7 and 8 - Skeletal Muscle Flashcards Preview

Physiology Exam 1 > 7 and 8 - Skeletal Muscle > Flashcards

Flashcards in 7 and 8 - Skeletal Muscle Deck (83):

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



What is the myosin filament?

Thick filament



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?



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?



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