Bisc 206 Exam 3 (Ch. 10)

Contractile proteins

(Actually shift and move in contractions). Produce tension.

Regulatory proteins

(Help determine if contraction will occur). Control when the muscle fiber can contract.

Dystrophin

(Links thin filaments to sarcolemma). Structural protein found in skeletal and cardiac muscle fibers that anchors the sarcolemma to the endomysium and the myofibrils; this is the arrangement found in normal muscle tissue. In the absence of functional --, the sarcolemma breaks down and the muscle fiber is destroyed. As muscle fibers die, they are replaced with fatty and fibrous connective tissue. This process causes the whole muscle to enlarge while it's actually losing function.

Actin

(Part of thin filament). A thin filament consists of many subunits of the contractile protein --. This bead-shaped protein has an area, called the active site, that can bind to a myosin head. Multiple -- subunits string together like beads on a necklace to form the largest part of the thin filament. This -- "string" appears as two intertwining strands in the functional thin filament. A bead-shaped contractile protein found in muscle fibers and motile cells.

Structural proteins

(Provide reinforcement to retain sarcomere structure). Hold the myofilaments in their proper places and ensure the structural stability of the myofibril and the muscle fiber.

Titin

(Stabilizes myosin). Massive structural protein, shaped like a spring that can uncoil when stretched and recoil to its original shape when the stretching force is removed. -- runs through the core of the thick filament, which helps to stabilize the myofibril structurally (you can't see -- through the thick filaments in the figures because it is inside them).

Myosin

(Thick filament). A club-shaped contractile protein found in muscle fibers and cells that are motile. Looks somewhat like two golf clubs twisted together, with two globular "heads" and two intertwining polypeptide chains making up a "tail." The heads protrude from the -- tail on a "neck." The neck of each -- protein is flexible where it meets the tail at a point called the hinge. Each -- head includes a site that binds to a thin filament, among other functional components. The -- proteins are arranged within the thick filament in such a way that clusters of -- heads are found at each end, with only -- tails found in the middle. The tails point toward the M line.

Distensibility

A property of a cell by which it can be stretched without sustaining damage. Most cells will rupture when stretched, but muscle cells are -- —they can be stretched up to three times their resting length without damage.

End-plate potential

A depolarization at the motor end plate of a skeletal muscle fiber that can trigger an action potential.

Axon terminal

A knob-like structure at the end of an axon that contains synaptic vesicles with neurotransmitters. Synaptic bulb.

Thick filaments

A myofilament composed of many molecules of the protein myosin. Have the largest diameter.

Thin filament s

A myofilament composed of molecules of the proteins actin, troponin, and tropomyosin. Made of both contractile and regulatory proteins.

Elastic filaments

A myofilament that consists of the structural protein titin. Thinnest type. Composed of single massive structural protein called titin. Serve several purposes in addition to holding the thick filaments in place. Some of these functions include resisting excessive stretching and providing elasticity to the muscle fiber—that is, helping it to "spring" back to its original length after it is stretched.

Motor neuron

A neuron that transmits motor impulses from the central nervous system to a muscle or gland cell. A single neuron that communicates with many muscle fibers, each connection is known as a synapse.

Acetylcholine (ACh)

A neurotransmitter involved in a wide variety of processes, including those of the autonomic nervous system and muscle contraction. The neurotransmitter in the axon terminals of motor neurons that connect to skeletal muscle fibers.

Wave summation

A phenomenon in which repeated stimulation of a muscle fiber by a motor neuron results in muscle twitches with progressively greater tension. In general, repeated stimulation of a muscle fiber by a motor neuron results in twitches with progressively greater tension. This happens because the pumps in the SR membranes have inadequate time to pump all of the released calcium ions back into the SR before the fiber is restimulated, and so the concentration of calcium ions in the cytosol increases with each stimulus. This phenomenon is known as --, so named because the waves of contraction add together. The amount of tension produced during -- depends on the frequency of stimulation by the motor neuron. Depending on the number of times per second the muscle fiber is stimulated, we end up with one of two states, unfused or fused tetanus. Successive stimuli leads to increased strength of contraction. 2nd stimulus given before complete relaxation.

Power stroke

A pivoting motion of a myosin head in which it moves from its cocked position to a relaxed position, pulling actin with it as it relaxes.

Sodium-potassium pump

A primary active transport pump consisting of a protein ATPase enzyme in the plasma membrane that pumps three Na+ from the cytosol into the extracellular fluid and two K+ from the extracellular fluid into the cytosol. This pump maintains concentration gradients of sodium and potassium ions across the sarcolemma, using the energy from ATP hydrolysis. Specifically, the pump moves three sodium ions out of the cell, making the concentration of sodium ions low in the cytosol and higher in the extracellular fluid. The pump also moves two potassium ions into the cell, making the concentration of potassium ions higher in the cytosol and lower in the extracellular fluid. ATP hydrolysis is necessary because this pump moves the ions against their concentration gradients. The sarcolemma of a skeletal muscle fiber has millions of these pumps, which work together constantly to maintain steep gradients of sodium and potassium ions. These gradients are critical because they are involved in the generation of the resting membrane potential and the events that trigger a muscle contraction.

Elasticity

A property of a cell by which it will return to its resting length after is has been stretched. Often, -- is mistaken for stretch, but distensibility, not --, refers to stretch.

Excitability

A property of a cell in which it is responsive in the presence of various stimuli. Muscle cells are this, or responsive, in the presence of various stimuli; these might include chemical signals from the nervous or endocrine systems, mechanical stretch signals, or local electrical signals. Such stimuli generate electrical changes across the plasma membrane of the muscle cell.

Calmodulin

A protein in the cytosol of smooth muscle cells that binds to calcium ions during a contraction.

Action potential

A quick, temporary change in the membrane potential of a cell in a single region of the plasma membrane. During an --, the membrane potential goes from its negative resting value to a more positive value and then back to its negative resting potential again. All of this happens extremely rapidly, within a few milliseconds—much faster than the muscle contraction itself. -- are generated by the opening and closing of gated sodium and potassium ion channels in the plasma membrane in response to a stimulus. When these channels open (or close) the membrane experiences rapid and dramatic changes in the movement of both sodium and potassium ions. This activity in turn alters the membrane potential, producing the --. Note that the total intracellular and extracellular concentrations of sodium and potassium ions change very little during an --. This is because only a few ions have to cross the sarcolemma to cause large changes in its membrane potential. Any minor shifts in ion concentration are corrected by the action of the Na+/K+ pump. These are propagated, meaning they are spread along the length of a cell's plasma membrane.

Troponin

A second regulatory protein is the smaller, globular -- that holds the tropomyosin in place. A regulatory protein with three subunits that binds tropomyosin and calcium ions in a thin filament.

Crossbridge cycle

A series of events in a muscle fiber during which a myosin head grabs onto a series of actin subunits in the thin filament and pulls the thin filament progressively closer to the M line of the sarcomere.

Axon

A single extension of a neuron that can generate action potentials; generally carries information away from the cell body. The motor neuron extends a long, membrane-covered "arm" called an -- to the muscle fiber.

Glycogen

A storage form of glucose. -- granules are found in the cytosol of both muscle fibers and liver cells.

Triad

A t-tubule and two adjacent terminal cisternae in a muscle fiber.

Steps of the crossbridge cycle (after preparation)

ATP hydrolysis "cocks" the myosin head. The myosin head binds to actin. The power stroke occurs when the phosphate detaches from the myosin head and myosin pulls actin toward the center of the sarcomere; ADP leaves the myosin head at the end of the power stroke. ATP breaks the attachment of myosin to actin.

Excitability

Ability to respond to stimuli by producing action potentials.

Steps of muscle relaxation

Acetylcholinesterase degrades the remaining ACh, and the final repolarization occurs. The sarcolemma returns to its resting membrane potential, and calcium ion channels in the SR close. Calcium ions are pumped back into the SR, returning the calcium ion concentration in the cytosol to its resting level. Troponin shifts and pulls tropomyosin back into position to block the active sites of actin, and the muscle relaxes.

Contraction period

Actin, myosin binding. This period is marked by a rapid increase in tension as crossbridge cycles occur repeatedly. The amount of tension produced during the -- phase, and the duration of this phase, depend on the type of muscle fiber.

Relaxation period

Actin, myosin detachment. During the -- period, tension decreases due to the decreasing calcium ion concentration in the cytosol. It takes the pumps in the SR from 10 to 100 ms to pump calcium ions from the cytosol back into the SR. Then, as tropomyosin once again blocks the active sites of actin, the muscle fiber relaxes.

Muscle fibers

Alternate name for a skeletal muscle cell.

Event sequence of excitation phase

An action potential arrives at the axon terminal and triggers Ca2+ channels in the axon terminal to open. Calcium ion entry triggers exocytosis of synaptic vesicles. Synaptic vesicles release acetylcholine into the synaptic cleft. Acetylcholine binds to ligand-gated ion channels in the motor end plate. Ion channels open and sodium ions enter the muscle fiber. Entry of sodium ions depolarizes the sarcolemma locally, producing an end- plate potential.

Ca++, Neuron action potential

An increase in -- in the cytoplasm leads to start of contraction cycle. -- stops leading to decrease in Ca++ in the cytoplasm leading to contraction cycle ending.

Excitation-contraction coupling

An increase in Ca++ in the cytoplasm leads to start of contraction cycle. Neuron action potential stops leading to decrease in Ca++ in the cytoplasm leading to contraction cycle ending.

Myoglobin

An oxygen-binding protein in muscle cells that increases the amount of oxygen immediately available to the cell. Found in the cytosol, binds to oxygen that has diffused into the muscle fiber from the extracellular fluid, and releases it as the available oxygen is depleted by mitochondria performing oxidative catabolism.

One muscle fiber

Approximately equals a muscle cell. Contains several myofibrils. Made up of myofibrils, which are composed of myofilaments. The arrangement of myofilaments within the myofibrils creates sarcomeres, the functional units of contraction.

Muscle tension

At the organ level, this force often creates movement, but it also maintains posture, stabilizes joints, generates heat, and regulates the flow of materials through hollow organs.

Fascia

Band/sheet of dense irregular CT; surrounds muscle and other organs. Remember that skeletal muscles are enclosed by a layer of thick connective tissue called --, which anchors them to the surrounding tissues and holds groups of muscles together.

Troponin and tropomyosin

Both form part of thin filament. Help to switch on and off the process of muscle contraction.

H zone

Both thick and thin filaments in the outer portion of the A band, and only thick filaments in the middle of the A band. This middle area, which is slightly lighter than the rest of the A band. Only thick filaments. The middle portion of the A band in which only thick filaments are found.

Glycolysis

Breakdown of glucose into pyruvic acid. Series of reactions that takes place in the cytosol of all cells, including muscle fibers. During this process, glucose is broken down to produce two ATP per molecule of glucose. Requires no oxygen directly, which is why it is sometimes called anaerobic catabolism. However, the fate of the product of -- depends on the availability of oxygen to the muscle fiber. The product is always two molecules of a compound known as pyruvate. If oxygen is abundant, this compound then enters the mitochondria for oxidative catabolism, which at that point will be occurring simultaneously with -- as long as glucose is available. If oxygen is not abundant, the pyruvate is converted into two molecules of the compound lactic acid. About 20% of this lactic acid diffuses out of the muscle fiber into the bloodstream, where much of it is converted into glucose by the liver. The remaining lactic acid was once thought to be a dead-end product, but current evidence indicates that a great deal of it enters the mitochondria and is used for oxidative catabolism.

Muscle/twitch contraction

Brief response of all fibers in a single motor unit. A single cycle of contraction and relaxation of a muscle fiber generated by a single action potential.

Steps after stimulation, in preparation for muscle contraction

Calcium ions bind to troponin. Tropomyosin moves, and the active sites of actin are exposed.

Pyruvic acid, fatty acids, amino acids

Can all be used as inputs for aerobic respiration.

Contraction for smooth muscles

Cells get smaller rather than shorter. Ca++ (from interstitial fluid) binds to calmodulin rather than troponin. Single unit smooth muscle: cells arranged in a network, contract together. Multiunit smooth muscle: each cell contracts independently.

Neurotransmitter

Chemical released from vesicles in the neuron. Chemical messengers produces by neurons that communicate with target cells. The -- in the axon terminals of motor neurons that connect to skeletal muscle fibers is acetylcholine.

Perimysium

Covers a muscle fascicle (bundle of muscle fiber). The epimysium blends with a deeper layer of connective tissue, the --, to form tendons, which bind the muscle to its attaching structure. The -- surrounds individual fascicles, or groups of muscle fibers.

Endomysium

Covers a muscle fiber. Skeletal muscle tissue consists of many skeletal muscle fibers and the -- surrounding each of them.

Epimysium

Covers entire muscle, continuous with tendon. Deep to the fascia is another layer of connective tissue, the --, which surrounds the whole muscle. The -- blends with a deeper layer of connective tissue, the perimysium, to form tendons, which bind the muscle to its attaching structure.

Sarcoplasm

Cytoplasm of muscle fiber. Like cytoplasm, contains cytosol and all organelles in the muscle cell. Rich in glycogen, myoglobin, and mitochondria.

Factors causing fatigue

Deficit of ATP. Depletion of key metabolites. Decreased availability of oxygen to muscle fibers. Accumulation of certain chemicals. Environmental conditions.

Latent period

Delay between stimulus and contraction. The 1- to 2-ms (millisecond) time that it takes for the action potential to spread through the sarcolemma. It begins with the start of the action potential, and by the end, the action potential has spread past the T-tubules and triggered the release of calcium ions from the terminal cisternae of the SR. These ions then bind to troponin, and tropomyosin moves away from the active sites of actin. The myofibril is now ready to enter a crossbridge cycle. Note that the sarcolemma completes the repolarization phase of the action potential at the end of this period.

Voltage

Difference in electrical potential between two points.

Creatine phosphate pathway

Direct phosphorylation of ADP; short energy bursts. An immediate energy source for certain cell types that donates a phosphate group to adenosine diphosphate (ADP). When contraction begins, the main immediate energy source of the muscle fiber is stored ATP. This ATP is rapidly consumed, but is regenerated almost immediately by a reaction using a compound called --. --, found primarily in muscle fibers, is about 5-6 times more abundant than ATP in the cytosol. During the --, --, with the help of the enzyme creatine kinase, donates a phosphate group to ADP, producing ATP. ATP produced by this reaction provides the muscles with enough energy for about an additional 10 seconds of maximal muscle activity. Though this may not sound like much, it's enough for a short burst of activity, such as a 100-meter sprint.

Motor units

Each muscle fiber innervated by only one neuron. The group of muscle fibers innervated by a single motor neuron. As a motor neuron approaches a muscle, its axon branches out and innervates multiple muscle fibers. A single motor neuron along with the muscle fibers it innervates is called a --. When the motor neuron fires an action potential, all of the muscle fibers within its -- respond and produce about the same amount of tension. Note that this applies only to a --, not to an entire muscle. An average -- consists of about 150 muscle fibers, but this number can vary widely with the degree of motor control needed for the muscle. -- contain one class of muscle fiber, either type I or type II. Those with type I fibers are called slow --, and those with type II fibers are called fast --. One neuron can innervate several muscle fibers (--). Neurons that innervate skeletal muscle approximately equal somatic motor neurons.

Terminal cisternae

Enlarged regions of the SR where it contacts t-tubules.

Ca++

Events of contraction cycle continue as long as -- is available.

Properties of muscle tissue

Excitability and contractility Distensibility and elasticity Conductivity

Transverse (T) tubules

Hollow inward extensions of the muscle fiber sarcolemma that surrounds myofibrils; filled with extracellular fluid. Dive into the muscle fiber and surround each myofibril, forming a tunnel-like network within the muscle fiber. These tunnels are continuous with the exterior of the cell and so are filled with extracellular fluid.

"Oxygen debt"

Extra oxygen consumed to restore resting conditions. Excess postexercise oxygen consumption, the persisting increased rate of breathing during the recovery period after completing exercise.

Type II fibers

Fast-twitch fibers that are often larger in diameter and contract more rapidly than type I fibers, but they are quickly fatigued. -- fibers rely more heavily on glycolytic energy sources, and they have less myoglobin, fewer mitochondria, and a less extensive blood supply than type I fibers. Due to their low myoglobin content, -- fibers are lighter in color than type I fibers, and so these muscle fibers are sometimes called "white muscle." (This visible difference between white muscle and red muscle explains the "white meat" and "dark meat" of chicken.) -- fibers have two subtypes: --a (also known as fast oxidative glycolytic, or FOG) and --x (fast glycolytic, or FG). These subtypes have progressively faster, stronger twitches and rely increasingly on glycolytic energy sources, with type --x fibers having extremely fast, powerful twitches and using glycolytic catabolism almost exclusively.

Striated

Feature alternating light and dark bands. Seen in skeletal and cardiac muscle cells. The arrangement of thin and thick myofilaments forms a pattern within the myofibril—there are regions where we find only thin filaments, regions where the thin and thick filaments overlap, and regions that have only thick filaments. This pattern forms alternating light and dark bands.

Sarcomere

Functional unit of skeletal muscle. The functional unit of muscle contraction; consists of the area of the myofibril from one Z-disc to the next Z-disc. Where muscle tension is produced, includes a full A band and a half of two I bands. Contains contractile, regulatory and structural proteins.

Synaptic cleft

Gap between neuron and muscle fiber. The small space between the axon terminal of a presynaptic neuron and its target cell. The narrow space between the axon terminal and the muscle fiber into which ACh is released. It is filled with collagen fibers and an extracellular gel that anchors the neuron in place. It also contains enzymes that break down ACh.

Anaerobic cellular respiration

Glycolytic catabolism, a series of ATP-producing reaction that occur in the cytosol of cells in which glucose is broken down into two molecules of pyruvate; these reactions do not require oxygen to proceed. When immediate energy sources are depleted, muscle fibers turn to glycolysis, also known as --, to make ATP. Muscle fibers that depend primarily on glycolysis as their means of ATP production have large quantities of glycogen in their cytosol. Regardless of the amount of stored glycogen, though, glycolysis by itself can provide adequate ATP for only about 30-40 seconds of sustained muscle contraction. Glycolysis: breakdown of glucose into pyruvic acid. In the absence of oxygen pyruvic acid converted into lactic acid. Enough energy for 30-40 seconds. Lactic acid converted back into glucose in the liver.

Unfused tetanus

If the fiber is stimulated about 50 times per second, it can only partially relax between contractions. A sustained contraction called -- results. During --, the tension pulsates, decreasing slightly and then increasing a bit more with each successive twitch until a level of maximal tension is reached. A type of wave summation in which a muscle fiber is stimulated rapidly and only allowed to partially relax between contractions.

Fused tetanus

If the fiber is stimulated at a higher rate of 80-100 times per second, the muscle fiber does not have time to relax between stimuli because the calcium ion concentration in the cytosol remains high. The availability of calcium ions allows more and more crossbridges to form, contributing to the increase in tension. This increase results in a condition called --, in which the tension remains constant at a maximal level. Note that -- is possible only because of the extremely short refractory period of the skeletal muscle fiber. This period is so short that the fiber does not have to relax between stimuli, and so can respond to a new stimulus before the last twitch is over, producing a sustained contraction. A type of wave summation in which a muscle fiber is stimulated rapidly and the muscle fiber is not allowed to relax between contractions. Tension is highest here.

Absence, glucose

In the -- of oxygen pyruvic acid converted into lactic acid. Lactic acid converted back into -- in the liver.

Excitation phase

Involves transmission of a signal from the motor neuron to the sarcolemma of a muscle fiber. This phase, which occurs at the neuromuscular junction. Action potential travels down neuron. Neurotransmitter (acetylcholine) released at synapse. Acetylcholine (ACh) binds to receptors on motor end plate. Muscle fiber produces a muscle action potential that travels down T-tubules. Ca++ released from the sarcoplasmic reticulum. Events of contraction cycle continue as long as Ca++ is available.

Smooth

Lack striations.

Events of the sliding filament mechanism

Length of filaments does not change. Length of sarcomere shortens. A band (myosin present): same length. H zone (myosin only), I band (thin filament only): shorter. Z discs: separate sarcomere. M-line: supports, stabilizes thick filaments.

Tetany

Maximum sustained contraction. Requires multiple, frequent, repeating stimuli.

Contractions, tension, stretched, compressed

Muscle -- increase the tension in the whole muscle. Amount of -- produced depends on beginning length of sarcomeres. -- fibers leads to less overlap leads to less tension. -- fibers lead to extensive overlap leads to less tension.

Fatigue

Muscle fiber physiologically unable to contract. An inability to maintain a given level of intensity of a particular exercise.

Receive

Muscle fibers -- action potential signals from neurons.

Respond

Muscle fibers -- by releasing Ca++ via muscle action potentials.

Myofilaments

Muscle proteins that make up a myofibril in a muscle fiber. Consists of one or more of the following types of proteins; contractile, regulatory, and structural. The arrangement of these within the myofibrils allows muscles to contract.

Muscle cells

Muscle tissue consists of these, sometimes called myocytes, and the surrounding extracellular matrix, the endomysium.

ATP

Needed to power contraction cycle AND pump Ca++ back into the sarcoplasmic reticulum. Necessary for contraction and relaxation of the muscle fiber.

Smooth muscle

No regular arrangement of thick, thin filaments. Widely distributed in the body. Much of it is found lining hollow organs, but it is also present elsewhere, such as in the arrector pili muscles in the dermis and the iris of the eye. Lack motor end plates, the SR is much less extensive, and T-tubules are absent. It does contain actin and myosin. Lack striations.

Smooth muscle tissue

Non-striated, involuntary. Consists of smooth muscle cells, which are long and flattened with two pointed ends and which have a single, centrally located, oval nucleus. These cells line nearly every hollow organ, and are found as well in the eyes, the skin, and the ducts of certain glands. Many -- muscle cells are linked to one another by gap junctions in their plasma membranes. Like cardiac muscle tissue, -- muscle tissue is involuntary. Can undergo mitosis.

Titin and dystrophin

What are the two structural proteins?

Skeletal, cardiac, smooth

What are the types of muscle tissue?

Aerobic respiration

Occurs in presence of O2. Oxidative catabolism, a series of reactions that occur in the mitochondria in the presence of oxygen during which electrons are removed from carbon-based compounds and the energy released is used to fuel the synthesis of ATP. Longer-lasting muscle activity, such as running a 5K or 10K race, requires a muscle fiber to use mostly -- to generate ATP. During this process, electrons are removed from carbon-based compounds, and the energy liberated is then used to fuel the synthesis of ATP. Oxidative catabolism produces more ATP than does glycolysis; the amount of ATP depends on the type of fuel used by the muscle fiber. Muscle fibers can use multiple fuels for oxidative catabolism, including the products of glycolysis, as well as fatty acids and amino acids. Glucose is generally the preferred fuel for muscle fibers—the one they use first. As glucose becomes less available, muscle fibers will catabolize fatty acids and amino acids if necessary. The final step in this process is the transfer of electrons to a molecule of oxygen, which is why this type of metabolism is also called --. In fact, oxidative catabolism, which takes place in nearly all cells, is the main reason we need oxygen for survival. To meet this requirement, muscle fibers must have a plentiful supply of oxygen during activity. Much of this oxygen diffuses into the cytosol from the bloodstream. The rest is bound to the oxygen-binding protein myoglobin, which is similar to hemoglobin, the oxygen-carrying protein in the blood. Myoglobin, which is found in the cytosol, binds to oxygen that has diffused into the muscle fiber from the extracellular fluid, and releases it as the available oxygen is depleted by mitochondria performing oxidative catabolism. Oxidative catabolism can provide ATP for hours, as long as oxygen and fuels are available. After only 1 minute of skeletal muscle activity, oxidative catabolism is the predominant source of ATP for almost all muscle fibers. After several minutes, nearly 100% of the ATP is produced by this -- process. This is why exercise that lasts continuously for at least several minutes is often called "--." Produces large amounts of ATP. Pyruvic acid, fatty acids, amino acids can all be used as inputs for aerobic respiration. Energy produced as long as inputs available.

Excitation-contraction coupling

Once the muscle fiber is excited by such stimulation, a process called -- conveys this excitation to the parts of the fiber that produce the contraction, the myofilaments. It's at this point that the sliding-filament mechanism occurs and the sarcomere contracts. The linking of muscle fiber excitation via an action potential, with contraction via the release of calcium ions from the sarcoplasmic reticulum. In this phase, the end-plate potential leads to an action potential in the sarcolemma, which in turn triggers events that result in contraction.

Sarcolemma

Plasma membrane of muscle fiber. Composed of a phospholipid bilayer with multiple specialized integral and peripheral proteins. Is not confined to the exterior of the cell. Rather, it forms inward extensions called T-tubules. It also contains terminal cisternae and the triad. Extension into the muscle fiber (T tubules).

Neuromuscular junction

Point of contact between somatic motor neuron and muscle fiber. The location where a neuron communicates with a muscle fiber. The synapse of a motor neuron with a muscle fiber. This is how neurons communicate with muscle fibers. At the --, a signal called a nerve impulse, or neuronal action potential, is transmitted from the neuron to the muscle fiber's sarcolemma. Each -- consists of three parts: the axon terminal, synaptic cleft, and motor end plate.

Functions of muscle tissue

Produce body movements Stabilize body position Store and move substances within body Generate heat Generating a force called muscle tension.

Zone of overlap

Region of overlapping filaments, where muscle tension is generated during a muscle contraction.

Motor end plate

Region of sarcolemma containing receptors for neurotransmitters. The specialized region of the skeletal muscle fiber plasma membrane that contains receptors for ACh. A specialized region of the sarcolemma whose folded surface contains many receptors for ACh. Recall that a receptor is a protein within the plasma membrane that binds to a specific ligand. These receptors are actually ligand-gated ion channels; ACh is the ligand.

Event sequence of contraction cycle/phase

Release of Ca++ begins. Ca++ binds to 1 subunit of troponin. Tropomyosin shifts in position. Actin binding sites exposed (1) ATP hydrolysis: reorients and energizes myosin head. (2) Formation of cross-bridge (myosin binds to actin). (3) Power stroke: myosin head bends. Thin filament is pulled toward M-line. (4) Detachment of myosin from actin. Additional ATP is needed for myosin to release from actin.

ATP hydrolysis

Reorients and energizes myosin head.

Electrical gradient

Separation of charges, unequal numbers of positive and negative ions separated by a barrier. As with all --, an -- represents a source of potential energy. When the barrier separating the ions is removed, they follow their --, creating a flow of electrical charges, and the potential energy becomes kinetic energy. For this reason, we can call the -- an electrical potential, because we are referring to its potential energy.

Synapse

Site of communication between neuron and muscle fiber. The location where a presynaptic neuron communicates with its target cell.

Fast-twitch fibers

Skeletal muscle fibers with high myosin ATPase activity that proceed more rapidly through their crossbridge cycles; generate rapid but generally short-duration contractions. Found in muscles that must move body parts rapidly, such as those that move the eyeballs.

Slow-twitch fibers

Skeletal muscle fibers with low myosin ATPase activity that proceed relatively slowly through their crossbridge cycles; generate slower but generally longer-lasting contractions. Found in muscles that require slow, sustained contractions, such as the postural muscles of the back.

Type I fibers

Slow-twitch fibers that are small to intermediate in diameter. -- fibers contract more slowly and less forcefully than other fibers, but they can maintain extended periods of contraction. This ability requires the continual generation of large quantities of ATP via oxidative catabolism; for this reason, -- fibers are also called slow oxidative fibers. To support oxidative catabolism, these fibers have large quantities of myoglobin, many mitochondria, and a well-developed blood supply. The high myoglobin content of -- muscle fibers makes them red, so they are sometimes known as "red muscle."

Muscle metabolism

Source of ATP energy.

Intercalated discs

Specialized structures that contain gap junctions and modified tight junctions. -- unite cardiac muscle cells and permit them to coordinate contraction so that the heart contracts as a unit. Specialized structures that connect adjacent cardiac muscle cells and contains gap junctions and desmosomes. These -- join cardiac muscle cells to one another physically and electrically, which permits the heart to contract as a unit.

Fast-twitch, slow-twitch, type I, type II

What are the types of skeletal muscle fibers?

Sarcoplasmic reticulum

Stores Ca++, encircles myofibrils. The specialized smooth ER of a muscle fiber that stores calcium ions. A modified smooth endoplasmic reticulum that forms a weblike network surrounding each myofibril. The structure of the -- varies in the three types of muscle tissue. Modified smooth ER. Modified to store Ca++.

Cardiac muscle tissue

Straited, involuntary. Like skeletal muscle tissue, consists of striated muscle cells. For example, -- muscle cells are shorter and wider, are branched, and generally have only a single nucleus (although some have two nuclei). Notice also what look like dark, wavy "lines" that join the -- muscle cells. These "lines" represent intercalated discs, which are specialized structures that contain gap junctions and modified tight junctions. Intercalated discs unite -- muscle cells and permit them to coordinate contraction so that the heart contracts as a unit. -- muscle tissue is found only in the heart and is involuntary, meaning that the brain does not have conscious control over its contraction. Like skeletal muscle fibers, -- cells are striated and consist of sarcomeres. They also have both T-tubules and extensive networks of sarcoplasmic reticulum. However, notable structural differences between -- cells and skeletal muscle fibers do exist. -- cells are typically shorter, branched cells with one or two nuclei and abundant myoglobin. Mitochondria account for about 30% of their cytoplasmic volume. Branched cells connected by intercalated discs.

Skeletal muscle tissue

Straited, voluntary. Made up of long, multinucleated (containing more than one nucleus) muscle cells that are arranged parallel to one another. Some -- muscle cells are quite long, extending nearly the entire length of the muscle. In keeping with their shape, these cells are often called muscle fibers (the terms cell and fiber are often used interchangeably). -- muscle fibers are mostly found attached by connective tissue to the skeleton, where their contraction can produce the movement of a body part. However, this contraction occurs only when muscle fibers are stimulated by the nervous system. Although we're not always conscious of the contractions of -- muscle fibers, they can be controlled voluntarily, or by conscious thought. Multinucleated (fusion of embryonic myoblasts).

Refractory period

Temporarily unable to respond to new stimulus. Between the start of the latent period and the start of the contraction period, there is an interval of about 5 ms during which the muscle fiber cannot respond to another stimulus. This period, called the -- period, is a property of all excitable cells. Cardiac muscle and smooth muscle have -- periods as long as their contractions, so the cells must fully relax before they can contract a second time. However, skeletal muscle fibers have a much shorter -- period, allowing them to maintain a sustained contraction phase.

Contractility

The ability of cells to contract. You might think that "contracting" means "shortening," but the term contraction actually refers to the ability of proteins within muscle cells to draw together. As you will discover in later modules, a muscle cell does not necessarily shorten when it contracts.

A bands

The component of the sarcomere that appears darker because it contains thick filaments and areas in which the thick and thin filaments overlap. The dark region of a striation, which contains thick filaments. Thick filaments block more light than thin filaments, making the -- appear darker in micrographs. This middle area, which is slightly lighter than the rest of the --, is called the H zone. Consists of both thin and thick filaments, and each thick filament is surrounded by six thin filaments. The dark band that contains both thick and thin filaments.

I bands

The component of the sarcomere that appears lighter because it contains only thin filaments. The light region of a striation. It appears lighter because it contains only thin filaments, which allow more light to pass through them. The light band that contains only thin filaments.

Z discs

The component of the sarcomere that contains structural proteins to which thin and elastic filaments attach. The dark line in the I bands is known as a --. The -- is composed of structural proteins that have many functions: They anchor the thin filaments in place and to one another, they are an attachment point for elastic filaments, and they attach myofibrils to one another across the whole diameter of the muscle fiber. Line bisecting the I band. Both thin and elastic filaments anchor to these, which also attach myofibrils to one another.

Membrane potential

The difference in voltage between the extracellular fluid and the cytosol in the area near the plasma membrane. There is a thin layer of negative ions in the cytosol and a thin layer of positive ions in the extracellular fluid. Away from the plasma membrane, positive and negative ions are present in equal numbers. This means that the cytosol and extracellular fluid are always electrically neutral. Unequal distribution of charged ions across a cell membrane leads to electrical potential. Voltage: difference in electrical potential. Cells spend energy to create the unequal distribution. Excitable cells use changes in the potential for communication.

Steps of excitation-contraction coupling

The end-plate stimulates an action potential. The action potential is propagated down the T-tubules. T-tubule depolarization leads to the opening of calcium ion channels in the sarcoplasmic reticulum, and calcium ions enter the cytosol. Leads to repolarization.

Contraction cycle/phase

The generation of tension by a muscle cell. Myofilaments slide past one another during the --. Recall that the myosin heads of the thick filaments attach to the actin of the thin filaments at their active sites and pull them toward the middle of the sarcomere. But before the contractile proteins can do their work, the regulatory proteins must let them. In a resting muscle fiber the regulatory protein tropomyosin curls around actin, blocking its active sites. Calcium ions initiate the mechanism that moves the attached troponin and tropomyosin away from the active sites. At rest, the concentration of calcium ions in the cytosol is very low. However, as we just saw, once excitation-contraction coupling has occurred, the concentration of calcium ions in the cytosol increases dramatically.

Tropomyosin

The long, ropelike regulatory protein called -- spirals around the two actin strands so that, at rest, it covers the active sites on actin. A filamentous regulatory protein that covers the active sites of actin subunits in a thin filament.

Sliding filament mechanism

The mechanism of contraction of a muscle cell in which the thin and thick filaments slide past one another while generating tension. The I bands and H zone narrow. This happens because the myosin heads of the thick filaments "grab" the thin filaments and pull them toward the M line. This pulling action brings the Z-discs closer together and causes the sarcomere as a whole to shorten. Remember, though, that none of the filaments themselves actually shorten—the thin filaments simply move toward the M line. The size of the A band, however, remains unchanged. The A band doesn't change in size because the myosin heads are actually doing the pulling. Think about a rope that you're climbing—you get progressively closer to the top, but the rope itself never changes in size. Sarcomeres are arranged end to end within each myofibril. As many sarcomeres simultaneously contract, the whole muscle fiber contracts.

Length-tension relationship

The relationship between the length of the sarcomeres of a muscle fiber while at rest and the amount of tension that can be generated by a contraction. The second factor that determines the amount of tension produced by a twitch contraction is the number of crossbridges that can form within each sarcomere of the muscle fiber. The number of crossbridges depends on the length of the sarcomere prior to contraction, a principle known as the --. The optimal length of the sarcomere is the length of the muscle fiber at which the most crossbridges can form, allowing the fiber to generate almost 100% of the tension that is possible to produce.

Muscle relaxation

The return of a muscle cell to its resting length due to the decreasing concentration of calcium ions in the cytosol.

Multiunit smooth muscle

The second, and rarer, of the two types. It is found in such locations as the muscles in the eye and the arrector pili muscles in the dermis. -- smooth muscle consists of individual muscle cells whose plasma membranes are not joined by gap junctions. This characteristic allows each cell to contract independently of the others, permitting precise control of contractions. As with skeletal muscle, the amount of tension produced by -- smooth muscle varies with the number of muscle cells activated. Smooth muscle cells that are able to contract individually. Responds primarily to nerve stimulation.

Endomysium

The surrounding extracellular matrix of muscle tissue, blends with surrounding connective tissue and so is often referred to as connective tissue. The -- holds the muscle cells together within muscle tissue and transmits tension generated by muscle cells to neighboring cells.

Resting membrane potential

The voltage difference across the plasma membrane of a cell when it is not being stimulated. The cell in this state is said to be polarized, which simply means that the voltage difference across the plasma membrane of the cell is not at 0 mV, but rather measures to either the positive or the negative side (or pole) of zero. Different cell types have different --. Changes in this potential are responsible for the electrical events of a muscle fiber. Two types of ions are involved in generating this: sodium and potassium ions.

Myofibrils

Thread-like, contractile. Long, cylindrical organelles composed of muscle proteins in a muscle fiber. Contained in the sarcoplasm. All three types of muscle cells contain --, although they are arranged differently in smooth muscle cells than in skeletal and cardiac cells. -- are essentially bundles of specialized proteins, including those involved in muscle contraction. A -- measures about 1 μm in diameter, or about 11001100 the thickness of a human hair. Each muscle cell has hundreds to thousands of -- —they make up about 50-80% of its volume. Other organelles such as mitochondria are found packed between the --. The most abundant organelle in the sarcoplasmic reticulum. Primarily made up of proteins involved in contraction. Composed of hundreds of thousands of protein bundles called myofilaments. Contain sarcomeres laid end-to-end.

Myoglobin content and source of ATP.

What do the three types of skeletal muscle fibers vary in?

Fascia, epimysium, perimysium, endomysium

What is the skeletal muscle organization?

Excitation

What leads to contraction?

Contractile, regulatory, and structural

What types of proteins does the sarcomere contain?

Conductivity

When a muscle cell is excited, the electrical changes across the plasma membrane do not stay in one place. Instead, they are rapidly -- along the entire length of the plasma membrane, similar to how an electrical impulse is -- through a copper wire.

Single unit smooth muscle

Unitary, also called visceral smooth muscle, the predominant type of smooth muscle in the body and is found in nearly all hollow organs, including the uterus. -- smooth muscle consists of hundreds to thousands of muscle cells whose plasma membranes are linked electrically via gap junctions. Action potentials spread rapidly through the cells via the gap junctions, causing the muscle cells to contract in a coordinated wave as a single unit. Smooth muscle cells that contract together as a single unit. Able to respond to multiple types of stimulation.

Sarcolemma, sarcoplasm, sarcoplasmic reticulum, sarcomere

What are the parts of the microscopic anatomy of the muscle tissues?

Creatine phosphate pathway, anaerobic cellular respiration, aerobic respiration

What are the three sources of ATP?

Thick, thin, and elastic

What are the three types of myofilaments?

Striated and smooth

What are the two basic forms of muscle cells?

Actin and myosin

What are the two contractile proteins?

Troponin and tropomyosin

What are the two regulatory proteins?

Glucose that enters from the bloodstream Glycogen

What are the two sources of glucose for glycolysis for muscle fibers?

M line

You can also see that both the A and I bands have a dark line running down their middle. The dark line in the middle of the A bands is called the --. The -- consists of structural proteins that hold the thick filaments in place and serve as an anchoring point for the elastic filaments. Middle line of the A band; contains structural proteins that hold the thick filaments in place. Myosin tails point toward this.