close

Вход

Забыли?

вход по аккаунту

?

291

код для вставкиСкачать
JEZ 780
THE JOURNAL OF EXPERIMENTAL ZOOLOGY 278:119–132 (1997)
Estimation of the Size and Directional Output of
Functional Groups of Interneurons Underlying
Abdominal Positioning Behaviors in Crayfish
LAWRENCE D. BREWER AND JAMES L. LARIMER*
Department of Zoology, University of Texas at Austin, Austin, Texas
ABSTRACT
Quantitative studies were made of a large population of interneurons that controls postural flexion and extension of the crayfish abdomen. The number of interneurons needed
to produce a motor program was estimated by stimulating a single abdominal positioning interneuron and recording interneuronal activity that was evoked from rostral and caudal connectives
in an isolated abdominal nerve cord. We also examined the role that these functional groups
have in producing a stronger motor output in either a rostral or caudal direction and thus specifying various abdominal geometrics. The average number of interneurons responding to stimulation of a single abdominal positioning interneuron was 32 (range: 3–50; n = 27). The average
number of interneurons that decreased activity was 10 (range: 2–32). Of 653 activated interneurons from 20 preparations, approximately 43% fired between 2 and 5 Hz, 33% fired between 6
and 15 Hz, and 25% fired >15 Hz. The size of a recruited group was usually but not always
correlated with the strength of its motor response or with the direction of motor bias. Therefore,
the contribution of a group may depend upon the number of active elements as well as synaptic
efficacy. J. Exp. Zool. 278:119–132, 1997. © 1997 Wiley-Liss, Inc.
Vertebrate as well as invertebrate movements
are encoded within the central nervous system
(CNS) by populations of cells (Hensler, ’88; Lee et
al., ’88; Zecevic et al., ’89; Churchland and Sejnowski, ’92; Georgopoulos et al., ’92; Tsau et al.,
’94). For example, in crayfish and lobsters groups
of synaptically interacting premotor interneurons
control postural movements of the segmented abdomen (Miall and Larimer, ’82a; Jellies and
Larimer, ’85, ’86; Jones and Page, ’86; Larimer
’88; Murphy et al., ’89). Abdominal positioning is
a relatively simple behavior involving extension
and flexion, however, the number of abdominal
positions ranging from full extension to full flexion is quite large. Many of these movements are
produced by different intensities of motor output
in different segments or directions of the abdominal nerve cord, sometimes referred to as a motor
bias (Kennedy et al., ’67; Jones and Page,’86;
Larimer and Pease, ’88). A biased motor output is
considered a major mechanism to produce the almost infinite number of abdominal geometries routinely seen in animals such as crayfish and
lobsters.
In crayfish hundreds of premotor abdominal positioning interneurons (APIs) have been described
both physiologically and morphologically in abdominal ganglia one through six and many are
© 1997 WILEY-LISS, INC.
considered identified cells (Miall and Larimer,
’82a, b; Larimer and Jellies, ’83; Larimer and
Moore, ’84; Jellies and Larimer, ’85, ’86; Larimer
and Pease, ’88; Murphy et al., ’89). An API may
originate in any of the six abdominal ganglia with
one or more axons projecting in the rostral and/or
caudal directions. All axons that project caudally
terminate in the sixth abdominal ganglion (A6),
and axons that project rostrally extend through
A1 into the thoracic ganglia and some terminate
in the brain (see Larimer and Jellies, ’83: Fig. 7;
Miall and Larimer, ’82b; Larimer and Moore, ’84).
Furthermore, each API extends dendrites into
most if not all ganglia that it projects through.
This type of organization has the potential for extensive recruitment and other synaptic interactions among large groups of cells throughout the
abdominal nerve cord. Numerous experiments
have indicated that APIs operate as members or
elements of a group to produce abdominal positioning movements (Miall and Larimer, ’82a; Jellies and Larimer, ’85, ’86; Murphy et al., ’89).
Using population statistics (Lincoln index and
*Correspondence to: James L. Larimer, Department of Zoology,
University of Texas at Austin, Austin, TX 78712. E-mail: neuroserf
@mail.utexas.edu
Received 7 November 1996; Revision accepted 23 December 1996
120
L.D. BREWER AND J.L. LARIMER
a maximum likelihood method) Larimer and Pease
(’88) estimated that there are about 360 APIs in
abdominal ganglia one through six. However, the
number of APIs that are active during any given
behavior is not known. We have designed a
method to estimate the number of interneurons
or APIs that are synaptically recruited when current is injected into a single API in an isolated
abdominal nerve cord. The number of recruited
interneurons was estimated by recording from the
connectives both rostral and caudal to the ganglion where the impaled API was located. The
number of interneurons or other APIs that are
excited can be determined since the axon(s) of each
API projects through the abdominal nerve cord
into the thoracic nerve cord and/or terminates in
the sixth and last abdominal ganglion. This study
also addressed the question of whether interneuronal activity can be used to predict the direction
of motor bias. Motor bias is defined as having a
stronger motor output in either the rostral or caudal ganglia. The direction of motor bias was compared to the size and firing frequency of the
recruited interneuronal group.
The use of isolated nerve cords offers the advantage of eliminating the confusion of sensory
feedback in the system. For example, in an intact
behaving animal it would be almost impossible to
distinguish APIs from sensory and other cells. Our
method, however, does not provide a direct measure of the number of interneurons activated in a
freely behaving animal; rather, these results represent an estimate of the number of interneurons
required to produce a fictive behavior. The motor
outputs produced by this method are very similar
if not identical to motor outputs observed in restrained, semi-intact preparations (Jellies and
Larimer, ’86; Murphy et al., ’89) (Larimer, personal observation). This type of motor output has
also been shown to produce abdominal movements
in behaving animals (Kennedy et al., ’67; Larimer
and Eggleston, ’71).
The results reported here support the model proposed by Larimer (’88) that APIs operate within
distributed circuits and that functional groups
arise from synaptic recruitment. Evidence indicated that in addition to the size of an interneuronal group, firing frequency, synaptic efficacy, and
local circuits also influence abdominal positioning motor outputs including motor bias.
MATERIALS AND METHODS
Both male and female crayfish, Procambarus
clarkii, were used in these experiments. All indi-
viduals had a rostral-telson length of approximately 8–15 cm. The animals were obtained from
Waubun Laboratories (Schriever, LA), maintained
in dechlorinated tap water at 18°C, and fed dry
commercial cat food.
Animals were anesthetized on ice and then dissected in van Harreveld’s saline (van Harreveld,
’36). Isolated abdominal nerve cords with motor
roots attached were pinned ventral side up in a
Sylgard-lined glass Petri dish filled with fresh saline. The first and sixth abdominal ganglia were
removed and the 1–2 (rostral) and 5–6 (caudal)
connectives were desheathed and teased into
bundles for extracellular recording (Fig. 1). The
third abdominal ganglion was desheathed with
fine forceps for intracellular impalement with microelectrodes. All APIs were impaled in the third
abdominal ganglion (A3). This ganglion was selected because a large number of abdominal positioning elements were identified in A3 in earlier
studies.
A WPI 767 intracellular probe with bridge balance (up to 1,000 MΩ) was coupled to microelectrodes for stimulating and recording from impaled
APIs. Microelectrodes were sometimes filled with
3 M KCl (20–40 MΩ). Alternatively, microelectrode
tips were filled with an aqueous 3% Lucifer Yellow CH solution (Sigma, St. Louis, MO), and the
shanks were filled with 1 M lithium chloride (50–
150 MΩ) (Stewart, ’78).
In most experiments, following physiological examination, the fluorescent dye Lucifer Yellow CH
(Sigma) was injected into impaled APIs by passing hyperpolarizing pulses between 3 and 5 nA
for 500–750 ms every second or 1.5 s for 5–30 min.
Nerve cords were then fixed in 4% paraformaldehyde for 12–24 h, rinsed in Sorenson’s buffer (3 ×
30 min washes), dehydrated in an ethanol series
from 50 to 100%, and cleared in methyl salicylate
for 30 min. Dye-filled APIs were viewed with a
compound microscope under near ultraviolet (UV)
light (430 nm) and drawn using a camera lucida.
Suction electrodes were used to record spontaneous and evoked interneuronal activity from axon
bundles in the rostral and caudal connectives (Fig.
1). Fictive tonic abdominal flexion motor activity
was usually recorded from the third superficial
motor roots of the second abdominal ganglion
(A2F) and from the fourth abdominal ganglion
(A4F). Extensor activity was recorded from the
second motor root of the fourth abdominal ganglion
(A4E) (Fig. 1). All extracellular electrical activity
was amplified using differential AC amplifiers (AM Systems, model 1700; Everett, WA).
SIZE AND OUTPUT OF GROUPS OF INTERNEURONS
Fig. 1. Diagram showing an isolated abdominal nerve cord
preparation with the first and sixth ganglia removed. A microelectrode (IN) was used to stimulate, record, and dye-fill
APIs in the third abdominal ganglion (A3). Suction electrodes
were used to monitor flexor motor activity from the superficial third roots (R3) of the second (A2F) and fourth (A4F)
abdominal ganglia. Extensor motor activity was monitored
from the second root (R2) of the fourth abdominal ganglion
(A4E). Suction electrodes were also used to record interneuronal activity from the rostral (RC) and caudal connectives
(CC), which were teased into bundles.
121
Any changes in interneuronal impulse activity
in the ventral nerve cord in response to stimulation of an impaled API were recorded from the
rostral and caudal connectives (Fig. 1). This allowed us to determine the number of interneurons recruited during a fictive motor program (see
below). Furthermore, stimulation of some APIs
produces a stronger motor output (bias) in the rostral or caudal direction. Hence, we were also able
to correlate this directional motor bias with interneuronal activity in the nerve cord.
Motor activity was displayed and photographed
on a Tektronix 5111A storage oscilloscope (Beaverton, OR) or recorded directly with an Astro-Med
Dash IV chart recorder (West Warrick, RI). Interneuronal activity was stored on tape using a Vetter
FM VHS recorder (Rebersburg, PA) and later displayed with the Dash IV using a pretriggered data
capture module. Waveforms (action potentials) were
sampled at 10 kHz per channel with 64 kilosamples
of total memory per channel. This digital, high frequency record was used to identify individual waveforms. Specific interneuronal records from each
animal taken from the Dash IV were copied as
transparencies. Each unique waveform (impulse) on
the transparency served as a template to identify
similar waveshapes from the complete chart recording (Fig. 2A–C). Data were analyzed 1 s before, 1 s
during, and 1 s after API stimulation. Most stimulations were of 1 s duration.
Only enough current was injected to produce a
ficitive motor output typical of a naturally produced and observable abdominal positioning movement. The following considerations led us to this
stimulus paradigm. First, stimulating different
APIs with a standard current cannot be used because many APIs require different amounts of current to give a motor output. Also, different APIs
were impaled at different locations. Second, one
cannot simply use a standard firing frequency for
each API because some APIs give a normal-looking motor output at 50 Hz while others require
90 Hz. Third, it is generally known that with crayfish and other motor systems, an increase in
stimulus intensity results in an increase in motor neuronal recruitment and firing frequency
(Atwood and Wiersma, ’67; Evoy and Kennedy, ’67;
Davis and Kennedy, ’72). At extremely low stimulus intensities little or no motor output occurs,
and at extremely high stimulus intensities a very
strong motor output is produced that is probably
not characteristic of behaving animals. Therefore,
the best criterion was to set the stimulus strength
to give a normal-looking motor program that
122
L.D. BREWER AND J.L. LARIMER
would have produced an observable movement in
an intact animal.
Occasionally coincidental impulses that occurred
during high neuronal activity caused distorted
waveforms; hence, these data could not be analyzed.
While the visual overlay method is time consuming, it was found to be much more reliable than
one computer software package that we tested
which was designed for this kind of analysis.
Interpretation of interneuronal data
from the connectives
Fig. 2. Chart record used to identify action potentials produced by specific interneurons from the abdominal nerve cord.
A: Electrophysiological records of interneuronal activity from
two rostral connective bundles (RC) and one caudal connective bundle (CC) before, during, and after stimulation of an
API (IN = intracellular electrode). The open arrow refers to a
portion of the record shown in B (prestimulus) and the solid
arrow refers to a portion of the record shown in C (stimulus
= 6 nA). B, C: Chart record of interneuronal activity sampled
at 10 kHz. B: The open arrowhead refers to a waveform that
was used as a template. A transparency of the template is
used to identify other waveforms which match the template
as shown in C. C: An example of comparing the template
Interneuronal activity was characterized by the
number of interneurons participating in a motor
program as well as by their firing frequencies.
These data are presented in histogram format.
The contribution of the impaled cell is not included
in these figures even though they were participating in these behaviors. Criteria for determining if a neuron was participating in a behavior
using this type of experimental paradigm are not
well established so we were forced to design our
own criteria. These criteria and their derivations
are discussed below.
Stimulation of each impaled API evoked activity that can readily be divided into two groups:
1) recruited interneurons that were silent before API stimulation and 2) interneurons that
were spontaneously active before API stimulation and increased their activity during the
stimulus. Interneurons were further pooled into
categories according to their firing frequency:
2–5, 6–10, 11–15, 16–20, and >20 Hz.
Spontaneously active interneurons were considered as participating in a fictive positioning behavior if they meet the following criteria. 1)
Interneurons that increased activity to above 10
Hz must have increased their firing rate by at
least 50% (e.g., prestimulus rate = 10 Hz and firing rate during stimulus = 15 Hz); or interneurons that increased their firing rate by 40–49%
were included if their poststimulus firing rate decreased to their prestimulus rate. 2) Interneurons
that fired below 11 Hz were only included if their
activity increased by 100% (i.e., doubled), or if
their firing rate increased by at least 60% and
their poststimulus firing rate returned to their
with other waveforms. The open arrowhead refers to a waveform that matches the template and is therefore considered
to be an action potential from the same neuron. The solid
arrowheads refer to waveforms which do not match the template and are therefore considered to be action potentials from
different neurons.
SIZE AND OUTPUT OF GROUPS OF INTERNEURONS
prestimulus rate. 3) Low activity cells that only
increased their rates from 1 to 2 Hz were ignored.
4) Interneurons which were inhibited were also
analyzed. Interneurons that had a prestimulus
rate below 11 Hz were counted as inhibited if their
rates decreased by at least 50%. Cells that had a
prestimulus rate above 10 Hz were considered to
be inhibited if their firing rates decreased by at
least 40% during the stimulus.
These criteria were determined in part by analyzing data from a single experiment in which the
same API was stimulated three times (in preparation). By repetition we could observe how interneurons recorded from the connectives responded
or failed to respond to repeated stimulation. This
also allowed us to estimate the “noise” that was
present in the abdominal nerve cord. While these
criteria are not perfect (i.e., some cells that are
included may not be part of the abdominal positioning system and vice versa), we feel that they
are reasonable based on these observations.
In order to accurately determine the number of
APIs participating during a fictive movement, activity from neurons that have bidirectional axons
should only be counted once. However, this task
was not always possible and we may have underestimated the number of bidirectional neurons due to several reasons: 1) branch point
failure (Grossman et al., ’79a, b; Nicholls et al.,
’92) (see Discussion for further explanation); 2)
axonal inhibition in other parts of the nerve
cord; 3) waveform distortion; and 4) axonal damage during dissection.
In some experiments it was only possible to
record from one rostral and caudal hemiconnective. Since APIs and nerve cord activity for flexion and extension are almost entirely bilaterally
symmetrical, counts of interneurons from these
experiments were doubled.
two groups: those that were recruited and those
that showed increased activity. Interneurons
classed as recruited were silent before API stimulation. Other interneurons were spontaneously
active before stimulation but increased their firing rate during the stimulus. The average number of interneurons recruited in response to
stimulation of an API was 18 (range: 3–50; n =
27 preparations). The average number of interneurons that increased activity was 14 (range: 3–
32) and the average number of interneurons that
decreased activity was 10 (range: 2–32). The number of interneurons activated and inhibited during each of these API stimulations is summarized
in Table 1. Stimulation of most APIs produced a
flexor motor output, one API produced an extension motor program, and activation of two other
APIs excited both flexor and extensor motor neurons. This type of mixed motor output has been
TABLE 1. Number of recruited interneurons, number of
interneurons that increased (+) activity, and number of
interneurons that decreased (–) activity in response to
intracellular stimulation of 27 APIs1
Comments
a
a
a
a, b
a
a
a
a
c
RESULTS
Single APIs were impaled and depolarized to
give a motor output typical of those underlying
postural movements. At the same time recordings
were made of evoked axonal activity from the rostral and caudal connectives. Several features regarding the interneuronal circuitry controlling
abdominal positioning were determined. First, we
estimated the size of each interneuronal group and
the firing frequency of each unit. Second, we studied the relationship between interneuronal group
activity and the production of different motor outputs in different segments of the nerve cord.
Evoked interneuronal activity was divided into
123
a
a
a, d
c
Average (SD)
1
Recruited
30
22
8
8
6
50
26
10
38
8
8
13
11
7
12
3
18
6
10
26
28
34
28
22
17
20
23
18 (11.5)
+Activity
8
10
3
4
6
18
32
8
20
11
10
16
11
16
17
11
13
20
16
16
22
20
14
15
20
10
10
14 (6.3)
–Activity
8
4
3
3
6
8
18
2
14
7
9
8
12
10
9
11
9
8
9
10
10
10
32
10
11
7
14
10 (5.7)
All APIs produced fictive abdominal flexion unless otherwise indicated. a: Numbers were doubled since this represents a recording from
a hemiconnective; b: flexion-producing ingibitor interneuron (this interneuron decreased motor activity); c: mixed motor output (excitatory
flexor and extensor motor neurons were activated); d: extension-producing interneuron.
124
L.D. BREWER AND J.L. LARIMER
observed previously (Murphy et al., ’89). Stimulation of another type of API activated a group of
interneurons that decreased or inhibited flexor
motor activity.
The number of interneurons recruited and the
number that only increased activity were categorized according to their firing frequency
(2–5, 6–10, 11–15, 16–20, and >20 Hz). Figure 3
summarizes how interneurons responded during
stimulation from 20 experiments in which 653 interneurons were considered to be affected by API
stimulation. Forty-three percent of all interneurons that were recruited or that increased activity fired between 2 and 5 Hz. Twenty-four percent
of affected interneurons fired between 6–10 Hz.
The total percentage of affected interneurons that
fired above 10 Hz was about 33%. Of this percentage about half of these fired above 20 Hz.
Only very weak motor outputs resulted unless at
least one or two interneurons fired at about 20
Hz or more. Interneurons that increased their firing frequencies usually did not exceed 35 Hz, but
a few fired as high as 70 Hz. The ranges of firing
frequencies of recruited interneurons observed in
these experiments were similar to those observed
in semi-intact animals (Jellies and Larimer, ’86;
Murphy et al., ’89).
The latency from stimulus to firing of most recruited interneurons was between 20 and 75 ms.
Those interneurons that showed longer latencies
tended to fire at low frequencies, generally between 2 and 5 Hz. Following API stimulation,
most of the evoked interneuronal activity returned
to prestimulus firing rates within 20 ms; however,
occasionally some interneurons continued to fire
higher than their prestimulus rate.
How group size and firing frequency
affected motor output
Representative examples of the morphology and
motor output of some impaled APIs are shown below (Figs. 4 and 5). As indicated in the model proposed by Larimer (’88), stimulation of each API
activated a set of interneurons. As might be expected, larger interneuronal groups produced
stronger motor outputs than smaller interneuronal groups, as defined by more motor neurons
firing at high frequency. However, in a few instances (n = 4 of 27), the size and firing frequency
of the interneuronal population were not always
correlated with the strength of motor output; some
large groups produced a weaker motor output than
some smaller groups.
Flexion-producing interneuronal
population
Fig. 3. Percentage of interneurons that were recruited
(lined bars) or increased (+) activity (open bars) according to
firing frequency (Hz) in response to stimulation of an API. A
total of 653 interneurons from 20 preparations were considered to be affected by stimulation of an API. Approximately
43% of all interneurons fired at very low frequencies (2–5
Hz) and 15% fired above 20 Hz. Usually only very weak motor outputs were produced unless a group contained at least
one or two cells that fired at about 20 Hz.
Stimulation of the API shown in Figure 4A produced a strong flexor motor output (Fig. 4B). This
API is one of the most commonly encountered
flexion-producing APIs (Jellies and Larimer, ’85;
Larimer and Jellies, ’83; Larimer and Moore, ’84;
Larimer and Pease, ’88). Analysis of nerve cord
activity identified 64 interneurons that were affected by API stimulation. Of these interneurons
34 were recruited (i.e., neurons that were previously silent), 20 increased activity, and 10 were
inhibited. The distribution according to firing
frequency of interneurons that were recruited
and increased activity is presented in Figure
4C. Most interneurons (n = 27) fired at low frequencies (2–5 Hz); however, a relatively large
population of APIs (n = 16) fired at higher frequencies (>15 Hz). This group appeared to have
a high level of synaptic interactions that produced a strong motor response. This API may
be important in evoking natural flexion due to
its extensive ability to recruit other neurons.
Stimulation of other APIs recruited very few if
any interneurons at higher frequencies (i.e., Fig.
5C, column A).
SIZE AND OUTPUT OF GROUPS OF INTERNEURONS
125
Different flexion-producing interneuronal
groups provide different and
unexpected motor outputs
While the strength of motor output was usually correlated with the size and firing frequency
of the interneuronal group, the examples presented in Figure 5 show that these criteria did
not always indicate the strength of motor activity. The interneuronal groups and the motor
outputs produced by stimulating two different
flexion APIs were compared. Each API had two
axons, one that projected in the rostral direction and one that projected in the caudal direction (Fig. 5A 1, B 1).
During current injection the firing frequencies
of the impaled APIs shown in Figures 5A 1, B 1 were
50 and 60 Hz, respectively. Stimulation of the API
shown in Figure 5A1 produced a strong flexor motor output in the fourth abdominal ganglion (A4F)
and an even more robust motor output in the second abdominal ganglion (A2F) (Fig. 5A2). Stimulation of the API shown in Figure 5B 1 produced a
weaker motor response, especially in the second
abdominal ganglion (A2F) (Fig. 5B2). Stimulation
of the API (Fig. 5B1) that produced the weaker
motor output activated more than six times as
many interneurons at frequencies above 10 Hz
(Fig. 5C, column B) than stimulation of the API
that produced the stronger motor output (Fig. 5C,
column A). Thus the size of an interneuronal group
did not always indicate the strength of a motor
response.
Figure 4.
Fig. 4. Interneuronal group activated in response to stimulation of an identified flexion-producing API. A: Morphology
of a flexion-producing API. This API was located entirely on
one side of the midline and had an axon that projected in
both the rostral and caudal directions. Dashed line represents the midline of the ganglion. Rostral is toward the top.
B: Flexor motor output (A4F and A2F) produced during stimulation of this API. The motor neuron with the largest action
potential firing in trace A4E is the extensor peripheral inhibitor. I = current; IN = intracellular electrode; A4F = flexor
output from fourth abdominal ganglion; A4E = extensor output from fourth abdominal ganglion; A2F = flexor output from
second abdominal ganglion. C: Frequency histogram of the
number of interneurons, according to firing frequency, that
were recruited or increased (+) activity in response to stimulation of the API. Each column represents the combined number of interneurons that were recruited (lined bar) and
increased activity (open bar) at a particular frequency. Hence,
the total number of affected interneurons as shown in column A at 2–5 Hz was 27. Twenty of these interneurons
were recruited and seven showed increased activity. A relatively large number of interneurons (n = 16) were fired
above 15 Hz.
126
L.D. BREWER AND J.L. LARIMER
Comparison between the strength and
direction of interneuronal activity
and motor output
Interneuronal group activity in the rostral and
caudal connectives was compared with the direction and strength of motor output in an effort
to explain motor bias. A motor output that is
stronger in either the more rostral or caudal
motor roots is defined as biased. Rostral motor
output refers to motor activity anterior to the
site of intracellular impalement (A3) and caudal motor output refers to motor activity posterior to A3 (see Fig. 1). The activity (firing
frequency) and the axonal direction of the impaled API were considered when assessing the
strength of group activity; however, the contribution of the impaled API itself was not included in the following frequency histograms.
Only the number of recruited interneurons and
the number of interneurons that increased activity are included in these figures.
In 8 of 12 experiments the direction and
strength of interneuronal activity were closely
correlated with the direction and strength of
motor output (see Table 2). Interneuronal activity resulting from stimulation of the remaining four APIs did not reflect the strength of
motor output (see Table 2). However, in 10 of
12 experiments the strength and direction of
motor output were correlated with the direction
of the stimulated APIs axon (Table 2).
Fig. 5. Comparison of two interneuronal groups activated
by different flexion-producing interneurons. A1, B1: Morphology of two flexion-producing APIs. Each API had an axon
that projected in both the rostral and caudal directions.
Dashed line represents the midline of the ganglion. Rostral
is toward the top. A2B2: Flexor motor outputs produced during stimulation of the command elements in A 1 and B1, respectively. Stimulation of the API shown in A1 produced a
slightly stronger flexor motor output (A2) than stimulation of
the API shown in B1 (B2). The motor output in root A2F was
much stronger in A2 than in B2. During current injection the
firing frequencies of the impaled APIs shown in A1 and B1
were 50 and 60 Hz, respectively. The intracellular trace (IN)
was not bridge balanced in A 2. See Figure 4 for explanation
of the abbreviations. C: Frequency histogram of the number
of interneurons, according to spiking frequency, that were recruited or increased (+) activity in response to API stimulation. Data from columns A and B are counts of interneurons
resulting from stimulation of the APIs shown in A 1 and B 1,
respectively. Very few interneurons were active above 10 Hz
during stimulation of the API shown in A1 (column A), but a
large population of interneurons was activated above 10 Hz
during stimulation of B1 (column B). The population that contained a larger number of interneurons that fired at frequencies above 10 Hz (column B) produced a weaker motor output
than the smaller population (column A). This figure follows
the same format as explained in Figure 4.
TABLE 2. Direction of the stimulated APIs axon, and
direction and strength of both the interneuronal
and motor activity1
Axonal
direction
Nerve cord
activity
Motor
activity
R and C2
R and C2
R and C2
C2
C2
C2
R2
R2
R and C3
R and C3
R and C3
R3
R=C
R=C
R<C
R<C
R<C
R<C
R=C
R>C
R<C
R<C
R>C
R=C
R=C
R =C
R<C
C only
R<C
R<C
R=C
R>C
R=C
R=C
R=C
R>C
1
All 3 variables are presented in relation to each other from 12 experiments. All impaled APIs were flexion-producing. R = rostral; c =
caudal; > = stronger than; < = weaker than; = indicates no bias observed.
2
Direction and strength of activity in nerve cord were reflected in
motor output.
3
Direction and strength of activity in verve cord did not reflect motor
output.
SIZE AND OUTPUT OF GROUPS OF INTERNEURONS
Specific example of interneuronal activity
and motor output that was correlated
Stimulation of an API with a single axon that
projected in the caudal direction (Fig. 6A) resulted
127
in a stronger output from the caudal flexor motor
roots (Fig. 6B, trace A4F), while rostral motor output was considerably weaker (Fig. 6B, trace A2F).
The number of interneurons responding to API
stimulation from the rostral and caudal connectives was about equal, 12 and 14, respectively,
but the number of interneurons firing above 10
Hz was four times greater in the caudal direction
(Fig. 6C). Eight axons from the caudal connectives
fired above 10 Hz, but only two axons from the
rostral connectives fired at higher frequencies.
Hence, both interneuronal and motor activity were
biased (stronger) in the caudal direction.
Specific example of interneuronal activity
and motor output that was not correlated
Stimulation of an API with axons that projected
in both the rostral and caudal directions (Fig.
7A) resulted in an equally robust output from
the rostral and caudal flexor motor roots (Fig.
7B). However, interneuronal activity in the caudal connectives was stronger than that in the rostral connectives (Fig. 7C). Six axons fired above
20 Hz in the caudal connectives, while only one
axon fired above 20 Hz in the rostral connectives. Hence, the stronger interneuronal activity in the caudal connectives might be expected
to produce a stronger caudal motor output. However, this was not observed; instead, motor output
was not biased in either direction. Inconsistencies
of this kind suggest that factors such as synaptic efficacy and the type of interneurons recruited are of importance in explaining bias as
well as the impulse traffic traveling in a particular direction.
Fig. 6. Stimulation of a flexion-producing interneuron that
resulted in both interneuronal and motor output activities
that were biased in the caudal direction. A: Morphology of a
flexion-producing API that projected an axon in the caudal
direction. Dashed line represents the midline of the ganglion.
Rostral is toward the top. B: Stimulation of this API resulted
in a stronger caudal (A4F) than rostral (A2F) flexor motor
output. The intracellular trace (IN) was not bridge balanced.
See Figure 4 for explanation of the abbreviations. C: Frequency histogram of the number of interneurons, according
to spiking frequency, that were recruited or increased (+)
activity in the rostral (column A) and caudal (column B)
connectives as a result of intracellular stimulation of the
API. Each column represents the combined number of interneurons that were recruited and increased activity at
a particular frequency. Hence, the total number of affected
interneurons as shown in column A at 2–5 Hz was six.
Two of these interneurons were recruited and four showed
increased activity.
128
L.D. BREWER AND J.L. LARIMER
DISCUSSION
The number of interneurons that comprised a
functional group controlling the fictive abdominal
positioning movements examined here ranged
from 11 to 68. A total of 360 APIs have been estimated in abdominal ganglia one through six by
Larimer and Pease (’88); therefore, only a subset
of this total is active during a fictive behavior.
Only APIs in ganglia two through five were examined in this study, and from these ganglia
Larimer and Pease (’88) estimated that 214 APIs
are present. Since the APIs from ganglia one and
six were not included, the number of APIs participating in an abdominal positioning behavior
is greater than that reported here. However, we
feel our sample gives us some indication of the
organization and functional activity of a group of
interneurons participating in a specific behavior.
While we cannot be sure that every interneuron
counted participated in a fictive behavior, previous evidence suggests that most of these interneurons are APIs that are synaptically recruited
into functional groups (Jellies and Larimer, ’86;
Murphy et al., ’89).
Usually interneuronal groups that contained
relatively large numbers of interneurons firing at
high frequencies (>15 Hz) produced strong motor
outputs. Thus coding for abdominal positioning
movements appears to be determined by the total spike frequency emanating from a recruited
group of APIs. This type of coding also controls
the direction of cockroach turning during escape
(Liebenthal et al., ’94). Little is known about how
APIs are recruited, but it is probable that the
strength of sensory or descending inputs influences the number and firing frequencies of the
APIs that form a functional group.
However, the total number of spikes did not al-
Figure 7.
Fig. 7. Stimulation of a flexion-producing interneuron that
resulted in an interneuronal activity that was stronger in
the caudal connectives, but the motor output was equally
strong in the rostral and caudal directions. A: Morphology of
a flexion-producing API that projected an axon in the rostral
direction. Dashed line represents the midline of the ganglion.
Rostral is toward the top. B: Stimulation of this API resulted
in a flexor motor output that was equally strong in the caudal (A4F) and rostral (A2F) motor roots. See Figure 4 for
explanation of the abbreviations. C: Frequency histogram of
the number of interneurons, according to spiking frequency,
that were recruited or increased (+) activity in the rostral
(column A) and caudal (column B) connectives as a result of
intracellular stimulation of the API. Interneuronal activity
was stronger in the caudal connectives, but the motor outputs were equally strong in the rostral and caudal directions.
This figure follows the same format as explained in Figure 6.
SIZE AND OUTPUT OF GROUPS OF INTERNEURONS
ways indicate the strength of motor output. In a
few instances strong motor outputs occurred even
though relatively few interneurons fired at high
frequencies and vice versa (Fig. 5). Several explanations can be offered to account for these results.
Strong synaptic inputs from only a few interneurons could have produced a robust motor output.
The large number of interneurons firing at low
frequencies could also have contributed toward a
strong motor output through temporal and/or
spatial summation. The relatively weak motor outputs may have occurred even though interneuronal group activity was strong because very few
of these interneurons may have had synaptic inputs onto the motor neurons, or the connections
were comparatively weak (Fig. 5). There is also
the possibility that inhibitory inputs onto the interneurons or the motor neurons resulted in a
weaker motor output. Thus other factors in addition to the size of the recruited group may influence motor neuron firing. These and other possible
explanations are further discussed in the following section.
Interneuronal activity and motor bias
Kennedy et al. (’67) found that stimulation of certain APIs, then called command neurons, produced
a stronger motor output in either the rostral or caudal direction (motor bias). The production of motor
bias programs is believed to be an important
mechanism of achieving a wide variety of abdominal movements. Larimer and Pease (’88) found a
strong correlation between the direction of motor
bias and the axonal projection of a stimulated API;
however, there were exceptions to this rule. The
results reported here confirm and extend the findings of Larimer and Pease (’88). We have added a
new dimension to the analysis of bias by comparing the direction and magnitude of bias with the
direction, size, and firing frequencies of the interneuronal groups that underlie fictive behaviors.
The strength and direction of interneuronal activity were compared with the strength and the
direction of motor output from interneuronal
groups recruited by stimulation of 12 different
APIs (Table 2). Six of these 12 groups exhibited a
motor bias, and in 5 of these 6 the strength and
direction of interneuronal activity reflected the direction of motor bias. The most likely mechanisms
that could produce a motor bias would include a
larger population of APIs firing in one direction
than another, stronger synaptic inputs in one direction, or inhibitory inputs onto the APIs or onto
motor neurons, in particular ganglia. For example,
129
in one experiment there was a rostral motor bias
but the nerve cord activity was equally robust in
the rostral and caudal directions (Table 2). In this
case we would predict that there were stronger
synaptic inputs onto the motor neurons in the
more rostral ganglia, or inhibitory inputs onto
the interneurons or motor neurons in the more
caudal ganglia resulted in a weaker caudal motor output. However, we cannot rule out the possibility that some of the interneuronal activity
in the caudal direction may have been primarily concerned with other motor systems (Burdohan and Larimer, ’95).
Some of the data from Table 2 can be interpreted in another manner. Six of the 12 APIs
stimulated had bidirectional axons. Stimulation
of three of these APIs resulted in a correlation
between interneuronal and motor activity. Stimulation of the remaining three APIs produced a
stronger interneuronal activity in the rostral or
caudal direction, but the motor output was not
biased (Table 2). Therefore, we must conclude that
the direction of interneuronal activity does not always indicate the direction of a motor bias.
Several explanations can be offered to account
for these discrepancies. First, APIs are involved
in coordinating with several motor systems (Murchison and Larimer, ’90, ’92; Chrachri et al., ’94;
Burdohan and Larimer, ’95); therefore, some of
these interneurons may have been involved primarily with motor systems other than abdominal
positioning. Second, the synaptic efficacy of APIs
could greatly determine the strength of motor output. Third, flexor inhibitory interneurons are frequently encountered and some APIs frequently
receive inhibitory postsynaptic potentials (personal observation). Therefore, an API providing
excitatory inputs onto motor neurons in one ganglion may be inhibited in another ganglion. One
inhibitory interneuron was analyzed (Table 1), and
stimulation of this flexor inhibitor evoked activity in 12 interneurons that fired at frequencies
that ranged from 3 to 33 Hz. Little is known about
how these inhibitory interneurons interact within
the abdominal positioning system. Fourth,
neuromodulators have been shown to affect
swimmeret and abdominal positioning motor
systems (Livingstone et al., ’80; Harris-Warrick
and Kravitz, ’84; Kravitz, ’88; Ma et al., ’92;
Barthe et al., ’93; Chrachri and Neil, ’93). It is
conceivable that a modulator could be affecting
some ganglia more than others. Fifth, local interneurons may be important modulators within the
abdominal positioning system. At least two iden-
130
L.D. BREWER AND J.L. LARIMER
tified local nonspiking interneurons have been
shown to inhibit local excitatory flexor motor activity in P. clarkii (Jellies and Larimer, ’85; Toga
et al., ’90). Sixth, interneurons with bidirectional
axons could also be important in producing a motor bias. Based on previous morphological data,
about one third of all APIs have bidirectional
axons, thus about two thirds of the remaining
APIs have axons that course only in the rostral
or caudal direction (Miall and Larimer, ’82a, b;
Larimer and Jellies, ’83; Larimer and Moore, ’84;
Jellies and Larimer, ’85, ’86). Only occasionally
was it possible to show that a given recruited API
has a bidirectional axon based on analyzing firing frequency in the rostral and caudal connectives. Therefore, the number of recruited APIs
with bidirectional axons was probably underestimated. We can speculate that the inability to detect these recruited APIs may have been due to
several factors: 1) axonal inhibition in other parts
of the nerve cord, 2) axonal damage during dissection, 3) waveform distortion, or 4) branch point
failure (Grossman et al., ’79a, b; Nicholls et al.,
’92). Conduction block or branch point failure
could produce a motor bias since one axon firing
at a high frequency could produce a vigorous motor response in one direction while the other axon
from the same interneuron firing at a lower frequency would produce a weaker response in the
other direction. In this manner the parts of the
one neuron could function as two separate units.
Organization of the abdominal
positioning system
Why are so many interneurons required for this
behavior? At least two explanations can be offered.
First, APIs are known to participate in several
behaviors, including swimmeret (Murchison and
Larimer, ’90, ’92) and uropod movements (Takahata and Hisada, ’85, ’86a, b; Burdohan and
Larimer, ’95). Since many APIs have axonal projections that course through the thoracic ganglia
(Larimer and Jellies, ’83; Larimer and Moore, ’84)
they probably interact with other neural centers
as well (Barthe et al., ’93; Chrachri and Neil, ’93;
Chrachri et al., ’94). This type of distributed organization has been observed in a wide variety of
animals (Cleary and Byrne, ’93; Otto and Hennig,
’94; Tsau et al., ’94; Wu et al., ’94).
A second reason for the large number of APIs
may be that no single API codes for an entire behavior. Rather, positioning behaviors are encoded
by ensembles of interneurons (APIs) organized
into functional groups that act in concert (Larimer,
’88). During any one movement, a subset (group)
of the total population of APIs is activated. Within
a group each API makes only a fractional contribution toward a behavior (Larimer, ’88); however,
some APIs probably make larger contributions
than others based on firing frequency and their
synaptic efficacy with motor neurons (discussed
below). Each API may belong to more than one
group, and as a result the contribution of any one
API during different behaviors may be quite variable. For example, an API may fire at a high frequency while acting as a member of one group,
but as part of another group the same API may
fire at a lower frequency. The combination of many
interneurons or APIs into a seemingly infinite
number of groups provides the variety required
to account for the large repertoire of abdominal
positioning behaviors.
While the number of groups formed are extremely large, we have recently completed studies that indicate that at least some of these groups
may have some degree of cohesion (Brewer and
Larimer, ’94). For example, repetitively stimulating the same API with a microelectrode in the
same animal consistently activates the same
group of interneurons provided synaptic fatigue
does not occur. Furthermore, stimulating the same
API in different animals activates a similar but
not identical number of interneurons (Brewer and
Larimer, ’94).
Comparison of abdominal positioning
system with other motor systems
Several large populations of neurons that control behavior in invertebrates as well as vertebrates are now being studied. As with the
abdominal positioning system, other systems
appear to use subsets from the larger interneuronal population to perform particular tasks.
During cockroach escape behavior a large population of thoracic interneurons (over 100) controls
leg movements. These thoracic interneurons
project through several ganglia to affect leg motor neurons and are organized into parallel and
serial groups (Ritzmann and Pollack, ’86, ’90;
Casagrand and Ritzmann, ’91). Furthermore,
some interneurons or groups affect motor neuronal
output more strongly than other groups (Ritzmann
and Pollack, ’90). Several other systems are also
organized similarly, including the interneurons
that control the head movements of crickets during eye cleaning (Hensler, ’88), the local bending
reflex of leeches (Lockery and Kristan, ’90), and
the descending propriospinal neurons that control
SIZE AND OUTPUT OF GROUPS OF INTERNEURONS
fictive scratching in turtles (Berkowitz and Stein,
’94a, b). Hence, the interneuronal organization
that controls these behaviors is very similar to
the abdominal positioning system.
In summary, abdominal positioning movements
are encoded by a synaptically recruited function
group of interneurons that represents a subset
formed from a larger population of APIs. Our observations show that some APIs recruited large
groups of interneurons while others recruited
much smaller groups. Usually, large groups with
interneurons firing at high frequencies produced
strong motor outputs while smaller groups with
interneurons firing at lower frequencies produced
weaker motor outputs. However, on occasion some
large groups with interneurons firing at high frequencies produced relatively weak motor outputs,
and some smaller groups produced relatively
strong motor outputs. The contribution of a group
therefore may depend on such factors as group
size, firing frequency, synaptic efficacy, the presence of inhibitory inputs, neuromodulation, and
the role of local circuits.
ACKNOWLEDGMENTS
Figures were prepared by Gwen Gage, Janet
Young, and Kristina Schlegel. We thank Drs.
Wesly Thompson and John Burdohan for reading
an early version of the manuscript. This research
was supported by NIH grant NS05423, a Jacob
Javits award to J.L.L.
LITERATURE CITED
Atwood, H.L., and C.A. Wiersma (1967) Command interneurons in the crayfish nervous system. J. Exp. Biol.,
46:249–261.
Barthe, J.-Y., M. Bevengut, and F. Clarac (1993) In vitro,
proctolin and serotonin induced modulations of the abdominal motor system activities in crayfish. Brain Res.,
623:101–109.
Berkowitz, A., and P.S.G. Stein (1994a) Activity of descending propriospinal axons in the turtle hindlimb enlargement
during two forms of fictive scratching: Broad tuning to regions of the body surface. J. Neurosci., 14:5089–5104.
Berkowitz, A., and P.S.G. Stein (1994b) Activity of descending propriospinal axons in the turtle hindlimb enlargement
during two forms of fictive scratching: Phase analyses. J.
Neurosci., 14:5105–5119.
Brewer, L.D., and J.L. Larimer (1994) Estimation of identified interneurons forming a functional group controlling abdominal positioning in the crayfish. Soc. Neurosci. Abstr.,
20:1407.
Burdohan, J.A., and J.L. Larimer (1995) Interneurons involved in the control of multiple motor centers in crayfish.
J. Exp. Zool., 273:204–215.
Casagrand, J.L., and R.E. Ritzmann (1991) Localization of
ventral giant interneuron connections to the ventral me-
131
dian branch of thoracic interneurons in the cockroach. J.
Neurobiol., 22:643–658.
Chrachri, A., and D.M. Neil (1993) Interaction and synchronization between two abdominal motor systems in crayfish.
J. Neurophysiol., 69:1373–1383.
Chrachri, A., D.M. Neil, and B. Mulloney (1994) Statedependent responses of two motor systems in the crayfish,
Pacifastacus leniusculus. J. Comp. Physiol. A., 175:371–380.
Churchland, P.S., and T.J. Sejnowski (1992) The Computational Brain. MIT Press, Cambridge, MA.
Cleary, L.J., and J.H. Byrne (1993) Identification and characterization of a multifunction neuron contributing to defensive arousal in Aplysia. J. Neurophysiol., 70:1767–1776.
Davis, W.J., and D. Kennedy (1972) Command interneurons controlling swimmeret movements in the lobster.
II. Interactions of effects on motoneurons. J. Neurophysiol., 35:13–19.
Evoy, W.H., and D. Kennedy (1967) Central nervous organization underlying control of antagonistic muscles in
the crayfish. I. Types of command fibers. J. Exp. Zool.,
165:223–238.
Georgopoulos, A.P., J. Ashe, N. Smyrnis, and M. Taira
(1992) The motor cortex and the coding of force. Science, 256:1692–1695.
Grossman, Y., I. Parnas, and M.E. Spira (1979a) Differential
conduction block in branches of a bifurcating axon. J.
Physiol., 295:283–305.
Grossman, Y., I. Parnas, and M.E. Spira (1979b) Ionic mechanisms involved in differential conduction of action potentials at high frequency in a branching axon. J. Physiol.,
295:307–322.
Harris-Warrick, R.M., and E.A. Kravitz (1984) Cellular
mechanisms for modulation of posture by octopamine and
serotonin in the lobster. J. Neurosci., 4:1976–1993.
Hensler, K. (1988) Intersegmental interneurons involved in
the control of head movements in crickets. J. Comp. Physiol.
A., 162:111–126.
Jellies, J., and J.L. Larimer (1985) Synaptic interactions between neurons involved in the production of abdominal posture in the crayfish. J. Comp. Physiol. A, 156:861–873.
Jellies, J., and J.L. Larimer (1986) Activity of crayfish abdominal positioning interneurons during spontaneous and
sensory-evoked movements. J. Exp. Biol., 120:173–188.
Jones, K.A., and C.H. Page (1986) Postural interneurons in
the abdominal nervous system of lobster. II. Evidence for
neurons having both command and driver roles. J. Comp.
Physiol. A, 158:273–280.
Kennedy, D., W.H. Evoy, B. Dan, and J.T. Hanawalt (1967)
The central nervous organization underlying control of antagonistic muscles in the crayfish. II. Coding of position by
command fibers. J. Exp. Zool., 165:239–248.
Kravitz, E.A. (1988) Hormonal control of behavior: Amines
and the biasing of behavioral output in lobsters. Science,
241:1775–1781.
Larimer, J.L. (1988) The command hypothesis: A new view
using an old example. Trends Neurosci., 11:506–510.
Larimer, J.L., and A. Eggleston (1971) Motor programs
for abdominal positioning in crayfish. Z. Vgl. Physiol.,
74:388–402.
Larimer, J.L., and J. Jellies (1983) The organization of
flexion-evoking interneurons in the abdominal nerve
cord of the crayfish, Procambarus clarkii. J. Exp. Zool.,
226:341–351.
Larimer, J.L., and D. Moore (1984) Abdominal positioning in-
132
L.D. BREWER AND J.L. LARIMER
terneurons in crayfish: Projections to and synaptic activation by higher CNS centers. J. Exp. Zool., 230:1–10.
Larimer, J.L., and C.M. Pease (1988) A quantitative study of
command elements for abdominal positioning behavior in
the crayfish, Procambarus clarkii. J. Exp. Zool., 247:45–55.
Lee, C., W.H. Rohrer, and D.L. Sparks (1988) Population coding of saccadic eye movements by neurons in the superior
colliculus. Nature, 332:357–360.
Liebenthal, E., O. Uhlmann, and J.M. Camhi (1994) Critical
parameters of the spike trains in a cell assembly: Coding of
turn direction by the giant interneurons of the cockroach.
J. Comp. Physiol. A, 174:281–296.
Livingstone, M., R.M. Harris-Warrick, and E.A. Kravitz (1980)
Serotonin and octopamine produce opposite postures in lobsters. Science, 208:76–79.
Lockery, S.R., and W.B. Kristan (1990) Distributed processing of sensory information in the leech. II. Identification of
interneurons contributing to the local bending reflex. J.
Neurosci., 10:1816–1829.
Ma, P.M., B.S. Beltz, and E.A. Kravitz (1992) Serotonin-containing neurons in lobsters: Their role as “gain-setters” in
postural control mechanisms. J. Neurophysiol., 68:36–54.
Miall, R.C., and J.L. Larimer (1982a) Interneurons involved in abdominal posture in crayfish: Structure, function and command fiber responses. J. Comp. Physiol. A,
148:159–173.
Miall, R.C., and J.L. Larimer (1982b) Central organization of
crustacean abdominal posture motoneurons: Connectivity
and command fiber inputs. J. Exp. Zool., 224:45–56.
Murchison, D., and J.L. Larimer (1990) Dual motor output interneurons in the abdominal ganglia of the crayfish Procambrus clarkii: Synaptic activation of motor
outputs in both the swimmeret and abdominal positioning systems by single interneurons. J. Exp. Biol.,
150:269–293.
Murchison, D., and J.L. Larimer (1992) Synaptic interactions among neurons that coordinate swimmeret and abdominal movements in the crayfish. J. Comp. Physiol. A,
170:739–747.
Murphy, B.F., M.L. McAnelly, and J.L. Larimer (1989) Abdominal positioning interneurons in crayfish: Participation
in behavioral acts. J. Comp. Physiol. A, 165:461–470.
Nicholls, J.N., A.R. Martin, and B.G. Wallace (1992) From
Neuron to Brain, Ed. 3. Sinaurer Associates, Inc., Sunderland, MA, pp. 140, 450.
Otto, D., and R.M. Hennig (1993) Interneurons descending
from the cricket subesophageal ganglion control stridulation and ventilation. Naturwissenschaften, 80:36–38.
Ritzmann, R.E., and A.L. Pollack (1986) Identification of thoracic interneurons that mediate giant interneuron-to-motor pathways in the cockroach. J. Comp. Physiol. A,
159:639–654.
Ritzmann, R.E., and A.L. Pollack (1990) Parallel motor pathways from thoracic interneurons of the ventral giant interneuron system of the cockroach, Periplaneta americana. J
Neurobiol., 21:1219–1235.
Stewart, W.W. (1978) Functional connections between cells
as revealed by dye-coupling with a highly fluorescent
napthalamide tracer. Cell, 14:741–759.
Takahata, M., and M. Hisada (1985) Interactions between
the motor systems controlling uropod steering and abdominal posture in crayfish. J. Comp. Physiol. A, 157:547–554.
Takahata, M., and M. Hisada (1986a) Local nonspiking interneurons involved in gating of the descending motor pathway in crayfish. J. Neurophysiol., 56:718–731.
Takahata, M., and M. Hisada (1986b) Sustained membrane
potential change of uropod motor neurons during the fictive abdominal posture movement in crayfish. J. Neurophysiol., 56:702–717.
Toga, T., M. Takahata, and M. Hisada (1990) An identified
set of local nonspiking interneurons which control the activity of abdominal postural motoneurones in crayfish. J.
Exp. Biol., 148:477–482.
Tsau, Y., J.-Y. Wu, H.P. Hopp, L.B. Cohen, D. Schiminovich,
and C.X. Falk (1994) Distributed aspects of the response to
siphon touch in Aplysia: Spread of stimulus information and
cross correlation analysis. J. Neurosci., 14:4167–4184.
van Harreveld, A. (1936) A physiological solution for freshwater crustaceans. Proc. Soc. Exp. Biol., 34:428–432.
Wu, J.-Y., L.B. Cohen, and C.X. Falk (1994) Neuronal activity during different behaviors in Aplysia: A distributed organization? Science, 263:820–823.
Zecevic, D., J.-Y. Wu, L.B. Cohen, J.A. London, H.P. Hopp,
and C.X. Falk (1989) Hundreds of neurons in the Aplysia
abdominal ganglion are active during the gill-withdrawal
reflex. J. Neurosci., 9:3681–3689.
Документ
Категория
Без категории
Просмотров
3
Размер файла
157 Кб
Теги
291
1/--страниц
Пожаловаться на содержимое документа