17.7
Regulación neuronal
La digestión comienza con una fase cefálica que prepara al sistema digestivo para recibir alimentos. Cuando nuestro cerebro procesa información visual…
- [Narrador] Durante la primera etapa de la digestión,
la etapa cefálica, el cerebro responde a
estímulos relacionados con la información
visual u olfativa de la comida
antes de que ingrese a la boca.
Tal control de estado se lo diseñó
para preparar al sistema digestivo
al originar impulsos neutrales en los nervios craneales
que provocan que las glándulas salivares generen más saliva.
El nervio vago también se activa
y aumenta la producción de los jugos gástricos
en el estómago.
Una vez que se ingirió la comida, la digestión
se convierte en un reflejo innato
controlado por una rama del sistema nervioso autónomo
llamado sistema nervioso entérico o SNE,
el cual controla los movimientos del músculo liso
que tritura y empuja la comida a través del tracto digestivo
desde el esófago hasta el ano.
La llegada de la comida al estómago en la etapa gástrica
estira las paredes y se detecta la distensión
a través de mecanorreceptores en el SNE.
Otros receptores sensoriales, los quimiorreceptores,
detectan cambios en el pH en el estómago
mientras la comida se transforma en quimo,
una mezcla líquida de comida y jugos gástricos.
Durante la etapa intestinal,
cuando el quimo se mueve hacia el duodeno,
el comienzo del intestino delgado,
se activan receptores similares.
Tanto en la etapa gástrica como en la intestinal,
la activación los receptores del SNE se relaciona con
la secreción de hormonas digestivas, de esta manera
estableciendo una relación neuroendocrina en la digestión.
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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.