Muscles Flashcards
What is skeletal muscle responsible for?
Voluntary movement of bones that underpins locomotion
Control of inspiration through contraction and relaxation of diaphragm
Skeletal muscle pump helps venous blood return to the heart
What is the neurotransmitter used at neuromuscular junctions?
Acetylcholine
What is the sarcoplasmic reticulum? What is a triad?
Specialised endoplasmic reticulum in skeletal muscle cells
T tubule runs through two areas near the SR which forms a triad- this is where the wave of depolarisation caused by an AP travels
What does the arrival of an action potential at a neuromuscular junction cause?
Increase in intracellular calcium
What are the three classes of skeletal muscle?
Slow oxidative (I) Fast oxidative (IIa) Fast glycolytic (IIX/B)
What muscle type is fatigueable?
Fast glycolytic
What are the differences between the appearance of different muscle types?
Fast glycolytic is white whereas the two oxidative types have a red appearance
Types of muscle in order of most mitochondria to least
Fast oxidative, slow oxidative, fast glycolytic
Types of muscle in order of highest to lowest glycogen content
Fast glycolytic, fast oxidative, slow oxidative
What are the differences between slow and fast muscle fibres?
Slow fibres are half the diameter of fast fibres and take longer to contract after nerve stimulation
Fast fibres take 10msec or less to contract
What is aerobic exercise?
Typically sustained, low level exercise stimulating slow fibres
Conversion of IIx to IIa
Increased muscle fatigue resistance but no change in muscle strength
What is anaerobic exercise?
Typically brief, intense exercise stimulating fast fibres
No change in the number of muscle fibres but enlargement of myofibril size by addition of new myofilaments- increased diameter of muscle fibres which is called hypertrophy
How is energy delivered in the initial stage of exercise?
Body relies on stored energy and anaerobic glycolysis
How is energy released in the intermediate stages?
Muscle cells have reserves of ATP and phosphocreatine
Build up of AMP, ADP and phosphate stimulates metabolic pathways involved in energy production
Creatine is recycled into P-creatine in mitochondria at rest
What enzyme catalyses the formation of ATP + creatine from ADP + PCr?
Creatine kinase
What enzyme catalyses the formation of ATP + ADP from ADP + ADP as it accumulates from the breakdown of ATP?
Adenylate kinase
At what two points of the glycolysis pathway does glycogen enter during anaerobic energy release?
Glycogenolysis of glycogen to produce glucose-1-phosphate
Glucose-1-phosphate conversion to glucose-6-phosphate
How does glucose enter the glycolysis pathway?
Uptake from blood via GLUT4 transporters
What is muscle fatigue and how is it caused?
Inability to maintain a desired power output
Build up of pyruvate from anaerobic energy release which is converted into lactic acid
H+ from lactic acid lowers cell pH and causes fatigue
What are the advantages and disadvantages of anaerobic respiration in muscles?
Advantage- produces ATP in the absence of oxygen
Disadvantage- ATP yield is low and toxic products are generated
How much ATP does aerobic and anaerobic respiration yield per glucose molecule?
30
2
What happens during extended periods of exercise?
Lactate and alanine used by the liver to generate new glucose
Lactate can be released by non-exercising muscles, body acts to replenish glucose stores
Mobilisation of non-muscle lipids by increasing circulating fatty acids which are taken up by muscles
Break down of triacylglycerols stored in muscle
What is central fatigue?
Minor factor in trained exercise
What is peripheral fatigue?
At the level of the muslce fibre, with multiple factors
What is high and low frequency fatigue?
High- alteration in cell Na/K balance, particularly relevant to type II fibres
Low- reduced release of Ca2+ from sarcoplasmic reticulum which is more apparent at low frequency stimulation, involves type I fibres
What is ATP depletion?
Intense stimulation can cause large drops in ATP near sites of cross bridge formation and ATPases
How is cardiac muscle different to skeletal muscle?
Striated like skeletal muscle but myocytes are shorter and are more branched
Myocytes join together at the intercalated disc through gap junctions which enables electrical coupling
What is smooth muscle responsible for?
Mechanical control of organ systems
Control of blood vessel and airway diameter
Can involve circulating hormones, autonomic NS input or inflammatory mediators
What are the two types of smooth muscle?
Multi unit- electrical isolation of muscle cells allows finer motor control
Unitary- gap junctions permit coordinated contraction
Smooth muscle structure
Non striated, multiple actin fibres join up at spots known as dense bodies and thick filaments are interspersed around thin filaments
How is intracellular calcium increase different in cardiac muscle compared to skeletal muscle?
There is no mechanical interaction between voltage gated Ca2+ channels in the T tubule and ryanodine receptors in the SR
Influx of Ca2+ through T tubule channels activates ryanodine receptors- this is called calcium induced calcium release
How is Ca2+ removed from the cytoplasm?
Across the cell membrane by means of plasma membrane calcium ATPase or the electrogenic Na/Ca exchanger
Back into the sarcoplasmic reticulum via the sarco/endoplasmic reticulum calcium ATPase
How does the structure of smooth muscle cells change how their intracellular calcium is increased?
Smooth muscle lacks lacks T tubules and tryad or dyad structures
Instead have shallow invaginations called caveolae
Peripheral SR encircles the caveolae and central SR runs through the cell
How is intracellular calcium increased in smooth muscle?
Change in vm or AP can activate L-type calcium channels
Leads to CICR via activation of ryanodine receptors in the SR membrane
Activation of Gq coupled receptors leads to IP3 production and stimulation of IP3 receptors in the SR membrane
What is the role of calcium and troponin in the formation of cross bridges in skeletal and cardiac muscle?
When Ca2+ falls, it dissociates from TnC and the subsequent movements of TnT, tropomyosin and TnI block any further myosin-actin interactions
When Ca2+ rises it binds to TnC, conformational changes causes TnT to pull tropomyosin and TnI out of the way so myosin and actin can interact
What molecules tonically inhibit cross bridge formation in smooth muscle?
Calponin and Caldesmon
There is no troponin in smooth muscle cells
Stimulation of contraction involves stimulation of calmodulin by Ca2+
What are the downstream effects of activation of calmodulin in smooth muscle?
Activates myosin light chain by phosphorylating it
Removes inhibitory effects of calponin and caldesmon so cross bridge formation is facilitated
How is contraction stopped in smooth muscle?
De-phosphorylation of myosin light chain by myosin light chain phosphatase
