7 and 8 - Skeletal Muscle Flashcards
What is a sarcolemma?
The cell membrane of a striated muscle fiber cell
What is a sarcomere?
The basic functional unit of muscle - from one Z line to the next which includes both thick and thin filaments
What is the sarcoplasmic reticulum?
The smooth endoplasmic reticulum of muscle cells (myocytes)
What is the structural difference between the sarcoplasmic reticulum and the smooth ER of a normal cell?
The only difference is the medley of proteins they have
What is the functional difference between the sarcoplasmic reticulum and the smooth ER of a normal cell?
The endoplasmic reticulum synthesizes molecules, while the sarcoplasmic reticulum stores and pumps calcium ions
What are transverse tubules or T tubules?
Deep invaginations in the sarcolemma of muscle cells that allow depolarization of the membrane to quickly penetrate to the interior of the cell.
Where are T tubules typically located?
In skeletal muscle cells, T-tubules are typically located at the junction overlap between the A and I bands of the sarcomere
T tubules assume the conformation of a triad. What does this mean? What else is in the triad?
The T tubule joins together with a pair (2) of terminal cisternae to form a triad
What are terminal cisterae?
Bulbous enlarged areas of the sarcoplasmic reticulum
What is a muscle fiber?
Another name for a muscle cell - they are used interchangeably
A muscle fiber runs the entire length of a muscle so it is very long
What is a myofibril?
A myofibril is a basic rod-like unit of a muscle
But I though the muscle fiber ran the entire length of the muscle? How do the smaller myofibrils come into play?
Although a muscle fiber runs the entire length of the muscle, it is actually multinucleated and is composed of repeating myofibrils
What are the smaller myofibrils composed of?
These cylindrical structures are made up of an end-to-end chain of a repeating unit, the sarcomeres
What is titin?
Titin is a very large protein molecule that acts as a framework to hold the myosin and actin filaments in place
What are the attachments of titin?
One end of a titin molecule is attached to the Z line and the other end is attached to the myosin thick filament
What is the function of titin?
It acts as a spring that can change its length as the sarcomere contracts and relaxes - MUSCLE CONTRACTION
What is dystrophin?
A large protein that forms a rod that connects the thin actin filaments to a transmembrane protein
What is the function of dystrophin?
It adds strength to the muslces by connecting fibrils to the extracellular matrix
What is a ryanodine receptor?
Ca++ release channels
Ryanodine receptors form a class of intracellular calcium channels in muscles and neurons - It is the major cellular mediator of calcium-induced calcium release
What is a dihydropyridine (DHP) receptor?
L-type Ca++ channels in the T-tubules that act as voltage sensors
These receptors mechanically couple with the Ca++ release channels (ryanodine receptors) in the sarcoplasmic reticulum
What is the actin filament?
Thin filament
Actin-Thin
What is the myosin filament?
Thick filament
Myosin-Thick
What is the Z-disc or the Z line?
The dark vertical lines that connect the thin filaments
What is a cross bridge?
The small extensions off the thick filament that seem to be “reaching out” to the thin filaments
What is the sliding filament theory of muscle contraction?
The length of the filament does not change, rather the thin and thick filaments overlapping each other causes contraction
What does the sliding filament theory mean for the I band, H band and A band?
The A band remains unchanged
(It extends the length of the thick filament, and since the actual length of the thick filament does not change, the A band does not change)
I band decreases
(length between each thick filament)
H band decreases
(length between each thin filaments)
What is the role of tropomyosin in the sliding filament theory?
Tropomyosin attaches to actin (thin filament) and covers the active binding site so that myosin can’t bind there)
There are three subunits in the troponin complex. What are they?
Troponin I = binds to actin
Troponin T = binds to tropomyosin
Troponin C = binds to Ca++ ions
What is the role of the troponin complex in the sliding filament theory?
Responds to Ca++ when appropriate
What happens when the troponin complex binds to Ca++
When Ca++ binds to troponin C, the interaction causes tropomyosin to move, exposing the active site on actin so that myosin can bind
What happens when Ca++ is pumped back into the sarcoplasmic reticulum?
Tropomyosin can move back to cover the active site on actin
What must be present in order to detach the myosin head from the active site on actin?
ATP
How does ATP accomplish this?
The hydrolysis of ATP to ADP “cocks the myosin head” and allows binding to actin in a different conformation
Phosphate release causes the “power stroke” where the myosin head comes back to the natural (contracted) conformation and ADP is release
This cycle starts over when a new ATP attaches
What is the role of the sarcoplasmic reticulum in skeletal muscle contraction?
It is a reservoir for the Ca++ that will be released to cause skeletal muscle contraction
How does the sarcoplasmic reticulum aid in skeletal muscle relaxation?
Ca++ is pumped back into the sarcoplasmic reticulum via a Ca++ ATPase pump
Once the Ca++ is back in the sarcoplasmic reticulum, it binds to the protein calsequestrin
What is rigor mortis?
Muscle stiffness after death
What causes rigor mortis?
After death as ATP in muscle decreases there is no ATP to detach the myosin from the actin and the muscle becomes stiff
There are many steps in the process of excitation/contraction coupling. What is the general purpose of this process?
A
What is the first step in the process of excitation/contraction coupling?
First, an action potential travels down a motor neuron
What happens when the action potential reaches the presynaptic nerve terminal?
Acetylcholine (ACh) is released
What does the ACh do upon release from the presynaptic terminal?
It binds to a nicotinic acetylcholine receptor on the postsynaptic cell (which is a muscle cell)
WHat happens once ACh binds?
There is a net sodium influx into the post synaptic terminal which causes an EPP (end plate potential)
What does the EPP at the postsynaptic terminal do?
It provides enough depolarization for the voltage-gated Na+ channels to open
Once the voltage gated Na+ channels open, a __________ can initiate on the ___________.
Action potential
Muscle cell
Where does the action potential spread?
Across the entire surface of the muscle cell and down the T tubules
What is found within the membrane of T tubules that is relevant to the spread of the action potential?
Within the membrane of the T tubules you will find many DHPR (dihydropyridine receptors)
What is the importance of the DHPR receptors?
They change their conformation in response to depolarization and can therefore link to an intracellular protein known as the Ryanodine receptor
What does a change in the DHPR cause?
Changes in the Ryanodine receptor
What changes will you see in the Ryanodine receptor?
The Ryanodine receptors will open up and allow Ca++ in the sarcoplasmic reticulum to diffuse out
What happens when there is Ca++ allowed to diffuse out?
It will bind to Troponin C, which alters the conformation of Troponin C
What does the altered conformation of Troponin C allow for?
It causes tropomysoin to move, which then allows actin and myosin to bind
The ATP-actin-myosin cycle will continue as long as ______ and ______ are present in the cytoplasm
Ca++ and ATP
What is another name for the DHP receptor?
L-type Ca++ receptor
What is another name for the ryanodine receptor?
Ca++ release channel
How do action potentials spread throughout skeletal muscle?
They travel down T-tubules
What are DHP receptors or L-type Ca++ receptors again?
They are receptors on the surface of the T-tubule membranes that are sensitive to depolarization during the spread of an action potential
DHP receptors signal to ryanodine receptors or Ca++ release channels that an action potential is occurring and that Ca++ needs to be released
What are ryaodine receptors or Ca++ release channels again?
Ryanodine receptors are found in the sarcoplasmic reticulum of muscle fibers and is sensitive to signaling by DHP receptors
Once signaled, ryanodine receptors release Ca++ out of the sarcoplasmic reticulum which is necessary for muscle contraction
Which is longer, the electrical events of an action potential or the mechanical event of a muscle twitch?
The mechanical event of the muscle twitch is always longer
What determines the strength of muscle contraction?
The rate of stimulation of action potentials in the muscle cell
Is there a long refractory period in muscle cells which inhibits the summation of action potentials in the muscle cells?
No - the refractory period of the muscle cell is very short, so a motor neuron can initiate a second muscle action potential while the Ca++ level in the myofibril is still increasing from the first action potential
What does tetany or tetanization mean?
A continuous state of full activation of a muscle cell
What about the relative rates of action potentials and muscle twitches allows for tetany?
Since the AP is short and the muscle twitch is long, you can get continuous muscle twitch (contraction) because the quick AP can go so quick that the muscle twitches overlap and provide continuous muscle contraction
What is the role of phosphocreatine in regenerating ATP for muscle contraction?
The high energy bond in phosphocreatine provides the energy to re-phosphorylate ADP to ATP
What is the role of creatine kinase in regenerating ATP for muscle contraction?
Creatine kinase functions to take the energy released from the high energy bond in phosphocreatine in order to rephosphorylate ADP to ATP
What is the role of glycogen in regenerating ATP for muscle contraction?
Glycogen is stored in the muscle cells and can be metabolized to pyruvate for the rapid production of ATP
What is a motor unit?
A motor unit is the smallest group of muscle contraction
A motor neuron innervates one set of muscle fibers
What is a motor neuron pool?
A motor neuron pool consists of many motor neurons, each of which innervate a motor unit within the muscle
Fine control muscles have ______ motor units while muscles that do not need fine control have ________ motor units
Fine control = Small motor units
No fine control = Large motor units
Usually an action potential in a motor neuron will cause an action potential in all the muscle fibers of that _____________
Motor unit
What is an example of a motor unit that requires fine control?
Extraocular eye muscles - they have very small motor units
What is an example of a motor unit that does not require fine control?
Large postural muscles (deep back muscles) - they have very large motor units
How do muscles produce graded contractions?
There are two ways:
1 - Recruit more muscle fibers to increase the strength of the muscle contraction
2 - Make the fibers that are already contracting work harder
What is the most severe type of muscular dystrophy?
Duchenne muscular dystrophy
Why is Duchenne muscular dystrophy the most severe?
It has the most severe reduction of dystrophin
What is dystrophin again?
A protein of the dystrophin-glycoprotein complex that holds the thin actin filaments of a sarcomere to a transmembrane protein
What is the function of the dystrophin-glycoprotein complex?
It adds strength to muscles
What is the effect of a severe reduction in dystrophen, such as we see in Duchenne muscular dystrophy?
Progressive weakness of skeletal and cardiac muscle - usually leads to death by 30 years of age
What is the other type of muscular dystrophy that is caused by a mutation in dystrophin?
Becker muscular dystrophy
What is the difference between Becker and Duchenne?
Becker is less severe since functional dystrophi is still present, it is just altered or found in a reduced amount
There is another type of muscular dystrophy that does not involve a mutation for the gene for dystrophin, What is it?
Limb-girdle dystrophy
Where is the problem in the dystrophin-glycoprotein complex of Limb-girdle dystrophy?
There are many different types of Limb-girdle dystrophies that are associated with mutations of genes coding for other omponents of the dystrophin-glycoprotein complex and other muscle proteins
What is an isometric muscle contraction?
The muscle does NOT shorten during contraction
- Constant length
- Increased tension
What is an isotonic muscle contraction?
The muscle does NOT experience increased tension during contraction
- Shortened length
- Constant tension
