Smooth and Cardiac Muscles tissue

Smooth Muscles Tissue

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 smooth muscle cell is 3-10 µm thick and 20-200 µm long. The cytoplasm is homogeneously eosinophilic and consists mainly of myofilaments. The nucleus is located in the center and takes a cigar-like shape during contraction. The cell membrane forms small pouch-like invaginations into the cytoplasm (caveolae) which are functionally equivalent to the T-tubules of the skeletal musculature. The smooth muscle cells are anchored to the surrounding connective tissue by a basal lamina.

The smooth muscle fibers group in branching bundles. As opposed to skeletal muscle fibers. These bundles do not run strictly parallel and ordered but consist in a complex system. Thus the cells can contract much stronger than striated musculature. The actin filaments are stretched between dense bodies in the cytoplasm and attachment plaques at the cell membrane. The myosin filaments lie between the actin filaments. Furthermore intermediate filaments such as desmin and vimentin support the cell structure.

Functions Smooth Muscles tissue

Smooth musculature is found in (almost) all organ system such as hollow organs (e.g. stomach, bladder), in tubular structures (e.g. vessels, bile ducts), in sphincters, in the uterus, in the eye etc. In addition it plays an important role in the ducts of exocrine glands. It fulfills various tasks such as sealing orifices (e.g. pylorus, uterine os) or the transport of the chyme through wavelike contractions of the intestinal tube. On the one hand smooth muscle cells contract slower than skeletal muscle cells, on the other hand they are stronger, more sustained and require less energy.

Myofibroblasts represent a special type of smooth muscle cell which additionally have qualities of fibrocytes. They produce connective tissue proteins such as collagen and elastin for which reason they are also referred to as fixed (or stationary) connective tissue cells. Myofibroblasts are found, among others, in alveolar septa of the lung and scar tissue.

Cardiac Muscles

Cardiac Layers
 

Endocardium

Inner layer of the heart, contains blood vessels. Has 3 sub-layers.

1. Endothelium - innermost portion a simple squamous epithelium.
2. Smooth Muscle and Connective Tissue - middle layer of the endocardium is mix of connective tissue and smooth muscle.
3. Subendocardial Layer - outer layer of the endocardium is loose connective tissue joining the endocardium and myocardium.

Myocardium

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

Epicardium

1. Outer layer of the heart, contains blood vessels and lymphatics.
2. Visceral layer of pericardium rather thin.

Intercalated Discs

1. Seen in longitudinal sections.
2. Connect the individual muscle cells.
3. Permit the conduction of electrical impulses between the cells.
   

Histology "step-like" appearance due to

• Transverse part - crossing fibres at right angle to myofibrils.
• Lateral part - runs in parallel to myofibrils.

Junctional Components

• Fascia adherens – major portion of transverse component. Anchoring sites for actin, and connect to the closest sarcomere.
• Macula adherens – (desmosomes) transverse and lateral components. Bind individual myocytes to one another. stop separation during contraction by binding intermediate filaments, joining the cells together.
• Gap junctions - lateral component. Allow action potentials to spread between cardiac cells by passage of ions between cells, producing depolarization of the heart muscle. Allows muscle to act as syncytium.

Macula adherens junctions are also called desmosomes.

Purkinje fibres

Modified cardiac muscle cells. Compared to ordinary cardiac muscle cells:

1. Contain large amounts of glycogen.
2. Fewer myofibrils.
3. Thicker cellsextend from the atrioventricular node, pierces the fibrous body, divides into left and right bundles, and travels, beneath the endocardium, towards the apex of the heart.
4. Bundle branches contact cardiac muscle cells through specialisations similar to intercalated discsconduct stimuli faster than ordinary cardiac muscle cells (2-3 m/s vs. 0.6 m/s) discovered in 1839 by Jan Evangelista Purkyně).