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Movements of Small Intestine, Function: These waves propel the…

- - - - 2 types
        
        eccentric,
        
        concentric contractions.
        
        Therefore, by
        ‘rhythmic segmental contractions’,
        
        the food is divided(segmented)
        
        and mixed together with digestive juices thoroughly (again and again)
        
        and finally chyme is formed.
    - - These contractions are initiated by pacemaker cells which are located
        in 2nd part of duodenum near the entry of common bile duct. A basic
        electrical rhythm of ‘slow wave’ can be demonstrated which is
        conducted caudally by the longitudinal muscle layer; coordination of
        this rhythm is achieved by Myenteric plexus.
        
        The ‘frequency’ of their contractions is directly related to the
        frequency of the ‘slow waves’ initiated by pacemaker cells within the
        wall of the small intestine. It is not influenced by either neural activity
        or circulating hormones.
        
        The ‘strength’ of the contractions is proportional to the frequency of
        the spike generated by the ‘slow waves’. This frequency is controlled
        by the amplitude of the ‘slow wave’. Thus, the greater the ‘slow wave’
        amplitude, the greater the frequency of spike generated, and the
        greater the strength of the contraction. ‘Slow wave’ amplitude is increased by GIT hormones released during digestion e.g. gasrtin, ‘CCK-PZ’ and motilin; whereas secretin and glucagon decrease the slow wave amplitude.
        
        5-HT released during contractions make the smooth muscle sensitive to
        distension. Distension of a short segment causes proximal segment
        contraction and distal segment relaxation.
  - - - 2.‘Peristalsis’ is a coordinated reaction in which a wave of contraction
        preceded by a wave of relaxation passes down a hollow viscus.
        
        The
        contents of viscus as they are propelled along would always enter a segment which had actively relaxed and enlarged to receive them.
        
        It can occur in either direction from a stimulated point but it normally
        dies out rapidly in oral direction, while continuing for considerable
        distance towards the anus.
        
        This is called Polarity of the intestine or
        Law of the Gut.
        
        4.Peristalsis always proceeds from the proximal to the distal end of the
        GIT.
  - - - Along with segmental contraction,
        permits longer contact of the chyme
        with the enterocytes and promotes absorption.
- - - - Peristaltic contractions are usually superimposed upon rhythmic
        segmental contractions.
        Both of these contractions can occur in the
        absence of extrinsic innervation but are dependent upon the integrity
        of myenteric nerve plexus. Therefore,
        
        strong emotions by activating parasympathetic component via
        vagus nerve, increase muscular contraction and increase the tone
        of the small intestine; and
        
        anger, fear and pain by activating sympathetic component via
        splanchnic nerve, decrease muscular contraction and decrease the
        tone of the small intestine.