An investigation into the cholinergic receptor control of gastric muscle activity through the action of neurotransmitters
Introduction
Different types of neurotransmitters have different effects on the activity of the smooth muscle. Neurotransmitters exert their effects on smooth muscle through binding to different types of neurotransmitter receptors present in smooth muscles. Smooth muscles are involuntary muscles in the body that do not have the ability of controlling their own movements. Smooth muscles are unstriated because they lack the visible cross striations available in other types of muscles like the skeletal muscles (KCUMB, 2012).
Smooth muscles are small in diameter 2-10mm. The muscle fibers that make the single unit muscle are complied together into sheets. Smooth muscles are found lining the walls of the hollow organs like esophagus, bronchi, stomach and the walls of blood vessels. Since smooth muscles are involuntary, they stimulated by some involuntary neurogernic impulses. These impulses are mediated by neurotransmitters. The slow, rhythmical contractions of smooth muscles are the ones used in the control of internal organs like intestines (KCUMB, 2012).
Nerve stimulation of muscle fiber causes depolarization. Excitation of smooth muscles is closely followed by contraction. The excitation, contraction-coupling system is called electromechanically coupling. Calcium ions are the link in the electromagnetic coupling. Upon smooth muscle excitation, calcium ions permeate the extracellular space into the intracellular water of smooth muscles causing contraction. Smooth muscles contract slowly and are not prone to fatigue (KCUMB, 2012).
The depolarization phase of smooth muscle results in an increase in, calcium ions permeability. An influx of calcium ions into the smooth muscle triggers a release of calcium ions from intracellular stores that result in the rise of calcium concentrations. This rise in calcium ion concentrations is responsible for triggering of contractile activity. When calcium is stored in intracellular stores, the muscle relaxes (Scharrer et al, 2011).
Neurotransmitters exert their effect on muscle contraction and relaxation through binding to smooth muscles receptors. Such receptors include alpha and beta-adrenergic receptors, cholinergic receptors, and cholinergic receptors. Other receptors that neurotransmitters bind in smooth muscles are muscarinic and nicotinic receptors (Scharrer, et al, 2011).
Objective of the experiment
The purpose of this experiment was investigating the function of cholinergic receptor control on the activity of a gastric smooth muscle an earthworm through the action of neurotransmitters. The experiment also investigated the differences in rate of excitation of gastric smooth muscles of earthworms by different neurotransmitters.
Results
After studying the effects of the various neurotransmitters on smooth muscle activity, it was observed that the contractile activity of the earthworm gastric smooth muscle occurred in a series of heightened contractions followed by relaxations. Under normal conditions, the earthworm smooth muscle contracted three times in a minute. When carbachol neurotransmitter was used on the earthworm gastric muscle, it caused contractions on the circular and the longitudinal muscle strips of the earthworm gastric muscle. These contractions were higher than the contractions recorded on normal conditions in earthworm gastric muscle.
When Atropine was used together with carbachol in the experiment, there was an increase in the number of muscle contractions higher than the contractions recorded in normal conditions. The use of carbachol and theophylline neurotransmitters together, in the experiment resulted in a decline in the number of muscle contractions than those recorded on normal conditions of the smooth muscle. Adrenaline also resulted in a decline in the number of muscle contractions in the smooth muscle at rates lower than the contractions in normal conditions.
Table showing the Effects of carbachol, theophylline, and adrenalin
Frequency of contractions per minute
Different neurotransmitters have different effects on the smooth visceral muscles. In the experiment, the use of carbachol resulted in a slight increase in the number of contractions of the smooth muscle from the earthworm. The contractions of the smooth muscle cells induced by carbachol consisted both of an initial phase of activation followed by a spontaneous relaxation phase. These results observed in the experiment are consistent with the mode of action of carbachol neurotransmitter.
Carbamylcholine is a cholinomimetec drug that binds and activates the receptor for acetylcholine. It is therefore, a cholinergic agonist. Carbamylcholine also stimulates both nicotinic and muscarinic receptors. Activation of choline receptors results in more binding of acetylcholine in the receptors, which is a potent neurotransmitter that increases muscular contractions. The binding of acetylcholine in the cell membrane receptors results in the opening of the calcium channels in the cells resulting in polarization, when calcium binds to calmodulin, the cells contracts (Scharrer et al, 2011).
The use of both carbachol and atropine neurotransmitters resulted in a slight increase in muscle contractions. These results are not consistent with the mode of action of atropine and carbachol. Although carbachol is a neurotransmitter that activates acetylcholine receptors eventually resulting in the increase of contractions in muscle, atropine cancels this effect (KCUMB, 2012).
Atropine is an antagonist that competes for the muscarinic acetylcholine receptor Atropine is an anti-cholinergic neurotransmitter. The use of both atropine and carbachol should not result in an increase in muscular contractions unless the concentration of the carbachol is more than that of atropine (KCUMB, 2012). Atropine should cancel out the effect of the carbachol and the muscle of the earthworms should only record few contractions.
The use of carbachol and theophylline together resulted in a decrease in the number of muscle contractions. Although carbachol is cholinergic neurotransmitter that enhances muscular contractions, its effects are cancelled by the presence of theophyline. Theophyline is also a competitive phosphodiesterase inhibitor. Inhibition of phospohodiesterase results in the raising of the intracellular cyclic AMP levels in the body. Carbachol can induce smooth muscle contractions in smooth muscle, but this effect is cancelled by theophyline. Theophyline cause an increase in the level of cyclic AMP levels in the body that causes muscle relaxation (Scharrer et al, 2011).
The use of adrenaline in the experiment reduced the number of muscular contractions of the muscle fiber from the earthworm. These results are consistent with the mode of action of adrenalin. According to Scharrer et al, (2011), Adrenalin is both a hormone and a neurotransmitter that binds to adrenergic receptors. Adrenalin is therefore, a non-selective agonist of the adrenergic receptors. Binding of Adrenaline triggers many physiological changes in the body.
The Binding of adrenalin to smooth muscles adrenergic receptors results in the relaxation of smooth muscles. Binding of adrenalin to smooth muscles also results in the activations of adenyl cyclase in the inner surfaces of cell membranes and the production of cyclic AMP. Cyclic AMP triggers relaxation in smooth muscle (Scharrer et al, 2011).
Earthworm gastric muscle is ideal for study because it has some distinctive features like non-striation. Earthworm gastric muscle contract spontaneously and are controlled by neurotransmitters and hormones. Another advantage of the use of earthworm gastric smooth muscle in the experiment is that the muscle does not require a heated tissue chamber to maintain temperature since earthworms are not warm-blooded animals (Aamodt, 2001).
Earthworms do not require lengthy approvals for use by the bodies that approve animals for use in research since they are invertebrates. The animals are also not costly. Another advantage of the use of earthworms is that earthworm gastric intestinal smooth muscles of earthworms are similar to mammalian smooth muscles. These muscles can therefore shed light on the properties and physiology of mammalian smooth muscles (Aamodt, 2001).
Conclusion
Smooth visceral muscles of the earthworm are ideal for the study of the smooth muscle physiology. This experiment was successful and it was possible to observe the differences in neurotransmitter function on smooth muscles depending on the receptors they bind on smooth muscles. The major problem experienced in the experiment is that tissue preparation was very challenging because the gut tissue of earthworms is very hard. This hardness made some sections of the gut tissues tear during the stretching process. This problem can be avoided in the future through careful preparation of the earthworm muscles to avoid tearing the tissues.
The experiment could not be completed with only one earthworm because the earthworms were slippery and motile during the preparation steps. This made the dissection process take time than required. However, the only inconsistent reports obtained in the experiment were those using atropine and carbachol neurotransmitters. The experiment can be enhanced through the careful consideration of each preparation stage in the experiment to minimize errors during the experiment.
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