Bio 141 - Exam 4 (Chapter 10)

List and describe the events that occur at the neuromuscular junction leading to the stimulation of muscle contraction in a muscle fiber.

- A nerve signal triggers opening of Ca2+ channels in the synaptic knob, so Ca moves down its concentration gradient into the synaptic knob, where it binds with membrane proteins exposed on external surfaces of synaptic vesicles - The binding of Ca2+ to synaptic vesicles triggers merging of synaptic vesicles w/ the synaptic knob plasma membrane, resulting in exocytosis of ACh into the synaptic cleft. ACh is released from approx 300 vesicles per nerve signal. - ACh diffuses across the synaptic cleft to bind w/ ACh receptors w/in the motor end plate. This causes excitation of muscle fibers.

List and explain the sequence of 5 steps in smooth muscle contraction.

1.Stimuli triggers opening of voltage-gated Ca2+ channels. Ca2+ enters the sarcoplasm from the interstitial fluid. 2. Ca2+ binds to calmodulin to form a Ca2+-calmodulin complex. 3. Ca2+-calmodulin complex activates MLCK, a phosphorylating enzyme. 4. Activated MLCK phosphorylates (adds phosphate to) myosin head, activating myosin (a slow process) 5. Activated myosin heads bind to thin filaments to form crossbridges. Myosin ATPase hydrolyzes ATP to provide energy. The process is repeated, the force generated is transferred to anchoring filaments, nd the smooth muscle cell shortens. Similarities w/ skeletal muscle contraction: initiated by Ca2+, involves sliding of thin filament past thick, and requires ATP.

Explain how a fast-twitch fiber differs from a slow-twitch fiber,

A fast-twitch fiber has a fast genetic variation of myosin ATPase (the enzyme that splits ATP), whereas a slow-twitch fiber has a slow genetic variation. Additionally, fast-twitch fibers have both a fast rate of action potential propagaton along teh sarcolemma and are quick in Ca2+ release adn reuptake by the sarcoplasmic reticulum. These fibers initiate a contration more quickly following stimluation (0.01 milliseconds vs at least 0.02 of a slow-twitch fiber). Fast twitch fibers: produce a strong contraction, initiate a contraction more quickly following stimulation, and produce a contraction of shorter duration. Ex of Slow Twitch Fiber: Trunk and lower limb muscles

Define a muscle twitch and recognize the 3 phases of a muscle twitch

A muscle twitch is a single brief contraction period then relaxation period of a skeletal muscle in response to a single stimulus. Three phases: - Latent period: delay that occurs after the stimulus is applied and before the contraction begins. (Delay is accounted for by time necessary for the occurrence of all event - Contraction period: repetitive power strokes pull rthe thin filaments past teh thick filaments, shortening the sarcomeres. Muscle tension increases during contraction. - Relaxation period: begins with the release of crossbridges resulting from return of Ca2+ back to the sarcoplasmic reticulum. Muscle tension decreases in relaxation

Explain how creatine kinase (CK) is used as a diagnosis of myocardial infarction

CK can be found in elevated levels in the blood of patients who have suffered from myocardial infarction (heart attack). It serves as a diagnostic tool for identifying damage to the heart.

Explain how an oxidative fiber differs from a glycolytic fiber.

Fast-twitch fibers fall into one of two categories: - Oxidative fibers specialize in providing ATP through aerobic cellular respiration and have features to support this (including extensive capillary network, lots of mitochondria, and large supply of red pigment myoglobin). They are fatigue-resistant. Sometimes called red fibers. - Glycolytic fibers provide ATP via glycolysis. They have few mitochondria, less capillary networks, and less myoglobin. They are sometimes called white fibers. These fibers tire more easily (are fatigable).

List the events that allow muscle relaxation to occur

For a muscle to relax, must terminate the nerve signals propagating the motor neuron. ACh is not released, so the ACh remaining in the synaptic cleft is hydrolyzed by acetlycholinesterase. ACh receptors close, and both end plate potentials cases. Ca2+ channels close, tropnonin returns to its original shape, and tropomyosin moves over the binding sites on actin, preventing crossbridge formation. The muscle then relaxes.

Define muscle fatigue and explain what causes it

Muscle fatigue is the reduced ability or inability of the muscle to produce muscle tension. Primary cause is excessive or sustained exercise which results in decrease in glycogen stores. Other causes: - Excitation at the neuromuscular junction. (Insufficient Ca2+ or a decrease in synaptic vesicles. Both limit the ability of somatic motor neurons to stimulate a muscle) - Excitation-contraction coupling. (A change in ion concentration that interferes w/ 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. - Crossbridge cycling. (Increased phosphate ion concentration can cause fatigue. Elevated P concentration interferes with P release from myosin head during crossbridge cycling, and slows the rate of cycling. Also, occurs when lower amounts of Ca2+ are availble for release.

What is muscle tension?

Muscle tension is the force generated when a skeletal muscle is stimulated to contract.

Define muscle tone and explain its functions

Muscle tone is the resting tension in a muscle generated by involuntary nervous stimulation of the muscle. (Motor units are stimulated randomly to maintain constant tension; the specific motor units being stimulated changes continuously so motor units do not become fatigued) Muscle tone establishes constant tension on the muscles' tendon, stabalizing the position of bones and joints.

Explain the functions of blood vessels and nerves serving a muscle

Skeletal muscle is highly vascularized and innervated. The extensive network of blood vessesls delivers oxygen and nutrients to muscle fibers, while removing waste products produced by muscle fibers. The extensive innervation allows for the voluntary control of skeletal muscle.

Define oxygen debt and explain why it occurs

Oxygen debt is the amount of additional O2 consumed following exercise to restore pre-exercise conditions. The additional O2 is used to replace O2 on hemoglobin in the blood and skeletal muscles, replenish glycogen in skeletal muscle fibers, replenish ATP and creatine phosphate, and convert lactate back to glucose. Oxygen debt occurs when the need for oxygen exceeds availability.

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

The length and intensity of activity determines the type of ATP supply used. For short, intense bursts of energy, the body uses available ATP and phosphate transfer. For slightly longer (50-60 seconds), glycolysis. Finally, for longer activity, all three means, but primarily aerobic respiration.

Explain the arrangement of the three connective tissue layers associated with a skeletal muscle

There are three layers of connective tissue that occur within muscles: - The Epimysium is a layer of dense irregular connective tissue that surrounds the whole skeletal muscle - The perimysium surrounds the fascicles and contains an array of blood vessels and nerves that supply nutrients to muscle fibers - The endomysium is a delicate, areolar connective tissue layer that surrounds and electrically insulates each muscle fiber. It contains reticular protein fibers to help bind together neighboring muscles.

Describe 3 pathways used for generating ATP in muscle cells: - Aerobic cellular respiration (long-term means of ATP) - Anaerobic respiration (short-term ATP) - creatine-phosphate complex formation (immediate ATP)

- Aerobic respiration: the long-term means of supplying ATP is through aerobic cellular respiration in the mitochondria. The pyruvate made available during glycolysis is oxidized completely to CO2 through the intermediate state and citric acid cycle. This results in the transfer of chemical bond energy to coenzymes, NADH and FADH2. The energy is then used to generate ATP in the electron transport system through oxidative phosphorylation. This nets 30 ATP molecules. (ATP can also be aerobically generated from triglycerides. Amount of ATP is dependent on how long the fatty acid chain is. However, sufficient O2 must be available, or fat cannot be burned.) - Anaerobic respiration provides ATP by way of glycolysis (which occurs in the cytosol and does not require O2). Glucose is made available either directly from glycogen stores in muscle fibers or delivered by blood. Through several enzymatic steps, glucose is broken down to 2 pyruvate molecules. Net energy released during glycolysis is 2 ATP molecules per glucose molecule. - Creatine-phosphate complex formation is when the enzyme creatine kinase transfers P from creatine phosphate to ADP, yielding ATP and creatine. This provides an additional 10-15 seconds of energy during maximum exertion. (During times of rest, as ATP accumulates, the pathway reverses, creatine kinase transfers P from ATP to creatine, yielding ADP and creatine phosphate for the purpose of increasing teh amount of creatine phosphate fro the next contraction.)

Describe the events that occur on the muscle membrane (sarcolemma) in terms of: - motor end plate with ACh receptor - binding of ACh with ACh receptor - Action potential generation on sarcolemma - Calcium release from sarcoplasmic reticulum

- Binding ACh to ACh receptors in the motor end plate triggers the opening of the chemically gated ion channels. Na+ rapidly diffuses into and K+ slowly out of the muscle fiber. An end-plate potential (EPP) is producd when sufficient Na+ enters at the motor end plate and the membrane potential changes from -90mv to -65mv. - The EPP initiates an action potential to be propagated along the sarcolemma and T-tubules. First, voltage-gated Na+ channels open, and Na+ moves in to cause depolarization. Second, voltage-gated K+ channels open, and K+ moves out to cause repolarization. - The action potential is then propagated along the T-tubules to trigger the opening of Ca2+ channels located in the terminal cisternae of the sarcoplasmic reticulum. Ca2+ diffusees out of the cisternae into the sarcoplasm.

Describe the 5 general functions of skeletal muscles

- Body movement (contraction of muscles produce movement) - Maintenance of posture (Contraction of specific muscles stabilizes joints and keeps the body's posture) - Protection and support (skeletal muscle is arranged to protect the body, such as the abdominal cavity and pelvic floor) - Regulating elimination of materials (spincters control passage of waste at orifices) - Heat production (help maintain normal body temperature through muscle movement)

Describe the 5 characteristics of skeletal muscle tissue

- Excitability: the ability of a cell to respond to stimulus - Conductivity: involves an electrical charge that travels along the plasma membrane as voltage gated channels open during action potential. - Contractability: exhibited when contractile proteins slide past one antoher. - Elasticity: ability of a muscle to return to its original length following shortening or lengthening. - Extensibility: the lengthening of a muscle cell.

Define the terms fascicle and muscle fiber

- Fascicle: bundles of muscle fibers - Muscle fiber: the term for skeletal muscle cells due to their potentially extraordinary length.

Briefly describe the structure of a sarcomere with reference to the arrangement of the thick and thin filaments in relation to the following: - I Band - A Band - H Zone - M line - Z disc - Connectin (Titin) - Dystrophin

- I Band: extends from both directions of a Z disc and is bisected by the Z disc. Contains only thin filaments; appears light in a microscope; At max muscle shortening, thin filaments are pulled parallel along the thick filament, causing the I band to disappear. - A Band: The central region of the sarcomere that contains the entire thick filament. Thin filaments partially overlap the ends. Appears dark in a microscope. Does not change in length during muscle contraction. - H Zone: The most central portion of the A band in a resting sarcomere. Does not have thin filament overlap. During max muscle shortening, the zone disappears when thin filaments are pulled past thick filaments. - M line: thin, transverse protein meshwork in the center of the H zone. Serves as an attachment site for thick filament and keeps thick filament aligned during contraction and relaxation. - Z disc: Deliniates ends of each sarcomere. Composed of specialized proteins that are positioned perpendicular to the myofilaments and serve as anchors for thin filament. Looks like a zigzagged line from a longitudinal view. - Connectin (Titin): a cablelike protein that ezrends from the Z discs to the M line through teh core of each thick filament. Stabilizes the position of thick filament and maintains thick filament aligtment. Portions of connectin are coiled and springlike, contributing to muslce fiber elasticity. - Dystrophin: part of a protein complex that anchors myofibrils that are adjacent to the sarcolemma to proteins w/in the sarcolemma. Proteins of the sarcolemma extend to connective tissue. Thus, dystrophin links internal myofilament proteins of a muscle fiber to external proteins.

Describe the functions of the following structures of skeletal muscle fiber: - T-tubules - sarcoplasmic reticulum - glycogen -mitochondria - myoglobin - sarcolemma

- Sarcolemma: the plasma membrane of a skeletal muscle fiber. - T-Tubules: deep invaginations of the sarcolemma, which extend into the skeltal muscle fiber as a network of narrow membranous tubules to the sarcoplasmic reticulum. - Sarcoplasmic reticulum: the ER of the muscle. - Mitochondria: Skeletal muscle fibers have a high demand for energy . The typical skeletal muscle fiber containsappx 300 mitochondria. - Glycogen: skeletal muscle fibers also contain glycogen stores (by way of granules called glycosomes) for use as an immediate fuel molecule. - Myoglobin: unique to muscle tissue. A reddish, globular protein similar to hemoglobin. It binds oxygen when the muscle is at rest and releases it for use during contraction. Provides the means to enhance aerobic cellular respiration and the production of ATP.

Compare and contrast the three muscle fiber types

- Slow oxidative Fibers (SO) - Fast oxidative fibers (FO) - Fast glycolytic fibers (FG)

Describe structure and function of a tendon, an aponeurosis, and a facia

- Tendon: attaches muscle to bone; is a thick, cord-like structures composed of dense regular connective tissue, formed by the three connective tissue layers as they merge and extend past the muscle fibers. - Aponeurosis: a thin, flattened sheet of dense irregular tissue sometimes formed by the connective tissue instead of a tendon. - Fascia: > Deep fascia is an additional expansive sheet of dense irrwegular connective tissue that is external to the epimysium; separates individual muscles; binds together muscles with similar functions; contains nerves, blood vessels, and lymph vessels. > Superficial fascia: composed of areolar connective tissue and adapose connective tissue that separates muscle from skin.

List the protein components of the thick and thin filament

- Thick filament consists of bundles of 200-500 myosin protein molecules. Each myosin protein molecule consists of two strands, each with a globular head and elongated tail. (ATP attaches to the head. The tail points towards the center of thick filament, and the head points towards the end of thick filament. - Thin filament is roughly 1/2 the diameter of thick filament. Composed of two strands of actin protein twisted around each other to form a helical shape. In each strand of actin, roughly 300-400 small, spherical molecules (G-actin) are connected to form a fibrous strand (F-actin).

Briefly describe the thick filaments of myofilaments in terms of: - myosin protein - myosin head binding site with actin - mysoin head with ATP hydrolysis - molecular structures of thick filament

- Thick filament is composed of 200-500 mysoin protein molecules. - The myosin head contains a binding site for actin of thin filament. - Mysosin head also has a catalytic site where ATP attaches and is split into ADP & P. (Sometimes referred to as ATPase).

Briefly describe the thin filaments of myofilaments in terms of: - functions of protein actin, troponin, and tropomyosin - myosin binding site on actin - molecular structures of thin filament

- Thin filament is composted of two strands of actin. The myosin head attaches to the myosin binding site of actin during muscle contraction. - Tropomyosin is a short, thin, twisted filament that is a stringlike protein. It covers small regions of the actin strands including myosin binding sites in noncontracting muscles. - Troponin is a globular or "ball-like" protein attached to tropomyosin (contains the binding site for Ca2+)

Define motor unit

A motor unit is the single motor neuron and the muscle fibers it controls.

Describe the events that occur at the neuromuscular junction in terms of: - Motor neuron - action potential (nerve impulse) - synaptic vesicles - neurotransmitter acetylcholine (ACh) - synaptic cleft - removal of ACh by enzyme acetylcholineesterase

A neuromuscular junction is the location in which the skeletal muscle fiber is innervated by a motor neuron. The synaptic knob is an expanded tip of an axon, which is enlarged and flattened to cover a larger surface area. The synaptic knob cytosol houses numerous synaptic vesicles filled with molecules of neurotransmitter acetylcholine (ACh). Ca pumps are embedded w/in the plasma membrane of the synaptic knob. Prior to arrival of the nerve signal, Ca pumps establish a concentration gradient with more Ca2+ outside the cell than within. When voltage gated Ca2+ channels are opened, Ca2+ flows down its concentration gradient from teh intersititial fluid into the synaptic knob, triggering the exocytosis of ACh from the vesicles. The motor end plate has numerous ACh receptors Binding of ACh opens gated ion channels, allowing Na+ entry into muscle fiber and K+ out. The synaptic cleft is a fluid-filled psace separating the synaptic knob and the motor end plate. The enzyme acetylcholinesterase (AChE) resides there and breaks down ACh when it is released into the synaptic cleft.

Define a neuromuscular junction

A neuromuscular junction is the specific location, usually in the mid-region of the skeletal muscle fiber where it is innervated by a motor neuron. Typically has the following parts: synaptic knob, motor end plate, and synaptic cleft.

Explain how the thin and thick filaments are organized into the sarcomere

A sarcomere is the repeating microscopic cylindrical unit of myofilaments within myofibrils. Each sarcomere is composed of overlapping thick and thin filaments.

Describe how changes in the strength of the stimulus and the frequency of the stimulus can alter the response of a muscle and how it can lead to tetany

Changes in stimulus strength increases the number of motor units to contract (since they vary in sensitivity to stimulation). The increase in muscle tension that occurs with an increase in stimulus intensity is recruitment. Changes in stimulus frequency allow for less time for relaxation b/t contractions. Tetany is the continuous contraction of muscle fibers that lacks relaxation.

List and describe the steps that occur when intracellular calcium levels rise in a muscle cell

If Ca2+ is still present and levels rise, the myosin binding sites are still exposed, so the cycle continues: attach, pull, release, reset. The muscle will move into a contracted state.

Define isometric and isotonic contractions

Isometric contraction: when muscle tension is insufficient to overcome resistence and there is no muscle movement. (The muscle contracts and tension increases, but muscle length stays the same.) (Ex: Holding a baby in one position, pushing on a wall) Isotonic contractions: when muscle tension results in movement of the muscle. Ex: lifting a baby, swinging a tennis racquet, walking. Isotonic contractions have two classes: - Concentric Contractions ( when muscle tension is greater than resistance; shortening of muscle fibers) - Eccentric contractions (when muscle exerts LESS force than needed to move the load; muscle lengthens)

Define single-unit and multi-unit innervation in smooth muscle

Multiunit smooth muscle cells receive stimulation to contract individually. (Examples: arrector pilli, wall of larger air passages, walls of larger arteries) Single-unit smooth muscle cells receive stimluation to contractin unison. (Ex: walls of digestive, urinary, and reproductive tracts, blood vessels)

Explain where smooth muscle is located throughout the body

Smooth muscle tissue is found in the walls of organs of many body systems: - Cardiovascular system: blood vessels - Respiratory system: bronchioles - Digestive system: stomach, small and large intestine - Urinary system: ureters, bladder - Female reproductive system: uterus - Others: iris of the eye, arrector pili muscles, etc

What are somatic motor neurons?

Somatic motor neurons are nerve cells that transmit nerve signals from the brain or spinal cord to control skeletal muscle activity.

Describe rigor mortis

Soon after the heart stops beating, ATP levels are exhausted, and the sarcoplasmic reticulum loses its ability to return Ca2+. As a result, the Ca2+ presnt in the sarcoplasm, plus the Ca2+ that continues to leak out of the sarcoplasmic reticulum, triggers sustained muscle contraction. (ATP is needed to detach the mysoin head of the thick filament from the mysoin binding site of actin. Since it is not available, the crossbridges cannot detach.) Skeletal muscles lock into a contracted position and the body of the deceased becomes rigid, known as rigor mortis. (After 15-24 hours, rigor mortis disappears becuase lysosomal enzymes are released, causing autolysis (self-destruction) of the myofibrils.

Explain the cellular arrangement of filaments in a smooth muscle cell

The cytoskeletal network is composed of intermediate filaments, which are linked with dense bodies at points where they interact w/in sarcoplasm of the smooth muscle. They are linked with dense plaques at points where they attach on the inner surface of the sarcolemma. Contractile proteins are arranged b/t dense bodies and dense plaques. Z discs are absent.

Describe the inverse relationship b/t the size of a motor unit and the degree of control of skeletal muscles in an organ or body part.

The size of the motor unit is inversely related to the amount of control. For example, the motor neurons innervating the eye muscles are very small b/c greater control is needed. In contrast, a single motor neuron controls several thousand individual skeletal muscle fibers in our lower limbs, where less precise control is needed.

Explain the sliding filament model by describing the structure and relative lengths of the I band, A band, and H zone of the sarcomere in a relaxed vs contracted muscle

The sliding filament model is a description of the repetitive movement of thin filaments sliding past thick filaments. The following changes can be noted: - H Zone disappears - I band narrows in width and may disappear - Z discs in one sarcomere move closer together - Thin and thick filaments do NOT shorten.