| Talk in general about the smooth muscle tissue | Smooth muscular tissue is unvoluntary and non striated. It is composed of smooth muscle cells (leïomyocytes) associated to connective, vascular and nervous elements. These cells participate, through contraction, to the regulation of the organism major vital functions; they contribute to the composition of vessel walls, as well as digestive, respiratory, urinary, genital tracts… |
| Talk about the smooth muscle cells. | Smooth muscle cells (or fibers) are elongated fusiform (spindle-shaped) cells of variable size, depending on their localization.
Each fiber contains only one centrally located ovoid nucleus containing one or 2 nucleoli. The sarcoplasm is acidophilic and low in myoglobin content. It presents two regions: one region where the major cell organelles are confined (Golgi, REG, mitochondria) at the 2 poles of the nucleus, and the other region (the peripheral cytoplasm) which is occupied by myofilaments which organization is different from that of the SSMC filaments and that do not form sarcomeres. |
| How do smoot muscle cells organize their myofilaments? | They are grouped in irregular bundles oriented in parallel direction to the main axis of the cell: the thin myofilaments are actin filaments associated to tropomyosin. The thick myosin myofilaments are much less numerous than the thin ones. The contractile proteins (actin and myosin) bundles intertwine each other and are embedded or anchored to ovoid dense areas called dense bodies. The dense bodies contain α-actinin and are dispersed on the sarcolemma and in the sarcoplasm. |
| Talk about the sarcolemma of the smooth muscle cells. | The sarcolemma is rich in invaginations called caveolae, and is a plasma membrane surrounded by a basal lamina.
Caveolae are numerous and have a regular distribution between the dense bodies on the sarcolemma. They contain essentially Ca++ ions; they may function in a similar manner to T tubules or systems of striated muscle cells. |
| Talk about the mechanism of smooth muscle contraction | The mechanisms of contraction are however slightly different than those observed in rhabdomyocytes: the Ca++ ions released in the cytosol get fixed on a protein called calmodulin. The Ca++/calmodulin complex activates a kinase that allows myosin phosphorylation, and consequently actin- myosin interaction. |
| Talk about the innervation of smooth muscle cells. (single unit) | Innervation allows the observation of 2 types of smooth muscle:
- Single-unit (or unitary) smooth muscles: Are the most numerous. Their cells contract together in a rhythmic or peristaltic way because they are electrically coupled to each other by Gap junctions. When a muscle cell is stimulated, stimulation is spread to adjacent fibers so that the whole mass of muscle fibers contract together as a single unit. This unitary type is characterized by a relatively slow contraction.
Eg : Baby delivery. |
| Talk about the innervation of smooth muscle cells. (multi unit) | - Multi-unit smooth muscles: They are composed of discrete distinct muscle fibers that operate independently of each other. Gap junctions are rare and the fibers are stimulated by their own separate motor nerve endings each. Contraction of these muscles is tonic and rapid because stimulation leads to independent contraction of every fiber. |
| Talk about heterogeneity of smooth muscle cells. (visceral and vascular) | 1- Visceral smooth muscle cells: These correspond to the type described above. Still differences exist depending on the localization of the cells.
2- Vascular smooth muscle cells: They are localized in the tunica media of the large elastic arteries (aorta, pulmonary artery). They are different from visceral myocytes, especially concerning the nature of cytoskeletal and enzymatic proteins. |
| Talk about heterogeneity of smooth muscle cells. (myoepithelial and myoepitheloid) | 3- Myoepithelial cells: of epithelial origin, they are smooth muscle cells localized between the basement membrane and glandular cells of acini of some exocrine glands. They are rich in myofilaments and their contraction helps glandular cells expulse their secretion product.
4- Myoepitheloïd cells: They are smooth muscle cells that have undergone a particular differentiation that makes them look like epithelial and secretory cells. Their cytoplasm contains both myofilaments and secretory granules. |
| Talk about heterogeneity of smooth muscles (myofibroblast) | 5- Myofibroblasts: They are cells having an intermediate morphology between that of smooth muscle cells and that of fibroblasts. They are present in many organs, are contractile and play an important role in the healing and tissue repair process. |
