Smooth muscle contraction mechanism

2021-06-03 03:07 PM

Another important property of smooth muscle, especially that of the single visceral smooth muscle of many hollow organs, is its ability to return to nearly its original force of contraction a few seconds or minutes after it is broken. lengthen or shorten.


Smooth muscle has actin and myosin filaments, chemically like the actin and myosin filaments of skeletal muscle. However, it does not contain troponin complex, which controls muscle contractions in skeletal muscle, so the mechanisms that control muscle contractions are of two different types. This issue will be explained in more detail later.

Studies have shown that actin and myosin filaments in smooth muscle act as the mechanism of skeletal muscle. Specifically, this process is activated by Ca ions, and adenosine triphosphate (ATP) is broken down to adenosine diphosphate (ADP) to provide energy for muscle contraction.

However, there are major differences between smooth muscle structure and skeletal muscle, such as the difference between muscle excitation-contraction, Ca channelling, maintenance of muscle contraction, and energy expended by muscle contraction.

Structural features

The arrangement of actin and myosin filaments is different between smooth muscle and skeletal muscle. Under the electron microscope, many actin filaments are bundled together by dense bodies. Some dense bodies are attached to cell membranes, others are in muscle cells. The dense bodies on the cell membrane are attached to each other via intracellular protein bridges, which are the basis for transmitting muscle contraction forces between cells.

Interspersed between the actin filaments are myosin filaments. Myosin filaments are twice the diameter of actin filaments. Under the electron microscope, the number of actin filaments is 5-10 times that of myosin filaments.

Figure. The physical structure of smooth muscle. The filament in the upper left show’s actin filaments radiating from dense bodies. The filaments at the bottom left and right demonstrate the relationship of myosin filaments to actin filaments.

The figure on the right depicts a smooth muscle unit. Many actin filaments radiate from dense bodies. The ends of the actin filaments overlap the myosin filaments. This structure is similar to striated muscle but does not follow the rules of striated muscle. In fact, dense bodies play a similar role to the Z-discs of striated muscle.

Another difference is that myosin filaments have "side polar" bridges arranged so that each side attaches to one side of the bridge. This arrangement allows the myosin filaments to pull one actin filament in one direction and the other actin filaments in the opposite direction. This allows smooth muscle fibres to shorten by 80% of their total length while in skeletal muscle by less than 30%.

Compare smooth and skeletal muscle contractions

While skeletal muscle contracts and relaxes very quickly, skeletal muscle contracts very slowly, which can last for hours or even days. Therefore, it is clear that the structure and composition between the two types of muscles will be different.

Slow Myosin Bridge

The velocity of Myosin bridges in smooth muscle is the attachment and release of actin filaments, and reattachment and release, many times slower than in skeletal muscle. In fact, this frequency in skeletal muscle is 1/10 to 1/300. However, it is this slow attachment that increases the force of contraction in smooth muscle. Another reason for this slow binding is that smooth muscle Myosin bridges use less ATP energy.

The energy required to maintain muscle contraction is low

This energy requirement is only 1/10 to 1/300 of that of skeletal muscle because the binding and release of myosin to actin use only 1 ATP, no matter how long the smooth muscle contraction lasts. This plays an important role in total body energy because organs such as intestines, bladder, gall bladder… muscle contraction almost continuously.

Time from stimulation to contraction and elongation

In general, this period is from 0-100 milliseconds, maximum contraction after 0.5 seconds, stretching for 1-2 seconds, total time 1-3 seconds, 30 times longer than skeletal muscle. However, this stimulation time depends on the type of smooth muscle, it can be from 0.2-30 seconds depending on the type. This is explained by slow Myosin bridges and slower response to Ca ions than skeletal muscle.

The maximum contraction force of smooth muscle is many times stronger than that of skeletal muscle

Although smooth muscle has fewer myosin fibres and because of slow Myosin bridges, the maximum contraction force of smooth muscle is many times stronger than that of skeletal muscle. For example, for the same cm2, smooth muscle can pull 4-6kg while skeletal muscle can only pull 2-3kg. This is due to the ability to maintain the bond between actin and myosin filaments.

The latching mechanism facilitates prolonged smooth muscle contractions. Once the smooth muscle has developed full contraction, the amount of continued stimulation can often drop to much lower than the initial level even though the muscle still maintains full contractile force. Furthermore, the energy consumed to sustain contraction is usually very small, sometimes as little as 1/300 of the energy required for equivalent sustained skeletal muscle contraction. This mechanism is called the “latching mechanism”. The importance of the latching mechanism is that it can sustain prolonged contractions in smooth muscle for many hours with little use of energy. Fewer further excitatory signals are required from nerve fibres or hormonal sources. Tension-Relaxation of smooth muscle. Another important property of smooth muscle, especially the single visceral smooth muscle of many hollow organs, is its ability to return to approximately its original force of contraction seconds or minutes after it has been lengthened or shortened. For example, a sudden increase in the volume of fluid in the urinary bladder, thereby stretching the smooth muscle in the bladder wall, causes an immediate large increase in pressure in the bladder. However, over the next 15 seconds or so, despite continued stretching of the bladder wall, the pressure returned almost exactly back to its original level. Then, when the volume is increased by another step, the same effect occurs again. In contrast, when the volume drops suddenly, the pressure drops sharply at first but then increases over a few seconds or minutes to or near the initial level. These phenomena are known as stress relaxation and reverse stress relaxation. Their importance is that, except in the short term,