Changes in Striation Pattern on Contraction

When a muscle cell contracts, the relationship among the stripes changes. This change can best be appreciated at the level ofthe electron microscope, as shown diagrammatically in Figure 10-2. Through the electron microscope, a myofibril can be seen to consist of two kinds of longitudinally oriented filaments, called thick and thin filaments. Both the thick and thin filaments are arrayed in parallel groups. As shown in Figure 10-2, the Z line corresponds to the position where the thin filaments of one sarcomere join onto those of the neighboring

(a) Relaxed muscle

Sarcomere

Thin filaments

J . Cross bridges

Z line

Sarcomere

Thin filaments

J . Cross bridges

M line

Z line

M line Thick filaments

A band I band

M line

Z line

M line Thick filaments

(b) Contracted muscle

A band I band ianrnmoro fchnntonl

(b) Contracted muscle ianrnmoro fchnntonl

M line

Z line

M line

A band (constant)

I band (shorter)

Figure 10-2 Schematic representation of the relationships between thick and thin filaments of a myofibril in a relaxed muscle (a) and a contracted muscle (b). In a contracted muscle, the sarcomere is shorter, because the degree of overlap of thick and thin filaments is greater.

M line

Z line

M line

A band (constant)

I band (shorter)

Figure 10-2 Schematic representation of the relationships between thick and thin filaments of a myofibril in a relaxed muscle (a) and a contracted muscle (b). In a contracted muscle, the sarcomere is shorter, because the degree of overlap of thick and thin filaments is greater.

sarcomere and where cross-connections are made among the parallel thin filaments. The thick filaments within a sarcomere arejoined to each other at the M line. It is clear from comparing Figure 10-2a with Figure 10-1d that the lighter I band corresponds to the region occupied only by thin filaments, and the darker A band corresponds to the spatial extent of the thick filaments. The darker regions at the two edges of the A band correspond to the region of overlap of the thick and thin filaments. The thick filaments bear thin fibers that appear to link to the thin filaments in the region of overlap, forming cross-bridges between the thick and thin filaments.

Upon contraction, the length of each sarcomere shortens that is, the distance between successive Z lines diminishes. However, the width of the A band is unaffected by contraction; thus, only the I band becomes thinner during a contraction. In terms of the thick and thin filaments, this observation can be explained by the sliding filament hypothesis, which is illustrated in Figure 10-2b. Neither the thick nor the thin filaments change in length during a contraction; rather, shortening occurs because the filaments slide with respect to one another, so that the region of overlap increases. In order to understand

Figure 10-3 The overall structure of a single molecule of the thick filament protein, myosin. The flexible fibrous tail is connected to the globular head region via a hinged point. The globular head includes a region that can bind and split a molecule of ATP.

Head groups of myosin molecules form cross bridges

Head groups of myosin molecules form cross bridges

Figure 10-4 The structure of a thick filament. The fibrous tails of individual myosin molecules polymerize to form the backbone of the filament. The globular heads radiate out perpendicular to the long axis of the filament to form the cross-bridges to the thin filament. The myosin molecules reverse orientation at the M line, at the midpoint of the filament.

Figure 10-4 The structure of a thick filament. The fibrous tails of individual myosin molecules polymerize to form the backbone of the filament. The globular heads radiate out perpendicular to the long axis of the filament to form the cross-bridges to the thin filament. The myosin molecules reverse orientation at the M line, at the midpoint of the filament.

how the sliding occurs, it will be necessary to examine the molecular makeup of the thick and thin filaments.

Figure 10-3 The overall structure of a single molecule of the thick filament protein, myosin. The flexible fibrous tail is connected to the globular head region via a hinged point. The globular head includes a region that can bind and split a molecule of ATP.

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