skeletal muscle: whole muscle function Flashcards
organization of skeletal muscle
- explain it
Relative movements of thin (actin) and thick (myosin) filaments in striated muscle
binding of ATP to myosin head group-> Dissociation of myosin head group from actin -> hydrolysis of ATP and change in angle of myosin head group -> Binding of myosin head-group to actin -> THE POWER STROKE- release of Pi changes the angles of myosin head group-> thin filaments move relative to the thick filament
fast and slow msucle fibers
muscle contains mixture of fast (type 2) and slow (type 1) muscle fibers
- fast/ slow relates to velocity of shortening
- Velocity of shortening (i.e how rapidly thick and thin filaments slide past each other) depends upon the rate of cross-bridge cycling
- Each cross-bridge power stroke is coupled to the hydrolysis of an ATP molecule
- The rate of cross-bridge cycling depends upon the rate of ATP hydrolysis
- ATPase activity is determined by the predominant
myosin heavy chain isoform in the fibre
fast twitch used
- Use anaerobic metabolism for fuel
- Provides short bursts of speed
- Fires rapidly
- Fatigues more quickly
- Great for sprinters
slow twitch used
- Uses oxygen for fuel
- Provides continuous energy
- Offers extended muscle contraction
- Fires slowly
- High endurance
- Great for marathoners
skeletal muscle myosins
6 human skeletal muscle myosins (chromosome 17):
MYH1 -> MyHC IIx (type IIb fast fibers)
MYH2 -> MyHC IIa /type IIa fast fibers)
MYH3 -> Embryonic MyHC
MYH4
MYH8 -> Perinatal MyHC
MYH13-> extraocular MyHC I (type I slow fibers)
2 human cardiac muscle myosins (chromosome 14):
MYH6
MYH7 -> MyHC I (type I slow fibers)
Mutation in MYH2 (MyHC IIa)
Autosomal Dominant Myopathy With Congenital Joint Contractures, Opthalmoplegia and Rimmed Vacuoles
- Normal early development
- Joint contractures
- Restricted eye movements
- Muscle weakness and atrophy (shoulder, pelvic girdle, back, hands)
- Progressive from 30-50 years leading to impaired ambulation
-> In the motor domain core affecting communication between ATP-binding site and neck region of the globular head
-> Normal filament assembly, but slow actin-myosin dissociation during cross bridge cycle
-> Weakness due to small number (absence) and hypotrophy of type 2a fast fibres
fast fibres
- MyHC type II predominates
- High myosin ATPase activity gives rapid cross-bridge cycling
- High Ca2+ATPase (pump) activity in the SR gives rapid reuptake of Ca2+ and short (10 ms) contraction time
slow fibers
- MyHC type I predominates
- Low myosin ATPase gives slower (4x less) cross-bridge cycling
- Low Ca2+ATPase (pump) activity in the SR gives slower reuptake of Ca2+ and long (100ms or more) contraction time
skelteal muscle function reuqires…
ATP
(1) “Cocking” the myosin head (hydrolysis)
(2) Cross-bridge detachment (binding)
(3) Active Ca2+ reuptake into the SR (hydrolysis)
ATP required to drive muscle contraction
- ATP hydrolysis increases 20–several hundred folds during contraction, depending upon fiber type
- Fibres contain sufficient preformed ATP for a few twitches
- ATP production must meet increased demand
source of ATP:
aerobic: : ATP generated oxidative phosphorylation
anaerobic: ATP generated by glycolysis
source of ATP: creatine phosphate- SHORT TERM
creatine phosphate + ADP <-> (with creatine kinase) Creatine + ATP
- Short-term (seconds) only as creatine phosphate stores are
limited - During relaxation increasing ATP levels drive the reaction to the left, replenishing creatine phosphate
- To maintain contraction, a store of phosphate is required in the blood (creatine phosphate)
- Reversible reaction
- At rest, creatine phosphate concentration is 5x that of ATP
- When a contraction begins, ATP levels fall, and ADP levels increase driving the reaction to the right, maintaining ATP concentration
sources of ATP: oxidative phosphorylation (moderate exercise)
Glucose (+ glycogen -> Glycolysis) + fatty acids + oxygen -> oxidative Phosphorylation - ATP
At moderate levels of activity, oxygen supply is sufficient to support ATP production by oxidative phosphorylation
- 5-10 Min:Glucose from glycogen stores
- 10-30 Min:Glucose and fatty acids from the blood
- >30 Min:Fatty acids from blood
repaying oxygen debt
Following intense exercise, oxygen consumption by muscle remains elevated
- Increased production of ATP by oxidative phosphorylation restores glycogen and creatine phosphate reserves
- Metabolism of accumulated lactic acid
- Restoration of blood and interstitial fluid [O2]
Slow-Oxidative:
Slow myosin ATPase, high oxidative capacity (rich in mitochondria)
Fast-Oxidative:
Fast myosin ATPase, high oxidative/intermediate glycolytic capacities
Fast-Glycolytic:
Fast myosin ATPase, high glycolytic capacit
skeletal muscle fiber types: oxidative
Oxidative fibres (Slow, Type I):
- Rich in mitochondria for oxidative phosphorylation
- Associated with many blood vessels to supply O2
- Contain O2–binding myoglobin to aid diffusion and store O2 (“red muscle”)
skeletal muscle fiber types: glycolytic
Glycolytic fibres (Fast, Type II):
- High concentration of glycolytic enzymes and substrate
- Associated with few blood vessels
- Contain little myoglobin (“white muscle”)
