Histology
Understanding the structure and architecture of skeletal muscle begins at the level of the muscle cell, or fiber, itself. Muscle fibers are multinucleated cells with a cylindrical shape, and they have diameters ranging between 10 to 100 µm. Fibers range a few millimeters to several centimeters in length. This wide variation in muscle fiber structure has important implications for their contractile mechanics.
A covering of endomysium surrounds individual muscle fibers, and fibers are arranged into larger units, visible to the naked eye, known as fascicles. A connective tissue sheath known as the perimysium surrounds fascicles. Finally, whole muscles made of numerous fasciculi are surrounded by the epimysium.
The connective tissue surrounding muscle carries the vast array of blood vessels supplying the tissue with a rich vascular network. The membranes are intimately, but loosely, associated with muscle to allow the changes in length and diameter that occur with muscle contraction. The epimysium, perimysium, and endomysium also provide a broad attachment surface for muscles to join with tendons.
The perimysium becomes continuous with the endotenon sheath of tendon at the myotendinous junction, and it allows the tendon to transmit the force that is generated by muscle contraction to result in limb locomotion. Tendons are considered more fully later in the chapter.
The arrangement of muscle fibers and fascicles in muscles varies, and this is apparent on gross inspection of muscle anatomy. Fibers can be arranged either parallel or at an angle to the longitudinal axis of muscle; the latter predominates and includes the unipennate, bipennate, fusiform, and multipennate types.
The specific orientation of muscle fibers within a muscle affects a muscle’s physiological cross-sectional area (i.e., the estimated sum of the cross sectional areas of all the fibers).
Orientation of muscle fibers plays an important role in determining the contractile properties of a given muscle. Force production is proportional to the physiological cross sectional areas of muscles and fiber orientation, but the speed and absolute amount of shortening are proportional to the muscle fiber length.
Shorter fibers have a lower velocity of shortening and a greater force production compared to longer fibers. Although a pennate arrangement of fibers results in a small reduction of the contractile force conducted to the tendon, pennate muscles have a greater number of fibers acting in parallel.
Therefore, a pennate muscle will be more powerful (i.e., have a greater force of contraction) than a muscle of equal mass with a parallel arrangement. Contraction of the pennate muscle–tendon unit causes the tendon to move along the axis of force, increasing pennation of the muscle fibers.
Appointment
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