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THE SMOOTH MUSCLE OF THE GASTRO-INTESTINAL TRACT
CHAPTER
IX
THE SMOOTH MUSCLE OF THE GASTRO-INTESTINAL TRACT
I
H A D N O T BEEN WORKING LONG O N THE PROBLEMS of peristalsis before I became impressed with the need for learning everything possible about smooth muscle. More and more it seemed to me that if I knew just what this tissue would do under certain conditions, I could explain many of the activities of the gut. I felt still surer of this when I found how autonomous the muscle is, and how strong its tendency is to contract rhythmically, even in the absence of nervous stimuli. It would seem well, then, in this book to enumerate briefly some of the properties and peculiarities of smooth muscle. Although from now on the term "muscle" will be used, it must not be forgotten that there are nerve cells scattered among the contractile fibers, connecting them and modifying their reactions. To be exact, we should probably use the term "musculoneural ap paratus" except in those instances in which we refer to denervated muscle, but the word is long and unwieldy, and after this explanation, I shall use the shorter term "muscle." ANATOMIC AND PHYSIOLOGIC CHARACTERISTICS
As is well known, smooth muscle is made up of spindle-shaped cells which vary in size, shape, number of nuclei, and other details, in different animals and in different parts of the same animal (Ranvier, p. 433; Pompilian; Lapicque, 1905; McGill; Schultz, 1895; Botazzi and Grünbaum; Paukul; Schiefferdecker). As a rule it contracts more sluggishly than striated muscle does; it takes longer to get started, and it is slower in recovering its original length. Incidentally, the greatest difficulty in working with this type of muscle arises from the fact that one can never be sure what its original length was be176
SMOOTH MUSCLE OF GASTRO-INTESTINAL TRACT
177
cause of the constant changes in tone. After a number of strong stimuli, or sometimes after only one, the muscle may become refrac tory (Parker, 1919, p. 168; Jennings; Woodworth, p. 39), but after a long rest it seems again to get on a hair trigger so that it will respond powerfully and explosively to a slight stimulus. That is the condition of the digestive tract after a night's rest, and it probably has much to do with the fact that most of us have the daily bowel movement in the morning, immediately after breakfast. With an animal's abdo men open under a bath of salt solution, one can often start a rush wave down the bowel by pinching the duodenum. For some time afterwards similar pinches will have no effect, but if one waits long enough the bowel will again become so sensitive that the slightest stimulus will start a wave. TYPES OF SMOOTH MUSCLE
Another characteristic of smooth muscle is its ability to maintain a firm and lasting contraction without fatigue. This is seen in the muscles which close the shells of bivalves, and it is seen in the wall of the colon. It is interesting that the muscle in a bivalve consists of two functionally very different parts: one which closes the shell and the other which locks it closed. By cutting first one and then the other it can be shown that neither can do the work of the other (Parnas). Similarly, if a man of average strength tries to hold his arm out per pendicularly to his body for ten minutes, he soon finds it a painful and fatiguing experiment. The deltoid was not designed for such heavy work, but the glutei and back muscles are carrying much heavier loads all day and they do not complain (Sherrington, 1915, p. 191). One learns from this that there are all kinds of muscles, all suited to different purposes. Some, like those in the wings of insects, must contract 300 times a second; others, like those in the wings of a hen, have little to do. Those persons who think all muscle is the same forget the differences between the white and dark meats of chicken, between the heart and the gizzard, and between tenderloin, round steak and tongue. I have gone into these differences at length because I believe there are similar differences between the muscle in the cardiac and pyloric ends of the stomach (Alvarez, 1 9 1 6 a \ 1 9 1 7 a b ) , and between that
178
I N T R O D U C T I O N T O GASTRO-ENTEROLOGY
in the small intestine and that in the cecum (Alvarez and Stark weather, 1918b) and colon (Alvarez, 1918 a ). The muscle on the lesser curvature near the cardia is soft to the touch like coagulated fibrin; that in the pars pylorica is tough like gizzard and has a dif ferent color. If one stimulates the two with an electric current or with a pinch, one gets two entirely different contraction curves; and if they are put into warm oxygenated Locke's solution they show two different types of rhythmic activity. These differences should be expected when it is remembered that the upper and lower ends of the stomach have different kinds of work to do. The upper end serves largely as a hopper to hold the food, the lower as the mill to do the heavy work. More of these local peculiarities will be described later. RESPONSE TO TENSION
Another characteristic of smooth muscle in hollow organs is its responsiveness to tension. Most of the motor activities of the stomach and bowel are brought about and regulated largely by the internal pressure due to the presence of food or gas. Cannon (1911 a , p. 187) showed that during rhythmic segmentation in the small intestine the muscle fibers that are stretched tend to contract. Their contraction increases the pressure in neighboring segments, and so the process goes on. Cannon showed also that the waves in the stomach tend to appear at those places where the internal pressure balances the local tonus of the muscle. If the pressure is too little or too great there may be no waves (Cannon, 191Γ, p. 189; Straub, 1900; Wislocki and O'Con nor) . When a man has been purged, his bowels are not likely to move for a few days. This has been supposed to be due to an astringent or constipating action of the purge, but I have considerable evidence to show that it is due simply to the lack of tension in the colon. The bowel has to fill to a certain point before the muscle fibers will be stretched enough so that they will contract well. As Cannon has pointed out, these reactions to stretching are purely local and are not brought about by nervous reflexes. RESPONSE TO DIRECT IRRITATION Smooth muscle shortens under the influence of direct irritation. Thus one finds spasmodic contraction of the cardia, pylorus, ileo-
SMOOTH MUSCLE OF GASTRO-INTESTINAL TRACT
179
cecal sphincter and anus when there is ulcération or inflammation nearby. Hourglass contractions of the stomach appear opposite ulcers on the lesser curvature, and a shrunken and irritable cap is produced by ulcers of the duodenum. From the point of view of the physician anxious to make an early diagnosis in cases of cancer of the digestive tract, it is unfortunate that carcinomatous growths com monly fail to stimulate the adjacent muscle to contraction in the way that benign lesions often do. Of late much new light is being thrown on the peculiar way in which stimuli are transmitted from nerves to smooth muscle. In striated muscle every little fiber seems to have its motor nerve supply: some fibril which brings the stimuli necessary for contraction. But now some physiologists suggest that there is no such thing as a direct electric transmission of nerve impulses to smooth muscle; instead there is transmission through chemical substances which are formed at the ends of the nerves (Rosenblueth and Cannon; Rosenblueth, Davis and Rempel). As a result, in smooth muscle, a stimulus arriv ing by way of any one nerve fiber can touch off contraction of a fairly large mass of muscle. The studies of Rosenblueth (1932) indicated that, when smooth muscle is stimulated through a nerve, a quantal nervous impulse liberates a quantal amount of a chemical mediator which may be called "M." This M combines with some substance, H, in the muscle according to the reaction, M + H <=± MH. Free M is destroyed locally, and as it disappears, the muscle relaxes. Because the process of destruction of M takes place slowly, there often is time for some of it to diffuse into the surrounding structures. With this new con cept the problems of spatial and temporal summation of effects can now be attacked more intelligently. One can now understand why the type of response of a smooth muscle can depend on the number of nerve impulses delivered in a unit of time, regardless of the number of nerve fibers stimulated. Bozler, however, has brought forward some evidence in favor of the view that smooth muscle is directly excitable by an electric cur rent. He argued that if it were influenced only indirectly through a nerve net, it wouldn't make any difference how the current went through, but actually, Bozler found the threshold for electric stimuli more than twenty times higher for a current passing at right angles
180
I N T R O D U C T I O N T O GASTRO-ENTEROLOGY
to the fibers than for one passing longitudinally in the direction of the fibers. Actually some histologists have demonstrated a fine nervous network, fibers from which reach every muscle fiber in the wall of the bowel. Boeke (1932) made a valuable study of the anatomy and the in nervation of smooth muscle. In some animals many of the fibers are not pointed at the ends but have club-shaped, rounded heads. Sometimes one fiber will penetrate another as if the other were folded about it. In young hedgehogs he could easily study the de velopment of separate fibers from a syncytial mass. The smooth muscle fibers showed a fibrillar structure, exceedingly fine myofibrillae lying immersed in a granular myoplasm. There are peculiar boundary fibrils on the surface of the muscle fibers. Authors differ as to the innervation of smooth muscle fibers. Some think there are practically no nerves to them, while a few histologists have found a rich plexus of nerves. Boeke found in some preparations a rich plexus in which several nerve fibers go to each muscle fiber. This plexus is hard to stain and probably this is why many observers have failed to see it. Even in one preparation parts will show the fibers and other parts will not. These very fine nerve fibers have been found surrounding the ganglion cells of the plexus of Auerbach. Boeke thought that those investigators who have been able to demonstrate the rich nervous plexus must be more nearly right than were those who failed, which is logical. The so-called interstitial cells appear to be part of this conducting system. Evans (1926) stated that there is no drug which has an action on smooth muscle like that of curare on striated muscle. In order to block the effects of vagai stimulation, atropine has to be used in doses so large that it probably then injures the muscle. I found that nico tine, even in fatal doses, had no effect on the motor nerve endings in the bowel, and according to Evans, cocaine actually increases the sensitiveness of smooth muscle to sympathetic excitation. There are many other puzzles, such as the facts that the uterine muscle is unresponsive to electric stimulation during anestrus, and its excitability can be restored by injecting the animal with theelin. More needs to be learned about the metabolism of smooth muscle. Prasad found that isolated intestinal muscle in the presence of oxygen
SMOOTH MUSCLE OF GASTRO-INTESTINAL T R A C T
181
oxidized about 1 mg. of carbohydrate per gram per hour. In the process lactic acid was produced. Sodium iodo-acetate in a 1:10,000 concentration interfered with the utilization of glucose by the muscle (Prasad, 193 5 a ' b ). A slowing up of the movements of a bit of intes tinal muscle did not appear to be due to an accumulation of acid but to an exhaustion of the labile carbohydrate store. Fatigue in smooth muscle appears to affect first the contractile tissue and not the myoneural junction as in striated muscle. Fischer (1944), in a big review article, stated that after eighteen years of research there is but little precise knowledge in regard to smooth muscle. One reason is that there are so many different types of muscle included under this term. What may be true of one type may not be true of another. The muscles of the iris, the retractor penis, the arteries and prob ably the nictitating membrane are free of nerve cells. Large parts of the uterus also do not contain nerve cells. In the muscle layer of the gut a few ganglion cells are scattered between the muscular elements, but their number is so small that the preparation of strips practically ganglion-free is possible for physiologic experiments. However, one must always check later with a histological examination. Smooth muscle devoid of any nerv ous tissue is found in cultures of embryonic gut. Complicating the problem of studying smooth muscle is the fact that the behavior of this tissue varies so markedly with the seasons, or in the two sexes, or with pregnancy. Even in the case of the muscles of the gallbladder and of the intestinal tract, the influence of sex hormones has been demonstrated. The pilomotors are unexcitable electrically after denervation. The nerve-free muscle of the chick amnion, however, is electrically ex citable, but strong currents are needed. Carey (1940) studied the microscopic lines of contracture that appear in some sections of smooth muscle taken from the active bowel of an animal. He showed that in the contracted regions the nuclei are short and thick and round while in the stretched regions in between they are pulled out so that they are long and narrow. Carey spoke of differences between contractions which he believed were traveling and those that appeared to be stationary. It would have been interesting to see if the distances between contractions
182
I N T R O D U C T I O N T O GASTRO-ENTEROLOGY
were shorter in the jejunum and longer in the ileum. The lines of contraction were sometimes parallel and sometimes interlacing, sug gesting the presence of a syncytium. Evidently, even in these micro scopic bits of tissue, seen with a magnification of 200 times or more, there must have been some form of conduction of an impulse to contract. Further work should be done with this technic. Those who would like to go more fully into the study of the physi ology of smooth muscle can gain entrée to the subject by reading the encyclopedic articles of Grützner (1904), Schultz (1895, 1903), Evans (1926), and Trendelenburg (1917 a ). There will be found much about latent periods, refractory periods and all such details. SUMMARY
The student of intestinal activity should know something about the behavior and properties of smooth muscle. Smooth muscle has no basic length, and its tonus is always changing. When contracted it may be only a fourth as long as when relaxed. There are different types of smooth muscle designed for different types of work. Smooth muscle in the several parts of the digestive tract has different prop erties suited to the function to be performed. Some can be distin guished by touch or sight, and some by chemical analysis or physio logic behavior. Differences in rhythmic activity in different parts of the digestive tract appear to be due largely to differences in the neuromuscular structure in the several regions. Smooth muscle probably has no nerve supply in the sense that voluntary muscle has, and it is probable that impulses are transmitted from the nerves to the muscle largely through chemical substances which are formed at the ends of the nerves. There are many peculiar features about the way in which smooth muscle responds to nerve stimulation, and much remains to be learned about the phenomenon. Carey (1940) studied the curious contraction bands observed in microscopic sections of smooth muscle. Boeke (1932) and others have found that under unusually favor able conditions a rich plexus of fine nerve fibers can be demonstrated around every smooth muscle fiber. The "interstitial" cells apparently belong to this conducting system.
IX
THE SMOOTH MUSCLE OF THE GASTRO-INTESTINAL TRACT
I
H A D N O T BEEN WORKING LONG O N THE PROBLEMS of peristalsis before I became impressed with the need for learning everything possible about smooth muscle. More and more it seemed to me that if I knew just what this tissue would do under certain conditions, I could explain many of the activities of the gut. I felt still surer of this when I found how autonomous the muscle is, and how strong its tendency is to contract rhythmically, even in the absence of nervous stimuli. It would seem well, then, in this book to enumerate briefly some of the properties and peculiarities of smooth muscle. Although from now on the term "muscle" will be used, it must not be forgotten that there are nerve cells scattered among the contractile fibers, connecting them and modifying their reactions. To be exact, we should probably use the term "musculoneural ap paratus" except in those instances in which we refer to denervated muscle, but the word is long and unwieldy, and after this explanation, I shall use the shorter term "muscle." ANATOMIC AND PHYSIOLOGIC CHARACTERISTICS
As is well known, smooth muscle is made up of spindle-shaped cells which vary in size, shape, number of nuclei, and other details, in different animals and in different parts of the same animal (Ranvier, p. 433; Pompilian; Lapicque, 1905; McGill; Schultz, 1895; Botazzi and Grünbaum; Paukul; Schiefferdecker). As a rule it contracts more sluggishly than striated muscle does; it takes longer to get started, and it is slower in recovering its original length. Incidentally, the greatest difficulty in working with this type of muscle arises from the fact that one can never be sure what its original length was be176
SMOOTH MUSCLE OF GASTRO-INTESTINAL TRACT
177
cause of the constant changes in tone. After a number of strong stimuli, or sometimes after only one, the muscle may become refrac tory (Parker, 1919, p. 168; Jennings; Woodworth, p. 39), but after a long rest it seems again to get on a hair trigger so that it will respond powerfully and explosively to a slight stimulus. That is the condition of the digestive tract after a night's rest, and it probably has much to do with the fact that most of us have the daily bowel movement in the morning, immediately after breakfast. With an animal's abdo men open under a bath of salt solution, one can often start a rush wave down the bowel by pinching the duodenum. For some time afterwards similar pinches will have no effect, but if one waits long enough the bowel will again become so sensitive that the slightest stimulus will start a wave. TYPES OF SMOOTH MUSCLE
Another characteristic of smooth muscle is its ability to maintain a firm and lasting contraction without fatigue. This is seen in the muscles which close the shells of bivalves, and it is seen in the wall of the colon. It is interesting that the muscle in a bivalve consists of two functionally very different parts: one which closes the shell and the other which locks it closed. By cutting first one and then the other it can be shown that neither can do the work of the other (Parnas). Similarly, if a man of average strength tries to hold his arm out per pendicularly to his body for ten minutes, he soon finds it a painful and fatiguing experiment. The deltoid was not designed for such heavy work, but the glutei and back muscles are carrying much heavier loads all day and they do not complain (Sherrington, 1915, p. 191). One learns from this that there are all kinds of muscles, all suited to different purposes. Some, like those in the wings of insects, must contract 300 times a second; others, like those in the wings of a hen, have little to do. Those persons who think all muscle is the same forget the differences between the white and dark meats of chicken, between the heart and the gizzard, and between tenderloin, round steak and tongue. I have gone into these differences at length because I believe there are similar differences between the muscle in the cardiac and pyloric ends of the stomach (Alvarez, 1 9 1 6 a \ 1 9 1 7 a b ) , and between that
178
I N T R O D U C T I O N T O GASTRO-ENTEROLOGY
in the small intestine and that in the cecum (Alvarez and Stark weather, 1918b) and colon (Alvarez, 1918 a ). The muscle on the lesser curvature near the cardia is soft to the touch like coagulated fibrin; that in the pars pylorica is tough like gizzard and has a dif ferent color. If one stimulates the two with an electric current or with a pinch, one gets two entirely different contraction curves; and if they are put into warm oxygenated Locke's solution they show two different types of rhythmic activity. These differences should be expected when it is remembered that the upper and lower ends of the stomach have different kinds of work to do. The upper end serves largely as a hopper to hold the food, the lower as the mill to do the heavy work. More of these local peculiarities will be described later. RESPONSE TO TENSION
Another characteristic of smooth muscle in hollow organs is its responsiveness to tension. Most of the motor activities of the stomach and bowel are brought about and regulated largely by the internal pressure due to the presence of food or gas. Cannon (1911 a , p. 187) showed that during rhythmic segmentation in the small intestine the muscle fibers that are stretched tend to contract. Their contraction increases the pressure in neighboring segments, and so the process goes on. Cannon showed also that the waves in the stomach tend to appear at those places where the internal pressure balances the local tonus of the muscle. If the pressure is too little or too great there may be no waves (Cannon, 191Γ, p. 189; Straub, 1900; Wislocki and O'Con nor) . When a man has been purged, his bowels are not likely to move for a few days. This has been supposed to be due to an astringent or constipating action of the purge, but I have considerable evidence to show that it is due simply to the lack of tension in the colon. The bowel has to fill to a certain point before the muscle fibers will be stretched enough so that they will contract well. As Cannon has pointed out, these reactions to stretching are purely local and are not brought about by nervous reflexes. RESPONSE TO DIRECT IRRITATION Smooth muscle shortens under the influence of direct irritation. Thus one finds spasmodic contraction of the cardia, pylorus, ileo-
SMOOTH MUSCLE OF GASTRO-INTESTINAL TRACT
179
cecal sphincter and anus when there is ulcération or inflammation nearby. Hourglass contractions of the stomach appear opposite ulcers on the lesser curvature, and a shrunken and irritable cap is produced by ulcers of the duodenum. From the point of view of the physician anxious to make an early diagnosis in cases of cancer of the digestive tract, it is unfortunate that carcinomatous growths com monly fail to stimulate the adjacent muscle to contraction in the way that benign lesions often do. Of late much new light is being thrown on the peculiar way in which stimuli are transmitted from nerves to smooth muscle. In striated muscle every little fiber seems to have its motor nerve supply: some fibril which brings the stimuli necessary for contraction. But now some physiologists suggest that there is no such thing as a direct electric transmission of nerve impulses to smooth muscle; instead there is transmission through chemical substances which are formed at the ends of the nerves (Rosenblueth and Cannon; Rosenblueth, Davis and Rempel). As a result, in smooth muscle, a stimulus arriv ing by way of any one nerve fiber can touch off contraction of a fairly large mass of muscle. The studies of Rosenblueth (1932) indicated that, when smooth muscle is stimulated through a nerve, a quantal nervous impulse liberates a quantal amount of a chemical mediator which may be called "M." This M combines with some substance, H, in the muscle according to the reaction, M + H <=± MH. Free M is destroyed locally, and as it disappears, the muscle relaxes. Because the process of destruction of M takes place slowly, there often is time for some of it to diffuse into the surrounding structures. With this new con cept the problems of spatial and temporal summation of effects can now be attacked more intelligently. One can now understand why the type of response of a smooth muscle can depend on the number of nerve impulses delivered in a unit of time, regardless of the number of nerve fibers stimulated. Bozler, however, has brought forward some evidence in favor of the view that smooth muscle is directly excitable by an electric cur rent. He argued that if it were influenced only indirectly through a nerve net, it wouldn't make any difference how the current went through, but actually, Bozler found the threshold for electric stimuli more than twenty times higher for a current passing at right angles
180
I N T R O D U C T I O N T O GASTRO-ENTEROLOGY
to the fibers than for one passing longitudinally in the direction of the fibers. Actually some histologists have demonstrated a fine nervous network, fibers from which reach every muscle fiber in the wall of the bowel. Boeke (1932) made a valuable study of the anatomy and the in nervation of smooth muscle. In some animals many of the fibers are not pointed at the ends but have club-shaped, rounded heads. Sometimes one fiber will penetrate another as if the other were folded about it. In young hedgehogs he could easily study the de velopment of separate fibers from a syncytial mass. The smooth muscle fibers showed a fibrillar structure, exceedingly fine myofibrillae lying immersed in a granular myoplasm. There are peculiar boundary fibrils on the surface of the muscle fibers. Authors differ as to the innervation of smooth muscle fibers. Some think there are practically no nerves to them, while a few histologists have found a rich plexus of nerves. Boeke found in some preparations a rich plexus in which several nerve fibers go to each muscle fiber. This plexus is hard to stain and probably this is why many observers have failed to see it. Even in one preparation parts will show the fibers and other parts will not. These very fine nerve fibers have been found surrounding the ganglion cells of the plexus of Auerbach. Boeke thought that those investigators who have been able to demonstrate the rich nervous plexus must be more nearly right than were those who failed, which is logical. The so-called interstitial cells appear to be part of this conducting system. Evans (1926) stated that there is no drug which has an action on smooth muscle like that of curare on striated muscle. In order to block the effects of vagai stimulation, atropine has to be used in doses so large that it probably then injures the muscle. I found that nico tine, even in fatal doses, had no effect on the motor nerve endings in the bowel, and according to Evans, cocaine actually increases the sensitiveness of smooth muscle to sympathetic excitation. There are many other puzzles, such as the facts that the uterine muscle is unresponsive to electric stimulation during anestrus, and its excitability can be restored by injecting the animal with theelin. More needs to be learned about the metabolism of smooth muscle. Prasad found that isolated intestinal muscle in the presence of oxygen
SMOOTH MUSCLE OF GASTRO-INTESTINAL T R A C T
181
oxidized about 1 mg. of carbohydrate per gram per hour. In the process lactic acid was produced. Sodium iodo-acetate in a 1:10,000 concentration interfered with the utilization of glucose by the muscle (Prasad, 193 5 a ' b ). A slowing up of the movements of a bit of intes tinal muscle did not appear to be due to an accumulation of acid but to an exhaustion of the labile carbohydrate store. Fatigue in smooth muscle appears to affect first the contractile tissue and not the myoneural junction as in striated muscle. Fischer (1944), in a big review article, stated that after eighteen years of research there is but little precise knowledge in regard to smooth muscle. One reason is that there are so many different types of muscle included under this term. What may be true of one type may not be true of another. The muscles of the iris, the retractor penis, the arteries and prob ably the nictitating membrane are free of nerve cells. Large parts of the uterus also do not contain nerve cells. In the muscle layer of the gut a few ganglion cells are scattered between the muscular elements, but their number is so small that the preparation of strips practically ganglion-free is possible for physiologic experiments. However, one must always check later with a histological examination. Smooth muscle devoid of any nerv ous tissue is found in cultures of embryonic gut. Complicating the problem of studying smooth muscle is the fact that the behavior of this tissue varies so markedly with the seasons, or in the two sexes, or with pregnancy. Even in the case of the muscles of the gallbladder and of the intestinal tract, the influence of sex hormones has been demonstrated. The pilomotors are unexcitable electrically after denervation. The nerve-free muscle of the chick amnion, however, is electrically ex citable, but strong currents are needed. Carey (1940) studied the microscopic lines of contracture that appear in some sections of smooth muscle taken from the active bowel of an animal. He showed that in the contracted regions the nuclei are short and thick and round while in the stretched regions in between they are pulled out so that they are long and narrow. Carey spoke of differences between contractions which he believed were traveling and those that appeared to be stationary. It would have been interesting to see if the distances between contractions
182
I N T R O D U C T I O N T O GASTRO-ENTEROLOGY
were shorter in the jejunum and longer in the ileum. The lines of contraction were sometimes parallel and sometimes interlacing, sug gesting the presence of a syncytium. Evidently, even in these micro scopic bits of tissue, seen with a magnification of 200 times or more, there must have been some form of conduction of an impulse to contract. Further work should be done with this technic. Those who would like to go more fully into the study of the physi ology of smooth muscle can gain entrée to the subject by reading the encyclopedic articles of Grützner (1904), Schultz (1895, 1903), Evans (1926), and Trendelenburg (1917 a ). There will be found much about latent periods, refractory periods and all such details. SUMMARY
The student of intestinal activity should know something about the behavior and properties of smooth muscle. Smooth muscle has no basic length, and its tonus is always changing. When contracted it may be only a fourth as long as when relaxed. There are different types of smooth muscle designed for different types of work. Smooth muscle in the several parts of the digestive tract has different prop erties suited to the function to be performed. Some can be distin guished by touch or sight, and some by chemical analysis or physio logic behavior. Differences in rhythmic activity in different parts of the digestive tract appear to be due largely to differences in the neuromuscular structure in the several regions. Smooth muscle probably has no nerve supply in the sense that voluntary muscle has, and it is probable that impulses are transmitted from the nerves to the muscle largely through chemical substances which are formed at the ends of the nerves. There are many peculiar features about the way in which smooth muscle responds to nerve stimulation, and much remains to be learned about the phenomenon. Carey (1940) studied the curious contraction bands observed in microscopic sections of smooth muscle. Boeke (1932) and others have found that under unusually favor able conditions a rich plexus of fine nerve fibers can be demonstrated around every smooth muscle fiber. The "interstitial" cells apparently belong to this conducting system.