Muscle Tissue and Physiology

A band

The component of the sarcomere that appears darker because it contains thick filaments and areas in which the thick and thin filaments overlap.

I band

The component of the sarcomere that appears lighter because it contains only thin filaments.

Z-disc

The component of the sarcomere that contains structural proteins to which thin and elastic filaments attach. - dark line in the I bands - 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.

thick filaments, thin filaments, and elastic filaments

There are three types of myofilaments, each with characteristic contractile, regulatory, and/or structural proteins

dehydration synthesis

What is the type of chemical reaction used to rebuild ADP into ATP?

myogram

a chart of the timing and strength of a muscle's contraction - Notice in this myogram that the muscle twitch has three distinct phases—the latent, contraction, and relaxation periods

sarcolemma

plasma membrane of a muscle fiber - composed of a phospholipid bilayer with multiple specialized integral and peripheral proteins.

axon terminal, synaptic cleft, and motor end plate

Each NMJ or neuromuscular junction consists of three parts...

Heat dissipation

Energy is lost as heat during both glycolytic and oxidative catabolism. Exercise increases the rate of catabolic processes, so it also raises the amount of heat generated. The body must lose this heat and return to the pre-exercise temperature level to maintain temperature homeostasis.

Terminal cisternae

Enlarged regions of the sarcoplasmic reticulum where it contacts T-tubules.

muscle tension

Force exerted by a contracting muscle. - this force often creates movement, but it also maintains posture, stabilizes joints, generates heat, and regulates the flow of materials through hollow organs.

Skeletal Muscle Fiber

- shaped like thin cylinders and are much longer than most cells - appear striated, or striped, when viewed through a microscope - contains multiple sausage-shaped nuclei on the inner surface of its sarcolemma. As you may recall, this feature is a result of the embryonic origin of the fiber, as each muscle fiber arises from the fusion of multiple embryonic cells called myoblasts

Tropomyosin

A filamentous regulatory protein that covers the active sites of actin subunits in a thin filament.

thick filaments

A myofilament composed of many molecules of the protein myosin.

wave summation

A phenomenon in which repeated stimulation of a muscle fiber by a motor neuron results in muscle twitches with progressively greater tension.

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.

muscle twitch

A single cycle of contraction and relaxation of a muscle fiber generated by a single action potential.

Isotonic concentric contractions

A type of muscle contraction in which the tension generated is greater than that of the external load, and so the muscle cell shortens with the contraction. - miometric contraction - Let's consider the example of holding a 10-lb weight in your hand and lifting it with your forearm. As you begin to pick up the weight (which is the external load), the motor units in your forearm flexors generate force to flex your forearm at the elbow. When they generate force of more than 10 lb, the muscle shortens, and an isotonic concentric contraction results.

Isotonic eccentric contractions

A type of muscle contraction in which the tension generated is less than that of the external load, and so the muscle cell lengthens with the contraction. - pliometric contraction - A muscle is able to lengthen while it's contracting because the elastic filaments in its myofibrils allow it to stretch considerably. Consider the example of the 10-lb weight that you have just lifted. As you slowly set the weight down (see Figure 10.27b), the force produced by your muscle becomes less than the load of the weight. However, your motor units are still generating tension, even though the sarcomeres are stretching and lengthening.

cardiac muscle cells

are shorter and wider, are branched, and generally have only a single nucleus (although some have two nuclei) - have intercalated discs

muscle relaxation

The return of a muscle cell to its resting length due to the decreasing concentration of calcium ions in the cytosol. - has three components: ACh release stops, the remaining ACh in the synaptic cleft is broken down, and the calcium ion concentration in the cytosol returns to its resting level.

length-tension relationship

The second factor that determines th​​e a​​mount 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 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.

types I and II

The speed of a twitch contraction is combined with its predominant energy source (oxidative versus glycolytic catabolism) to give us two main classes of skeletal muscle fibers

Contraction period.

This period is marked by a rapid increase in tension as crossbridge cycles occur repeatedly. The amount of tension produced during the contraction phase, and the duration of this phase, depend on the type of muscle fiber, which we discuss shortly.

the excitation phase, excitation-contraction coupling, and the contraction phase

the overall process of muscle contraction can be broken down into three parts or steps

sarcoplasm

cytoplasm of a muscle cell - like cytoplasm, contains cytosol and all organelles in the muscle cell

muscular fatigue

defined as the inability to maintain a given level of intensity of a particular exercise

Striations

Alternating light and dark bands seen in skeletal and cardiac muscle cells.

Acetylcholinesterase (AChE)

An enzyme located in the synaptic cleft that degrades acetylcholine.

creatine phosphate

An immediate energy source for certain cell types that donates a phosphate group to adenosine diphosphate (ADP) with the help of the enzyme creatine kinase

Pyruvic acid is converted back to lactic acid.

The "rest and recovery" period, where the muscle restores depleted reserves, includes all of the following processes EXCEPT __________.

excess postexercise oxygen consumption (EPOC)

(EPOC) The persisting increased rate of breathing during the recovery period after completing exercise. - results from the responses necessary to correct the disturbances to homeostasis that were brought on by exercise. To return to homeostasis, the body must accomplish several goals, which include dissipating heat, restoring ion concentrations, and correcting blood pH.

Oxidative catabolism

(aerobic 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. - 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. - can provide ATP for hours, as long as oxygen and fuels are available.

Glycolytic catabolism

(anaerobic catabolism) A series of ATP-producing reactions 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. - As you may recall, glycolysis is a 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 - 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 glycolysis 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.

Fused tetanus

(complete tetanus) A type of wave summation in which a muscle fiber is stimulated rapidly and the muscle fiber is not allowed to relax between contractions. 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.

triad

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

Actin

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

myosin

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 myosin tail on a "neck." The neck of each myosin protein is flexible where it meets the tail at a point called the hinge. Each myosin head includes a site that binds to a thin filament. - The myosin proteins are arranged within the thick filament in such a way that clusters of myosin heads are found at each end, with only myosin tails found in the middle

bloodstream and a storage form of glucose called glycogen Glycogen granules are found in the cytosol of both muscle fibers and liver cells. Muscle fibers that depend primarily on glycolysis as their means of ATP production have large quantities of glycogen in their cytosol.

A muscle fiber has two potential sources of glucose for glycolysis

spasm

A muscle that is unable to relax - very common and may be due to factors such as dehydration, electrolyte imbalances, muscle injury, or muscle overload. These spasms vary in degree, ranging from a "tight" muscle (also known as a muscle "knot") to a full spasm that impairs function and is typically very painful.

Thin filament

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

Elastic filament

A myofilament that consists of the structural protein titin. - the thinnest type - titin is shaped like a spring that can uncoil when stretched and recoil to its original shape when the stretching force is removed - 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.

Troponin

A regulatory protein with three subunits that binds tropomyosin and calcium ions in a thin filament. smaller, globular

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. - A mu​​scle contraction i​​s simply a succession of crossbridge cycles and the resulting production of tension.

Isometric contractions

A type of muscle contraction in which the tension generated is equal to that of the external load, and so the muscle cell remains at a constant length. - This time, lift the 10-lb weight by abducting your arm at the shoulder rather than flexing your forearm (see ​Figure 10.27c​). The initial lifting involved an isotonic concentric contraction; however, now your arm is stationary in an abducted position and your forearm is stationary in a supinated position. So, the muscle is neither shortening nor lengthening, even though it is still producing 10 lb of force to hold the weight in the air.

(1) reactions in the cytosol that immediately add a phosphate group to ADP, (2) glycolytic catabolism in the cytosol, and (3) oxidative catabolism in the mitochondria.

ATP. Regeneration may be accomplished through three processes

recruitment

An increase in the number of motor units of a skeletal muscle that are stimulated in order to produce a contraction with greater tension. Slow motor units are typically activated first, followed by fast motor units if additional tension is needed. For example, if you were lifting thistextbook off your desk, you would likely use your slow motor units for the most part. However, if you were to lift a stack of textbooks from the floor, your nervous system would first activate the slow motor units and then the fast motor units.

myoglobin

An oxygen-binding protein in muscle cells that increases the amount of oxygen immediately available to the cell.

refractory period

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. Cardiac muscle and smooth muscle have refractory 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 refractory period, allowing them to maintain a sustained contraction phase

Smooth muscle cells lack motor end plates. The sarcoplasmic reticulum is much less extensive. T-tubules are absent.

Beyond contractile proteins, there are three other important structural differences between skeletal muscle fibers and smooth muscle cells:

Both unfused and fused tetanus generate more tension than a single twitch contraction. Tension is highest during fused tetanus, as the muscle fiber produces a constant tension several times greater than that created during a single twitch.

Both unfused and fused tetanus generate more tension than a single twitch contraction. Tension is highest during fused tetanus, as the muscle fiber produces a constant tension several times greater than that created during a single twitch.

Correction of blood pH

Certain products of metabolism, including lactic acid and carbon dioxide, can cause the pH of the blood to decrease (which means acidity increases) from its normal value of 7.35-7.45. The body must return the pH of the blood to the pre-exercise level in order to maintain homeostasis.

endomysium

Connective tissue surrounding a muscle fiber - holds the muscle cells together within muscle tissue and transmits tension generated by muscle cells to neighboring cells.

Restoration of intracellular and extracellular ion concentrations.

During muscle contractions, the ATP-consuming pumps in the sarcolemma and SR must work harder to maintain the normal concentrations of calcium and sodium ions in the cytosol and of potassium ions in the extracellular fluid.

Relaxation period

During the relaxation 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.

Transverse tubules

Hollow inward extensions of the muscle fiber sarcolemma that surround myofibrils; filled with extracellular fluid. - dive into the muscle fiber and surround each myofibril, forming a tunnel-like network within the muscle fiber - continuous with the exterior of the cell, which is why they have extracellular fluid

Unfused tetanus

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

myofibrils

Long, cylindrical organelles composed of muscle proteins in a muscle fiber. - all three muscle types have this (bit different in smooth though) - essentially bundles of specialized proteins, including those involved in muscle contraction. - Each muscle cell has hundreds to thousands, make up 50-80% of volume - most abundant organelle in the sarcoplasm

myofilaments

Muscle proteins that make up a myofibril in a muscle fiber. consist of one or more of the following types of proteins: - contractile proteins, which produce tension - regulatory proteins, which control when the muscle fiber can contract - structural proteins, which hold the myofilaments in their proper places and ensure the structural stability of the myofibril and the muscle fiber.

muscle cells​, sometimes called myocytes, and the surrounding extracellular matrix, the ​endomysium​.

Muscle tissue consists of....

Note that the great majority of mature skeletal muscle fiber nuclei are amitotic, meaning that they generally do not undergo mitosis under normal conditions. The precise structural and biochemical changes depend on the type of training chosen—endurance training and resistance training are the two basic types.

Note that the great majority of mature skeletal muscle fiber nuclei are amitotic, meaning that they generally do not undergo mitosis under normal conditions. The precise structural and biochemical changes depend on the type of training chosen—endurance training and resistance training are the two basic types.

Of the three types of contractions, isotonic eccentric contractions require the greatest amount of tension, and therefore produce the greatest amount of force. As a result, activities that require a large number of isotonic eccentric contractions (e.g., running downhill) have a greater tendency to produce exercise-induced injuries than do those using more isotonic concentric or isometric contractions (e.g., running on a flat surface).

Of the three types of contractions, isotonic eccentric contractions require the greatest amount of tension, and therefore produce the greatest amount of force. As a result, activities that require a large number of isotonic eccentric contractions (e.g., running downhill) have a greater tendency to produce exercise-induced injuries than do those using more isotonic concentric or isometric contractions (e.g., running on a flat surface).

isotonic concentric, isotonic eccentric, and isometric contractions

On the basis of muscle length during a contraction, we can classify contractions into three types:

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.

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.

many different stimuli may elicit a smooth muscle cell contraction, including mechanical (stretch), hormonal, and nervous stimuli. In addition, some smooth muscle cells are able to depolarize and contract spontaneously. These cells, known as pacemaker cells, have unstable membrane potentials that cause them to spontaneously depolarize in a rhythmic fashion. Their depolarizations cause the firing of an action potential that spreads through the surrounding muscle cells and initiates a wave of contraction. Pacemaker cells of this type are responsible for the waves of contraction that move through the stomach and intestines. The triggering event for a smooth muscle contraction is the same as that for skeletal muscle—calcium ions flooding the cytosol. However, calcium ions come from two sources in a smooth muscle cell: They are released from the SR and they enter from the extracellular fluid. Calcium ions also play a different role in smooth muscle contraction because these cells lack troponin.

Smooth Muscle Contraction and Relaxation

- Single-unit smooth muscle (or unitary), also called visceral smooth muscle (VISS-er-ul), is the predominant type of smooth muscle in the body and is found in nearly all hollow organs, including the uterus. Single-unit 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. - Multi-unit smooth muscle is 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. Multi-unit 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 multi-unit smooth muscle varies with the number of muscle cells activated.

Smooth muscle tissue varies in structure and organization with each organ of the body in which it is found. However, from all this variation, two general types emerge:

- Peristalsis. In many organs, such as the small intestine, the smooth muscle tissue is arranged into two differently oriented layers of fibers, the circular and longitudinal layers (​Figure 10.29a​). These two layers alternately contract and relax, producing rhythmic waves called peristalsis (pehr-ih-STAL-sis; "constricting around"). Peristalsis propels materials through hollow organs of the digestive, urinary, and reproductive systems. - Formation of sphincters. In both the digestive and urinary systems, smooth muscle forms rings called sphincters. Sphincters are usually contracted but relax periodically to allow substances to pass through them. - Regulation of flow. Smooth muscle controls the flow of materials through certain hollow organs by changing the diameter of the passages. For example, the smooth muscle found in the walls of all but the smallest blood vessels controls both blood pressure and blood flow to organs and tissues (see ​Chapter 18​). In addition, the smooth muscle in the walls of the respiratory tract regulates air flow to the smaller airway passages (see ​Chapter 21​).

Some functions of smooth muscle include the following:

M line

The dark line in the middle of the A bands - consists of structural proteins that hold the thick filaments in place and serve as an anchoring point for the elastic filaments

voltage

The difference in electrical potential between two points.

1. An action potential arrives at the axon terminal and triggers Ca2+ channels in the axon terminal to open. 2. Calcium ion entry triggers exocytosis of synaptic vesicles. 3. Synaptic vesicles release acetylcholine into the synaptic cleft. 4. Acetylcholine binds to ligand-gated ion channels in the motor end plate. 5. Ion channels open and sodium ions enter the muscle fiber. 6. Entry of sodium ions depolarizes the sarcolemma locally, producing an end-plate potential.

The 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, proceeds with the following steps

sarcomere

The functional unit of muscle contraction; consists of the area of the myofibril from one Z-disc to the next Z-disc. - each includes a full A band and half of two I bands

motor unit

The group of muscle fibers innervated by a single motor neuron. - An average motor unit consists of about 150 muscle fibers, but this number can vary widely with the degree of motor control needed for the muscle - Muscles that require fine control, such as those around the larynx (voice box) or in the fingers, will have multiple small motor units, often containing as few as 10 muscle fibers per motor unit. This gives the nervous system more precise control over the amount and rate of tension produced. Conversely, large, powerful muscles, such as the postural muscles of the back and the gastrocnemius in the calf, can have 2000-3000 muscle fibers in each motor unit. - Motor units contain one class of muscle fiber, either type I or type II. Those with type I fibers are called slow motor units, and those with type II fibers are called fast motor units.

switch on and off the process of muscle contraction.

The two regulatory proteins tropomyosin and troponin help to....

Latent period

The latent period is 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.

Excitation-contraction coupling

The linking of muscle fiber excitation via an action potential, with contraction via the release of calcium ions from the sarcoplasmic reticulum.

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.

Principle of myoplasticity

The principle stating that the structure of a muscle will change in accordance with its functional use.

motor end plate

The specialized region of the skeletal muscle fiber plasma membrane that contains receptors for acetylcholine. - ligand-gated ion channels; ACh is the ligand.

sarcoplasmic reticulum (SR)

The specialized smooth endoplasmic reticulum of a muscle fiber that stores calcium ions. - forms a weblike network surrounding each myofibril. The structure varies in the three types of muscle tissue

1. The end-plate potential stimulates an action potential. 2. The action potential is propagated down the T-tubules. 3. T-tubule depolarization leads to the opening of calcium ion channels in the sarcoplasmic reticulum, and calcium ions enter the cytosol. As calcium ions flood the cytosol, the sarcolemma is repolarizing.

The steps of excitation-contraction coupling are as follows:

Electrophysiology

The study of electrical changes in the body's cells and the physiological processes that accompany these changes.

Electrochemical Gradients

The sum of the electrical gradient and chemical (concentration) gradient for an ion.

Type I fibers

Type I fibers are slow-twitch fibers that are small to intermediate in diameter. Type I 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, type I 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 type I muscle fibers makes them red, so they are sometimes known as "red muscle."

Type II fibers

Type II fibers are fast-twitch fibers that are often larger in diameter and contract more rapidly than type I fibers, but they are quickly fatigued. Type II 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, type II 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.) Type II fibers have two subtypes: types IIa (also known as fast oxidative glycolytic, or FOG) and IIx (fast glycolytic, or FG). These subtypes have progressively faster, stronger twitches and rely increasingly on glycolytic energy sources, with type IIx fibers having extremely fast, powerful twitches and using glycolytic catabolism almost exclusively.

white fast twitch fibers

Which type of muscle fiber has a large quantity of glycogen and mainly uses glycolysis to synthesize ATP?

in the T-tubules preventing action potentials from reaching the terminal cisternae.

While researching muscle contraction, you discover that action potentials are not reaching the terminal cisternae of the sarcoplasmic reticulum, and in turn calcium ions are not being released into the cytosol. You hypothesize that there is must be a mutation...

Resistance training

also called strength training, features a moderate increase in the frequency of motor unit activation and a large increase in force production—in other words, fewer repetitions with heavier weight. Resistance training typically involves the use of either free weights or a resistance-exercise machine. Whereas many of the changes due to endurance training are primarily biochemical, several changes that result from resistance training are in large part anatomical. both the number of myofibrils and the diameter of the muscle fibers increase, a change called hypertrophy (hy-PER-troh-fee). With hypertrophy comes a decreased proportion of mitochondrial proteins and a lower blood supply. However, this decrease is a function of the fiber enlarging rather than actually losing mitochondria or blood vessels. As you might expect, resistance training can decrease the capacity for endurance.

Cardiac muscle cells more closely resemble skeletal muscle fibers than smooth muscle cells, as we saw in Figure 10.1. Like skeletal muscle fibers, cardiac muscle cells are striated and consist of sarcomeres. They also have both T-tubules and extensive networks of sarcoplasmic reticulum. However, notable structural differences between cardiac muscle cells and skeletal muscle fibers do exist. Cardiac muscle cells are typically shorter, branched cells with one or two nuclei and abundant myoglobin. Mitochondria account for about 30% of their cytoplasmic volume. In addition, cardiac muscle cells are connected by intercalated discs that contain desmosomes and gap junctions (see Chapter 4). These discs join cardiac muscle cells to one another physically and electrically, which permits the heart to contract as a unit. Unlike skeletal muscle fibers, cardiac muscle cells do not require stimulation from the nervous system to generate action potentials, because their electrical activity is coordinated by a small population of pacemaker cells. Like those in smooth muscle, these cells rhythmically and spontaneously generate action potentials that trigger the remaining cardiac muscle cells to have action potentials as well. This quality allows cardiac muscle tissue to be autorhythmic—it sets its own rhythm. In this way, cardiac muscle tissue is similar to single-unit smooth muscle in that all the cells depolarize and contract with every contraction cycle.

cardiac

smooth muscle tissue

consists of smooth muscle cells, which are long and flattened with two pointed ends and which have a single, centrally located, oval nucleus - line nearly every hollow organ, and are found as well in the eyes, the skin, and the ducts of certain glands. - many are linked to one another by gap junctions in their plasma membranes

First 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.

describe sarcomere contraction

Smooth

lacks striations

skeletal muscles are enclosed by a layer of thick connective tissue called fascia, which anchors them to the surrounding tissues and holds groups of muscles together. Deep to the fascia is another layer of connective tissue, the epimysium, which surrounds the whole muscle. The epimysium blends with a deeper layer of connective tissue, the perimysium, to form tendons, which bind the muscle to its attaching structure. The perimysium surrounds individual fascicles, or groups of muscle fibers. The muscle fiber is then of course made up of myofibrils, which are composed of myofilaments. The arrangement of myofilaments within the myofibrils creates sarcomeres, the functional units of contraction.

layers to a muscle

atophy

loss of muscle mass from disuse - In addition, the amount of oxidative enzymes decreases, and the fiber has a lower capacity for oxidative catabolism. The result is a decline in both strength and endurance. Muscle atrophy is a particular problem for people who are bedridden or have lost the use of their limbs.

H zone

middle of A band with only thick filaments

Muscle fiber

muscle cell - skeletal muscle cells that are quite long, extending nearly the entire length of the muscle - mostly found attached by connective tissue to the skeleton, where their contraction can produce the movement of a body part

- Depletion of key metabolites. Certain metabolites, such as creatine phosphate, glycogen, and blood glucose, are depleted by intense activity, rendering the muscle fiber less able to replenish the ATP consumed by the activity. - Decreased availability of oxygen to muscle fibers. Exercise increases the oxygen requirement of muscle fibers because of the need for more ATP. The amount of oxygen bound to myoglobin may also be depleted by intense exercise, and the amount of oxygen taken in by the lungs may be inadequate to replace it. The muscle fiber must then rely more heavily on the less efficient process of glycolysis to generate ATP. Keep in mind that we are referring only to the amount of oxygen available to muscle fibers relative to what they need; the actual amount of oxygen in the body does not decrease significantly. - Accumulation of certain chemicals. Many chemicals that may contribute to fatigue are produced by muscle fibers. For example, calcium ions accumulate in the mitochondria, where they interfere with the mitochondrial ability to carry out oxidative catabolism. Also, as ATP is split, phosphate ions and ADP accumulate in the cytosol and interfere with excitation-contraction coupling. - Environmental conditions. Severe environmental conditions, particularly extreme heat, disrupt the body's homeostasis, leading to more rapid muscular fatigue. Sweating in response to excessive heat may cause electrolyte disturbances that can interfere with the production of action potentials in a muscle fiber.

muscular fatigue results from

Most obvious is that a relationship exists between the rate of ventilation and the level of carbon dioxide in the blood: As the rate and depth of ventilation increase, more carbon dioxide is exhaled, helping to return the pH of the body's fluids to normal. However, the relationship may be less obvious for temperature and electrolyte homeostasis. To make it easier to understand, consider facts you already know—the Na+/K+ pump restores ion gradients, pumps move calcium ions back into the sarcoplasmic reticulum, and sweating produces cooling. These three mechanisms have one thing in common: They all consume ATP. Since oxidative catabolism—the most efficient way to generate ATP—requires oxygen, it follows that the body's cells will need "extra" oxygen after exercise.

note

Relaxation of smooth muscle begins with removal of calcium ions from the cytosol. As this concentration declines, calcium ions dissociate from calmodulin, and MLCK is inactivated. At this point, the muscle cell either relaxes or enters the latch state. During the latch state, the muscle cell maintains tension while consuming very little ATP. The latch state is very important in terms of the ability of smooth muscle to maintain a state of energy-efficient sustained contraction, as is found in sphincters, which must stay contracted to remain closed.

note

- Contractility - Excitability (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.) - Conductivity - Distensibility (they can be stretched up to three times their resting length without damage) - Elasticity (can also return to their original shape after being stretched)

properties of muscle cells

To begin with, smooth muscle cells lack striations and sarcomeres, which gives them their characteristic "smooth" appearance (Figure 10.29b). In addition, the actin filaments are arranged obliquely in the sarcoplasm and are anchored to proteins called dense bodies. Some dense bodies are associated with the sarcolemma, where they attach to the dense bodies of other smooth muscle cells and transmit tension from one cell to the next. Other dense bodies are found in the sarcoplasm, where they are bound to scaffold-like intermediate filaments that connect the dense bodies to one another. Several thin filaments radiate from each dense body, and these thin filaments surround one thick filament. The ratio of thin to thick filaments is higher in smooth muscle than in skeletal muscle, with more thin filaments for every thick filament. Differences are also found in the structure and composition of thin and thick filaments in smooth muscle cells. Both types of filaments are longer than those in skeletal muscle fibers, and the thin filaments lack troponin. You can see in Figure 10.30a that another difference is found in the my

smooth muscle structure

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.

1. ATP hydrolysis "cocks" the myosin head into its high-energy position. The ADP and phosphate remain attached to the myosin head when it is cocked. 2. The myosin head binds to actin. Note that the resulting crossbridge is at about a 90° angle relative to the thick filament. 3. 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. Notice that the myosin crossbridge is now at about a 45° angle relative to the thick filament 4. ATP breaks the attachment of myosin to actin by binding to myosin.

steps in a crossbridge cycle

1. Calcium ions bind to troponin. 2. Tropomyosin moves, and the active sites of actin are exposed. When the active sites of actin are exposed, the myosin heads are able to bind tightly to them. A myosin head bound to an actin subunit is known as a cross bridge.

steps in preparation for muscle contraction

1. Depolarization stage: In response to a stimulus, voltage-gated sodium ion channels open and sodium ions enter the cell, making the membrane potential less negative. 2. Repolarization stage: Sodium ion channels close while voltage-gated potassium ion channels open and potassium ions leave the cell, making the membrane potential more negative again.

steps to how an action potential is generated in a muscle cell

1. Acetylcholinesterase degrades the remaining ACh, and the final repolarization occurs. 2. The sarcolemma returns to its resting membrane potential, and calcium ion channels in the SR close. 3. Calcium ions are pumped back into the SR, returning the calcium ion concentration in the cytosol to its resting level. 4. Troponin shifts and pulls tropomyosin back into position to block the active sites of actin, and the muscle relaxes.

steps to muscle relaxation

Calcium ions bind a protein in the cytosol called calmodulin (kal-MOD-yoo-lin​; ​​​​​​Cam​).​ The calcium ion-Cam complex activates an enzyme associated with myosin called myosin light-chain kinase (MLCK). MLCK causes the activation of myosin ATPase. Crossbridge cycles then ensue.

steps to smooth muscle contraction Instead, when calcium ions flood the cytosol, the following series of events occurs:

skeletal muscle tissue

striated, voluntary, multinucleated - muscle cells that are arranged parallel to one another

zone of overlap

the densest, darkest area on a light micrograph where thick and thin filaments overlap - where tension is generated during a muscle contraction.

Muscle tone

the state of balanced muscle tension that makes normal posture, coordination, and movement possible - serve important functions, including maintaining erect posture, stabilizing joints, generating heat, and ensuring that the muscle is ready to respond if movement is initiated. If the tone in a skeletal muscle is abnormally low, generally because of a nervous system disorder, a condition known as hypotonia results. A hypotonic muscle looks flattened and feels soft and loose rather than firm. Often, the joints around the affected muscles are hyperextended, as the muscles are no longer able to generate enough tension to stabilize them. Alternately, a muscle may have abnormally high tone, a condition called hypertonia. A hypertonic muscle often feels rock hard, and it may shorten so much that it causes painful joint contractures (see Chapter 8). Normal hypertonia occurs during shivering, which takes place in response to a drop in body temperature, as more motor units are activated to generate additional heat.

smooth, skeletal, cardiac

three types of muscle tissue

Endurance Training

training with a large increase in the frequency of motor unit activation and a moderate increase in force—in other words, more repetitions with lighter weight. Endurance training involves activities such as cycling, jogging, cross-country skiing, and distance swimming. shows that after endurance training the muscle fiber has increased amounts of oxidative enzymes, more mitochondria and mitochondrial proteins, and a greater number of blood vessels. These adaptations enhance the oxidative capabilities of the muscle dramatically, resulting in a better ability to use fatty acids and other fuels to make ATP, and so an increased resistance to fatigue. These changes occur even in type II fibers, but are more dramatic in type I fibers.