Special sensory proteins called “receptors” are found on the surface of some cells that line the upper respiratory tract.2 Locations in the body where these receptors are found include the back of the throat, the trachea (windpipe), and the upper bronchi where the trachea branches to the lungs. When these receptors are stimulated, they send a signal to sensory nerve fibers, such as those found in the vagus nerve.2 The sensory fibers transmit information to the brain for interpretation. The part of the brain that monitors the throat and upper airway region has been called the “cough center.” When the cough center receives a signal that the lining of airway is irritated the lining of the airway, muscles in the throat and chest receive action signals that trigger the cough mechanism, which is a 3-part process:4
- First, a volume of air is inhaled.
- Second, the opening to the trachea (the epiglottis) closes as the chest constricts, compressing the air within the lungs.
- Third, the epiglottis opens, allowing a rapid burst of air to be expelled through the mouth.
During a cold, mucus production in the airways increases, and coughing is how the body expels excess mucus from accumulating in the airways and lungs. Cough also can result from another common cold symptom, postnasal drip.2 Increased nasal secretions can run down the back of the throat and irritate sensitive cough touch points, creating the feeling that you need to clear your throat. This type of irritation also signals the brain to trigger the cough reflex. For many people, postnasal drip is especially annoying at night.
There are different ways that nonprescription products can help to relieve a cough and chest congestion (Table 1). Centrally acting cough suppressant medications are so named because they can act directly on the brain’s cough center.2 The most common of these is dextromethorphan (DM), which is found in many over-the-counter cough remedies. It can help reduce the sensitivity and activity of the cough center and prevent it from triggering a cough. Another class of ingredients is expectorants like guaifenesin, which help to make mucus thinner and easier to clear from the airways.2 The soothing action of a liquid cough medications can also play a role in their ability to provide relief. Liquid formulations can have a high viscosity, or thickness, can coat the throat and help reduce the irritation that is signaling the brain’s cough center.4
An understanding of the anatomy of cough and the nature of the cough reflex is an important part of knowing how to best provide relief.