Bronchodilators: Beta 2-agonists and muscarinic antagonists

In obstructive lung diseases like asthma, where individuals suffer from reversible narrowing of the airways, medications like bronchodilators are helpful in keeping the airways open.

Now, based on their mechanism of action, bronchodilators can be broadly divided into four main groups; β2-agonists, muscarinic antagonists, leukotrienes antagonist and methylxanthines.

In this video, we will focus on the bronchodilators like β2-agonist and muscarinic antagonist which mimics or inhibits the regulatory effects of the autonomic nervous system on bronchial smooth muscle.

So, if we take a look at the lungs, you’ve got the trachea, which branches off into right and left bronchi, and then continues to branch into thousands of bronchioles.

In the bronchioles you’ve got the lumen, the mucosa, which includes the inner lining of epithelial cells, as well as the lamina propria which contains many cells like the type 2 helper cells, B cells, and mast cells.

Surrounding the lamina propria, there is a layer of smooth muscles and submucosa. These muscles are innervated by the nerves of the autonomic nervous system, which means they can’t be controlled consciously.

The autonomic nervous system is made up the sympathetic system which is involved in the “fight or flight” response, like running from angry raccoons, and parasympathetic system which is involved in the “rest and digest” response, like taking a nap after a big dinner.

So let’s say that racoons are chasing you, the sympathetic nerves activates and release norepinephrine which bind to β2 adrenergic receptors on the smooth muscles in the respiratory tract, causing them to relax. The diameter of the airways increase and more oxygen gets to the lungs.

When you’re resting, there’s less need for the extra oxygen, so the parasympathetic nerves release acetylcholine, which bind to muscarinic M3 receptors in the respiratory tract, causing smooth muscle contraction and narrowing the airways.