Muscle Flashcards
What are the 3 types of muscle?
Skeletal, cardiac and smooth muscles
What is skeletal muscle?
Muscles used for posture and voluntary movements
What is cardiac muscle?
Muscles responsible for rhythmic contractions of the heart
What is smooth muscle?
Muscles that cause involuntary contractions
Muscles are connected to bones by […]
tendons
A single muscle cell is called a […]
Muscle fiber
A bundle of muscle fibers is called a […]
fascicle
Describe the appearance of muscle fibers.
They are long thin cells with many nuclei
Describe how muscle fibers are generated during development?
They result from the fusion of many small precursors called myoblasts. Each myoblast contributes a nuclei explaining why the fibers are multinucleated
Name 2 advantages of having more than one nucleus in a muscle fiber.
- Since the muscle fibre is very long If there was only one nucleus, the products would have to be exported all over the cell. It is more effective to have many nuclei that can serve their fibre section.
- Muscle fibres are long and make lots of proteins, which are responsible for muscle contraction. The amount of protein a cell can make is limited by how much mRNA is made. One nucleus means two copies of every gene, whereas many nuclei will allow for way more protein synthesis.
Within each muscle fibres are many […]
Myofirbils
What are myofibrils?
They are long, thin fibers that comprise the inside of a muscle fiber. They are the contractile unit of the muscle.
Describe the striations in each myofibril, including the major components.
The light bands are called I-bands and the dark bands are called A-bands. In the centre of each I-band is a dark line called the Z-line. At the centre of each A-band is the M-line, which is within the H-zone.
What is a sarcomere?
It is a section on the myofibril that goes from one Z-line to the next Z-line. This is the area over which contraction occurs.
Describe the structure of the filaments in each sarcomere.
Each sarcomere contains thick filaments and thin filaments. The thick filaments extend from one end of the A band to the other and the thin filaments extend across the I band and part way into the A band. The areas where they overlap have crossbridges sticking out of the thick filaments.
In the I-band, are there thick filaments or thin filaments or both?
Thin filament
In the H-zone, are there thick filaments or thin filaments or both?
Thick filaments
In the A-band, are there thick filaments or thin filaments or both?
Both
What are thin filaments made of?
Actin
Describe the structure of each actin filament.
Each actin filament is formed from two chains of globular actin subunits, twisted into a helix
What are thick filaments made of?
Myosin
Describe the structure of each myosin fiber and filament.
A myosin protein has a long fibre with an double-head end that form the crossbrideges. The filament is made from many myosin proteins together.
In a given sarcomere, how many thick and thin filaments are there?
There are about 6 thin filaments for each think filament or 3 thick filaments for each thin filament
Explain the sliding filament model.
it is the process by which muscles contract. The Z-lines are pulled closer together when the head groups of the thick filaments attach to the thin filaments and pull them OVER themselves.
Is the sliding filament model synchronous for all fibres in a muscle? Explain why or why not.
No, At a given time, the head groups are all at different stages of the cycling process when the muscle is contracting. If they all worked at the same time, it would be hard to do anything.
When the sarcomere contracts, are thick and thin filaments changing length?
No - only their placement is changing to shorten the sarcomere and therefore the myofibril.
What energy source drives muscle contraction?
The hydrolysis of ATP.
The amount of tension a muscle fibre can develop depends on […]. Explain
Fiber length
because the length-tension relation reflects the degree of overlap between thick and thin fibres.
If the muscle fibres are very contracted, can the muscle produce force?
The myosin will not be able to move the actin any further (Z-line are too close) and so the muscle can’t produce any force.
If the muscle fibres are very lengthened…
The myosin will have a hard time pulling actin filaments, so the muscle can’t produce as much force
Describe the 5 steps of the cross-bridge cycle.
- The myosin head group has an ATP binding and then goes through hydrolysis to form ADP + Pi, which gives it a high affinity for actin fibers.
- Myosin binds to actin, which causes a conformational change to the headgroup that causes the Pi to come off.
- The ADP is removed. This triggers another conformational change which allows for the power stroke that pulls the thin filament over the thick filament.
- The headgroup binds another ATP, causing the headgroup to detach from the actin filaments.
- The ATP is hydrolyzed, which allows the headgroup to reposition itself and for the cycle to restart.
How does voluntary muscle contraction get initiated?
Neurons from the brain activate motor neurons in the spinal cord. They are the neurons whose axons extend into the muscle fibers to communicate with them.
What is a motor unit?
A single motor neuron and all the fibres it innervates
Each motor neuron can synapse with [one/many] muscle fibers.
many
Each muscle fiber can be associated with [one/many] motor neurons.
one
How does the number of fibers innervated by a single motor neurons vary?
In certain parts of the body, like the eyes, a single motor neuron can innervate around 10 fibers, while in other parts of the body, like the hand, it can innervate around 100.
What does the the number of fibers innervated by a single motor neuron say about the function of that part of the group?
A small number of fibres per neuron allows small muscles with fine, precise movements. A large number of fibre per neuron allow large muscles that need to generate large, imprecise force.
How do the synapses in muscle fibers compare to those in the brain?
They are much larger than those in the brain.
What is the muscle synapse called?
The neuromuscular junction.
What type of neurotransmitter is released at the neuromuscular junction?
ACh: Acetylcholine
What is the postsynaptic membrane called at the neuromuscular junction?
End plate
Describe the location of the end plate relative to the whole muscle fiber.
It’s about in the middle of the fiber
What types of receptors are present on the end plate? What type of receptors are they?
There are nicotinic acetylcholine receptors which are ion channels
Name the 5 steps of neuromuscular transmission.
- An action potential occurs in the motor neuron
- Acetylcholine is released at the presynaptic terminal and binds to the receptors
- Na+ influx through activated nicotinic acetylcholine receptors
- End plate potential
- The action potential spreads through the fiber
Explain how the action potential in the motor neuron occurs.
It propagates at very high speeds to reach all parts of the fiber at about the same time
Explain how Na+ influx through nicotinic acetylcholine occurs.
There’s a great amount of Na+ that flows in because the synapse is so big and so many neurotransmitters are released. This allows AP to always be fired in muscles
Name the two major structures inside the muscle fiber aside from the myofibrils.
The T-tubules and the sarcoplasmic reticulum.
Describe the structure of the T-tubules.
They are holes in the outer membrane of the muscle fiber that are continuous with the outside of the cell.
Describe the structure of the sarcoplasmic reticulum.
It is a network of compartments on the inside of the cell and that are flush with the T-tubules.
What are the two key proteins involved in excitation-contraction coupling?
Ryanodine receptors and DHP receptors.
Describe what DHP receptors are and where they are found.
They are calcium ion channels that are located in the T-tubule membrane and touch the ryanodine receptors. They open when the T-tubule membrane gets depolarized.
Describe what Ryanodine receptors are and where they are located.
They are found in the membrane of the sarcoplasmic reticulum and touch the DHP receptors. They are also Ca2+ ion channels.
What is inside the sarcoplasmic reticulum? How does this differ from the contents of the general cytoplasm?
There is an extremely high concentration of Ca2+ ions. This is different from the cytoplasm of the muscle fiber, which has a very low concentration of Ca2+.
Describe the first part of the excitation-contracting coupling process (up to ion release).
When an action potential is fired after neuromuscular transmission and the depolarization of the endplate, the action potential spreads throughout the fiber and depolarizes the membrane, including the membrane of the T-tubules since they’re flush with the membrane. When the T-tubulus get depolarized, this activates the voltage-gated Ca2+ DHP receptors and causes a change in conformation. This pushes on the Ryanodine receptor and causes it to change conformation as well. This causes Ca2+ to flow out of the SR and into the cytoplasm of the tubule.
How does the timing work in terms of activation of different Ryanodine and DHP receptors across a muscle fiber? How does this affect muscle activity?
The excitation-contraction coupling process happens simultaneously across the whole fiber, allowing the whole muscle to contract at the same time.
Name the two proteins involved in excitation-contraction coupling AFTER calcium ion release.
Troponin and tropomyosin.
Describe the location and appearance of tropomyosin in the muscle fiber.
It is a long, filamentous protein that lines and wraps around the thin actin filaments.
Describe the location and appearance of troponin in the muscle fiber.
It is a small globular protein that is dotted along thin actin filaments at regular intervals. It binds Ca2+.
Describe the function of tropomyosin when the muscle is relaxed.
The headgroups of myosin are blocked from binding to the actin filaments by tropomyosin, as it covers the binding sites.
Describe the process of excitation-contracting coupling AFTER the release of Ca2+ ions.
After Ca2+ is released into the cytoplasm, it binds to troponin, which has a Ca2+ binding site. It then changes its shape and pushes away the tropomyosin. This allows the myosin headgroups to now bind to the actin and cause the muscle to contract (assuming there is sufficient ATP).
What is a twitch?
The contraction of a muscle fiber in response to a single action potential.
Describe the timing of the action potential vs the motion in a twitch.
The action potential occurs very shortly while the motion of the twitch takes more time to be activated
What process accounts for the time taken for the relaxation of muscle contractions?
The process where Ca2+ pumps pump back the ions into the SR
What is the latent period in a twitch?
When the action potential occured but the muscles isn’t moving yet
Force generated by a muscle is called […]
tension
The tension exerted by a whole muscle is controlled by […] and […]
recruitment, summation
What is recruitment?
An increase in the number of active muscle fibers to generate muscle tension.
What is summation?
It is the additive effects of several closely spaced twitches to generate muscle tension.
Explain summation in the muscle fiber when it gets stimulated at a regular, relatively large interval.
It creates unfused tetanus- each stimulation will cause a twitch, and the twitches will build on each other such that the fiber will reach a steady state of oscillation between contraction and relaxation.
Explain summation in the muscle fiber when it gets stimulated repeatedly at a very short interval.
It will create fused tetanus- each stimulation will cause a twitch, and the twitches are so close together that the muscle fiber will just remain contracted. (how we use muscles)
Tetanus is defined as […]
The sustained contraction of the muscle fiber.
Compare the amount of tension generated by a twitch to the tension generated by unfused tetanus.
The peaks of the unfused tenanus consist of twice as much force as a twitch.
Describe how recruitment works (in general) in the generation of muscle force.
To increase the amount of force being generated by a single muscle, the number of muscle fibers (all or nothing) that contract can be increased according to the motor unit of the muscle group.
Explain how the motor unit of the muscle group affects recruitment.
The motor unit, which can be small for something like the eye or large for something like the leg, determines the steps by which recruitment can occur. So a smaller motor unit means more fine-tuned adjustments, while a larger motor unit means more large-scale adjustments.
What is the main energy source for skeletal muscle metabolism?
ATP
Explain the first step of skeletal muscle metabolism.
The muscle has a stock of ATP, which gets converted into ADP when used. But since ATP concentrations have to remain high for the muscle to contract, the existing ATP is just enough to give the muscle time to start generating more elsewhere.
Explain the second step of skeletal muscle metabolism.
The muscle has a stock of creatine phosphate, which can take its phosphate and attach it to the ADP from step 1 to make more ATP using the enzyme creatine kinase. This converts the creatine phosphate to creatine and gives the muscle enough time for other energy-generating mechanisms to come in.
What are the two possible forms of ATP production in skeletal muscle excluding creatine phosphate?
Glycolysis and oxidative phosphorylation
How does glycolysis work in muscle cells?
Muscles can use either glucose from the blood or glycogen from the muscle itself to generate ATP (and pyruvate + lactic acid).
How does oxidative phosphorylation work in muscle cells?
The pyruvate producted by glycolysis can be used to produce more energy.
What are the 3 types of skeletal muscle fibers?
Fast glycolytic fibres, Slow oxidative fibers and fast oxidative fibers
Describe the characteristics of fast glycolytic fibers in terms of ATPase activity, myoglobin, and amount of force/unit time
Myosin with high ATPase activity
No myoglobin
For generation of large force over short periods
Describe the characteristics of slow oxidative fibers in terms of ATPase activity, myoglobin, and amount of force/unit time
Myosin with low ATPase activity
Myoglobin facilitates oxygen transport from the blood
For generation of low levels of force over long periods
Describe the characteristics of fast oxidative fibers as they compare to the two other types of skeletal muscle.
Myosin with high ATPase activity (F.G.F.)
Myoglobin facilitates oxygen transport from the blood (S.O.F.)
What is the difference between fast myosin and slow myosin? How does it affect muscle fiber activity?
Fast myosin makes the contraction fast in muscles (Fast cross-bridge cycle)
Slow myosin makes the contractions in slow in muscles (Slow cross-bridge cycle)
Which types of skeletal muscle fibers use fast myosin vs slow myosin?
FAST: white muscle - producing force (chicken wings)
SLOW: Red muscle - endurant muscles (chicken leg)
What is the major energy-generating mechanism of fast glycolytic fibers?
Glycolysis
What is the source of glucose for glycolysis in fast glycolytic fibres? Why?
Glycogen is already stored in the muscle. This allows a faster acquisition of glucose.
Aside from energy, what is the major output of glycolysis in fast glycolytic fibres?
The glycolysis causes the production of pyruvate which then changes into lactic acid
Explain the function of lactic acid in fast glycolytic fibers.
It will build up and change the pH of the muscle more acid, stopping the proteins in the cross-bridge cycle of functioning
How do concentrations of ATP change in the muscle when it is intensely used?
The muscle will easily be fatigued, but not because of ATP depletion
What is the cause of fatigue in high-intensity, short-duration activity?
- Accumulation of lactic acid
- When the fibers are highly active, the concentration of physiological ions can change enough to prevent action potential being fired
What is the major energy-generating mechanism of slow oxidative fibers?
It is Oxidative phosphorylation
What is the source of glucose in slow oxidative fibers?
From the blood
What are the 2 major inputs for the energy generation process in slow oxidative fibers?
Glucose and oxygen
What is the role of myoglobin in slow oxidative fibers?
It’s an oxygen-binding protein used to facilitate the transport of oxygen from the blood to the mitochondria
Describe the energy generation mechanism of fast oxidative fibers.
It will use glycolysis and oxidative phosphorylation like the slow oxidative fibers but rapidly use ATP.
What is the purpose of muscle fatigue?
Protects muscles from damage
What is the cause of muscle fatigue in response to low-intensity, long-duration activity?
The depletion of glycogen is most likely detected, causing the muscle to feel fatigued
What is the effect of ATP on muscle fatigue
It’s depletion is never the cause
Describe the changes in muscle physiology that can be expected after high-intensity, short-duration exercise.
It increases the diameter of fast glycolytic fibers by making more myofibrils (stronger contractions), Causing hypertrophy.
Describe the changes in muscle physiology expected after low-intensity, long-duration exercise.
Aerobic exercise causes an increase in fiber mitochondria and vascularization. In all, the metabolism of S.O.F. gets more efficient but not bigger
What is the cause of muscle soreness after exercise?
It is due to inflammation in response to tissue damage.
Physiological muscle changes in response to exercise are caused by the release of […], released by […]
factors
damage tissues
What do skeletal and smooth muscle have in common in terms of structure?
Contraction involves myosin thick filaments and actin thin filaments
Describe how the structure of smooth muscle differs from that of skeletal muscle (3 ways).
- Not highly ordered myosin/actin
- Lack striation
- Don’t produce force
Smooth muscle contraction is activated by […]
Ca 2+
Where does the Ca2+ that drives smooth muscle contraction come from?
from the SR or flowing into the cell through membrane calcium channels
Describe the steps by which smooth muscle gets activated.
Ca2+ attaches to calmodulin, myosin light chain kinase, activation of myosin
What types of signals regulate the activity of smooth muscle?
Activated by the autonomous nervous system
