Chapter 6

How much energy is available from each source?

ATP- 1-2 seconds of contraction, Phosphocreatine- 5-8 seconds, Glycolysis- 1 minute, Oxidative metabolism- carbs(2-4 hours) Fats(>4 hours)

What is a motor unit?

All the muscle fibers innervated by a single nerve fiber.

What is the mechanism behind a Treppe?

when a muscle beings to contract after a long period of rest, its initial strength of contraction may be as little as one half its strength 10 to 50 muscle twitches later. The strength of contraction increases to a plateau.

Muscle denervation-

when a muscle loses its nerve supply it no longer receives the contractile signals that are required to maintain normal muscle size.

What is Skeletal Muscle tone?

when muscles are at rest, a certain amount of tautness usually remains.

Recovery of muscle contraction in Poliomyelitis-

when some but not all nerve fibers to a muscle are destroyed, the remaining nerve fibers branch off to form new axons that then innervate many of the paralyzed muscle fibers.

Fast fibers(type 2)-

white muscle-glycolytic- have a large diameter, low myoglobin content, low capillary density, few mitochondria, high glycolytic enzyme content

Tropomyosin-

wrapped spirally around the sides of the F-actin helix, in resting state the lie on top of the active sites of the actin strands so that attraction cannot occur between the actin and myosin filaments to cause a contraction.

What is the difference between an isotonic contraction and an isometric contraction?-

Isometric contractions do not shorten muscle, just creates tension when the force of contraction is less than the load. whereas isotonic contractions shorten muscle at a constant tension. Force of contraction is greater than the load.

Which types of muscle have the shortest and longest duration of isometric contractions?

Ocular muscle has the shortest contraction, then gastrocnemius, then the longest contraction duration is Soleus muscle.

Muscular dystrophy-

Progressive weakness and degeneration of muscle fibers, which are replaced by fatty tissue and collagen. Lack of dystrophin or mutated forms of the protein cause muscle cell membrane destabilization, activation of multiple pathophysiological processes, including altered intracellular calcium handling, and impaired membrane repair after injury.

Cross-bridges

Small projections from the sides of the myosin filaments. The interaction between the cross-bridges and the actin filaments that cause contraction

Sarcolemma

a thin membrane enclosing a skeletal muscle fiber. Consists of a true cell membrane called the plasma membrane and an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils. At each end of the muscle fiber, this surface layer of the sarcolemma fuses with the tendon fiber.

Z-disks-

attached to the end of the actin filaments, extend in both directions to interdigitate with the myosin filaments, attaches the myofibrils to one another all the way across the muscle fiber

Phosphocreatine

carries a high-energy phosphate bond similar to the bonds of ATP. Instantly cleaved, and it is released energy causes bonding of a new phosphate ion to ADP to reconstitute the ATP. Can only cause maximum muscle contraction for about 5-8 seconds

Myofibrils

composed of actin and myosin filaments. These are large polymerized protein molecules that are responsible for the actual muscle contraction.

List the three places in the contracting muscle that require the greatest amount of energy-

cross-bridge cycle, calcium pump(sarcoplasmic reticulum) and Sodium/potassium pump(biggest user of ATP in body because most common)

Explain the mechanisms of muscle fatigue

fatigue results mainly from inability of the contractile and metabolic processes of the muscle fibers to continue supplying the same work output. Transmission of the nerve signal throughout the neuromuscular junction, can diminish at least a small amount after intense prolonged muscle activity, thus further diminishing muscle contraction.

Troponin-

intermittently attached along the sides of the tropomyosin molecules. Protein molecules that are complexes of three loosely bound protein subunits, each play a specific role in controlling muscle contraction. Connects actin and topramyacin with a third arm for calcium.

Sarcoplasm

intracellular fluid between myofibrils. Contains large quantities of potassium, magnesium, and phosphate, plus multiple protein enzymes.

T-tubule-

invaginations of the sarcolemma filled with extracellular fluid. Penetrate the muscle fiber, branch and form networks, transmit APs deep into the muscle fiber.

How efficient is muscle contraction?

it is less than 25%, with the remainder becoming heat. One half of the energy in foodstuffs is lost during the formation of ATP, and then only 45% of the energy in ATP itself can later be converted to work.

Titin-

keep the myosin and actin filaments in place, filamentous molecules of a protein, one of the largest protein molecules in a body. Because it is filamentous it is very springy, keep myosin and actin in place so that the contractile machinery of the sarcomere will work.

A bands-

myosin filaments and are anisotropic to polarized light

Adjustment of muscle length-

occurs when muscles are stretched to greater than normal length. The stretching causes new sarcomeres to be added to the ends of muscle fibers, however conversely if they continually remain shortened the sarcomeres at the end of the muscle fibers can actually disappear.

Sarcoplasmic reticulum

pecialized endoplasmic reticulum of skeletal muscles. Located in the sarcoplasm surrounding the myofibrils of each muscle fiber. Special organization that is extremely important in regulating calcium storage, release and reuptake and therefore muscle contraction

Oxidative metabolism-

provides 95% of energy for sustained contraction, primary fuels: carbohydrates (2-4 hours) and fats (>4 hours)

Glycolysis

reconstitutes ATP and phosphocreatine, does not require O2 but produces toxins-limited to 1 minute.

Slow fibers(type 1)

red muscle- oxidative- have small diameter, high myoglobin content, high capillary density, many mitochondria, low glycolytic enzyme content.

What determines the amount of tension developed by the contracting muscle?

the amount of actin and myosin filament overlap

Muscle atrophy-

the decrease of the total mass of a muscle. When a muscle remains unused for many weeks, the rate of degradation of the contractile proteins is much more rapid than the rate of replacement. Therefore, muscle atrophy occurs.

Muscle hypertrophy

the increase of the total mass of a muscle. Caused from an increase in the actin and myosin filaments in each muscle fiber, causing enlargement of the individual muscle fibers.

Sarcomere-

the portion of the myofibril that lies between two successive Z disks.

Myosin

thick filaments, dark bands, anisotropic to polarized light

Actin

thin filaments, light band, isotropic to polarized light

Hyperplasia-

under rare conditions of extreme muscle force generation, the actual number of muscle fibers has been observed to increase, in addition to the fiber hypertrophy process. The mechanism is linear splitting of previously enlarged fibers.

Describe length-tension relations for the intact, whole skeletal muscle. Why is it not the same as for a single sarcomere?

When the length of the muscle is ½ normal the tension is at zero, when it is normal the tension significantly increases. When the length is 2x normal the tension does drop somewhat during the contraction but does not drop all the way down to zero and then continues to move upwards from there the whole muscle has a large amount of connective tissue in it, in addition the sarcomeres in different parts of the muscle do not always contract the same amount, therefore, the curve has somewhat different dimensions from those shown of the individual muscle fiber.

Describe length-tension relations for a single sarcomere in a skeletal muscle

When the muscle is at its least amount of length it has a minimal tension

Describe the 6 chemical events that control the movement of the myosin head

1. Before the contraction begins, the heads of the cross-bridges bind with ATP. The ATPase activity of the myosin head immediately cleaves the ATP but leaves the cleavage products, the ADP plus phosphate ion, bound to the head. Conformation of the head is such that it is perpendicular toward the actin filament but is not yet attached to the actin. 2. When the troponin-tropomyosin complex binds with calcium ions, active sites on the actin filament are uncovered and the myosin heads then bind with these sites. 3. The bond between the head of the cross-bridge and the active site of the actin filament causes a conformational change in the head, prompting the head to tilt toward the arm of the cross-bridge and the providing the power stroke for pulling the actin filament. 4. Once the head of the cross-bridge tilts, release of the ADP and phosphate ion that were previously attached to the head is allowed. At the site of release of the ADP, a new molecule of ATP binds. This binding of new ATP causes detachment of the head from actin. 5. After the head has detached from the actin, the new molecule of ATP is cleaved to being the next cycle. Leading to a new power stroke. Brings the head back to his perpendicular position and ready to begin a new power stroke cycle. 6. When the cocked head binds with the new active site on the actin filament, it becomes uncocked and once again provides a new power stroke.

When it is at its maximum strength of contraction-

2.0-2.2 micrometers in length, it has maximum tension developed. When the length is increased over 2.2 micrometers in length the tension starts to drop significantly.

How much energy used by the muscle is obtained from oxidative metabolism?

95%

Describe relationship of contraction velocity to load.

A skeletal muscle contracts rapidly when it contracts against no load- to a state of full contraction in about -.1 second for the average muscle. When loads are applied, the velocity of contraction becomes progressively less as the load increases.

Describe mechanisms of force summation and tetanization

Summation adds together the individual twitch contractions to increase the intensity of overall muscle contraction. Can happen in two ways 1. By increasing the number of motor units contracting simultaneously, which is called multiple fiber summation 2. By increasing the frequency of contraction, which is called frequency summation and can lead to tetanization.

I bands

actin filaments that are isotropic to polarized light.

Rigor mortis

all of the muscles go into a state of contracture and become rigid. The rigidity results from loss of all of the ATP, which is required to cause separation of the cross-bridges from the actin filaments during the relaxation process. It stays as such for approx. 15-25 hours when results from autolysis caused by enzymes released by lysosomes.

sliding filament mechanism of muscle contraction

basic mechanism of muscle contraction. In a relaxed state, the ends of the actin filaments extending from two successive Z disks barely overlap one another. In the contracted state these actin filaments have been pulled inward among the myosin filaments, so their ends overlap one another to their maximum extent. The Z disks have been pulled by the actin filaments up the ends of the myosin filaments, thus muscle contraction occurs by a sliding filament mechanism.