|
| |
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online August 8, 2008
Journal of Experimental Biology 211, 2624-2637 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.019711
Characterization of a descending pathway: activation and effects on motor patterns in the brachyuran crustacean stomatogastric nervous system
Institute of Neurobiology, Ulm University, D-89069 Ulm, Germany
* Author for correspondence (e-mail: wstein{at}neurobiologie.de)
Accepted 2 June 2008
 |
Summary |
|---|
 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
We have documented the actions of two identified neurons located in the brain on the STNS motor circuits. We show that these neurons provide exteroceptive chemosensory information to the motor circuits and we outline their axonal projection patterns, their firing activity and their effects on three motor patterns. Backfill stainings and activity measurements in vivo and in vitro show that two neurons located in cluster 17 of the brain project via the inferior ventricular (IV) nerve to the STNS. These IV neurons started to burst rhythmically when chemosensory stimuli were applied to the first antennae. When rhythmically activated in vitro, gastric mill rhythms were elicited or, if already active, entrained by the IV neuron activity. In addition, IV neuron stimulation excited the esophageal motor neuron and inhibited several pyloric neurons such that the timing of the IV neuron activity was imposed on all motor rhythms. The IV neurons were thus capable of synchronizing the activities of different motor circuits, which demonstrates the regulation of motor patterns by higher-order neuronal centers.
Key words: central pattern generation, Cancer crabs, stomatogastric ganglion, descending control
|
INTRODUCTION |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
;
Pearson, 2004), flying
(Wilson, 1961), breathing
(Lieske et al., 2000) and the
grinding of food (Marder,
2000; Marder and Bucher,
2001; Marder and Calabrese,
1996) are usually produced by central pattern generators (CPGs).
These CPGs are often multifunctional and comprise a large number of different
motor outputs. They are controlled by sensory feedback and upstream neural
structures (Brodfuehrer and Thorogood,
2001; Fleischer,
1981; Perrins et al.,
2002; Rossignol et al.,
2006; Spirito,
1975) in order to generate the appropriate behavior for the
situation. The regulation of motor activity by higher neuronal centers,
particularly those in the brain, is as yet poorly understood.
There are only a few systems in which CPG circuit neurons, their
connectivity and their input pathways have been studied in sufficient detail
to allow an investigation of motor pattern regulation at the cellular level.
One such system is the stomatogastric nervous system (STNS) of decapod
crustaceans (Marder and Bucher,
2001; Marder and Bucher,
2007; Nusbaum,
2002; Selverston and Moulins,
1987). The STNS is an extension of the central nervous system and
contains several CPGs that control rhythmic movements of the crustacean
foregut. Two of these CPGs are located in the stomatogastric ganglion (STG):
the gastric mill circuit controls the chewing movements of three teeth in the
gastric mill, and the pyloric CPG generates the rhythmic movement of the
pyloric filter apparatus (Hartline and
Maynard, 1975; Maynard and
Dando, 1974).
The motor circuits are heavily modulated by projection neurons that descend
from the paired commissural ganglia (CoG; see
Fig. 1B)
(Nusbaum and Beenhakker,
2002), which in turn receive input from the brain
(Fleischer, 1981;
Kirby and Nusbaum, 2007). The
inferior ventricular nerve (ivn) is an unpaired direct connection
between the brain and STNS (Fig.
1A,B) and it contains only a few axons
(Böhm et al., 2001;
Dando and Selverston, 1972;
Orlov, 1929). In crayfish, the
activity on the ivn correlates with the amount of ingested food
(Böhm et al., 2001),
inspiring the hypothesis that some of the neurons on the ivn transmit
motor pattern-associated information either from the STNS to the brain or
vice versa. The somata of two ivn axons, called pyloric
suppressor (PS) neurons or inferior ventricular neurons (IV neurons), are
located either in the ivn or in the brain, close to the insertion
point of the ivn. While it has been shown that these neurons can
affect STNS motor patterns (Christie et
al., 2004; Claiborne and
Selverston, 1984; Marder and
Eisen, 1984a), it is unclear whether they receive information from
the STNS or relay information to it, and what kind of information they
process.
|
|
MATERIALS AND METHODS |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
Solutions
C. pagurus physiological saline had the following composition
(mmoll–1): NaCl, 440; MgCl2, 26; CaCl2,
13; KCl, 11; Trizma base, 10; maleic acid, 5, pH 7.4–7.6. In some
experiments, high divalent saline (5x Ca2+/5x
Mg2+) was applied exclusively to the CoG to block polysynaptic
connections (Blitz and Nusbaum,
1999; Smarandache and Stein,
2007).
Electrophysiology and preparations
Electrophysiological experiments were performed using standard methods as
described previously (Bartos and Nusbaum,
1997; Blitz and Nusbaum,
1997; Stein et al.,
2006).
Intact animal preparation (Fig. 1A)
In in vivo experiments the activities of the dorsal ventral nerve
(dvn) and the ivn were recorded extracellularly. For
identification of the IV neuron spikes, we determined the conduction direction
of spikes on the ivn with two hook electrodes attached to the
ivn (see Fig. 1C,D).
Silver wire (diameter 75 µm) with PTFE isolation (Goodfellow, Cambridge,
UK) was used for the hook electrodes. The activities of the motor neurons of
the pyloric and gastric CPGs were monitored via a single
extracellular hook electrode at the dvn. Differential signals were
recorded, filtered and amplified with an AM Systems amplifier (model 1700;
Carlsborg, WA, USA). In these experiments, crabs were mounted in a turnable
holder (courtesy of H. G. Heinzel, University of Bonn, Germany) to access
dvn and ivn on the dorsal and ventral side of the crab,
respectively. During the experiments, crabs were chilled to 12–14°C
by surrounding the carapace with ice. Crabs were not in contact with seawater.
Previous studies (Heinzel et al.,
1993) have demonstrated that in these experimental conditions
gastric mill and pyloric activity are generated consistently and
neuromodulators keep their effects on the network. In general, activity
generated under these conditions is similar to that in the intact animal and
stress levels are usually sufficiently low to permit data collection. After
dissection, animals remained undisturbed in the holder for at least 20 min to
allow acclimatization. Only animals that showed normal eye and antennal
movements as well as a typical motor activity (as estimated by the pyloric
rhythm) were used in these experiments. For recording from the dvn,
the dorsal carapace and the hypodermis above the STNS were opened. To test
what kind of information the IV neurons relay from the brain to the motor
circuits in the STG, we applied visual, tactile and chemosensory stimuli.
Fleischer showed that the gastric mill rhythm is enhanced in the dark and
suppressed when the crabs are illuminated
(Fleischer, 1981). Therefore,
we used a very simple stimulation protocol, namely turning the illumination on
and off to estimate the impact of visual stimuli on ivn activity.
Tactile stimuli were presented by touching the first antenna, the second
antenna or the mouth area, in particular the maxillae and maxillipeds.
Chemosensory stimuli were given by applying a mixture of seawater and crab
food to the first antennae [which contain olfactory as well as chemosensory
receptors (Brock, 1930)] and
to the chemoreceptors around the mouth
(Garm et al., 2005). Here, we
did not distinguish between olfactory and chemoreceptive function. Chopped
crab meat was used to stimulate the chemoreceptors and to feed the crabs,
while seawater served as a control in these experiments. We tested the impact
of food intake on ivn activity in feeding experiments. For this, a
tube was inserted through the esophagus into the foregut and food was directly
pressure injected into the stomach. Pressure injection rather than voluntary
food intake was used to avoid olfactory and mechanical stimuli to antennae and
claws and to avoid visual cues that could interfere with the measurements.
Isolated nervous system preparation (Fig. 1B)
The dissection of the STNS was performed in physiological saline at
4°C as described previously
(Stein et al., 2005). Briefly,
the isolated STNS was pinned down in a silicon elastomer-lined (Elastosil
RT-601, Wacker, Munich, Germany) Petri dish and continuously superfused
(7–12 ml min–1) with chilled physiological saline
(10–13°C). In preparations with intact brains, the ophthalmic artery
that contains the STG and carries hemolymph to the brain was superfused with
chilled saline for 5–10 min prior to the dissection of the STNS to keep
the brain alive (M. P. Nusbaum, personal communication).
Extracellular recordings in vitro were obtained as described
previously (Smarandache and Stein,
2007); that is, by electrically isolating individual sections of
STNS nerves from the bath by building a petroleum jelly-based cylindrical
compartment around a section of the nerve. The action potentials propagating
through the nerve were recorded by placing one of two stainless steel
electrode wires within this compartment. The second wire was placed in the
bath as a reference electrode. The differential signal was recorded, filtered
and amplified with an AM Systems amplifier (model 1700).
To facilitate intracellular recordings the STG was desheathed and
visualized with white light transmitted through a darkfield condenser (Nikon,
Tokyo, Japan). Microelectrodes (15–25 M
) were filled with a
solution containing 0.6 moll–1 K2SO4
and 0.02 moll–1 KCl. Intracellular current injections were
accomplished using an NPI NEC 10L amplifier (NPI, Tamm, Germany) in bridge or
single-electrode discontinuous current-clamp mode. Sample rates in
discontinuous current-clamp mode ranged from 2 to 4 kHz. STG neurons were
identified by estimating their activity patterns, synaptic interactions and
axonal projection pathways in combination with current injections, as
described previously (Bartos and Nusbaum,
1997; Blitz and Nusbaum,
1997; Weimann et al.,
1991).
The pyloric cycle period was defined as the duration between the onset of impulse bursts of the two pyloric dilator (PD) neurons and the onset of the subsequent PD neuron bursts. Intracellular recordings were exclusively used to determine the mean number of action potentials per burst for several pyloric neurons, including the PD neurons and the lateral pyloric (LP, one cell) and pyloric constrictor (PY, 3–5 cells) neurons, and inferior cardiac (IC, one cell) and ventricular dilator (VD, one cell) neurons. The activities of the LP and PY neurons could also be monitored on the lateral ventricular nerve (lvn), while IC and VD neuron activities were recorded from the medial ventricular nerve (mvn; Fig. 1B). The activities of the PD neurons were monitored on the pyloric dilator nerve (pdn).
The gastric mill rhythm was monitored by the activity of the lateral gastric (LG, one cell) neuron, the dorsal gastric (DG, one cell) neuron and the gastric mill motor neurons (GM, four cells). The latter were exclusively measured intracellularly. The gastric mill rhythm was considered to be spontaneously active when the LG neuron (a member of the CPG) produced bursts of action potentials. The gastric mill cycle period was defined as the duration between the onset of a LG neuron burst and the onset of the subsequent LG neuron burst. The LG neuron was recorded either intracellularly from its soma or extracellularly from the lateral gastric nerve (lgn). The DG neuron was recorded either intracellularly or extracellularly from the dorsal gastric nerve (dgn; Fig. 1B).
The esophageal motor output was monitored by intra- and extracellular recordings from the esophageal motor neurons (OMNs). OMNs are a pair of bilaterally symmetrical neurons with one soma located in each CoG (Fig. 1B). Their axons project through the inferior esophageal nerve (ion) and can usually be identified by their large spike amplitudes.
Activities were measured as the number of action potentials (spikes) per burst, instantaneous spike frequency or as a sliding average with a bin width of 1 s. Mean values were determined from 10 consecutive cycles of gastric mill or pyloric activity. To activate the IV neurons, we stimulated the ivn extracellularly with 10 consecutive stimulus trains of 40 or 20 Hz stimulation frequency and different train durations (2–6 s) and intertrain intervals (1–20 s). For these experiments we used both preparations without brains and preparations with brains attached. In the latter preparations the STNS and the brain were exclusively connected via the ivn. The paired circumesophageal commissures (coc) had been transected.
Data analysis
Data were recorded onto computer hard disk using Spike2 (version
5.03–6.04; CED, Cambridge, UK) and a micro 1401AD board (CED). Data were
analyzed using Spike2 script language. Individual scripts are available at
http://www.neurobiologie.de/spike2.
Final figures were prepared with CorelDraw (version 12 for Windows). Graphics
and statistics were generated using Excel (Microsoft) or Plotit (version 3.2,
Scientific Programming Enterprises, Haslett, MI, USA). All data were tested
for normal distribution. Statistical tests for data were one-way ANOVA for
repeated measures (in the case of normal distribution) and Wilcoxon
signed-rank test for non-parametric data. For the former, data are presented
as means ± s.d. and for the latter as box-and-whisker plots showing
medians, lower and upper quartiles, and minimums and maximums. N
refers to the number of animals, while n gives the number of trials.
For all statistical tests, significance with respect to control is indicated
on the figures using the following symbols: *P<0.05,
**P<0.01, ***P<0.001.
Nerve fills
We used Lucifer Yellow-CH (Sigma-Aldrich, Munich, Germany),
CoCl2 and NiCl2 fills of the ivn and other
nerves within the STNS to determine the projections of the IV neurons
(Kirby and Nusbaum, 2007).
Both anterograde and retrograde axon fillings were observed. In these
experiments, a VaselineTM well was built around the ivn, ion,
superior esophageal nerve (son) or stomatogastric nerve
(stn), and the saline within the well was replaced with distilled
water. Then, we transected the nerve within the well and, after several
minutes, removed the distilled water and replaced it with a solution of 10%
Lucifer Yellow, 10% CoCl2 or 10% NiCl2 in distilled
water. The preparation was then incubated at 4°C for 12–72h.
Finally, the dye was removed from the well and the preparation was fixed in 4%
paraformaldehyde (Merck, Hohenbrunn, Germany) in phosphate buffer, dehydrated
in an ascending ethanol series and mounted in methyl salicylate (Fluka Chemie
GmbH, Buchs, Switzerland) for viewing.
The exact location of the IV neurons in the brain was determined from frontal sections of the brain after Lucifer Yellow backfills of the ivn. For this, the brain was embedded in paraffin and sectioned on a microtome. The thickness of the brain slices was 10 µm.
|
RESULTS |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
The locations of the somata that project towards the brain (ascending fibers) within the ivn were determined with NiCl2 backfills. Somata were located in the esophageal ganglion (OG) and in the CoG. In seven animals, we transected the CoG and filled the ivn towards the STNS. In five of the seven animals we found three stained somata in the OG. In two animals only two somata in the OG were stained. In five animals, the ivn was filled towards the STNS with intact CoG. In two preparations, the dye did not reach the CoG, but in three preparations one soma in each CoG was clearly visible.
We then filled the ivn towards the brain and determined the number of axons within the ivn. In N=9 of 11 frontal sections of ivn backfills, we found eight axons (Fig. 2C). In two animals the ivn contained seven axons. In each of the 11 backfills, we observed two large stained somata in the brain. Their axons could be traced to the ivn. No other somata were stained, which indicates that only two neurons descended from the brain to the STNS.
|
;
Cazalets et al., 1987;
Christie et al., 2004;
Claiborne and Selverston,
1984; Dando and Selverston,
1972; Sigvardt and Mulloney,
1982a; Sigvardt and Mulloney,
1982b). The IV neuron somata of C. pagurus were located
in the brain, similar to the situation in C. borealis
(Christie et al., 2004). They
were located in the dorso-medial area of the brain (cell 1: 178.57±47.1
µm; cell 2: 221.43±48.5 µm; total thickness of brain:
575±67.7 µm; N=9; Fig.
2B) in cell cluster 17 [nomenclature after Sandeman et al.
(Sandeman et al., 1992)].
There were no obvious dendritic ramifications of the IV neurons within the
brain. We used NiCl2 backfills of the ivn to quantify the
size of IV somata. Cell bodies had an average length of 119.48±28.8
µm and a width of 90.89±19.3 µm (N=21).
Fig. 2D shows a schematic
summary drawing of the axonal projections of all ivn units in the
brain and the location of the IV neuron somata in cluster 17. Our ivn backfills (N=8) towards the STNS indicated that axons contained within the ivn projected to the ion, son and stn. To determine the axonal projections of the IV neurons we thus backfilled the ion, son or stn with CoCl2 or NiCl2 and followed the stained axons to the brain. In each backfill of the son (N=3), ion (N=9) and stn (N=5), the somata of the IV neurons were stained. In addition, when we traced the stained axons, we found that axons that connected to the IV neuron somata also projected to the son, ion and stn in each preparation (Fig. 2E). Together, these results show that the IV neurons projected via the son and ion to the CoG and via the stn to the STG.
Activation of the descending IV neurons in the intact animal
As yet, it is unclear whether and if so what kind of information the IV
neurons relay from the brain to the STNS. Data from the crayfish
Orconectes limosus indicate that the activity on the ivn
increases after food intake and that this increase correlates with the amount
of food intake (Böhm et al.,
2001). It is, however, unclear whether this increase resulted from
the activity of the descending IV neurons (and thus originated in the brain)
or whether ascending ivn units started firing to a greater extent.
Furthermore, since voluntary food intake activates a variety of sense organs,
such as tactile, visual and olfactory organs, the sensory modalities
stimulated during food intake are unknown. We thus recorded the ivn
extracellularly in intact animals with two hook electrodes and used the delay
between the spikes on both electrodes to determine their conduction direction
and to differentiate between ascending and descending units. Corresponding to
the results of our backfill stainings, all of our recordings (N=15)
showed that only two neurons descended from the brain to the STNS. All other
spikes ascended from the STNS to the brain
(Fig. 1C).
We then tested the impact of mechanical, visual and chemosensory stimuli and that of food intake on the activity of the ivn units. First, we repeated the experiments previously reported on O. limosus, and fed the animal. Instead of relying on voluntary food intake we injected food directly into the stomach via a tube inserted into the esophagus, thus circumventing the activation of sense organs outside of the stomach. When we injected food, the activity of the neurons on the ivn recording remained unchanged (N=12; Fig. 3A and B right). By contrast, the pyloric rhythm sped up (Fig. 3A). In the example shown in Fig. 3A, the pyloric cycle period decreased from 1.06s before feeding to 0.77 s after feeding. The median pyloric cycle period decreased significantly from 1.62 s (lower quartile 1.25 s; upper quartile 1.91 s) to 1.28 s (lower quartile 1.02 s; upper quartile 1.56 s; P<0.01, N=12, Wilcoxon signed-rank test). The phasing of the LP neuron also changed. Both the onset and the end of its activity were significantly delayed. The median phase onset changed from 0.36 before feeding to 0.38 after feeding (N=12, P<0.04; Fig. 3B left); the end of its activity phase changed from 0.66 before feeding to 0.70 after feeding (N=12, P<0.01, Wilcoxon signed-rank test; Fig. 3B left). The phasing of the PD neurons did not change. Together, these results indicate that filling the stomach with food activated sense organs within the stomach that in turn affected the pyloric circuit in the STG. This effect, however, was not mediated via the ivn.
|
To examine whether the IV neurons relay sensory information from the brain
to the STNS we tested visual (1), tactile (2) and chemosensory stimuli (3).
For all sensory modalities, we used very simple stimulation procedures. (1)
Fleischer has shown that the gastric mill rhythm is very sensitive to changes
in illumination; that is, it is suppressed in bright light and enhanced in
darkness (Fleischer, 1981).
Therefore, we used a very simple stimulation protocol, namely turning the
illumination on and off, to test whether changes in illumination affected the
activity of the IV neurons. However, in all 13 tested animals, the IV neurons
did not respond to these illumination stimuli. (2) Similarly, tactile stimuli
applied with a paintbrush or a pair of forceps to the antennae or the mouth
area did not have any effect (N=13). (3) By contrast, we found that
during chemosensory stimulation the IV neurons started to burst in
N=13 of 15 animals immediately after the onset of stimulation
(Fig. 3C). Action potentials in
ivn originating in the STNS did not occur rhythmically and their
activity remained unchanged (Fig.
3C). In these experiments, we applied a mixture of seawater and
crab food to the first antennae. The maximum period of the rhythmic IV bursts
was 37.66 s and the minimum 4.43 s, the maximum burst duration of the IV
neurons was 11.39 s and it was 1.13 s at minimum. The maximum spike frequency
of both IV neurons was 42.03 Hz and the minimum was 9.23 Hz (N=13;
Fig. 3D). By contrast, when we
applied seawater only in control, the activity of the IV neurons remained
unchanged (N=13). In three of the 13 animals that responded to
chemosensory stimulation, a gastric mill rhythm occurred at the same time
(Fig. 3C,E). In these rhythms a
LG neuron burst occurred with each IV burst, in cases where IV neuron spike
frequency exceeded 30 Hz (Fig.
3E). The delay between the first IV spike within a burst and the
first action potential of the LG neuron within its burst averaged
1.91±0.7 s (N=3).
Together, these results indicate that the IV neurons started to burst rhythmically when chemosensory stimuli were applied to the first antennae. In a subset of experiments, rhythmic IV burst coincided with rhythmic activity of the gastric mill neuron LG. Changes in illumination and tactile stimuli applied to the antennae and mouth did not affect the IV neurons.
In vitro oscillations of the IV neurons
To investigate the actions of an enhanced IV neuron activity on the motor
circuits in the STNS, we used the isolated nervous system preparation
(Christie et al., 2004). In 34
preparations, the brain was left attached to the STNS, connected solely
via the ivn. The coc were transected. In 64
additional preparations, the brain was removed. We recorded from the
ivn, the son, the ion, the stn and the
different motor nerves of the STG. To identify IV neuron spikes, we measured
the direction of spike propagation on the ivn with two extracellular
electrodes. The spikes of the IV neurons could also be monitored on the
son and the stn recordings
(Fig. 4A). Intriguingly, we
were unable to monitor IV activity on the ion
(Fig. 4A; but see Discussion),
although the backfills showed that the IV axon projects through that nerve.
Nevertheless, the spikes observed on the ivn, son and stn
could be attributed to the IV neurons, because our backfills of the
son and stn demonstrated that only the IV neurons project
axons through the ivn, son and stn.
|
ivn stimulation inhibits the pyloric rhythm
In other crustacean species, IV neuron activity is known to weaken or
terminate the pyloric rhythm (Christie et
al., 2004; Dando and
Selverston, 1972; Marder and
Eisen, 1984a; Sigvardt and
Mulloney, 1982a; Sigvardt and
Mulloney, 1982b). To examine the effects of the IV neurons on the
motor circuits in Cancer pagurus, we stimulated the ivn
extracellularly (Christie et al.,
2004). These experiments were performed in the isolated nervous
system either with the brain attached or without the brain. According to the
IV neuron spike frequencies observed in intact animals and in isolated
preparations, we used stimulation frequencies between 10 and 40 Hz. We then
characterized the effects on the pyloric rhythm by measuring pyloric cycle
period and the activities of the different pyloric neurons. Since the action
potentials of some pyloric neurons are difficult to separate in extracellular
recordings, we used intracellular recordings of single pyloric neurons to
determine their response to ivn stimulation.
Stimulation frequencies of 10 and 15 Hz (10 Hz: N=2; 15 Hz:
N=5) did not yield significant changes in the pyloric motor pattern
(data not shown). Stimulation frequencies of 20 Hz and above, by contrast,
clearly affected the pyloric rhythm. We found consistent results for both 20
and 40 Hz stimulation. ivn stimulation caused a small but significant
decrease in pyloric cycle period from 1.08±0.4 to 1.04±0.3 s
(N=11, P<0.005, ANOVA) during 20 Hz stimulation and from
1.05±0.3 to 0.95±0.2 s (N=14, P<0.05,
ANOVA; Fig. 5B) during 40 Hz
stimulation. Conversely, the activity of the PD motor neuron, which is
electrically coupled to the pacemaker neuron anterior burster
(Eisen and Marder, 1982;
Maynard and Selverston, 1975;
Miller and Selverston, 1982)
decreased from 4.3±2.6 spikes burst–1 to
3.96±2.4 spikes burst–1 (N=11,
P<0.002, ANOVA) during 20 Hz stimulation and from 4.12±1.9
to 3.47±1.6 spikes burst–1 (N=14,
P<0.001, ANOVA; Fig.
5A,B) during 40 Hz stimulation. The activity of the LP motor
neuron also decreased from 5.04±2.1 to 4.04±2.0 spikes
burst–1 (N=15, P<0.001, ANOVA) during 20
Hz ivn stimulation and from 4.82±2.2 to 2.88±2.1 spikes
burst–1 (N=17, P<0.001, ANOVA;
Fig. 5A,B) during 40 Hz
stimulation.
|
The activity of the pyloric neurons VD and IC decreased during ivn stimulation. In the VD neuron, the number of spikes per burst decreased from 2.48±1.6 to 1.28±0.7spikesburst–1 (N=11, P<0.05, ANOVA) during 20 Hz stimulation and from 3.03±1.8 to 0.82±0.6 spikes burst–1 (N=13, P<0.05, ANOVA; Fig. 5B) during 40 Hz stimulation. The activity of the IC neuron changed from 4.80±1.1 to 4.23±1.2 spikes burst–1 (N=7, P<0.05, ANOVA) during 20 Hz stimulation and from 5.48±1.3 to 3.16±1.0 spikes burst–1 (N=10, P<0.001, ANOVA; Fig. 5B) during 40 Hz stimulation.
In contrast to findings in other crustacean species, we found no postsynaptic potentials in pyloric neurons that were elicited by the ivn stimulation (see Discussion).
ivn stimulation elicits gastric mill rhythms
Gastric mill rhythms typically have a 10-fold longer cycle period than
pyloric rhythms (Bartos et al.,
1999; Weimann and Marder,
1994; Weimann et al.,
1991). Our results demonstrate that the cycle period of the
rhythmic IV neuron activity in intact animals and in isolated ganglion
preparations ranged within the same order of magnitude. For investigating the
effects of such rhythmic IV neuron activity on the gastric mill rhythm, we
thus applied 10 trains of ivn stimuli with 20 or 40 Hz intratrain
frequency and durations between 2 and 6 s, respectively. Intertrain intervals
ranged from 1 to 20 s, resulting in stimulation periods of 3 to 26 s. The
gastric mill rhythm was monitored with either extracellular or intracellular
recordings.
In all preparations without spontaneously active gastric mill rhythms
(N=31), 40Hz ivn stimulations elicited gastric mill rhythms,
when the stimulation period was longer than 4 s (example shown in
Fig. 6A). Stimulation periods
shorter than 4 s or train durations of 3 s or less did not elicit gastric mill
rhythms (Fig. 6Bi). If present,
gastric mill rhythms persisted for the duration of the stimulation. Every
stimulus train elicited a LG neuron burst, such that LG neuron was time locked
to the stimulus, with the stimulus preceding the LG neuron bursts. In the
example shown in Fig. 6A, the
delay between the start of the ivn stimulus and the LG neuron burst
was 2.68±0.29 s (n=10). For all animals, the delay averaged
3.65±1.1 s (N=31). As a consequence, the gastric mill period
corresponded to the stimulus period. This was true over a broad range of
stimulus periods, as shown in Fig.
6Bi (6
N32; regression line slope 1.137,
R2=0.978).
|
; Sigvardt
and Mulloney, 1982a; Sigvardt
and Mulloney, 1982b). By contrast, we observed that in C.
pagurus, ivn stimulation excited the GM neurons
(Fig. 6A,B; N=13). The
GM neurons started to burst during ivn stimulation, in time with the
LG neuron (Fig. 6Bii). The
phase plot for the gastric mill neurons LG, DG and GM, during ivn
stimuli (stimulus duration 6 s, intertrain interval 6 s) is shown in
Fig. 6Bii (N=13).
Here, the beginning of the LG neuron burst was defined as the start of a
gastric mill cycle. On average, the LG neuron burst ended at phase
0.22±0.09 (N=31). The onset of GM neuron activity averaged at
0.04±0.02 and its end at 0.20±0.03 (N=13), revealing
that the gastric mill neurons LG and GM were active simultaneously. The burst
of the antagonistic DG neuron started at phase 0.34±0.09 and ended at
0.51±0.13 (N=29). The small standard deviations show that the
gastric mill rhythms of different animals were quite similar when identical
stimulus protocols were used. We also measured burst durations and spike frequencies of LG, DG and GM neurons during stimulus trains (Fig. 6Biii; stimulus regime as in Fig. 6Bii). The burst duration during ivn stimulation was 2.55±1.0 s for the LG neuron (N=31), 1.77±0.6 s (N=29) for the DG neuron and 2.63±0.9 s for the GM neuron (N=13); the spike frequency was 7.83±3.8Hz for the LG neuron (N=21), 11.39±4.4Hz (N=21) for the DG neuron and 3.55±2.2 Hz for the GM neuron (N=13).
While we observed many postsynaptic potentials (PSPs) in GM neurons during ivn stimulation, none were time locked to single ivn stimuli, indicating that the GM neurons were activated indirectly via interneurons. Similarly, the gastric mill neurons LG and DG showed no PSPs that were time locked to the stimulus, although both neurons exhibited marked depolarizations caused by the ivn stimulus trains.
In contrast to the 40 Hz stimulations, 20 Hz stimuli did not elicit gastric mill rhythms. LG and the GM neurons, however, received excitation, but were not depolarized above spike threshold (LG: N=31, GM: N=13; data not shown).
|
When we transected the CoG, ivn stimulation no longer elicited gastric mill rhythms (N=10, data not shown). It thus appears that the actions of the IV neurons on the gastric mill rhythm were mainly mediated via their projections to the CoG (see Fig. 2E), indicating an involvement of descending projection neurons located in the CoG.
ivn stimulation affects the esophageal rhythm
Our backfills revealed that the IV neurons project axons not only to the
STG but also to the CoG (via the son and ion;
Fig. 2E). In the CoG,
information from the brain, the thoracic ganglion and several sense organs
converges (Beenhakker and Nusbaum,
2004; Kirby and Nusbaum,
2007). In addition, the motor control circuit for the esophageal
rhythm is located in the CoG (Spirito,
1975). This rhythm can be monitored by the activity of the OMNs.
Like the pyloric rhythm, the esophageal rhythm is usually spontaneously active
in the isolated STNS (Stein et al.,
2005). The OMNs are a bilaterally symmetrical pair of neurons,
with one soma residing within each CoG. Each OMN projects via the
ion to the OG (Fig.
1B) to innervate esophageal muscles in the region of the OG (D. M.
Blitz, M. P. Nusbaum and W.S., unpublished observation). Its activity can be
recorded either with extracellular recordings of the ion or with
intracellular recordings from the OMN somata.
When we stimulated the ivn with 10 stimulus trains (40 Hz intratrain stimulation frequency; duration 6 s, intertrain interval 6 s), we observed that IV neuron activity affected the esophageal rhythm. The original recording of OMN on the ion in Fig. 7A shows that each ivn stimulus train tonically activated OMN. OMN activity started on average 0.7±0.4s (N=22) after the beginning of IV neuron stimulation. As a consequence of its mostly tonic activity, OMN burst duration corresponded to the duration of the stimulus train. The number of spikes per time bin (6 s) increased significantly from 12.0 (median) during rhythmic OMN activity before stimulation to 51.4 (median) during stimulation (Fig. 7B, left; N=11, P<0.001, Wilcoxon signed-rank test).
In between stimulus trains and after the end of the stimulation, OMN resumed its regular bursting activity. The maximum instantaneous firing frequency of OMN, as measured during the last burst before stimulation and during stimulation, did not change significantly (N=11; Fig. 7B, right). The excitation of OMN appeared to be mediated monosynaptically, since it persisted when the CoG were bathed in high divalent saline (N=3, data not shown).
It is currently unknown whether or not OMN is a member of the esophageal CPG. Hence, the excitation of OMN may not reflect an influence of the IV neurons on the esophageal CPG. Indeed, we did not find a consistent effect of ivn stimulation on the immediate phasing of the esophageal rhythm (N=11). By contrast, when we compared OMN activity before and after the end of the stimulus trains, we found that the number of OMN spikes per burst was significantly increased after stimulation (Fig. 7C; median OMN spikes per burst pre-stimulation, 14.41; median post-stimulation, 24.88; N=21, P<0.05, Wilcoxon signed-rank test). The esophageal rhythm also sped up after ivn stimulation; its period decreased significantly from 4.82 s (median) before to 2.96 s (median) after stimulation (Fig. 7C; N=21, P<0.001, Wilcoxon signed-rank test). Additionally, OMN burst duration was reduced (Fig. 7C; median pre-stimulation, 2.44 s; median post-stimulation, 1.26 s; N=21, P<0.001, Wilcoxon signed-rank test) and intraburst spike frequency was enhanced (Fig. 7C; from 6.72Hz (median) to 16.34Hz (median), N=21, P<0.001, Wilcoxon signed-rank test). By contrast, OMN duty cycle did not change (Fig. 7C; P>0.2, N=21, Wilcoxon signed-rank test).
As a consequence of the excitatory influences of the IV neurons on OMN and the gastric mill neuron LG, and their inhibitory actions on pyloric rhythm, the timing of the IV neurons was imposed on all investigated motor patterns. This is shown in Fig. 7A by the simultaneous activation of the OMN and LG neurons and the concurrent inhibition of the pyloric neuron VD. Thus, IV neuron activity synchronized the motor activities of the pyloric, gastric mill and esophageal rhythms.
|
DISCUSSION |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
The activity of the motor circuits in both vertebrates and invertebrates is
regulated by sensory feedback and upstream neural structures
(Brodfuehrer and Thorogood,
2001; Fleischer,
1981; Perrins et al.,
2002; Spirito,
1975), ensuring that the resulting behavior is appropriate for the
situation at hand (for reviews, see
Grillner, 2003;
Rossignol et al., 2006). In
the STNS, most of the regulatory neurons are located in the CoG, which are
connected to the brain and to the thoracic ganglion via the
coc. Backfills of the coc have localized the brain neurons
that are likely candidates for influencing the STNS motor patterns
(Kirby and Nusbaum, 2007).
Indeed, stimulating axons within the coc affects motor activity, for
example, during locomotion (Atwood and
Wiersma, 1967; Bowerman and
Larimer, 1974a; Bowerman and
Larimer, 1974b). The individual neurons responsible for these
effects, as well as their actions on the motor circuits and their input from
the brain, remain to be identified in most cases.
By contrast, previous work has demonstrated the influence of two identified
neurons in the brain, the IV (or PS) neurons, on the STNS motor patterns in
various lobster and crab species (Christie
et al., 2004; Dando and
Selverston, 1972; Hooper et
al., 1990; Meyrand et al.,
1994; Sigvardt and Mulloney,
1982a; Sigvardt and Mulloney,
1982b). The IV neurons project to the STNS via a
unilateral nerve, the ivn. Here, we show for the first time that
their somata are located in cell cluster 17 of the brain and that these
neurons provide exteroceptive sensory information to the motor circuits in the
STNS. We also outline their axonal projection pattern in the crab, C.
pagurus. The IV neurons respond with rhythmic bursts to chemosensory
stimulation of the first antennae. They project to the CoG and the STG, and
they have a pronounced impact on the esophageal, gastric mill and pyloric
motor patterns. Their effects include a synchronization of these rhythms by
providing excitation to the esophageal and gastric mill rhythms and inhibition
to pyloric neurons.
IV neurons relay exteroceptive information from the brain to the motor circuits in the STNS
So far, it has not been clear whether the IV neurons transfer information
from the brain to the STNS and, if so, what kind of information they relay. In
only one study was the activity of neurons projecting through the ivn
assessed in a behavioral approach: in O. limosus, the activities of
all neurons on the ivn were measured during feeding. ivn
activity increased during food intake
(Böhm et al., 2001). Yet
the different spikes were not classified into descending and ascending spikes,
leaving the question of whether the IV neurons or neurons that ascend from the
STNS to the brain responded to the food intake. In addition, animals may have
been exposed to several different sensory stimuli during feeding, such as
mechanical, olfactory and visual stimuli. We thus used the conduction delay
between two hook electrodes attached to the ivn in intact animals to
identify the IV neuron action potentials while we applied different sensory
stimuli in order to determine whether the IV neurons indeed relay information
from the brain to the STNS.
First, we tested the impact of food intake on the ivn units. In
contrast to the experiments on O. limosus where food intake was
voluntary, we injected food directly into the stomach. This circumvented the
activation of sense organs outside of the stomach. In contrast to O.
limosus, neither the descending IV neurons nor the ascending neurons were
activated (Fig. 3A,B). However,
the pyloric rhythm sped up and the phasing of the pyloric neurons was altered.
Clearly, since no neurons on the ivn were affected in C.
pagurus, the effect on the pyloric rhythm must have been mediated
via different pathways. The most likely scenario is that local
sensory feedback from within the stomach, such as that via the
mechanosensitive ventral cardiac neurons
(Beenhakker et al., 2004), was
activated and thus affected the pyloric rhythm.
While neither mechanosensory stimuli applied to the mouth and antennae nor
changes in illumination altered the activity of the ivn units,
chemosensory stimulation of the first antennae activated the IV neurons. The
first antennae are responsible for olfactory and chemosensory perception
(Brock, 1930). In these
experiments, we applied a mixture of seawater and crab food to the antennae.
In a subset of these applications, a gastric mill rhythm started shortly after
the beginning of the application. The finding that the bursts of the gastric
mill protractor motor neuron LG were time locked to the IV neuron bursts
(Fig. 3E), and that IV neuron
bursts preceded those of the LG neuron argues for the hypothesis that the IV
neurons contributed to the activation of the LG neuron. An indirect activation
of the gastric mill rhythm, however, cannot be excluded, because in intact
animals the brain and the STNS are connected not only via the
ivn but also via the coc. In fact, it is also
possible that the IV neurons were entrained into the gastric mill via
ascending pathways from the STNS. This scenario appears unlikely, though,
since in most experiments the IV neurons showed rhythmic activity even in the
absence of a gastric mill rhythm.
Further support for the hypothesis that the IV neurons elicited the gastric
mill rhythm comes from our experiments with isolated nervous systems. In these
experiments the coc had been transected, leaving the ivn as
the only connection between the brain and STNS. Although we never found
rhythmic activity of ascending units on the ivn and, thus,
information related to the gastric mill rhythm could not have been transferred
from the STNS to the brain, the IV neurons were rhythmically active in more
than half of the preparations. In three preparations, the gastric mill motor
neuron LG was activated and a gastric mill rhythm started whenever IV spike
frequency exceeded 30 Hz. The isolated preparation thus showed the same
frequency dependence as the intact animal. A possible reason for this
dependence could be the neurotransmitter complement of the IV neurons. In the
close relative C. borealis these neurons contain the co-transmitters
histamine and the peptide FLRF-amide
(Christie et al., 2004).
Peptide release often requires high-frequency trains of presynaptic action
potentials (Marder, 1998).
Our in vivo experiments yielded a low occurrence of gastric mill
rhythms during chemosensory stimulation. Fleischer showed that optical
information can suppress the gastric mill rhythm for periods of up to 40h
(Fleischer, 1981). It is thus
conceivable that even the dissection of the animal or the illumination applied
when implanting the electrodes reduced the occurrence of gastric mill rhythms
and that this effect lasted throughout the duration of the experiment,
possibly via a reduction of IV neuron firing rate.
Identification of IV neurons
Our extracellular recordings in the isolated STNS show that action
potentials originating in the brain descend via the ivn to
the STNS and that they occur either tonically or rhythmically. These action
potentials could be generated by the IV neurons or by one of the other six
neurons projecting axons through the ivn. In the latter case action
potentials would travel towards the soma (antidromically), a situation known
to exist in the STNS (Bucher et al.,
2003). Several facts argue against this hypothesis, though. For
example, in lobsters, rhythmic activity on the ivn was identified to
be generated by the IV neurons with intracellular recordings from the IV
neuron somata. Furthermore, in those species where the IV neuron somata are
located in the ivn root rather than in the brain, rhythmic activity
was present even in the absence of the brain. By contrast, in C.
borealis and C. pagurus, where the IV neuron somata are located
in the brain, rhythmic activity was never present if the brain was removed
(C. borealis: W.S., unpublished observations). In addition, our
extracellular recordings reveal that spikes that descended from the brain
via the ivn were present on both son and the
stn. At the same time, backfills from the son to the brain
prove that only two axons project from the son to the brain, namely
those of the IV neurons. Thus, the descending action potentials on the
ivn must indeed be generated by the IV neurons.
The backfill stainings demonstrate that the IV neurons project one axon
each via the son and ion to the CoG. Yet we were
unable to detect the IV neuron spike on the ion. This corresponds to
findings in C. borealis (Christie
et al., 2004) where stimulation of the IV neurons activates the
modulatory projection neuron 1 (MCN1) in the CoG solely via its axon
in the son but not the ion
(Christie et al., 2004). In
H. americanus Lucifer Yellow staining of the PS neurons show that
these neurons project an axon through the ion, but ion
spikes that correlate with the PS soma potentials have never been observed
(Cazalets et al., 1990).
Furthermore, action potentials elicited by antidromic stimulation of the
ion do not invade the soma. The function of the axon that projects
through the ion is thus unclear. One possibility could be that this
axon is functionally inactivated unless the adequate neuromodulatory
environment is present. The region of the STNS where the IV neuron axon
divides into stn and ion branches is under modulatory
control (Goaillard et al.,
2004), which may lead to a functional compartmentalization of the
IV neurons. An example of such a compartmentalization is found in Aplysia
californica. In the mechanosensory neuron B21 of this mollusk, spike
initiation and propagation is regulated via synaptic control
(Cropper et al., 2004;
Evans et al., 2003). Depending
on synaptic activity and the local membrane potential, action potentials are
either actively propagated to a lateral process of the cell or do not enter
this part of the neuron. A similar mechanism in the IV neurons could gate
spike propagation to the ion, depending on the neuromodulatory
conditions. Neuromodulators like octopamine, for example, act on active
properties of axons in this region of the STNS and they affect spike
propagation (Goaillard et al.,
2004).
Effect on the motor patterns
The effects of the IV (or PS) neurons on the different motor patterns
(pyloric, gastric mill and esophageal rhythms) were described previously in
different crustacean species (Claiborne
and Selverston, 1984; Dando
and Selverston, 1972; Faumont
et al., 2005; Hooper et al.,
1990; Marder and Eisen,
1984a; Marder and Eisen,
1984b; Russell and Hartline,
1981; Sigvardt and Mulloney,
1982a; Sigvardt and Mulloney,
1982b). It appears that the effects of the IV neurons on the
pyloric rhythm are more similar among brachyuran species than between
Brachyura, Astacidea and Palinura. For example, we did not observe any PSPs in
pyloric neurons, which is similar to C. borealis, but is in contrast
to the findings for the spiny lobster P. interruptus
(Hooper et al., 1990;
Marder, 1984;
Russell and Hartline, 1981;
Sigvardt and Mulloney, 1982a)
and H. gammarus (Cazalets et al.,
1987; Faumont et al.,
2005; Meyrand et al.,
1991; Meyrand et al.,
1994). Also, in C. pagurus and C. borealis, ivn
stimulation inhibited VD and IC neurons, which contrasts with the findings for
P. interruptus (Hooper et al.,
1990; Russell and Hartline,
1981; Sigvardt and Mulloney,
1982b). In all species
(Christie et al., 2004;
Dando and Selverston, 1972;
Sigvardt and Mulloney, 1982b),
however, the activity of the PD motor neuron decreased in response to IV
activation (Fig. 5A,B).
While the effects of the IV neurons on most pyloric neurons were consistent
in all experiments, the PY neurons showed three different responses to
ivn stimulation: PY activity increased during ivn
stimulation or it decreased or it did not change. Different responses of the
PY neurons to ivn stimulation have also been reported in the lobster
P. vulgaris (Hooper et al.,
1990): here, ivn stimulation caused long-term
inactivation, long-term depolarization or no long-lasting response in
different PY neurons. The findings in C. borealis were different,
though. Here, the PY neurons started to fire tonically during ivn
stimulation (Christie et al.,
2004). In this study, however, PY activity was analyzed
extracellularly, so that possible differences in PY responses may not have
been detected. Until now, little was known about the interactions between the
PY neurons and about their different actions, but they have been divided into
subsets in Panulirus (Hartline et
al., 1987) on the basis of their responses to stimulation of an
afferent pathway within the STNS. Our results further support this
subdivision.
One of the particularly noticeable effects of the PS neuron in the lobster
H. gammarus is its action of combining the pyloric, gastric mill and
esophageal rhythms into a single, conjoint motor pattern
(Meyrand et al., 1991), which
is entrained by PS activity. In C. pagurus, IV neuron stimulation
either started or entrained the gastric mill rhythm
(Fig. 6A,B) such that each LG
neuron burst was time locked to the ivn stimulus trains. Thus, the
gastric mill period corresponded to the stimulus period. The pyloric rhythm,
by contrast, continued in between ivn stimulus trains, but received
phasic inhibitory input during the trains. While the pyloric and gastric mill
circuits are located in the STG, the third spontaneously active motor rhythm
in the isolated STNS of the crab – the circuit generating the esophageal
rhythm – resides within the CoG
(Spirito, 1975). We found that
the IV neurons projected via the son and ion to the
CoG and that they monosynaptically excited the OMN such that OMN activity
increased during ivn stimulation
(Fig. 7B). These results
correspond to the findings in C. borealis
(Christie et al., 2004). The
excitation of OMN did not change the phasing of the esophageal rhythm,
however. Similar to the pyloric rhythm, the esophageal rhythm continued to
oscillate in between ivn stimulus trains. It thus appears that in
C. pagurus, the IV neurons cannot conjoin the pyloric, gastric mill
and esophageal rhythms. Rather, they impose their timing on the pyloric and
esophageal rhythms by providing inhibitory and excitatory drive, respectively,
to the motor neurons. Nevertheless, we found that the properties of the
esophageal rhythm were different after the end of stimulation from what they
had been before stimulation (Fig.
7B). This indicates that the IV neurons possessed (limited) access
to the esophageal CPG, the effects of which outlasted the stimulation.
In preparations where the CoG, and thus also the IV neuron projections to
these ganglia had been removed, ivn stimulation did not elicit a
gastric mill rhythm. One possible explanation for this absence of the gastric
mill rhythm is that the rhythm depended on excitation provided by one of the
CoG projection neurons (Nusbaum and
Beenhakker, 2002) that were activated during ivn
stimulation. In C. borealis, two identified projection neurons, the
modulatory commissural neuron 1 (Coleman
and Nusbaum, 1994; Coleman et
al., 1992) and the commissural projection neuron 2
(Blitz and Nusbaum, 1999),
appear to contribute to the response of the gastric mill neurons
(Christie et al., 2004). The
fact that we did not observe PSPs in gastric mill neurons that were time
locked to the ivn stimulus supports the hypothesis of an indirect
excitation via projection neurons, especially since the GM neurons
received many excitatory PSPs during the ivn stimulus train that were
not time locked to the stimulus. These PSPs most probably originated from
commissural projection neuron 2 (Blitz and
Nusbaum, 1999), the only known source of excitatory input to the
GM neurons (Stein et al.,
2005). Our results thus indicate that projection neurons located
in the CoG are involved in the processing of exteroceptive sensory
information. These neurons are well known to process proprioceptive and
mechanosensory information (Beenhakker and
Nusbaum, 2004; Simmers and
Moulins, 1988a; Simmers and
Moulins, 1988b; Smarandache
and Stein, 2007) from sense organs within the stomatogastric
system. It is thus conceivable that they represent a point of multisensory
convergence, processing information from proprioceptors and exteroceptors. Our
study provides an initial characterization of identified neurons located
within the C. pagurus brain that relay exteroceptive chemosensory
information to the STNS via CoG neurons known to regulate STNS motor
patterns. More generally, we provide a framework for future
electrophysiological experiments aimed at characterizing the processing of
descending information from the brain, which should, in turn, benefit our
understanding of the principles that underlie the regulatory control of motor
pattern generation at the cellular level.
LIST OF ABBREVIATIONS
|
Acknowledgments |
|---|
|
References |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
Atwood, H. L. and Wiersma, C. A. G. (1967).
Command interneurons in the crayfish central nervous system. J.
Exp. Biol. 46,249
-261.
Bartos, M. and Nusbaum, M. P. (1997). Intercircuit control of motor pattern modulation by presynaptic inhibition. J. Neurosci. 17,2224 -2256.
Bartos, M., Manor, Y., Nadim, F., Marder, E. and Nusbaum, M.
P. (1999). Coordination of fast and slow rhythmic neuronal
circuits. J. Neurosci.
19,6650
-6660.
Bässler, U. (1993). The femur-tibia control system of stick insects – a model system for the study of the neural basis of joint control. Brain Res. Brain Res. Rev. 18,207 -226.[CrossRef][Medline]
Beenhakker, M. P. and Nusbaum, M. P. (2004).
Mechanosensory activation of a motor circuit by coactivation of two projection
neurons. J. Neurosci.
24,6741
-6750.
Beenhakker, M. P., Blitz, D. M. and Nusbaum, M. P.
(2004). Long-lasting activation of rhythmic neuronal activity by
a novel mechanosensory system in the crustacean stomatogastric nervous system.
J. Neurophysiol. 91,78
-91.
Blitz, D. M. and Nusbaum, M. P. (1997). Motor
pattern selection via inhibition of parallel pathways. J.
Neurosci. 17,4965
-4975.
Blitz, D. M. and Nusbaum, M. P. (1999).
Distinct functions for cotransmitters mediating motor pattern selection.
J. Neurosci. 19,6774
-6783.
Böhm, H., Dybek, E. and Heinzel, H. G. (2001). Anatomy and in vivo activity of neurons connecting the crustacean stomatogastric nervous system to the brain. J. Comp. Physiol. A Neuroethol. Sens. Neural Behav. Physiol. 187,393 -403.[CrossRef][Medline]
Bowerman, R. F. and Larimer, J. L. (1974a).
Command fibres in the circumoesophageal connectives of crayfish I. Tonic
fibres. J. Exp. Biol.
60, 95-117.
Bowerman, R. F. and Larimer, J. L. (1974b).
Command fibres in the circumoesophageal connectives of crayfish II. Phasic
fibres. J. Exp. Biol.
60,119
-134.
Brock, F. (1930). Das Verhalten der ersten Antennen von Brachyuren und Anomuren in bezug auf das umgebende Medium. J. Comp. Physiol. A Neuroethol. Sens. Neural. Behav. Physiol. 11,774 -790.
Brodfuehrer, P. D. and Thorogood, M. S. E. (2001). Identified neurons and leech swimming behavior. Prog. Neurobiol. 63,371 -381.[CrossRef][Medline]
Bucher, D., Thirumalai, V. and Marder, E.
(2003). Axonal dopamine receptors activate peripheral spike
initiation in a stomatogastric motor neuron. J.
Neurosci. 23,6866
-6875.
Cazalets, J. R., Nagy, F. and Moulins, M. (1987). Suppressive control of a rhythmic central pattern generator by an identified modulatory neuron in crustacea. Neurosci. Lett. 81,267 -272.[CrossRef][Medline]
Cazalets, J. R., Nagy, F. and Moulins, M. (1990). Suppressive control of the crustacean pyloric network by a pair of identified interneurons. I. Modulation of the motor pattern. J. Neurosci. 10,448 -457.[Abstract]
Christie, A. E., Stein, W., Quinlan, J. E., Beenhakker, M. P., Marder, E. and Nusbaum, M. P. (2004). Actions of a histaminergic/peptidergic projection neuron on rhythmic motor patterns in the stomatogastric nervous system of the crab Cancer borealis. J. Comp. Neurol. 469,153 -169.[CrossRef][Medline]
Claiborne, B. J. and Selverston, A. I. (1984). Histamine as a neurotransmitter in the stomatogastric nervous system of the spiny lobster. J. Neurosci. 4, 708-721.[Abstract]
Coleman, M. J. and Nusbaum, M. P. (1994). Functional consequences of compartmentalization of synaptic input. J. Neurosci. 14,6544 -6552.[Abstract]
Coleman, M. J., Nusbaum, M. P., Cournil, I. and Claiborne, B. J. (1992). Distribution of modulatory inputs to the stomatogastric ganglion of the crab, Cancer borealis. J. Comp. Neurol. 325,581 -594.[CrossRef][Medline]
Cropper, E. C., Evans, C. G., Jing, J., Klein, A., Proekt, A., Romero, A. and Rosen, S. C. (2004). Regulation of afferent transmission in the feeding circuitry of Aplysia. Acta. Biol. Hung. 55,211 -220.[CrossRef][Medline]
Dando, M. R. and Selverston, A. I. (1972). Command fibres from the supra-oesophageal ganglion to the stomatogastric ganglion in Panulirus argus. J. Comp. Physiol. A Neuroethol. Sens. Neural. Behav. Physiol. 78,138 -175.[CrossRef]
Eisen, J. S. and Marder, E. (1982). Mechanisms
underlying pattern generation in lobster stomatogastric ganglion as determined
by selective inactivation of identified neurons. III. Synaptic connections of
electrically coupled pyloric neurons. J. Neurophysiol.
48,1392
-1415.
Evans, C. G., Jing, J., Rosen, S. C. and Cropper, E. C.
(2003). Regulation of spike initiation and propagation in an
Aplysia sensory neuron: gating-in via central depolarization.
J. Neurosci. 23,2920
-2931.
Faumont, S., Combes, D., Meyrand, P. and Simmers, J. (2005). Reconfiguration of multiple motor networks by short- and long-term actions of an identified modulatory neuron. Eur. J. Neurosci. 22,2489 -2502.[Medline]
Fleischer, A. G. (1981). The effect of eyestalk hormones on the gastric mill in the intact lobster, Panulirus interruptus. J. Comp. Physiol. A Sens. Neuroethol. Behav. Physiol. 141,363 -368.[CrossRef]
Garm, A., Shabani, S., Høeg, J. T. and Derby, C. D. (2005). Chemosensory neurons in the mouthparts of the spiny lobsters Panulirus argus and Panulirus interruptus (Crustacea: Decapoda). J. Exp. Mar. Biol. Ecol. 314,175 -186.[CrossRef]
Goaillard, J. M., Schulz, D. J., Kilman, V. L. and Marder,
E. (2004). Octopamine modulates the axons of modulatory
projection neurons. J. Neurosci.
24,7063
-7073.
Grillner, S. (2003). The motor infrastructure: from ion channels to neuronal networks. Nat. Rev. Neurosci. 4,573 -586.[Medline]
Hartline, D. K. and Maynard, D. M. (1975).
Motor patterns in the stomatogastric ganglion of the lobster Panulirus
argus. J. Exp. Biol. 62,405
-420.
Hartline, D. K., Gassie, D. V. and Sirchia, C. D. (1987). PY cell types in the stomatogastric ganglion of Panulirus. In The Crustacean Stomatogastric System (ed. A. Selverston and M. Moulins), pp.75 -77. Berlin: Springer.
Heinzel, H. G., Weimann, J. M. and Marder, E. (1993). The behavioral repertoire of the gastric mill in the crab, Cancer pagurus: an in situ endoscopic and electrophysiological examination. J. Neurosci. 13,1793 -1803.[Abstract]
Hooper, S. L., Moulins, M. and Nonnotte, L.
(1990). Sensory input induces long-lasting changes in the output
of the lobster pyloric network. J. Neurophysiol.
64,1555
-1573.
Katz, P. S., Eigg, M. H. and Harris-Warrick, R. M.
(1989). Serotonergic/cholinergic muscle receptor cells in the
crab stomatogastric nervous system. I. Identification and characterization of
the gastropyloric receptor cells. J. Neurophysiol.
62,558
-570.
Kirby, M. S. and Nusbaum, M. P. (2007). Central nervous system projections to and from the commissural ganglion of the crab Cancer borealis. Cell Tissue Res. 328,625 -637.[CrossRef][Medline]
Lieske, S. P., Thoby-Brisson, M., Telgkamp, P. and Ramirez, J. M. (2000). Reconfiguration of the neural network controlling multiple breathing patterns: eupnea, sighs and gasps. Nat. Neurosci. 3,600 -607.[CrossRef][Medline]
Marder, E. (1984). Roles for electrical
coupling in neural circuits as revealed by selective neuronal deletions.
J. Exp. Biol. 112,147
-167.
Marder, E. (1998). From biophysics to models of network function. Annu. Rev. Neurosci. 21, 25-45.[CrossRef][Medline]
Marder, E. (2000). Motor pattern generation. Curr. Opin. Neurobiol. 10,691 -698.[CrossRef][Medline]
Marder, E. and Bucher, D. (2001). Central pattern generators and the control of rhythmic movements. Curr. Biol. 11,986 -996.[CrossRef]
Marder, E. and Bucher, D. (2007). Understanding circuit dynamics using the stomatogastric nervous system of lobsters and crabs. Annu. Rev. Physiol. 69,291 -316.[CrossRef][Medline]
Marder, E. and Calabrese, R. L. (1996).
Principles of rhythmic motor pattern generation. Physiol.
Rev. 76,687
-717.
Marder, E. and Eisen, J. S. (1984a).
Electrically coupled pacemaker neurons respond differently to same
physiological inputs and neurotransmitters. J.
Neurophysiol. 51,1362
-1374.
Marder, E. and Eisen, J. S. (1984b).
Transmitter identification of pyloric neurons: electrically coupled neurons
use different transmitters. J. Neurophysiol.
51,1345
-1361.
Maynard, D. M. and Dando, M. R. (1974). The structure of the stomatogastric neuromuscular system in Callinectes sapidus, Homarus americanus and Panulirus argus (Decapoda Crustacea). Philos. Trans. R. Soc. Lond., B, Biol. Sci. 268,161 -220.[Medline]
Maynard, D. M. and Selverston, A. I. (1975). Organization of the stomatogastric ganglion of the spiny lobster. J. Comp. Physiol. A Neuroethol. Sens. Behav. Physiol. 100,161 -182.[CrossRef]
Meyrand, P., Simmers, J. and Moulins, M. (1991). Construction of a pattern-generating circuit with neurons of different networks. Nature 351, 60-63.[CrossRef][Medline]
Meyrand, P., Simmers, J. and Moulins, M. (1994). Dynamic construction of a neural network from multiple pattern generators in the lobster stomatogastric nervous system. J. Neurosci. 14,630 -644.[Abstract]
Miller, J. P. and Selverston, A. I. (1982).
Mechanisms underlying pattern generation in lobster stomatogastric ganglion as
determined by selective inactivation of identified neurons. IV. Network
properties of pyloric system. J. Neurophysiol.
48,1416
-1432.
Nusbaum, M. P. (2002). Regulating peptidergic modulation of rhythmically active neural circuits. Brain Behav. Evol. 60,378 -387.[CrossRef][Medline]
Nusbaum, M. P. and Beenhakker, M. P. (2002). A small-systems approach to motor pattern generation. Nature 417,343 -350.[CrossRef][Medline]
Orlov, J. (1929). Über den histologischen Bau der Ganglien des Mundmagennervensystems der Crustaceen. Cell Tissue Res. 8,493 -541.
Pearson, K. G. (2004). Generating the walking gait: role of sensory feedback. Prog. Brain Res. 143,123 -129.[Medline]
Perrins, R., Walford, A. and Roberts, A.
(2002). Sensory activation and role of inhibitory reticulospinal
neurons that stop swimming in hatchling frog tadpoles. J.
Neurosci. 22,4229
-4240.
Rossignol, S., Dubuc, R. and Gossard, J. P.
(2006). Dynamic sensorimotor interactions in locomotion.
Physiol. Rev. 86,89
-154.
Russell, D. F. and Hartline, D. K. (1981). A multiaction synapse evoking both EPSPs and enhancement of endogenous bursting. Brain Res. 223,19 -38.[CrossRef][Medline]
Sandeman, D., Sandeman, R., Derby, C. and Schmidt, M. (1992). Morphology of the brain of crayfish, crabs, and spiny lobsters: a common nomenclature for homologous structures. Biol. Bull. 183,304 -326.[Abstract]
Selverston, A. I. and Moulins, M. (1987). The Crustacean Stomatogastric System: A Model for the Study of Central Nervous Systems. Germany: Springer-Verlag.
Sigvardt, K. A. and Mulloney, B. (1982a).
Properties of synapses made by IVN command-interneurones in the stomatogastric
ganglion of the spiny lobster Panulirus interruptus. J.
Exp. Biol. 97,153
-168.
Sigvardt, K. A. and Mulloney, B. (1982b).
Sensory alteration of motor patterns in the stomatogastric nervous system of
the spiny lobster Panulirus interruptus. J. Exp. Biol.
97,137
-152.
Simmers, J. and Moulins, M. (1988a). A
disynaptic sensorimotor pathway in the lobster stomatogastric system.
J. Neurophysiol. 59,740
-756.
Simmers, J. and Moulins, M. (1988b). Nonlinear
interneuronal properties underlie integrative flexibility in a lobster
disynaptic sensorimotor pathway. J. Neurophysiol.
59,757
-777.
Smarandache, C. R. and Stein, W. (2007).
Sensory-induced modification of two motor patterns in the crab, Cancer
pagurus. J. Exp. Biol. 210,2912
-2922.
Spirito, C. P. (1975). The organization of the crayfish oesophageal nervous system. J. Comp. Physiol. A Neuroethol. Sens. Behav. Physiol. 102,237 -249.[CrossRef]
Stein, W., Eberle, C. C. and Hedrich, U. B. S. (2005). Motor pattern selection by nitric oxide in the stomatogastric nervous system of the crab. Eur. J. Neurosci. 21,2767 -2781.[CrossRef][Medline]
Stein, W., Smarandache, C. R., Nickmann, M. and Hedrich, U.
B. (2006). Functional consequences of activity-dependent
synaptic enhancement at a crustacean neuromuscular junction. J.
Exp. Biol. 209,1285
-1300.
Weimann, J. M. and Marder, E. (1994). Switching neurons are integral members of multiple oscillatory networks. Curr. Biol. 4,896 -902.[CrossRef][Medline]
Weimann, J. M., Meyrand, P. and Marder, E.
(1991). Neurons that form multiple pattern generators:
identification and multiple activity patterns of gastric/pyloric neurons in
the crab stomatogastric system. J. Neurophysiol.
65,111
-122.
Wilson, D. M. (1961). The central nervous control of flight in a locust. J. Exp. Biol. 38,471 -490.[Abstract]
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati 
Twitter What's this?
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
