This type of drug reduces the tone of smooth muscle in the lungs’ BRONCHIOLES and therefore increases their diameter. Such drugs are used in the treatment of diseases that cause bronchoconstriction, such as ASTHMA and BRONCHITIS. As bronchiolar tone is a balance between sympathetic and parasym pathetic activity, most bronchodilators are either B2 receptor agonists or cholinergic receptor antagonists – although theophyllines may also be useful. (See Autonomic Nervous System).