PHYSIOLOGY OF SMOOTH MUSCLE
Smooth muscle cells are spindle shaped. They have one centrally placed nucleus per cell and are usually organized in small clusters of cells. Fine collagenous, reticular connective tissue separates each cell and the bundles.
One way to distinguish smooth muscle from striated muscle is the absence of the regular pattern of sarcomeres (no A, I-bands or Z lines). Smooth muscle cells also have only one nucleus. In that sense, they are like cardiac muscle cells.
Smooth muscle cells can be distinguished from connective tissue by their organized appearance. They are usually in a homogeneous bundle or sheet of cells rather than scattered single cells.
TYPES OF SMOOTH MUSCLE
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups:
- SINGLE-UNIT (UNITARY) SMOOTH MUSCLE
- The autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by junctions to neighbouring cells such that the whole bundle or sheet contracts as a syncytium.
- They are in sheets lining the walls of hollow organs.
- The smooth muscle cells may be organized in 2-3 layers. One layer may run circularly around the lumen; another layer may be running longitudinally along the length of the organ. In the stomach, there is a layer that runs more diagonally or obliquely.
- Each of these works together to propel food along the organ, or in the case of the stomach, churn the food.
- MULTIUNIT SMOOTH MUSCLE
- Innervate individual cells; as such, they allow for fine control and gradual responses, much like motor unit recruitment in skeletal muscle.
- Rarely possess gap junctions, and thus are not electrically coupled. As a result, contraction does not spread from one cell to the next, but is instead confined to the cell that was originally stimulated.
- Stimuli come from autonomic nerves or hormones but not from stretching.
- Found around large blood vessels, in the respiratory airways, and in the eyes.