Skeletal muscle fibers attach at their ends to collagen fibers. The latter may be part of a tendon, a fibrous sheet (aponeurosis), the periosteum, or a raphe. It is by these attachments that the contractile activity of the muscle fibers is translated into useful work in moving or stabilizing parts of the body.
Skeletal muscle fibers are richly innervated by motor neurons that originate in the spinal cord or brain stem. The axons of the neuron branch as they near the muscle, giving rise to twigs that end on individual muscle fibers. The motor unit or neuromotor unit is the name given to a neuron and the specific muscle cells it innervates; a single neuron may innervate from several to a hundred or more muscle fibers.
Figure 1, musculotendinous junction, H&E x350.
In this figure, the muscle fibers appear to terminate directly on the tendon. The muscle fibers (M) are in the left half of the figure. They stain redder than the tendon (T), which typically stains pale pink with eosin. The skeletal muscle fibers have been cut obliquely, and the boundaries between individual muscle cells are not distinct. In many places, however, there is a slight separation of the muscle fibers, and along with the orientation of the muscle cell nu-
Figure 2, musculotendinous junction, electron micrograph x24,600.
At the actual junction between the muscle cell and tendon, the end of the cell becomes serrated, and the cytoplasmic projections of the muscle cell interdigitate with the collagen fibrils of the tendon. This arrangement is, at best, only suggested in Figure 1, but it is evident in an electron micrograph of the junction, such as that shown here.
This figure shows four finger-like projections of the muscle cell. The external lamina (BL) is directly adjacent to
Figure 3, neuromuscular junction, Golgi stain x280.
In the neuromuscular junction shown here, a special stain has been used to visualize the neural elements. This staining procedure does not reveal the skeletal muscle cells to best advantage. They are horizontally disposed in the illustration; cross-striations (arrows) are visible in some muscle fibers. The nerve (TV) enters the field from the left, initially dips downward, then turns in an upward direction. As it does, it can be seen to divide into smaller branches, clei, this separation tends to show their general direction. Cross-striations can be identified at right angles to the direction of the fibers. In contrast, the tendon gives no obvious indication of how the collagen fibers are arranged. This needs to be surmised from the orientation of the fibroblast nuclei, which usually have their long axes parallel to the direction of the fibers. Although the nuclei of the fibroblasts are readily identified, the cytoplasm of these cells is not distinguishable from the collagen of the tendon.
the plasma membrane (PM) of the muscle cell and follows the finger-like projections. Actin filaments (arrows) of the terminal sarcomeres extend into the finger-like projections and insert into densities on the inner face of the plasma membrane (arrowheads). Note that the terminal sarcomeres possess only one Z disk (ZD). Also to be seen within the muscle cell cytoplasm are triads (Tr), glycogen granules (G), and mitochondria (Mi). External to the cell are fibrils of the tendon (T).
and finally, as it nears the muscle cells, the nerve fiber branches to make contact with several muscle cells. At the terminal between nerve fiber and each muscle cell, the nerve fiber arborizes to form a disk-like structure, the motor end plate (MEP), on the surface of the muscle cells. The motor end plate is the physiologic contact between nerve and muscle; it is, in fact, a neuromuscular synapse at which the neurotransmitter acetylcholine is liberated. This transmitter initiates the sequence of muscle membrane events that leads to contraction of the muscle.
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