2 - Vascular smooth muscle structure and function

Summary

INTRODUCTION

Smooth muscle has an important role in regulating the function of a variety of hollow organ systems including the: vasculature, airways, gastrointestinal tract, uterus and reproductive tract, bladder and urethra and several other systems. Smooth muscle has two fundamental roles: 1) to alter the shape of an organ and 2) to withstand the force of an internal load presented to that organ. In order to achieve these fundamental objectives smooth muscles have developed mechanisms of mechanical coupling, which enable the development of powerful and coordinated contractions at a relatively low energy cost. For example, smooth muscle in the gastrointestinal tract must undergo intermittent but coordinated phasic contractions to propel the bolus of food through the alimentary canal. Whereas in the airways and vasculature the smooth muscle is more often in various states of tonic contraction, but can be dynamically regulated to relax or contract in response to specific neuro-hormonal and haemodynamic signals. In keeping with the aims of this text, this chapter will focus on the principle mechanisms through which vascular smooth muscle functions.

SMOOTH MUSCLE (VASCULAR) STRUCTURE

Vascular smooth muscle cells have classically been envisaged as fusiform cells, on average 200 microns long × 5 microns in diameter, with a large central nucleus surrounded by an abundant array of endoplasmic reticulum and golgi apparatus, with the cytosol and plasma membrane tapering toward the poles. Although the dimensions of the vascular smooth muscle cell narrow toward their ends there is clear evidence that the end-to-end junctions coupling smooth muscle cells are complex and contain a significant number of membrane invaginations to provide increased surface area for both mechanical tight junctions and electrical coupling via gap junctions (Figure 2.1).