Muscle Physiology Flashcards
What are the three types of muscle?
- Skeletal
- Cardiac
- Smooth
Where is skeletal muscle found and what is it responsible for?
Found in the arms, legs and back
-Responsible for voluntary movement
Where is cardiac muscle found and what is it responsible for?
Found in the heart
- Responsible for involuntary movement the rhythmic contractions of the heart
Where is smooth muscle found and what is it responsible for?
Found in the blood vessels, uterus, gut and other internal organs
-Responsible for involuntary contractions
What is the basis of all voluntary movement/ how does the muscle work?
-The muscle is attached to a tendon
-When the muscle contracts it pulls on the tendon and the tendon pulls across the join which moves our limb
-Muscle is attached to tendons which in turn attach to bone on both sides of a joint. Contraction of skeletal muscle pulls on the tendons resulting in flexion of the joints
What is the skeletal muscle made up of?
It is made up of muscle fibers(cells) that are orientated in the same direc tion and that are packaged together into fascicles surrounded by CT
What is a fascicle?
A bundle of muscle fibers
What are the striations along the muscle fibers?
These are the underlying mechanism that allows the muscle fibers to contract
Why are the striations of the skeletal muscle highly ordered?
Being highly ordered allows the entire muscle to contract at the same time, this means the muscle fibers will have maximal efficiency and force
Do muscle fibers all contract at the same time?
Yes
T/F: The striations of each muscle fibre are aligned in the fascicles?
True
T/F: skeletal muscles are multinucleated?
True
How do muscle fibers become multinucleated?
During embryonic development, the cells that turn into muscle fibers are myoblasts. During development, individual myoblasts fuse together to form one long muscle fiber because each myoblast contains a nucleus the mature muscle fiber is multinucleated
What is the benefit of muscle fibers being multinucleated?
- Muscle fibers don’t need to export mRNA or proteins from the nucleus to rest of the fiber. Instead the nuclei offer multiple sites for transcription/translation and allows mRNA and proteins to be made down the entire length of the muscle fiber
- Muscles use a lot of protein to contract. The more nuclei, the more copies of the gene and the more mRNA and protein the cell has. This enables the muscle fibers to make lots of protein
What are muscle fibers made of?
Myofibrils
Characteristics of the myofibril
- Extend the entire length of the muscle fiber
-When the myofibril contracts it causes the muscle fiber to shorten
-The contractile element inside the muscle
-All myofbril contract at the same time
-Have the striated appearance
Why are muscle fibers striated?
Because the myofibrils are striated
What is the dark band of the myofibril called?
A-band
What is the light band of the myofibril called?
I-band
What is the dark line in the middle of the I band called?
Z-line
What is a sarcomere?
Distance from one Z-line to the next
Describe contraction from sarcomere to muscle fiber?
When sarcomeres get shorter(distance between two Z-lines decreases), this causes the myofibrils to get shorter which then causes the muscle fiber to contract
What is the I-band made up of?
Thin filaments (actin filaments)
What is the A-band made up of?
Thick filaments (myosin filaments)
What causes the Z-line?
Thin filaments from the I-band are attached to the Z-line and extend to the A-band
What causes the M-line?
The ends of the thick filaments are attached to each other at the M-line by proteins causing it to be dark
Where do thick filaments project to?
Toward the I-bands
What is the darkest region of the sarcomere?
Where the thick and thin filaments overlap and contraction occurs here
What are crossbridges?
These are the head groups of the myosin filaments, they extend off of the myosin and interact with the actin filament for contraction
What is the arrangement of thick and thin filaments in a cross section where they overlap?
One thick filament is surrounded by six thin filaments
One thin filament is surrounded by three thick filaments
If you took a cross section of the I-band what would you see?
Only thin filaments
If you took a cross section of the H-band what would you see?
Only thick filaments
What is actin?
Actin are globular proteins that bind to each other to form long chains, typically two chains wrap around each other to form a helix
How is actin useful in other cells?
Cells can make actin to change shapes or move places and then can break apart the actin into different parts of the cell so other parts can follow along
What are the two parts of myosin?
- Long thin fiber
- Two head groups
What are thick filaments?
Bundles of myosin molecules with head groups pointing in the same direction (toward the I-band)
T/F: Thin and thick filaments point in opposite directions?
True
How do thin and thick filaments cause muscle to contract?
- Muscle activates to contract
- Head groups of the myosin reach out and grab a hold of the thin filaments and pull them toward each other
- Then myosin head groups let go of the actin and then reach out again and grab and pull. They continue to do this.
- As the myosin head groups reach, grab and pull the actin filaments get pulled overtop of the thick filaments bringing the Z-lines closer together
Are the myosin head groups all in sync as they pull on the actin filaments?
No, the myosin head groups pull and let go of the actin filaments at different times
What would happen if the myosin head groups were in sync during contraction?
They would all reach out and grab the actin at the same time and then they would all pull it at the same time but then they would also all release the myosin at the same time which would cause the muscle to relax instead of contract
Do the thick and thin filaments change length during contraction?
No, only the length of the sarcomere changes, the thin filaments just slide over top of the thick filaments
Does contraction occur down the entire myofibril at each sarcomere of the myofibril?
Yes, contraction of sarcomeres shortens the entire myofibril
Does muscle contraction require energy?
Yes
What happens if we stretch a muscle fiber too much ?
As we stretch the fiber out we pull the sarcomeres apart pulling the thin filaments away from the thick filaments, this means there are less myosin heads that can grapb actin and pull on it. This muscle fiber will thus generate lower tension
What happens if we stratch a muscle fiber so much that the thick and thin filaments no longer overlap at all?
No myosin head groups will be able to pull on the actin so no tension will be generated and no contraction will occurs
What happens is we scrunch up a muscle ?
The thick and thin filaments will be so overlapped to the point where they cannot contract because there in no room for the Z-lines to get even closer
T/F: Muscle fibers have an optimal range for contraction?
True
Describe the steps of the cross-bridge cycle
- Myosin dissociates from actin and ATP binds to the myosin head groups
- Hydrolysis of ATP changes the conformation of the head groups
3.The myosin head groups bind to actin - Loss of ADP + Pi changes the conformation of the head groups, this pulls the actin filament
What happens in Rigor Mortis?
When people die they stop making ATP. ATP is the step in the cycle that allows myosin to let go of the actin filaments. If ATP is gone all of the myosin head groups stay attached to the actin resulting in the human being stuck in the position they died in
How the contraction of voluntary muscle is initiated?
The brain produces an output from the motor region of the cerebral cortex. These neurons axons will activate motor neurons in the spinal cord that have axons going to the skeletal muscles
What is the motor unit?
A single motor neuron and all of the muscle fibers it innervates
Where are the motor neurons cell bodies located?
Gray matter of the ventral horn
Where are the motor neurons axons located?
Exit the spinal cord via the ventral roots and go to the skeletal muscle fibers
A single motor neuron can synapse onto…..
Multiple muscle fibers
A single muscle fiber can be innervated by……
One motor neuron
Motor neurons synapse to where?
The middle of the muscle fiber
Where are small motor units found and what does this mean?
Found in the eyes where we need precise movement.
The number of fibers innervated by one motor neuron is small(ex, 5)
This means that muscle fibers in the eye can be activated in increments of 5 depending on how much force is required
Where are large motor units found?
The thigh can have one motor neuron that innervates 1000 muscle fibers this is because thighs need to generate a lot of force rather than precise/controlled movement
T/F: Synapses in the brain are larger than synapses in the muscles?
False, synapses in muscles are much stronger and thus are more visible
What is the postsynaptic cell in the muscle?
The muscle fiber
What is the neuromuscular junction?
The synapse that occurs between the motor neuron and the muscle fiber
What do the presynaptic vesicles contain at the neuromuscular junction?
Acetylcholine neurotransmitter
What is the end plate?
Region of the muscle fiber that is directly across from the presynaptic terminal
What type of receptors are found along the end plate and what type of receptor are they?
Nicotinic acetylcholine receptors
These are ionotropic because they form ion channels
Nicotinic because they can be activated by nicotine
Describe the neuromuscular transmission
- Action potential is initiated in the spinal cord
- Action potential propagates down the axon to the presynaptic terminal
- Presynaptic terminal depolarizes which opens up the calcium channels and calcium flows in
- vesicles fuse to the plasma memebrane and release acetylcholine into the synaptic cleft
- Acetylcholine binds to the nicotinic acetylcholine receptors opening their ion channels
- Ion channels are permeable to sodium which flows into the endplate
How does sodium entering the endplate affect the muscle fiber?
Influx of sodium causes the muscle fiber to depolarize (Endplate potential)
EPSP vs Endplate Potential
EPSP:
- A single EPSP hardly depolarizes the postsynaptic membrane (about 1mV)
-Multiple EPSPs are needed to reach the action potential threshold
Endplate potential:
- A lot of ACh is released during an action potential which creates an enormous endplate potential (30-40 mV)
-The action potential threshold is reached 100% of the time
T/F: The muscle fiber will always fire an action potential when the motor neuron is activated?
True, this is due to the large endplate potential
How does the action potential propagate down the entrire muscle fiber from the endplate?
The entire muscle fiber is covered in voltage-gated sodium channels
The depolarization of the postsynaptic membrane will activate the voltage-gated sodium channels in the vicinity which will cause the action potential to zip down both end of the muscle fiber
How does the muscle fiber contract at the same time if the action potential starts at the endplate?
The action potential zips down both ends of the muscle fibers so fast that the muscle contracts all at the same time
Describe how the muscle acts as an on/off switch
On: An action potential is fired the muscle will contract 100% of the time
Off: No action potential, no contraction
What is the sarcoplasmic reticulum?
-Intracellular storage site made out of plasma membrane
-Stores Ca2+
What are T-tubules?
- Holes in the external plasma membrane that are connected to tubes that go deep into the muscle fiber
-T-tubules are continuous with the outside of the cell
What are the Ryanodine receptors where are they found?
-Located in the membrane of the SR
-Ion channels that have pore permeable to calcium ions
-When open they diffuse calcium outside of the SR into the cytoplasm
What are DHP receptors, where are they found?
-Located in the membrane of the T-tubules
-Voltage-gated calcium channels
-Open when the membrane is depolarized
Decribe the calcium levels in a relaxed muscle?
-Low calcium concentration in the cytoplasm
-High calcium contration in the SR
Describe the calcium concentration in a contracted muscle?
-High calcium concentration in the cytoplasm
-Low calcium concentration in the SR
Describe how the calcium triggers the muscle to contract
- When the endplate is activated and the action potential propagate along the muscle fibers external membrane it will go down into the T-tublules.
- When the action potential goes down into the T-tubules that depolarizes the membrane in the T-tubule which activates the voltage-gated calcium channels (DHP)
- When these calcium channels get activated they not only open but they also change their shape. The change in shape pushes on the physical linkage betwee
- This then triggers the Ryanodine receptors to open and Ca ions to flow out of the SR into the cytoplasm
Do the DHP receptors cause the muscle to contract?
No, the DHP receptors do not allow enough calcium ions into the muscle fiber cytoplasm to result in a contraction
-These receptors are mainly voltage sensors that detect the action potential and activate the Ryanodine receptors
What causes the muscle to contract?
When the Ryanodine receptor activates and allows the influx of Ca2+ into the cytoplasm.
The spike in calcium triggers the muscle fiber to contract
What is troponin and where is it found?
-Globular protein dotted down the length of the actin filament
-Has a binding site for Ca2+
Describe the function of troponin in a contracted muscle
Ca2+ released from the SR binds to the Troponin. This results in the Troponin changing conformations which pushes the tropomyosin molecule away from the myosin binding sites on the actin filament. This then allows the myosin to bind and contract the muscle
What is Tropomyosin and where is it found ?
-Long thin protein associated with the actin filaments
-Wraps itself around the actin and covers the myosin head group binding site (relaxed muscle)
What is twitch?
Contraction of a muscle fiber in response to a single action potential
Why does the muscle fiber contraction lag behind the action potential ?
Because the action potential has to trigger the release of Ca2+, which then has to bind to troponin which then has to push tropomyosin away (takes time)
T/F: The muscle fiber contraction is instantaneous?
False, the muscle fiber rises up over 30 ms becuase it has to overcome things like inertia
Why does the muscle decay back down slowly?
That is how long it takes the get the calcium concentration back down to normal in the muscle fiber
What is tension?
Force generated by a muscle
What is recruitment?
Process of activating more motor units in a muscle to generate more force
What is summation?
The additive effect of multiple contractions/action potentials/twitches
What is an unfused tetanus?
Action potentials are applied more rapidly to muscle fibers, however there is still enough time between the action potentials for the muscle fibers to partially relax
-These muscle fibers will get more and more contracted until it reaches a steady state where it will just oscillate between two tensions
What is a fused tetanus?
When a whole bunch of twitches pile on top of each other due to extremely fast action potentials.
-Individual twitches cannot be dissected
-Muscle fiber goes from relaxed to fully contract
Why do fused tetanus cause the muscle to remain contracted?
Since action potentials are happening so fast there is no time for calcium to get pumped back into the SR, the calcium concentration in the muscle fiber will stay at a high concentration
What type of sustained contraction do muscles in our body typically do?
Fused tetanus
Why are less motor units recruited in weak contractions ?
If you want to pick up a feather less force is required which is why only a small number of motor uits are activated
When are more motor units activated?
For larger contractions (ex. picking up something super heavy)
Describe skeletal muscle energy metaboolism in the first fraction of a second of the muscle contraction
- Muscle already contains ATP in the muscle fiber
- When the muscle first starts contracting it will start consuming the ATP that it already has and converting it to ADP
- The ATP only lasts for a few twitches
What does the muscle do after it runs out of its initial supply of ATP in the muscle?
- The muscle contains creatine phosphate which will stick its phosphate onto the ADP and convert it back to ATP (catalyzed by creatine kinase)
- Creatine phosphate concentration will decrease but ATP concentration remains stable
-This can keep the ATP going for a few seconds
How are ATP levels sustained in muscles during prolonged muscle activity ?
- Glycolysis
- Oxidative Phodphorylation
How does glycolysis produce ATP and where does it occur?
Takes a glucose molecule and breaks in in half which releases 2 ATP molecules
-Occurs in the cytoplasm and in anaerobic conditions
How does oxidative phosphorylation produce ATP and where does it occur?
-Breaks down the prodcuts of glycolysis(pyruvate) this process generates a lot of ATPs
-This occurs in the mitochondria and is aerobic
-Slower process
Where does glucose come from for glycolysis and oxidative phosphorylation?
- Blood
- Internal storage of glucose as glycogen in the muscle
Do all muscle fibers use both glycolysis and oxidative phosphorylation to generate ATP?
-Some muscle fibers use only glycolysis
-Other muscle fibers depend on both glycolysis and oxidative phosphorylation
Why do muscles need ATP to be maintained?
For the crossbridge cycling process
What are the two steps muscle has built in before glycolysis and oxidative phosphorylation are activated?
- ATP already in the muscle
- Creatine phosphate
What is glycogen and how is it used?
-Long strings of glucose molecules
-When the muscle fiber is contracting it will start chopping up the glycogen molecules and feeding them into glycolysis and oxidative phosphorylation
What are the three types of skeletal muscle fibers?
- fast glycolytic fibers
- Slow oxidative fibers
- Fast oxidative fibers
T/F: Different muscles in the body emphasize different types of muscle fibers?
True
Describe the fast glycolytic fibers and when they are activated?
-Myosin with high ATPase activity (goes through crossbridge cycle fast)
-No myoglobin(white muscle)
Activated when a. large force over a short period of time is needed (ex. weightlifting)
Describe the slow oxidative fibers and when they are activated?
-Myosin with low ATPase(goes through crossbridge cycling slower)
-Myoglobin facilitate oxygen transport from blood
Activated when low levels of force over long periods of time are needed (ex. standing)
Describe the fast oxidative fibers?
-Intermediate properties between fast glycolytic and slow oxidative fibers
-Fast myosin and oxidative metabolism
What is the difference between Fast and Slow fibers?
Fast:
- Have a type of myosin that goes through the cross bridge cycling really fast (consumes ATP very fast and allows muscles to contract rapidly
Slow :
- Type of myosin that goes through the cross bridge cycle at a slower rate (consumes less ATP)
What is the difference between glycolytic and oxidative fibers?
Glycolytic:
-Fiber mainly gets it energy from glycolysis
Oxidative:
-Fiber mainly gets its energy from oxidative phosphorylation
Why does fast glycolytic fibers lack mitochondria, myoglobin and capillaries?
Mitochondria:
- Not needed since they obtain there energy mainly from glycolysis which occurs in the cytoplasm
Myoglobin/Capillaries:
- Since glycolysis is anaerobic these fibers to not require myoglobin to transport O2
-These fibers also get their glucose mainly from glycogen and thus don’t need to surrounded by capillaries
What do fast glygolytic fibers fatigue rapidly?
The pyruvate they produce via glycolysis is converted into lactic acid which fatigues the muscle fibers
Why do slow oxidative fibers have a lot of mitochondria and capillaries/myoglobin?
Mitochondria:
-This is where the oxidative phosphorylation occurs
Myoglobin:
-Binds to oxygen and transports it from the blood to the mitochondria for oxidatice phosphrylation
Capillaries:
-Blood supply that contains oxygen and glucose
Why are slow oxidative fibers red?
Myoglobin has red colour
Describe recruitment order of muscle fibers when you pick up a very light object?
-Slow oxidative fibers will be activated
Describe the recruitment order of fibers when you pick up a moderately heavy object?
- Slow oxidative fibers will be activated first
- Fast oxidative fibers will be activated second
Describe the recruitment order of fibers when you pick up a very heavy object?
- Slow oxidative fibers
- Fast oxidatve fibers
- Fast glycolytic fibers
What fibers are activated when runners run a long distance?
-Slow ooxidative fibers and fast oxidative fibers
What fibers are activated when someone is weightlifting?
Fast glycolytic fibers along with the oxidative fibers
Why is muscle fatigue important?
Protects the muscle from damage by causing the muscle to stop function before the ATP concentration is diminished
How does the muscle fiber action potential in fast glycolytic fiber lead to muscle fatigue?
When the muscle is very active there are changes in the concentration of ions that are large enough for the muscle fibers to lose their ability to fire action potentials
How does the lactic acid build up in the fast glycolytic muscle fibers lead to muscle fatigue?
The acid part of the lactic acid causes a decrease of pH in the muscle fiber. The proteins in the muscle including actin and myosin have evolved to operate optimally within a narrow pH range. When the pH drops these protein don’t function anymore and the muscle fibers won’t contract
T/F: The fatigue processes in fast glycolytic fibers recover rapidly?
True
What is a possibility for how slow oxidative fibers fatigue?
Glycogen levels drop which could cause a process that leads to muscle fiber fatigue
What is central command fatigue?
- When you start getting tired it requires more and more motivation to keep pushing yourself
-Your brain is designed to tell whatever systems are driving you forward that we need to dial it back because if we keep pushing ourselves we’re going to start to damage the muscle
How do fast glycolytic fibers get stronger and bigger?
When you lift heavy weights the muscle fibers start making more myofibrils
-More myofibrils = more force can be generated = stronger muscles
-more myofibrils makes your muscles fibres bigger which causes your muscles to look larger
What type of muscle fibers have the most myofibrils?
Fast glycolytic fibers since they generate the most force/tension
Does the strength of the slow oxidative muscle fibers change when activated?
No, the strength of these muscle fibers does not change which is why runners appear lean rather than muscular
What changes in the slow oxidative fibers during exercise?
The efficiency of the muscle fibers to utilize and process energy
-The fiber improves its ability to extract glucose and create ATP
What is muscle dammage and what is the cause?
-Inflammed muscle
-Can be due to muscle dammage
What are muscle cramps and why does dehydration cause them ?
Spontaneous contraction of the muscle
-Dehydration causes less fluid between muscle fibers which changes ion concentration and can cause the motor neuron to depolarize and cause a muscle contraction
How are smooth muscles controlled?
Autonomic nervous system
Describe the smooth muscle cells?
-Elongated
-Non-striated
-Do not contract rapidly
-Do not have highly ordered actin + myosin filaments
How does contraction occur in smooth muscle>?
Myosin molecules found inside the smooth muscle cells are activated and then they start pulling on the actin filaments
Are skeletal muscles or smooth muscle contractions more ordered?
Skeletal muscle
What activates smooth muscle contraction?
Ca2+
Where does calcium come from in smooth muscle cells?
Different sources:
-Can come from an SR
-Can have calcium channels on the surface of the cell allowing calcium to flow in
Calcium can build up slower in the cell since don’t contract as rapidly
Where does calcium bind in smooth muscle and what is the result of this binding?
Calcium binds to calmodulin and activates it
The activation of calmodulin causes the myosin light chain kinases to also activate
What do the mysoin light chain kinases do?
Phosphorylates the smooth muscle myosin and activates it leading to the contraction of the muscle
How are smooth muscle cells activated?
By extracellular signals released by the autonomic nervous system including hormones and neurotransmitters. These bind to a metabotropic receptor on the outside of the smooth muscle cell which triggers intracellular events that result in the contraction of the smooth muscle
