Smooth Muscle

Our second topic in our Muscle section of Veterinary Physiology 1 deals with smooth muscle. This post will be discussing the major structural and functional characteristics of the various types of smooth muscle and how these compare with skeletal muscle. I will also explain the sequence of electrical, chemical and structural events which occur when smooth muscle contracts. 

Structure and Function

Smooth muscle is usually found in the walls of various hollow tubes found in the body. This includes places such as the gastrointestinal tract, blood vessels and airways. The contraction of smooth muscle changes the shape of the organ which squeezes its contents along. Compared to skeletal and cardiac muscle, smooth muscle contractions are:

• slower
• can be sustained for longer
• and require less energy per unit of force

Smooth muscle cells are small and spindle shaped. They have a single central nucleus and contain bundles of actin and myosin filaments when extend diagonally. Smooth muscle cells don't have any T-tubules or distinct striations and the cells aren't separated from each other. When the cells contract they become a globular shape. 

The Structure of Smooth Muscle
source:http://commons.wikimedia.org/wiki/File%3ASmooth_muscle_contraction1.png
Please see this website if you'd like to use this picture

  Mechanisms of Contraction 

Excitation:

The trigger for contraction is the elevation of Ca 2+ levels which mainly involves Ca 2+ entry from the extracellular fluid. This occurs via ligand, voltage or mechanically gated channels and is initiated by many different signals. These signals include:

• spontaneous activity of pacemaker smooth muscle cells
• the stretch of smooth muscle cells
• neurally released transmitters
• hormones or paracrines (locally generated chemicals)

Excitation-Contraction Coupling:
This involves 6 steps:

1.  Ca 2+ ions enter the cell through gated ion channels
2. Ca 2+ entry triggers Ca 2+ release from the Sarcoplasmic Reticulum (SR)
3. Ca 2+ binds to calmodulin (CaM)
4. Ca 2+-CaM activates MLCK (Myosin light chain kinase)
5. MLCK phosphorylates myosin light chains resulting in increased myosin ATPase activity
6. Activated myosin starts crossbridge cycling

Note how smooth muscle contraction is regulated by myosin where as skeletal muscle contraction is regulated by actin.    

Contraction:

Smooth muscle has the slowest contraction and relaxation times of all the muscle types. This is due to the more basic structure of smooth muscle. However, smooth muscle can sustain its contractions for longer than the other muscle types. 

Contraction in smooth muscle follows the basic sliding filament action that is seen in skeletal muscle. However, there are some differences between smooth and skeletal muscle contraction:

• Actin and myosin filaments are longer in smooth muscle
• Smooth muscle myosin filaments have no bare zone. This allows them to slide along actin over a longer distance.  
• Smooth muscle myosin has a slower rate of crossbridge cycling and this leads to a slower contraction phase.

Relaxation:


In smooth muscle, relaxation involves 4 steps:

1. Ca 2+ is pumped out of the cell or into the SR, decreasing cytoplasmic Ca 2+
2. Ca 2+ unbinds from calmodulin, inactivating MLCK
3. Myosin phosphatase removes phosphate from myosin, decreasing myosin phosphatase activity
4. Reduced myosin ATPase activity results in less crossbridge cycling.

Note that unlike skeletal muscle, removing intracellular Ca2+ is not enough to stop contraction. Dephosphorylated myosin or actin forms a stable 'latch' state in smooth muscle, this means that detachment takes time. This is why smooth muscle can contract for long periods of time without fatigue.


And that's it :) If you have any questions please feel free to ask.