Structure of Muscle

MUSCLE, STRUCTURE OF. Muscle is the tissue by which the various parts of the body are moved. Thus it forms the main bulk of the limbs, back, neck and body wall. Most of the viscera too possess well developed muscular coats. Muscle in all instances is built up of a number of long fibres. These are of three well defined types. Those forming the skeletal muscles are of large size, even in some instances up to r2cm. in length, their diameter varying from o•or to o•Imm. Microscopically, these are characterized by a decided transverse marking, and they are therefore known as striated muscle fibres. From the fact that they comprise those muscles which are under the control of the will they are also called voluntary muscle fibres. The second variety of muscle is made up of much smaller fibres varying in different parts from 0.05 to o.r5mm. in length and about 0.005mm. in diameter. These fibres show no transverse striations nor are they directly under the control of the will. They are therefore termed smooth or involuntary muscle. Lastly, there is a third type of muscle found in the heart which lies intermediate in structure between these two varieties. In this the fibres are small and show distinct transverse striations. Longitudinal striations are also present though somewhat less marked. In most respects this form of muscle fibre resembles smooth muscle more closely than striated muscle.

Voluntary or Striated Muscle.

Each muscle fibre of which this is composed is a syncytium or plasmodium, i.e., a structure containing a number of nuclei, which has been formed from a single cell by proliferation of its nucleus without subdivision of the protoplasm. It is thus an assemblage of cells possessing a common protoplasm. Each fibre generally runs parallel to the length of the muscle and if this is short extends the whole length. Thus the one end of the fibre may be attached to tendon when the end is rounded off. The other end may also terminate in ten don or in the fibrous covering of bone in which case it is again rounded. In long muscles, however, the fibre may only extend a certain distance along the muscle, and it is then found to termi nate in a tapering or bevelled end. In some of the long muscles some fibres may both arise and terminate in the substance of the muscles. In such a case both ends are bevelled. All the fibres in

a muscle are arranged parallel to one another.

The outer surface of each muscle fibre consists of a tough homogeneous membrane called the sarcolemma. The main muscle substance is composed of several parts, viz., the fibrillae, the sarcoplasm and the nuclei. Under the action of reagents the muscle fibre may be split into a number of longitudinal elements. These are the fibrillae. They possess alternate bands of light and dark substance which give them a striated appearance. When viewed under polarized light the dark substance is found to be doubly refracting or anisotropic, the light band is singly refracting or isotropic. According to many observers, in the centre of each isotropic segment there is a thin transverse disc of anisotropic material and in the centre of each anisotropic segment a thin disc of isotropic substance. The fibrillae are arranged in the muscle fibre parallel to one another and with the alternate light and dark bands at approximately the same level across the fibre, thus giving to the whole muscle fibre its typical transverse striation. The fibrillae are united by interfibrillar substance to form bun dles, of which there may be a considerable number in each muscle fibre. The bundles lie in a surrounding layer of sarcoplasm which apparently represents the remaining portion of unaltered proto plasm of the syncytium. This structure of muscle is best seen in the transverse sections of the fibres. A number of areas separated by a clear protoplasm are then to be seen. The areas are formed by the bundles of fibrillae seen in transverse section, the inter mediate substance is the sarcoplasm. In some muscles, apparently, each fibrilla is surrounded by sarcoplasm, in which case the fibril lae are easily isolated from one another and can be readily exam ined. This is the case in the wing muscles of insects. The nuclei of the fibre are arranged close under the sarcolemma. Each is sur rounded by a small quantity of sarcoplasm and in shape is an elongated ellipse. In most cases the muscle fibres do not branch, though in a few instances, such as the superficial muscles of the tongue, branching is found.