Airway Smooth Muscle

It's too late in the afternoon, I can't be bothered trying to figure out a nice introduction for this blog post. (Not like I ever do anyway.)

Describe the functional role of smooth muscle in the vascular, gastric, reproductive and urinary systems systems at a simple level.

• Arteries: Vasoconstriction, regulation of mean arterial pressure, distribution of cardiac output
• Veins: Venoconstriction, venous capacitance and cardiac filling
• Gastric: Propulsion of gut contents
• Reproductive: Contraction of the uterus pushes out the foetus
• Urinary: Contraction of bladder
• Excretory: Contraction of rectum

Recall and explain the activation of ASM.

See previous posts on smooth muscle in general:

• Physiology of Smooth Muscle
• Excitation Contraction Coupling
• Calcium Sensitisation

Mediators that activate airway smooth muscle: ACh, endothelin, LTC₄ and PGF₂

Mediators that inhibit activation of airway smooth muscle: NA, NO, VIP (vasoactive intestinal peptide) and PGE₂

Also note that unlike vascular smooth muscle, airway smooth muscle receives input from both sympathetic and parasympathetic nerves. Airway smooth muscle can also use calcium from both inside and outside of the cell (which is apparently different to other smooth muscle, such as gastric smooth muscle).

List the possible functional roles of ASM
Explain the potential function of ASM in:

We are still uncertain about the functions of airway smooth muscle. It is activated during inspiration and has phasic contractions, but the function of this is unknown. Airway resistance is decreased when ASM is inhibited, but this doesn't appear to have any beneficial effect either. There are, however, a few hypotheses about the role of ASM:

Ventilation/perfusion matching

ASM may play a role in matching ventilation to perfusion. Airways with low blood perfusion have a lower CO₂, and low CO₂ has been found to increase contraction of airway smooth muscle. Contraction of airway smooth muscle may shunt air towards areas with better perfusion. The problem with this hypothesis is that smooth muscle contraction doesn't really increase until CO₂ levels drop considerably.

Dead space regulation

Anatomical dead space, as mentioned here, here and here, consists of the conducting zone in the lungs where air passes through but is not exchanged. When we breathe in, the air that we breathe in is actually a mixture of fresh air and exhaled air from the last breath that's been sitting in the dead space. If dead space is decreased by bronchoconstriction, then the amount of rebreathed air in each breath will be reduced. The downside to this is that bronchoconstriction also increases resistance, making it harder to breathe. The optimum ratio of dead space to tidal volume is usually around 20%. It's possible that ASM can regulate the dead space volume to fit changes in tidal volume during exercise (where airways dilate, reducing resistance but increasing dead space) and other conditions.

Cough and airway stability

Even though coughing is mainly controlled by other muscles, such as the diaphragm, coughing also increases ASM tone and bronchoconstriction. While bronchoconstriction reduces air flow (the amount of volume flowing through in a given amount of time), it also increases flow velocity (which I think is the distance each of those air particles travels in a given amount of time). Increased flow velocity can increase the clearance of airways.

Another effect of an increase in ASM tone during cough is that it may also help to stabilise the airways. An increase in stiffness may reduce airway compression, which occurs during forced exhalation. (Forced exhalation occurs not only during coughing, but also crying, shouting, and so on.) A reduction in airway compression helps to keep the airways open. The overall effects of this, though, are yet to be determined.

Foetal breathing

Airway smooth muscle appears early on in foetal development. In both pig and human foetuses, airway smooth muscle shows peristaltic (i.e. contraction that moves along the length of the tube) contraction. It has been suggested that these peristaltic waves move the lung fluid and amniotic fluid into the periphery of the lung, where it is needed to help the lungs to grow. Therefore, it's possible that airway smooth muscle may have been very important in aiding lung growth in the foetus, but it may not be as useful in adults.