In physiology, a stimulus (plural stimuli) is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity.When a stimulus is applied to a sensory receptor, it normally elicits or influences a reflex via stimulus transduction.he endocrine system is affected largely by many internal and external stimuli. One internal stimulus that causes hormone release is blood pressure. Hypotension, or low blood pressure, is a large driving force for the release of vasopressin, a hormone which causes the retention of water in the kidneys.
Macrophages can be activated by cytokines such as interferon-gamma (IFN-gamma) and bacterial endotoxins, such as lipopolysaccharide (LPS). Activated macrophages undergo many changes which allow them to kill invading bacteria or infected cells.
Activation of pepsinogen in the stomach. The peptidase in the stomach is pepsin. Pepsin works optimally in the acidic environment of the stomach, being active at pH 2-3, but becoming inactivated when the pH is above 5. The chief cells at the base of the gastric glands secrete the zymogen, which is called pepsinogen.Activation of pepsinogen starts when hydrocholoric acid (HCl) , which is secreted by the parietal cells partially activates pepsinogen (pepsinogen* in figure).This partially active enzyme then cleaves the peptide from other pepsinogen molecules to form active pepsin.he chief cells at the base of the gastric glands secrete the zymogen, which is called pepsinogen. Activation of pepsinogen starts when hydrocholoric acid (HCl) , which is secreted by the parietal cells partially activates pepsinogen (pepsinogen* in figure).