Myokogy Flashcards
What are skeletal muscle cells characterized by?
Skeletal muscle cells are multinucleated
Multinucleated means having more than one nucleus.
What is the name of the plasma membrane in muscle cells?
Sarcolemma
Sarcolemma translates to ‘muscle husk’.
Where are the nuclei located in skeletal muscle cells?
Just beneath the plasma membrane
The nuclei are pushed aside by myofibrils.
What organelles nearly fill the cytoplasm of skeletal muscle cells?
Myofibrils
Myofibrils are long ribbon-like structures.
What gives skeletal muscle cells their striped appearance?
Alternating light and dark bands along the length of myofibrils
This banding pattern is a characteristic feature of skeletal muscle.
What is the midline interruption in the light I band called?
Z line
The Z line is a darker area in the I band.
What is located in the center of the dark A band?
H zone
The H zone is a lighter central area within the A band.
The larger thick filaments also called myosin filaments
are made mostly of bundled molecules of the protein myosin
they also contain ATP enzymes which split ATP to generate the power for muscle contraction.
Notice that the thick filaments extend the entire length of the dark A band. Also
notice that the mid parts of the thick
filaments are smooth
but their ends are studded with small projections. These projections
sometimes called cross bridges because they link the thick and the thin filaments together during contraction. The thin
filaments are composed of the contractile protein called actin
plus some regulatory proteins that play a role in allowing
(or preventing) the myosin head binding to actin. The thin filaments
also called actin filaments
(which is actually a disc like membrane).
Notice that the light | band is an area that includes parts of two adjacent sarcomeres and contains only the thin filaments.
Although the thin filaments overlap the ends of the thick filaments
they do not extend into the middle of the relaxed
sarcomere
and thus the central region (the H zone) looks a bit lighter. When contraction occurs
filaments slide toward each other into the center of the sarcomeres
these light zones disappear because the actin and
myosin filaments are completely overlapped.
Another very important muscle fiber organelle - the sarcoplasmic reticulum SR
is a specialized smooth endoplasmic
reticulum. The interconnecting tubules and sacs of the SR surround each and every myofibril as the sleeve of a loosely
crocheted sweater surrounds your arm. The major role of this elaborate system is to store calcium and to release it “on
demand” when the muscle fiber is stimulated to contract. As you will see
calcium provides the final “go” signal for
contraction.
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What is the first special functional property of muscle cells?
Irritability
Irritability is the ability to receive and respond to a stimulus.
What is the second special functional property of muscle cells?
Contractility
Contractility is the ability to shorten forcibly when an adequate stimulus is received.
What must happen for skeletal muscle cells to contract?
They must be stimulated by nerve impulses.
What is a motor unit?
One motor neuron and all the skeletal muscle cells it stimulates.
What are the components involved in skeletal muscle contraction?
Nerve impulses and motor neurons.
Fill in the blank: The ability of muscle cells to receive and respond to a stimulus is called _______.
irritability
True or False: One motor neuron can stimulate only a single muscle cell.
False
One motor neuron may stimulate a few muscle cells or hundreds of them.
What happens if ATP is used up in muscle cells?
The muscle will not be able to contract.
What is the role of the neuromuscular junction in muscle contraction?
It serves as the site where the motor neuron communicates with the muscle cell.
When a long threadlike extension of the neuron
called the nerve fiber or axon
number of axonal terminals
each of which forms junctions with the sarcolemma of a different muscle cell.
These junctions are called Neuromuscular Junctions (literally
“nerve-muscle”).
Although the nerve endings and the muscle cells membranes are very close
they never touch. The gap between them
the synaptic cleft
is filled with tissue (interstitial) fluid.
When the nerve impulse reaches the axonal terminals
a chemical referred to as a Neurotransmitter is released. The
specific neurotransmitter that stimulates skeletal muscle cells is acetylcholine
(as” e-til-ko len)
Acetylcholine diffuses across the synaptic cleft and attaches to receptors (membrane proteins)
which are part of the
sarcolemma. If enough acetylcholine is released
the sarcolemma at that point becomes temporarily permeable to sodium
ions (Na-)
which rush into the muscle cell. This sudden inward rush of sodium ions gives the cell interior an excess of
positive ions
which upsets and changes the electrical conditions of the sarcolemma. This “upset” generates an electrical
current called an action potential. Once begun
the action potential is unstoppable; it travels over the entire surface of
the sarcolemma
conducting the electrical impulse from one end of the cell to the other. The result is contraction of the
muscle cell.
Mechanism of Muscle Contraction:
The Sliding Filament Theory
What causes the filaments to slide? This question brings us back to the cross bridges (myosin heads) that
protrude all around the ends of the thick filaments. When muscle fibers are activated by the nervous system
as previously described
the cross bridges attach to myosin binding sites on the thin filaments
sliding begins. Energized by ATP
each cross bridge attaches and detaches several times during a contraction
acting much like a tiny oar to generate tension and pull the thin filaments toward the center of the
sarcomere. As this event occurs simultaneously in sarcomeres throughout the cell
the muscle cell shortens.
The attachment of the myosin cross bridges to actin requires calcium ions
and the action potential leading
to contraction causes the sarcoplasmic reticulum to release stored calcium ions into the sarcoplasm.
When the action potential ends
calcium ions are immediately reabsorbed into the SR storage areas
muscle cell relaxes and settles back to its original length. This whole series of events takes just a few
thousandths of a second.
It should be mentioned that while the action potential is occurring
acetylcholine
is broken down by enzymes present on the sarcolemma. For this reason
a single nerve impulse produces
only one contraction. This prevents continued contraction of the muscle cell in the absence of additional
nerve impulses. The muscle cell relaxes until stimulated by the next round of acetylcholine release.
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In a relaxed muscle cell
the regulatory
proteins forming part of the actin
myofilaments prevent myosin binding.
When an action potential seeps along its
sarcolemma and a muscle cell is excited
calcium ions are released from
intracellular storage areas (the sacs of the
sarcoplasmic reticulum).
b)
The flood of calcium acts as the final trigger
for contraction
because as calcium binds to
the regulatory proteins on the actin
filaments
they change both their shape and
their position on the thin filaments
this
action exposes myosin binding sites on the
actin
to which the myosin heads can attach
and the myosin heads immediately begin
seeking out binding sites
- Protein complex
Myosin myotilament
(a)
Actin myofilament
Ca 2+
- Protein complex
Myosin myotilament
Actin myofilament
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What happens to myosin heads when ATP is split?
Myosin heads split ATP and become reoriented and energized.
What do myosin heads bind to during muscle contraction?
Myosin heads bind to actin, forming crossbridges.
What conditions are necessary for the contraction cycle to continue?
The contraction cycle continues if ATP is available and Ca²⁺ level in the sarcoplasm is high.
What occurs when myosin heads bind ATP?
As myosin heads bind ATP, the crossbridges detach from actin.
What is the action of myosin heads during the power stroke?
Myosin heads rotate toward the center of the sarcomere (power stroke).
What does the ‘all-or-none’ law of muscle physiology state?
It states that a muscle cell will contract to its fullest extent when it is stimulated adequately; it never partially contracts.
How do skeletal muscles produce graded responses?
Skeletal muscles react to stimuli with graded responses, or different degrees of shortening, by changing the speed of muscle stimulation and by changing the number of muscle cells being stimulated.
What happens during increasingly rapid stimulation of muscles?
Nerve impulses are delivered to the muscle at a very rapid rate, causing the cells to not relax completely between stimuli, resulting in stronger and smoother contractions.
What is tetanus in muscle contraction?
When the muscle is stimulated so rapidly that no evidence of relaxation is seen and the contractions are completely smooth and sustained, the muscle is said to be in tetanus, or tetanic contraction.
What is the primary role of tetanus in muscle contractions?
To produce smooth and prolonged muscle contractions.
How does the number of stimulated cells affect muscle contraction strength?
The strength of muscle contraction depends on how many cells are stimulated.
What happens when only a few muscle cells are stimulated?
The contraction of the muscle as a whole will be slight.
What occurs during the strongest muscle contractions?
All motor units are active and all muscle cells are being stimulated.
What can muscle contractions vary between?
Muscle contractions can be slight or vigorous depending on the work to be done.
What are the types of muscle contractions listed?
a) Twitch
b) Summing of contractions
c) Incomplete tetanus (unfused)
d) Complete tetanus (fused)
What is aerobic respiration?
Aerobic respiration is the metabolic pathway that uses oxygen to regenerate ATP, primarily at rest and during light exercise.
What are the end products of aerobic respiration?
The end products of aerobic respiration are carbon dioxide (CO2) and water (H2O).
How much ATP is produced from one glucose molecule during aerobic respiration?
Aerobic respiration produces 36 ATP per 1 glucose molecule.
What is a requirement for aerobic respiration to continue?
Aerobic respiration requires continuous delivery of oxygen and nutrient fuels to the muscle.
What is the speed of ATP production in aerobic respiration?
Aerobic respiration is fairly slow in terms of ATP production.
What is the primary energy source for muscle contraction?
ATP is the only energy source that can be used directly to power muscle activity.
How long can muscles store ATP for contraction?
Muscles store very limited supplies of ATP, only 4-6 seconds worth.
What are the three pathways for ATP regeneration in working muscles?
- Direct phosphorylation of ATP by creatine phosphate.
What is creatine phosphate (CP)?
Creatine phosphate is a unique high energy molecule found in muscle fibers.
How does creatine phosphate help in ATP regeneration?
Interactions between CP and ADP result in transfers of a high-energy phosphate group from CP to ADP, regenerating ATP.
How much creatine phosphate do muscle cells store compared to ATP?
Muscle cells store perhaps five times as much creatine phosphate as ATP.
How long do creatine phosphate supplies last during muscle activity?
Creatine phosphate supplies are exhausted in about 20 seconds.
What is glycolysis?
Glycolysis is a pathway for glucose breakdown that occurs without oxygen, making it an anaerobic part of the metabolic pathway.
Where does glycolysis occur?
Glycolysis occurs in the cytosol.
What is produced from glucose during glycolysis?
During glycolysis, glucose is broken down to pyruvic acid, and small amounts of energy are captured in ATP bonds (2 ATP per 1 glucose molecule).
What happens to pyruvic acid when enough oxygen is present?
When enough oxygen is present, pyruvic acid enters aerobic pathways in the mitochondria to produce more ATP.
What occurs when muscle activity is intense and oxygen is inadequate?
When muscle activity is intense and oxygen is inadequate, pyruvic acid is converted to lactic acid, and the process is referred to as anaerobic glycolysis.
How much ATP does anaerobic glycolysis produce compared to aerobic respiration?
Anaerobic glycolysis produces only about 5 percent as much ATP from each glucose molecule as aerobic respiration.
What is the speed of anaerobic glycolysis compared to aerobic respiration?
Anaerobic glycolysis is approximately 2 ½ times faster than aerobic respiration.
How long can anaerobic glycolysis provide ATP for strenuous muscle activity?
Anaerobic glycolysis can provide most of the ATP needed for 30-40 seconds of strenuous muscle activity.
What are the main shortcomings of anaerobic glycolysis?
The main shortcomings are that it uses large amounts of glucose for a small ATP harvest, and the accumulation of lactic acid promotes muscle fatigue and soreness.
What occurs when muscles are exercised strenuously for a long time?
Muscle fatigue occurs when a muscle is unable to contract even though it is still being stimulated.
What happens to a muscle without rest during prolonged activity?
The muscle begins to tire, contracts more weakly, and eventually ceases to react and stops contracting.
What is believed to cause muscle fatigue?
Muscle fatigue is believed to result from oxygen debt that occurs during prolonged muscle activity.
What happens to oxygen intake during vigorous muscle activity?
A person is not able to take in oxygen fast enough to supply the muscles with all the oxygen they need.
What factors affect how long a muscle can work without becoming fatigued?
The work a muscle can do and how long it can work depend on the quality of its blood supply.
What accumulates in muscles when they lack oxygen?
Lactic acid begins to accumulate in the muscle via the anaerobic mechanism.
What happens to the muscle’s ATP supply during fatigue?
The muscle’s ATP supply starts to run low, leading to less effective contractions.
What causes a muscle to stop contracting altogether?
Increasing acidity in the muscle and lack of ATP cause the muscle to contract less effectively and finally stop.
Does true muscle fatigue commonly occur in most people?
True muscle fatigue rarely occurs because most people feel fatigued and slow down or stop before it happens.
In which group of people does true muscle fatigue commonly occur?
True muscle fatigue commonly occurs in marathon runners, some of whom may collapse when their muscles can no longer work.
What must happen to oxygen debt after vigorous muscle activity?
Oxygen debt must be ‘paid back’ during the recovery period after activity.
What occurs during the recovery period after activity?
The individual breathes rapidly and deeply until the muscles receive enough oxygen to eliminate lactic acid and restore ATP and creatine phosphate reserves.
What is another way to get oxygen into the muscle?
Hydration is another way to get oxygen into the muscle.
What are the two types of muscle contractions?
Isotonic and Isometric
What happens during isotonic contractions?
The myofilaments slide successfully, the muscle shortens, and movement occurs.
Examples include bending the knee, rotating the arms, and smiling.
What are isometric contractions?
Contractions in which the muscles do not shorten, and tension in the muscle keeps increasing.
Example: Trying to lift a 400-pound dresser alone or pushing against a wall with bent elbows.
What is muscle tone?
A state of partial contraction in skeletal muscles that cannot be consciously controlled.
What is muscle tone?
Muscle tone is the state of continuous partial contractions of the muscle, resulting from the stimulation of different motor units by the nervous system.
What happens to muscles during inactivity?
Muscle inactivity leads to muscle weakness and wasting.
This supports the saying ‘use it or lose it!’
What are the effects of regular exercise on muscles?
Regular exercise increases muscle size, strength, and endurance.
What type of exercise is aerobic exercise?
Aerobic exercise includes activities like jogging, biking, or participating in an aerobics class.
What are the benefits of aerobic exercise?
Aerobic exercise leads to stronger, more flexible muscles, greater resistance to fatigue, improved body metabolism, enhanced digestion and elimination, better neuromuscular coordination, and a stronger skeleton.
What happens to the heart during aerobic exercise?
The heart enlarges (hypertrophies) to pump more blood with each beat.
Does aerobic exercise significantly increase muscle size?
No, aerobic exercise does not cause much increase in muscle size, even with prolonged activity.
What type of exercise leads to bulging muscles?
Bulging muscles are primarily the result of resistance, or isometric, exercises.
How much time is typically required for resistance exercises?
Resistance exercises usually require very little time, often just a few minutes every other day.
