Contraction of Muscle and Locomotion Muscle is an organ in which chemical energy is converted to mechanical work. A great deal is known about muscle b a t we are just beginning t o be able to put the details into a coherent picture. Cross-striated muscle is essentially a. bundle of muscle fibers. The diameter of these fibers is somewhat less than that of s single hair. In a long muscle, the length of these filaments amounts to several inches. The muscle fibers can be torn lengthwise into even thinner units which are called fibrils. These fibrils are about 1 rr wide. The fiber is thus a bundle of some 100 of these fibrils. The fibrils are shill not homogeneous;. They exhibit periodical struetnres along their entire length. Under an electron microscope, one sees about every 3 A apart d o n g the fibril the so-called Z-discs. From each Z-disc, hundreds of fine 1-rr long filaments emanate in bothdirections. These fine filaments originating in the Z-discs do not fill in the total space in the fibril. The 1-rr gap between the suceesnive Z-membranefilament formations is filled with yet another kind of filament which is much l.hicker than the one originating from the Z-membrane. The fibril is then a succession of segments containing the thin and thick filaments. Muscle contracts because the thin filaments climb along the thick filaments pulling their Z-discs with them and, au a result, t,he fibril becomes shorter. Evidence is presented in this paper which strongly indicates that the energy converting machinery in muscle is the Z-disc thin filament unit. The thin filaments in such a unit shorten and elongate in succession while using the energy of ATP, and as a. result move stepwise along the thick filament, thus causing muscle shortening. This explains how the shortening and elongation of the thin filaments takesplaeeon themolecular level. Shortening is the result of a small decrease in the helical portion of the molecules constituting the thin filaments. This decresse in helical content is elicited by s m d mechanical deformations starting probably a t tho base of the filaments. The research of the past few years revealed that the chemical composition of the filaments in the flagella,of some primit,ive organisms (e.g., flagellates and the tail of sperm) is very similar lo that of the thin filaments of muscle. Contraction and elongation of the filaments in flagellum-again using the energy of ATP-creates a wavy motion which propels the primitive organism. According to our present knowledge, nature invented the flagellum-bearing microbes some 2 or 3 billion years ago. Thus flagellurnis the ancient organelleof lowmotion. I t appears that nature is using bs~icallythe same material and principle to produce movement in the muscles of higher organisms. The main difference,of course, is that in muscle the shortening and elongat,ing filaments produce a linear motion, not a wavy one, and that t,he filaments do not wiggle in sea water but inside the muscle fiber. To achieves contract,ian of the muscle from the motion of the thin filaments, nature introduced an innovat,ion (not present in flragellum) by creating thick filaments onto which the bhin filarnent,~can attach and, a8 they carry out their mntraction-elongation cycles, move closer together, thus causing the whole mnsele to shorten.
. . .Taken from a paper "Contraction of Muscle and Locomotion in Biology" by K. Leki, National I x d t n t e ai Arthritis and Metabolic Diseases, presented before the 158th Meeting of the American Chemical Society, New Yolk 1969. Volume 46, Number 11, November 1 9 6 9
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