September 12, 2007

Smooth Muscles: Slow and Steady

Unlike sprinters, marathoners need to be slow and steady to win a race. Likewise, in our body, there are muscles that work almost untiringly. These muscles, called smooth muscles, lack striations, so characteristic of skeletal muscles (the muscles that we see on our and Arnold Schwarzenegger's body). Smooth muscles are involuntary in nature and are found surrounding blood vessels, viscera (hollow organs like stomach, intestine, uterus, ureters etc); in the muscles of the iris and many other places. They may be of two types: single unit or unitary (also known as visceral smooth muscle) and multiunit smooth muscle. In the former variety, the cells are arranged in sheets, juxtaposed to each other. Moreover, there are many canal like connections between the cells. These connections called gap junctions, allow cytosols of adjacent cells to be in direct communication with each other, via ion channels. Thus an impulse occurring in one cell, excites its immediate neighbors, allowing the impulse to propagate. Hence, visceral smooth muscles may be construed as a functional syncitium (continuous).

The visceral smooth muscle are always ready for a job and they are always on their toes. This is manifested in the form of 'tone', a form of semi contracted state. Thus, they are in a position to both relax or further contract, as the situation demands. They are wicked as a naughty child and they do not have a resting membrane potential. The resting potential always vary, and averages -50 milli volts, approximately; the interior of the cell being negative. There is a kind of electrical storm (quite like solar flares), from adjoining structures, which makes it further difficult to measure their potentials. Its also hard to attribute its length. When you stretch it, a tension develops in it, which dies out after some time and the muscle remains elongated. This flexible plastic like behavior has led to the term 'plasticity', in regard to this phenomenon seen in smooth muscles. They are capable of contracting when stretched. This is essential in peristalsis (forward propulsion), seen in the intestines or ureters, as they propel food and urine respectively.

Unlike their skeletal muscle counterparts, which use mitochondria for their energy needs, they draw their energy from glycolysis, for mitochondria are relatively scanty in them. In response to a stimulus, calcium ions (Ca++) enter the smooth muscle cells, from extra cellular fluid. (This is in direct contrast to skeletal muscles, where calcium ions come from within the cell, from an organelle called sarcoplasmic reticulum, which is poorly developed in smooth muscles). Once inside, it binds to a molecule called calmodulin within the cell, to form calcium-calmodulin complex. This complex activates calmodulin dependent myosin light chain kinase. As is expected, this kinase phosphorylates myosin light chains, which results in enhanced ATPase activity. The cells contract. Strangely, undoing of this phosphorylation by phosphatases, such as myosin light chain phosphatase, does not relax the muscle immediately. This is because when these muscles contract, a gear like mechanism, called 'latch bridge' occurs that allows it to remain contracted for some time. Thus we can see the plentiful ways smooth muscles adopt in order to conserve energy.

Things are very different in smooth muscles that can not be described in simplistic terms of classical cells.
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