Overlapping actin, myosin filaments amount in relation with the force of muscle contraction

2021-06-04 04:08 PM

The whole muscle has a large amount of connective tissue in it. In addition, the contractile units in different parts of the muscle do not always contract by the same amount.

The figure shows the effect of contractile unit length and amount of overlapping myosin-actin fibres on triggering an increase in contractile force by a contractile muscle fibre. On the right are different degrees of overlap of myosin and actin filaments at different contractile unit lengths.

At point D on the diagram, the actin filament has been fully pulled away from the tip of the myosin filament, without overlapping actin-myosin. At this point, the increase in contractile force by muscle activity is zero. Then, as the contractile unit shortens and the actin filament begins to overlap the myosin filament, the contractile force gradually increases until the contractile unit length decreases to about 2.2μm. At this point, the actin filament has overlapped all the cross-bridges of the myosin filament but has not yet reached the centre of the myosin filament. With further shortening, the contractile unit maintains the same contractile force until point B is reached, where a contractile unit is approximately 2μm in length. At this point, the ends of the two actin filaments begin to overlap in addition to the overlap of myosin filaments. As the contractile unit length decreases from 2μm to about 1.65μm, at point A, the intensity of the contraction rapidly decreases. At this point, the two Z-discs of the contractile unit contact the ends of the myosin filaments. Then, when contraction is still in progress with a shorter contractile unit length, the ends of the myosin filaments are pinched and, as shown in the figure, the intensity of the contraction approaches zero, but the unit The muscle contraction has now contracted to its shortest length.

Figure. Stretch diagram - full contractile force, showing maximum contraction strength when the contraction unit is 2.0 to 2.2 micrometres in length. In the upper right are the relative positions of actin and myosin filaments at different contractile unit lengths from point A to point D.

Effect of muscle length on contractile force in whole intact muscle

The curve depicts the contractile force of an intact, whole muscle rather than a single muscle fibre.

The whole muscle has a large amount of connective tissue in it; In addition, the contractile units in different parts of the muscle do not always contract by the same amount. Thus, the curve has a slightly different dimension than the representations for individual muscle fibres, but it exhibits the same general pattern for slopes within the normal range of contractility, as stated.

Note that when the muscle is at its normal resting length, that is, at a contraction unit about 2μm in length, it contracts during activity with approximately maximum contractile force. However, the increase in contractile force that occurs during contraction, called active contractility, decreases when the muscle is stretched beyond its normal length - that is, to a unit length of contraction. about 2.2μm larger. This phenomenon is demonstrated by the decrease in the length of the arrow in the figure at a length greater than that of normal muscle.

Figure. Relationship of muscle length to the force of contraction in the muscle both before and during contraction.

Relation of contraction speed to load level

A striated muscle contracts rapidly when it contracts without resistance to load - to a state of full contraction in about 0.1s on average for the muscle. As the load level is applied, the rate of contraction gradually becomes less as the load is increased. That is, when the load is increased to equal the maximum force that the muscle can exert, the rate of contraction becomes zero and the contraction is fruitless, despite the contraction of the muscle fibres.

Figure. Relation of load to the rate of contraction in a striated muscle with a cross-section of 1 square centimetre and a length of 8 cm.

This decrease in the rate of contraction with load is caused by the fact that a load on a contracting muscle is the opposite force that opposes the force exerted by the contraction. Therefore, the actual force that can be used to cause speed is shortened that.