Evidence concerning Crossbridge Attachment during Muscle Contraction

Abstract

IT is widely assumed that in contracting striated muscle the head ends of the myosin molecules, called crossbridges, attach to the I-filaments and pull them towards the middle of the sarcomere. There is direct evidence that crossbridges do move axially as the activated muscle shortens¹ and become attached to the I-filaments in rigor, when the substrate is absent^2,3, but there is no direct evidence of their attachment in active muscle. We set out to detect crossbridge attachment by measuring the intensities of the equatorial X-ray diffraction reflexions in active muscle. From these reflexions one can derive the distribution of diffracting mass between the A and I-filaments³; any lateral movement of the activated crossbridge should thereby be revealed.