Physiology - Chapter 9

What is the function of creatine phosphate in skeletal muscle contraction?

Creatine phosphate is a high-energy compound and is built up in resting skeletal muscles. Creatine phosphate generates ATP by serving as a source of phosphate group for ADP phosphorylation. This reaction occurs in sliding filaments, and therefore is the speediest way to make ATP available to muscles.

What types of stimuli can trigger an increase in cytosolic Ca2+ in smooth muscle cells?

Cytosolic calcium ion concentration allows the phosphorylation of myosin. Smooth muscle cytosolic calcium ion concentration is increased or decreased by graded depolarization or hyperpolarization in membrane potentials. This results in an increased or decreased number of open calcium ion channels and affects the degree of contraction.

List the factors responsible for skeletal muscle fatigue.

Failure of conduction of the muscle action potential into the fiber along the T-tubule causes muscle fatigue. Conduction failure halts the release of calcium from the sarcoplasmic reticulum. This conduction failure results from the build-up of potassium ions in the small volume of the T-tubule during the repolarization of repetitive action potentials. The increased external potassium concentration results in a persistent depolarization of the membrane potential and causes a failure to produce action potentials in the T-tubular membrane. Accumulation of lactic acid in muscles causes their acidification which in turn alters the conformation and activity of several muscle proteins such as actin and myosin, as well as the proteins involved in calcium release. The lactic acid build-up also affects the function of the Ca2+-ATPase pumps of the sarcoplasmic reticulum which is one of the factors responsible for the impaired relaxation of the fatigued muscle. Muscle fatigue is also caused by inhibition of cross-bridge cycling. During intense activity, ADP and Pi accumulate within muscle fibers and inhibit cross-bridge cycling. It results in delayed detachment of the cross-bridge from actin.

Describe isometric, concentric, and eccentric contractions.

Isometric contractions involve the contractions in which muscles develop tension but do not shortens or lengthen. This occurs in the supporting conditions of the load in a constant position. The concentric contractions occur when the tension exceeds the load, and the shortening occurs, while the eccentric contractions are those in which unsupported load is greater than the tension generated by cross-bridges.

What fuel molecules are metabolized to produce ATP during skeletal muscle activity?

Muscle also use of blood glucose and plasma fatty acids as an energy source. Both of these are delivered to muscles by circulating blood. Adipose tissue is the source of plasma fatty acids for muscle contraction. The amount of fat burned increases when more time is spent of exercise.

What effect does increasing the frequency of action potentials in a skeletal muscle fiber have upon the force of contraction? Explain the mechanism responsible for this effect.

Successive action potentials during the phase of mechanical activity, muscle tension increased and known as summation. Tetanic contractions or tetanus is maintained contractions in response to receive stimulations. Increasing the frequency of action potential in a skeletal muscle fiber, no oscillation produces, and the muscle fiber does not relax. The level of muscle tension increases by summation until a maximum fused tetanus is achieved. The further increase in frequency will not affect the muscle.

Describe the changes that occur in skeletal muscles following a period of (a) long-duration, low-intensity exercise training; and (b) short-duration, high-intensity exercise training.

The aerobic exercises are relatively low in intensity but are long duration exercises. This indicates the activities like long distance running as in marathons. The muscle requires energy to work for more time without getting fatigue. So, in the muscle fibers there is increase in the no. of mitochondria as they are the power house of the cell. Also, the blood supply needs to increase hence the capillaries around these muscle fibers also increases. Due to these changes in the fibers, the endurance capacity of the muscle increases. It will produce minimum fatigue. Hence, these changes improve the over-all health of the muscle by increase in the oxygen delivery to the cells and production of energy. The strength training, is a high intensity but short duration exercises. It requires strong contractions of the muscle fibers. For example, in activities like weight lifting. More myofibrils are formed due to the increased synthesis of actin and myosin filaments. Also, there is increase in the diameter of the muscle fibers. Synthesis of glycolytic enzymes increases which increase the glycolytic activity. Although powerful, but the endurance of the muscle is less and gets fatigued easily.

Compare the mechanisms by which an increase in cytosolic Ca2+ concentration initiates contractile activity in skeletal, smooth, and cardiac muscle cells?

The calcium ions in the skeletal ions in the skeletal and cardiac muscles bind to a calcium-binding protein called troponin that allows the movement of tropomyosin. This results in the sliding of thick and thin filament resulting in contraction. In smooth muscle fibers, the cross-bridge cycling is controlled by calcium-regulated enzymes that phosphorylate myosin. It results in the binding of actin to myosin to undergo cross-bridge cycling. Smooth muscle fibers lack calcium-binding troponin that is an important factor for the contraction of skeletal and cardiac muscles. The smooth muscle contraction depends upon the availability of phosphorylated myosin that results in contraction.

What three events in skeletal muscle contraction and relaxation depend on ATP?

The cross-bridge movement, dissociation of myosin from actin, and the returning of the cross-bridge to its pre-power stroke position need the use of ATP in skeletal muscle contraction and relaxation.

Describe the sequence of events by which an action potential in a motor neuron produces an action potential in the plasma membrane of a skeletal muscle fiber.

The most neuromuscular junction is located near the middle of a muscle fiber, and newly generated muscle action potential propagates from this region in both directed toward the ends of the fiber. Every action potential in a motor neuron generates an action potential in each muscle fiber

What happens to skeletal muscle fibers when the motor neuron to the muscle is destroyed?

The muscle fibers become denervated and diameter of the muscle fibers will keep getting smaller. Also, the muscle fibers contain contractile protein which helps in the contraction. The amount of the contractile proteins also decreases when the fibers are denervated. The changes in the skeletal muscle due to cut off of the nerve supply is called as Denervation Atrophy. The muscle fibers in these conditions shrink and becomes angular. At an extreme stage of this condition, occurs in the pathologies like peripheral neuropathy and motor neuron.

Describe the physical state of a muscle fiber in rigor mortis and the conditions that produce this state

The physical state of the rigor mortis is the gradual stiffening of the muscle that do not allow the dissociation of the muscles. It happens due to the unavailability of the ATP due to a decline in cellular metabolism.

Describe the events that result in the relaxation of skeletal muscle fibers

The reduced concentration of calcium ions in the cytosol results in the blocking action of tropomyosin and ends cross-bridges. This causes the removal of calcium from troponin and muscle relaxes.

Describe the length-tension relationship in skeletal muscle fibers.

The relationship between the fiber length and tension produced during contraction. By increasing the stretch in the muscle fiber, the tension and length increases, while it comes back to original with the release of stress.

Describe the location, structure, and function of the sarcoplasmic reticulum in skeletal muscle fibers.

The sarcoplasmic reticulum in a muscle fiber is homologous to the endoplasmic reticulum found in most cells. A series of sleeve-like segments are formed around each myofibril by these structures, and at the end of each segment, two enlarged regions are present. These regions are known as terminal cisternae and are connected by a series of smaller tubular elements. Ca 2+ is stored in the terminal cisternae and is released into the cytosol during contractionSR has an important role in the contraction of muscle.

Describe the effect of increasing the load on a skeletal muscle fiber on the velocity of shortening

The shortening velocity of the muscle fiber depends upon the breakdown of the ATP molecule. A decrease in the load results in the high velocity of shortening, while the increase of load lowers the velocity of shortening

Describe the difference between single-unit and multiunit smooth muscles

The single-unit smooth muscle contains gap junctions that allow jumping of the action potential and all muscle tissue act as a single unit. They perform both electrical and mechanical activity. In single-unit, the contractile response can be produced by stretching the muscle fiber. Multiunit smooth muscle has no, or few gap junctions, and the muscle unit behaves as multiple units as each cell responds independently. They have bunches of the autonomic nervous system. Stretching does not produce any contractile activity in the multiunit smooth muscles.

How does the organization of thick and thin filaments in smooth muscle fibers differ from that in striated muscle fibers?

The thick and the thin filaments are not organized into myofibrils and there is no regular alignment of these filaments into sarcomeres that results in the absence of banding pattern. Their contraction occurs by the sliding filament model.

Describe the organization of myosin, actin, tropomyosin, and troponin molecules in the thick and thin filaments.

The thick filaments are comprised almost entirely of the protein myosin. The thin filaments are principally composed of the protein actin, as well as other proteins tropomyosin and troponin. Each thick filament is covered by a hexagonal array of six thin filaments.

Describe the structure and function of the transverse tubules.

The transverse tubular system is associated with terminal cisternae of the adjacent segment of the sarcoplasmic reticulum. Transverse tubules are also called T tubules and are involved in the conduction of impulse from the sarcolemma to down in the cell especially in sarcoplasmic reticulum

What prevents cross-bridges from attacking to sites on the thin filaments in a resting skeletal muscle?

The two types of filaments present in skeleton muscle cells are thick and thin filaments. The thin filaments are principally composed of the protein actin, as well as other proteins tropomyosin and troponin. Chains of tropomyosin molecules are arranged end to end along the thin actin filament. These tropomyosin molecules partially cover the myosin-binding site on each actin monomer. Each tropomyosin molecule is held in this blocking position by a small globular protein, troponin. Troponin interacts with both actin and myosin. One molecule of troponin binds to each molecule of tropomyosin and regulate the access to myosin-binding sites. Troponin and tropomyosin cooperatively block the interaction of cross-bridges with the thin filament.

What component of skeletal muscle fibers accounts for the differences in the fibers' maximal shortening?

The velocity at which a single muscle fiber shortens is determined mainly by the load on the fiber and the type of the fiber (fast or slow fiber). Fast skeletal muscle fibers have myosin with high ATPase activity while the slow fibers have myosin with lower ATPase activity. Therefore, fast and slow fibers differ in the maximal rates at which they split ATP. The rate of ATP hydrolysis determines the maximal rate of cross-bridge cycling and thus the maximal shortening velocity.

List the three types of muscle cells and their locations.

Three types of muscle cells are cardiac, skeletal, and smooth muscle cells.

How are skeletal muscles arranged around joints so that a limb can push or pull?

Through connective tendon, contracting muscle gives the force on bone. The contractive muscle only exerts pulling force so it leads to shortening of the muscle. At least two muscles needed for opposing motion such as flexion and extension of the limb. A group of muscle causes oppositely directed movement at a joint is called antagonists. The contraction of biceps muscles cause flexion of joint and the triceps cause extension of the arm. These antagonist's muscle cause flexion of joint and the triceps cause extension of the arm. These antagonist's muscles are not only responsible for flexion and extension also helpful for rotation of the limb.

Define motor unit and describe its structure.

Together, a motor neuron and the muscle fibers it innervates are called a motor unit. The axons of motor neurons are myelinated and are the largest-diameter axons in the body. When an action potential occurs in a motor neuron, all the muscle fibers in its motor units are stimulated to contract.

Compare and contrast the transmission of electrical activity at a neuromuscular junction with that at a synapse.

When an action potential in a motor neuron arrives at the axon terminal, it depolarizes the plasma membrane, opening voltage-sensitive Ca2+ channels and allowing calcium ions to diffuse into the axon terminal from the extracellular fluid. This Ca2+ binds to proteins that enable the membrane of acetylcholine containing vesicles to fuse with the neuronal plasma membrane, thereby releasing acetylcholine. Acetycholine diffuses from the axon terminal to the motor end-plate, where it binds to ionotropic receptors. The binding of acetylcholine opens ion channels, and both sodium and potassium ions can pass through these channels. Because of the difference in electrochemical gradients across the plasma membrane move, Na+ moves in than K+ out, producing a local depolarization of the motor end-plate is known as an end-plate potential. The EEP is analogous to EPSP. The magnitude of EPP is, however, much larger than that of EPSP. IPSP may occur in the case of neuron-neuron synapses, but all neuromuscular junctions are excitatory.

Summarize the characteristics of the three types of skeletal muscle fibers.

Slow oxidative (SO) skeletal muscle fibers appear dark red as they have large amounts of myoglobin and many blood capillaries. The presence of many large mitochondria indicates that they generate ATP mainly by aerobic respiration. Therefore, they are called oxidative fibers. Myosin heads of these fibers hydrolyze ATP relatively slowly, and the contraction cycle occurs at a slower pace than in "fast" fibers. Therefore, these fibers called "slow." The slow fibers are very resistant to fatigue and can sustain prolonged contractions for many hours. Fast oxidative-glycolytic (FOG) fibers are the largest fibers and contain large amounts of myoglobin and many blood capillaries. Therefore, they also appear dark red. These fibers generate considerable ATP by aerobic respiration and are moderately resistant to fatigue. They have a higher intracellular glycogen level and also generate ATP by anaerobic glycolysis. Myosin heads of FOG fibers hydrolyze ATP three to five times faster than the myosin ATPase in slow oxidative fibers. This imparts them a faster speed of contraction. Fast glycolytic (FG) fibers have low myoglobin content. These fibers appear white as they have relatively few blood capillaries and mitochondria. They contain large amounts of glycogen and generate ATP mainly by glycolysis. They combine high myosin-ATPase activity with high glycolytic capacity. Due to their ability to hydrolyze ATP rapidly, these fibers contract strongly and quickly. However, fast glycolytic fibers fatigue quickly.

In what ways does the neural control of smooth muscle activity differ from that of skeletal muscle?

Smooth muscle fibers differ from skeletal muscle fibers in the activity of neurotransmitters as they increase or decrease the muscle tension. They have multiple receptors for multiple types of neurotranmitters.

Upon what three factors does the amount of tension developed by a whole skeletal muscle depend?

The first factor is the amount of tension developed by each fiber in the muscle. The tension developed in them depends on the length and diameter of the fiber. Also, it depends on the frequency at which it receives action potential and the fatigability of the muscle. Based on these parameters the muscle is able to develop the total tension. Another factor is the number of active fibers present in the skeletal muscle. As the organization of the fibers forms a motor unit, it depends on how many fibers in the motor unit are active and, in that manner, how many active motor units are present. More the amount of active motor units presents, more tension is developed in the skeletal muscle. If the fibers are inactive, then the amount of tension developed will be far less, because there will be decrease in the force of the contraction. The other factor that contributes to the development of muscle tension is the neural control of the muscle. If the muscle in denervated, action potential will not be produced and the motor units will not be recruited. Hence, there won't be any contraction in the muscle. So, no development of tension in the fibers as well as the muscle.

What are the two sources of Ca2+ that lead to the increase in cytosolic Ca2+ that triggers contraction in smooth muscle?

The first source of calcium is the sarcoplasmic reticulum that is present near the plasma membrane and releases calcium in response to action potential on the plasma membrane. The second source is extracellular calcium that enters through the voltage-gated calcium ion channels and increases the cytosol calcium level.

Describe the four steps of one cross-bridge cycle.

The four steps of the cross-bridge cycle are attachment of the cross-bridge to thin filament, movement of the cross-bridge, detachment of the cross-bridge, and energizing the cross-bridge that allows again attachment to thin filament

What is an end-plate potential, and what ions produce it?

The junction of an axon terminal with the motor end-plate is known as a neuromuscular junction, and local depolarization of the motor end-plate is known as an end-potential. Na+ and K+ ions will produce this EPP.