17.7
Regulação Neural
A digestão começa com uma fase cefálica que prepara o sistema digestivo para receber alimentos. Quando o nosso cérebro processa informações visuais ou…
Durante a primeira parte da digestão, a fase cefálica, o cérebro responde a estímulos relacionados, como informações visuais ou olfativas sobre comida antes mesmo de entrar na boca. Tal controle condicionado é projetado para preparar o sistema digestivo ao disparar impulsos neurais nos nervos cranianos que inervam glândulas salivares para produzirem mais saliva. O nervo vago também é ativado e aumenta a produção de sucos gástricos no estômago Uma vez que a comida é engolida, a digestão torna-se um reflexo não condicionado governado por um ramo do sistema nervoso autônomo chamado de sistema nervoso entérico, ou SNE, que governa os movimentos do músculo liso que agita e impulsiona a comida ao longo do trato digestivo, do esôfago ao ânus.
A chegada da comida no estômago durante a fase gástrica estica as paredes e a distensão é detetada por mecanorrecetores no SNE. Outros recetores sensoriais, quimiorrecetores, detetam mudanças no pH do estômago à medida que a comida é transformada em quimo, uma mistura líquida de alimentos e sucos gástricos. Durante a fase intestinal, quando o quimo se move para o duodeno, o início do intestino delgado, recetores semelhantes são ativados.
Em ambas as fases gástrica e intestinal, a ativação de recetores SNE está conectada à secreção de hormônios digestivos, estabelecendo um relacionamento neuroendócrino na digestão.
View the full transcript and gain access to JoVE Core videos
Q1: What happens during the cephalic phase of digestion?
During the cephalic phase, the brain responds to visual or olfactory information about food before it enters the mouth. This triggers neural impulses in cranial nerves that stimulate salivary glands to produce more saliva and activate the vagus nerve to increase gastric juice production in the stomach. The cephalic phase is a conditioned response modified by appetite and desire, which are controlled by the hypothalamus and amygdala.
Q2: How does the enteric nervous system control digestion after food is swallowed?
Once food is swallowed, the enteric nervous system (ENS) governs digestion as an unconditioned reflex. The ENS directs smooth muscle movements that churn and propel food along the digestive tract through monogastric digestion and peristalsis. Unlike the cephalic phase, this process occurs automatically regardless of food type and does not require conscious learning or brain involvement.
Q3: What role do mechanoreceptors and chemoreceptors play in the gastric and intestinal phases?
Mechanoreceptors detect stretching and distension of the stomach and duodenum walls when food arrives. Chemoreceptors detect changes in pH and chemical composition of chyme, the liquid mixture of food and gastric juices. This sensory information from both receptor types is used to coordinate digestive steps and signal the release of digestive hormones through a neuroendocrine relationship.
Q4: Why is the enteric nervous system sometimes called the little brain?
The enteric nervous system contains 200-600 million neurons and uses many of the same neurotransmitters as the central nervous system and brain. Because of these similarities, ENS neurons are susceptible to the same neurological insults as brain neurons, creating links between gut disorders and neurological disorders like Parkinson's disease.
Q5: How does the enteric nervous system communicate with the brain?
The enteric nervous system functions mostly independent of the brain, with approximately 90% of communication flowing from the ENS to the brain rather than the reverse. These messages provide the brain with information about satiety, nausea, bloating, and other digestive states, establishing a bidirectional gut-brain axis that influences overall digestive regulation.
Q6: What is the difference between conscious and unconscious digestive processes?
The brain directs skeletal muscles for conscious processes like swallowing and defecation. The enteric nervous system, however, directs smooth muscle movements and sensory detection throughout the digestive tract automatically. This division allows voluntary control over entry and exit of food while the ENS handles involuntary propulsion and chemical processing.
Q7: How does appetite influence the cephalic phase of digestion?
Appetite and desire, controlled by the hypothalamus and amygdala, modify preparatory responses during the cephalic phase. Individuals produce more saliva and experience stomach rumblings in anticipation of preferred foods like apple pie compared to less desirable foods like broccoli. This learned response demonstrates how emotional and visceral brain areas personalize digestive preparation.