Digestion begins with a cephalic phase that prepares the digestive system to receive food. When our brain processes visual or olfactory information about food, it triggers impulses in the cranial nerves innervating the salivary glands and stomach to prepare for food.
The cephalic phase is a conditioned or learned response to familiar foods. Our appetite or desire for a particular food modifies the preparatory responses directed by the brain. Individuals may produce more saliva and stomach rumblings in anticipation of apple pie than of broccoli. Appetite and desire are products of the hypothalamus and amygdala—brain areas associated with visceral processes and emotion. After the cephalic phase, digestion is governed by the enteric nervous system (ENS) as an unconditioned reflex. Individuals do not have to learn how to digest food; it happens regardless of whether it is apple pie or broccoli.
The ENS is unique in that it functions (mostly) independent of the brain. About 90% of the communication are messages sent from the ENS to the brain rather than the other way around. These messages give the brain information about satiety, nausea, or bloating.
The ENS, as part of the peripheral nervous system, is also unique in that it contains both motor and sensory neurons. For example, the ENS directs smooth muscle movements that churn and propel food along the digestive tract—from the esophagus to the anus. The brain, though, directs the skeletal muscles that perform conscious processes like swallowing and defecation.
Sensory ENS neurons detect changes in the stomach and intestine. Mechanoreceptors detect stretching and distension of the stomach and duodenum linings when food enters these cavities. Chemoreceptors then detect changes in the chemical composition of the chyme—like pH levels and the presence of proteins and fats. This information is used to propel each step in digestion and to coordinate with the endocrine system to release digestive hormones.
The ENS contains 200-600 million neurons and is sometimes referred to as the “little brain.”.It uses many of the same neurotransmitters as the central nervous system (CNS) and brain. Because of this, neurons in the ENS are susceptible to the same neurological insults as brain neurons, creating a link between gut disorders and neurological disorders. For example, in people with Parkinson’s disease, neurons in both the ENS and brain show similar disease symptoms such as alpha-synuclein inclusions and possibly Lewy bodies.
