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1. Smooth Muscle Logo

Smooth muscle relies on actin and myosin and the cross-bridge cycling mechanism to cause cell shortening in the same way as skeletal and cardiac muscle. However, the contractile proteins are not so highly ordered or organised and so the striations characteristic of skeletal and cardiac muscles are not seen, hence the 'smooth' appearance of the muscle when viewed under a microscope. The filaments are also rather longer which enables smooth muscle to undergo greater degrees of shortening than is the case for striated muscle. The length- active tension curve has a similar bell shaped to that of striated muscle which is good evidence that a similar sliding filament mechanism underlies shortening and force generation in smooth muscle
Micrograph of a human colonic smooth muscle cell. The cell was isolated from the circular muscle layer of the colon by proteolytic enzymes.

Figure 1 - Micrograph of a human colonic smooth muscle cell. The cell was isolated from the circular muscle layer of the colon by proteolytic enzymes.




Basic properties Top

There are several important things to remember which should help you to recognise just how different smooth muscle is from striated muscles. Smooth muscle:
  • Does not have T-tubules
  • Relies heavily on Ca2+ entering from the extracellular space (through channels) to raise cellular calcium levels and so support prolonged contraction.
  • Cells are electrically coupled by gap junctions proteins (connexins) allowing electrical activity to propagate through the tissue.
  • Has an internal Ca2+ store (sarcoplasmic reticulum) that can be released in response to some transmitter substances.
  • Can exhibit action potentials or slow wave electrical activity (which cause contraction) but smooth muscle can exist in a partially contracted state (tone) for long periods without any obvious electrical events.
  • Electrical activity may be initiated and coordinated by a specialised type of cell, the interstial cells of Cajal (ICC). These cells may serve as GI pacemakers and they have been found in arterial smooth muscle.
  • Does not have troponin complex (on actin filament of striated muscle). Instead calcium binds to Calmodulin (related structurally to troponin) which activates the enzyme myosin light chain kinase (MLCK). MLCK has to phosphorylate the light chains of myosin in order for actin and myosin to interact and begin to cycle.
  • Smooth muscle can generate similar levels of stress (force per unit area) as skeletal muscle
  • Contraction, particularly maintained contraction (tone), is very energy efficient, using only 1% of the energy of striated muscle.

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Copyright © 1998 University of Bristol. All rights reserved.
Author: Phil Langton 		Last modified: 9 Jun 1999 20:13
Authored in CALnet