Gut Motility




What factors control the motility of smooth muscles in the earthworm gut? Unlike striated muscle in which the contractile proteins are in a parallel arrangement along the length of the muscle, the contractile proteins of smooth muscles have an irregular arrangement. This has several important consequences:

The individual muscle cells that compose smooth muscle are arranged into functional groups that are connected internally by gap junctions to allow rapid communication and integration. The control of smooth muscle contraction is significantly more complex than that of skeletal muscle and is under both hormonal and neural control. During this week's lab you will investigating the role of two neurotransmitters in regulating contraction in the crop gizzard of an earthworm.

The gut has two layers of smooth muscle.

Alternate contractions in these two layers create a wave of contraction that moves down the gut to push food toward the anus. Like cardiac muscle, smooth muscle is myogenic (i.e. contracts autonomically) and can create spontaneous waves of depolarization that result in waves of contraction.

Cells in the gut produce serotonin, a neurotransmitter that can act on the smooth muscles directly as well as stimulate nerve cells. It is one of the two neurotransmitters you will be studying. The other is acetylcholine (ACh). One of its functions is to stimulate gut contraction. There are also neurotransmitters that inhibit gut contraction. From this, it should be obvious that regulation of contraction in the crop gizzard is complex. As with striated muscle, calcium ions play an important but indirect role in the regulation of muscle contraction. They move into the cell through proteins in the cell membrane or sarcoplasmic reticulum known as calcium channels.

Let's briefly examine the role that ACh plays in muscle contraction.

ACh is released from a nerve terminal and attaches to a receptor in the cell membrane. What is the condition of the calcium channel at this time?

ACh stimulates a G protein to activate another protein located in the cell membrane. This protein produces 2 different second messengers.

How do the second messengers affect the calcium channels? Click here to examine their condition before the secondary messangers arrive.

Calcium levels rise.

At this point, we will take a slight digression to review the structure of one of the contractile proteins, myosin. What are the three major parts of the heavy chains of myosin?

How many light chain myosin molecules are associated with each of the heavy chains? The head of the myosin is capable of attaching to actin and causing contraction, but must be activated by the presence of a phosphate group being added to the light chain. For this reason, the light chain myosin is considered a regulator protein.

How does an increase in the calcium level in a cell lead to the attachment of a phosphate to the light chain? Click to find out. Explain what you discovered in the box below.

How does the muscle relax? Explain what you discovered in the box below.

Calcium is also involved in another pathway that affects the binding properties of actin through an entirely different series of proteins not outlined above.

From this, it should be clear that the regulation of smooth muscle is very complex and there is still more to learn about the proteins and pathways involved.

Experimental Design Question:

You decide to test the effects of different concentrations of ACH on the strength of smooth muscle contraction. You will be using 5 different concentrations. What statistical test will you use to evaluate you results? Statistics Guide Use the back button to return to this document.

To learn more about the roles of ACh and serotonin read the paper by Krajniak and Klohr (1999) which is available from the Biology 109 Moodle page.