<P> The digestive system can respond to external stimuli, such as the sight or smell of food, and cause physiological changes before the food ever enters the body . This reflex is known as the cephalic phase of digestion . The sight and smell of food are strong enough stimuli to cause salivation, gastric and pancreatic enzyme secretion, and endocrine secretion in preparation for the incoming nutrients; by starting the digestive process before food reaches the stomach, the body is able to more effectively and efficiently metabolize food into necessary nutrients . Once food hits the mouth, taste and information from receptors in the mouth add to the digestive response . Chemoreceptors and mechanorceptors, activated by chewing and swallowing, further increase the enzyme release in the stomach and intestine . </P> <P> The digestive system is also able to respond to internal stimuli . The digestive tract, or enteric nervous system alone contains millions of neurons . These neurons act as sensory receptors that can detect changes, such as food entering the small intestine, in the digestive tract . Depending on what these sensory receptors detect, certain enzymes and digestive juices from the pancreas and liver can be secreted to aid in metabolism and breakdown of food . </P> <P> Intracellular measurements of electrical potential across the membrane can be obtained by microelectrode recording . Patch clamp techniques allow for the manipulation of the intracellular or extracellular ionic or lipid concentration while still recording potential . In this way, the effect of various conditions on threshold and propagation can be assessed . </P> <P> Positron emission tomography (PET) and magnetic resonance imaging (MRI) permit the noninvasive visualization of activated regions of the brain while the test subject is exposed to different stimuli . Activity is monitored in relation to blood flow to a particular region of the brain . </P>

Give an immediate plant response to a stimulus