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Comparative morphology of interstitial cells of Cajal Ultrastructural characterization

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Gastrointestinal Peristalsis: Joint Action of Enteric Nerves,
Smooth Muscle, and Interstitial Cells of Cajal
Intestinal Disease Research Programme and Department of Medicine, McMaster University, Hamilton, ON L8N 3Z5, Canada
ICC; pacemaker; intestinal motility; rhythmicity
Peristalsis is a propulsive motor pattern orchestrated by neuronal excitation and
inhibition in cooperation with intrinsic muscular control mechanisms, including those residing in
interstitial cells of Cajal (ICC). Interstitial cells of Cajal form a network of cells in which electrical
slow waves originate and then propagate into the musculature initiating rhythmic contractile
activity upon excitaton by enteric nerves. Interstitial cells of Cajal have now been isolated and their
intrinsic properties reveal the presence of rhythmic inward currents not found in smooth muscle
cells. In tissues where classical slow waves are not present, enteric cholinergic excitation will evoke
slow wave-like activity that forces action potentials to occur in a rhythmic manner. Intrinsic and
induced slow wave activity directs many of the peristaltic motor patterns in the gut. Microsc. Res.
Tech. 47:239–247, 1999. r 1999 Wiley-Liss, Inc.
Peristaltic motor activity is a motor pattern orchestrated by complex sequencing of neuronal excitation
and inhibition in cooperation with intrinsic muscular
control mechanisms, including those residing in interstitial cells of Cajal (ICC). Peristalsis is defined as
waves of contraction propagating along the gastrointestinal tract for various distances as a means of mixing
and propelling its content distally. Both the type of
neuronal activity and the type of intrinsic myogenic
control mechanism differ widely throughout the gastrointestinal tract. Interestingly, peristalsis is often
equated with the peristaltic reflex (Grider and FoxxOrenstein, 1999). The peristaltic reflex as evoked by
pinching or pulling at a point along the intestine, is of
obvious physiological interest but is ‘‘unlikely to occur
during the normal passage of liquid contents in the
small intestine. . .;thus may be rarely, if ever, activated
in vivo’’ (Tonini et al., 1996). Physiological activation of
peristalsis will in most cases involve the stretching of a
segment of stomach, intestine, or colon and it will occur
by neuronal pathways that contain additional mechanisms to those required for the ascending excitatory
reflex (Tonini et al., 1996). When peristaltic motor
activity occurs, in particular in the stomach and proximal small intestine, the waves of contraction always
have rhythmicity to it (Fig. 1). This rhythmicity is
determined by electrical slow wave activity in the
musculature, referred to as pacemaker activity (Huizinga et al., 1997). This review focuses on the role of
interstitial cells of Cajal in the generation of peristalsis.
Interstitial cells of Cajal are highly branching cells that
occur in networks associated with the plexuses of the
enteric nervous system. ICC are in electrical communication with the smooth muscle cells. ICC are the cells in
which pacemaker activity is initiated (Koh et al., 1998;
Thomsen et al., 1998) and then, together with smooth
muscle cells, generate the tissue slow wave activity.
Therefore, ICC can be considered to be part of the
myogenic control system. That the ICC and smooth
muscle cells originate from the same mesenchymal
precursor cell supports this classification (Klüppel et
al., 1998; Torihashi et al., 1997). The fact that interstitial cells of Cajal are always very intimately associated
with neural structures (Berezin et al., 1988; Thuneberg, 1982) indicates interaction between the myogenic
and neural control systems at the level of ICC. In fact,
interstitial cells of Cajal may mediate some types of
inhibitory neurotransmission (Ward et al., 1998). Therefore, ICC are a crucial link in both neural and myogenic
control mechanisms for peristalsis.
The entrance of interstitial cells of Cajal into the
arena of the physiology of gastrointestinal motility was
greatly facilitated by the work of Lars Thuneberg and
Juri Rumessen (Thuneberg, 1982). They continued
work done by Taxi and others reviewed elsewhere
(Thuneberg, 1989; Thuneberg et al., 1995). Thuneberg
not only introduced the ICC to investigators in this
field, but also introduced the means of identifying
them. Furthermore, he provided or re-introduced many
theories of physiological functions of ICC. Electronmicroscopy identified the ICC as a special network of cells
distinct from the enteric nervous system. Together with
Faussoni-Pelligrini, hypotheses were provided on the
role of several independent ICC networks and their
relationship with enteric nerves and smooth muscle
cells (Faussone Pellegrini et al., 1977; FaussonePellegrini, 1992). Many investigators provided evidence for a role of ICC in the generation of slow waves
by selectively removing those sections of the tissue that
harboured the ICC. In this way, the ICC network
Contract grant sponsor: Medical Research Council of Canada.
*Correspondence to: Dr. Jan D. Huizinga, McMaster University, HSC-3N5C,
1200 Main Street West, Hamilton, ON L8N 3Z5 Canada. E-mail:
Received 1 July 1999; accepted in revised form 22 August 1999
pioneered by Thuneberg (Thuneberg and Peters, 1987).
After a few days in culture, ICC regain a morphology
that is similar to that in situ. Nevertheless, positive
identification of these cells requires more than morphological similarities. Optimization of measurements of
kit mRNA in single cells led to the detection of kit
mRNA in most cells identified as ICC by morphological
criteria (Huizinga et al., 1999a ; Thomsen et al., 1998).
The cells identified as interstitial cells of Cajal produced very regular inward currents, associated with
membrane potential oscillations (Fig. 2) (Lee et al.,
1999; Thomsen et al., 1998). This result, recently
confirmed by others (Koh et al., 1998), provided conclusive evidence that ICC are pacemaker cells generating
the rhythmic electrical oscillatory activity that is determining the rhythmic contractile activity of the gut
musculatures. ICC, by initiating slow wave activity and
through communication with the enteric nervous system (although few mechanistic details of this are
known yet), play a crucial role in many, if not all, types
of peristaltic motor activity.
Fig. 1. Slow wave driven peristalsis implies that ICC play a role in
peristaltic motor activity. When the stomach of the mouse is filled with
barium contrast fluid, peristaltic activity in the distal stomach causes
emptying of the stomach and filling of the proximal small intestine.
This causes activation of cholinergic nerves resulting in excitation of
the musculature. This leads to contractions of short sections of the
circular muscle layer (see arrowheads), which propagate in aboral
direction. The frequency and propagation velocity of this motor
activity are identical to the frequency and propagation velocity of the
electrical slow wave activity. The direction of propagation is indicated
with an arrow and the position of the pylorus with a small arrow. The
frequency of slow waves is ⬃ 40 per minute in the mouse small
intestine, hence three sequential contractions are seen. The moment
depicted, slow waves occurred only in the contracted segments;
moments later the slow waves will be positioned more distally at all
three sites. The slow wave activity in the mouse has been shown to
originate in interstitial cells of Cajal (Thomsen et al., 1998). See also
Der-Silaphet et al. (1998).
responsible for pacemaker activity was identified as
being in the Auerbach plexus area in the stomach and
small intestine and in the submuscular plexus area in
the colon, at the border of submucosa and circular
muscle (Huizinga et al., 1997; Sanders, 1996). The
search for the physiological significance of ICC has
made important steps forward since the discovery that
ICC harbor the Kit protein (Maeda et al., 1992). Studies
on mutant mice that do not express functional Kit
proteins showed an absence of slow wave activity as
well as an absence of several types of ICC including
those responsible for pacemaking activity, as reviewed
recently (Huizinga et al., 1997; Sanders, 1996). Although this evidence was compelling, it was still not
clear whether the ICC played an essential role in
generating the slow wave activity or whether they were
the cells from which slow waves originated and, hence,
truly the pacemaker cells of the gut. Such confirmation
required the investigation of properties of single isolated ICC and the required isolation procedures were
Interstitial cells of Cajal in culture have now been
shown to generate slow wave activity. Although we still
know very little about the electrophysiological properties of ICC, evidence points to a nonselective cation
channel that is responsible for the initiation of the slow
wave and hence could be called the pacemaker channel
(Koh et al., 1998; Thomsen et al., 1998). Full characterization of this channel awaits further experimentation.
Is this slow wave activity in isolated ICC identical to
that observed in tissue?
Slow waves as observed in tissue are periodic oscillations of the cell membrane potential that are omnipresent and have characteristic frequencies in each organ in
each animal, between 3 and 50 cycles per minute. In
ICC isolated from the mouse small intestine, the frequency is about 10 cycles/minute at room temperature,
similar to the tissue slow wave activity at that temperature. The slow waves cause alternation of brief periods
of high and low excitability. The period of high excitability corresponds to the plateau phase of the slow wave.
Upon excitatory stimulation, the plateau phase of the
slow wave rises above threshold for activation of L-type
calcium channels and, consequently, action potentials
(fast membrane potentials changes, the depolarization
being due to calcium influx through L-type calcium
channels) are generated (Huizinga et al., 1997). The
calcium influx causes contraction of smooth muscle
cells. Action potentials occur only superimposed on the
plateau phase of the slow waves. Action potentials are a
typical manifestation of smooth muscle activity and
ICC may not generate them normally. Contractions in
ICC are often seen to occur only in part of the cell,
suggesting that contractions are mediated by local
calcium release from the SR instead of through general
activation of L-type calcium channels. The intensity
(i.e., amplitude and frequency) of action potential generation is directly related to the force of contraction of
the musculature. The slow waves determine the maximum frequency of contractions. The slow waves also
determine propagation characteristics of the contrac-
Fig. 2. Spontaneous rhythmic inward currents from an ICC isolated from the adult mouse small
intestine. The activity was recorded at a holding potential of 0 mV, with an acquisition frequency of 66 Hz.
The recording was obtained immediately after the whole cell configuration was established. The ICC had
been 4 days in culture.
tions. When measured in vivo, the slow wave activity is
seen to propagate in an aboral direction at a velocity of
approximately 2 cm/second. The reason for propagation
is that the slow wave exhibits an intrinsic frequency
gradient (Diamant and Bortoff, 1969; Szurszewski,
1987). That is, when pieces of tissue are taken out of an
organ and examined, the slow wave frequency will be
higher in the proximal part compared to the distal part.
In vivo, however, because of the electrical coupling
between cells in the muscle layers, the slow waves are
entrained; the proximal slow waves pace the distal slow
waves and hence the proximal slow waves lead. There
is a certain phase lag (time lag) between the start of a
proximal slow wave and the premature activation
(pacing) of a distal one and hence the slow waves
appear to propagate. When excited sufficiently, a slow
wave is associated with circular muscle contraction.
Slow waves occur virtually simultaneously at any point
across the circumference of an organ and hence a ring
contraction of circular muscle develops. This leads to an
increase in intraluminal pressure. Because the slow
waves and, therefore, the associated circular muscle
contraction ring propagate distally, the (semi-) occlusion propagates distally. When the contraction is forceful enough it will push contents anally (Huizinga et al.,
1997). The origin of the frequency gradient likely comes
from ICC at the proximal side having some different
intrinsic properties compared to ICC at distal sites.
The identification of a nonselective cation channel as
generating the pacemaker current is consistent with
tissue data that suggested this (Liu et al., 1995; Malysz
and Huizinga, 1997). The hypothesis is that rhythmic
calcium release from the SR will trigger the pacemaker
current. In addition we identified typical whole cell
outward current profiles in ICC (Lee et al., 1999) and a
beginning is made in unraveling the individual K⫹
currents. A Ba2⫹-sensitive inwardly rectifying K⫹ current in ICC that may be involved in slow wave repolarization and maintenance of a negative potential between slow waves was found by Ward and co-workers
(Horowitz et al., 1999; Koh et al., 1998). Attempts have
been made to characterize ICC from the dog colon
(Langton et al., 1989) and L-type calcium channels as
well as a 4-amino- pyridine resistant voltage activated
K channel were found (Lee and Sanders, 1993).
If action potentials were to be generated without any
type of underlying depolarization, propagation of the
action potentials would occur randomly in all directions. This is well demonstrated in the feline small
intestine (Lammers et al., 1999) and myometrium
(Lammers et al., 1994). Propagation patterns were
studied by Lammers and co-workers with a battery of
240 electrodes each 1 mm apart. Propagation appeared
random. In addition, in 17-day pregnant rats, marked
asynchrony of bursts in different areas of the myometrium was observed suggesting non-uniform spatial
and temporal distribution of excitation across the organ
(Lammers et al., 1994). A major determinant of propagation patterns was the refractory period following an
action potential, which could stop colliding action potentials (Lammers et al., 1994). The generation of an
action potential results in a brief increase in intracellular calcium, a propagating action potential results in a
wave of high intracellular calcium apparently propagating from one cell to another. This was demonstrated in
the longitudinal muscle of the guinea pig colon (Stevens
et al., 1999). The calcium waves propagated from cell to
cell in a random fashion. Propagation was restricted by
refractory periods associated with the action potentials.
The guinea pig colon does not have regular slow wave
activity and it will be interesting to see how calcium
waves are modified in tissue with regular slow wave
activity. In the myometrium, synchronization may be
improved during expulsion of the fetus by enhanced
synchronization due to increased numbers of gap junctions and oxytocin receptors and uniform excitation by
oxytocin (Huizinga et al., 1992).
Because action potentials propagate over limited
distances and propagate in random directions, slow
waves have evolved to control tissue excitability temporally and spatially. This forces action potential generation into rhythmic patterns, furthermore leading to
apparent propagation of action potential activity in
Fig. 3. Distention results in periodic neural excitation inducing slow wave driven peristalsis in a segment of the mouse small
intestine in vitro. Top three traces: electrical
activity from 3 different sites 1 cm apart,
recorded simultaneously. Bottom three traces:
intraluminal pressure changes recorded simultaneously at intraluminal sites of the
electrodes. At bottom, pulsatile outflow was
recorded (j). The segment of intestine was
distended by setting the intraluminal pressure at 4 cm H2O. Action potentials occurred
on 9–11 slow waves followed by 2–6 slow
waves without action potentials. The accompanying intraluminal pressure changes were
accordingly periodic. Outflow occurred with
high amplitude propagating pressure waves
(j). The direction and speed of propagation
were verified by visual inspection of propagation of indentations in the muscle wall due to
the circular muscle contractions. Reproduced
from Huizinga et al. (1998) with permission of
the publisher.
aboral direction. In various tissues, the initiation of the
slow wave is an exclusive property of interstitial cells of
Cajal. In other tissues, slow wave-like activity develops
serving the same function but with smooth muscle cells
and enteric nerves playing a critical role in its initiation
(see below).
It has always been seen as problematic that a small
group of cells such as the ICC can critically influence a
much larger group of smooth muscle cells. The idea is
that any current generated by the ICC would dissipate
into the large mass of cells to which they are coupled.
The explanation has to lie in a special nature of the
three-dimensional coupling characteristics between ICC
and smooth muscle cells (Publicover, 1995). It may also
be that close apposition contacts or gap junctions that
are coupling ICC to smooth muscle cells are of a special
nature. Dye coupling has shown that dye spreads easily
from ICC to smooth muscle cells but not as readily from
smooth muscle cells to ICC, suggesting a rectifying type
of junctional coupling (Liu et al., 1998). A difference in
coupling between smooth muscle cells and between ICC
and smooth muscle cells was also observed when studying transfer of electrical current (Dickens et al., 1999).
Motor Patterns Determined by Cholinergic
Excitation and Slow Wave Activity
It is likely that most patterns of peristalsis are
induced by some form of distention of the bowel wall.
Weems and Seygal (1981) showed that a segment of cat
ileum subjected to intraluminal pressures from 5–10
cm H2O developed periodic motor activity with time
intervals of ⬃ 8 minutes. These bursts of motor activity
led to an outflow of contents in an aboral direction. The
bursts of motor activity were abolished by tetrodotoxin
and atropine and, therefore, initiated, at least in part,
by activity of intrinsic cholinergic neurons. This excitation moves along the intestine directed by the enteric
nervous system. When cholinergic nerves excite the
musculature, intrinsic slow waves generated by the
musculature surpass the threshold for action potential
generation due to depolarization, and action potentials
occur, superimposed on each slow wave. As a consequence, propagating rhythmic contractile activity at
the slow wave frequency occurs. In addition, in vivo,
such slow wave-driven peristalsis was shown to occur
after distention by content received from the stomach
(Fig. 1) (Der-Silaphet et al., 1998).
The associations between intraluminal pressure, electrical slow wave activity, and pulsatile outflow were
demonstrated in an in vitro model of distention (Fig. 3).
The intraluminal pressure was allowed to rise to between 2 and 4 cm H2O in a segment of proximal
intestine of the mouse (Huizinga et al., 1998). A bursting pattern of action potential generation was elicited
that was sensitive to TTX. Action potentials occurred on
several sequential slow waves, alternating with slow
waves without action potentials. Slow waves with
superimposed action potentials were associated with
increases in intraluminal pressure (Fig. 3). The slow
waves and the transient increases in intraluminal
pressure propagated along the intestine and caused
outflow of fluid in a pulsatile manner at the slow wave
frequency. The direction of propagation was always in
an aboral direction. When the tissue was mounted in
the opposite direction, the aboral end fixed to the
pressure column and the oral end free to allow outflow,
no outflow at the oral end occurred. In fact, the pressure
dropped in the oral column identifying flow in aboral
direction and a negative pressure gradient developed.
The unidirectional propagation of slow waves is due to
their intrinsic frequency gradient (Diamant and Bortoff, 1969; Huizinga et al., 1997). Hence, the correlation
between propagating waves of transient intraluminal
pressure (peristalsis) and slow waves can be easily
demonstrated in segments of intestine in vitro.
The above-described models of distention-induced
peristalsis illustrate that two different mechanisms
occur concurrently to induce aboral propagation of
motor activity, one being a periodic wave of neural
excitation that propagates aborally, the other, slow
wave-driven peristalsis driving rhythmic motor activity
in the same direction. These mechanisms may be aided
by a neural reflex initiating ascending excitation oral to
the distended segment (Tonini et al., 1996).
The stomach has very regular periodic peristaltic
waves of contraction when in the process of emptying.
This peristaltic activity has underlying slow wave
activity (Haba and Sarna, 1993; Van Nueten et al.,
1990). Bauer provided evidence that these slow waves
originated from the Auerbach’s plexus area (Bauer et
al., 1985), which has a dense plexus of interstitial cells
of Cajal (Christensen et al., 1992). In the guinea pig
antrum, slow wave activity was recently shown in situ
to be generated by interstitial cells of Cajal associated
with Auerbach’s plexus (Dickens et al., 1999; Huang et
al., 1999). These slow waves have the typical insensitivity to nifedipine and voltage changes. Interestingly, in
circular muscle preparations without Auerbach’s plexus,
slow wave-like activity is still recorded but this activity
is abolished by hyperpolarization (induced by chromakalim) and not by nifedipine. The authors speculate that
the slow wave activity might be related to the presence
of intermuscular ICC. It is likely, however, that the
rhythmicity of this activity resides in the smooth
muscle cells themselves. Interesting evidence for the
role of ICC as pacemaker activity has come from recent
work from Hirst and coworkers (Dickens et al., 1999)
and D.F. Van Helden (personal communication). In the
antrum of the guinea pig, normal slow wave activity
consisting of a voltage-independent pacemaker component and a voltage-dependent plateau is generated. An
electrical pulse can generate a premature slow wave. In
preparations without the myenteric plexus, slow wave
activity does not occur but an electrical pulse generates
the second component after a short delay. This provides
interesting indirect evidence that the ICC produced the
component that triggered the slow wave, and that the
ICC-induced pacemaker activity can be replaced by an
electrically induced depolarization.
Motor Patterns Determined by Cholinergic
Excitation in the Absence of Classic Slow Waves
Mice with a spontaneous mutation in the Kit gene
(WWv) produce an abnormal Kit protein that prevents
the normal development of ICC in the Auerbach’s
plexus area of the small intestine and this musculature
does not generate the classical slow wave. The small
intestine upon receiving content from the stomach, does
not exhibit regular rhythmic peristalsis but does generate some propulsive motor activity that, with great
delay, moves content aborally (Der-Silaphet et al.,
1998). What is the nature of this activity? Action
potentials do not have a fixed frequency and can occur
at entirely different frequencies at sites close to each
other. How can electrical activity with such properties
produce coordinated, propagating action potentials and,
consequently, a propagating wave of increased intraluminal pressure? Although the WWv mouse, without
marked stimulation of the musculature, does not show
normal slow wave-driven peristalsis, significant distention in vitro has shown that regular rhythmic peristalsis can develop. The musculature, in the absence of
classical slow waves, has the capability to generate
slow wave-like activity that produces ‘‘slow wavedriven’’ peristalsis predominantly in the aboral direction (Fig. 4). In the W mutant mice, the frequency of the
slow wave-like activity is remarkable constant at ⬃ 40
cpm, not different from that of normal slow wave
activity! This oscillatory activity may originate in
smooth muscle cells in response to neural stimulation,
but it is also possible that ICC of the deep muscular
plexus are involved (Hara et al., 1986; Jimenez et al.,
Cholinergic activity can cause general depolarization
of a short segment of intestine that is so strong that
action potentials are generated continuously, overruling the usual restrictions of slow wave activity if
present. Such activity was noticed in the rat ileum,
where a very strong contraction occurred in one part of
the ileum and ‘‘moved’’ distally without immediate
relaxation of the preceding segment. A motor pattern
occurred that truly resembled the ‘‘squeezing of tooth
paste,’’ in that an entire segment of intestine contracted
for a few seconds. The driving force behind such a
pattern of propulsive contractile activity is clearly the
enteric nervous system. This motor activity occurred
about 3 times per 10 minutes and nitrergic activity was
shown to be part of the mechanism that controlled this
frequency (Bercik et al., 1994).
In the mid to distal mouse colon, slow wave activity is
hardly ever observed (Bywater, personal communication). In this region, Bywater and colleagues (1989)
noticed atropine-sensitive bursts of action potentials
superimposed on slow, transient depolarizations of ⬃17
mV. These complexes occurred every 3 minutes and
propagated aborally along the colon at a velocity of ⬃ 1
mm/s (Bywater et al., 1989; Lyster et al., 1995). The
action potentials within a burst occurred associated
with fast oscillations occurring at ⬃ 2/s. The periodicity
of the bursts was determined, at least in part, by
nitrergic enteric nerves responsible for electrical quiescence in between the bursts of activity. Interestingly,
the periodic depolarizations were seen not to be sensitive to atropine, but due to periodic inhibition of
otherwise continuous activity of nitrergic nerves (Spencer et al., 1998a,b). This periodic inhibition of nitric
oxide release was accompanied by inhibition of neural
activity (ATP release?) normally leading to inhibitory
junction potentials. Bywater suggested that this motor
Fig. 4. Distention induced periodic activity in the W/Wv mouse.
A,B: Top three traces: electrical activity from 3 different sites 1 cm
apart, recorded simultaneously. Bottom two traces: intraluminal
pressure changes recorded simultaneously at intraluminal sites of
electrodes 1 and 3. A: Without sufficient distention, action potentials
occurred uncoordinated at the three sites. Pressure changes were also
uncoordinated and no outflow occurred. B: Distention by 3.5 cm H2O of
intraluminal pressure evoked periodic activity. The periods of activity
of ⬃ 10 seconds duration occurred simultaneously at the different
recording sites. Slow wave-like activity appeared on which action
potentials were superimposed. The ‘‘slow waves’’ were related 1:1 to
increases in intraluminal pressure and outflow pulses (j). Hence, the
outflow was pulsatile in nature with each transient outflow associated
with an individual propagating pressure wave of sufficient amplitude.
The ‘‘slow waves’’ were different from classical slow waves in that they
were blocked by L-type calcium blockers and hyperpolarization.
Reproduced from Huizinga et al. (1998) with permission of the
pattern represents the peristaltic activity of the mid to
distal mouse colon (Bywater et al., 1998).
Generation of propagating bursts of action potentials
was also observed in the human colon, both in vitro and
in vivo. In vitro, carbachol induces bursts of action
potentials, often without any appreciable underlying
depolarization (Huizinga and Waterfall, 1988). Interestingly, the development of such bursts is impaired in
diverticular disease (Huizinga et al., 1999b). Normally,
in vivo, bursts of action potentials (6–25-second duration) at 3 per minute or less, with very similar characteristics to those seen in vitro, are seen to propagate anally
and produce flow of content upon muscarinic stimulation (Bueno et al., 1980; Schang et al., 1986). Reduced
occurrence of such bursts is associated with constipation (Bueno et al., 1980). The periodicity of the motor
complexes may reside in periodic presynaptic inhibition
of nitric oxide release, which causes periodic depolarization. Since during this period cholinergic activity dominates, action potentials appear on top of the depolarizations, causing the periodic contractile activity. It is yet
to be examined how such patterns of electrical activity
relate to slow wave-like activity also generated by the
human colon (Rae et al., 1998a).
The guinea pig ileum is electrically quiescent without
any stimulus applied to it, despite the presence of a
network of interstitial cells of Cajal. All obvious morphological features of this ICC network are similar to those
in the intestine of other species, such as the mouse
(Christensen and Rick, 1987; Jessen and Thuneberg,
1991; Komuro and Zhou, 1996; Komuro et al., 1996;
Zhou and Komuro, 1992). Electrical activity, however,
does not show randomly propagating action potentials.
Action potentials occur on transient depolarizations.
Transient depolarizations occur in single isolated
smooth muscle cells (Kohda et al., 1998) as well as in
tissue evoked by cholinergic stimulation (Bolton, 1971).
There appear to be several types of oscillatory activity
occurring in the guinea pig (Huizinga et al., 1999c). In
vitro, distention can be shown to produce a propagating
contraction that empties the segment under investigation (Tonini et al., 1996) (Fig. 5). This contraction occurs
rhythmically with sustained distention (Huizinga et
al., 1999c) and cholinergically induces slow wave-like
activity underlying the propagating contraction. The
generation of this slow wave activity may involve
interstitial cells of Cajal, but this point is still to be
investigated. Hence, myogenic and neural activity work
in concert to produce peristalsis. Neural activity includes ascending excitation as well as other more
complex stretch sensitive neural circuitries (Kunze et
al., 1998; Tonini et al., 1996).
In the small intestine, segmental and propulsive
patterns of contractile activity can occur at irregular
intervals. Both occur at the slow wave frequency and
hence are likely to involve interstitial cells of Cajal.
What are the factors determining the motor pattern
when aborally propagating slow wave activity is likely
underlying both phenomena? Once again, it is likely the
Fig. 5. Distention-induced peristalsis in
the guinea pig small intestine. The experimental setup as used by Costa and co-workers to
initiate peristalsis and ascending excitation
in a segment of intestine in vitro. Influx of
fluid builds up distention of a segment of
intestine (bottom trace). This distention will
trigger a neural reflex initiating ascending
excitation in a rhythmic fashion (top trace).
At a certain threshold, a propagating wave of
intraluminal pressure clears the content from
the ‘‘anal’’ part of the intestine (middle trace).
It can be seen that evoking ascending excitation is not enough to evoke peristalsis. A
complex array of neural activities is triggering this peristalsis of which ascending excitation is only a part. This experimental setup is
similar to that used with Figures 3 and 4
except that all recordings were made from the
‘‘anal’’ part and distention was maintained
just above threshold, which results in rhythmic appearance of propagating contractions
at the slow wave frequency. When this approach was taken with the guinea pig, cholinergically induced slow wave activity was associated with the peristaltic contractions
(Huizinga et al., 1999c). Reproduced from
Tonini et al. (1996) with permission of the
pattern of nerve stimulation that determines the pattern of contraction. If a segment of intestine receives a
more or less uniform level of excitation, all slow waves
bear action potentials and the propagating nature of
the slow waves sweeps the action potentials along the
intestine and causes a propagating contraction. When
the level of excitation is not uniform, action potentials
will only occur in those regions where excitation produces sufficient depolarization for contraction generation. If such excitation were to occur over a significant
period of time in one spot, contractile activity would be
rhythmic at the slow wave frequency because only with
every passing slow wave would action potentials be
generated. It would not be propagating since the adjacent section would not be excited, hence no action
potentials would be generated there. In this way a
pattern of segmental contractile activity will be generated.
An interesting new idea related to changing motor
patterns comes from research from Thuneberg’s laboratory. Thuneberg discovered that distention results in a
dramatic increase in special communication structures
between smooth muscle cells and between smooth
muscle cells and interstitial cells of Cajal (Thuneberg
and Peters, 1999). Membrane extrusions from one cell
form pegs that push into a neighbouring cell, forming a
peg and socket junction. The quantity of such contacts
can change from undetectable levels to hundreds of
contacts per cell. Interestingly, the junctions are polarized with respect to ICC; the peg is always from smooth
muscle cells, the socket always in ICC. Although this
dramatic increase in peg and socket junctions occurs in
experimental stretch, it still has to be discovered when
they actually form in vivo. Furthermore, their physiological role has yet to be established. Thuneberg
speculates that distention may induce peg and socket
junctions to facilitate a motor pattern that is primarily
segmental. Evidence for this is based on the observation that whenever tissue is fixed while engaged in
segmental contractile activity, peg and socket contacts
are abundant. Another possibility is that the junctions
assist the role of ICC as stretch receptors. If stretch
sensed by ICC is to be transmitted to the smooth muscle
cells, existence of peg and socket junctions may greatly
facilitate this.
It has become clear from studies into distentioninduced peristalsis in the intestine and colon that the
control of inhibitory innervation involving nitric oxide
determines, to a great extent, the periodicity of rhythmic neural patterns of excitation (Der-Silaphet et al.,
1998; Rae et al., 1998b; Spencer et al., 1998a). Since
evidence is emerging that ICC are involved in inhibitory innervation to the gut musculature (Sanders,
1996; Ward et al., 1998), it will be interesting to study
the specific role of ICC in rhythmic patterns of neural
excitation. A well-studied motor pattern involving rhythmic activity of the enteric nervous system is the ‘‘migrating myo-electric complex’’ or MMC, which occurs in the
empty stomach and intestine. The initiation of the
MMC resides in the enteric nervous system and in part
the CNS through vagal pathways (Hall et al., 1982,
1984; Rudge et al., 1990; Sarna et al., 1984; Szurszewski, 1969). In humans, every 1.5 hours a section of
stomach or intestine receives intense stimulation from
cholinergic motor neurons that derive from the vagus or
the enteric nervous system. Two propagating activities
are superimposed. First, the MMC, as a band of neural
excitation, travels slowly from the LES towards the
distal intestine. Secondly, within the segment of stomach or intestine under activation, rhythmic slow wave
driven peristalsis plays an important role in moving
contents anally (Hall et al., 1982). It would be interesting to study the specific interactions between those
nerves involved in initiating the MCC and interstitial
cells of Cajal. Interestingly, the MMC is one of the
peristaltic motor patterns not requiring distention for
its initiation.
The existence of multiple control levels for gastrointestinal motility has always been recognized. In 1899,
Bayliss and Starling noticed myogenic waves of contractions with associated transit of content, in addition to
their discovery of the law of the intestine (Bayliss and
Starling, 1899). The characteristics of myogenic control
have been further elucidated by Alvarez (Alvarez, 1948)
and, subsequently, by many others notably Diamant
and Bortoff (1969) and Szurszewski et al. (1970). A
synthesis of myogenic and neural control mechanisms
was given by Costa and Furness (1982) and Weisbrodt
(1987). The present review incorporates the interstitial
cells of Cajal into the control mechanisms of peristalsis.
No significant motor activity occurs without excitation
of the musculature by enteric nerves. Once activated,
however, the gut musculature exhibits its intrinsic
control mechanisms. Interstitial cells of Cajal generate
slow wave activity passed on to involve the whole
musculature. This slow wave activity permits action
potentials to occur in a rhythmic and propagating
manner, and such slow wave-driven peristaltic activity
is central to many motor patterns of peristalsis.
The Medical Research Council of Canada is acknowledged for supporting most of the research from my
laboratory mentioned in this review. Yaohui Zhu recorded the spontaneous inward currents from an ICC
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ultrastructure, morphology, interstitial, characterization, comparative, cajal, cells
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