Why do skeletal muscles appear striated

At such junctions, the T-tubules are in close contact with the sarcoplasmic reticulum, which forms a network surrounding each myofibril. The part of the sarcoplasmic reticulum associated with the T-tubules is termed the terminal cisternae because of its flattened cisternal arrangement. When an excitation signal arrives at the neuromuscular junction, the depolarization of the sarcolemma quickly travels through the T-tubule system and comes in contact with the sarcoplasmic reticulum, causing the release of calcium and resulting in muscle contraction.

Smooth muscle forms the contractile portion of the wall of the digestive tract from the middle portion of the esophagus to the internal sphincter of the anus. It is found in the walls of the ducts in the glands associated with the alimentary tract, in the walls of the respiratory passages from the trachea to the alveolar ducts, and in the urinary and genital ducts.

The walls of the arteries, veins, and large lymph vessels contain smooth muscle as well. Smooth muscle is specialized for slow and sustained contractions of low force. Instead of having motor units, all cells within a whole smooth muscle mass contract together. Smooth muscle has inherent contractility, and the autonomic nervous system, hormones and local metabolites can influence its contraction. Since it is not under conscious control, smooth muscle is involuntary muscle.

Smooth muscle fibers are elongated spindle-shaped cells with a single nucleus. In general, they are much shorter than skeletal muscle cells.

The nucleus is located centrally and the sarcoplasm is filled with fibrils. The thick myosin and thin actin filaments are scattered throughout the sarcoplasm and are attached to adhesion densities on the cell membrane and focal densities within the cytoplasm. Since the contractile proteins of these cells are not arranged into myofibrils like those of skeletal and cardiac muscle, they appear smooth rather than striated. Smooth muscle fibers are bound together in irregular branching fasciculi that vary in arrangement from organ to organ.

These fasciculi are the functional contractile units. There is also a network of supporting collagenous tissues between the fibers and the fasciculi. Cardiac muscle shares important characteristics with both skeletal and smooth muscle.

Functionally, cardiac muscle produces strong contractions like skeletal muscle. However, it has inherent mechanisms to initiate continuous contraction like smooth muscle. The rate and force of contraction is not subject to voluntary control, but is influenced by the autonomic nervous system and hormones.

Histologically, cardiac muscle appears striated like the skeletal muscle due to arrangement of contractile proteins. It also has several unique structural characteristics:. Collagenous tissues are found surrounding individual cardiac muscle fibers. There is abundance vascularization within this supporting tissue, which is required to meet the high metabolic demands of cardiac muscle.

The cardiac muscle fibers are joined end to end by specialized junctional regions called the intercalated discs. The intercalated discs provide anchorage for myofibrils and allow rapid spread of contractile stimuli between cells.

Such rapid spread of contraction allows the cardiac muscles to act as a functional syncytium. The intercalated discs contain three types of membrane-to-membrane contact:.

In addition to the contractile cells, there is a specialized system made up of modified muscle cells whose function is to generate the stimulus for heartbeat and conduct the impulse to various parts of the myocardium. This system consists of sinoatrial node, atrioventricular node, bundle of His, and Purkinje fibers.

How are fibers connected? Location and number of nuclei How are fibers connected? Thick filaments occur only in the A band of a myofibril. Thin filaments attach to a protein in the Z disc called alpha-actinin and occur across the entire length of the I band and partway into the A band. The region at which thick and thin filaments overlap has a dense appearance, as there is little space between the filaments.

Thin filaments do not extend all the way into the A bands, leaving a central region of the A band that only contains thick filaments. This central region of the A band looks slightly lighter than the rest of the A band and is called the H zone.

The middle of the H zone has a vertical line called the M line, at which accessory proteins hold together thick filaments. Both the Z disc and the M line hold myofilaments in place to maintain the structural arrangement and layering of the myofibril. Myofibrils are connected to each other by intermediate, or desmin, filaments that attach to the Z disc. Thick and thin filaments are themselves composed of proteins.

Thick filaments are composed of the protein myosin. The tail of a myosin molecule connects with other myosin molecules to form the central region of a thick filament near the M line, whereas the heads align on either side of the thick filament where the thin filaments overlap. The primary component of thin filaments is the actin protein. Two other components of the thin filament are tropomyosin and troponin.

Actin has binding sites for myosin attachment. Strands of tropomyosin block the binding sites and prevent actin-myosin interactions when the muscles are at rest. Troponin consists of three globular subunits. In addition to three different types of muscles discussed above, there are also differences in skeletal muscle function.

Different skeletal muscles in animals are described as white and red muscle. These different types of skeletal muscles are recruited depending on whether a fast and short versus steady and prolonged locomotion is needed by the animal. Describe the differences between white and red muscle. Based on the article about swimming in fish you read linked above , answer the following questions.

Where is the most of the red muscle found in the fish studied in the above article and what is the functional significance of this muscle location? Hint: review Figures 5 and 6 of the article.

Question 6. Which is more efficient — red or white muscle? The cells of cardiac muscle, known as cardiomyocytes, also appear striated under the microscope. Unlike skeletal muscle fibers, cardiomyocytes are single cells typically with a single centrally located nucleus.

A principal characteristic of cardiomyocytes is that they contract on their own intrinsic rhythms without any external stimulation. Cardiomyocyte attach to one another with specialized cell junctions called intercalated discs. Intercalated discs have both anchoring junctions and gap junctions.

Attached cells form long, branching cardiac muscle fibers that are, essentially, a mechanical and electrochemical syncytium allowing the cells to synchronize their actions. The cardiac muscle pumps blood through the body and is under involuntary control. The attachment junctions hold adjacent cells together across the dynamic pressures changes of the cardiac cycle. Smooth muscle tissue contraction is responsible for involuntary movements in the internal organs.

It forms the contractile component of the digestive, urinary, and reproductive systems as well as the airways and arteries. Each cell is spindle shaped with a single nucleus and no visible striations Figure.

Watch this video to learn more about muscle tissue. In looking through a microscope how could you distinguish skeletal muscle tissue from smooth muscle? The three types of muscle cells are skeletal, cardiac, and smooth.

Their morphologies match their specific functions in the body. Skeletal muscle is voluntary and responds to conscious stimuli. The cells are striated and multinucleated appearing as long, unbranched cylinders. Cardiac muscle is involuntary and found only in the heart. Each cell is striated with a single nucleus and they attach to one another to form long fibers.

Cells are attached to one another at intercalated disks. The cells are interconnected physically and electrochemically to act as a syncytium.