Autonomic Nervous System Overview

Gastrointestinal Tract (GIT) Motility

• The GIT's motility is controlled by the Central Nervous System (CNS), Autonomic Nervous System (ANS), and the Enteric Nervous System (ENS)
• Phasic and tonic contractions are two types of contractions with different functions
• Phasic contractions occur rhythmically and are determined by slow waves, followed by relaxation
  • Occur in the esophagus and small intestine
• Tonic contractions are maintained contractions, not associated with slow waves
  • Occur in sphincters
  • Intensity can increase or decrease

Learning Objectives

• Describe the composition of the CNS, ANS & ENS, and their relationship.
• Differentiate between phasic and tonic contractions in the GIT, and where they occur.
• Explain the voluntary and pharyngeal stages of swallowing.
• Compare and contrast primary and secondary peristaltic movements.
• Detail the functions of the stomach, including the causes of retropulsion and pyloric spasm.
• Explain the motility of the small intestine, including segmentation and peristalsis, and the roles of hormones in regulating motility.
• Describe the main functions of the large intestine, and how mass movements occur.
• Discuss the roles of hormones in regulating gastrointestinal motility.

Neural Control of GIT Functions

• The ENS, a component of the intrinsic system, consists of two plexuses:
  • The myenteric plexus (Auerbach's plexus) which controls GIT movements.
  • The submucosal plexus (Meissner's plexus) which controls GIT secretions and local blood flow.
• The extrinsic component, the ANS, influences GIT function:
  • The parasympathetic nervous system increases motility and secretions (stimulates GIT functions)
  • The sympathetic nervous system inhibits motility and secretions.
• The ENS functions independently.

Sensory Nerve Endings

• Many sensory nerve fibers in the gut respond to stimuli like irritation, distension, and chemicals
• These stimuli may cause excitation or inhibition of intestinal movements or secretions.

Swallowing (Deglutition)

• Swallowing involves three stages:
  • Voluntary (mouth)
  • Pharyngeal
  • Esophageal
• The voluntary stage involves the tongue squeezing food posteriorly toward the pharynx.
• The pharyngeal stage involves the stimulation of swallowing receptor areas, resulting in impulses traveling to the brain stem swallowing center.
  • The palate, palatopharyngeal folds and epiglottis are involved to prevent food from entering the nasal cavities, trachea and lungs.
• The esophageal stage involves primary peristalsis initiated in the pharynx, to propel the food to the stomach via the esophagus.

Esophageal Stage

• Primary peristalsis is a continuation of the pharyngeal stage peristaltic wave that moves food from the pharynx to the stomach.
• Secondary peristalsis occurs in response to food stuck in the esophagus
• The process continues until all food that entered the esophagus is emptied into the stomach

Motor Functions of the Stomach

• The stomach stores large quantities of food.
• Mixes the food with gastric secretions to form chyme.
• Empties chyme into the small intestine.

Retropulsion

• Weak peristaltic constrictor waves (mixing waves) in the stomach
• Constrictor waves in the mid-upper stomach that move towards the antrum
• Propel food under higher pressure towards the pylorus

Pylorus

• The pylorus opening is small and only a limited amount of antral contents is expelled per peristaltic wave.
• Peristaltic wave contracts the pyloric muscle which impedes contents from emptying into the duodenum (important in the mixing process).

Motility of the Small Intestine

• The small intestine utilizes mixing contractions (segmentations) and propulsive movements (peristalsis).
  • Segmentation involves localized concentric contractions that mix chyme to increase its contact with digestive enzymes.
• Peristalsis propels chyme through the intestines.
  • The gastro-enteric and gastro-ileal reflexes enhance peristaltic activity.

Hormones and Intestinal Motility

• Certain hormones increase intestinal motility, while others inhibit it.
  • Gastrin, CCK, insulin, motilin, and serotonin increase intestinal motility
  • Secretin, Glucagon inhibit intestinal motility.

Large Intestine

• Absorption of water and electrolytes from chyme into solid feces
• Storing fecal matter until expulsion.
  • Mixing through haustrations, and
• Propulsive movements (mass movements).

Defecation

• The reflexes are transmitted by the autonomic nervous system.
• Irritation can initiate intense mass movements (e.g., in ulcerative colitis).

Pyloric Spasm

• Pyloric spasm is a condition where the pylorus becomes more constricted than normal.
  • This leads to food staying in the stomach longer than normal, which increases stomach acidity
  • This can cause vomiting and pain