ZOO 235 - Skeletal Muscle

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Typically, both ends of skeletal muscleare connected to bone. Muscles areconnected to bone through___Exceptions include the skeletal musclesthat are attached to facial skin. .tendons
Skeletal muscle contains many ____, whichare supported by layers of connective tissue.As you probably expect, skeletal muscle alsocontains blood vessels and nerves. . fascicles
The fascicles contain many muscle cells called ___
.(muscle fibers).
____often run the length of themuscle tissue. Note, this is much longer thanmost cells. Each muscle fiber contains manynuclei and mitochondria. .Muscle fibers
The plasma membrane is called the ___
In specific regions, the sarcolemma foldsinward and tunnels deep into the interior ofthe fiber forming___ .transverse tubules or Ttubules
The cytoplasm of the muscle fiber is called the___
. sarcoplasm
How does the anatomy of the muscle fiberrelate to its function? For example, the sarcolemma of the musclefiber propagates ___ as occurs inneurons. .action potentials
Under the microscope, skeletal muscle (andcardiac muscle) appear striped or striated. This striated appearance is due to the orderlyarrangement of ____ inside the myofibrils. .myofilaments (myosin andactin proteins)
Myofibrils are composed of many repeatingunits called _____ . sarcomeres
Each sarcomere is bordered by the ___ Inthe middle of each sarcomere is the M line. . Z lines
Under a microscope, a sarcomere has distinctlight and dark regions.1. The ___ is the darkest region2. The __ is a lighter region in the centerof the A band3. The ___ is the brightest region 1. A band (A is foranisotropic).2. H zone (H is for Heller which is "light" in German)3.I band (I is forisotropic).
The contractile elements of the muscle fiberare the myofilaments. .
Thin filaments are called ___ which areattached at the Z line. .actin,
Thick filaments are called ___ which areattached at the M line. .myosin,
___ filaments span the I band and extendinto the A band. .Actin
___ filaments span the length of the Aband. . Myosin
A ________, the fundamental repeating unit ofthe myofibril that gives skeletal muscle itsstriated appearance, is bordered by ________.A) crossbridge : Z linesB) sarcomere : M linesC) sarcomere : I bandsD) sarcomere : Z linesE) crossbridge : A bands
Answer: D
A cross-sectional view of a skeletal muscle fiberthrough the H zone would reveal the presence ofwhat?A) actin, but no myosinB) actin and myosinC) myosin, but no actinD) actin and titinE) actin, myosin and titin
Answer: C
To form actin, __- actinpolymerizes into a single strand called __-actin. Note, each actin has myosin binding sites, which are important in developing muscle contraction. ..G (globular actin); F (filamentous actin)
A double helix of ___ is an actin strand. . F-actin
Two important regulatory molecules on actin filaments are tropomysosin and troponin.• ___ is a long fibrous molecule that blocks the myosin binding sites. . Tropomysin
The ___ 1. Supports protein in muscle2. Anchors thick filaments between M-line and Z-line3. Provides structural support and elasticity Titin
During contraction, the___ molecules slideover the ___ molecules and move furtherinto the H zone. Thus, the H zone and I bandshorten. The A band remains the same.This pulls the Z lines closer together,shortening the ___ . actin; myosin ; sarcomere.
Shortening the sarcomeres causes the __and ___ to shorten. Thisis muscle contraction. . myofibrils; muscle fibers
• Myosin head undergoesconformation changes swivelingback-and-forth ____ -energy form• ADP and Pi bound to myosin• High affinity for actin-Process relies on ATP hydrolysis - High
• Myosin head undergoesconformation changes swivelingback-and-forth___-energy form (rigor)• No ATP, ADP or Pi bound tomyosin• Myosin remains bound to actin-Process relies on ATP hydrolysis – Low
Asynchronous Cycling– If all the myosin molecules were synchronized,there would be times during the ___ cyclethat the ___ would passively fall back towards their original positions.– This does not happen during a continuouscontraction because the myosin molecules are out of phase.• During contraction thick and thin filaments do not break contact. .crossbridge; filaments
What triggers the rise of intracellular Ca2+ thatinitiates the cross-bridge cycle?
An action potential (AP) initiated at theneuromuscular junction.
Random Student QuestionWhich of the following statements regarding theshortening of a skeletal-muscle fiber is not true?When a skeletal-muscle fiber shortens, thea) Sarcomeres shortenb) Distance between Z lines decreasesc) Myofilaments shortend) Myofilaments slide past each othere) Length of the A bands remains the same
Answer: c, Vander’s 12th - see Ch. 9, section A.2
Inside the sarcoplasm are the ___which contain contractilemachinery (myosin and actin proteins). myofibrils,
Two important regulatory molecules on actin filaments are tropomysosin and troponin.• __ is a complex of 3 different proteins that binds tropomyosin, actin and Ca2+.Ca2+ binding to troponin shifts tropomyosin away from the myosin binding sites. Troponin
____ transmission works by thesame mechanism as neuron-neuron synapses. Neuromuscular
Some unique features of NMJs include:•Only one NMJ/muscle fiber•Very highly folded ____ side(motor endplate)•The nerve terminal spreads out over alarge area on the ____ •Large postsynaptic response to a ___ action potential, which almostalways results in an action potential in thesarcolemma. postsynaptic ; sarcolemma; neuronal
___ is an autoimmune disorderin which antibodies block the acetylcholinereceptors in the neuromuscular junction. Myasthenia gravis
What would be symptoms of myasthenia gravis?
– Muscle weakness• Difficulty lifting objects, difficulty swallowing,paralysis…
Once an action potential is triggered in themuscle, the action potential travels along thesarcolemma and into the ____ system. T-tubular system.
Inside the T-tubular system, the actionpotential activates voltage-gated Ca2+ channelscalled ___ receptors. dihydropyridine receptors (DHPR).
In addition to being a Ca2+ channels, DHPRare also believed to directly contact and openCa2+ channels on the sarcoplasmic reticulum(SR) called ___ receptors. ryanodine
The Ca2+ released from the ___ through ryanodine receptors causes muscle contraction. Muscle contraction can occur even if no Ca2+enters the cell from the DHPR, which is evidence for direct contact between the DHPR and ryanodine receptor. sarcoplasmic reticulum
the T-tubules are positioned where the ______ overlap. myofilaments
How is the signal to cause contraction terminated?– Sarcoplasmic Ca2+ concentration must be lowered1. Close the ___ receptors, which occurs at highconcentrations of Ca2+.2. Remove the _____ Ca2+. ryanodine ; extracellular
How is this Ca2+ removed? The usual method, membrane transporters. Ca2+ATPase in the ____ and Na+/Ca2+ exchanger in the ____. sarcoplasmic reticulum ; sarcolemma.
The Ca2+ channels we just discussed are named after the toxins/drugs that bind to them.– The ability of ____ to block Ca2+ channels is used to treat angina, hypertension and to delay premature labor. A common example is nifedipine. dihydropyridines
The Ca2+ channels we just discussed are named after the toxins/drugs that bind to them.- ___is a plant toxin. At low doses, it opensthe ___ receptor and at high doses, it inhibits the channel. Ryanodine; ryanodine
• This is an inherited disorder that causes the ryanodinereceptors to open upon exposure to certain anesthetics,particularly volatile anesthetics. Malignant Hyperthermia
What would happen if all of your ryanodine receptorsopened? • The massive contracture of nearly all your skeletal muscle causes a dramatic effect, including a potentially lethal rise in temperature.
Which of the following is NOT a specialization observed at the neuromuscularjunction?A) the high density of acetylcholine receptors in the motor end plateB) every action potential that reaches the axon terminal of the motor neuron willgenerate an action potential in the muscle fiber.C) the motor end plate is relatively large compared with other synapses.D) the nerve terminal fans out over a wide area of the sarcolemma.E) each muscle fiber is innervated by multiple motor neurons. Answer: E
Myasthenia gravis is an autoimmune disease - that is, a disease in which one'simmune system gradually attacks a part of one's own body, in this case thereceptors for acetylcholine at the neuromuscular junction. Which of the followingdrugs might be useful in treating this disease?A) A drug that inhibits acetylcholinesteraseB) A drug that inhibits release of acetylcholineC) Curare (a nicotinic acetylcholine acetylcholine antagonist)D) Atropine (a muscarinic acetylcholine receptor antagonist)E) All of the choices are correct Answer: A, see Ch. 9, section A.2
Malignant hyperthermia is a potentially fatal genetic disorder. When given inhaledanesthetics, patients with malignant hyperthermia can experience elevated bodytemperatures, skeletal muscle rigidity, and lactic acidosis. Which of the following molecularchanges could account for these clinical manifestations?a) reduction in the density of voltage-sensitive Na+ channels in the T tubularmembraneb) decreased voltage-sensitivity of dihydropyridine receptor channelsc) inhibition of ryanodine receptor channelsd) enhanced activity of the Ca2+ ATPase on the sarcoplasmic reticulume) prolonged opening of ryanodine receptor channels Answer: e (see first practice question for muscle), as long as ryanodine receptorchannels on the sarcoplasmic reticulum remain open, Ca2+ will continue to flood thesarcoplasm and stimulate contraction. This prolonged contraction results in heatproduction, muscle rigidity, and lactic acidosis. In contrast, factors that either inhibit Ca2+release or stimulate Ca2+ reuptake into the sarcoplasmic reticulum, or that prevent eitherdepolarization of the T tubule membrane or the transduction of the depolarization intoCa2+ release, would favor muscle relaxation.
A twitch can be measured by connecting twoends of a fiber or group of fibers to equipmentthat measures ____force. contractile
After stimulating a muscle fiber, there is a latencyperiod of a few ms before contraction begins.This is the period required for ___ to occur. EC coupling
During the contraction phase, the intracellularcalcium levels are increasing because release fromthe ____ exceeds reuptake. During relaxation, the rate of calcium reuptake exceeds the rate of release. sarcoplasmic reticulum
-Contractile elements = ____ sarcomeres
– Series elastic elements = ___ connective tissue, tendons
– Tension = ____ force exerted by contracting muscle
– Load = ____ force opposing contraction (such as weight tobe moved)
What type of twitch? • Contractile elements shorten, generatingtension• The series elastic elements lengthen• However, the load is greater than the tension• Thus, the muscle does not shorten• For example, trying to lift a ton with your armsor maintaining posture. Isometric Twitch (stays thesame length – isometric)
What type of twitch?• Contractile elements shorten, generatingtension• The series elastic elements lengthen• Because the load is less than the tension, themuscle shortens. Isotonic Twitch
Notice that during the ____ , the tension reaches a plateau. During this plateau, the force is constant.During the rising phase, the muscle is generating force, but not enough to move the load.At the plateau, there is enough force to move the load so the muscle shortens.Soon, the muscle relaxes (calciumreuptake is greater than release). isotonic twitch
Which of the following is a property of isometricskeletal muscle contraction?A) rapid shortening of the muscleB) rapid lengthening of the muscleC) load > force generated by muscleD) load = force generated by muscleE) load < force generated by muscle Answer: C, see page 335
The removal of calcium ions from the cytosol of skeletal muscle causesA) The myosin binding sites on actin to be uncovered by tropomyosinB) Tropomyosin to change conformation and thereby move troponinmolecules over cross-bridge binding sitesC) Troponin to change conformation and thereby expose cross-bridge bindingsitesD) The myosin binding sites on actin to be covered by tropomyosinE) None of the choices are correct Answer: D, see Ch. 9, section A.2
Factors Affecting Whole Muscle Contraction1. The number of fibers stimulated2. The force generated in individual musclefibers depends on the number of activecrossbridges. The number of crossbridges isinfluenced by:a) Frequency of __b) Fiber ___c) ___ of fiber a. stimulation b. diameterc. Length
Compare the duration of an action potential to that of a twitch. Multiple action potentials can be initiated during one twitch. Single isolated twitches are rareoutside the laboratory. Contractions are usually induced by multiple action potentials occurring at high frequency. Stimulating at higher frequenciescan cause summation. This is all called __ Frequency of Stimulation
If ___ twitches occur repetitively at a moderate frequency, the peak tension will increase. isometric
The cause of the increased tension during summation is that more calcium is entering the ___ - fiber “warms up” may also contribute. cytosol
Increasing the frequency of stimulation even more results in incomplete tetanus . The peaks during incomplete tetanus result because the ___ are able to move calcium back into the sarcoplasmic reticulum between action potentials. calcium pumps
During ___ tetanus, oscillations in tension due to pump activity are not observed andthe calcium remains high enough to sustain the maximum number of crossbridges. Thisis the maximum tetanic tension. fused
What frequency induces maximum tetanicstimulation? – ~50-60 Hz
What is the frequency of AC current transmittedfrom a power plant to homes and businesses(the line frequency)? – 50-60 Hz (typically 60 in the US)
As we discussed, _____influences the force a muscle fiber will generate. stimulation frequency
It may be easier to understand that ___ influences the force-generating capacity of afiber. The larger the fiber, the greater the force.• This is simply because there are more actin andmyosin filaments and hence, more crossbridgesthat can form. diameter
Another factor that influences the forcegeneratingcapacity of a fiber is its ____ length.
In short, ____ will be generatedwhen the actin and myosin filaments areoptimally aligned. maximum force
When learning control of whole musclecontraction, it is important to understand themotor unit.A motor neuron can ___ more than onemuscle fiber. The total number of muscle fibersinnervated by one motor neuron is the motorunit. innervate
Thus, an action potential in one motor neuroncan stimulate an ___ twitch in manyfibers, or just a few. all-or-none
Of course, a high frequency train of actionpotentials in one motor neuron can stimulate ____contraction in many fibers, or just a few. tetanic
If multiple____ are stimulated, thetension developed is additive.To generate a small tension, only a few will be activated.Moreover, the fibers innervated can be small orlarge, adding an additional variability to controlthe strength of contraction. motor units
Generally, small motor units innervate ____ fibers and large motor units innervatelarge diameter fibers. small diameter
Furthermore, small motor units are controlled bysmall motor neurons (smaller-than-average cellbodies and axon diameters) and large motor unitsare controlled by large motor neurons.• This is important because small motor neuronsare ____ more easily than large motorneurons. This takes us to the size principle. stimulated
Small motor units are stimulated first, becausesmall motor neurons are stimulated more easilythan large motor units. This develops a small level of _____. If more tension is needed more small motorunits are recruited. If still more tension is required, larger motor units are recruited and so on.Fine motor control is best achieved with smallmotor units and your hands have many smallmotor units. tension