Types of Cough Reflex. The pattern of the cough reflex depends on the nature of the evoking stimulus as well as on the part of the respiratory tract stimulated. There are two or more distinctive human cough reflexes with likely different evolutionary histories. The first is the involuntary cough reflex provoked by mechanosensory-responsive Aδ fibers. It is relatively insensitive to chemical stimuli and was derived as a protective mechanism against aspirated gastric content due to the change in the position of the larynx in humans and their primate ancestors. This type of cough reflex evolved a few million to several hundred thousand years ago. An alternative possibility is what has been identified in both conscious and anesthetized guinea pigs and presumably also in humans called the "true cough receptor" as described in originating from low threshold mechanosensitive afferent nerve fibers and is activated by light punctuate mechanical stimulation and rapid changes in pH evoke cough [6, 18]. The rapid changes in pH may hold for the protection against aspiration of gastric acid. The second type of involuntary cough reflex occurs when unmyelinated vagal sensory C-fibers are activated by tissue injury, inflammation or by chemicals such as capsaicin, bradykinin, SO2, and citric acid. The chemically-sensitive cough reflex was possibly a co-evolutionary response by different animal and plant species and occurred over hundreds of millions of years.