Defining Muscle
The basic function of muscle is to generate force. Secondarily, muscles can provide some shape and form to the organism. Anatomically and functionally, muscle can be divided into two types, smooth and striated. Striated or striped muscle can be further divided into skeletal muscle and cardiac (heart) muscle. Regardless of the type, all muscles share the following basic properties (Gowitzke and Milner 1988):
• Conductivity: A muscle has the ability to conduct an action potential.
• Irritability: When stimulated, the muscle will react.
• Contractility: A muscle can shorten or produce tension between its ends.
• Relaxation: A muscle can return to resting properties after contraction.
• Distensibility: A muscle can be stretched by a force outside of the muscle itself. The muscle is not injured as long as it is not stretched past its physiological limits.
• Elasticity: The muscle will resist elongation and will return to its original position after passive or active elongation. Elasticity is the opposite of distensibility.
Smooth muscle and striated muscle can easily be differentiated from each other in a variety of ways, including appearance. For example, smooth muscle is uni-nucleated and contains sarcomeres (the functional units of muscle) that are arranged at oblique angles to each other; under a light microscope smooth muscle appears to be relatively featureless as a result of the orientation of its sarcomeres. On the other hand, striated muscle contains protein arrays called myofibrils that are parallel to each other and thus form striations or stripes. Cardiac muscle can be easily identified as distinct from skeletal muscle by appearance and differences in function, such as an intrinsic ability to contract. (We will not go into detail on smooth and cardiac muscle because though interesting, such discussion is not within the scope of this book.)
Muscle Structure and Function
Skeletal muscle is found in many sizes and various shapes. The small muscles of the eye may contain only a few hundred cells, while the vastus lateralis may contain hundreds of thousands of muscle cells. The shape of muscle is dependent on its general architecture, which in turn helps to define the muscle’s function. Some muscles, such as the gluteal muscles, are quite thick; some, such as the sartorius, are long and relatively slender; and others, such as the extensors of the fingers, have very long tendons. These differences in muscle shape and architecture permit skeletal muscle to function effectively over a relatively wide range of tasks.
For example, thicker muscles with a large cross-sectional area can produce great amounts of force; longer muscles can contract over a greater distance and develop higher velocities of shortening; muscles with long tendons can form pulley arrangements that allow large external movement (e.g., grasping by the fingers) with relatively small movement of the muscles and tendons. Some long slender muscles such as the sartorius and biceps femoris are divided by transverse fibrous bands that form distinct sections or compartments (McComas 1996). Although fibers were previously believed to run the length of these muscles, because of these compartments the longest possible human muscle fiber is about 12 cm (4.7 in.) in length (McComas 1996).
The individual compartments can have different fiber type distributions and different cross-sectional areas (English and Ledbetter 1982). Each compartment has a separate innervation; however, individual motor neurons often innervate muscle fibers in adjacent compartments. But the functional outcomes of compartmentalization are not completely understood. One possible consequence of compartmentalization is that it could ensure that contraction occurs relatively synchronously and rapidly along the muscle belly. However, it is also possible to recruit compartments separately (English 1984).
Muscle fibers can be arranged into two basic structural patterns, fusiform and pinnate (also spelled pennate). Most human muscles are fusiform, with the fibers largely arranged in parallel arrays along the muscle’s longitudinal axis. In many of the larger muscles the fibers are inserted obliquely into the tendon, and this arrangement resembles a feather (i.e., pinnation). The fibers in a pinnate muscle are typically shorter than those of a fusiform muscle. The arrangement of pinnate muscle fibers can be single or double, as in muscles of the forearm, or multipinnate, as in the gluteus maximus or deltoid.
The fibers of a pinnated muscle pull on the tendon at an angle, and the amount of force actually exerted on the tendon can be calculated using the cosine of the angle of insertion. At rest, the angle of pinnation in most human muscles is about 10° or less and does not appear to have a marked effect on most functional properties such as force production (Roy and Edgerton 1992; Wickiewicz et al. 1983, 1984). However, during muscle contraction the angle of pinnation can vary and may change some functional parameters, at least in some muscles (Fukunaga et al. 1997; Otten 1988). It is possible that during muscle contraction the angle of pinnation increases enough to decrease speed of contraction and increase force production. It is also possible that hypertrophy, which adds sarcomeres in parallel and can alter the angle of pinnation, can alter functional properties (Binkhorst and van’t Hof 1973; Tihanyi, Apor, and Fekete 1982).
Pinnation offers a force advantage over fusiform fibers because with pinnation there are more fibers in a muscle of a given volume; thus the effective cross section of the pinnated muscle is larger. Pinnation also permits more sarcomeres to be arranged in parallel (at the expense of those in series), resulting in enhanced force production (Gans and Gaunt 1991; Roy and Edgerton 1992; Sacks and Roy 1982). Additionally, the central tendon moves a greater distance in comparison to the shortening length of the muscle fibers, allowing the fibers to operate over the optimum portion of their length-tension curves (Gans and Gaunt 1991; McComas 1996).
About 85% of the mass of a muscle is made up of muscle fibers; the remaining 15% is mostly connective tissue. Muscle is organized and largely shaped by the connective tissue, which is composed of a ground substance, collagen, and reticular and elastin fibers of varying proportions. In muscle, the connective tissue is largely responsible for transmitting forces, for example the transmission of forces from the muscle to the bone by the tendon. The connective tissues’ elasticity and distensibility help to ensure that the tension developed by the muscle is smoothly transmitted and that a muscle will return to its original shape after being stretched.
Thus, the connective tissue of a muscle provides a framework for the concept of series and parallel elastic components within a muscle. When a muscle is passively stretched or when it actively contracts, the resulting initial tension is largely caused by the elastic properties of the connective tissue. During a contraction, the muscle cannot actively develop force or perform work against a resistance until the elastic components are stretched out and the muscle tension and resistance (load) are in equilibrium.
There are three levels of muscle tissue organization: epimysium, endomysium, and perimysium. These three levels are a consequence of differing sizes and orientations of connective tissue fibers, particularly collagen. The outside surface of a muscle is covered by a relatively thick and very tough connective tissue, the epimysium, which separates it from surrounding muscles. Arteries and veins run through the endomysium. The collagen fibers of the epimysium are woven into particularly tight bundles that are wavy in appearance.
These collagen bundles are connected to the perimysium. The perimysium divides the muscle into bundles typically containing about 100 to 150 muscle fibers, which form a fasciculus or fascicle. However, muscles that function in producing small or very fine movements have smaller fascicles containing relatively few fibers and a larger proportion of connective tissue (Gowitzke and Milner 1988). The muscle fibers take on a polygonal cross-sectional shape that allows a greater number of fibers to fit into a fascicle (McComas 1996).
Typically the interstitial spaces between fibers are about 1 µm. The perimysium also forms connective tissue tunnels, the intramuscular septa, which run through the muscle belly and provide a pathway for larger arterioles, venules, and nerves. The perimysium contains many large collagen bundles that encircle the outer surface of the muscle fibers lying on the outside of a fascicle. Some of the collagen bundles encircle the fascicles in a cross pattern, adding stability to the structure of the fascicle. Underneath the thicker perimysial sheets of connective tissue is a much looser network of collagen fibers that run in various directions and connect with the endomysium. The endomysium, which is made up of collagen fibers 60 to 120 nm in diameter, surrounds each muscle fiber, again adding more stability. Capillaries run between individual muscle fibers and lie within and are stabilized by the endomysium. Many of the endomysial fibers connect with the perimysium and likely connect to the basement membrane, which lies on the outside of the muscle cell sarcolemma (McComas 1996).
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