Skeletal Muscles Physiology Flashcards
Why do isometric contractions happen?
Load greater than Muscle tension
The force generated by the muscle is not enough to lift the load so although there is tension building no contraction that results in shortening occurs.
Tropomyosin
This is a string that overlaps or blocks the binding sites on the actin for myosin.
What gives the skeletal muscle its striated appearance?
Striations are due to thick and thin myosin and actin filaments that run parallel to the long axis
What does the A band consist of?
A band = Dark band
It has both myosin and actin overlapping or stacked.
This is where cross-bridges occur between the myosin heads and actin filaments
What is the sliding filament model?
Model describes how muscle contractions occur
- The actin and myosin filaments of striated muscle slide over each other to shorten the length of the muscle fibers.
- Myosin-binding sites on the actin filaments are exposed when Ca2+ ions bind to troponin making the myosin binding site accessible
- Crossbridges can form between actin and myosin with ATP as an energy source.
- Hydrolysis of ATP (ADP & Pi ) in the myosin heads causes a conformation change and binds to the actin filaments.
- The release of ADP from the myosin heads causes a further change in shape and generates mechanical energy that causes the actin and myosin filaments to slide over one another.
What is the twitch contraction force dependent on?
Motor unit size
Fine muscle control = smaller motor units
Large muscle control = larger motor units
Describe the anatomy of the muscle from muscle belly to muscle cell
Epimysium -> Muscle belly -> Perimysium -> Fascicles -> Endomysium -> Muscle fibers/ cells -> Sarcolemma -> Sarcoplasmic reticulum -> Myofibrils -> Sarcomeres
What is the I band?
I band = Light band
I band only contains actin filament
No overlapping of myosin and actin
What are isotonic contractions?
Muscle tension greater than Load
The tension produced by the contraction results in the shortening or lengthening of the muscle and the movement of the load.
What is the T- tubule?
Transverse Tubule is an invagination of the sarcolemma which runs between two terminal cisternae of the sarcoplasmic reticulum.
Myofibril, Fasciculus and Muscle fiber
Order for largest to smallest
Fasciculus, Muscle fiber, Myofibril
Troponin
This is a regulatory protein with forms a complex with F-actin and tropomyosin. When bound to Calcium ions it will change conformation and cause tropomyosin to move off of the myosin-binding sites so that the myosin heads can bind and a contraction can take place.
What does the high-energy form of myosin mean?
ADP and Pi bound to myosin
High affinity for actin
What is G actin?
Little beads with myosin-binding sites for the heads of myosin on the thick filament
What is a Z line made up of?
Only F actin filaments
What are the two ways increases in the frequency of APs can increase tension?
Treppe
Wave Summation
What is a motor unit?
1 Motor neuron + all the muscle fibers it innervates
What shortens in the sarcomere during contraction?
I band shortens because the actin is being pulled towards the center
H zone shortens
Sacromere shortens
How are motor units recruited?
Activation of the motor neuron activates all muscle fibers in the motor unit that it innervates
The greater the stimulus the more motor unites are activated to produce a greater force.
What happens in Treppe?
A stimulus is received immediately after a muscle twitch just ended so the muscle does not have enough time to relax and reuptake the Ca2+ so the next contraction has
more Ca2+ present → more troponin deactivated→ more cross-bridges!!! → more tension
This increase in stimulus strength continues until the Maximum tension is reached and it Plateaus
What is fused tetanus?
Rapid stimulations by motor neurons without the muscle relaxing, producing a smooth sustained contraction.
What is an Isometric contraction?
This is a contraction that occurs, which does not result in the shortening or lengthening of the muscle.
What is unfused tetanus?
Successive stimulations at the same frequency, producing a series of contractions with increasing tension.
How is a muscle contraction terminated?
Ca2+ will leave troponin so tropomyosin can re-cover the myosin-binding sites
Ca2+ will be taken back up from the sarcoplasm by the sarcoplasmic reticulum via active transport using Ca2+ ATPase in the sarcoplasmic reticulum.
What is the H zone?
H zone = Only Thick MYOSIN filaments
No overlap of myosin and actin
Can purely isotonic contractions occur?
Rarely
Isometric** contractions always proceed to the **isotonic phase of contraction.
Isometric contractions** continue (tension increases) until tension **exceeds load
Tension remains constant as muscle shortens
What would happen if there was a competitive inhibitor on the acetylcholine receptors of a muscle cell?
Muscle paralysis as it won’t be able to contract if the receptors are not working and an action potential cannot be passed on.
What is the smallest functional unit of a muscle cell?
Sarcomere
What causes summation and tetanus?
Tension depends on [Ca2+]
If stimulations keep coming at high frequencies the [Ca2+] will keep increasing in the sarcoplasm.
Eventually, the system becomes saturated:
- All troponin has Ca2+ bound to it
- Cross bridge cycling is maxed out
- Maximum tetanic contraction takes place
Triad Region
- Terminal cisternae of each sarcoplasmic reticulum (x2)
- T-tubule
What is the contraction period of a muscle twitch?
The muscle contracts as the cross-bridges are cycle.
Tension within the muscle is increasing as more cross-bridges form.
What is a muscle twitch?
A single contraction of a motor unit is caused by a single action potential.
Crossbridges cycle
- The myosin head is bound to ATP and it is in the low energy state
- The myosin head hydrolyses ATP to ADP and inorganic phosphate and it is in its high energy configuration (Cocked/ extended position)
- The myosin head binds to actin forming a cross-bridge while still to ADP, but inorganic phosphate is released.
- When ADP is released the activated myosin head reverts to its low energy bent shape, which causes the myosin to pull the actin towards the center of the sarcomere.
- Cross bridge detachment occurs when another ATP binds to the myosin head causing it to lose affinity for the actin and release.
- A new cycle begins
What is the function of Sacrolemma?
Sarcolemma is the phospholipid bilayer of the muscle cell which will invaginate the muscle cell and aid in excitation of the muscle cells as it transmits signals from neuromuscular junctions.
Why is the T-tubule important?
It transmits the action potentials deep and throughout the muscle cell and makes contact with the SR so that Ca2+ can be released and a contraction can take place at the same time in all the cells.
What is F actin?
When G actin polymerizes into strands and then double helices it is referred to as F actin
Why is the relaxation phase so long?
It takes a while for the sarcoplasmic reticulum to bring down the [Ca2+] in the sarcoplasm
What are the two regulatory muscle proteins?
Tropomyosin & Troponin
What is Titin?
- This is an elastic protein
- Supports protein in the muscle
- Anchors thick filaments between the M line and the Z line
- Provides structural support and elasticity
What is a Motor endplate?
This is a highly folded area on the sarcolemma surface with a high density of Acetylcholine receptors.
It received action potentials from motor neurons
What is wave summation?
This is where a second stimulus comes to a partially relaxed muscle producing a stronger contraction.
Contractions can overlap and add up!
What is the M-line?
M-line = Myosin
Point where two thick filaments are linked
What does the low-energy form of myosin mean?
ATP is bound to myosin
Low affinity for actin
What is the length of a sacromere?
The distance between 2 Z-lines forms a functional unit of the muscle or sarcomere
What are the two regulatory muscle proteins?
Tropomyosin & Troponin
What happens in the latent phase?
This occurs right after a stimulus
It is a short period where nothing seems to be happening, but:
- Action potential has traveled across the sarcolemma and down the T-tubule setting off the voltage-gated Ca2+ channels of DHP and Rhodopsin.
- The Ca2+ binds to troponin causing it and tropomyosin to change conformation uncovering myosin-binding sites.
- The myosin has hydrolyzed the ATP attached to ADP and Inorganic phosphate and it becomes active.
- Activated myosin binds to actin
What is the relaxation phase of the twitch?
The relaxation phase is where the muscle is no longer contracting and Ca2+ is being taken back up by the SR.
Tension is decreasing down to ZERO.
What is a Threshold stimulus?
This activates a single motor unit to produce a very small force of contraction
Excitation- Contraction coupling
- An action potential is received at the receptors on the motor endplate of the sarcolemma.
- An action potential in the muscle cell is triggered
- Action potential propagates along the sarcolemma and down T tubules.
- Ca2+ release is triggered in the Sarcoplasmic reticulum
- Ca2+ concentrations increase in the sarcoplasm
- Ca2+ binds to troponin causing a shift in tropomyosin off of the binding sites
- Crossbridge cycling occurs
What are the phases of a muscle twitch
Latent phase
Contraction phase
Relaxation phase
Describe the structure of a thick myosin filament
Myosin molecule has a head and tail
- Tail points toward M line
- Head points towards I band
- Myosin head has an actin-binding site and nucleotide-binding site for ATP and ATPase
Note: There are cross-bridges at either end of the thick filament with a bare zone where there are no cross-bridges in the middle
Can purely isometric contractions occur?
Yes
What remains the same in a sarcomere during contraction?
A band stays the same length
This is an area of overlapping of myosin and actin so the myosin/ thick filament length does not change so the overlapping does not change
How is Ca2+ released when an action potential is received?
Voltage-gated opening DHP receptors of T tubules open Ca2+ channels called ryanodine receptors in the lateral sacs of the sarcoplasmic reticulum.
Why are larger motor units recruited last?
Larger motor units are recruited last because larger neurons are more difficult to depolarize to threshold than smaller motor units which are stimulated easier.
Why is ATP needed for the muscles to function?
ATP is the molecule needed to restart every cross-bridge cycle and for every contraction to take place.
What are 3 main sources of ATP for muscle function?
Direct phosphorylation of Creatine
Anaerobic respiration (Glycolysis)
Aerobic respiration (Kreb cycle & ETC)
What is the body’s immediate energy system?
ATP-PC system
Using Creatine to store energy
What is Creatine?
A molecule that acts as an energy storage molecule for ATP, specifically at rest.
The phosphate is removed from the ATP and added to creatine creating creatine phosphate.
Why is Creatine important at rest?
Muscles are highly metabolic so they need ATP all the time to function
Problem: the presence of ATP inhibits the oxidation of glucose used to make more ATP as it is a HIGH ENERGY molecule
Solution: by converting ATP to ADP and storing some of that energy as Creatine phosphate this lowers the [ATP] allowing the muscle fibers to continually make ATP
Law of mass action and Creatine
Le Chatlier’s principle
The more reactants or products that are on a particular side of the equation will determine which direction the reaction will proceed
More ATP → More Creatine phosphate + More ADP
Need ATP → More Creatine phosphate breakdown → ATP + Creatine
How long does the ATP-PC system last for?
10 seconds
After 10 seconds of high-intensity exercise another energy source such as anaerobic or aerobic respiration takes over to provide the muscle with ATP for cross-bridge cycle
Are all the fuel sources working together to provide energy at the same time?
Yes, they all are working at the same time but they work at different rates and based on the accessibility.
Direct phosphorylation of Creatine will take a shorter time to make ATP available
WHEREAS
Both aerobic and anaerobic respiration requires several steps to get glucose into the cell and oxidize before adequate energy can be gained.
What happens at the onset of high-intensity exercise?
Immediately the ATP stores are used up and Creatine phosphate is converted back to Creatine and ATP via Creatine Kinase
How does the muscle cell metabolism change with light/moderate exercise activity?
Oxidative phosphorylation occurs while anaerobic respiration is transient
Glycogen (supplies glucose) → Up to 30 mins of glucose and fatty acids in blood will be used
GLUT 4 glucose receptor is transiently increased so that more glucose can be take-up
Oxygen supply is kept at an adequate level
How is oxygen supply kept adequate in light to moderate exercise to allow Aerobic respiration?
Ven’s
Heart
Compressed to
Dila
- Increased ventilation to ensure that there is a good oxygen supply
- Increased heart rate to increase circulation and gas exchange
- Contractions of muscles increase to aid in blood returning to the heart to become reoxygenated
- Blood vessels in the muscles and body are dilated just to increase blood flow, O2, and nutrient supply.
How does muscle cell metabolism change with heavy intensity exercise?
Anaerobic glycolysis is KING
- Oxygen is not used as efficiently so the body must rely on anaerobic respiration for energy
- Lactate or Lactic acid is produced to make NAD+ available
- Net 2 ATP per glucose (little energy)
Light exercise vs high intensity
Light exercise uses energy more effectively as Aerobic takes over ATP production after a while.
High intensity is not as effective as glycolysis is predominantly used producing 2 ATP per glucose. It is not sustainable as lactic acid production causes fatigue/ burning and low ATP production.
What is fast-twitch fibers
Fast-twitch fibers have myosin with fast ATPase activity which allows them to contract faster
Ca2+ overwhelms the system to allow for contraction but it can be taken up quickly so the muscle can rest faster.
Contracts 2-3 times more rapidly than slow fibers (more crossbridges occuring)
What are slow twitch fibers?
The muscles relax slower as myosin ATPase activity is slower as it takes a longer time for the cross-bridges to cycle
It contracts slower because Ca2+ is pumped out slower. It also takes longer to be taken back up resulting in longer relaxation periods
What are the three skeletal muscle types?
Slow Oxidative
Fast oxidative
Fast glycolytic
What characteristics allow oxidative fibers to be able to use oxidative phosphorylation as a main energy source?
Many mitochondria allow for several Kreb and ETC cycles to occur simultaneously allowing for a lot of ATP
Myoglobin stores provide extra oxygen when oxygen availability is decreasing. This ensures that Oxidative phosphorylation occurs. (Red fibers)
Small diameter allows for gas exchange to take place quickly so the muscle cells can get their O2 supply to the mitochondria
Many capillaries allow for a lot of gas exchange and nutrient supply
Resistant to fatigue (no lactic acid, alot of ATP)
What characteristics allow glycolytic fibers to be able to use anaerobic glycolysis as the main energy source?
Fewer mitochondria mean it most likely gonna rely on glycolysis due to the limited amount of ATP being provided
Many glycolytic enzymes allow the glycolysis to occur faster as substrates can be acted on faster
High glycogen stores within the muscle and livers will provide ample amounts of glucose to breakdown.
Uses little oxygen making it anaerobic therefore making it less likely to rely of the mitochondria for ATP.
Large diameter means it takes a long time for gas exchange to occur this is the reason little Oxygen is used.
Quick to fatigue due to lactic acid being produced → pH increasing → affects the glycolytic enzyme activity → decreased ATP production efficiency
No myoglobin so its white fibers or WHITE MEAT
Properties of slow oxidative fibers
Low myosin ATPase activity means that contractions take longer to occur
Possess a large number of mitochondria so more ATP can be produced during each metabolic cycle, making the fiber more resistant to fatigue.
Relatively small-diameter means they do not produce a large amount of tension.
Extensively supplied with blood capillaries to supply O2 from the red blood cells in the bloodstream.
Possess myoglobin that stores some of the needed O2 within the fibers themselves (and gives SO fibers their red color).
These features allow SO fibers to produce large quantities of ATP, which can sustain muscle activity without fatiguing for long periods of time.
Why are slow oxidative fibers useful?
They can function for long periods without fatiguing makes them useful in
maintaining posture
producing isometric contractions
stabilizing bones and joints
making small movements that happen often but do not require large amounts of energy.
Pros of slow oxidative fibers
High amounts of mitochondria mean they can make a lot of ATP (HIGH Amounts)
Slow ATPase activity means they don’t use up that much ATP or glucose so there is sufficient ATP to maintain function.
Fatigue SLOWLY
Cons of slow oxidative fibers
They have very thin muscle fibers so they are the weakest fibers and cannot produce a lot of tension.
Lower number of sarcomeres less contraction power
What are fast glycolytic fibers?
These are muscle fibers that have fast ATPase activity, but it uses glycolysis as their main energy supply.
They are the largest and the strongest.
Properties of fast glycolytic fibers?
High myosin ATPase activity allows these muscle fibers to cycle through the cross-bridge cycle fast creating stronger, faster, and more forceful contractions.
High glycogen stores allow for a steady supply of glucose so that glycolysis can occur faster.
Many glycolytic enzymes allow for the substrate to be acted on faster.
Due to the lack of need for Oxygen, these cells lack myoglobin and have less blood vessel supply so they appear white
Larger diameter means that there are more muscle fibers and therefore sarcomeres present which allows for greater tensions and contractions.
Fatigues rapidly due to the small amount of ATP produced per glucose (2)
In what situation would fast glycolytic fibers be great?
Sprinting or powerlifting
You need a lot of strong force quickly, but it cannot be sustained
Cons of fast glycolytic
Fatigues quickly
What are fast oxidative fibers?
These fibers have intermediate myosin ATPase activity and high oxidative capacity so they are aerobic.
Properties of fast oxidative fibers
Intermediated myosin ATPase activity means that it’s fast but not as fast as the glycolytic.
Can do both oxidative and glycolytic processes. As the muscle fibers contract fast it will use up a lot of ATP, but oxidative phosphorylation in the mitochondria can take a while to produce ATP so the glycolytic cycle provides ATP quickly.
Glycogen stores are present in these muscles for the glycolytic cycle
These are slow to fatigue but more rapid than slow oxidative fibers
Intermediate fiber diameter with intermediate motor unit size and force generation.
Myoglobin and capiliaries presence are high
What is the Recruitment order of the three muscle fiber types during exercise?
Muscle fibers
Slow oxidative fibers are the smallest number of motor units so they would respond first to stimulus
Fast Oxidative fibers will be recruited next due to their intermediate size
Fast glycolytic fibers are the biggest and will require a greater stimulus and will be recruited last
What connects the T-tubules and sarcoplasmic reticulum?
Junctional feet or foot proteins are comprised of 2 integral membrane proteins: dihydropyridine (DHP) and ryanodine.
What is dihydropyridine?
Voltage-sensitive protein in the cell membrane of T tubules
What is ryanodine?
A foot protein that forms a calcium channel in the membrane of the sarcoplasmic reticulum.
How does an action potential cause the release of Ca2+ from the junctional feet?
Action potential → Propagate on sarcolemma → Reaches T-tubule → causes conformation change in dihydropyridine → causes a conformational change in ryanodine → releases Ca2+ from SR into sarcoplasm.
What happens in muscles during high intensity exercise?
Glycolytic fibers are used to attain energy fast while producing a lot of power due to thick fibers
After a while lactic acid builds up
Strong contractions cause compression of blood vessels limiting blood flow to the muscles
Neuromuscular fatigue occurs which results in the depletion of acetylcholine
Altered enzyme activity occurs as the pH decreases
How long does it take muscles to recover from high intensity exercise?
minutes to hours
What is the short term response of the muscle to low intensity exercises?
The energy reserves are depleted due to anaerobic respiration taking place which uses use glycogen stores.
It takes a long time to recover because you need a good bit of glucose in the blood to replace the stores
What are the long-term responses of muscles to aerobic exercises?
- There is increased oxidative respiration capacity as some fast glycolytic fibers convert to fast oxidative fibers due to the increased need as the fibers decrease their fiber diameter.
- The mitochondria will increase in number and in size.
- Increased vascularization of the muscle fibers to ensure an adequate supply of oxygen.
What is the long term response of muscles to high intensity exercise?
Due to glycolytic fibers being predominantly needed in high-intensity exercise with quick bursts of energy there will be a decreased oxidative phosphorylation capability
Some fast oxidative fibers will convert into fast glycolytic fibers
The mitochondria will decrease in number and in size reducing the ability for oxidative.
The fiber diameter increases to produce more power
Reduced resistance to fatigue due to lactic acid production
Disuse atrophy
This occurs when the muscle fibers are not being used as readily.
Decreased activity can cause decreased muscle size as myofibrils are lost
Muscular dystrophy can occur as muscles become progressively weaker and loses mass
What is within the tendon of a skeletal muscle that allows for the tension within the muscle to be monitored?
Tendons are composed of dense regular connective tissue so they have many collagen fibers running along the length of the tendon
Integrated throughout the collagen fibers are Type Ib afferent sensory receptors which detect the degree of tension generated by the muscle by detecting the degree of stretch/ tension of the collagen fibers
Whenever the collagen fibers stretch it causes the Golgi tendon organs to stretch sending a signal via action potentials back up the axon to the brain.
What are extrafusal muscle fibers?
These are the muscle fibers that attach to the tendons which are connected to the bones
This allows them to generate movement
What is the connective tissue capsule?
This is a protective layer that protects the intrafusal fibers which are proprioreceptive
What does proprioreceptive mean?
They tell the position of the muscle, ligaments, joints in a 3D space in relation to the body.
Allows you to know where your hand is and if it is extended or flexed
What is a muscle spindle?
A bunch of intrafusal fibers encased in a connective tissue capsule under the extrafusal fibers
Not connected to the tendons
What is the function of extrafusal fibers?
Connects to the tendons
Generate movement
Innervated by alpha motor neurons
What is the function of intrafusal fibers?
Proprioceptive
Picks up the length and velocity at which the muscle is being stretched
Contractile cells of the muscle spindle
Innervated by gamma motor units
Characteristics of smooth muscle
No striations, NO SARCOMERES
Contains actin and myosin
Involuntary control (Autonomic Nervous system)
Spindle shaped
1/10 the size of skeletal muscle
No sarcomeres
Dense bodies
Generally uninucleated
