Basic GI Motility Mechanisms

Overview
  • This page discusses some general principles regarding the physical basis and basic patterns of alimentary tract motility. The most basic concept is that contraction of muscle is the physical basis of all motility within the GI tract and that this muscular contraction is locally coordinated by the enteric nervous system with sizable modulation by the autonomic nervous system.
Muscular Geometry
  • Depending on how muscles are arranged within the GI tract wall, their contraction results in different patterns of motility. In general, muscular bundles in the alimentary tract are arranged in either a circular or longitudinal pattern. Circular arrangements are similar to that of a ring around the alimentary tract and contraction reduces the diameter of the segment. Longitudinal arrangements run length-wise down the long axis of the segment and contraction reduces the length of the segment.
Muscular Subtypes
  • Overview
    • Both skeletal and smooth muscle cells contribute to GI Motility, but do so in different segments of the tract.
  • Skeletal Muscle:
    • GI motility mediated by striated, skeletal muscle is only found at either termini of the alimentary tract: upper third of the esophagus, pharynx, and mouth as well as the outer anus.
    • Like all striated muscle, motility in these segments is controlled by motor neurons.
  • Smooth Muscle:
    • The remainder of the GI tract is controlled by smooth muscle which is not under conscious control. These segments are controlled by the myenteric plexus of the enteric nervous system although their action can be modulated by input from the autonomic nervous system.
Muscular Coordination
  • Overview:
    • Coordination of muscular contraction within the GI tract is locally performed by the myenteric plexus of the enteric nervous system. However, local coordination by the enteric nervous system can be tuned to fit the larger physiological requirements of the body through substantial efferent input by the autonomic nervous system as described in Autonomic GI Neural Control. Whatever the source of nervous control, contraction of smooth muscle cells within the GI tract can either occur tonically or occur rhythmically.
  • Tonic Contraction:
    • Tonic contraction may last several hours and is usually observed in sphincters which separate different anatomic segments of the alimentary tract.
  • Rhythmic Contraction:
    • Rhythmic contractions are characterized by alternating periods of contraction followed by relaxation. This is the pattern of contraction typically observed in most sections of the GI tract and is responsible for propulsion and mixing of the GI contents. The nervous regulation of rhythmic contractions is described in more detail in Electrical Basis of GI Rhythmic Contractions.