Muscle cell

“ A single skeletal muscle cell is an exceedingly slender fiber, much smaller than the finest thread; it may also be very short, not more than a twenty-fifth of an inch long, or it may be as much as an inch long. A muscle is made up of many of these fibers grouped side by side in bundles, and also, if the muscle is long, placed end to end. The fibers are held in place and fastened together by connective tissue. Lean meat consists of thousands of these muscle cells with their connective tissue fastenings. ”

“ Since quick movement is usually (but not always) associated with voluntary control, these striated muscle cells are often termed “voluntary” muscle fibres. The great increase in length of these cells is accompanied by the fragmentation of the originally single nucleus.
(b) Cell-modification in Relation to Secretion.—Just as the complex movements considered above were the result of a great development of the power of spontaneous movement possessed by all protoplasm, so cell-secretion is the result of a development of the metabolic processes underlying all vital phenomena. ”

“ It is probable that the contractile substance which makes up the larger part of the muscle cell does not contribute to the growth of the nucleus as does the protoplasm of embryonic cells—that so far as the growth of the nucleus is concerned it acts as does yolk, oil, membranes, fibers or other products of metabolism and differentiation. If only the sarcoplasm of the muscle cell and not its contractile substance is able to contribute to the growth of the nucleus, the small volume of the nuclei as compared with the entire cell would find a ready explanation. ”

“ Heart muscle cells are short, thick cylinders, which are joined end to end to form long fibers. By means of lateral branches the cells of one fiber anastomoses with cells of adjacent fibers. Each cell of heart muscle contains one centrally located nucleus. ”

“ The muscular fibres. The muscular elements of the Chordata undoubtedly belong to the myoepithelial type. The embryonic muscle-cells are at first simple epithelial cells, but soon become spindle-shaped: part of their protoplasm becomes differentiated into longitudinally placed striated muscular fibrils, while part, enclosing the nucleus, remains indifferent, and constitutes the epithelial element of the cells. The muscular fibrils are either placed at one side of the epithelial part of the cell, or in other instances (the Lamprey, the Newt, the Sturgeon, the Rabbit) surround it. ”

“ The microscopic examination of each of these types of muscle shows it to be made up of fibers, these fibers to be made up of muscle cells, the muscle cell to be inclosed in a delicate tubular sheath, or membrane, called the sarcolemma. This membrane, tough and elastic, isolates each fiber. The bundles of fibers, called fasciculi, are surrounded by a fibrous sheath, which is called the perimysium internum; and the entire muscle has likewise an investing sheath of connective tissue, called the perimysium externum. ”

“ This syncytial host is in the adult composed of a number of double elements, a nerve-cell element, and an epithelial element, such as muscle-cell, gland-cell, etc., connected together by nerves; and if such connection is always present as we pass from the adult to the embryo, if there is no period when, for example, the neural element exists alone free from the muscle-cell, no period when the two can be seen to come together and join, then it follows that when the single-layered blastula stage is reached, muscle-cell and nerve-cell must have fused together to form a neuro-muscular cell. ”

“ How muscles act.—A nerve runs from the brain, and touches every cell of the muscle. When we wish to move, the brain sends an order down the nerve. Then each muscle cell makes itself thicker and shorter. This pulls its ends together, and bends the joint. We can make muscle cells move when we wish to, but we cannot make any other kind of cell move. We make all our movements by means of our muscles. ”

“ Fig. 108—A striated muscle cell highly magnified, showing striations and nuclei. Attached to the cell is the termination of a nerve fiber. Within the sarcolemma are minute fibrils and a semiliquid substance, called the sarcoplasm. At each end the cell tapers to a point from which the sarcolemma appears to continue as a fine thread, and this, by attaching itself to the inclosing sheath, holds the cell in place. Most of the muscle cells receive, at some portion of their length, the termination of a nerve fiber. ”

“ A further stage in evolution is that the muscle-cells lose their connexion with the epithelium and come to lie entirely beneath it, forming a sub-epithelial contractile layer, developed chiefly in the tentacles of the polyp. The evolution of the ganglion-cells, is probably similar; an epithelial cell develops processes of nervous nature from the base, which come into connexion with the bases of the sensory cells, with the muscular cells, and with the similar processes of other nerve-cells ”

“ Thus one cell specializes in, say, secretion, another in contractility, another in receiving and carrying stimuli, and so forth, so that we have the gland cell, the muscle cell, and the nerve cell, each appropriately grouped with its fellows to constitute the particular tissue or organ—gland, muscle or brain—which has for its function that of its constituent cells. ”

“ The muscle-cell consists of two portions, a contractile part which lies next the sub-cuticle, and which usually, to some extent, wraps round the second or medullary half. The latter consists of a fibrillar spongioplasm, in the meshes of which lies a clear structureless hyaloplasm. The nucleus always lies in the medullary half. ”

“ The energy of muscular contraction is derived from the oxidation of energy-yielding compounds, like sugar. We must conclude that the polarization-changes at the cell-surface influence the chemical processes in the muscle-cell. Stimulation is known to increase many times the rate of oxidation in muscle-cells. ”

“ These prove (Pl. 12, fig. 10) that the cells in this situation have become elongated in a longitudinal direction, and, in fact, form typical spindle-shaped embryonic muscle-cells, each with a large nucleus. Every muscle-cell extends for the whole length of a protovertebra, and in the present stage, or at any rate in stage I, acquires a peculiar granulation, which clearly foreshadows transverse striation (Pl. 12, figs. 11-13) . ”

“ We do not have, then, in these organs the fine subdivision of control that there is in the skeletal muscles, for a single skeletal motor cell in the spinal cord connects with one or, at the most, a very few muscle fibers. If we consider the uses to which smooth muscles are put, we shall see that finely subdivided control is not important, as it is in the case of the skeletal muscles, where the movements have to be graded very accurately. Smooth muscles are present in the stomach and intestines and other abdominal organs which make movements. ”

“ But if this be so, then the second and third forms have a similar composition, except so far as the matrix of the cells has become fibrillated, or vacuolated, or marked off into masses corresponding with the several nuclei. By a parity of reasoning, muscular tissue may also be considered a cell aggregate, in which the inter-nuclear substance has become converted into striated muscle; while, in the nerve fibres, a like process of metamorphosis may have given rise to the pellucid gelatinous nerve substance. ”

“ Your muscles are made of muscle cells, and the protoplasm in them moves, and when the muscle cells all move 119together, that moves your arm or your leg or your head or some other part of your body.
Since your muscle cells devote themselves to moving, they do not try to do much else; so other cells digest the food which the blood carries to the muscle cells. Yet other cells build a good thick skin to protect the soft muscles, and yet another set of cells thinks for the muscles, and tells them where and when and how to move. Each set of cells has its own work. ”

“ A muscle, for instance, may suffer very considerable wasting and recover from that wasting without any loss or renewal of its elementary fibres. And it is obvious that the metabolism of which we are now speaking does not involve any such shifting of histological units. On the other hand, we find these histological units, the muscle fibre or the gland cell, for instance, living on their internal medium, the blood, or rather on the lymph, which is the middleman between themselves and the actual blood flowing in the vascular channels. ”

“ The Muscle. mechanical work is obtained by movement resulting from a change, it is supposed, in the elastic tension of the framework of the living cell. In the fibrils existing in the cell a sudden alteration of elasticity occurs, resulting in an increased tension on the points of attachment of the cell to the neighbouring elements of the tissue in which the cell is placed. These yield under the strain, and the cell shortens between those points of its attachment. This shortening is called contraction. ”

“ We have, in the skeleton, a complicated apparatus of parts hinged and movable upon one another; the agent moving these parts is the same agent that we find in the heart walls propelling the blood through the circulation, in the alimentary canal squeezing the food along its course, and universally in the body where motion occurs, except in the case of the creeping phagocytes, and the ciliary waving of ciliated epithelium. This agent is muscle. We have, in muscular tissue, a very wide departure from the structure of the primordial cell ”

“ It extends upwards and downwards by the continuous conversion of fresh cells of the splanchnic layer into muscle-cells. At the same time it grows rapidly in thickness, but it requires some little patience and care to unravel the details of this growth; and it will be necessary to enter on a slight digression as to the relations of the muscle-plates to the surrounding connective tissue.
As the muscle-plates grow dorsalwards and ventralwards their ends dive into the general connective tissue, whose origin has already been described (Pl. 13, fig. 1) . ”

“ These muscle-cells have for their special function this one of contraction, and massed together in great numbers they form the strongly contractile muscular tissue and muscles of the body on which the animal's power of motion depends. The cells which line certain parts of the alimentary canal are the ones on which the function of digestion chiefly rests. And so we might continue our survey of the whole complex body. The point of it all is that the thousands of cells which compose the many-celled animal body are differentiated and specialized ”

“ In short, the size of cells is directly proportional to the rate and duration of growth and inversely proportional to the rate of division. It is well known that muscle cells and nerve cells cease to divide at a relatively early age, whereas epithelial and gland cells, mesenchyme, blood and sex cells continue to divide for a longer period, if not throughout life ”

“ Each cell has its specific energy in Johannes Müller's sense, and in its extended form there is no more general law for all the operations of stimuli than this law of specific energy. To take examples, whether a muscle be stimulated by a chemical, mechanical, thermal or electrical stimulus the result is in each case the same—namely, a twitching of the muscle. ”

“ The presence of collagen fibrils varies between the adaxial and the abaxial connective tissue layers. au = auricle, bl = basal lamina, cc = central cavity, cf = collagen fibril, ci = cilium, ct = connective tissue, ec = exterior cavity, ep = epithelial cell, fa = fascicle, ic = interradial cavity, ip = interpyramidal muscle, lo = lobe, mf = muscle fiber, np = nerve process, nu = nucleus, ph = pharynx, po = postural muscle, pr = protractor muscle, pv = perivisceral coelom, py = pyramid, re = retractor muscle, to = tooth. ”

“ Fig. 112—Muscle cells from the heart, highly magnified (after Schäfer) . The Muscular Stimulus.—The inactive, or resting, condition of a muscle is that of relaxation. It does work through contracting. It becomes active, or contracts, only when it is being acted upon by some force outside of itself, and it relaxes again when this force is withdrawn. Any kind of force which, by acting on muscles, causes them to contract, is called a muscular stimulus. Electricity, chemicals of different kinds, and mechanical force may be so applied to the muscles as to cause them to contract. ”

“ In the embryo ray the primitive muscle cells first appear, then they swell out anteriorly and shrivel up posteriorly until each loses the characteristic striated muscle structure and becomes an electric plate lying in a little compartment embedded in a jelly-like substance. ”

“ Also, the distance through which muscles can make movements depends on their length, so a muscle that has to pull for a considerable way must be long, and since single muscle cells are short there will have to be a good many cells end to end to make the whole muscle long enough for its task. The actual make-up and arrangement of muscles in the body depends in part, therefore, on the thickness and length needed for the particular work to be done, and in part on the architecture of the part of the body where the muscle is located. ”