Cardiac Muscle Tissue, Skeletal Muscle Tissue and Smooth Muscle Tissue

Visceral smooth muscle cells

Are located in the walls of the digestive tract, the gallbladder, the urinary bladder and many other internal organs. it lacks the direct contact with any motor neuron. Are arranged in sheets and layers. Within each layer, adjacent muscle cells are connected by gap junctions. As a result, whenever one muscle cell contracts, the electrical impulse that triggered the contraction can travel to adjacent smooth muscle cells. For this reason, the contraction spreads in a wave that soon involves every smooth muscle cell in the layer. Smooth muscle can also contract in response to chemicals, hormones, local concentrations of oxygen or carbon dioxide or physical factors such as extreme stretching or irritation.

Cardiac Muscle Tissue

Are small. Have a single, centrally placed nucleus. Cells are typically branches T tubules are short and broad and there are NO triads. The T tubules encircle the sarcomeres at the Z lines rather than at the zones of overlap. The Sarcoplasmic reticulum lacks terminal cisternae, and its tubules contact the plasma membrane as well as the T tubules. Action potential triggers the release of calcium from the SR and the contraction of sarcomeres. However, it must also make the sarcolemma more permeable to extracellular calcium ions. Because their contractions require both intracellular and extracellular calcium ions. Are more sensitive to changes in the extracellular calcium ion concentration than other muscle fibers. Are almost totally dependent on aerobic metabolism for the energy they need to continue contracting. They have energy reserves in the form of glycogen and lipid inclusions. The sarcoplasm of this contains large numbers of mitochondria as well as abundant reserves of myoglobin, which stores the oxygen needed to break down energy reserves during times of peak activity.

Functional Syncytium

Because cardiac muscle cells are mechanically, chemically, and electrically connected to each other, the entire tissue resembles a single, enormous muscle cell. For this reason, cardiac muscle cells are called this.

Plasticity

Because the thick and thin filaments are scattered and are not organized into sarcomeres in smooth muscle, tension development and resting length are not directly related. A stretched smooth muscle soon adapts to its new length and retains the ability to contract on demand. This ability to function over a wide range of length is called this. Smooth muscle can contract over a range of lengths four times greater than that of skeletal muscle. This is especially important in the digestive organs, such as the stomach, that change greatly in volume. Despite the lack of sarcomeres, smooth muscle contractions can be just as powerful as those of skeletal muscles. Like skeletal muscle fibers, smooth muscle cells can undergo sustained contractions.

Functions of Smooth Muscle

Differs from other muscle tissue in (1) excitation-contraction coupling (2) length-tension relationships (3) control of contractions (4) smooth muscle tone. The triggers for smooth muscle contractions is the appearance of free calcium ions in the cytoplasm. On stimulation, a surge of calcium ions enters the cell from the extracellular fluid, and the sarcoplasmic reticulum releases additional calcium ions. The net result is a rise in calcium ion concentrations throughout the cell. Once in the sarcoplasm, the calcium ions interact with calmodulin, a calcium binding protein. Calmodulin then activates the enzyme myosin light chain kinase, which in turn enables myosin heads to attach to actin.

Smooth Tissue

Forms sheets, bundles, or sheaths around other tissues in almost every organ. Plays a role in Integumentary, Cardiovascular, Respiratory, Digestive, Urinary and Reproductive.

Structural Characteristics of Smooth Muscle

The internal organization of this is very different. Are relatively long and slender. Each cell is spindle shaped and has single, centrally located nucleus. Has no T Tubules and the sarcoplasmic reticulum forms a loose network throughout the sarcoplasm. Lack myofibrils and sarcomeres. As a result, this tissue has no striations and is called nonstriated muscle. Thick filaments are scattered throughout the sarcoplasm of this. The myosin proteins are organized differently than in a skeletal muscle cell and smooth muscle cells have more myosin heads per thick filament. Thin filaments in this are attached to dense bodies, structures distributed throughout the sarcoplasm in a network of intermediate filaments composed of the protein desmin. Adjacent cells are bound together at dense bodies, transmitting the contractile forces from one cell to cell throughout the tissue. Although they are surrounded by connective tissue, the collagen fibers never unite to form tendons or aponeuroses, as they do in skeletal muscles.

Intercalated Discs

The sarcolemmas of two adjacent cardiac muscle cells are extensively intertwined and bound together by gap junctions and desmosomes. These connections help stabilize the position of adjacent cells and maintains the three-dimensional structure of the tissue. The gap junctions allow ions and small molecules to move from one cell to another. These junctions create a direct electrical connection between two muscle cells. An action potential can travel across this. Two muscle cells can "pull together" with maximum efficiency because their myofibrils are essentially locked together at this site.

Cardiac Muscles Functions

This tissue contracts without neural stimulation. This property is called automaticity. Specialized cardiac muscle cells called pacemaker cells normally determine the timing for contractions Cardiac muscle cell contractions last about ten times as long as do those of skeletal muscle fibers. They also have longer refractory periods and do not readily fatigue. The nervous system can alter the pace set by the pacemaker cells and adjust the amount of tension produced during a contraction. The properties of cardiac muscle sarcolemmas differ from those of skeletal muscle fibers. As a result, individual twitches cannot undergo wave summation, and cardiac muscle tissue cannot produce tetanic contractions. This difference is important, because a heart in a sustained tetanic contraction could not pump blood.

Pacesetter Cells

Visceral smooth muscle layers show rhythmic cycles of activity in the absence of neural stimulation. For example, these cycles are characteristic of the smooth muscle cells in the wall of the digestive tract, where these cells spontaneously trigger the contraction of entire muscular sheets.

Multiunit smooth muscle cells

are innervated in motor units comparable to those of skeletal muscles but each smooth muscle cell may be connected to more than one motor neuron. Are found in the iris, portions of the male reproductive tract, within the walls of large arteries, and in the arrector pili muscles of the skin.