the external muscle layer of the mammalian esophagus consists of   striated muscle fibers , which extend from the pharyngoesophageal junction to the thoracic or abdominal portion [ 13 ] . this is in contrast to the tunica muscularis of the stomach , small intestine , and large intestine which are constituted entirely of smooth muscle .   in humans and cats , the upper and lower portions of the esophagus are composed of striated and smooth muscle fibers , respectively , with mixed composition of these fibers in the middle portion .   in dogs , ruminants , and rodents including mice , rats , and hamsters , these striated muscle fibers were hitherto considered as  classical  skeletal muscle fibers , innervated exclusively by excitatory vagal efferents that arise from motor neurons originating in the nucleus ambiguus and terminate on motor endplates [ 46 ] . the major function of the esophagus is propulsion of swallowed food into the stomach by peristalsis .   to accomplish effective peristalsis , contraction of each muscle needs to be well coordinated [ 8 , 9 ] . the peristaltic motility of small and large intestines is mainly regulated by the intrinsic nervous system , which consists of intrinsic sensory neurons , interneurons , and motor neurons that project into the smooth muscle layers [ 8 , 1012 ] . in contrast , it has been shown that peristalsis in the esophageal  skeletal  muscle is controlled by the swallowing center in the medulla oblongata through a mediation of extrinsic vagus nerves [ 7 , 1316 ] . however , recent studies have revealed that intrinsic neurons also play roles in regulating the motility of the esophageal striated muscle . morphological studies have shown that esophageal striated muscle receives dual innervation from both vagal motor fibers originating in the brainstem and varicose intrinsic nerve fibers originating in the myenteric plexus , which is called  enteric coinnervation  of esophageal motor endplates [ 3 , 17 ] . we have also provided evidence that a local neural reflex pathway in the esophagus , consisting of primary afferent neurons and myenteric neurons , can modulate motility of the striated muscle portion of the esophagus [ 1821 ] . thus , both extrinsic and intrinsic neural regulations would be important in coordinating the peristaltic motility of the esophagus . in addition to the neural mechanisms , contractile properties of smooth muscles are beneficial in coordinating motility of small and large intestines . for instance , relatively long duration time of each contraction enables summation of contractions , which in turn results in effective compression of the intraluminal space [ 2224 ] . furthermore , coupling of neighboring cells by gap junctions would be essential to adjust the timing of muscle contractions . on the other hand , skeletal muscle fibers are insulated from each other   and thus may be unfavorable for coordinated motility .   to compensate the disadvantages of striated muscle we speculated that the striated muscle of the esophagus also has specific properties to establish coordinated peristaltic motility . hence , the aim of the present study was to clarify the contractile properties of esophageal muscle with focus on the similarities to cardiac muscle .   for this purpose , we compared the mechanical responses of isolated preparations from the esophagus , soleus muscle , and heart in rats . male wistar rats ( rattus norvegicus , 12 weeks of age and weighing 200400  g ) were used for the experiments . they were maintained in plastic cages at 22   2c with a 12:12 h light - dark cycle ( light on 07:0019:00  h ) and given free access to laboratory chow and water . the experiments were approved by the animal care and use committee of gifu university .    during experiments , tissues were maintained in krebs ' solution consisting of ( mm ) : nacl 118.4 , kcl 4.7 , cacl2 2.5 , mgso4 1.2 , kh2po4 1.2 , nahco3 25 , and glucose 11.7 . high potassium krebs ' solution consisting of nacl 4.7 , kcl 118.4 , cacl2 2.5 , mgso4 1.2 , kh2po4 1.2 , nahco3 25 , and glucose 11.7 was used for inducing high potassium contracture . tetrodotoxin was used as a blocker of voltage - dependent sodium channels on striated muscle cells . tetrodotoxin , halothane , and caffeine were obtained from sigma - aldrich ( st . louis , mo , usa ) . tetrodotoxin was initially dissolved in citrate buffer and then dissolved in krebs ' solution to the desired concentration . caffeine was initially dissolved in krebs ' solution to the desired concentration and then krebs ' solution was exchanged for caffeine - containing krebs ' solution in an organ bath . the abdominal and thoracic cavities were opened , and the stomach , esophagus up to the larynx , heart , and lungs were removed and immediately placed in a dish containing precooled aerated krebs ' solution .   a 5  mm long segment of the middle part of the thoracic esophagus was carefully dissected out for mechanical recording .   the heart was also dissected out and retrogradely perfused through the aorta with krebs ' solution for washing out intracardiac blood . the right ventricle was longitudinally cut into strips of 1  cm in length , 2  mm in width , and 1  mm in thickness .   for estimating mechanical activity of the representative skeletal muscle , hind limbs were exposed and the soleus was excised from both legs . the soleus muscle was also cut into strips of the same size as that of heart muscle strips .   for connecting the heart and soleus muscle strips to the isometric tension transducer , a silk - snare was placed at an end of each strip . the whole segment of each muscle preparation was transferred to a 3 ml thermostatically controlled ( 35c ) organ bath containing krebs ' solution bubbled with 95%  o2 + 5%  co2 gas mixture , maintained at ph 7.4 .   for recording contractile responses of the esophageal segments , two l - shaped stainless - steel pins were introduced into the esophageal lumen ; one pin was fixed to the bottom of the organ bath and the other was connected to the bar of the isometric force transducer ( t7 - 30 - 240 ; orientec , tokyo , japan ) .   for recording contractile responses of the muscle strips isolated from the heart and the soleus muscle , one end was fixed to an l - shaped hook at the bath side and the other end was connected to an isometric tension transducer via a placed silk - snare . the contractile responses were recorded isometrically on powerlab system ( adinstruments , bella vista nsw , australia ) through an ac amplifier ( as1202 , nec , tokyo , japan ) . an initial resting tension of 1  g was applied to each preparation , which was subsequently allowed to equilibrate for at least 30 minutes .    for inducing muscle contractile response , electrical field stimulation ( efs ) was applied through a pair of platinum electrodes placed on either sides of each preparation . efs was applied using an electronic stimulator ( model sen-3201 , nihon kohden , tokyo , japan ) connected to the electrodes .   for efs with single pulses , square - wave pulses of supramaximal intensity ( 80  v ) and 10  ms in duration were applied at intervals of 2 minutes .   for repeated multipulse efs , square - wave pulses of supramaximal intensity ( 80  v ) and 10  ms in duration were applied at frequency of 20  hz for 1  s.    data are presented as means  standard deviation ( s.d ) . the significance of differences between mean values was determined by one - way analysis of variance followed by tukey - kramer 's test . a p - value less than  .05 denotes the presence of a statistically significant difference . to clarify the contractile properties , the mechanical responses to efs were recorded in esophageal , skeletal , and cardiac muscle preparations . spontaneous contractions occurred without electrical stimulations in some preparations of cardiac muscle but not in preparations of esophageal and skeletal muscles ( data not shown ) . efs with single pulses ( voltage : 80  v , duration : 10  ms ) evoked twitch - like contractions in all types of muscle preparations ( figure 1(a ) ) . the contractile responses were abolished by tetrodotoxin ( 1  m ) in esophageal and skeletal muscles , whereas the blocker failed to affect the contractions in cardiac muscle ( data not shown ) . the amplitude of the twitch - like contraction of esophageal muscle was larger than that of skeletal muscle and was comparable to that of cardiac muscle ( figure 1(a ) ) . the duration of the twitch - like contraction of esophageal muscle was comparable to that of skeletal muscle but shorter than that of cardiac muscle ( figure 1(b ) ) .    to determine whether esophageal muscle shows summation of contractions , repeated efs ( 80  v-10  ms , 20  hz for 1  s ) skeletal muscle and cardiac muscle preparations were also used to obtain positive and negative controls of summation , respectively . repeated multipulse efs evoked summation of contraction in skeletal muscle , resulting in tetanic contractions ( figure 2(b ) ) . in contrast , cardiac muscle did not show summation of contraction ( figure 2(c ) ) . esophageal muscle preparation responded to all 20 individual stimulations and summated transiently in response to the initial several stimulations ( figure 2(a ) ) . however , the response observed in esophageal muscle was different from that in skeletal muscle .   maximal amplitude of tetanic contractions was 2 - 3 times larger than that of twitch - like responses induced by single - pulse efs in skeletal muscle ( figure 2(b ) ) . esophageal muscle reached a peak of tetanic contractions after 3 - 4 pulses , the amplitude of which was about 1.5 times larger than that of twitch - like responses ( figure 2(a ) ) . then an amplitude similar to that of the twitch - like contraction was sustained until the end of the stimulations ( figure 2(a ) ) .    to clarify the involvement of gap junctions in the contractile responses , we examined the effects of an inhibitor of gap junctions , halothane , on the efs - induced contractions in esophageal and cardiac muscles . application of halothane ( 50  mm ) inhibited the contractions in cardiac muscle ( figure 3(b ) ) . in contrast , esophageal muscle contractions were not affected by halothane ( figure 3(a ) ) . we then compared the effects of high potassium stimulation on the contractile responses in the three muscles . application of high potassium ( 118.4  mm ) in the organ bath evoked phasic contractions in esophageal and skeletal muscles ( figure 4(a ) ) . in contrast , cardiac muscle showed long - lasting contractions , which were sustained at least for 2 minutes .   exogenous application of caffeine ( 20  mm ) , a releaser of calcium from intracellular stores , caused two - phase contractions in esophageal and skeletal muscles ( figure 4(b ) ) . first - phase contraction occurred just after application of caffeine and sustained the tension for about 5 minutes . after the first contraction , the tension was increased again , inducing the second - phase contraction ( figure 4(b ) ) . in the present study , we investigated the contractile properties of esophageal striated muscle by comparison with those of other types of striated muscle , that is , skeletal and cardiac muscles . the major findings are ( 1 ) efs with single pulses evoked twitch - like contractile responses in esophageal muscle , similar to those in skeletal muscle in duration and similar to those in cardiac muscle in amplitude , ( 2 ) efs with a high frequency induced a transient summation of contractions followed by a sustained contraction with an amplitude comparable to that of twitch - like contraction induced by the single - pulse efs , ( 3 ) an inhibitor of gap junctions did not affect the esophageal contractions , and ( 4 ) both high potassium stimulation and application of caffeine induced contractions of esophageal muscle in a manner analogous to that of skeletal muscle . these findings suggest that contractile properties of esophageal striated muscle are not identical to those of skeletal muscle . the specific characteristics revealed in the present study might be associated with effective peristalsis in the striated muscle portion of the esophagus . it has generally been accepted that striated muscle fibers in the esophagus are a type of  skeletal  muscle [ 3 , 5 , 6 ] .   in agreement with this , esophageal striated muscle showed properties similar to those of skeletal muscle ( soleus muscle ) in terms of duration of twitch - like contraction , sensitivity to tetrodotoxin , and responses to high potassium or caffeine . it was of interest to determine whether esophageal striated muscle has the ability to contract spontaneously , but it never showed contractile activity unless appropriate stimulus was applied . this is also in line with the general concept that esophageal muscle fibers are skeletal muscles . however , summation of the contractions during tetanic stimulation was not obvious in the esophageal muscle , although large summation of contractions is one of the important characteristics of skeletal muscles . moreover , large amplitude of twitch - like contraction in the esophageal muscle after application of a single - pulse stimulus is analogous to that in cardiac , but not skeletal , muscle . it is therefore thought that esophageal striated muscle is similar but not identical to skeletal muscle . low amplitude in twitch - like contraction is a potential basis for large summation of contractions in skeletal muscle since it provides a wide margin to summate sequential contractions . it is thus reasonable to expect that the modest summation of contractions in the esophageal muscle may be related , at least in part , to the large amplitude of contraction in response to single - pulse stimulation .   in the case of cardiac muscle , the large amplitude of twitch - like contraction can contribute to generation of sufficient ventricular pressure during isovolumic contraction and to effective reduction of ventricular end - systolic volume [ 2830 ] . the absence of summation is also essential to ensure diastolic intervals for ventricular filling .   considering that properties of cardiac muscle are adequately utilized to accomplish effective pumping out of blood in the heart in contrast to small and large intestines , where intraluminal contents are usually chyme , the esophagus needs to propel solid foods rapidly .   for this purpose , rapid and large contraction of the esophageal muscle in response to neural messages may be convenient . furthermore , esophageal function is specialized for propulsion , but not agitation or retention [ 7 , 16 ] . it is therefore most probable that keeping the maximal contraction at a particular portion of the esophagus is dispensable but that dilation of muscle after propelling the intraluminal contents is required to receive additional foods or to prevent stagnation . accordingly , it seems likely that only a transient summation of contractions , which is one of the specific properties of esophageal muscle , is suitable for the function of the esophagus . in the cardiac muscle preparation , efs applied at 20  hz caused one complete twitch - like contraction followed by small rise in tension . it is well established that the lack of summation in cardiac muscle depends on a long - lasting refractory period of the action potential , which results in substantial overlapping between the refractory period and duration of contraction .   however , the refractory period of esophageal muscle would not overlap with its contraction . this is based on the fact that esophageal muscle contractions induced by high - frequency stimulation were maintained after the transient summation , with the amplitudes being comparable to those of twitch - like contractions .   taking into consideration the fact that strength of striated muscle contraction is correlated with intracellular calcium concentration , it can be postulated that esophageal muscle has the ability to keep calcium level at a constant level during repeated stimulation . since caffeine , a calcium releaser from the intracellular calcium store , caused similar tetanic contractions in esophageal and skeletal muscles , it is possible that calcium uptake into the store is unique in the esophageal striated muscle . electrical coupling of cardiac muscle cells through gap junctions is essential for coordinated contraction [ 32 , 33 ] .   in the present study , we tried to elucidate the possible involvement of gap junctions in coordination of the contractions of esophageal striated muscles . an inhibitor of gap junctions , halothane , failed to affect efs - induced contractile responses in esophageal muscle , whereas the inhibitor blocked those in cardiac muscle . gap junctional protein connexin 43 , which is a major connexin expressed in cardiac muscles , was not detected in esophageal muscles in our immunohistochemical analysis ( unpublished observation ) .   at present , the most probable conclusion is that esophageal muscles do not couple to each other through gap junction channels . however , gap junction channels are formed by a family of more than 20 connexin proteins [ 34 , 35 ] , and halothane might not inhibit all of the gap junction proteins . hence , there remains the possibility that the halothane - insensitive channels are operated in the esophageal muscles . we characterized the contractile properties of esophageal muscle in comparison with those of skeletal and cardiac muscles in rats . the results demonstrate that esophageal muscle has properties that are similar but not identical to those of skeletal muscle . it is thought that some specific properties of esophageal muscle may be beneficial for peristaltic motility in the esophagus .
the external muscle layer of the mammalian esophagus consists of striated muscles . 
 we investigated the contractile properties of esophageal striated muscle by comparison with those of skeletal and cardiac muscles . 
 electrical field stimulation with single pulses evoked twitch - like contractile responses in esophageal muscle , similar to those in skeletal muscle in duration and similar to those in cardiac muscle in amplitude . 
 the contractions of esophageal muscle were not affected by an inhibitor of gap junctions . 
 contractile responses induced by high potassium or caffeine in esophageal muscle were analogous to those in skeletal muscle . 
 high - frequency stimulation induced a transient summation of contractions followed by sustained contractions with amplitudes similar to those of twitch - like contractions , although a large summation was observed in skeletal muscle . 
 the results demonstrate that esophageal muscle has properties similar but not identical to those of skeletal muscle and that some specific properties may be beneficial for esophageal peristalsis .