Location and Setting of the Mars InSight Lander, Instruments, and Landing Site

Knowing precisely where a spacecraft lands on Mars is important for understanding the regional and local context, setting, and the offset between the inertial and cartographic frames. For the InSight spacecraft, the payload of geophysical and environmental sensors also particularly benefits from knowing exactly where the instruments are located. A ~30 cm/pixel image acquired from orbit after landing clearly resolves the lander and the large circular solar panels. This image was carefully georeferenced to a hierarchically generated and coregistered set of decreasing resolution orthoimages and digital elevation models to the established positive east, planetocentric coordinate system. The lander is located at 4.502384°N, 135.623447°E at an elevation of -2,613.426 m with respect to the geoid in Elysium Planitia. Instrument locations (and the magnetometer orientation) are derived by transforming from Instrument Deployment Arm, spacecraft mechanical, and site frames into the cartographic frame. A viewshed created from 1.5 m above the lander and the high-resolution orbital digital elevation model shows the lander is on a shallow regional slope down to the east that reveals crater rims on the east horizon ~400 m and 2.4 km away. A slope up to the north limits the horizon to about 50 m away where three rocks and an eolian bedform are visible on the rim of a degraded crater rim. Azimuths to rocks and craters identified in both surface panoramas and high-resolution orbital images reveal that north in the site frame and the cartographic frame are the same (within 1°).

Ileo-cecal junction: a valve or a sphincter? An experimental study in the opossum

The characteristics of the ileo-junction (UIC) were examined in seven opossums in vivo, and the effect of the UIC on colo-ileal reflux in eight opossums in vitro. Electromyography and intraluminal manometry were studied during intestinal distensions, and administration of phenylephrine, isoproterenol and carbachol. In vitro studies used preparations of ileum, UIC, and colon, attached to a propulsion evaluation system. Fluid flow across the UIC was studied basally and after phenylephrine, isoproterenol and carbachol. A high pressure zone in the UIC was not observed in vivo. Colonic distension increased the pressure and electrical spike bursts in the ileum and UIC, while ileal distension had the opposite effect. Myoelectric and contractile activities were inhibited by adrenergic agonists and stimulated by carbachol. In vitro studies demonstrated aborally migrating ileal contractions initiated by fluid injections into the ileum, and cecal contractions elicited by fluid injections into the colon. The UIC only prevented colo-ileal reflux when it was undergoing contraction as part of ileal or colonic activity. These findings suggest that the opossum UIC does not have valvular properties and ileal fluid propulsion is the main factor in the prevention of colo-ileal reflux.

Choledochoduodenal flow: effect of the sphincter of Oddi in opossums and cats

The aim of this study was to test in vivo (a) whether the sphincter of Oddi acts as a resistor or also as a pump, (b) the effect of an IV infusion of cholecystokinin (CCK) on choledochoduodenal flow, and (c) the ability of the choledochoduodenal junction to prevent duodenobiliary reflux in two animal species, opossums (n = 11) and cats (n = 8). Opossums were implanted with bipolar electrodes on the sphincter of Oddi and the adjacent duodenum. Cats were not. Experiments were performed in vivo using a propulsion evaluation system to test whether the Sphincter of Oddi was able to pump fluid from the bile duct to the duodenum against pressure gradients. In 5 opossums and 4 cats, choledochoduodenal flow was evaluated during the IV infusion of CCK (20 ng.kg-1.min-1). The opossum sphincter of Oddi moved fluid against duodenal pressure gradients of 6-45 cm H2O. The spike-burst frequency (6.4 +/- 1.7 min-1) was maximal at peak bile duct pressures and decreased as bile duct pressure decreased (4.9 +/- 1.6 min-1; P less than 0.001). Pressure pulses in the bile duct were observed at a frequency that correlated with sphincter of Oddi spike-burst frequency (r = 0.84; P less than 0.001). In cats, choledochoduodenal flow occurred only along a hydrostatic gradient; the sphincter of Oddi never acted as a pump but only as a resistor. Infusion of CCk significantly increased the frequency of sphincter of Oddi contractions in opossums, but the transfer of fluid from bile duct to duodenum was significantly reduced. In cats, the rate of fluid flow from the bile duct to the duodenum during CCK infusion did not differ from control values. Reflux of duodenal fluid into the biliary tree was never observed, even at duodenal pressures as high as 100 cm H2O. In conclusion, in vivo, the sphincter of Oddi is able to pump fluid from the bile duct to the duodenum against a pressure gradient in opossums, but, in cats, choledochoduodenal flow requires a bilioduodenal pressure gradient. The increase in sphincter of Oddi contraction frequency induced by CCK in opossums resulted in a decrease in active transsphincteric flow. Duodenobiliary reflux could not be elicited in opossums and cats under the conditions of these experiments.

Gallbladder and gastrointestinal motility after hemorrhagic shock

The alterations in fasting gallbladder and gastrointestinal motility during hemorrhagic shock were investigated. Eight opossums implanted with a gallbladder cannula, gastrointestinal bipolar electrodes, and a carotid catheter were subjected to hemorrhagic shock of 30 mm Hg for 60 minutes by the removal of arterial blood. Shed blood was reinfused after the shock period. Fasting gallbladder volume and gastrointestinal electrical activity were studied before, immediately after, and 24 hours after hemorrhagic shock. Control measurements demonstrated a slow-wave frequency maximal in the duodenum (18.1 +/- 1.1 waves/min), with a plateau in the proximal third of the small bowel, decreasing thereafter. The migrating motor complex (MMC) had a duration of 118 +/- 28 minutes. The average volume of the gallbladder before shock was 5.4 +/- 1.5 ml. Gallbladder volume fluctuated with the MMC, being maximal during phase I and minimal in phase III. The volume of blood removed to reduce mean arterial pressure to 30 mm Hg was 45 +/- 5 ml/kg. Immediately after the shock and blood reinfusion, slow-wave frequency decreased by 40% in the antrum and 25% in the small bowel. The MMC was of shorter duration (91 +/- 22 minutes; p less than 0.05), and gallbladder volume increased to 7.0 +/- 1.7 ml (p less than 0.05). Fluctuations in gallbladder volume during the MMC were absent. Twenty-four hours after shock, slow-wave frequency, MMC, and gallbladder volume had returned to normal and were not different from control measurements. Ischemic damage to the gastrointestinal tract is postulated as the cause of gallbladder dysfunction and altered intestinal motility after hemorrhagic shock.

The propulsive behavior of the opossum sphincter of Oddi

This study investigates whether the phasic contractions of the opossum sphincter of Oddi (SO) delay bile flow by acting as a resistor or facilitate bile flow by acting as a pump. The common bile duct (CBD) and an adjacent segment of duodenum from eight opossums were studied in a propulsion evaluation system in vitro. This system required the production of hydrostatic work by the SO to transfer fluid from the CBD to the duodenum when the pressure in the duodenum was equal or greater than the pressure in the CBD. Fluid movement from the CBD to duodenum and duodenum to CBD was studied at pressure gradients up to 50 cmH2O before and after sodium nitroprusside (10(-7) M) inhibition of smooth muscle contractile activity. All preparations propelled fluid from the CBD to the duodenum against a pressure gradient ranging from 10 to 50 cmH2O. The SO emptied the CBD in a monoexponential fashion, with a time constant of 1.52 +/- 0.7 min, until CBD pressure was reduced to 8.5 +/- 3.2 cmH2O, when propulsion ceased. Superimposed on the CBD pressure waveform were pressure pulses of 1-2 cmH2O in amplitude that resulted from the contractions of the SO. CBD pressure was higher at the start than at the end of a periodic pressure pulse, whereas CBD pressure was stable between pulses. The frequency of the pressure pulses was greatest at the maximal CBD pressure (9.4/min) and decreased significantly when the basal pressure was reached (1.5/min, P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term influence of enteric infection on jejunal propulsion in guinea pigs

The intrinsic fluid-propelling behavior of guinea pig jejunal segments was reportedly altered by primary infection with Trichinella spiralis from day 10 to day 20 postinoculation. After that time propulsive behavior returned to normal. The objective in the investigation reported here was to determine if jejunal responsiveness to an infection is influenced by prior exposure to the parasite. Accordingly, jejunal propulsion was examined in guinea pigs reinfected 2 mo after the initial inoculation with the parasite. Propulsive behavior was measured in vitro by attaching oral and aboral ends of jejunal segments to a propulsion evaluation system that imposed input-output conditions of constant capacitance and negligible resistance. Propulsive complexes produced by segments removed from animals 60 days after primary inoculation were generally similar in configuration to those produced by segments from uninfected hosts. The pronounced net aboral propulsive behavior previously observed 10-20 days after primary infection was not observed in segments from animals 60 days postinfection or from uninfected controls. After secondary inoculation the fluid volume expelled per propulsive complex began to increase within 1 day and net aboral propulsive behavior was clearly evident 2 days postinoculation. As net aboral propulsive behavior was induced approximately five times faster after secondary as compared with primary inoculation with T. spiralis, it is evident that the overall intrinsic mechanisms that are responsible for an earlier onset of net aboral propulsive behavior are responsive for at least 2 mo after initial exposure to the parasite.

Intrinsic jejunal propulsion in the guinea pig during parasitism with Trichinella spiralis

The onset and duration of alterations in intrinsic fluid propelling behavior of guinea pig jejunum caused by infection with Trichinella spiralis was determined using an in vitro monitoring system. Isolated intestinal segments from uninfected hosts produced propulsive complexes of approximately 30-s duration at 125-s intervals. The maximum intraluminal pressure produced at the oral and aboral ends of the segments was approximately 25 cmH2O and the maximum rate of fluid ejection in both the oral and aboral directions was approximately 0.28 ml/s. Segments removed from guinea pigs 10 and 15 days after being inoculated with infective parasite larvae ejected significantly more fluid in the aboral as compared with the oral direction. The maximum aboral pressure developed 10 days postinoculation was 45.7 cmH2O and the maximum aboral fluid ejection rate was 0.60 ml/s. Analogous oral values were 29.0 cmH2O and 0.40 ml/s. This net aboral propulsion was not observed on days 7, 20, 30, or 60 postinoculation. We conclude that precise and highly predictable alterations in intrinsic propulsive behavior of the small intestine are induced by primary infection. These changes are expressed by day 10 postinoculation and are reversed upon spontaneous termination of the intestinal phase of parasitism.

Ileal and colonic propulsive behavior: contribution of enteric neural circuits

Experiments were conducted in vitro to determine the extent to which the intrinsic propulsive behavior of cat terminal ileum and colon is controlled by the enteric nervous system (ENS) and the myogenic control system. Oral and aboral ends of terminal ileal and colonic segments, 17 cm in length, were connected to a propulsion evaluation system that imposed input-output conditions of constant capacitance and negligible resistance. To determine the effect of inhibiting one or more components within the ENS on spontaneously occurring patterns of propulsive behavior, segments were arterially perfused with tetrodotoxin (3.1 X 10(-7) M), atropine sulfate (10(-6) M), tubocurarine chloride (1.5 X 10(-6) M), hexamethonium chloride (2.8 X 10(-5) M), or methysergide (2.8 X 10(-5) M). Results indicated that when cat ileal and colonic segments are evaluated under conditions requiring significant amounts of hydrostatic work to effect fluid ejection 1) the unmodulated myogenic control system is unable to induce significant amounts of fluid expulsion, 2) cholinergic excitatory motor neurons of the ENS are required for the generation of the intrinsic patterns of propulsive behavior normally observed to expel fluid, and 3) the spontaneous occurrence of intrinsic propulsive patterns requires the operation of ENS integrative mechanisms utilizing nicotinic cholinergic and serotonergic synapses.

Functional and structural changes in intestinal smooth muscle after jejunoileal bypass in rats

Active stress and cross-sectional area of intestinal muscle were assessed in tissues taken from unoperated rats, from rats that had undergone bypass of 70% of the small bowel, and from rats that had undergone transection and anastomosis of the bowel. Thirty-five days after operation, muscle from the intestine of transected and bypassed animals elicited active stresses that were equal to or greater than those developed by muscle taken from unoperated animals. The total cross-sectional areas of the in-continuity segment and the area and thickness of the muscle layers of both the in-continuity and bypassed segments were greater when compared with unoperated animals. Significant differences also existed among tissues taken from bypassed and transected animals. Additionally, transection induced increases in active stress, area of muscle in the distal intestine, and circular muscle thickness in the mid- and distal intestine when compared with tissues from unoperated animals. These findings support the hypothesis that intestinal bypass induces increases in functioning smooth muscle tissue.

Induction in vitro of a specific pattern of jejunal propulsive behavior by cholecystokinin

Experiments were conducted in vitro to determine whether arterial infusion of the C-terminal octapeptide of cholecystokinin (CCK-8) into cat jejunal segments could elicit propulsive behavior when a segment must do hydrostatic work to expel fluid. Oral and aboral ends of jejunal segments, 17 cm in length, were connected to a propulsion evaluation system that imposed input-output conditions of constant capacitance and negligible resistance. Infusion of 1 X 10(-10) to 3 X 10(-9) M CCK-8 produced an initial simultaneous ejection of approximately equal volumes of fluid from both ends of the segments. This reduction in luminal volume lasted for approximately 4 min. Concurrent with this initial reduction in luminal volume were phasic ejections of fluid that often did not occur simultaneously or with equal magnitude from both ends of a segment. This pattern persisted throughout the period of CCK-8 infusion. Arterial infusion of atropine sulfate (10(-6) M) or hexamethonium sulfate (2.8 X 10(-7) M) prevented the induction of propulsive behavior by CCK-8. These results indicate that arterial infusion of CCK-8 can induce a specific pattern of jejunal propulsive behavior by triggering neural activity in the enteric nervous system and that CCK-8 may have a physiological role in regulating propulsion.

Comparison of colonic and ileal propulsive capabilities under conditions requiring hydrostatic work

The intrinsic ability of cat colonic segments to do hydrostatic work to expel fluid was investigated in vitro and compared with the propulsive ability of ileal segments evaluated under identical conditions. Colonic segments spontaneously produced propulsive complexes at an average interval of 4.8 min when basal intraluminal pressure was set at 5 cmH2O. These complexes produced a net ejection of fluid in the aboral direction. When the capacitance of the evaluation system was set at 0.025 ml/cmH2O, colonic segments ejected only 5% of their content. This percentage increased to 23% when evaluation capacitance was increased to 0.125 ml/cmH2O. Peak aboral pressure associated with these complexes was approximately 37 cmH2O at both capacitances. Ileal segments aborally ejected almost twice as much fluid (44% of their luminal content) as did colonic segments at a capacitance of 0.025 ml/cmH2O even though ileal luminal content was five times less than colonic content. The aborally ejected volume per complex did not change when ileal segments were evaluated at two different capacitances even though higher pressures were required to eject fluid at the lower capacitance than at the higher capacitance. These results indicate that the propulsive behavior of ileal and colonic segments has some qualitative aspects that are similar but that only the propulsive ability of colonic segments is pressure limited under the conditions tested. Possible mechanisms that might produce this limitation are considered.

An intracellular characterization of neurones and neural connexions within the left coeliac ganglion of cats

Intracellular recordings were made in vitro from neurones located within the left coeliac ganglion of the cat solar plexus. Thirty percent of the neurones within left coeliac ganglia were identified as efferent neurones. Within this neuronal population, splenic-efferent and renal-efferent neurones were identified specifically. Neurones within left coeliac ganglia were characterized as either phasic (fast adapting) neurones or tonic (slowly adapting) neurones depending upon their prolonged firing behaviour. Electrophysiological properties of neurones varied considerably. The wide range of values obtained for both input resistance and input capacitance suggest that sizeable differences in either specific membrane resistance or cell geometry exist within the over-all neurone population. Frequency distributions of input resistance, time constant, input capacitance and current threshold for tonic and phasic neurones were found to be significantly different. Compound excitatory post-synaptic potentials were produced by stimulation of the ipsilateral splanchnic nerves in 69% of the neurones tested and in 3% of the neurones tested upon stimulation of the contralateral splanchnic nerves. Electrical stimulation of nerve fibres located in the coeliac plexus, the superior mesenteric plexus or the left renal nerves generated excitatory synaptic potentials in neurones located within left coeliac ganglia. It is concluded that neurones within the left coeliac ganglion are innervated by splanchnic nerve fibres primarily contained within the left splanchnic nerves, receive excitatory synaptic input from splenic, renal and other peripheral preganglionic fibres and have extremely varied electrophysiological properties.

A study of renal-efferent neurones and their neural connexions within cat renal ganglia using intracellular electrodes

1. Intracellular recordings were made, in vitro, fron neurones located within left renal ganglia and left coeliac ganglia of cat solar plexus. 2. Forty-three percent of the neurones of the renal ganglia tested were identified by antidromic activation as renal-efferent neurones. 3. Electrical stimulation of all nerve trunks emanating from renal ganglia, other than the renal nerves, did not antidromically activate renal ganglia neurones. Neurones not antidromically activated were designated as non-efferent neurones. 4. Neurones within the renal ganglia were also characterized as phasic or tonic neurones depending on their pattern of discharge. 5. Electrical stimulation of renal nerves produced excitatory synaptic potentials (e.p.s.p.s) in 18% of the renal-efferent neurones. 6. Compounded e.p.s.p.s were produced in 50% of the renal ganglia neurones tested by stimulation of the ipsilateral splanchnic nerves and in 84% of the neurones upon stimulation of the vertebral nerve. 7. Synaptic responses demonstrating characteristics typical of those induced by activation of multisynaptic neural pathways were produced upon stimulation of renal and coeliac nerves. 8. The results of this study indicate that the electrophysiological properties of renal-efferent neurones vary considerably from neurone to neurone and that these neurones receive synaptic inputs from a variety of preganglionic fibres and possibly from other neurones having their soma located within the solar plexus.

The intestine as a fluid propelling system

The ability of intestinal segments to propel fluid from their lumens can be characterized in dynamic terms by measuring the movement of luminal fluid ejected into attached systems that predispose the amount of work the segment must do to affect fluid transfer. Studies employing this approach have demonstrated that the propulsive capability intrinsic to intestinal and colonic segments is not uniform along the bowel. Differences in propulsive behavior observed to occur solely as the result of the operation of subsystems intrinsic to particular intestinal regions suggest (a) that external control inputs can alter the propulsive state of a region, and (b) that this altered state may exist for hours after the external input to the system is terminated. Consideration of the propulsive performance of segments as an emergent property of interactions among intrinsic subsystems has contributed to the formalization of precise questions heretofore not asked about the organization and function of intestinal subsystems and their associated interactions. Knowledge of propulsive behavior and its control is still in its infancy. Further studies employing basic concepts and procedures similar to those reviewed above will advance our understanding of intestinal propulsion and of the intestine as a complex system.

Fluid propulsion by cat intestinal segments under conditions requiring hydrostatic work

The intrinsic ability of duodenal, jejunal, and ileal segments of cats to propel fluid by doing hydrostatic work was investigated in vitro. Segments of terminal ileum demonstrated two functional states. In the net propulsive state, propulsive complexes occurred at an average interval of 7.9 min. These complexes consisted of an initial phase characterized by the simultaneous ejection of equal fluid volumes from both ends of a segment at a frequency of 8/min and a second phase that ejected a net volume of fluid from the aboral end. Ileal segments not in the net propulsive state demonstrated complexes consisting only of the 8/min ejections. Segments of proximal duodenum and midjejunum failed to produce net propulsion when evaluated under similar conditions. These results indicate that 1) ileal segments can intrinsically produce net fluid transport in the aboral direction when hydrostatic work is required for propulsion, 2) propulsive activity intrinsic to ileal segments is regulated by intrinsic control mechanisms having an effective oscillatory period of 8 min, and 3) basic differences exist in the intrinsic control structures between ileal and other intestinal regions.

An intracellular analysis of some intrinsic factors controlling neural output from inferior mesenteric ganglion of guinea pigs

1. In vitro studies were conducted on neurons within the inferior mesenteric ganglion (IMG) of guinea pigs to investigate how intrinsic features of the spike-generating process interact with preganglionic inputs to produce the output firing patterns of these neurons. Intracellular-electrode techniques were used to monitor and control electrical activity of IMG neurons. Preganglionic inputs were activated either synchronously by stimulating an attached nerve trunk or asynchronously by leaving the ganglion attached to a segment of terminal colon and activating the colonic-IMG mechanosensory system. 2. Ninety-seven percent of the neurons studied demonstrated an afterspike hyperpolarization (ASH). The ASH process was activated only by the occurrence of a spike and did not have a synaptically induced component. Further activation of this process was produced by two or more spikes having interspike intervals less than the duration of an ASH following a single spike. An aftertrain hyperpolarization (ATH) resulted from this progressive activation. The amplitude of both the ASH and the ATH decreased when the resting membrane potential was hyperpolarized by current injection or by increasing the external potassium ion concentration. 3. Neuronal excitability was reduced during the ASH. From this observation it was concluded that when IMG neurons operate in the occasional-firing mode, the ASH process prevents output frequency from greatly exceeding the reciprocal of the ASH duration produced by a single spike. 4. Two types of synaptically induced slow depolarizations were observed: a slow, long-latency depolarization and a short-latency depolarization (SLD). These depolarizations differed in their latency, onset, and duration. Both were capable of converting synchronous, preganglionic input from subthreshold (non-spike-activating) to threshold (spike-activating) activity. 5. Neurons having resting potentials more positive than -60 mV were capable of firing in the rhythmic-firing mode; 40% of these neurons demonstrated tonic- and 60% phasic-firing behavior. Frequency-current relations for tonic-discharging neurons were linear from the rhythmic-firing threshold to current levels approximately 2.5 times the threshold value. Minimal frequency for tonic firing and the slope of the linear portion of the frequency-current relation were indirectly related to the duration of the ASH. 6. This study suggests that sympathetic, noradrenergic neurons of the IMG can operate in either the occasional- or rhythmic-firing mode. In the physiologic state in vivo, most IMG neurons probably do not produce action potentials in excess of 10-15 Hz because of their intrinsic properties which regulate firing in both modes of operation.

Modulation of colonic motility by peripheral neural inputs to neurons of the inferior mesenteric ganglion

Colonic motility is known to be regulated, in part, by postganglionic sympathetic neurons whose cell bodies are located in the inferior mesenteric ganglion (IMG). Several studies have demonstrated that the neural activity of postganglionic neurons located in the IMG results from the synaptic integration of neural input received not only from the central nervous system via the inferior splanchnic nerves but also from sensory receptors located in the periphery and received via the lumbar colonic, intermesenteric, and hypogastric nerves. To investigate some of the interactions among colonic motility, peripheral neural input to the IMG, and postganglionic neural activity, intracellular recordings were made from single neurons located in the IMG of in vitro IMG-colon preparations obtained from guinea pigs. Recordings of colonic intraluminal pressures were simultaneously obtained with these neurophysiological recordings. Gross afferent input to the IMG from colonic mechanoreceptors was found to be nonadapting, to the correlated with physiological levels of intraluminal pressure, and to vary with propulsive motor activity. Both peripheral and central preganglionic inputs to the IMG reduced or abolished colonic motiligy, but with varying degrees of effectiveness. It is concluded that the firing patterns of sympathetic postganglionic neurons affecting motility of the terminal colon result primarily from the synaptic integration of inputs from preganglionic fibers of both central and peripheral origin.

A study of peripheral input to and its control by post-ganglionic neurones of the inferior mesenteric ganglion

1. Intracellular recordings were made, in vitro, from neurones of guinea-pig inferior mesenteric ganglia (IMG) attached, via the lumbar colonic nerves, to segments of distal colon. 2. 'Spontaneous' synaptic input from colonic afferent fibres was observed in 79% of the neurones tested. In any given preparation, the level and pattern of this synaptic input to different neurones varied considerably. 3. Superfusion of colonic segments with drugs (papaverine, isoprenaline, and adenosine triphosphate) which reduce colonic motility decreased colonic afferent input to IMG neurones. 4. Superfusion of colonic segments with acetylcholine or stimulation of pelvic nerves, both of which increase colonic motility, increased colonic afferent input to IMG neurones. 5. Superfusion of colonic segments with either atropine or tubocurarine reduced the level of 'spontaneous', colonic afferent input. However, distension of these relaxed segments increased the colonic afferent input. 6. Repetitive stimulation of preganglionic inputs to the IMG inhibited afferent input from drug relaxed segments of colon that were moderately distended by the injection of air into the lumen. Superfusion of the colon with phentolamine blocked this inhibition. 7. The results of this study suggest that IMG neurones receive afferent input from mechanoreceptors located in the distal colon and that the mechanosensitivity of this afferent pathway is in part controlled by efferent noradrenergic neurones of the IMG. The IMG-colon neural circuitry can therefore be considered to form a feed-back control system which participates in the regulation of colonic motility.

A study of pace-maker activity in intestinal smooth muscle

1. Electrical activity of longitudinal muscle from cat intestine was recorded in the double sucrose gap.2. Approximately 20% of the preparations demonstrated slow, spontaneous fluctuations of membrane voltage, slow waves. This activity, although quite uniform in a given preparation, showed considerable inter-preparation variation with respect to amplitude, frequency and wave form.3. Application of steady hyperpolarizing current decreased slow-wave frequency and increased slow-wave amplitude while depolarizing currents increased frequency and decreased amplitude.4. Some preparations with no spontaneous slow-wave activity developed slow waves when the membrane was hyperpolarized into a given range which, depending on the preparation, varied in size from 10 to 40 mV. Step or ramp depolarization of the membrane from hyperpolarized levels triggered slow waves in some preparations.5. When the membrane potential of a slow-wave generating preparation was clamped at the resting potential, spontaneous inward-directed current transients were observed.6. No changes in membrane conductance were observed during the course of a slow wave.7. The slow-wave pattern was simulated for individual preparations by applying the membrane current measured under voltage clamp to the passive membrane resistance and capacitance measured independently under current clamp.8. In addition to the defined slow-wave activity, voltage-dependent oscillations in membrane potential were sometimes observed.9. Application of 10(-5)M ouabain irreversibly blocked slow waves and produced a membrane depolarization equal to or slightly greater than the slow wave crest. Repolarization of the membrane to the resting potential, or hyperpolarization, failed to restore slow-wave activity.10. Removal of external potassium produced a reversible sequence of events almost identical to those following ouabain application.11. Replacement of 50% of the external sodium chloride with sucrose produced no changes in slow-wave activity with respect to rates of rise or fall, maximum amplitude or frequency. Sucrose replacement of all external sodium chloride eliminated slow waves after 5 min; however, activity could be restored by a slight hyperpolarization. Longer exposures to the modified bath abolished activity.12. Following a conditioning exposure to potassium-free Krebs solution, readmission of potassium at normal concentration produced a mean hyperpolarization of 20.5 mV and in spontaneous preparations an arrest of activity.13. Pump current in sodium-loaded, non-spontaneously active preparations was measured by voltage clamp and was observed to be voltage-dependent.14. The results of this study indicate that an electrogenic pump is present in longitudinal muscle of cat duodenum, and that oscillations in the level of pump current produce slow waves.