Clase 46 Fisiología Gastrointestinal - Propulsión y Mezcla Parte 2 (2/2) (IG:@doctor.paiva)

Understanding Gastrointestinal Physiology: Propulsion and Mixing

Overview of Intestinal Movements

• The class focuses on gastrointestinal physiology, specifically the second part concerning propulsion and mixing in the intestines.

• Discussion includes movements of the small intestine, colon, and defecation processes.

Types of Movements in the Small Intestine

• The small intestine consists of three parts: duodenum, jejunum, and ileum, ending at the ileocecal valve which separates it from the large intestine.

• Two main types of contractions occur simultaneously: mixing contractions (segmentation) and propulsive contractions.

• Mixing contractions are localized concentric contractions that segment food to enhance mixing with intestinal secretions.

Propulsive Movements

• Propulsive movements consist of peristaltic waves traveling at 1 to 2 cm/s; they are stronger in proximal regions.

• These waves move chyme through the small intestine at a net speed of approximately 1 cm/min, taking about 3 to 5 hours for chyme to travel from pylorus to ileocecal valve.

Regulation of Intestinal Activity

• Intestinal motility is regulated by neural and hormonal signals; distension in both duodenum and stomach stimulates peristalsis via gastroenteric reflexes.

• Hormones such as gastrin, cholecystokinin (CCK), insulin, motilin, and serotonin promote intestinal motility while secretin and glucagon inhibit it.

Reflexes Affecting Peristalsis

• The gastroileal reflex facilitates passage through the ileocecal valve when food enters the stomach.

• Increased irritation or inflammation can lead to heightened peristalsis known as "peristaltic rush," often seen during infectious diarrhea.

Structural Adaptations for Absorption

• Muscular contraction forms folds in the intestinal mucosa increasing surface area for absorption due to local nervous reflexes responding to chyme presence.

Ileocecal Valve Functionality

• The ileocecal valve prevents reflux from the cecum into the ileum; it can withstand pressures up to 50–60 cm water column.

Functions and Movements of the Colon

Overview of Colon Functions

• The primary functions of the colon include water and electrolyte absorption, which occurs mainly in the proximal part, and storage of fecal matter for later evacuation through defecation, primarily in the distal section.

Types of Movements in the Colon

• The movements of the colon are categorized into propulsion movements and mixing movements. Mixing movements involve circular muscle contractions occurring every 25 centimeters along the colon.

• Longitudinal muscle contractions, known as teniae coli, create sac-like pouches called haustra that facilitate mixing and movement within the colon.

Haustral Contractions

• Each haustrum reaches its peak intensity within approximately 30 seconds before fading over a span of 60 seconds. This process helps organize and slowly push fecal content through the large intestine for proper absorption.

Impact of Motility on Feces Consistency

• Reduced motility leads to increased water absorption from feces, resulting in harder stools and potential constipation. Conversely, increased motility results in less water absorption, leading to softer stools or diarrhea.

Propulsive Movements

• While haustral contractions assist with fecal propulsion, mass movements are more effective at moving feces through the colon. These occur when there is irritation or distension in the colon due to stool presence.

Defecation Process

Mechanism of Defecation

• Defecation involves eliminating waste through the rectum. The rectum typically does not contain feces due to a weak functional sphincter between it and the sigmoid colon.

Sphincter Control During Defecation

• Two anal sphincters control defecation:

• The internal anal sphincter (involuntary control via autonomic nervous system).

• The external anal sphincter (voluntary control via somatic nervous system).

Reflexes Involved in Defecation

• The defecation reflex begins when stool enters the rectum causing distension. This triggers signals through intrinsic neural pathways that relax both anal sphincters.

Reinforcement of Reflex Action

Understanding the Defecation Reflex and Its Mechanisms

The Role of Nervous System in Defecation

• The pelvic nerves play a crucial role in enhancing the enteric reflex, leading to effective defecation by stimulating parasympathetic responses.

• Skeletal motor nerves are involved in controlling both internal and external anal sphincters, with parasympathetic stimulation facilitating relaxation for defecation.

• The intrinsic enteric reflex is triggered by rectal distension, activating peristaltic waves in the descending colon and rectum while relaxing the internal anal sphincter.

Interaction Between Reflexes

• Both intrinsic enteric reflexes and parasympathetic responses work synergistically to enhance bowel movements during defecation.

• Signals from the rectum travel to the spinal cord, initiating additional reflexes such as deep inspiration and abdominal muscle contraction, which aid in pushing fecal matter downwards.

Voluntary Control During Defecation

• When an individual feels ready to defecate, they can voluntarily relax the external anal sphincter while also stimulating the defecation reflex through deep breathing techniques.

• Deep inhalations lower the diaphragm and contract abdominal muscles, increasing intra-abdominal pressure to facilitate fecal movement towards the rectum.

Additional Reflexes Affecting Intestinal Activity

• Beyond basic defecation mechanisms, other autonomic reflexes can influence intestinal activity significantly.

• Important nervous reflexes include peritoneo-intestinal (due to peritoneal irritation), nephro-intestinal (from kidney irritation), and vesico-intestinal (from gallbladder irritation).

Implications of Irritation on Intestinal Function