪ Lipid bilayer
▪ Membrane separates/insulates each fiber from one another during
depolarization
• Sarcoplasm
o Contains enzymes, fat, glycogen particles, and nuclei
• Sarcoplasmic Reticulum (SR)
o Network of tubular channels and vesicles
o Gives cell structural integrity
o Important in spreading muscle contraction of signal from outside to inside of
muscle through T-tubule system
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Muscle Blood Supply
• Exercising increases blood flow and blood supply
• Vessels surround each fiber to deliver oxygenated blood
• During exercise
o 70 times increase in muscle oxygen uptake
▪ increased blood flow
• ilkig atio = pulsatile lood flo
o contraction = decreased flow
o relaxation = increased flow
• increased dormant capillary dilation
o up to 4 capillaries/muscle fiber
• Endurance training increases capillary to muscle fiber ration which leads to about a 40%
difference in VO2max with training
• Increased muscle capillary number = increased VO2max
• Intense aerobic training leads to increased blood flow during exercise which leads to
increased vascular stretch and shear stress on vessel walls which leads to stimulation of
capillary development
• What does increased capillarization help?
o Increases exercise capacity:
▪ Faster removal of heat, metabolic byproducts, enhanced delivery of
oxygen, nutrients, hormones from muscle
Cellular Organization
• Muscle fibers
o Single cells
o Multinucleated
o Surrounded by the sarcolemma
• Myofibrils
o Contractile element
o Surrounded by the sarcoplasm
• Cellular organelles
o Lie between myofibrils (mitochondria, sarcoplasmic reticulum, etc.)
Organization of Skeletal Muscle: Myofibrils
• Myofibrils make up muscle fibers
• Myosin and actin filaments make up myofibrils
• Myosin: thick filaments; dark bands
• Actin: thin filaments; light bands
• Myosin and actin overlap
• Sarcomere
o Part of myofibril between z-disks
o Shortens during contraction
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• Z-disk
o Cross-wise/perpendicular
o Attach to z-disk of surrounding myofibrils
o Attach end of actin
• A-band
o Length of myosin
• I-band
o Only actin
• H-Zone
o Only myosin
• M-line
o Middle line
What Are Striations?
• The striated pattern is due to the regular organization of the contractile proteins
• The sarcomere is the functional unit
• The banding pattern is only seen in polarized light or in stained tissue
o I – isotropic = appearing light
o A – anisotropic = appearing dark
• The sarcomere is made up of a complete A band and the 2 halves of the I band adjacent
to it
Which Band Shortens I or A?
• The I band shortens
• The A band appears dark (myosin)
• The I band appears light (actin)
o So, when the I band filament is pulled into the A band, the I band is obscured
The Actin Filament
• F-actin
o Double-stranded helix backbone
o Composed of polymerized G-actin molecules
o ADP bound to each G-actin (active sites)
▪ Myosin heads bind to active sites during contraction
• Troponin
o Protein molecule with 3 subunits
▪ I – binds actin
▪ T – binds tropomyosin
▪ C – binds Calcium
• Tropomyosin
o Wrapped around F-actin Helix
o Covers active sites during rest
o Prevents interaction with myosin
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