Smooth muscle provides the contractile force for most of our internal organs, including the digestive tract, urinary bladder, uterus, and blood vessels. Structurally, smooth muscle cells are usually long and spindle-shaped and, unlike skeletal muscle, each cell has only a single nucleus. The actin and myosin filaments in smooth muscle are not arranged in a regular pattern like that in skeletal muscle, and the cells therefore have a "smooth" appearance, rather than the striated appearance of skeletal muscle, when viewed under a microscope.
In many organs, smooth muscle cells are arranged in sheets, with individual cells in electrical contact with one another through gap junctions. As a result, an action potential generated in one smooth muscle cell can quickly spread to all the cells in the sheet. Thus, the cells in the sheet can contract in a coordinated fashion. In the digestive tract, a coordinated, spreading wave of smooth muscle contraction will push the contents through its central lumen. This process is called peristalsis.
The muscle is anchored to a device that applies force to stretch the muscle. The force of the contraction is then measured on a chart recorder. Using this apparatus, researchers discovered that external stimuli were not necessary to activate the contractile mechanism—simply stretching the muscle was enough to trigger action potentials and contraction.
The cells of visceral smooth muscle are connected via gap junctions. These gap junctions provide an electrical coupling between the muscle cells that allows for a rhythmic wave of contraction to spread to adjacent muscle cells, and eventually across the entire tissue. This can occur even in the absence of neural stimulation.
In addition to the rhythmic self-excitation described, many smooth muscle tissues also respond to input from the autonomic nervous system. The autonomic nervous system has two components: the sympathetic division, which uses norepinephrine as a neurotransmitter, and the parasympathetic division, which uses acetylcholine as a neurotransmitter.
The isolated smooth muscle preparation can be used to study the response of intestinal smooth muscle cells to this autonomic input.
We'll first add norepinephrine to the cells. Norepinephrine decreases the activity of the cells.A wash step clears out the norepinephrine and allows us to test the effect of acetylcholine. Acetylcholine increases the activity of the cells.
The graph shown here reveals the response patterns for the two neurotransmitters. Norepinephrine decreases the activity, while acetylcholine increases the activity of the cells.
The results of this experiment show how the sympathetic and parasympathetic divisions of the autonomic nervous system work in opposition to each other, causing an increase or decrease in activity. In the digestive tract, norepinephrine hyperpolarizes muscle cells, which slows digestion, and acetylcholine depolarizes muscle cells, which facilitates digestion.
An interesting property of smooth muscle cells is that stretching alone can depolarize the smooth muscle membrane, activating the contractile mechanism. This property is important for organs such as the digestive tract, where food entering the tract will stretch the muscle cells, thereby triggering a wave of peristalsis that can push the food along the hollow core of the tract. The same rhythmic contractile properties also act to control the flow of urine from the bladder, and help expel the baby out of the uterus during childbirth.