MUSCLE TYPES

Muscle types Smooth muscle Because vertebrate smooth muscle is located in the walls of many hollow organs, the normal functioning of the cardiovascular, respiratory, gastrointestinal, and reproductive systems depends on the constrictive capabilities of smooth muscle cells. Smooth muscle is distinguished from the striated muscles of the skeleton and heart by its structure and its functional capabilities. As the name implies, smooth muscle presents a uniform appearance that lacks the obvious striping characteristic of striated muscle. Vascular smooth muscle shortens 50 times slower than fast skeletal muscle but generates comparable force using 300 times less chemical energy in the process. These differences in the mechanical properties of smooth versus striated muscle relate to differences in the basic mechanism responsible for muscle shortening and force production. As in striated muscle, smooth muscle contraction results from the cyclic interaction of the contractile protein myosin (i.e., the myosin cross bridge) with the contractile protein actin. The arrangement of these contractile proteins and the nature of their cyclic interaction account for the unique contractile capabilities of smooth muscle. Structure and organization Smooth muscle contains spindle shaped cells 50 to 250 micrometres in length by five to 10 micrometres in diameter. These cells possess a single, central nucleus. Surrounding the nucleus and throughout most of the cytoplasm are the thick (myosin) and thin (actin) filaments. Tiny projections that originate from the myosin filament are believed to be cross bridges. The ratio of actin to myosin filaments (approximately 12 to 1) is twice that observed in striated muscle and thus may provide a greater opportunity for a cross bridge to attach and generate force in smooth muscle. An increased probability for attachment may, in part, account for the ability of smooth muscle to generate, with far less myosin, comparable or greater force than striated muscle. Smooth muscle differs from striated muscle in lacking any apparent organization of the actin and myosin contractile filaments into the discrete contractile units called sarcomeres. Recent advances have shown that a sarcomere-like structure may nonetheless exist in smooth muscle. Such a sarcomere-like unit would be composed of the actin filaments that are anchored to dense, amorphous bodies in the cytoplasm as well as dense plaques on the cell membrane. These dense areas are composed of a-actinin, a protein, found in the Z lines of striated muscle, to which actin filaments are known to be attached. Thus, force generated by myosin cross bridges attached to actin is transmitted through actin filaments to dense bodies and then through neighbouring contractile units, which ultimately terminate on the cell membrane. Relaxed smooth muscle cells possess a smooth cell membrane appearance, but upon contraction, large membrane blebs (or eruptions) form as a result of inwardly directed contractile forces that are applied at discrete points on the muscle membrane. These points are presumably the dense plaques on the cell membrane to which the actin filaments attach. As an isolated cell shortens it does so in a corkscrewlike manner. It has been hypothesized that, in order for a single cell to shorten in such a unique fashion, the contractile proteins in smooth muscle are helically oriented within the muscle cell. This helical arrangement agrees with earlier speculation that the contractile apparatus in smooth muscle may be arranged at slight angles relative to the long axis of the cell. Such an arrangement of contractile proteins could contribute to the slower shortening velocity and enhanced force-generating ability of smooth muscle. The contractile proteins interact to generate a force that must be transmitted to the tissue in which the individual smooth muscle cells are embedded. Smooth muscle cells do not have the tendons present in striated muscles, which allow for transfer of muscular force to operate the skeleton. Smooth muscles, however, are generally embedded in a dense connective tissue matrix that connects the smooth muscle cells within the tissue into a larger functional unit. Other organelles of the cell interior are related to energy production and calcium storage. Mitochondria are located most frequently near the cell nucleus and at the periphery of the cell. As in striated muscles, these mitochondria are linked to ATP production. The sarcoplasmic reticulum is involved in the storage of intracellular calcium. As in striated muscle, this intracellular membrane system plays an important role in determining whether or not contraction occurs by regulating the concentration of intracellular calcium. Muscle types Cardiac muscle The heart is the pump that keeps blood circulating throughout the body and thereby transports nutrients, breakdown products, antibodies, hormones, and gases to and from the tissues. The heart consists mostly of muscle, the myocardial cells (collectively termed the myocardium), arranged in ways that set it apart from other types of muscle. The outstanding characteristics of the action of the heart are its contractility, which is the basis for its pumping action, and the rhythmicity of the contraction. Heart muscle differs from its counterpart, skeletal muscle, in that it exhibits rhythmic contractions. The amount of blood pumped by the heart per minute (the cardiac output) varies to meet the metabolic needs of the peripheral tissues (muscle, kidney, brain, skin, liver, heart, gastrointestinal tract). The cardiac output is determined by the contractile force developed by the muscle cells of the heart (myocytes) as well as the frequency at which they are activated (rhythmicity). The factors affecting the frequency and force of heart muscle contraction are critical in determining the normal pumping performance of the heart and its response to changes in demand. Structure and organization The heart is a network of highly branched cardiac cells 110 micrometres in length and 15 micrometres in width, which are connected end to end by intercalated disks. The cells are organized into layers of myocardial tissue that are wrapped around the chambers of the heart. The contraction of the individual heart cells produces force and shortening in these bands of muscle with a resultant decrease in the heart chamber size and the consequent ejection of the blood into the pulmonary and systemic vessels. Important components of each heart cell involved in excitation and metabolic recovery processes are the plasma membrane and transverse tubules in registration with the Z lines, the longitudinal sarcoplasmic reticulum and terminal cisternae, and the mitochondria. The thick (myosin) and thin (actin, troponin, and tropomyosin) protein filaments are arranged into contractile units (that is to say, the sarcomere extending from Z line to Z line) that have a characteristic cross-striated pattern similar to that seen in skeletal muscle. Muscle types Primitive contractile systems Cilia and flagella Unicellular organisms such as the paramecium, a protozoan that lives in freshwater ponds and streams, propel themselves by the action of cilia. Cilia occur in large numbers and move in a coordinated way. Ciliated cells within the vertebrate body propel fluid and mucus along interior passages, such as the lining of the respiratory tract. Flagella are structurally similar to cilia, except that they are longer (sometimes up to 50 times longer) than cilia and usually number only one or two per cell. Sperm cells of most higher organisms move using flagella. Many types of unicellular algae and protozoans use flagella in swimming through the water. Both cilia and flagella contain a regular pattern of tubules extending along their lengths; there is an outer ring of nine pairs of tubules surrounding a central pair of tubules. Each tubule is composed of filaments comprising a string of globular subunits. The movement of a cilium or a flagellum requires energy, which is obtained from the breakdown of adenosine triphosphate (ATP), catalyzed by a protein attached to the outer tubules, dynein. Some types of bacteria have flagella whose motion seems to depend on a cellular particle called the basal body, to which the flagellum is attached. Such flagella derive their energy from a difference in hydrogen ion concentration across the cell membrane. Amoeboid motion Amoeboid movement occurs as an extension of the cytoplasm, called a pseudopod (false foot), flows outward, deforms the cell boundary, and is followed by the rest of the cell. Many pseudopodia may be formed at the same time, and their actions do not seem to be coordinated. Although amoeboid motion is characteristic of the amoeba, a unicellular protozoan, it is also found in nonmuscle cells of multicellular organisms. These cells contain myosin and actin, which differ in some aspects of their structure from the corresponding proteins in muscles because of variations in the genes that encode them.