Circular smooth muscle contributes to esophageal shortening during peristalsis

Abstract

Aim: To study the angle between the circular smooth muscle (CSM) and longitudinal smooth muscle (LSM) fibers in the distal esophagus.

Methods: In order to identify possible mechanisms for greater shortening in the distal compared to proximal esophagus during peristalsis, the angles between the LSM and CSM layers were measured in 9 cadavers. The outer longitudinal layer of the muscularis propria was exposed after stripping the outer serosa. The inner circular layer of the muscularis propria was then revealed after dissection of the esophageal mucosa and the underlying muscularis mucosa. Photographs of each specimen were taken with half of the open esophagus folded back showing both the outer longitudinal and inner circular muscle layers. Angles were measured every one cm for 10 cm proximal to the squamocolumnar junction (SCJ) by two independent investigators. Two human esophagi were obtained from organ transplant donors and the angles between the circular and longitudinal smooth muscle layers were measured using micro-computed tomography (micro CT) and Image J software.

Results: All data are presented as mean ± SE. The CSM to LSM angle at the SCJ and 1 cm proximal to SCJ on the autopsy specimens was 69.3 ± 4.62 degrees vs 74.9 ± 3.09 degrees, P = 0.32. The CSM to LSM angle at SCJ were statistically significantly lower than at 2, 3, 4 and 5 cm proximal to the SCJ, 69.3 ± 4.62 degrees vs 82.58 ± 1.34 degrees, 84.04 ± 1.64 degrees, 84.87 ± 1.04 degrees and 83.72 ± 1.42 degrees, P = 0.013, P = 0.008, P = 0.004, P = 0.009 respectively. The CSM to LSM angle at SCJ was also statistically significantly lower than the angles at 6, 7 and 8 cm proximal to the SCJ, 69.3 ± 4.62 degrees vs 80.18 ± 2.09 degrees, 81.81 ± 1.75 degrees and 80.96 ± 2.04 degrees, P = 0.05, P = 0.02, P = 0.03 respectively. The CSM to LSM angle at 1 cm proximal to SCJ was statistically significantly lower than at 3, 4 and 5 cm proximal to the SCJ, 74.94 ± 3.09 degrees vs 84.04 ± 1.64 degrees, 84.87 ± 1.04 degrees and 83.72 ± 1.42 degrees, P = 0.019, P = 0.008, P = 0.02 respectively. At 10 cm above SCJ the angle was 80.06 ± 2.13 degrees which is close to being perpendicular but less than 90 degrees. The CSM to LSM angles measured on virtual dissection of the esophagus and the stomach on micro CT at the SCJ and 1 cm proximal to the SCJ were 48.39 ± 0.72 degrees and 50.81 ± 1.59 degrees. Rather than the angle of the CSM and LSM being perpendicular in the esophagus we found an acute angulation between these two muscle groups throughout the lower 10 cm of the esophagus.

Conclusion: The oblique angulation of the CSM may contribute to the significantly greater shortening of distal esophagus when compared to the mid and proximal esophagus during peristalsis.

Keywords: Circular smooth muscle; Esophageal contraction; Esophageal shortening; Gastroesophageal junction; Gastroesophageal reflux disease.

Figure 1

A photograph of a dissected cadaveric esophagus. The esophagus is cut longitudinally along the longitudinal muscle fibers after dissecting the serosa. The mucosa and sub mucosa were later dissected. The esophagus was later folded back so that the longitudinal fibers were on top of the circular muscle fibers. Measurements of the angle of the circular smooth muscle fibers with respect to the longitudinal smooth muscle fibers were made starting from the squamocolumnar junction.

Figure 2

A human esophagus obtained using micro-computed tomography. The longitudinal muscle fibers were virtually dissected to expose the underlying circular muscle fibers. Measurements were made starting from the squamocolumnar junction.

Figure 3

The average angle of the circular smooth muscle with respect to the longitudinal smooth muscle. A: On cadaveric esophagi. The x-axis represents the distance from the squamocolumnar junction with 0 being the squamocolumnar junction. At 90 degrees the circular smooth muscle and longitudinal smooth muscle are perpendicular to each other. Therefore the smaller the angle the greater the deviation from the perpendicular and the greater the resulting shortening during peristaltic contraction; B: Measured using micro computed tomography. The x-axis represents the distance from the squamocolumnar junction with 0 being the squamocolumnar junction. CT: Computed tomography.