HUMAN PHYSIOLOGY - Chapter 9 - Exam 2

___ (number) of EPPs is normally more than sufficient to depolarize the muscle plasma membrane adjacent to the end-plate membrane to its threshold potential.

1

Factors Contributing to the Fatigue of Skeletal Muscle During High-Intensity, Short-Duration Exercise (3)

1.) Conduction Failure 2.) Lactic acid buildup 3.) Inhibition of cross-bridge cycling

When a muscle fiber length is _____ percent of L(0) the fiber develops no tension when stimulated. Stretching a fiber to ____ percent of L(0) pulls the filaments apart to the point where there is no overlap.

60 175

The shortening velocity is dependent on the rate of _____ _____.

ATP splitting (or hydrolysis)

The Ability of a Muscle Fiber to Generate Force and Movement Depends on the interaction of the Contractile Proteins _____ and _____.

Actin, myosin

Unfused Tetanus

At low stimulation frequencies tension may oscillate as the muscle fiber partially relaxes between stimuli.

Nerves to smooth muscles are derived from the _____ _____ of the nervous system.

Autonomic Division

Changes in cytosolic _____ concentrations control the contractile activity in smooth muscles.

Calcium

Isotonic Contraction

Contraction in which the muscle changes length while the load on the muscle remains constant.

Most of the strength gains during weight lifting is due to the _____ portion of the movement.

Eccentric

When more sodium moves into the postsynaptic terminal than potassium moving out, it produces a local depolarization of the motor end plate known as a(n) ___ _____ ______.

End-plate potential (EPP)

All neuromuscular junctions are ______. (excitatory or inhibitory)

Excitatory

The lumen of the T-tubule is continuous with the _____ ____ surrounding the ____ ______.

Extracellular fluid Muscle fiber

Fibers containing myosin with high ATPase activity are classified as ____ _____, or type _____ fibers. Fibers containing myosin with lower ATPase activity are called ____ ____, or type ____ fibers.

Fast fibers II Slow fibers I

Recruitment of motor units leads to increases in both _____ and _____.

Force Velocity

Sarcoplasmic Reticulum

Forms a series of sleeveliie segments around each myofibril. At the end of each segment are two enlarged regions known as lateral sacs that are connected to each other by a series of smaller tubular elements. The Transverse tubule (T-tubule) lies directly between the lateral sacs of adjacent segments of the sarcoplasmic reticulum.

Actin Molecule

Globular protein composed of a monomer. Polymerizes with other actin monomers to form a polymer made up of two intertwined helical chains. Chains make up the core of a thin filament. Each molecule contains a binding site for myosin.

Cardiac muscle contractions are _____. [Fixed or Graded]

Graded

Tension generated by a smooth muscle cell can be _____ by varying cytosolic calcium concentration.

Graded

Glycolytic Fibers

Have few mitochondria. Possess a high concentration of glycolytic enzymes and a large store of glycogen. Surrounded by very few blood vessels. Contain little myoglobin. Also known as white muscle fibers.

Along with neurotransmitters, the binding of _____ to their smooth muscle receptors may lead to either an increased or decreased contractile activity.

Hormones

Step 4 in the cross-bridge cycle: Following the dissociation of actin and myosin, the ATP bound to myosin is _____. If ____ is still present at this time, the cross-bridge can reattach to a new _____ _____ in the thin filament and the cross-bridge cycle repeats.

Hydrolyzed Calcium Actin monomer

Calcium-mediated Changes in Smooth Muscle vs Striated Muscle

In smooth muscle the calcium-mediated changes in the thick filaments turn on cross-bridge activity. In striated muscle, calcium mediates changes in thin filaments.

Adjacent cells within cardiac muscles are joined end to end at structures called _____ _____, within which are desmosomes that hold the cells together and to which the myofibrils attach.

Intercalated disks

Depolarization during cardiac muscle cell action potentials is partly due to an influx of calcium through voltage-gated channels. These calcium channels are known as __-_____ _____ _____ and are modified versions of the dihydropyridine (DHP) receptors that act as the voltage sensor in skeletal muscle cell excitation-contraction coupling.

L-type Ca(+2) channels

Glycolytic fibers generally have ____ diameters than oxidative fibers.

Larger

A _____ _____ occurs when a phosphorylated cross-bridge becomes dephosphorylated while still attached to actin.

Latch state

Soreness most often results from _____ contractions.

Lengthening

I Bands

Lies between the ends of the A bands of two adjacent sarcomeres. Contains the portions of the thin filaments that do not overlap the thick filaments.

Skeletal muscle cells have _____ nuclei, while smooth muscle cells have _____ nuclei.

Many One

Contraction of cardiac muscle cells occurs in response to a _____ _____ _____ that propagates through the T-tubules.

Membrane action potential

____ are undifferentiated, mononucleated cells that fuse to form a muscle fiber.

Myoblasts

High-Frequency Fatigue

Occurs in muscle fibers that fatigue rapidly if continuously stimulated, but also recover after a brief rest. Accompanies high-intensity, short-duration exercise. Examples: Weight lifting, sprinting

Sarcomere

One unit of repeating thick and thin filaments in each myofibril. Thick Filaments located in the middle. Contains two sets of thin filaments, one at each end.

Sliding-Filament Mechanism

Overlapping thick and thin filaments in each sarcomere move past each other, propelled by movements of the cross-bridges. No change in the lengths of either the thick or thin filaments. Each myosin cross-bridge attached to a thin filament actin molecule moves in an arc. Swiveling motion of many cross-bridges forces the thin filaments attached to successive Z lines to move towards the center of the sarcomere.

At moderate levels of muscular activity, most of the ATP used for muscle contraction is formed by ______ _____. During the first 5 to 10 minutes, breakdown of muscle glycogen to _____ provides the major fuel contributing to this ATP formation. For the next 30 minutes, blood-borne fuels become dominant, ____ ____ and ____ _____ contributing approximately equally.

Oxidative phosphorylation Glucose Blood glucose Fatty acids

Cross-bridge cycling in smooth muscle is controlled by a calcium-regulated enzyme that _____ the protein _____.

Phosphorylates Myosin

Fused Tetanus

Produced at high stimulation frequencies. No oscillations.

The process of increasing the number of motor units that are active in a muscle at any given time is called _____.

Recruitment

Many local factors, such as nitric acid (NO) induce smooth muscle _____.

Relaxation

Large amounts of calcium are released from the sarcoplasmic reticulum in cardiac muscle cells. This occurs because _____ receptors in the cardiac sarcoplasmic reticulum lateral sacs are calcium channels that are opened by the binding of trigger calcium in the cytosol.

Ryanodine

There are no T-tubules connected to the plasma membrane in smooth muscle. Portions of the _____ _____ are located near the plasma membrane however, forming associations similar to the relationship between T-tubules and the lateral sacs in skeletal muscle.

Sarcoplasmic reticulum

In the absence of external lengthening forces, a fiber will only ____ when stimulated; it will never _____.

Shorten Lengthen

A Band Cross Section

Shows a regular arrangement of overlapping thick and thin filaments. Each thick filament is surrounded by a hexagonal array of six thin filaments. Each thin filament is surrounded by a triangular arrangement of three thick filaments.

Junctional Feet (Foot Processes)

Structures which connect the lateral sacs of the sarcoplasmic reticulum with the T-tubules. This junction involves two integral membrane proteins: one in the T-tubule membrane, one in the membrane of the sarcoplasmic reticulum. During a T-tubule action potential changed amino acid residues within the DHP receptor protein induce a conformational change which acts via the foot process to open the ryanodine receptor channel.

Hypertrophy

The compensation for a loss of muscle tissue occurs through this satellite cell-midated increase in size.

Muscle Fatigue

The decline in muscle tension as a result of previous contractile activity. Additional characteristics: decreased shortening velocity, slower rate of relaxation.

Motor End Plate

The region of the muscle fiber plasma membrane that lies directly under the terminal portion of the axon.

Why does skeletal muscle fatigue during prolonged periods of activity, but smooth muscle doesn't?

The smooth muscle form of myosin has a very low rate of ATPase activity. Because of this, smooth muscle shortening is much slower than that of skeletal muscle. Due to this slow rate of energy usage, smooth muscle does not undergo fatigue during prolonged periods of activity.

Cross-Bridges

The space between overlapping thick and thin filaments is bridged by cross-bridges. Portions of myosin molecules that extend from the surface of the thick filaments towards the thin filaments. During muscle contraction cross-bridges make contact with the thin filaments and excerpt force on them.

What is the primary reason for cardiac muscles not being able to initiate multiple cardiac action potentials during the time frame of a single twitch? [The reason, not the mechanism]

This allows the heart to function as an oscillating pump. It must alternate between being relaxed (filling with blood,) and contracting (ejecting the blood.)

As the axon of a post-ganglionic autonomic neuron enters the region of smooth muscle cells, it divides into many branches, each branch containing a series of swollen regions known as _____.

Varicosities

A single heartbeat corresponds to the initiation and conduction of _____ action potential(s.) [number]

one

Table 9-3: Characteristics of the Three Types of Skeletal Muscle Fibers

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Twitch

The mechanical response of a muscle fiber to a single action potential.

Unlike skeletal muscle cells, smooth muscle tension can be _____ by neural activity.

Decreased

Smooth muscle thin filaments are anchored either to the plasma membrane, or to cytoplasmic structures known as _____ _____ (which are functionally similar to the Z lines in skeletal muscle fibers.)

Dense bodies

To relax a contracted smooth muscle, myosin must be _____ (because it then can no longer bind to actin.)

Dephosphorylated

Stretching smooth muscles leads to membrane _____, and therefore _____.

Depolarization Contraction

Low-Frequency Fatigue

Develops more slowly with low-intensity, long-duration exercise. Example: Long-distance running

A variety of paracrine compounds can stimulate smooth muscle cells to _____ (often in response to tissue injury.)

Divide

____-____ fibers (type ___ ) combine low myosin-ATPase activity with high oxidative capacity. ____-____-___ fibers (type ___ ) combine high myosin-ATPase activity with high oxidative capacity and intermediate glycolytic capacity. ____-____ fibers (type ___ ) Combine high myosin-ATPase activity with high glycolytic capacity.

Slow-oxidative, I Fast-oxidative-glycolytic, IIa Fast-glycolytic, IIb

Comparing sizes, Smooth muscles are much _____ than skeletal muscles.

Smaller

In smooth muscles tropomyosin is present in the thin filaments, but the regulatory protein _____ is absent.

Troponin

General Smooth Muscle Classes (2)

1.) Single-unit smooth muscles 2.) Multiunit smooth muscles

Cross-Bridge Cycling is initiated when ____ enters the cytoplasm.

Calcium

Smooth muscle contraction occurs by a _____-_____ mechanism.

Sliding-filament

Cross-Bridge Cycle (4)

1.) Attachment of the cross-bridge to a thin filament. 2.) Movement of the cross-bridge (producing tension in the thin filament.) 3.) Detachment of the cross-bridge from the thin filament. 4.) Energizing the Cross-bridge (so it can repeat the cycle.)

Sequence of Events that Occur After a Rise in Cytosolic Calcium in a Smooth Muscle Fiber (6)

1.) Calcium binds to calmodulin 2.) The calcium-calmodulin complex binds to another cytosolic protein, myosin light-chain kinase, activating the enzyme. 3.) Active myosin light-chain kinase uses ATP to phosphorylate myosin light chains in the globular head of myosin. 4.) Phosphorylation of myosin drives the cross-bride away from the thick filament backbone, allowing it to bind to actin. 5.) Cross-bridges go through repeated cycles of force generation as long as myosin light chains are phosphorylated. 6.) Myosin is dephosphorylated by myosin light-chain phosphatase, which is continuously active in smooth muscle during periods of rest and contraction.

Factors Contributing to the Fatigue of Skeletal Muscle During Low-Intensity, Long-Duration Exercise (2)

1.) Changes in regulation of the ryanodine receptor channels through which calcium exits the sarcoplasmic reticulum. 2.) Depletion of fuel substrates.

ATP Roles in the Cross-Bridge Cycle (2)

1.) Energy released from ATP hydrolysis provides energy for cross-bridge movement. 2.) ATP binding to myosin breaks the link formed between actin and myosin during the cycle (allowing the cycle to repeat.)

Roles of ATP in Smooth Muscle (2)

1.) Hydrolyzing one ATP to transfer a phosphate onto myosin light chain (phosphorylation) starts a cross-bridge cycling. 2.) After phosphorylation, one ATP per cycle is hydrolyzed to provide the energy for force generation.

Characteristics of an isotonic twitch depend upon the magnitude of the load being lifted. At heavier loads: 1.) The latent period is _____. 2.) The velocity of shortening is ______. 3.) The duration of the twitch is ______. 4.) The distance shortened is _____.

1.) Longer 2.) Slower 3.) Shorter 4.) Less

Ways a muscle fiber can form ATP (3)

1.) Phosphorylation of ADP by creatine phosphate. 2.) Oxidative phosphorylation of ADP in the mitochondria. 3.) Phosphorylation of ADP by the glycolytic pathway in the cytosol.

Some fast fibers have contraction times as short as ___ msec.

10

Some slower fibers may take ____+ msec contraction times.

100

T-tubules and lateral sacs surround the myofibrils at the region of the sarcomeres where the __ ___ and the __ ____ meet.

A bands I bands

Tetanus (Tetanic Contraction)

A maintained contraction in response to repetitive stimulation.

Motor Unit

A motor neuron plus the muscle fibers it innervates.

Relationships Between Numbers of Smooth Muscle Cells, Neurotransmitters, and Neurons (Due to Varicosities)

A number of smooth muscle cells are influenced by the neurotransmitters released by a single neuron. A single smooth muscle cell may be influenced by neurotransmitters from more than one neuron.

Step 3 in the cross-bridge cycle: The binding of ____ to ____ breaks the link between actin and myosin.

ATP Myosin

Vesicles at axon terminals of a motor neuron contain the neurotransmitter _______.

Acetylcholine (ACh)

Dissociation of actin and myosin by ATP is _____ regulation. The binding of ATP at one site of myosin _____ the affinity for actin bound at another site.

Allosteric Decreases

Isometric Contraction

Also known as a constant length contraction. Occurs when a muscle develops tension but does not shorten or lengthen. Occurs when the muscle supports a load in a constant position or attempts to move an otherwise supported load that is greater than the tension developed by the muscle.

Eccentric Contraction

Also known as a lengthening contraction. Occurs if an unsupported load is greater than the tension generated by cross-bridges. Load pulls the muscle to a longer length in spite of the opposing force produced by the cross-bridges. Occurs when an object being supported by muscle contraction is lowered.

Excitation-Contraction Coupling

An action potential in a skeletal muscle fiber lasts 1 to 2 ms. Once begun the mechanical activity following an action potential may last 100+ ms. Produces a state of increased cytosolic calcium concentration which continues to activate the contractile apparatus long after the electrical activity in the membrane has ceased.

Latent Period

An interval of a few milliseconds that follows the action potential. During the latent period the processes associated with excitation-contraction coupling are occurring.

Multiunit smooth muscles have few or no gap junctions. Each cell responds independently, and the muscles behave as multiple units. These muscles are richly innervated by branches of the _____ _____ _____. Stretching of these muscles DOES NOT induce contractile responses.

Autonomic nervous system

Why is the magnitude of a single EPP much larger than an EPSP?

Because neurotransmitter is released over a larger surface area, binding to many more receptors and opening many more ion channels.

In smooth muscles in which action potentials occur, the membrane opens voltage-gated _____ channels.

Calcium

Lateral sacs store and release _____ following membrane excitation.

Calcium

Cardiac muscle action potentials and twitches are both prolonged due to the long-lasting _____ _____. Because the plasma membrane remains refractory to additional stimuli as long as it is depolarized, it is not possible to initiate multiple cardiac action potentials during the time frame of a single twitch.

Calcium Current

Thick Filaments

Composed almost entirely of myosin protein. Located in the middle of each sarcomere. Orderly, parallel arrangement produces an A band. Myosin molecules in each end of the filament are orientated in opposite directions, so the tails are directed toward the center of the filament.

Myosin Molecule

Composed of two large polypeptide heavy chains and four smaller light chains. Polypeptides combine to form a molecule that consists of two globular heads (containing heavy and light chains,) and a long tail formed by the two intertwined heavy chains. Tail of each myosin molecule lies along the axis of the thick filament. The two globular heads extend out to the sides, forming the cross-bridges. Each head contains two binding sites: one for actin, one for ATP.

Smooth muscle filaments are oriented slightly _____ to the long axis of the cell. When the fiber shortens, the regions of the plasma membrane between the points where actin is attached to the membrane _____ _____.

Diagonally Balloon out

The maximum force a muscle generates decreases by 30 to 40% between the ages of 30 to 80. This decrease in tension-generating capacity is due primarily to a decrease in average _____ _____.

Fiber diameter

Oxidative Fibers

Fibers containing numerous mitochondria and thus have a high capacity for oxidative phosphorylation. Most of the ATP produced is dependent upon blood flow to deliver oxygen and fuel molecules to the muscle. Surrounded by many small blood vessels. Contain large amounts of myoglobin (an oxygen-binding protein.) Also known as red muscle fibers.

Rigor Mortis

Gradual stiffening of skeletal muscles that begins several hours after death. ATP concentration in cells declines after heath due to the nutrients and oxygen metabolic pathways required to form ATP no longer being supplied by the circulation. Because of a lack of ATP the cross-bridges can not break.

When the initial shortening velocity (slope) of a series of isotonic twitches is plotted as a function of the load on a single fiber, the result is a ______ curve.

Hyperbolic

The second messenger _____ _____ ( __ ) can cause the release of calcium from the sarcoplasmic reticulum, producing a smooth muscle contraction without changing the membrane potential.

Inositol triphosphate (IP3)

Dihydropyridine (DHP) Receptor

Integral membrane protein found at the T-tubule. Acts as a voltage sensor.

Ryanodine Receptor

Integral protein embedded in the sarcoplasmic reticulum membrane. Includes the foot process and also forms a calcium channel.

Many smooth muscle cells are generally _____ to form sheetlike layers of cells.

Interconnected

_____ are small cylindrical bundles which constitute the thick and thin filaments.

Myofibrils

In smooth muscles the thick and thin filaments are not organized into _____, as in striated muscles, and there is no regular alignment of these filaments into _____.

Myofibrils Sarcomeres

Smooth muscles have 1/3 the _____ concentration that striated muscles do, but have twice as much _____ concentration.

Myosin Actin

Cross-Bridge Cycling begins with the binding of an energized ______ _______ to a thin filament ______ ______.

Myosin cross-bridge Actin molecule

The enzyme _____ _____-_____ _____ mediates the dephosphorylation of myosin.

Myosin light-chain phosphatase

Central Command Fatigue

Occurs when regions of the cerebral cortex fail to send excitatory signals to the motor neurons. May cause a person to stop exercising even though the muscles are not fatigued. Pushing the body past this limit results in the sensation that runners describe as a "Second Wind."

Muscle tension and load are _____ forces.

Opposing

Elevated oxygen consumption following exercise repays the _____ ____ - that is, the increased production of ATP by oxidative phosphorylation following exercise is used to restore the energy reserves in the form of creatine phosphate and glycogen.

Oxygen debt

Step 2 in the cross-bridge cycle triggers the release of strained conformation of the energized cross-bridge which produces the movement of the bound cross-bridge and the release of ____ and ____.

P(i) ADP

The membrane potential change occurring during the spontaneous depolarization to threshold is known as a _____ _____.

Pacemaker potential

Thin Filaments

Principally composed of actin protein. Also composted of troponin proteins and tropomyosin proteins that play important roles in regulating contraction. Two sets found in each sarcomere, one at each end. One end of each filament is anchored to a network of interconnecting proteins, known as the Z line. The other end of each filament overlaps a portion of the thick filament.

Process of Phosphorylation of ADP by Creatine Phosphate

Provides very rapid means of forming ATP at the onset of contractile activity. When chemical bond between creatine and phosphate is broken the amount of energy released is about the same as that released when the terminal phosphate bond in ATP is broken. The energy along with the phosphate group can be transferred to ADP to form ATP in a reversible reaction catalyzed by creatine kinase. Energy cannot be released by myosin to drive cross-bridge activity. During periods of rest, muscle fibers build up a concentration of creatine phosphate that is approximately five times that of ATP. When ATP concentration begins to fall and ADP con rations begin to rise, mass action favors the formation of ATP from creatine phosphate.

Tropomyosin and Troponin

Rod-shaped molecule composed of two intertwined polypeptides. Length is approximately equal to seven actin monomers. Chains of tropomyosin molecules are alined end to end along the actin thin filament. Tropomyosin molecules partially cover the myosin-binding site on each actin monomer, preventing the cross-bridges from making contact with actin. Each molecule is held in the blocking position by troponin. Troponin is made of three subunits: I (inhibitory,) T (tropomyosin-binding,) and C (calcium-binding) When calcium binds to troponin the tropomyosin moves away from the myosin-binding site on each actin molecule.

_____-_____ _____ _____ behave like an electrical fence; electrical activity occurring in one location is carried out to all of the other connected cells. Stretching of these muscles DOES induce contractile responses.

Single-unit smooth muscles

Cardiac muscles have structures that resemble those found in _____ muscles, but their size and nuclei resemble those found in _____ muscles.

Skeletal Smooth

Smooth Muscle Membrane Activation vs Skeletal Muscle Membrane Activation

Skeletal muscle membrane activation: dependent on a single input. Smooth muscle membrane activation: may be receiving multiple inputs, with contractile state of the muscle dependent on the relative intensity of the various inhibitory and excitatory stimuli.

Tension

The force exerted on an object by a contracting muscle.

Load

The force exerted on the muscle by an object.

Summation

The increase in muscle tension from successive action potentials occurring during the phase of mechanical activity.

Neuromuscular Junction

The junction of an axon terminal with the motor end plate.

L(0) (Optimal Length)

The length at which the fiber develops the greatest isometric active tension.

Contraction Time

The time interval from the beginning of tension development at the end of the latent period to the peak tension. Not all Skeletal muscle fibers have the same twitch contraction time.

The characteristics of the protein _____ (which is attached to the Z line at one end and the thick filaments at the other) is responsible for most of the _____ elastic properties of relaxed muscle fibers.

Titin Passive

Thin Filaments to Thick Filaments Ratio in the Region of Filament Overlap

Twice as many thin filaments than thick filaments.

Concentric Contraction

When tension exceeds the load, shortening occurs.

Table 9-2: Sequence of Events Between a Motor Neuron Action Potential and Skeletal Muscle Fiber Contraction

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