BMS 250 ch. 10.

Explain the two primary criteria used to classify skeletal muscle fiber types.

-the type of contraction generated -the primary means used for supplying ATP 1. Skeletal muscle fibers differ in the power, speed, and duration of the muscle contraction generated. Power is related to the diameter of a muscle fiber; large muscle fibers are more densely packed with myofibrils and produce a more powerful contraction. Speed has traditionally been described based upon whether the skeletal muscle fiber contains the relatively slow or fast genetic variant of myosin ATPase, the enzyme that splits ATP. 2.The second criterion used to differentiate skeletal muscle fibers is whether the primary means to supply ATP to the muscle is aerobic cellular respiration or anaerobic cellular respiration. Oxidative (red) fibers use aerobic cellular respiration and have several features that support these processes, including an extensive capillary network, large numbers of mitochondria, and a large supply of the red pigment myoglobin. These fibers are also called fatigue-resistant. Glycolytic (white) fibers use anaerobic cellular respiration and have fewer structures needed for aerobic cellular respiration—thus, they have sparse capillary networks, few mitochondria, and small amounts of myoglobin.

Distinguish between isometric and isotonic contractions, and give examples of both.

1. An isometric contraction occurs when muscle tension is insufficient to overcome the resistance (i.e., force generated is less than the load) and there is no movement of the muscle. The muscle contracts and muscle tension increases, but muscle length stays the same. e.g. pushing on a wall, holding a very heavy weight in the gym while your arm does not move, attempting to move a shovel load of snow that is too heavy, or holding a baby in one position 2.An isotonic contraction occurs when muscle tension is greater than the resistance (load) (i.e., force generated is greater than the load), and there is movement of the muscle. e.g. walking, lifting a baby, or swinging a tennis racket Isotonic contractions are differentiated into two subclasses based on whether the muscle is shortening or lengthening as it contracts. The shortening of muscle length is a concentric contraction. The lengthening of muscle is an eccentric contraction.

Explain the five general functions of skeletal muscle.

1. Body movement -skeletal muscle movement 2. Protection and support of internal organs -walls of abdominal cavity and pelvic floor 3. maintenance of posture -stabilizing joints 4.storage and movement of materials -gastrointestinal and urinary tracts -sphincters to regulate passage through orifices 5.heat production -exercise -shivering

Discuss what happens to each of the following to allow for skeletal muscle relaxation: ACh, action potential, Ca2+ concentration in sarcoplasm, and troponin-tropomyosin complex.

1. The first step in muscle relaxation is the termination (end) of the nerve signal in the motor neuron. -This prevents further release of ACh from the motor neuron's synaptic knobs. 2. Acetylcholinesterase (AChE) in the synaptic cleft continually hydrolyzes (i.e. breaks down) ACh. 3. The ACh receptors close, and both the end plate potentials at the motor end plate and the action potentials along the sarcolemma and T-tubules cease. 4. Voltage-gated Ca2+ channels in the SR and terminal cisternae close. 5. The Ca2+ already released from the sarcoplasmic reticulum is continuously returned into the terminal cisternae by Ca2+ pumps (active transport requiring ATP). After cessation of skeletal muscle fiber stimulation, the remaining Ca2+ in the sarcoplasm is transported back into storage within the sarcoplasmic reticulum, where it is bound by both calmodulin and calsequestrin. 6. Troponin returns to its original shape when Ca2+ is removed, and simultaneously the tropomyosin moves over the myosin binding sites on actin. This prevents myosin-actin crossbridge formation.

Distinguish between treppe, wave summation, incomplete tetany, and tetany that occur with an increase in frequency of stimulation.

1. Treppe is an increase in muscle tension that occurs when not all Ca2+ is returned to the sarcoplasmic reticulum prior to the next stimulus and muscle temperature increases. Consequently, more crossbridges may form upon subsequent stimulation, causing a stronger contraction. 2.The restimulated muscle displays a summation of contractile forces as the effect of each new wave is added to the previous wave. This effect is often called either wave summation because contraction waves are added together or "summed," or temporal summation because it depends upon increasing frequency of stimulation over time. (The voltage remains the same.) 3.Further increases in stimulation frequency allow less time for relaxation between contraction cycles, and now incomplete tetany (the tension tracing continues to increase and the distance between waves decreases). The voltage remains the same. 4.Stimulation frequency is further increased (40 to 50 stimuli per second) until ultimately the contractions of the muscle fiber "fuse" and form a continuous, smooth contraction that lacks any relaxation. The voltage remains the same.

Explain the sequence of steps in smooth muscle contraction

1.In response to stimulation, Ca2+ enters the sarcoplasm from both the interstitial fluid (ECF) and sarcoplasmic reticulum (inside the cell). 2. Ca2+ binds to calmodulin to form a Ca2+-calmodulin complex that then binds to MLCK, resulting in its activation. 3. The activated kinase (MLCK) phosphorylates the myosin head to both activate the myosin ATPase activity of the myosin head and to allow the myosin head to bind to actin forming a crossbridge. 4. Myosin ATPase hydrolyzes ATP to produce the power stroke. 5.The myosin head releases and reattaches to the actin repetitively, causing the thin filament to slide past the thick filament. -This sliding results in a pull on the attached dense bodies anchored to the intermediate filaments of the cytoskeleton and the dense plaques attached to the sarcolemma. -The anchoring intermediate filaments move inward and the entire smooth muscle cell shortens.

explain the three characteristics that allow smooth muscle to effectively fulfill its functions.

1.Initiation and Duration of Contraction slow developing contractions relatively long latent period because of phosphorylation of the myosin head via MLCK and ATPase activity long duration contractions because of slowness of Ca2+ pumps, dephosphorylation of the myosin head via MLCP, and myosin locking to actin (i.e. latchbridge mechanism) 2.Fatigue Resistant because of low ATP demand due to the latchbridge mechanism. because it gets most of its ATP from aerobic respiration It's necessary to be fatigue resistant to be able to maintain continuous tone in the GI tract and blood vessels. 3.Broad Length-Tension Curve no optimal length for smooth muscle contraction as is seen in skeletal muscle contraction which has sarcomeres and z disks Smooth muscle will contract at various degrees of stretch.

Describe the components of a neuromuscular junction.

A neuromuscular junction is the specific location, usually in the mid-region of the muscle fiber where it is innervated by a motor neuron. The neuromuscular junction has three parts. 1. synaptic knob contains Ca2+ pumps contains voltage-gated Ca2+ channels contains synaptic vesicles filled with the neurotransmitter acetylcholine (ACh) 2. motor end plate specialized region of the sarcolemma of a muscle fiber that has vast numbers of ACh receptors Binding of ACh to these receptors opens chemically gated Na+ and K+ channels, allowing Na+ entry into the muscle fiber and K+ to exit. 3. synaptic cleft fluid-filled space separating the synaptic knob and the motor end plate contains acetylcholinesterase (AChE) which quickly breaks down ACh molecules following their release into the synaptic cleft

Define a motor unit, and describe its distribution in a muscle and why it varies in size.

A single motor neuron and the muscle fibers it controls is called a motor unit. The number of skeletal muscle fibers a single motor neuron innervates—and thus the size of the motor unit—varies and can range from small motor units that have less than five muscle fibers to large motor units that have several thousand muscle fibers. The size of the motor unit determines the degree of control. There is an inverse relationship between the size of a motor unit and the degree of control.

Describe the structure and function of a tendon and an aponeurosis.

A tendon is a thick, cordlike structure composed of dense regular connective tissue. Tendons attach the muscle to bone, skin, or another muscle. An aponeurosis is a fibrous sheet or flat, expanded tendon.

Describe what occurs in a muscle during a single twitch, and relate each event to a graph of a twitch.

A twitch is defined as a single, brief contraction period and then relaxation period of a skeletal muscle in response to a single stimulation 1. There is a delay called a latent period (lag period) that occurs in a twitch after the stimulus is applied and before the contraction of the muscle fiber begins. -There is no change in fiber length during the latent period. -This delay can be accounted for by the time necessary for the occurrence of all of the events in excitation-contraction coupling, Ca2+ release from the sarcoplasmic reticulum (terminal cisternae) into the cytosol, the interaction between Ca2+ and troponin, and the movement of tropomyosin off the active sites on actin. 2. The contraction period begins as repetitive power strokes pull the thin filaments past the thick filaments, shortening the sarcomeres; muscle tension increases during muscle contraction. 3.The relaxation period begins with release of crossbridges resulting from return of Ca2+ back into the sarcoplasmic reticulum; muscle tension decreases during muscle relaxation. The contraction phase has a shorter duration than the relaxation phase because contraction is an active process that forms crossbridges and generates the power stroke, whereas relaxation is a passive process (except where Ca2+ is concerned) that depends upon the elasticity of connectin within muscle tissue to return to its original length.

Explain how the means of supplying ATP is related to intensity and duration of exercise.

ATP is also generated in aerobic cellular respiration from triglycerides - a storage form of fatty acids. The amount of ATP generated is dependent upon the length of the fatty acid chain. The longer the chain, the more ATP produced. Fatty acids are the preferential fuel molecule for generating ATP in most skeletal muscle tissue. The major drawback is that sufficient oxygen must be delivered continuously during sustained exercise for skeletal muscle tissue to use fatty acids to generate energy. need a lot of ATP and oxygen for prolonged intensity and duration of exercise

Describe the distribution of muscle fiber types in a muscle and how this relates to function.

Although most muscles contain a mixture of all three fiber types, the relative percentage of the muscle fiber types varies between different skeletal muscles of the body and reflects the function of the muscle. Example 1: The muscles of the eye and hand require swift but brief contractions so they contain a high percentage of FG fibers. Example 2: Many postural back and calf muscles, which contract almost continually to help us maintain an upright posture, are dominated by SO fibers. Variations are also present between individuals, and this is seen most dramatically in high-caliber athletes. Distance runners have higher proportions of SO fibers in their leg muscles. Sprinters and weightlifters have a higher percentage of FG fibers in their leg muscles.

Explain the events that occur in motor unit recruitment as the intensity of stimulation is increased.

An Experiment Designed To Show Motor Unit Recruitment The gastrocnemius muscle is stimulated repeatedly in a set of experiments to demonstrate motor unit recruitment, and each stimulation event is at a greater voltage. The frequency of stimulation remains the same, and the time between stimulation events is sufficient for the gastrocnemius muscle to contract and relax before it is stimulated again. Because motor units vary in their sensitivity to stimulation, each increase in voltage causes a greater number of motor units in the gastrocnemius muscle to contract. Consequently, the tension generated with each gastrocnemius muscle contraction increases until the point of maximum contraction is reached when all motor units in the gastrocnemius muscle have been stimulated. This increase in muscle tension that occurs with an increase in stimulus intensity is called recruitment, or multiple motor unit summation.

Explain the organization of myofibrils, myofilaments and sarcomeres.

Approximately 80% of the volume of a muscle fiber is composed of long, cylindrical structures termed myofibrils. A skeletal muscle fiber contains hundreds to thousands of myofibrils. Each myofibril is about 1 to 2 micrometers in diameter and extends the length of the entire skeletal muscle fiber. Each myofibril contains bundles of muscle protein filaments called myofilaments. Myofibril bundles contain two types of myofilaments. thick filaments - myosin thin filaments - actin

Define muscle fatigue and explain some of its causes.

Muscle fatigue is the reduced ability or inability of the muscle to produce muscle tension. The primary cause of muscle fatigue during excessive or sustained exercise (e.g., running a marathon) is caused by a decrease in glycogen stores. Muscle fatigue may be caused either by insufficient free Ca2+ at the neuromuscular junction to enter the synaptic knob or by a decreased number of synaptic vesicles to release neurotransmitter. Muscle fatigue may be due to a change in ion concentration (e.g. Na+ and K+) that interferes with the ability of the muscle fiber to conduct an action potential along the sarcolemma. This interferes with stimulating release of Ca2+ from the sarcoplasmic reticulum. Muscle fatigue may result from increased phosphate ion (Pi) concentration. Elevated Pi concentration in the muscle sarcoplasm interferes with Pi release from the myosin head during crossbridge cycling, and this slows the rate of cycling. Muscle fatigue also may occur when lower amounts of Ca2+ are available for release from the sarcoplasmic reticulum (part of which is due to its binding with the excess Pi). Lower Ca2+ levels results in less Ca2+ binding to troponin, producing a weaker muscle contraction. Lack of ATP is not currently thought to be a primary cause of muscle fatigue, but lack of ATP may still contribute to fatigue. This is because ATP levels are generally maintained through aerobic cellular respiration in mitochondria during sustained exercise. It remains to be determined if ATP may still be a factor because of its location in the cell—that is, within the mitochondria and not in proximity to myofilaments.

Describe muscle tone, and explain its significance.

Muscle tone is the resting tension in a muscle generated by involuntary nervous stimulation of the muscle. Limited numbers of motor units within a muscle are usually stimulated randomly at any given time to maintain a constant tension; the specific motor units being stimulated during rest change continuously so motor units do not become fatigued. The resting muscle tone establishes constant tension on the muscle's tendon, thus stabilizing the position of the bones and joints.

Distinguish between thick and thin filaments.

Myofibril bundles contain two types of myofilaments. thick filaments - myosin thin filaments - actin

Explain the organization of sarcomeres.

Myofilaments within myofibrils are arranged in repeating microscopic cylindrical units called sarcomeres. Each sarcomere is composed of overlapping thick filaments (myosin) and thin filaments (actin). Each sarcomere is delineated at both ends by Z discs. Z discs (also called Z lines) are composed of specialized proteins that are positioned perpendicular to the myofilaments and serve as anchors for the thin filaments.

Explain the length-tension relationship in skeletal muscle contraction.

One factor that influences the amount of tension a muscle can generate when stimulated is the amount of overlap of thick and thin filaments when the muscle begins its contraction. A muscle generates different amounts of tension dependent upon its length at the time of stimulation. A skeletal muscle fiber stimulated when it is at a normal resting length generates a maximum contractile force because there is optimal overlap of thick and thin filaments.

List and describe the similarities and differences between skeletal muscle and cardiac muscle.

Similarities Between Cardiac and Skeletal Muscle -Cardiac muscle cells are striated like skeletal muscle -cells. -Cardiac muscle cells contain sarcomeres like skeletal muscle cells. -Cardiac muscle cells contain a large number of mitochondria like skeletal muscle cells. Differences Between Cardiac and Skeletal Muscle -Cardiac muscle cells branch unlike skeletal muscle cells. -Cardiac are both shorter and thicker than skeletal muscle cells. -Cardiac muscle cells have intercalated discs unlike skeletal muscle cells. -Cardiac muscle cells can have only one nucleus (but may have two) unlike skeletal muscle cells which are always multinucleate. -Cardiac muscle cells use aerobic respiration almost exclusively to generate the ATP required for their unceasing work. -Cardiac muscle cells are stimulated by a specialized autorhythmic pacemaker.

List and describe the structures associated with energy production within skeletal muscle fibers.

Skeletal muscle fibers have a great demand for energy and contain several components that facilitate the production of ATP. -mitochondria for aerobic cellular respiration -glycogen stores for use as an immediate fuel molecule -Myoglobin binds oxygen when the muscle is at rest and releases it for use during muscular contraction. -Creatine phosphate provides muscle fibers with a means of supplying ATP anaerobically.

Explain the function of blood vessels and nerves serving a muscle.

Skeletal muscle is vascularized by an extensive network of blood vessels that deliver oxygen and nutrients and remove waste products. Skeletal muscle is innervated by or functionally connected to and controlled by motor neurons. Somatic motor neurons extend from the brain and spinal cord to skeletal muscle fibers.

Compare the microscopic anatomy of smooth muscle to skeletal muscle.

Smooth muscle cells are small and fusiform-shaped unlike skeletal muscle cells. Smooth muscle cells have one centrally located nucleus unlike skeletal muscle cells. Smooth muscle cells are much smaller than skeletal muscle cells. The small tapered ends of smooth muscle cells overlap the larger middle area of adjacent cells to provide for close packing of cells. The sarcolemma of smooth muscle cells contains various types of Ca2+ channels (e.g., voltage-gated, chemically gated, modality gated) that allow these cells to respond to different types of stimuli. Smooth muscle cells are lacking T-tubules but have invaginations called caveolae.

Explain the primary functional difference between multiunit and single-unit smooth muscle.

Smooth muscle is classified into two broad groups. Multiunit smooth muscle receives stimulation to contract individually. Multiunit smooth muscle is found in several places and are arranged in motor units. the iris and ciliary muscles of the eye the arrector pili muscles in the skin the wall of larger air passageways within the respiratory system the walls of larger arteries Degree of contraction is dependent on number of motor units activated. Single-unit smooth muscle (i.e. visceral smooth muscle cells) is the most abundant and are stimulated to contract in unison. Single-unit smooth muscle is found in several places. the walls of the digestive, urinary, and reproductive tracts smaller portions of the respiratory tract most blood vessels. Single-unit muscle cells usually form sheets and contract in response to the release of neurotransmitters from varicosities.

Briefly explain the different means of controlling smooth muscle.

Smooth muscle is controlled by the autonomic nervous system (components of the nervous system that control cardiac muscle, smooth muscle, and glands involuntarily). The response of smooth muscle to stimulation by the nervous system—that is, whether it contracts or relaxes—is dependent upon the specific neurotransmitter that is released. Smooth muscle within the walls of bronchioles, for example, contracts in response to the release of ACh, and relaxes in response to norepinephrine. Smooth muscle also contracts in response to being stretched (myogenic response). Smooth muscle exhibits what is called the stress-relaxation response. Smooth muscle is "stressed" by being stretched and it responds by contracting, but after a given period of time, it relaxes. Smooth muscle is also stimulated to contract by various hormones, a decrease in pH, low oxygen concentration, increased carbon dioxide levels, certain drugs, and pacemaker cells.

Identify organs of various body systems where smooth muscle is located.

Smooth muscle tissue is found in the walls of organs of many different body systems. cardiovascular system: blood vessels respiratory system: bronchioles (air passages) digestive system: stomach, small intestine, and large intestine urinary system: ureters female reproductive system: uterus other: iris of the eye, ciliary body of the eye, arrector pili

Summarize the changes that occur within a sarcomere during contraction.

The H zone disappears. The I band narrows in width and may disappear. The Z discs in one sarcomere move closer together. The thin and thick filaments do not shorten. A description of the repetitive movement of thin filaments sliding past thick filaments is called the sliding filament theory.

Identify and describe the three connective tissue layers associated with a muscle.

The epimysium is a layer of dense irregular connective tissue that surrounds the whole skeletal muscle. The perimysium surrounds the fascicles and contains extensive arrays of blood vessels and nerves that branch to supply muscle fibers within each individual fascicle. The endomysium is the innermost connective tissue layer that surrounds and electrically insulates each muscle fiber.

Explain the events that lead to release of the neurotransmitter ACh from a motor neuron.

The first physiologic event of skeletal muscle contraction is muscle fiber excitation by a motor neuron—an event that occurs at the neuromuscular junction and results in release of ACh and its subsequent binding to ACh receptors. There are three steps in this first event. 1.A nerve signal is propagated down a motor axon and triggers the opening of voltage-gated Ca2+channels within the synaptic knob. Calcium moves down its concentration gradient from the interstitial fluid (ECF) through the open channels into the synaptic knob (ICF). Calcium binds with synaptotagmin , a protein on the vesicle membrane. (1a) 2.The binding of Ca2+ to synaptic vesicles triggers the synaptic vesicles to merge with the synaptic knob plasma membrane resulting in exocytosis of ACh into the synaptic cleft. (1b) 3.ACh diffuses across the fluid-filled synaptic cleft to bind with ACh receptors within the motor end plate. (1c)

Describe immediate means that muscle fibers use to supply ATP for muscle contraction.

The immediate supply of ATP is generated by the phosphagen system, which uses molecules that contain a high-energy phosphate. The phosphagen system is not dependent upon the presence of oxygen; that is, the processes of this system are anaerobic. This system represents both the available ATP and the immediate means of forming ATP from other phosphate-containing molecules (i.e. myokinase and ADP, and creatine phosphate)

Describe long-term means that muscle fibers use to supply ATP for muscle contraction.

The long-term supply of ATP is provided by aerobic cellular respiration that occurs within mitochondria. The nutrient source for aerobic production of ATP is the pyruvate made available following anaerobic cellular respiration. Aerobic cellular respiration requires the presence of oxygen that is both delivered by the blood and provided by myoglobin.

Describe the sarcolemma, T-tubules, and sarcoplasmic reticulum of a skeletal muscle fiber.

The plasma membrane of a skeletal muscle fiber is called the sarcolemma Deep invaginations of the sarcolemma, called T-tubules or transverse tubules, extend into the skeletal muscle fibers as a network of narrow membranous tubules. The sarcoplasmic reticulum is an internal membrane complex that is similar to the smooth endoplasmic reticulum of other cells. -Segments of sarcoplasmic reticulum surround bundles of contractile proteins called myofibrils. The sarcoplasmic reticulum fits around the myofibril like a sleeve of membrane netting. At either end of individual sections of the sarcoplasmic reticulum are expanded sacs called terminal cisternae, which serve as reservoirs for calcium ions (Ca2+). Terminal cisternae are immediately adjacent to each T-tubule. Together, two terminal cisternae and a centrally placed T-tubule interact to form a structure called a triad that functions during muscle contraction.

Describe the steps in excitation-contraction coupling.

The second physiologic event of muscle contraction is excitation-contraction coupling—an event that involves the sarcolemma, T-tubules, and sarcoplasmic reticulum. This event links the steps of skeletal muscle stimulation at the neuromuscular junction (event 1) with the steps of muscle contraction (event 3). There are three steps in event 2: excitation-contraction coupling. development of an end-plate potential at the motor end plate (2a) initiation and propagation of an action potential along the sarcolemma and T-tubules (2b) release of Ca2+ from the terminal cisternae of the sarcoplasmic reticulum (2c) These events occur during what is known as the latent period.

Describe short-term means that muscle fibers use to supply ATP for muscle contraction.

The short-term supply for ATP is provided by anaerobic cellular respiration (glycolysis), a process that occurs in the cytosol and does not require oxygen.

Explain the relationship of skeletal muscle elasticity and muscle relaxation.

Through the natural elasticity of the muscle fiber, the muscle returns to its original relaxed position, a process facilitated by the release of passive tension that developed in connectin proteins that were compressed during contraction.

Define oxygen debt, and explain why it occurs.

When an individual participates in exercise during which the demand for oxygen exceeds the availability of oxygen, an oxygen debt is incurred. Oxygen debt is the amount of additional oxygen that must be consumed following exercise to restore pre-exercise conditions. This additional oxygen is needed for the following: to replace oxygen on hemoglobin molecules in the blood and myoglobin molecules in muscle to replenish glycogen stored in muscle fibers to replenish ATP and creatine phosphate in the phosphagen system to convert lactic acid (lactate) back to glucose

Describe the five characteristics of skeletal muscle tissue.

conductibility- Conductivity involves an electrical change that travels along the plasma membrane of the skeletal muscle cell. excitability- Excitability is the ability of skeletal muscle cells to respond to stimulation by the nervous system. Neurons release molecules called neurotransmitters that bind to receptors of a muscle cell. Contractility- contractility is exhibited when contractile proteins within skeletal muscle cells slide past one another, causing skeletal muscle cells to shorten. elasticity-Elasticity is due to specialized protein fibers within skeletal muscle cells that act like compressed coils when a muscle contracts. Extensibility- extensibility is the lengthening of a muscle cell. When you flex your elbow joint, you are contracting the biceps brachii on the anterior side of your arm, while the triceps brachii on the posterior side is extended with the motion.

Compare and contrast the three muscle fiber types.

slow oxidative (SO) fibers (get ATP from aerobic cellular respiration) small diameter, slow ATPase, slow and less powerful contractions, resistant to fatigue, appear red (have myoglobin) fast oxidative (FO) fibers (get ATP from aerobic cellular respiration) intermediate diameter, fast ATPase, fast and powerful contractions, resistant to fatigue, appear light red fast glycolytic (FG) fibers (get ATP from anaerobic respiration) large diameter, fast ATPase, fast and powerful contractions, not resistant to fatigue, appear pale or white (lack myoglobin)