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Macular labyrinthine input to the contralateral lateral reticular nucleus
Brahl Research, 131 (1977) 147-151 ,i~;)Elsevier/North-Holland Biomedical Press
147
Short Communications
Macular labyrinthine input to the contralateral lateral reticular nucleus
OTTAVIO POMPE1ANO and KATSUMASA HOSHINO &tituto di Fisiologia Umana, Cattedra 11, Universit~ di Pisa, Pisa (Italy)
(Accepted April 13th, 1977)
Neurons located in the precerebellar lateral reticular nucleus (NRL) 4 as well as in the main reticular formation (RF) 9 respond to lateral tilting of the animal. There are at least two mechanisms through which the macular input may affect the activity of medullary reticular neurons, namely of NRL neurons. The first is by way of a direct vestibuloreticular projection originating from Deiters' nucleus6; the second is by vestibulospinal excitation of ascending spinoreticular neurons terminating within the NRL (cf. ref. 8). While the first projection is uncrossed, the second projection is assumed to be mainly crossed. The present experiments were undertaken to determine whether the responses of medullary reticular neurons to tilt were due to crossed or uncrossed pathways. Nine adult cats were decerebrated under ether anesthesia and submitted to section of the right Vlllth cranial nerve. In a first group of 4 experiments, the animals were also submitted to complete section of the spinal cord at T10-T12. In a second group of two experiments, in addition to the surgical procedures reported above, the animals were submitted to a complete bilateral ablation of the cerebellar vermis and the fastigial nuclei ('cerebellectomized' preparations). In a third group of 3 experiments, the animals were submitted to cerebellectomy as described above, but the spinal cord was completely cut between C1 and C2. The animals were then positioned in a stereotaxic and spinal cord frame, mounted on a hydraulic driven tilting table, paralyzed with gallamine triethiodide (3-4 mg/kg, i.v.) and artificially respired. The activity of reticular neurons was recorded by means of tungsten electrodes (2 l0 M~2 impedance). Unit activity was stored on one channel of a tape-recorder (Philips) while the animals were submitted to 20° tilts in both directions in the median plane at constant velocity4,9. The activity of neurons was later analyzed by a computer (Correlatron 1024, Laben), which provided sequential pulse density histograms (800 msec sampling interval, 256 bins). The analogue output of the computer was plotted on an X - Y plotter (Hewlett-Packard 7035B), while digital data were printed for statistical evaluation of results. Statistical computations were sampled for 40 sec periods taken before tilt and from 20 to 25 sec after the animal had been tilted to the new posi-
148 tion. At the end of each experiment a representative track was marked by passing anodal current through the tip of the recording electrode (0.5 mA for 10-20 sec). The position of each track was then identified on histological serial sections, stained with Nissl and Weil methods, and the location of each recorded unit was determined by relating the experimental co-ordinates to the histologically verified position of the tip of the recording electrode. Among 155 units recorded from the reticular formation ipsilateral to the deafferented labyrinth, 101 units were located in the NRL, and a proportion were shown to project to the ipsilateral vermal cortex of the cerebellar anterior lobe, as in preparations with the cerebellum intact they could be antidromically activated on electric stimulation of this corticocerebellar area (mean latency of the antidromic spike: 0.57 ~ 0.10 msec, S.D.). The remaining 54 units were located in the main RF of the medulla. In all, 56 out of these 155 reticular units responded to lateral tilts of the head and body, in spite of the complete section of the VIllth nerve ipsilateral to the recording side. In particular, responses to tilt were found in 37 of the 69 reticular neurons recorded in cats with the cerebellum intact (group I experiments) and in 19 of 46 reticular neurons recorded after cerebellectomy (group II). However, none of the 40 units recorded in cerebellectomized preparations with complete section of the spinal cord at C1-C2 (group III) responded to 20 ° tilt. The most prevalent responses found among the medullary units studied in the first 2 groups of experiments were of the a-type (38/56 units) and the fl-type (16/56 units), where the a-responses were characterized by a steady increase in discharge rate during ipsilateral tilt and a reduction in firing rate on contralateral tilt, while the /3TABLE
1
Distribution of 155 units recorded in different preparations .from the medullary reticulur /ormation oJ the right side, ipsilateral to the labyrinthine deafferentation and classified according to their type 0/" response to lateral tilting N R L d a n d N R L v ; dorsal m a t i o n of the medulla.
and ventral part of the lateral reticular nucleus; RF, main
Prep.Trations
reticular
Localization Response type
Total
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ct
Group
1: decerebrate, with t r a n of the spinal cord at T 1 o - T 1 2
section
Group lI: decerebrate, cerebellectomized with transection of the spinal cord at T l o - T I ~ Group III: decerebrate, cerebellectomized with transection of the spinal cord at C 1 - C ~
NRL
d
6
NRL v NRLd RF NRL d
7 13 9
NRL v NRLd RF NRL d RF
t v
7 4
i v ~ v
[~
7
2
--
3
mlmber
No response 1
12
21
7 --9 -1 4 -I . . . . . . . . .
4 16
18 39
2 2
. . 11 -5 1 . . . . . . . . . . . . . . . . . . . . . .
. . . . . . .
16
30
12
19
4 16 I1 33 7
I0 29 17 33 7
for-
149 responses showed the opposite pattern of response. Moreover no unit showed a rate increase (v-response), while 2/56 units showed a rate decrease during both ipsilateral and contralateral tilt (6-response). The units showing a positional sensitivity were most frequently found in the ventral (parvicellular)than in the dorsal (magnocellular) part of the NRL, both in preparations with the cerebellum intact as well as in cerebellectornized preparations. They were also found in the nucleus ventralis and the nucleus reticularis parvicellularis of the main RF. Table I illustrates the distribution of the units responsive to tilt and those unresponsive to tilt within the reticular structures explored ipsilaterally to the VIllth nerve section. It is of interest that in the group I experiments the reticular units showing an ~- or a fl-type of response displayed symmetrical changes in firing rate during both Im~o./sec
?~,,¢v/,/
Imp./sec 75
so 25
Imp./sec 40 ,
130
i', ,
4~o
Imp
|
Imp./sec 20
C
/
\
/sec
fmp.lsec 2O
--1
/ 60sec
Fig. 1, Typical responses to lateral tilt of N R L units ipsilateral to the labyrinthine deafferentation. All the experiments were performed in decerebrate cats with section of the right VIIlth nerve. In A the animal was also submitted to complete section of the spinal cord at Tl0 Tie, while the cerebellum was intact (group I experiment). In B the animal was submitted to section of the spinal cord at T1, T12, but in addition the vermal cortex of the cerebellum and the fastigial nucleus of both sides were completely removed (group II experiment). Finally in C the animal was submitted to cerebellar ablation as described above and to complete section of the spinal cord between C1 and C2 (group Ill experiment). All the units were recorded from the right N R L . The responses of these units to 20' tilt to the ipsilateral (left records) and the contralateral side (right records) are shown as average sequential pulse density histograms, using 256 bins with 800 msec sampling intervals. Each record is the result of 3 trials. A: typical a-response of a single unit located in the ventral part of the N R L . B: asymmetrical response of a single unit located in the dorsal part of the N R L . C: absence of response to tilt of a single unit located in the ventral part of the N R L .
150 ipsilateral and contralateral tilts (Fig. I A), whereas in cerebellectomized group II experiments most of the responsive neurons showed an asymmetrical pattern of response, with tilt in one direction producing a greater response than tilt in the opposite direction (Fig. 1B). However, none of the medullary reticular units recorded in group IlI experiments, i.e., in cats submitted to cerebellar ablation and complete section of the spinal cord at C~-C~_, responded to tilt (Fig. 1C). In addition to the medullary reticular units recorded in this last group of experiments from the side of the VIIIth nerve section, 81 units were also recorded ipsilaterally to the intact labyrinth. Of these neurons, 40 were located within the NRL, of which only 5 responded to lateral tilt (three a-, one fl- and one y-response). The remaining 41 units were located in the main RF, of which 6 responded to tilt (five a- and one [# response). While there was no doubt about the exact location of the main RF units responsive to tilt, all the responsive neurons attributed to the N R L were located close to the dorsomedial border of the nucleus, a region whose limits are difficult to assess. The present experiments indicate that medullary neurons located in the NRL as well as in the main RF respond to lateral tilt of the whole animal even after section of the Vlllth nerve ipsilateral to the side of the recording. This finding indicates that macular influences are transmitted from the intact labyrinth to the contralateral reticular formation by crossed pathways. One of these pathways might cross through the cerebellum, since in animals with the cerebellum intact most of the units showing a reciprocal pattern of discharge during tilt in both directions displayed a magnitude of response which was similar during both ipsilateral and contralateral tilt, while in cerebellectomized preparations most of the reticular neurons responsive to tilt showed an asymmetrical pattern, with tilt in one direction producing a larger response than tilt in the opposite direction. This finding indicates the importance of the cerebellar loop in producing balanced responses of the reticular neurons to macular stimulation in spite of the absence of the ipsilateral labyrinth. There is no doubt, however, that macular influences can be transmitted from the intact labyrinth to the contralateral reticular formation in the absence of any cerebellar loop. The observation that these crossed responses can still be observed in cerebellectomized preparations with transection of the spinal cord at TLo-Ta2, but are totally suppressed following section of the spinal cord at C1- C2, indicates that the response of reticular neurons to volleys originating from the contralateral intact labyrinth utilizes a vestibulospinal mechanism, which acts on neurons of the crossed spinoreticular pathway located in the upper segments of the spinal cord. This conclusion is supported by the following observations: (1) neurons of the cervical spinoreticutar pathway transmit information to the N R L originating from static labyrinthine receptorsZ,:~; (2) electrical stimulation of the lateral vestibulospinal tract exerts a monosynaptic excitation on spinoreticular neurons whose ascending axons are mainly, if not exclusively, crossed2,a,5; (3) most of the crossed spinoreticular tract neurons labeled after injection of horseradish peroxidase (HRP) within the N R L are located in Rexed's laminae VII, VIII and IX 1, i.e., in the same area where the lateral vestibutospinal tract terminates v. A final comment concerns the observation that medullary reticular units ip-
151 silateral to the intact l a b y r i n t h r e s p o n d to tilt after cerebellar a b l a t i o n s and c o m p l e t e section o f the spinal c o r d at C1-Cz. These responses can be a t t r i b u t e d to activation o f a direct vestibuloreticular projection6; there is, however, some d o u b t a b o u t the imp o r t a n c e o f such a direct p r o j e c t i o n to the ipsilateral N R L 6, since all the responsive units were located in the m a i n R F and along the d o r s o m e d i a l b o r d e r o f the N R L , close to the s u r r o u n d i n g R F . This conclusion is strengthened by the o b s e r v a t i o n that no labeled cells were f o u n d in Deiters' nucleus following injection o f H R P within the ipsilateral N R L 1. It appears, therefore, that the degenerating fibers observed within the N R L following lesion o f the ipsilateral Deiters' nucleus 6 m a y in p a r t at least be a t t r i b u t e d to i n t e r r u p t i o n o f crossed fastigioreticular afferents to the N R L in their course t h r o u g h Deiters' nucleus a°. This investigation was s u p p o r t e d by Public H e a l t h Service Research G r a n t NS 07685-08 from the N a t i o n a l Institute o f N e u r o l o g i c a l and C o m m u n i c a t i v e D i s o r d e r s a n d Stroke, N . I . H . , U.S.A. and by a Research G r a n t from the Consiglio N a z i o n a l e delle Ricerche, Italy. Dr. K. H o s h i n o is on leave o f absence f r o m the D e p a r t a m e n t o de Ci~ncias Fisiol6gicas, F a c u l d a d e de Ci~ncias M6dicas e Biol6gicas, Botucatu, Brasil.
1 Corvaja, N., Grofov~i, 1., Pompeiano, O. and Walberg, F., The lateral reticular nucleus in the cat. I. An experimental anatomical study of its spinal and supraspinal afferent connections, Neuroscience, (1977)in press. 2 Coulter, J. D., Mergner, T. and Pompeiano, O., Macular influences on ascending spinoreticular neurons located in the cervical cord, Brain Research, 82 (1974) 322-327. 3 Coulter, J. D., Mergner, T. and Pompeiano, O., Effects of static tilt on cervical spinoreticular tract neurons, J. Neurophysiol., 39 (1976) 45-62. 4 Ghelarducci, B., Pompeiano, O. and Spyer, K. M., Activity of precerebellar reticular neurones as a function of head position, Arch. ital. Biol., 112 (1974) 98-125. 5 Grillner, S., Hongo, T. and Lund, S., The origin of descending fibers monosynaptically activating spinoreticular neurones, Brain Research, 10 (1968) 259-262. 6 Ladpli, R. and Brodal, A., Experimental studies of commissural and reticular formation projections from the vestibular nuclei in the cat, Brain Research, 8 (1968) 65 96. 7 Nyberg-Hansen, R. and Mascitti, T. A., Sites and mode of termination of fibers of the vestibulospinal tract in the cat. An experimental study with silver impregnation methods, J. eomp. Neurol., 122 (1964) 369-387. 8 Pompeiano, O., Macular input to neurons of the spinoreticulocerebellar pathway, Brain Researeh, 95 (1975) 351-368. 9 Spyer, K. M., Ghelarducci, B. and Pompeiano, O., Gravity responses of neurons in the main reticular formation, J. Neurophysiol., 37 (1974) 705-721. 10 Walberg, F. and Pompeiano, O., Fastigiofugal fibers to the lateral reticular nucleus. An experimental study in the cat, Exp. Neurol., 2 (1960) 40-53.
147
Short Communications
Macular labyrinthine input to the contralateral lateral reticular nucleus
OTTAVIO POMPE1ANO and KATSUMASA HOSHINO &tituto di Fisiologia Umana, Cattedra 11, Universit~ di Pisa, Pisa (Italy)
(Accepted April 13th, 1977)
Neurons located in the precerebellar lateral reticular nucleus (NRL) 4 as well as in the main reticular formation (RF) 9 respond to lateral tilting of the animal. There are at least two mechanisms through which the macular input may affect the activity of medullary reticular neurons, namely of NRL neurons. The first is by way of a direct vestibuloreticular projection originating from Deiters' nucleus6; the second is by vestibulospinal excitation of ascending spinoreticular neurons terminating within the NRL (cf. ref. 8). While the first projection is uncrossed, the second projection is assumed to be mainly crossed. The present experiments were undertaken to determine whether the responses of medullary reticular neurons to tilt were due to crossed or uncrossed pathways. Nine adult cats were decerebrated under ether anesthesia and submitted to section of the right Vlllth cranial nerve. In a first group of 4 experiments, the animals were also submitted to complete section of the spinal cord at T10-T12. In a second group of two experiments, in addition to the surgical procedures reported above, the animals were submitted to a complete bilateral ablation of the cerebellar vermis and the fastigial nuclei ('cerebellectomized' preparations). In a third group of 3 experiments, the animals were submitted to cerebellectomy as described above, but the spinal cord was completely cut between C1 and C2. The animals were then positioned in a stereotaxic and spinal cord frame, mounted on a hydraulic driven tilting table, paralyzed with gallamine triethiodide (3-4 mg/kg, i.v.) and artificially respired. The activity of reticular neurons was recorded by means of tungsten electrodes (2 l0 M~2 impedance). Unit activity was stored on one channel of a tape-recorder (Philips) while the animals were submitted to 20° tilts in both directions in the median plane at constant velocity4,9. The activity of neurons was later analyzed by a computer (Correlatron 1024, Laben), which provided sequential pulse density histograms (800 msec sampling interval, 256 bins). The analogue output of the computer was plotted on an X - Y plotter (Hewlett-Packard 7035B), while digital data were printed for statistical evaluation of results. Statistical computations were sampled for 40 sec periods taken before tilt and from 20 to 25 sec after the animal had been tilted to the new posi-
148 tion. At the end of each experiment a representative track was marked by passing anodal current through the tip of the recording electrode (0.5 mA for 10-20 sec). The position of each track was then identified on histological serial sections, stained with Nissl and Weil methods, and the location of each recorded unit was determined by relating the experimental co-ordinates to the histologically verified position of the tip of the recording electrode. Among 155 units recorded from the reticular formation ipsilateral to the deafferented labyrinth, 101 units were located in the NRL, and a proportion were shown to project to the ipsilateral vermal cortex of the cerebellar anterior lobe, as in preparations with the cerebellum intact they could be antidromically activated on electric stimulation of this corticocerebellar area (mean latency of the antidromic spike: 0.57 ~ 0.10 msec, S.D.). The remaining 54 units were located in the main RF of the medulla. In all, 56 out of these 155 reticular units responded to lateral tilts of the head and body, in spite of the complete section of the VIllth nerve ipsilateral to the recording side. In particular, responses to tilt were found in 37 of the 69 reticular neurons recorded in cats with the cerebellum intact (group I experiments) and in 19 of 46 reticular neurons recorded after cerebellectomy (group II). However, none of the 40 units recorded in cerebellectomized preparations with complete section of the spinal cord at C1-C2 (group III) responded to 20 ° tilt. The most prevalent responses found among the medullary units studied in the first 2 groups of experiments were of the a-type (38/56 units) and the fl-type (16/56 units), where the a-responses were characterized by a steady increase in discharge rate during ipsilateral tilt and a reduction in firing rate on contralateral tilt, while the /3TABLE
1
Distribution of 155 units recorded in different preparations .from the medullary reticulur /ormation oJ the right side, ipsilateral to the labyrinthine deafferentation and classified according to their type 0/" response to lateral tilting N R L d a n d N R L v ; dorsal m a t i o n of the medulla.
and ventral part of the lateral reticular nucleus; RF, main
Prep.Trations
reticular
Localization Response type
Total
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ct
Group
1: decerebrate, with t r a n of the spinal cord at T 1 o - T 1 2
section
Group lI: decerebrate, cerebellectomized with transection of the spinal cord at T l o - T I ~ Group III: decerebrate, cerebellectomized with transection of the spinal cord at C 1 - C ~
NRL
d
6
NRL v NRLd RF NRL d
7 13 9
NRL v NRLd RF NRL d RF
t v
7 4
i v ~ v
[~
7
2
--
3
mlmber
No response 1
12
21
7 --9 -1 4 -I . . . . . . . . .
4 16
18 39
2 2
. . 11 -5 1 . . . . . . . . . . . . . . . . . . . . . .
. . . . . . .
16
30
12
19
4 16 I1 33 7
I0 29 17 33 7
for-
149 responses showed the opposite pattern of response. Moreover no unit showed a rate increase (v-response), while 2/56 units showed a rate decrease during both ipsilateral and contralateral tilt (6-response). The units showing a positional sensitivity were most frequently found in the ventral (parvicellular)than in the dorsal (magnocellular) part of the NRL, both in preparations with the cerebellum intact as well as in cerebellectornized preparations. They were also found in the nucleus ventralis and the nucleus reticularis parvicellularis of the main RF. Table I illustrates the distribution of the units responsive to tilt and those unresponsive to tilt within the reticular structures explored ipsilaterally to the VIllth nerve section. It is of interest that in the group I experiments the reticular units showing an ~- or a fl-type of response displayed symmetrical changes in firing rate during both Im~o./sec
?~,,¢v/,/
Imp./sec 75
so 25
Imp./sec 40 ,
130
i', ,
4~o
Imp
|
Imp./sec 20
C
/
\
/sec
fmp.lsec 2O
--1
/ 60sec
Fig. 1, Typical responses to lateral tilt of N R L units ipsilateral to the labyrinthine deafferentation. All the experiments were performed in decerebrate cats with section of the right VIIlth nerve. In A the animal was also submitted to complete section of the spinal cord at Tl0 Tie, while the cerebellum was intact (group I experiment). In B the animal was submitted to section of the spinal cord at T1, T12, but in addition the vermal cortex of the cerebellum and the fastigial nucleus of both sides were completely removed (group II experiment). Finally in C the animal was submitted to cerebellar ablation as described above and to complete section of the spinal cord between C1 and C2 (group Ill experiment). All the units were recorded from the right N R L . The responses of these units to 20' tilt to the ipsilateral (left records) and the contralateral side (right records) are shown as average sequential pulse density histograms, using 256 bins with 800 msec sampling intervals. Each record is the result of 3 trials. A: typical a-response of a single unit located in the ventral part of the N R L . B: asymmetrical response of a single unit located in the dorsal part of the N R L . C: absence of response to tilt of a single unit located in the ventral part of the N R L .
150 ipsilateral and contralateral tilts (Fig. I A), whereas in cerebellectomized group II experiments most of the responsive neurons showed an asymmetrical pattern of response, with tilt in one direction producing a greater response than tilt in the opposite direction (Fig. 1B). However, none of the medullary reticular units recorded in group IlI experiments, i.e., in cats submitted to cerebellar ablation and complete section of the spinal cord at C~-C~_, responded to tilt (Fig. 1C). In addition to the medullary reticular units recorded in this last group of experiments from the side of the VIIIth nerve section, 81 units were also recorded ipsilaterally to the intact labyrinth. Of these neurons, 40 were located within the NRL, of which only 5 responded to lateral tilt (three a-, one fl- and one y-response). The remaining 41 units were located in the main RF, of which 6 responded to tilt (five a- and one [# response). While there was no doubt about the exact location of the main RF units responsive to tilt, all the responsive neurons attributed to the N R L were located close to the dorsomedial border of the nucleus, a region whose limits are difficult to assess. The present experiments indicate that medullary neurons located in the NRL as well as in the main RF respond to lateral tilt of the whole animal even after section of the Vlllth nerve ipsilateral to the side of the recording. This finding indicates that macular influences are transmitted from the intact labyrinth to the contralateral reticular formation by crossed pathways. One of these pathways might cross through the cerebellum, since in animals with the cerebellum intact most of the units showing a reciprocal pattern of discharge during tilt in both directions displayed a magnitude of response which was similar during both ipsilateral and contralateral tilt, while in cerebellectomized preparations most of the reticular neurons responsive to tilt showed an asymmetrical pattern, with tilt in one direction producing a larger response than tilt in the opposite direction. This finding indicates the importance of the cerebellar loop in producing balanced responses of the reticular neurons to macular stimulation in spite of the absence of the ipsilateral labyrinth. There is no doubt, however, that macular influences can be transmitted from the intact labyrinth to the contralateral reticular formation in the absence of any cerebellar loop. The observation that these crossed responses can still be observed in cerebellectomized preparations with transection of the spinal cord at TLo-Ta2, but are totally suppressed following section of the spinal cord at C1- C2, indicates that the response of reticular neurons to volleys originating from the contralateral intact labyrinth utilizes a vestibulospinal mechanism, which acts on neurons of the crossed spinoreticular pathway located in the upper segments of the spinal cord. This conclusion is supported by the following observations: (1) neurons of the cervical spinoreticutar pathway transmit information to the N R L originating from static labyrinthine receptorsZ,:~; (2) electrical stimulation of the lateral vestibulospinal tract exerts a monosynaptic excitation on spinoreticular neurons whose ascending axons are mainly, if not exclusively, crossed2,a,5; (3) most of the crossed spinoreticular tract neurons labeled after injection of horseradish peroxidase (HRP) within the N R L are located in Rexed's laminae VII, VIII and IX 1, i.e., in the same area where the lateral vestibutospinal tract terminates v. A final comment concerns the observation that medullary reticular units ip-
151 silateral to the intact l a b y r i n t h r e s p o n d to tilt after cerebellar a b l a t i o n s and c o m p l e t e section o f the spinal c o r d at C1-Cz. These responses can be a t t r i b u t e d to activation o f a direct vestibuloreticular projection6; there is, however, some d o u b t a b o u t the imp o r t a n c e o f such a direct p r o j e c t i o n to the ipsilateral N R L 6, since all the responsive units were located in the m a i n R F and along the d o r s o m e d i a l b o r d e r o f the N R L , close to the s u r r o u n d i n g R F . This conclusion is strengthened by the o b s e r v a t i o n that no labeled cells were f o u n d in Deiters' nucleus following injection o f H R P within the ipsilateral N R L 1. It appears, therefore, that the degenerating fibers observed within the N R L following lesion o f the ipsilateral Deiters' nucleus 6 m a y in p a r t at least be a t t r i b u t e d to i n t e r r u p t i o n o f crossed fastigioreticular afferents to the N R L in their course t h r o u g h Deiters' nucleus a°. This investigation was s u p p o r t e d by Public H e a l t h Service Research G r a n t NS 07685-08 from the N a t i o n a l Institute o f N e u r o l o g i c a l and C o m m u n i c a t i v e D i s o r d e r s a n d Stroke, N . I . H . , U.S.A. and by a Research G r a n t from the Consiglio N a z i o n a l e delle Ricerche, Italy. Dr. K. H o s h i n o is on leave o f absence f r o m the D e p a r t a m e n t o de Ci~ncias Fisiol6gicas, F a c u l d a d e de Ci~ncias M6dicas e Biol6gicas, Botucatu, Brasil.
1 Corvaja, N., Grofov~i, 1., Pompeiano, O. and Walberg, F., The lateral reticular nucleus in the cat. I. An experimental anatomical study of its spinal and supraspinal afferent connections, Neuroscience, (1977)in press. 2 Coulter, J. D., Mergner, T. and Pompeiano, O., Macular influences on ascending spinoreticular neurons located in the cervical cord, Brain Research, 82 (1974) 322-327. 3 Coulter, J. D., Mergner, T. and Pompeiano, O., Effects of static tilt on cervical spinoreticular tract neurons, J. Neurophysiol., 39 (1976) 45-62. 4 Ghelarducci, B., Pompeiano, O. and Spyer, K. M., Activity of precerebellar reticular neurones as a function of head position, Arch. ital. Biol., 112 (1974) 98-125. 5 Grillner, S., Hongo, T. and Lund, S., The origin of descending fibers monosynaptically activating spinoreticular neurones, Brain Research, 10 (1968) 259-262. 6 Ladpli, R. and Brodal, A., Experimental studies of commissural and reticular formation projections from the vestibular nuclei in the cat, Brain Research, 8 (1968) 65 96. 7 Nyberg-Hansen, R. and Mascitti, T. A., Sites and mode of termination of fibers of the vestibulospinal tract in the cat. An experimental study with silver impregnation methods, J. eomp. Neurol., 122 (1964) 369-387. 8 Pompeiano, O., Macular input to neurons of the spinoreticulocerebellar pathway, Brain Researeh, 95 (1975) 351-368. 9 Spyer, K. M., Ghelarducci, B. and Pompeiano, O., Gravity responses of neurons in the main reticular formation, J. Neurophysiol., 37 (1974) 705-721. 10 Walberg, F. and Pompeiano, O., Fastigiofugal fibers to the lateral reticular nucleus. An experimental study in the cat, Exp. Neurol., 2 (1960) 40-53.