Objectives: 

To unveil possible differences in the myenteric neurotransmission between the circular and longitudinal muscular layers of human sigmoid colon

Materials: 

Macroscopically normal samples of human sigmoid colon were obtained from 6 patients (aged 60-87 yrs) who underwent surgery for rectal cancer. After removing mucosa and submucosal layers, circular muscle strips (10x4 mm) were cut with the long axis lying in the direction of the muscle fibres. Longitudinally orientated strips were taken after removing the circular muscle. In vitro contractility was recorded under basal conditions and electrical field stimulation (EFS, 0.3 ms, 350 mA, 0.2-10 Hz, for 60 s every 9 min). Statistical differences were determined by two-way ANOVA, followed by Bonferroni post hoc test.

Results: 

Isolated strips contracted rhythmically in the absence of stimuli, but circular strips had spontaneous contractions of higher frequency and lower amplitude than the longitudinal ones. In the longitudinal muscle, EFS induced contractile responses at all tested frequencies but in the circular layer 2, 5 and 10 Hz had an inhibitory character, indicating a predominant inhibitory myenteric neurotransmission to the circular muscle. In keeping with this, after EFS a significant decrease in the basal tone and spontaneous activity was only recorded in the circular muscle. Blockade of excitatory cholinergic and adrenergic innervations caused a strong reduction in EFS-induced responses of the longitudinal layer, and the residual contraction was abolished by GR94800, an inhibitor of substance P. However, in the circular layer these inhibitors had little effects and EFS-induced responses were reverted by blockade of nitrergic and purinergic inhibitory nerves.

Conclusions: 

Our results show that in aged human sigmoid colon activation of myenteric plexus causes relaxation of the circular muscle but contraction of the longitudinal layer, which will favor peristaltic reflexes.

Supported by BFU2011-24365, RETICEF , ISCIII, GR10009-JEX, FEDER.