Question

What is the histological structure of skeletal , cardiac and smooth muscle

i nee 5 page at least

Answer 1

Ans. Skeletal muscle is an excitable, contractile tissue responsible for maintaining posture and moving the orbits, together with the appendicular and axial skeletons. It attaches to bones and the orbits through tendons. Excitable tissue responds to stimuli through electrical signals. Contractile tissue is able to generate tension of force. Skeletal muscle tissue is also extensible and elastic. Extensible tissue can be stretched and elastic tissue is able to return to its original shape following distortion.

Special terms are used to describe structures associated with skeletal muscle tissue. Muscle tissue terms often begin with myo-, mys-, or sarcoplasm. The cytoplasm of a muscle cells is referred to as sarcoplasm. The plasma membrane is called the sarcolemma and the endoplasmic reticulum is called the sarcoplasmic reticulum. A muscle fiber may also be referred to as a myofiber.

Individually, skeletal muscles cells are referred to as muscle fibers. The length of a skeletal muscle fiber varies by location. In the anterior thigh, a muscle fiber may be a meter long.Myofibrils are rod shaped subunits of muscle cells. The actin and myosin filaments making up the myofibrils are organized into sarcomeres. Under a microscope, sarcomeres give skeletal muscle a striated appearance.

Skeletal muscle tissue develops through the fusion of individual myoblasts, or early muscle cells. This fusion results in a characteristic multinucleated structure. Because the cells are fused and multinucleated, they form a structural syncytium. Nuclei of skeletal muscle tissue are oval-shaped and located at the periphery of the cell.

Skeletal muscle tissue is made up of a collection of muscle fibers wrapped in connective tissue sheaths. There are three types of connective tissue sheaths named for their location. Endomysium surrounds individual muscle fibers. It is made up of a delicate layer of reticular fibers and permits only small-diameter nerve fibers and capillaries.

Perimysium is a slightly thicker layer of connective tissue consisting mainly of type I and III collagen and surrounds a group of fibers. This fiber group is referred to as a fascicle or bundle. Fascicles are the functional units of skeletal muscle tissue.

Epimysium surrounds the entire collection of fascicles making up an individual muscle. This dense connective tissue made up of mainly type I collagen contains the neurovascular supply to the muscle.

Sarcolemma - cell plasma membrane of a muscle cell

Sarcoplasmic reticulum - modified endoplasmic reticulum. The sarcoplasmic reticulum resembles a piece of lace that surrounds the myofibril.

Terminal cisterna - enlarged regions on either end of the sarcoplasmic reticulum. The terminal cisternae sequester calcium to be used in the contraction cycle. There is a terminal cisterna on either side of a t-tubule.

T-tubules - also called transverse tubules. The t-tubules are pits along the surface of the muscle cells. Their walls are continuous with the sarcolemma, meaning the internal surface of the t-tubule is exposed to the extracellular matrix. The t-tubules are responsible for moving action potentials to the inner region of the muscle cell.

Cardiac muscle, the myocardium, consists of cross-striated muscle cells, cardiomyocytes, with one centrally placed nucleus.

Nuclei are oval, rather pale and located centrally in the muscle cell which is 10 - 15 µm wide.

Cardiac muscle cells excitation is mediated by rythmically active modified cardiac muscle cells.

Cardiac muscle is innervated by the autonomic nervous system .which adjusts the force generated by the muscle cells and the frequency of the heart beat.

Cardiac muscle cells often branch at acute angles and are connected to each other by specialisations of the cell membrane in the region of the intercalated discs.

Intercalated discs invariably occur at the ends of cardiac muscle cells in a region corresponding to the Z-line of the myofibrils.

Cardiac muscle does not contain cells equivalent to the satellite cells of skeletal muscle.

Endocardium Inner layer of the heart, contains blood vessels. Has 3 sublayers

Endothelium - innermost portion a simple squamous epithelium.

Smooth Muscle and Connective Tissue - middle layer of the endocardium is mix of connective tissue and smooth muscle.

Subendocardial Layer - outer layer of the endocardium is loose connective tissue joining the endocardium and myocardium.

Myocardium

Middle layer of the heart, thickest contains cardiomyocytes, blood vessels.

Muscular layer.

Epicardium

Outer layer of the heart, contains blood vessels and lymphatics.

Visceral layer of pericardium .

Red Blood Cells (orange cells) Cardiac Muscle Cells (blue)

Cardiac muscle cells are cut longitudinally.

At high magnification see both striations and the large nuclei of the cardiac muscle cells.

Follow the course of individual cardiac muscle cells and note fine, dark blue lines which seem to cross (traverse) the fibres.

Intercalated Discs that connect the individual muscle cells and permit the conduction of electrical impulses between the cells.

Cardiac muscle cells usually have a single nucleus. The cells are often branched, and are tightly connected by specialised junctions. The region where the ends of the cells are connected to another cell is called an intercalated disc. The intercalated disc contains gap junctions, adhering junctions and desmosomes.

Gap junctions allow the muscle cells to be electrically coupled, so that they beat in synchrony. through the wall of the heart, showing the striated appearance of the muscle, and the nuclei.

you can now see individual cardiac muscle cells, that branch. The densely staining regions at the ends of the muscle fibres are intercalated discs.

Smooth muscle is made up of cells that contain a single central nucleus. The cells stick together and are connected by specialised cell junctions, called gap junctions. The cells are spindle shaped, and the nucleus is central.

Smooth muscle is non-striated and consists of histologically distinct cells. It is capable of synchronous contractions that are based on a similar mechanism to the one in skeletal muscle. However, smooth muscle has other properties in addition to contractility. This article will describe the histology of smooth muscle, together with its properties.

The cytoplasm (sarcoplasm) is eosinophilic and contains anchoring points called focal densities. Their role is to provide one attachment point for the bundles ,which form a criss-cross pattern in smooth muscle. The contractile proteins are alpha actin and type II myosin, which form thick and thin filaments, respectively. Thick filaments do not occupy a fixed place within the sarcoplasm, but are rather scattered throughout. Smooth muscle cells also contain intermediate filaments, called desmin, which insert into focal densitirs.

In addition to the focal densities found within the cytoplasm, smooth muscle has a second set of proteins providing attachment points. Those second connections are provided by the focal adhesion densities, also known as dense bodies, which are located on the plasma membrane of myocytes. They usually exhibit a linear or branching configuration and are small, isolated, and electron-dense structures.

The ultrastructure of smooth muscle cells reveals distinct invaginations all around the plasma membrane. Some of them are irregularly shaped and it is believed they are involved in pinocytosis. In contrast, others have more regular shapes and are called caveolae. Scattered smooth endoplasmic reticulum fragments and cytoplasmic vesicles are located directly underneath and in close proximity to these caveolae. You can imagine these three structures to be the ‘T-system’ of the smooth muscle, similar to skeletal muscle. In reality, this muscle type does not posses such a system. In addition, adjacent cells form gap junctions, which allow small ions and molecules to pass from one cell to the next. This communication facilitates a synchronous smooth muscle contraction by allowing the spread of depolarization. Organelles are located at each end of the nucleus and consist of free ribosomes, glycogen granules, small Golgi apparatus, rough endoplasmic reticulum and many mitochondria.