Introduction To Muscle Physiology Flashcards
Organization of skeletal muscle from the gross to the molecular
Electron micrograph of muscle myofibrils showing the detailed organization of actin and myosin filaments. Note the mitochondria lying between the myofibrils.
Relaxed and contracted states of a myofibril showing ( top ) sliding of the actin filaments ( pink ) into the spaces between the myosin filaments ( red ) and ( bottom ) pulling of the Z membranes toward each other.
The side-by-side relationship between the myosin and actin filaments is maintained by a large number of filamentous molecules of a protein called titin. These springy titin molecules act as a framework that holds the myosin and actin filaments in place so that the contractile machinery of the sarcomere will work. One end of the titin molecule is elastic and is attached to the Z disk, acting as a spring and changing length as the sarcomere contracts and relaxes. The other part of the titin molecule tethers it to the myosin thick filament. The titin molecule may also act as a template for the initial formation of portions of the contractile filaments of the sarcomere, especially the myosin filaments.
Organization of proteins in a sarcomere. Each titin molecule extends from the Z disk to the M line. Part of the titin molecule is closely associated with the myosin thick filament, whereas the rest of the molecule is springy and changes length as the sarcomere contracts and relaxes.
Sarcoplasmic Reticulum Is a Specialized Endoplasmic Reticulum of Skeletal Muscle
Sarcoplasmic reticulum in the spaces between the myofibrils, showing a longitudinal system paralleling the myofibrils. Also shown in cross section are T tubules ( arrows ) that lead to the exterior of the fiber membrane and are important for conducting the electrical signal into the center of the muscle fiber
A, Myosin molecule. B, Combination of many myosin molecules to form a myosin filament. Also shown are thousands of myosin cross-bridges and interaction between the heads of the cross-bridges with adjacent actin filaments.
Actin filament composed of two helical strands of F-actin molecules and two strands of tropomyosin molecules that fit in the grooves between the actin strands. Attached to one end of each tropomyosin molecule is a troponin complex that initiates contraction.
subunits (troponin I) has a strong affinity for actin, another (troponin T) for tropomyosin, and a third (troponin C) for calcium ions.
Interaction of One Myosin Filament, Two Actin Filaments, and Calcium Ions to Cause Contraction
The walk-along mechanism for contraction of the muscle.
Length-tension diagram for a single fully contracted sarcomere showing the maximum strength of contraction when the sarcomere is 2.0 to 2.2 micrometers in length. At the upper right are the relative positions of the actin and myosin filaments at different sarcomere lengths from point A to point D.
Relationship of muscle length to tension in the muscle both before and during muscle contraction
A skeletal muscle contracts rapidly when it contracts against no load to a state of full contraction in about 0.1 second for the average muscle. When loads are applied, the velocity of contraction decreases progressively as the load increases, as shown in Figure 6-11 . When the load has been increased to equal the maximum force that the muscle can exert, the velocity of contraction becomes zero, and no contraction results, despite activation of the muscle fiber.
When a muscle contracts against a load, it performs work. To perform work means that energy is transferred from the muscle to the external load to lift an object to a greater height or to overcome resistance to movement.
Three Sources of Energy for Muscle Contraction
The first source of energy that is used to reconstitute the ATP is the substance phosphocreatine
The second important source of energy, which is used to reconstitute both ATP and phosphocreatine, is a process called glycolysis —the breakdown of glycogen previously stored in the muscle cells.
The third and final source of energy is oxidative metabolism , which means combining oxygen with the end products of glycolysis and with various other cellular foodstuffs to liberate ATP. More than 95% of all energy used by the muscles for sustained long-term contraction is derived from oxidative metabolism. The foodstuffs that are consumed are carbohydrates, fats, and protein. For extremely long-term maximal muscle activity—over a period of many hours—the greatest proportion of energy comes from fats but, for periods of 2 to 4 hours, as much as one half of the energy can come from stored carbohydrates.
Isometric Contractions Do Not Shorten Muscle, Whereas Isotonic Contractions Shorten Muscle at a Constant Tension
Isotonic and isometric systems for recording muscle contractions. Isotonic contraction occurs when the force of the muscle contraction is greater than the load, and the tension on the muscle remains constant during the contraction. When the muscle contracts, it shortens and moves the load. Isometric contraction occurs when the load is greater than the force of the muscle contraction; the muscle creates tension when it contracts, but the overall length of the muscle does not change.
Duration of isometric contractions for different types of mammalian skeletal muscles showing a latent period between the action potential (depolarization) and muscle contraction.
shows records of isometric contractions of three types of skeletal muscle—an ocular muscle, which has a duration of isometric contraction of less than 1/50 second; the gastrocnemius muscle, which has a duration of contraction of about 1/15 second; and the soleus muscle, which has a duration of contraction of about 1/5 second. These durations of contraction are highly adapted to the functions of the respective muscles. Ocular movements must be extremely rapid to maintain fixation of the eyes on specific objects to provide accuracy of vision. The gastrocnemius muscle must contract moderately rapidly to provide sufficient velocity of limb movement for running and jumping, and the soleus muscle is concerned principally with slow contraction for continual, long-term support of the body against gravity.
A motor unit consists of a motor neuron and the group of skeletal muscle fibers it innervates. A single motor axon may branch to innervate several muscle fibers that function together as a group. Although each muscle fiber is innervated by a single motor neuron, an entire muscle may receive input from hundreds of different motor neurons.
Tetany occurs because enough calcium ions are maintained in the muscle sarcoplasm, even between action potentials, so that a full contractile state is sustained without allowing any relaxation between the action potentials.
The maximum strength of tetanic contraction of a muscle operating at a normal muscle length averages between 3 and 4 kg/cm 2 of muscle, or 50 pounds/inch 2 . Because a quadriceps muscle can have up to 16 square inches of muscle belly, as much as 800 pounds of tension may be applied to the patellar tendon
