Nobel Institute for Neurophysiology, Department of Neuroscience, Karolinska Institutet, SE-171 77 Stockholm, Sweden
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ABSTRACT |
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Fagerstedt, Patriq, Grigori N. Orlovsky, Tatiana G. Deliagina, Sten Grillner, and Fredrik Ullén. Lateral Turns in the Lamprey. II. Activity of Reticulospinal Neurons During the Generation of Fictive Turns. J. Neurophysiol. 86: 2257-2265, 2001. We studied the neural correlates of turning movements during fictive locomotion in a lamprey in vitro brain-spinal cord preparation. Electrical stimulation of the skin on one side of the head was used to evoke fictive turns. Intracellular recordings were performed from reticulospinal cells in the middle (MRRN) and posterior (PRRN) rhombencephalic reticular nuclei, and from Mauthner cells, to characterize the pattern of activity in these cell groups, and their possible functional role for the generation of turns. All recorded reticulospinal neurons modified their activity during turns. Many cells in both the rostral and the caudal MRRN, and Mauthner cells, were strongly excited during turning. The level of activity of cells in rostral PRRN was lower, while the lowest degree of activation was found in cells in caudal PRRN, suggesting that MRRN may play a more important role for the generation of turning behavior. The sign of the response (i.e., excitation or inhibition) to skin stimulation of a neuron during turns toward (ipsilateral), or away from (contralateral) the side of the cell body was always the same. The cells could thus be divided into four types: 1) cells that were excited during ipsilateral turns and inhibited during contralateral turns; these cells provide an asymmetric excitatory bias to spinal networks and presumably play an important role for the generation of turns; these cells were common (n = 35; 52%) in both MRRN and PRRN; 2) cells that were excited during turns in either direction; these cells were common (n = 19; 28%), in particular in MRRN; they could be involved in a general activation of the locomotor system after skin stimulation; some of the cells were also more activated during turns in one direction and could contribute to an asymmetric turn command; 3) one cell that was inhibited during ipsilateral turns and excited during contralateral turns; and 4) cells (n = 12; 18%) that were inhibited during turns in either direction. In summary, our results show that, in the lamprey, the large majority of reticulospinal cells have responses during lateral turns that are indicative of a causal role for these cells in turn generation. This also suggests a considerable overlap between the command system for lateral turns evoked by skin stimulation, which was studied here, and other reticulospinal command systems, e.g., for lateral turns evoked by other types of stimuli, initiation of locomotion, and turns in the vertical planes.
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INTRODUCTION |
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In all classes of vertebrates,
the spinal cord contains neuronal networks that can generate the basic
motor pattern of locomotion. Descending supraspinal systems activate
these networks and modulate their activity, to produce alterations in
speed, steering maneuvers, postural corrections, and other adaptations
of the locomotor movements to the surrounding and the goals of the
organism (Grillner 1985; Orlovsky et al.
1999
).
Here we study the functional organization of the descending control
system for lateral turning movements in the lamprey, a lower vertebrate
belonging to the cyclostomes. Lampreys perform lateral turns
spontaneously (McClellan and Hagevik 1997;
Ullén et al. 1997
), but they can also be evoked by
various types of sensory stimuli, e.g., unilateral eye illumination
(negative phototaxis) (Ullén et al. 1995
,
1997
), illumination of tail skin photoreceptors (Deliagina et al. 1995
; Harden-Jones
1955
; Young 1935
), mechanical or electrical skin
stimulation (McClellan 1984
; McClellan and Hagevik 1997
), and olfactory stimulation (Kleerekoper
1972
; Kleerekoper and Mogensen 1963
;
Teeter 1980
). Neural correlates of turning movements
(fictive turns) can be evoked in vitro by electrical stimulation of the
skin of the head (Fagerstedt and Ullén 2001
; McClellan and Hagevik 1997
). A detailed analysis of the
motorneuron activity during turns is presented in the accompanying
paper (Fagerstedt and Ullén 2001
).
Higher vertebrates have a number of descending systems that can
influence spinal locomotor networks. However, in the lamprey, supraspinal commands are mediated essentially by reticulospinal (RS)
and vestibulospinal pathways. The RS system contains the largest number
of cells (Bussières 1994) and is also the only system that reaches the middle and caudal parts of the spinal cord. RS
neurons are located in four nuclei: the mesencephalic reticular
nucleus, and the anterior, middle, and posterior rhombencephalic reticular nuclei (MRN, ARRN, MRRN, and PRRN, respectively; Fig. 1A). They project mainly to
the ipsilateral spinal cord. Mauthner cells and accessory Mauthner
cells are situated lateral to the MRRN and project to the contralateral
spinal cord (Brodin et al. 1988
; Bussières
1994
; Bussières et al. 1999
; Ronan
1989
; Rovainen 1983
; Swain et al.
1993
).
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Recordings of the mass activity in the left and right RS pathways in
intact, freely behaving lampreys have shown that, during lateral turns,
the RS activity on the side toward which the animal turns is stronger
than that on the opposite side (Deliagina et al. 2000).
However, it has remained unclear which groups of RS neurons are
responsible for this asymmetry in descending influences. In the present
study, we used an in vitro preparation of the isolated lamprey brain
and spinal cord and performed intracellular recordings from RS cells in
the MRRN and the PRRN and from Mauthner cells, to elucidate the
possible roles of different groups of RS neurons for eliciting lateral turns.
Preliminary accounts of parts of this study have appeared in abstract
form (Fagerstedt et al. 1998; Ullén et al.
1998
).
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METHODS |
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Experiments were performed on 14 adult (15-30 cm) North
American silver lampreys, Ichthyomyzon unicuspis. The
preparation and surgery were described in detail in the accompanying
paper (Fagerstedt and Ullén 2001). In brief, the
preparation consisted of the head with exposed brain and rostral part
of the spinal cord (20-50 segments) and was mounted in an experimental
chamber, divided by a barrier into two pools, which were filled with
Ringer solution (Fig. 1B).
Fictive lateral turns were evoked by electrical stimulation of the skin
on one side of the head with bipolar silver wire electrodes (0.5 mm
diam). The activity of individual RS neurons in the two largest
reticular nuclei, MRRN and PRRN, and Mauthner cells were recorded
intracellularly with sharp glass micropipettes filled with 4 M
potassium acetate and 0.1 M potassium chloride (Fig. 1, A
and B). The activity of motoneurons was recorded
extracellularly with two pairs of glass pipette electrodes (20-30 µm
tip diameter) positioned over the ventral roots at the level of
segments 11-19 and segments 18-33, respectively, so that the distance
between the electrode pairs was around 10 segments (Fig.
1B). Data on ventral root activity at different rostrocaudal
levels during turns are presented in the accompanying paper
(Fagerstedt and Ullén 2001); here, only data from
the rostral ventral root electrodes will be included. Two additional
glass pipette electrodes (tip diameter 50-75 µm) were positioned on
the left and right dorsal surface of the spinal cord, close to the
caudal end of the preparation (segment 22-38; Fig. 1B).
They were used to determine to which side of the spinal cord each
recorded RS neuron projected and its conduction velocity. This was done
either by stimulating the RS neuron intracellularly, and recording the
arrival of the orthodromically propagated action potentials in the
spinal cord, or by stimulating the axon of the RS neuron
extracellularly, and recording the antidromically propagated action
potentials in the cell body.
Fictive locomotion, characterized by rhythmical bursts of
motoneuron activity alternating between the left and the right side was
evoked by administering D-glutamate (0.5-2 mM) to the
spinal cord pool. Fictive turns were evoked by electrical skin
stimulation (250- to 1,500-ms trains of 2-ms pulses at 10-50 Hz) and
were characterized by a transient increase of the burst duration and intensity on the turning side, and of the cycle duration (see RESULTS) (Fagerstedt and Ullén
2001). Turns toward the side of the recorded cell were
classified as ipsilateral, whereas turns in the opposite direction were
classified as contralateral. In 18 cells, the effect of high-frequency
intracellular stimulation (10-50 Hz for 10-30 s) on the locomotor
rhythm was also investigated.
Data acquisition and analysis of the ventral root activity was
described in the accompanying paper (Fagerstedt and Ullén 2001). In summary, a locomotor cycle was defined as starting
with the onset of a burst of ventral root activity on the side of the skin stimulus, and ending with the onset of the successive burst on the
same side; burst duration was defined as the time between onset and
termination of the same locomotor burst; and burst intensity as the
area of the rectified burst (burst amplitude) divided with its burst
duration. The activity of RS cells during ipsilateral and contralateral
fictive turns was analyzed separately. For neurons that fired action
potentials during the turn, the number of action potentials, the firing
frequency, and the total duration of the burst were measured. For two
activated cells, where a larger number of turns with varying amplitude
were recorded, a correlation analysis between turn amplitude in the
rostral segments and cellular response was performed. Turn amplitude
was determined either by looking at changes in burst intensity or in
cycle duration, since these two parameters to some extent vary
independently (Fagerstedt and Ullén 2001
). The
effects of intracellular stimulation on the locomotor rhythm were
evaluated by comparing the mean burst intensity, burst duration, and
cycle duration during the period of stimulation with the mean values of
the same parameters during a control period of approximately equal
duration (t-test), immediately preceding the stimulation.
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RESULTS |
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Response patterns in individual RS cells
Altogether, 72 RS neurons were recorded intracellularly in MRRN
(n = 42) and PRRN (n = 30) in 10 preparations. Their resting membrane potentials ranged between 58 and
80 mV. In around 50% of the cells, a spinal projection of the neuron
could be verified (see METHODS). The main reason that a
spinal projection could not be directly demonstrated in some cells is
most likely that the axon terminated rostral to the spinal surface
electrodes; earlier studies have shown that practically all larger
reticular cells do project to the spinal cord (Wannier
1994
). For simplicity, the term "RS neuron" will therefore
be used for all cells recorded in the reticular nuclei. The recorded RS
neurons practically never fired action potentials spontaneously.
Rhythmic modulation of the membrane potential in phase with ipsilateral
fictive locomotor activity was sometimes seen, but the amplitude was
low. All neurons were tested during 2-10 ipsilateral turns and 2-10
contralateral turns (see METHODS), with the exception of
two cells in PRRN and six cells in MRRN, which could only be tested for
turns in one direction.
As illustrated in Figs. 2 and 3, individual RS cells showed different characteristic patterns of responses during ipsilateral and contralateral turns. Excitatory responses consisted either of a subthreshold wave of depolarization or a depolarization with action potentials. During ipsilateral turns, most RS neurons (n = 53) received an excitatory input. Of these, 26 cells were depolarized and fired a burst of action potentials (Fig. 2A), while 25 neurons responded only with subthreshold depolarizations during the turn (Fig. 2C). Two cells responded with subthreshold depolarizations during some turns but were recruited and fired action potentials during other turns (see Fig. 6, A and B). The remaining cells (n = 13) were inhibited (not illustrated). Most RS neurons (n = 50) received an inhibitory input (Fig. 2D) during contralateral turns. The remaining cells (n = 20) received an excitatory input. Of the latter cells, six responded with subthreshold waves of depolarization (not illustrated), 13 responded with action potentials (Fig. 2B), while one cell responded with action potentials during some turns and with subthreshold depolarizations during the others (not illustrated).
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These results are summarized in Fig. 3, for the neurons recorded from MRRN and PRRN and also separately for the neurons from rostral and caudal subdivisions of these nuclei. The large majority of cells in MRRN were excited during ipsilateral turns (n = 33), and most of these (n = 23) also fired action potentials. Only five cells were inhibited (Fig. 3A). During contralateral turns, most MRRN cells (n = 24) were inhibited while the remaining cells (n = 17) were depolarized; 13 cells fired action potentials (Fig. 3A). No differences in the response characteristics were found between cells in rostral (n = 26; Fig. 3B) and caudal MRRN (n = 13; Fig. 3C). For three cells the location in MRRN was not noted.
Most RS neurons in PRRN were also excited during ipsilateral turns (n = 22), but only five of these fired action potentials. The remaining eight cells were inhibited (Fig. 2D). During contralateral turns, almost all (n = 26) PRRN cells were inhibited. Only four cells were depolarized, one of which fired action potentials (Fig. 3D). Cells in rostral PRRN (Fig. 3E) were more activated than cells in caudal PRRN (Fig. 3F) during turns in either direction. In the rostral PRRN, five cells fired action potentials during ipsilateral turns and one cell during contralateral turns (Fig. 3E), while none of the cells in caudal PRRN were activated during turning responses evoked by stimulation from either side (Fig. 3F).
Since several cells received subthreshold excitation during some turns and fired action potentials during others, these two responses will not be considered as distinct. In this way, four main types of cell responses during turns evoked by trigeminal skin stimulation could be distinguished. Cells with corresponding reponse patterns will be labeled TT1, TT2, TT3, and TT4 cells, respectively: 1) TT1 cells were excited during ipsilateral turns and inhibited during contralateral turns (Fig. 2, C and D); 2) TT2 cells were inhibited during ipsilateral turns and excited during contralateral turns; 3) TT3 cells were excited during turns in either direction (Figs. 2, A and B, and 6); and 4) TT4 cells, finally, were inhibited during turns in either direction.
Tables 1 and 2 show the response patterns of all recorded cells in MRRN and PRRN. In MRRN, TT3 cells and TT1 cells dominated. Ten of the total 15 TT3 cells in this nucleus fired action potentials during turns in either direction (Fig. 2, A and B). Of the 17 TT1 cells, nine were activated during ipsilateral turns (Table 1). The remaining cells consisted of four TT4 cells and one single cell with a TT2 response (Table 1). TT1 cells were the most common cell type also in PRRN (n = 18; Table 2). Only four of these cells were activated, however; the remaining 14 cells received subthreshold depolarization during ipsilateral turns (Fig. 2, C and D; Table 2). TT4 cells constituted the second most common cell type in PRRN (n = 8; Table 2). Only four TT3 cells and no TT2 cell were found in this nucleus. Two Mauthner cells were recorded. Both of these were TT3 cells and were activated during turns in either direction with relatively high firing frequency (see Fig. 4).
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Response characteristics of cells that were activated during fictive turns
Response characteristics of individual cells that fired action potentials during turns are summarized in Fig. 4. Of all 23 MRRN cells that were activated during ipsilateral turns (TT3 and TT1 cells; see Table 1 and above), 17 were selected for quantitative analysis, while the remaining cells were not included because of relatively unstable recordings or few recorded turns. The mean number of spikes per turn for these MRRN cells showed a wide distribution with a total mean of 6.7 (Fig. 4A). The mean spike train durations and firing frequencies likewise varied within a wide range with total means of 0.85 s (Fig. 4B) and 10.4 Hz (Fig. 4C), respectively.
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During contralateral turns 13 MRRN cells were activated (TT3 and TT2 cells; see Table 1 and above), of which nine were selected for quantitative analysis. The magnitude of these responses did not differ significantly from those seen during ipsilateral turns, however (see DISCUSSION). The total mean number of spikes per turn was 6.5 (Fig. 4D). The spike train durations had a total mean of 0.85 s (Fig. 4E), and the firing frequencies had a total mean of 13.0 Hz (Fig. 4F). The latter two parameters could only be calculated for cells that fired more than one spike per turn (n = 7).
Only four PRRN cells were activated during ipsilateral turns (TT1 cells; see Table 2). For all parameters, the responses of these cells were considerably weaker than for the MRRN cells. The total mean number of spikes per turn was 2.0 (Fig. 4A), while the total mean spike train duration was 0.27 s (Fig. 4B), and the total mean firing frequency was 5.2 Hz (Fig. 4C). A single PRRN cell (TT3 cell; see Table 2) fired action potentials during contralateral turns. The mean number of spikes fired by this cell was 1.8, with a mean train duration of 0.30 s and a mean firing frequency of 3.0 Hz.
The two recorded Mauthner neurons were strongly activated during turns in both directions. During ipsilateral turns, the total mean number of spikes was 16.2, with a mean train duration of 0.91 s and a mean firing frequency of 50.3 Hz (Fig. 4, A-C). During contralateral turns the values of the same parameters were 11.7 spikes, 0.80 s and 15.5 Hz, respectively (Fig. 4, D-F).
Responses during ipsilateral and contralateral turns were compared cell by cell in eight of the 10 MRRN TT3 cells that were activated during turns in both directions. Notably, these cells did not show similar changes of response parameters with change in turn direction (see DISCUSSION). Four cells fired less spikes per turn during contralateral than ipsilateral turns, while three cells fired more spikes during contralateral turns and one cell had the same number of spikes (Fig. 5A). Both the mean spike train duration and the mean firing frequency were decreased during contralateral turns in about one-half of the cells and increased in the remaining cells (Fig. 5, B and C).
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Correlation between turn amplitude and activity of RS neurons
We did not systematically try to evoke turns with large differences in amplitude, to study the correlation between the turn amplitude and the activity of RS neurons. Some observations could be made, however, which indicate that the generation of large turns is accompanied by both a recruitment of more RS cells and an increased response in already active cells.
Some neurons (two in PRRN, one in MRRN) were depolarized below threshold for action potentials in weaker turns but were activated during stronger turns, as illustrated in Fig. 6, A and B. In other neurons, an increase of turn amplitude was accompanied by an increase of the burst duration and frequency. Figure 6, C and D, shows an example of a MRRN TT3 cell that was activated during a weaker turn (Fig. 6C) and increased both number of spikes, train duration, and spiking frequency during a stronger turn (Fig. 6D).
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An analysis of the correlation between the cell response and the turn
amplitude was done for the two recorded Mauthner cells, where a larger
number of turns (n = 21) could be examined. This revealed strong positive correlations between parameters of the cell
response and turn amplitude on the contralateral side. The number of
spikes showed the strongest correlation with increases in cycle
duration during contralateral turns (k = 0.90), as did spike train duration (k = 0.92). Spike frequency showed
the strongest correlation with increase in burst intensity during
contralateral turns (k = 0.55). Correlations with
ipsilateral turn amplitude were low (0.3 < k < 0.3). One can note that the Mauthner cell is a contralaterally
projecting neuron (see DISCUSSION).
Intracellular stimulation of RS cells
Intracellular stimulation (10-50 Hz; see METHODS) of the recorded RS cells were performed in 18 cells (10 MRRN cells, seven PRRN cells, and one Mauthner cell). Significant effects on the locomotor rhythm were seen in 12 cases; both increases and decreases of the locomotor frequency were observed. Effects on burst intensity were predominantly excitatory on the ipsilateral side and inhibitory on the contralateral side of the spinal cord, but different cells influenced rostral and caudal ventral roots differently. Figure 7 shows an example of a TT3 cell from MRRN that evoked a strong response, similar to a fictive turn, when stimulated at 20 Hz. The locomotor frequency was decreased, and the burst intensity was increased on the ipsilateral side and decreased on the contralateral side of the spinal cord.
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DISCUSSION |
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General properties of the descending command for lateral turns
During lateral turns in the swimming lamprey, a mechanical wave
with larger amplitude than the normal locomotor waves is propagated rostrocaudally along the body, so that the orientation of the body is
shifted toward the new axis of swimming, initially in the rostral part,
then at progressively more caudal levels. The pattern of muscle
activity during turning is characterized by an increase in EMG burst
amplitude, duration, and proportion (burst duration divided by cycle
duration) on the side toward which the animal is turning, as well as by
an increase in cycle duration. An increase in burst amplitude on the
contralateral side is commonly seen after the initial turn cycle
(Fagerstedt and Ullén 2001; McClellan
1984
; McClellan and Hagevik 1997
;
Ullén et al. 1993
). All these changes in the
locomotor rhythm can be seen in the in vitro preparation of the
isolated brain and spinal cord, in which fictive lateral turns are
evoked by skin stimulation (Fagerstedt and Ullén
2001
; McClellan and Hagevik 1997
). The basic
components of the turn motor pattern are thus centrally generated.
In this study, the focus of interest is on the descending brain stem
commands causing lateral turns. Fibers reaching the spinal cord in
lamprey originate in the reticular nuclei (Brodin et al. 1988; Ronan 1989
; Rovainen 1983
;
Swain et al. 1993
), the vestibular nuclei
(Bussières 1994
; Bussières et al.
1999
; Rovainen 1979
), tectum, pretectum, and
scattered cell groups in diencephalon (Bussières 1994
). Of these, the reticulospinal and vestibulospinal systems are by far the largest, containing at least around 2,400 and 400 cells,
respectively, while only a few fibers from the other systems reach the
spinal cord (Bussières 1994
; Swain et al.
1993
). Furthermore, fictive turns can be evoked in
rhombencephalic preparations (Fagerstedt and Ullén
2001
), i.e., reticulospinal cells in the MRRN and the PRRN
together with the vestibulospinal system are sufficient to evoke
lateral turns. Vestibular fibers reach only the most rostral part of
the spinal cord (Rovainen 1979
). Recent experiments have also shown that vestibulospinal influences on spinal motorneurons are
3-5 times weaker than reticulospinal influences on the same segment,
and that the strength of the vestibulospinal influences tapers off
rapidly in more caudal segments, to become negligible beyond segment
10-12 (P. V. Zelenin, personal communication). For these
reasons we focused our attention on neurons in the two large
rhombencephalic reticular nuclei, the MRRN and the PRRN.
Skin stimulation in quiescent animals can evoke an avoidance behavior
that includes arousal, lateral turning, and initiation of locomotion
(McClellan 1984; McClellan and Grillner
1983
). Therefore the responses in RS neurons to the skin
stimulation employed in the present study most likely reflect commands
both for a general activation of the locomotor system and for a lateral
turn. Activation of the spinal locomotor network is presumably caused
by bilaterally symmetric commands transmitted via RS pathways, whereas
lateral turns are caused by asymmetric commands: recordings of mass
activity in RS pathways in intact lampreys have shown that initiation
of locomotion is preceded by a bilateral activation of the RS system, while turning movements are accompanied by an increase of RS activity on the side toward which the animal turns (Deliagina et al.
2000
). Additional evidence for the importance of ipsilateral
commands for turning was obtained in lesion experiments: a rostral
hemisection of the spinal cord prevents turning toward the lesioned
side (Fagerstedt and Ullén 2001
;
Ullén et al. 1997
). Finally, mathematical modeling has shown that, in many models of the lamprey spinal locomotor networks, turn-like alterations of ongoing locomotor activity can be
evoked by commands directed to one side of the spinal cord (Kozlov et al. 2001; McClellan and Hagevik
1997
; Ullén et al. 1998
). RS cells that
respond differentially to ipsilateral and contralateral turns were
therefore considered as candidates for generating the turning command
in the present study.
Role of different nuclear regions for the generation of turns
The largest proportion of cells that fired action potentials
during fictive turns was found in MRRN, followed by rostral PRRN, and
finally caudal PRRN, where few activated cells were found. No
difference in the level of activity was found between the rostral and
caudal halves of MRRN. The Mauthner cells were strongly activated during turns in either direction (see Fig. 4). These findings suggest that MRRN is an important nucleus for the generation of lateral
turns as a result of skin stimuli. In this preparation, however,
locomotion was not evoked from the brain stem. The level of excitation
of at least some RS cells was therefore somewhat lower than in vivo,
when swimming is initiated from the brain stem and accompanied by
depolarizing plateaus and spiking activity in reticular cells
(Kasicki et al. 1989; Viana Di Prisco et al. 1997). Since PRRN contains a large number of cells with an
asymmetric response during lateral turns, it is therefore possible that
the relative importance of PRRN for the generation of turn commands increases at higher levels of tonic RS activity.
Putative roles of different RS cell types for the generation of turns
All RS cells responded with either excitation or inhibition during turns in either direction; the sign of these responses was always the same. RS neurons therefore naturally fall into four groups with regard to their responses during lateral turns: TT1, TT2, TT3, and TT4 cells (see RESULTS). The distribution of these cell types in MRRN and PRRN reflected the overall pattern of excitability discussed above. More excited cells (e.g., spiking TT3 cells) occurred in MRRN, whereas TT4 cells were most common in caudal PRRN (see Tables 1 and 2).
TT1 cells are likely to play an important role for the generation of lateral turns. These cells could provide an excitatory bias to spinal networks involved in the ipsilateral turn generation, while they are inhibited during turns in the other direction. TT1 cells were common in both MRRN and PRRN, but TT1 cells that actually fired action potentials during fictive turns, and thus could contribute to the observed modulations of the locomotor pattern, were found mainly in MRRN (see Table 1). Increasing the excitability in the brain stem could presumably recruit more TT1 cells in PRRN (see above). Only one TT2 cell was recorded, in MRRN (Table 1), and whether it projected to the ipsilateral or contralateral spinal cord was unfortunately not determined.
TT3 cells were common in MRRN (Table 1). The activation of these cells could represent a general arousal and activation of the locomotor system. Some TT3 cells, however, were more strongly activated during ipsilateral turns than during contralateral turns, and may in addition contribute to the asymmetric turn command. In other TT3 cells, the opposite pattern was found; such cells could also contribute to the asymmetric turn command if they project contralaterally. Some TT3 cells, finally, showed no clear differences in their responses depending on turn laterality. These cells are probably not directly involved in the specification of turn direction. If they play a role for turn generation, they could possibly be involved in triggering the turning event, or influence the turn amplitude by exciting spinal networks involved in turn generation.
TT4 cells were predominantly found in PRRN, and especially in the caudal half of this nucleus. Since these cells were not active during normal fictive locomotion, nor during fictive turns, they appear unlikely to play a role for the control of the fictive turns studied in these experiments. One possibility would be that TT4 cells are involved in other types of behavior, and that they get inhibited during lateral turns to avoid behavioral conflict.
The present data clearly indicate that the generation of large turns involves both recruitment of additional RS neurons and larger responses in cells that are active already during turns with smaller amplitude. At the RS level, turn amplitude is thus most likely represented in the activity of the whole population of turn-related neurons, where the contribution of each individual cell will depend on its level of activity, as well as on the strength and specificity of its connections to the segmental networks in the spinal cord. One can note that several cells that were activated during turns in the present study produced turn-like changes of one or more parameters of the locomotor activity when stimulated intracellularly at a high frequency, supporting the notion that they are involved in the generation of turns.
The increase in burst intensity during turns can presumably in part be
explained by direct excitation of motoneurons, while effects on cycle
duration require that interneurons of the locomotor pattern generator
are affected. Cycle duration and burst intensity can, at least to some
extent, vary independently during turns (Fagerstedt and
Ullén 2001). Preliminary data in the present study
indicate that, at least in some cells, changes in burst intensity are
mainly reflected in the firing frequency of active RS command neurons,
while changes in cycle duration correlate best with the total duration
of the train of action potentials. In this way, an independent
regulation of these two parameters could be obtained by modulating
different aspects of the response in the same command neurons.
Relation of the lateral turn command system to other RS command systems
In this paper we have discussed the functional organization of a
descending command system for lateral turns evoked by skin stimulation.
The system for lateral turning is an integral part of the steering
system. Lateral turns can be evoked not only by skin stimuli, but also
by stimuli of a number of other modalities (visual, olfactory, lateral
line; see INTRODUCTION) and may also be performed
spontaneously. To what extent the control systems for lateral turns,
performed in different contexts, share common mechanisms at the RS
level has not been investigated. Visually evoked turns (negative
phototaxis) have been shown to be accompanied by the same types of
modulations of the locomotor pattern as turns evoked by skin
stimulation (Ullén et al. 1993;
Wallén et al. 1994
). Having one common system for
the generation of turns at the immediate supraspinal level would avoid
unnecessary redundancy.
A second, related issue concerns the relation of the command system for
lateral turns to the descending control systems for other forms of
behavior, such as initiation of forward and backward locomotion,
braking, and turns in the vertical planes. Since the RS system is the
main system for the transmission of all descending commands in the
lamprey, one can assume that, in general, different types of command
are encoded as different spatiotemporal patterns of activity in the
whole population of RS neurons. This raises the question of to what
extent the same RS neurons participate in the generation of different
types of commands; one extreme would be that all RS neurons are active
to some degree during all types of commands; the other extreme would be
that different commands involve separate, nonoverlapping subpopulations
of RS cells. In this regard, it is striking that all RS neurons
recorded in the present study responded during lateral turns, albeit in some cases below the threshold for the generation of action potentials. This suggests a considerable overlap between the command system for
lateral turns and other RS command systems. Lateral turns could, e.g.,
involve an asymmetric activation of the same ipsilateral and
contralateral RS neurons that are involved in initiation and maintenance of locomotion. If this were the case, unilateral activation of this pool of neurons should give an increase in cycle duration, while a bilateral activation of the same cells decreases the cycle duration. Observations on rostrally hemisected animals in vivo suggest
that at least some neurons involved in lateral turns may be separate
from the neurons initiating locomotion, however; these animals swim
along straight lines, although the descending activity is completely
unilateral, and perform normal spontaneous lateral turns toward the
intact side, with the same frequency as intact animals
(Ullén et al. 1997).
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ACKNOWLEDGMENTS |
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We thank D. Parker for valuable comments on the manuscript.
Support by grants from the National Institute of Neurological Disorders and Stroke (NS-38022), the Howard Hughes Medical Institute (75195-544801), the Swedish Medical Research Council (MFR 3026 and 11554), and the Royal Academy of Science is gratefully acknowledged. F. Ullén was supported by a postdoctoral grant from the Swedish Research Council for Engineering Sciences (TFR 282-96-905), the Swedish Brain Foundation, and by grants from Åke Wibergs Stiftelse and Magnus Bergwalls Stiftelse.
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FOOTNOTES |
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Present address and address for reprint requests: F. Ullén, Neuropediatrics, Dept. of Woman and Child Health, Astrid Lindgren Children's Hospital, SE-171 76 Stockholm, Sweden (E-mail: Fredrik.Ullen{at}neuro.ki.se).
Received 21 June 2000; accepted in final form 11 June 2001.
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