Skeletal Muscle Physiology II (Week 9) Flashcards
What are the steps of the cross-bridge cycle?
1) ATP hydrolysis
2) Cross-bridge formation
3) Power stroke
4) Detachment of myosin from actin
Describe step 1 of the cross-bridge cycle
ATP hydrolysis:
- myosin heads have the capacity to work as an enzyme (ATPase) to hydrolyze the ATP, ripping the phosphate off and hold onto to both (the phosphate and the ADP)
- hydrolysis causes the myosin head to pivot, lining up with a new actin monomer
Describe step 2 of the cross-bridge cycle
Cross-Bridge Formation
- the energized myosin head attaches to the myosin-binding site on actin
- phosphate is released
- creates a myosin-actin cross-bridge
Describe step 3 of the cross-bridge cycle
Power Stroke:
- as the myosin grabs the actin, it “pushes” the actin, causing the sarcomere to shorten (brings Z lines closer together)
- ADP is released
Describe step 4 of the cross-bridge cycle
Detachment of myosin from actin:
- if ATP is around, ATP distracts myosin, and it loses connection to actin
What happens when there is no more ATP to help myosin detach from actin?
rigor mortis
(several hours after death, all muscles of the body go into contracture)
What determines the amount of tension in a contracting muscle?
the amount of action and myosin filament overlap
Note: Up to a point… too much or too little overlap decreases tension
muscle creates tension when it contracts but the overall muscle length does not change
load > force generated by muscle
isometric contraction
muscle shortens against a fixed load
force generated by muscle contraction > the load
isotonic contraction
True or False: During muscle relaxation, calcium is re-sequestered into the SR, as L-type channels return to their resting membrane potential
True
__________ pumps calcium back into the SR
SERCA
____________ pumps calcium into the ECF
sodium/calcium exchanger
What binds/traps calcium within the SR?
calsequestrin and calreticulin
What are the two types of muscle fibers?
1) slow twitch fibers (type I)
2) fast twitch fibers (type II)
What are some characteristics of slow twitch muscle fibers?
- generally smaller
- smaller nerve fibers = slower action potentials
- more capillaries to supply higher amounts of oxygen
- lots of mitochondria to support high levels of oxidative metabolism
- lots of myoglobin (gives reddish appearance)
- major storage fuel = fat
What are some characteristics of fast twitch muscle fibers?
- generally larger
- larger nerve fibers = faster action potentials
- lots of SR for rapid Ca2+ release
- lots of glycolytic enzymes present
- energy can be derived from oxidative metabolism and anaerobic metabolism, based on subtypes…
Type IIA = fast oxidative glycolytic fibers
Type IIB = fast glycolytic fibers (fastest)
- major storage fuel = glycogen
True or False: Slow twitch fibers are very resistant to fatigue, whereas fast twitch fibers fatigue quickly
True
True or False: Fast twitch fibers are used for endurance activities
False.
Slow twitch fibres = low force/endurance (e.g., walking, jogging)
Fast twitch fibers = high force (e.g., jumping, sprint)
True or False: We are generally limited in how much we can change the number of muscle fiber types we have, as this is largely predetermined by our genetics. However, we can change the size of our muscle fibers (hypertrophy) with particular types of training
True
Ex: you can train your type I’s to hypertrophy by endurance training
Ex: you can train your type II’s to hypertrophy by heavy lifting
What are the three metabolic energy systems that allow us to recycle AMP and ADP back into ATP?
1) phosphagen system (or phosphocreatine system)
2) glycogen lactic acid system
3) aerobic system
True or False: Phosphorylated creatine molecule has a high energy phosphate bond, holding more energy than ATP. It serves as a rapidly mobilizable reserve of high energy phosphates.
True
Both _______ and _______ constitute the phosphagen system
phosphocreatine,
ATP
The phosphagen system provides _________ seconds of maximal power. After this, must be replenished.
8-10s
Ex: think of a 100m sprint
Once we run out of PCr, what can we use?
Glycogen
What is the fate of glucose after consumption?
can be broken down immediately (via glycolysis) into energy for the cells OR stored in the form of glycogen
True or False: All cells are capable of storing some glycogen, but some cells can store more (e.g., hepatocytes, muscle cells)
True
What percent of the weight of hepatocytes is glycogen?
5-8%
What percent of the weight of muscle cells are is glycogen?
1-3%
The initial stage of the glycogen lactic acid system is what?
glycolysis
REVIEW: During glycolysis, _________ is split into two __________ molecules, yield a net of ____ ATP
glucose,
pyruvate,
2
If we have sufficient oxygen, what happens after glycolysis?
pyruvate gets converted to acetyl CoA
which goes on to the Kreb’s cycle/CAC
byproducts of CAC = NADH, FADH2 –> go on to ETC to generate lots of ATP
Note: we need lots of mitochondria for both the CAC and ETC
If there is insufficient oxygen, what happens after glycolysis?
pyruvate breaks down into lactic acid
through this rxn, we also generate NAD+, which feeds back into glycolysis
Note: lactic acid leaves the muscle and goes into the interstitial fluid/blood –> travels to the liver as lactate, where it is repurposed (Cori cycle) into glucose, and gives it back to the muscle
The glycogen-lactic acid system provides _______ minutes of maximal contraction
1.3-1.6 minutes
How much ATP is lost during the Cori cycle?
4 ATP
Review: In the presence of oxygen, pyruvate is broken down into CO2, H2O, and energy via the CAC and ETC.
True or False: As long as nutrients in the body last, the AEROBIC system can be used for unlimited duration
True
What type of sports might the aerobic system be useful for?
Ex: marathon running, cross-country skiing
Note: athletic events over 4-5 hours deplete glycogen stores of the muscle and depend on energy from other sources (mainly fats)
Phosphocreatine can be used to replenish levels of ______
ATP
Glycogen-lactic acid system can be used to replenish both ________ and ________
phosphocreatine,
ATP
Oxidative metabolism can be used to replenish _________________
all systems; ATP, phosphocreatine, and glycogen-lactic acid system
True or False: After exercise is over, stored oxygen must be replenished by breathing extra amounts of oxygen above normal requirements
True
repaying “oxygen debt”
How many liters of oxygen are needed to restore the phosphagen and lactic acid system?
9 litres
total oxygen that must be “repaid” after stores are used up
11.5 litres
During heavy exercise, how long does it take to use up our stored oxygen?
~ 1 minute
What is muscle strength determined by?
size
note: maximum contractile force is between 3-4kg/cm^2 of muscle cross-sectional area
measure of the amount of work the muscle can perform IN A GIVEN PERIOD OF TIME
muscle power
What is muscle power determined by?
- strength of the muscle
- distance and rate of contraction
What does muscular endurance depend on?
nutritive support
(e.g., how much glycogen has been stored in the muscle prior to the exercise period?)
Note: how much can be stored may depend on the type and size of muscle fibre (e.g., type II can store more glycogen than type I; and type II B can store more than type IIA)
With respect to muscle hypertrophy, we are not increasing the number of muscle ________ but increasing the number of ___________
fibers,
myofilaments within a muscle fiber
Note: in very RARE circumstances, under extreme muscle force generation, the number of muscle fibers may increase due to linear splitting of previously enlarged fibers –> this is called HYPERPLASIA
occurs when muscles are stretched to a greater than normal length, and causes new sarcomeres to be added at the ends of the muscle fibers
muscle lengthening
True or False: If you haven’t stretched in a long time, you may lose your muscle length
True
What can be caused by maximal force development (e.g., weight lifting) leading to an increase in actin and myosin?
hypertrophy
formation of new muscle fibers (rare) that can occur with endurance training
hyperplasia
True or False: Hypertrophy but not hyperplasia results in increased force generation. With both, there is NO change in shortening capacity or velocity of contraction.
False
Both result in increased force generation
Both result in NO change in shortening capacity nor velocity of contraction
True or False: Muscle lengthening occurs with normal growth, results in NO change in force development, increases shortening capacity, and increases contraction velocity
True
When a muscle no longer receives contractile signals (no longer being stimulated) to maintain normal muscle size, this can lead to ______________
muscle atrophy
Note: we are NOT losing muscle fibers (yet), but we are losing actin and myosin filaments
What are some causes of muscle atrophy?
- denervation/neuropathy
- tenotomy (lose a tendon)
- sedentary lifestyle
- plaster cast
- space flight (micro-gravity)
True or False: Muscle atrophy leads to…
- degeneration of contractile proteins
- decreased max force of contraction
- decreased velocity of contraction
True
True or False: If contractile signals return and reverses muscle atrophy, full return to function can occur within 12 months
False
Full return to function can occur in as little as 3 months (e.g., someone starts exercising again) as nerve supply grows back
Chronic disuse of muscle (over months/years) can lead to the loss of __________
muscle fibers
Note: A lot more challenging to recover from
Note: when this happens, we get shortening of the muscle and fibrosis; fibers are replaced by fibrous and fatty tissue with little contractile proteins… can lead to loss of mobility
