Physiology Smooth muscles

Morphology: -Lack visible cross striations. -Actin and myosin are present. -There are dense bodies instead of Z lines. -Contain tropomyosin but toponin absent. -Poorly developed sarcoplasmic reticulum -Few mitochondria so depend on glycolysis in their metabolism.  

2 Types: -Visceral smooth muscle (unitary or single unit) 2 Types: -Visceral smooth muscle (unitary or single unit). -Multi-unit smooth muscle.   -Unitary or visceral smooth muscles (or syncytial smooth muscles): It occurs in large sheets, has low-resistance bridges between individual muscle cells, and functions in a syncytial fashion, they contract together as a single unit. It is found primarily in the walls of hollow viscera. The cell membranes are joined by gap junctions through whom ions can flow freely from one muscle cell to another.   Multi-unit smooth muscle: It is made up of individual units without interconnecting bridges. It is found in structures such as the iris of the eye, in which fine, graded contractions occur. It is not under voluntary control.  

Electrical & Mechanical Activity: Visceral smooth muscle: It is characterized by the instability of its membrane potential and by the fact that it shows continuous, irregular contractions that are independent of its nerve supply.   This maintained state of partial contraction is called tonus or tone. There is no true "resting" value for the membrane potential, but it averages about -50 mV, when the muscle active it becomes low and high during inhibition.

Superimposed on the membrane potential are waves of various types: There are -Slow sine wave-like. Sharp spikes. Pacemaker potentials. Thus, the excitation-contraction coupling in visceral smooth muscle is a very slow process compared with that in skeletal and cardiac muscle, in which the time from initial depolarization to initiation of contraction is less than 10 ms.  

Molecular Basis of Contraction; 1-Binding of Ach to Muscarinic reseptors. 2-Ca2+ influx from the ECF via Ca2+ channels. 3-Ca2+ binds to calmodulin, and the resulting complex activates calmodulin-dependent myosin light chain kinase. This enzyme catalyzes the phosphorylation of the myosin light chain. 4-The phosphorylation allows the myosin ATPase to be activated, and actin slides on myosin, producing contraction. 5-Myosin is dephosphorylated by myosin light chain phosphatase in the cell. 6-Relaxation of the smooth muscle. Dephosphorylation of myosin light chain kinase does not necessarily lead to relaxation of the smooth muscle. a latch bridge mechanism by which myosin cross-bridges remain attached to actin for some time after the cytoplasmic Ca2+ concentration falls. This produces sustained contraction with little expenditure of energy, which is especially important in vascular smooth muscle.

Stimulation of the smooth muscles: 1- Stretch: It contracts when stretched in the absence of any extrinsic innervations. Stretch is followed by a decline in membrane potential, an increase in the frequency of spikes and a general increase in tone.

Chemical mediators: 1-epinephrine or norepinephrine : The membrane potential usually becomes larger, the spikes decrease in frequency, and the muscle relaxes. Norepinephrine exerts both α and β actions on the muscle. The β action, reduced muscle tension in response to excitation, is mediated via cyclic AMP and is due to increased intracellular binding of Ca2+. The α action, which is also inhibition of contraction, is associated with increased Ca2+ efflux from the muscle cells.   2- Acetylcholin: Has an effect opposite to that of norepinephrine on the membrane potential acetylcholine causes the membrane potential to decrease and the spikes become more frequent , with an increase in tonic tension and the number of rhythmic contractions. Released by stimulation of cholinergic nerves (similar to cold and stretch in vitro). Other chemicals: like progesterone which decreases the activity and estrogen which increase it (in uterine smooth muscles). -Thermal stimuli : like cold which causes spasm.

Function of the Nerve Supply to Smooth Muscle: It has two important properties: (1) its spontaneous activity in the absence of nervous stimulation, and (2) Its sensitivity to chemical agents released from nerves locally or brought to it in the circulation.   The function of the nerve supply is not to initiate activity in the muscle but rather to modify it (control). It has dual nerve supply from 2 divisions of the autonomic nervous system. Stimulation of one division usually increases smooth muscle activity, whereas stimulation of the other decreases it. (i.e if noradrenergic increase ,the Acetylcholine decrease and visa versa).

Relation of Length to Tension; Plasticity: It is the variability of the tension it exerts at any given length. If a piece of visceral smooth muscle is stretched, it first exerts increased tension, if the muscle is held at the greater length after stretching, the tension gradually decreases. It is consequently impossible to correlate length and developed tension accurately. In intact humans, For example, the tension exerted by the smooth muscle walls of the bladder can be measured at different degrees of distention as fluid is infused into the bladder via a catheter Initially there is relatively little increase in tension as volume is increased, because of the plasticity of the bladder wall. However, a point is eventually reached at which the bladder contracts forcefully. This phenomenon is called stressrelaxation and reverse stress-relaxation. Its importance is that its ability to return to nearly its original force of contraction seconds or minutes after it has been elongated or shortened  

MULTI-UNIT SMOOTH MUSCLE: -It is nonsyncytial MULTI-UNIT SMOOTH MUSCLE: -It is nonsyncytial . -Contractions do not spread widely through it (discrete, fine and more localized). - Very sensitive to circulating chemical substances and is normally activated by chemical mediators (acetylcholine and norepinephrine). - Norepinephrine tends to persist in the muscle and to cause repeated firing of the muscle after a single stimulus rather than a single action potential. Therefore, the contractile response produced is usually an irregular tetanus rather than a single twitch. The simple muscle twitch resembles the twitch contraction of skeletal muscle except that its duration is ten times longer.