Muscle contraction shortening

Muscle cells contain contractile elements, myofibrils, which are essentially bundles of proteins found in the sarcoplasm (Figure 2.23). Muscle myofibrils contain isotropic sections (I-bands about 0.8 xm long) and anisotropic sections (A-bands about 1.5 xm long). The I-band is interrupted by a Z-line about 80 nm wide. The low-density central part of the A-band of is the H-zone. The M-line is situated in the centre of the H-zone. The entire structural unit of myofibrils, which is the span between two Z-lines, is called the sarcomere. In a relaxed state (relaxed muscle), it is about 2.5 [am long. During muscle contraction (shortening) it is shorter, 1.7-1.8 p.m. 

Figure 14.11 The sliding filament model of muscle contraction. The actin (red) and myosin (green) filaments slide past each other without shortening.

Figure 4. When a muscle contracts isotonically or a constant resisting force is imposed on it during a contraction, the velocity at which it shortens quickly comes to a constant. The force-velocity curve shows the relationship between the force applied to a muscle and the steady-state velocity of shortening. As in all other muscles, the force-velocity curve of smooth muscle is a rectangular hyperbola for all positive shortening velocities. In order to compare the behavior of muscles of different lengths and diameters, it is common to normalize force and velocity by dividing each by its maximum value and expressing the result as a percentage, nd...

In terms of muscle function, muscle is very adaptable. Depending on the type of stimulation, muscle can either twitch or contract tetanically for a variable length of time. If the ends are held fixed, then it contracts isometrically and the force produced is maximal. If one or both ends of the muscle are not held fixed then the muscle is able to shorten. The muscle can shorten at a fixed load (isotonic contraction) where the velocity of shortening is also constant. Power output (force X velocity) is maximum where the velocity of shortening is about one third of the maximal rate. Finally, the muscle can shorten at maximum velocity (unloaded shortening). However, the molecular basis of the interaction of myosin with actin to produce force, or shortening, is the same in each case. 

The ability of S-2 to act as a flexible link also explained another problem in muscle contraction. When muscle contracts its volume remains constant. As a muscle shortens, and the filaments slide past each other, the spacing between the filaments increases as part of this constant volume behavior. Therefore, the crossbridges have to be able to interact with actin over a wide range of filament spacings. The presence of the flexible link in S-2 would allow this to occur. 

Isotonic contraction occurs when the muscle shortens under a constant load. For example, when an object is lifted, the muscle contracts and becomes shorter although the weight of the object remains constant. In addition to moving external objects, isotonic contractions are performed for movements of the body, such as moving the legs when walking. 

Paul That s probably where I would be coming down. The speed of a smooth muscle is not set by the Ca2+ release. I would maintain that the speed of the muscle is set by its intrinsic unloaded shortening velocity, as modulated by its elasticity and whatever force it is facing. Generally, the Ca2+ transient is largely over before the smooth muscle contracts. I don t believe that the speed of the smooth muscle is regulated by means of Ca2+ waves. 

Each cell within vertebrate striated muscle contains within its sarcoplasm many parallel myofibrils which in turn are made up of repeating sarcomere units. Within the sarcomere are the alternating dark A band and light I band, in the middle of which are the H zone and Z line, respectively. A myofibril contains two types of filaments the thick filaments consisting of myosin which are present only in the A band, and the thin filaments consisting of actin, tropomyosin and troponin. When muscle contracts, the thick and thin filaments slide over one another, shortening the length of the sarcomere. 

To understand how a muscle contracts, consider the Interactions between one myosin head (among the hundreds In a thick filament) and a thin (actin) filament as diagrammed In Figure 3-25. During these cyclical Interactions, also called the cross-bridge cycle, the hydrolysis of ATP Is coupled to the movement of a myosin head toward the Z disk, which corresponds to the (+) end of the thin filament. Because the thick filament Is bipolar, the action of the myosin heads at opposite ends of the thick filament draws the thin filaments toward the center of the thick filament and therefore toward the center of the sarcomere (Figure 19-23). This movement shortens the sarcomere until the ends of the thick filaments abut the Z disk or the (—) ends of the thin filaments overlap at the center of the A band. Contraction of an Intact muscle results from the activity of hundreds of myosin heads on a single thick filament, amplified by the hundreds of thick and thin filaments In a sarcomere and thousands of sarcomeres In a muscle fiber. 

MAPK translocates to the surface membrane before contraction and then redistributes to the cytoplasm coincident with muscle cell shortening. Protein kinase C inhibitors block these translocations, suggesting a role for PKC in the activation of MAPK. In further support of a role for PKC in this process, PKC translocates to the surface membrane at the same time as MAPK. 

Huxley s model (1957) for muscle contraction states that the mechanical and enzymatic characteristics can be described by the overall apparent attachment and detachment rates of the cross-bridge. Contraction was described as a transition between free and attached states (Huxley, 1957). This simple yet elegant two-state model produces several specific predictions about the relationships between force production, ATPase rates, and shortening as a function of these two rate constants. Although this model is somewhat oversimplified given the actual number of biochemical states, and several of its assumptions may not strictly hold in... 

The sliding filament model explains the molecular basis by which muscular contraction occurs. During muscular contraction, thin filaments within the sarcomere of a myofibril are pulled towards the center of the sarcomere (called the H zone) by the thick filaments. In the process, the sarcomeric length shortens and the myofibril shortens. As a result, the muscle contracts (see Figure 8.11). Steps in the process include the following ... 

---