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Eighth nerve activation of cat pontine reticular neurons which project in or near the ascending medial longitudinal fasciculus
EXPERIMENTAL
NEUROLOGY
70, 706-711 (1980)
RESEARCH
NOTE
Eighth Nerve Activation of Cat Pontine Reticular Neurons Which Project in or near the Ascending Medial Longitudinal Fasciculus R. S. Departments
R. D.
REMMEL,
of Physiology Received
and Anatomy, Little Rock. April
AND L. B. MINOR'
SKINNER, University Arkansas
8. 1980: revision
of Arkansas 72205
received
for
Medical
Sciences,
June 30, 1980
One class of neurons in the nucleus reticularis pontis caudalis (NRPC) was antidromically identified by weak shocks to project an ascending axon into the region of the ipsilateral medial longitudinal fasciculus (IMLF); these are called NRPC-iMLF neurons. Some were excited orthodromically at monosynaptic or longer latencies by shocks to the region of the contralateral medial vestibular nucleus (cMVN). Some were excited at disynaptic or longer latencies by shocks to the ipsilateraI(18) or contralateral (~8) 8th nerve. Many showed convergence from several inputs. These NRPC-iMLF neurons are hypothesized to participate in vestibuloocular or other vestibular functions.
We identified electrophysiologically the 8th nerve and vestibular nucleus inputs to the nucleus reticularis pontis caudalis (NRPC) neurons whose axons ascend in the ipsilateral medial longitudinal fasciculus (iMLF). These NRPC-iMLF fibers in the cat were described by the use of autoradiography (4) and their somatic distribution was mapped out by the technique of antidromic excitation from the region of the ascending iMLF (11). Their somatic distribution extends in the NRPC for about 3 mm ventral and 3 mm rostra1 of the 6th nucleus and from about 0.5 to 1.6 mm lateral, i.e., more lateral usually than burst inhibitory eye-movement neurons (5) and omnipauser eye-movement neurons (3) in the cat. Probably Abbreviations: NRPC-nucleus reticularis pontis caudalis, iMLF-ascending longitudinal fasciculus ipsilateral to the recording site, cMVN-contralateral vestibular nucleus, i8-ipsilateral 8th nerve, c8-contralateral 8th nerve. * We thank G. Sammartino for histological assistance. This work was supported Institutes of Health grant EY01794 and work-study funds for L.B.M. from Brown
706 0014-4886/80/120706-06$02.00/O Copyright All rights
0 1980 by Academic Press. Inc. of reproductron m any form reserved.
medial medial by National University.
EIGHTH
NERVE-VESTIBULORETICULO-MLF
CONNECTIONS
707
homologous fibers in the monkey (1, 2) ascend immediately ventral or lateral to the iMLF, but join it more rostrally; some terminate in the midbrain near the oculomotor nucleus. Our study addresses three questions: (i) Do NRPC-iMLF cells receive input from the 8th nerves or the region of the contralateral medial vestibular nucleus (cMVN)? (ii) Is such input monosynaptic from the cMVN and disynaptic from the nerves? (iii) Is there bilateral convergence of inputs? All answers appear to be yes. In adult cats under ether anesthesia, both carotid arteries were ligated, the cat was decerebrated, and then the ether was discontinued. Life functions were supported as previously described (11, 12). The cerebellum was aspirated to expose the brain stem. Isolated constant-current shocks were 0.1 ms in duration. Stimulating electrodes were made from an insulated stainless-steel wire (cathode) placed inside an uninsulated concentric stainless-steel tube (anode). The wire tip was inserted into the brain or labyrinth, where it functioned as a local monopolar electrode, and the tube remained outside in the saline which covered the tissue. The tips of the two, contralateral and ipsilateral, 8th nerve electrodes (c8 and i8) were inserted into the round windows (12). The iMLF and cMVN stimulating electrodes were made from a 0.125-mm-diameter wire (Medwire Corp.) protruding 4 mm from a tube. Stereotaxic manipulators were tilted 45” caudal of the vertical plane. The iMLF electrode was inserted 1 mm caudal to the 4th nucleus and 0.4 mm lateral of midline (11). The cMVN electrode was inserted visually 1.5 mm caudal to the cerebellar peduncle and 3 mm lateral to the midline. While recording with the cMVN electrode, it was lowered until the nerve-evoked fields were maximal so that it was within the region where primary axons synapse monosynaptically upon ascending crossed vestibular-MLF neurons (12). The ipsilateral MVN was not stimulated because of possible excitation of cerebelloreticular fibers, which do not cross in the brain stem. Neurons were identified as antidromically excited from the iMLF electrode by previously described criteria (11). The predominantly negative spikes were considered to be from somas. Orthodromic responses were considered only when excited by 1200 PA for the cMVN and 5 1000 PA for the nerve electrodes. Orthodromic responses had to show a variable latency of >O. 15 ms near threshold and a decreased, more constant latency for stronger shocks. Orthodromic spikes could be collided with antidromic spikes. Previously described histological techniques (11, 12) showed that the iMLF electrode tips were within the MLF and that the cMVN electrode tips were 1.7 2 0.7 mm caudal to the 6th nucleus center and 2.2 f 0.6 mm ventral to the ventricular floor, i.e., within the region described in (12). In 16 cats, 245 NRPC-iMLF cells had loci and antidromic properties as previously described (11). Most were excited orthodromically from the
708
REMMEL,
0
0
SKINNER,
I
2 Loiency,
AND
3
rns,P
MINOR
5
)6
FIG. 1. Top-extracellular records from a neuron of the nucleus reticularis pontis caudalis which projects into the ipsilateral medial longitudinal fasciculus (NRPC-iMLF). The shock artifact is at the left and the spike(s) is indicated by a dot for each trace; other negative waves are nonunitary fields. iMLF (four traces in a vertical column): two antidromic responses at a constant latency and one miss to a 13-PA-threshold shock to the iMLF electrode. The fourth trace shows responses to four iMLF shocks at 1340 Hz. cMVN, c8, and i8 (three traces each in a vertical column): orthodromic responses to one shock to the contralateral medial vestibular nucleus (cMVN), contralateral (c8), and ipsilateral 8th nerve (i8) electrodes, respectively, with latency variation on successive trials (c8 pictures were double exposures). Horizontal bar (lower left): 1 ms (iMLF, top three), 2.5 ms (lower left and cMVN), and 5 ms (c8 and i8). Vertical bar: 0.5 mV. Bottom-histograms of the minimum latencies of orthodromic responses to shocks to the cMVN, c8. and i8 electrodes.
cMVN, c8, and/or i8 electrodes. The neuron in Fig. 1 responded antidromically at a constant 0.3%ms latency to a 13-PA-threshold iMLF shock and followed four shocks at 1340 shocks per second. The neuron responded at a variable latency to cMVN stimulation, with a 60-PA threshold. Suprathreshold shocks caused a decreased latency, the minimum being 1.20 ms (top trace), i.e., monosynaptic (see below). The neuron responded to c8 stimulation at a 670qA threshold (2.1 x the c8 nerve stimulus threshold for evoking any field potential in the MVN); the minimum latency was 2.2 ms (top trace), i.e., disynaptic (see below). The neuron responded to i8 stimulation with a 600-PA threshold and 2.7-ms minimum latency (top trace), i.e., not definitely disynaptic. The numbers of NRPC-iMLF cells orthodromically excited (+) from the cMVN, c8, and i8 electrodes are given in Table 1. The largest group (N = 91)
EIGHTH
NERVE-VESTIBULORETICULO-MLF TABLE
CONNECTIONS
709
1
Percentage of Neurons Activated by the Different Electrodes cMVN
c8
i8
N
%
0 + 0 + 0 0 + +
0 0 + + 0 + 0 +
0 0 0 0 + + + +
71 28 5 24 2 8 16 91
29 11 2 10 1 3 7 37
245
100
Total
Totals
N
%
Unexcited cMVN c8 i8
71 159 128 117
29 65 52 48
was excited by all electrodes. Half (8 + 16 + 91) were excited by at least one electrode on each side, indicating frequent bilateral convergence. Another large group (28 + 5 + 24) was excited by only contralateral cMVN and/or c8 stimulation. Only two cells were excited only by i8 stimulation. The 29% giving no orthodromic response might still have had subthreshold inputs. Inhibition of spontaneous activity was observed occasionally. These properties are similar to those of NRPC neurons in general, as determined through intracellular recording (8). The range of thresholds for responses of NRPC-iMLF neurons to cMVN stimulation was 12 to 200 PA (90 + 49, mean and SD), that for responses to c8 stimulation was 60 to 1000 PA (mean, 580 _t 250), and that for responses to i8 stimulation was 220 to 1000 PA (mean, 650 + 220). These unit thresholds were sometimes comparable and never >5x the 210 + 90-PA mean stimulus threshold for c8 nerve stimulation for evoking any field potential in the MVN. Latencies were measured between the start of the shock and the start of the spike for suprathreshold shocks. The range of minimum latencies to cMVN stimulation (Fig. 1, histograms) was 1.O to 7.0 ms (mean, 2.9 ? 1.1); the range for c8 stimulation was 1.4 to 6.9 ms (mean, 3.4 + l.l), i.e., one synaptic delay of 0.5 ms longer than from cMVN, on the average. The range for i8 stimulation was 1.5 to 18 ms (mean, 3.5 + 1.2). Some latencies seemed short enough to the monosynaptic from cMVN stimulation and disynaptic from the nerves. A neuron disynaptic rather than monosynaptic from cMVN stimulation should have a latency of ~0.5 ms for each synapse and a latency of 20.6 ms for conduction of vestibuloreticular fibers (7), i.e., ~1.6 ms; a neuron trisynaptic rather than disynaptic from a vestibular nerve stimulation should have, in addition, a latency of ~0.5 ms for another
710
REMMEL,
SKINNER,
AND
MINOR
synapse and ~0.3 ms for nerve conduction, i.e ., 22.4 ms. According to these criteria, at least 17 of 159 neurons were monosynaptic from cMVN stimulation (Fig. I), at least 27 of 128 were disynaptic from c8 stimulation, and at least 20 of 117 were disynaptic from i8 stimulation. (A few neurons had such short latencies (cl.6 ms) from the nerves that they might conceivably even have been monosynaptic.) For stimulus spread limits, Ranck (10) states that the 5200~PA cMVN shock excites axons and somas cl.5 mm distant. In fact the mean unit threshold for cMVN stimulation was only 90 PA and the lowest was only 12 PA. Therefore the shock to the cMVN electrode excited neural elements mainly within the vestibular-reticular region from which some ascending crossed MLF fibers arise (12). Because the cMVN (and the 8th nerves) contains a number of fiber types and fibers of passage, it is impossible by these stimulation techniques to say exactly which vestibular, auditory, or possibly other types were stimulated. We hypothesize that the predominant disynaptic inputs are vestibular because the MLF between the 4th and 6th nuclei was reported to carry only vestibular, oculomotor, and spinal motor signals (1,6,9) and because few, if any, acoustic striae fibers terminate in the NRPC. The described NRPC region in cats might be situated homologously to the paramedian pontine reticular formation of monkeys, which has many eye-movement neurons (1, 2). Our data that some NRPC-iMLF neurons appeared to have monosynaptic input from the cMVN electrode and disynaptic input from i8 or c8 electrodes, with some bilateral convergence, thus support the hypothesis that NRPC-iMLF neurons participate in vestibular or vestibuloocular functions. REFERENCES I. BOTTNER-ENNEVER, J. A., AND V. HENN. 1976. An autoradiographic study of the pathways from the pontine reticular formation involved in horizontal eye movements. Brain Res. 108: 155- 164. 2. COHEN, B. 1974. The vestibulo-ocular reflex. Pages 239-352 in H. H. KORNHUBER, Ed., Handbook of Sensory Physiology, Vol. 6 (part 1). Springer-Verlag, Berlin. 3. EVINGER, C., C. R. S. KANEKO, G. W. JOHANSON, AND A. F. FUCHS. 1977. Omnipauser cells in the cat. Dev. Neurosci. 1: 337-340. 4. GRAYBLEL, A. M. 1977. Direct and indirect preoculomotor pathwaysofthe brainstem: An autoradiographic study of the pontine reticular formation in the cat. J. Camp. Neural. 175: 37-78. 5. HIKOSAKA, O., AND T. KAWAKAMI. 1977. Inhibitory reticular neurons related to the quick phase of vestibular nystagmus-their location and projection. Exp. Brain Res. 27: 377-396. 6. KING, W. M., S. G. LISBERGER, AND A. F. FUCHS. 1976. Responses of fibers in medial longitudinal fasciculus (MLF) of alert monkeys during horizontal and vertical
EIGHTH
NERVE-VESTIBULORETICULO-MLF
CONNECTIONS
711
eye movements evoked by vestibular or visual stimuli. J. Neurophysiol. 39: 1135-1149. PETERSON, B. W., AND C. ABZUG. 1975. Properties of projections from vestibular nuclei to media1 reticular formation in the cat. J. Neurophysiol. 38: 1421- 1435. PETERSON, B. W., M. FILION, L. P. FELPEL, AND C. ABZUG. 1975. Responses of medial reticular neurons to stimulation of the vestibular nerve. Exp. Brain Res. 22: 335-350. POLA, J., AND D. A. ROBINSON. 1978. Oculomotor signals in medial longitudinal fasciculus of the monkey. J. Neurophysiol. 41: 245-259. RANCK, J. B. 1975. Which elements are excited in electrical stimulation of mammalian central nervous system: a review. Brain Res. 98: 417-440. REMMEL, R. S., J. POLA, ANDR. D. SKINNER. 1978. Pontomedullary reticular projections into the region of the ascending medial longitudinal fasciculus in cat. Exp. Brain Res. conjugate
7.
8. 9. 10. 11.
32: 31-37. 12.
REMMEL, R. S., AND R. D. SKINNER. 1979. Monosynaptic vestibular-nerve input to vestibular and adjacent reticular neurons projecting into the ascending media1 longitudinal fasciculus. Exp. Neural. 66: 200-204.
NEUROLOGY
70, 706-711 (1980)
RESEARCH
NOTE
Eighth Nerve Activation of Cat Pontine Reticular Neurons Which Project in or near the Ascending Medial Longitudinal Fasciculus R. S. Departments
R. D.
REMMEL,
of Physiology Received
and Anatomy, Little Rock. April
AND L. B. MINOR'
SKINNER, University Arkansas
8. 1980: revision
of Arkansas 72205
received
for
Medical
Sciences,
June 30, 1980
One class of neurons in the nucleus reticularis pontis caudalis (NRPC) was antidromically identified by weak shocks to project an ascending axon into the region of the ipsilateral medial longitudinal fasciculus (IMLF); these are called NRPC-iMLF neurons. Some were excited orthodromically at monosynaptic or longer latencies by shocks to the region of the contralateral medial vestibular nucleus (cMVN). Some were excited at disynaptic or longer latencies by shocks to the ipsilateraI(18) or contralateral (~8) 8th nerve. Many showed convergence from several inputs. These NRPC-iMLF neurons are hypothesized to participate in vestibuloocular or other vestibular functions.
We identified electrophysiologically the 8th nerve and vestibular nucleus inputs to the nucleus reticularis pontis caudalis (NRPC) neurons whose axons ascend in the ipsilateral medial longitudinal fasciculus (iMLF). These NRPC-iMLF fibers in the cat were described by the use of autoradiography (4) and their somatic distribution was mapped out by the technique of antidromic excitation from the region of the ascending iMLF (11). Their somatic distribution extends in the NRPC for about 3 mm ventral and 3 mm rostra1 of the 6th nucleus and from about 0.5 to 1.6 mm lateral, i.e., more lateral usually than burst inhibitory eye-movement neurons (5) and omnipauser eye-movement neurons (3) in the cat. Probably Abbreviations: NRPC-nucleus reticularis pontis caudalis, iMLF-ascending longitudinal fasciculus ipsilateral to the recording site, cMVN-contralateral vestibular nucleus, i8-ipsilateral 8th nerve, c8-contralateral 8th nerve. * We thank G. Sammartino for histological assistance. This work was supported Institutes of Health grant EY01794 and work-study funds for L.B.M. from Brown
706 0014-4886/80/120706-06$02.00/O Copyright All rights
0 1980 by Academic Press. Inc. of reproductron m any form reserved.
medial medial by National University.
EIGHTH
NERVE-VESTIBULORETICULO-MLF
CONNECTIONS
707
homologous fibers in the monkey (1, 2) ascend immediately ventral or lateral to the iMLF, but join it more rostrally; some terminate in the midbrain near the oculomotor nucleus. Our study addresses three questions: (i) Do NRPC-iMLF cells receive input from the 8th nerves or the region of the contralateral medial vestibular nucleus (cMVN)? (ii) Is such input monosynaptic from the cMVN and disynaptic from the nerves? (iii) Is there bilateral convergence of inputs? All answers appear to be yes. In adult cats under ether anesthesia, both carotid arteries were ligated, the cat was decerebrated, and then the ether was discontinued. Life functions were supported as previously described (11, 12). The cerebellum was aspirated to expose the brain stem. Isolated constant-current shocks were 0.1 ms in duration. Stimulating electrodes were made from an insulated stainless-steel wire (cathode) placed inside an uninsulated concentric stainless-steel tube (anode). The wire tip was inserted into the brain or labyrinth, where it functioned as a local monopolar electrode, and the tube remained outside in the saline which covered the tissue. The tips of the two, contralateral and ipsilateral, 8th nerve electrodes (c8 and i8) were inserted into the round windows (12). The iMLF and cMVN stimulating electrodes were made from a 0.125-mm-diameter wire (Medwire Corp.) protruding 4 mm from a tube. Stereotaxic manipulators were tilted 45” caudal of the vertical plane. The iMLF electrode was inserted 1 mm caudal to the 4th nucleus and 0.4 mm lateral of midline (11). The cMVN electrode was inserted visually 1.5 mm caudal to the cerebellar peduncle and 3 mm lateral to the midline. While recording with the cMVN electrode, it was lowered until the nerve-evoked fields were maximal so that it was within the region where primary axons synapse monosynaptically upon ascending crossed vestibular-MLF neurons (12). The ipsilateral MVN was not stimulated because of possible excitation of cerebelloreticular fibers, which do not cross in the brain stem. Neurons were identified as antidromically excited from the iMLF electrode by previously described criteria (11). The predominantly negative spikes were considered to be from somas. Orthodromic responses were considered only when excited by 1200 PA for the cMVN and 5 1000 PA for the nerve electrodes. Orthodromic responses had to show a variable latency of >O. 15 ms near threshold and a decreased, more constant latency for stronger shocks. Orthodromic spikes could be collided with antidromic spikes. Previously described histological techniques (11, 12) showed that the iMLF electrode tips were within the MLF and that the cMVN electrode tips were 1.7 2 0.7 mm caudal to the 6th nucleus center and 2.2 f 0.6 mm ventral to the ventricular floor, i.e., within the region described in (12). In 16 cats, 245 NRPC-iMLF cells had loci and antidromic properties as previously described (11). Most were excited orthodromically from the
708
REMMEL,
0
0
SKINNER,
I
2 Loiency,
AND
3
rns,P
MINOR
5
)6
FIG. 1. Top-extracellular records from a neuron of the nucleus reticularis pontis caudalis which projects into the ipsilateral medial longitudinal fasciculus (NRPC-iMLF). The shock artifact is at the left and the spike(s) is indicated by a dot for each trace; other negative waves are nonunitary fields. iMLF (four traces in a vertical column): two antidromic responses at a constant latency and one miss to a 13-PA-threshold shock to the iMLF electrode. The fourth trace shows responses to four iMLF shocks at 1340 Hz. cMVN, c8, and i8 (three traces each in a vertical column): orthodromic responses to one shock to the contralateral medial vestibular nucleus (cMVN), contralateral (c8), and ipsilateral 8th nerve (i8) electrodes, respectively, with latency variation on successive trials (c8 pictures were double exposures). Horizontal bar (lower left): 1 ms (iMLF, top three), 2.5 ms (lower left and cMVN), and 5 ms (c8 and i8). Vertical bar: 0.5 mV. Bottom-histograms of the minimum latencies of orthodromic responses to shocks to the cMVN, c8. and i8 electrodes.
cMVN, c8, and/or i8 electrodes. The neuron in Fig. 1 responded antidromically at a constant 0.3%ms latency to a 13-PA-threshold iMLF shock and followed four shocks at 1340 shocks per second. The neuron responded at a variable latency to cMVN stimulation, with a 60-PA threshold. Suprathreshold shocks caused a decreased latency, the minimum being 1.20 ms (top trace), i.e., monosynaptic (see below). The neuron responded to c8 stimulation at a 670qA threshold (2.1 x the c8 nerve stimulus threshold for evoking any field potential in the MVN); the minimum latency was 2.2 ms (top trace), i.e., disynaptic (see below). The neuron responded to i8 stimulation with a 600-PA threshold and 2.7-ms minimum latency (top trace), i.e., not definitely disynaptic. The numbers of NRPC-iMLF cells orthodromically excited (+) from the cMVN, c8, and i8 electrodes are given in Table 1. The largest group (N = 91)
EIGHTH
NERVE-VESTIBULORETICULO-MLF TABLE
CONNECTIONS
709
1
Percentage of Neurons Activated by the Different Electrodes cMVN
c8
i8
N
%
0 + 0 + 0 0 + +
0 0 + + 0 + 0 +
0 0 0 0 + + + +
71 28 5 24 2 8 16 91
29 11 2 10 1 3 7 37
245
100
Total
Totals
N
%
Unexcited cMVN c8 i8
71 159 128 117
29 65 52 48
was excited by all electrodes. Half (8 + 16 + 91) were excited by at least one electrode on each side, indicating frequent bilateral convergence. Another large group (28 + 5 + 24) was excited by only contralateral cMVN and/or c8 stimulation. Only two cells were excited only by i8 stimulation. The 29% giving no orthodromic response might still have had subthreshold inputs. Inhibition of spontaneous activity was observed occasionally. These properties are similar to those of NRPC neurons in general, as determined through intracellular recording (8). The range of thresholds for responses of NRPC-iMLF neurons to cMVN stimulation was 12 to 200 PA (90 + 49, mean and SD), that for responses to c8 stimulation was 60 to 1000 PA (mean, 580 _t 250), and that for responses to i8 stimulation was 220 to 1000 PA (mean, 650 + 220). These unit thresholds were sometimes comparable and never >5x the 210 + 90-PA mean stimulus threshold for c8 nerve stimulation for evoking any field potential in the MVN. Latencies were measured between the start of the shock and the start of the spike for suprathreshold shocks. The range of minimum latencies to cMVN stimulation (Fig. 1, histograms) was 1.O to 7.0 ms (mean, 2.9 ? 1.1); the range for c8 stimulation was 1.4 to 6.9 ms (mean, 3.4 + l.l), i.e., one synaptic delay of 0.5 ms longer than from cMVN, on the average. The range for i8 stimulation was 1.5 to 18 ms (mean, 3.5 + 1.2). Some latencies seemed short enough to the monosynaptic from cMVN stimulation and disynaptic from the nerves. A neuron disynaptic rather than monosynaptic from cMVN stimulation should have a latency of ~0.5 ms for each synapse and a latency of 20.6 ms for conduction of vestibuloreticular fibers (7), i.e., ~1.6 ms; a neuron trisynaptic rather than disynaptic from a vestibular nerve stimulation should have, in addition, a latency of ~0.5 ms for another
710
REMMEL,
SKINNER,
AND
MINOR
synapse and ~0.3 ms for nerve conduction, i.e ., 22.4 ms. According to these criteria, at least 17 of 159 neurons were monosynaptic from cMVN stimulation (Fig. I), at least 27 of 128 were disynaptic from c8 stimulation, and at least 20 of 117 were disynaptic from i8 stimulation. (A few neurons had such short latencies (cl.6 ms) from the nerves that they might conceivably even have been monosynaptic.) For stimulus spread limits, Ranck (10) states that the 5200~PA cMVN shock excites axons and somas cl.5 mm distant. In fact the mean unit threshold for cMVN stimulation was only 90 PA and the lowest was only 12 PA. Therefore the shock to the cMVN electrode excited neural elements mainly within the vestibular-reticular region from which some ascending crossed MLF fibers arise (12). Because the cMVN (and the 8th nerves) contains a number of fiber types and fibers of passage, it is impossible by these stimulation techniques to say exactly which vestibular, auditory, or possibly other types were stimulated. We hypothesize that the predominant disynaptic inputs are vestibular because the MLF between the 4th and 6th nuclei was reported to carry only vestibular, oculomotor, and spinal motor signals (1,6,9) and because few, if any, acoustic striae fibers terminate in the NRPC. The described NRPC region in cats might be situated homologously to the paramedian pontine reticular formation of monkeys, which has many eye-movement neurons (1, 2). Our data that some NRPC-iMLF neurons appeared to have monosynaptic input from the cMVN electrode and disynaptic input from i8 or c8 electrodes, with some bilateral convergence, thus support the hypothesis that NRPC-iMLF neurons participate in vestibular or vestibuloocular functions. REFERENCES I. BOTTNER-ENNEVER, J. A., AND V. HENN. 1976. An autoradiographic study of the pathways from the pontine reticular formation involved in horizontal eye movements. Brain Res. 108: 155- 164. 2. COHEN, B. 1974. The vestibulo-ocular reflex. Pages 239-352 in H. H. KORNHUBER, Ed., Handbook of Sensory Physiology, Vol. 6 (part 1). Springer-Verlag, Berlin. 3. EVINGER, C., C. R. S. KANEKO, G. W. JOHANSON, AND A. F. FUCHS. 1977. Omnipauser cells in the cat. Dev. Neurosci. 1: 337-340. 4. GRAYBLEL, A. M. 1977. Direct and indirect preoculomotor pathwaysofthe brainstem: An autoradiographic study of the pontine reticular formation in the cat. J. Camp. Neural. 175: 37-78. 5. HIKOSAKA, O., AND T. KAWAKAMI. 1977. Inhibitory reticular neurons related to the quick phase of vestibular nystagmus-their location and projection. Exp. Brain Res. 27: 377-396. 6. KING, W. M., S. G. LISBERGER, AND A. F. FUCHS. 1976. Responses of fibers in medial longitudinal fasciculus (MLF) of alert monkeys during horizontal and vertical
EIGHTH
NERVE-VESTIBULORETICULO-MLF
CONNECTIONS
711
eye movements evoked by vestibular or visual stimuli. J. Neurophysiol. 39: 1135-1149. PETERSON, B. W., AND C. ABZUG. 1975. Properties of projections from vestibular nuclei to media1 reticular formation in the cat. J. Neurophysiol. 38: 1421- 1435. PETERSON, B. W., M. FILION, L. P. FELPEL, AND C. ABZUG. 1975. Responses of medial reticular neurons to stimulation of the vestibular nerve. Exp. Brain Res. 22: 335-350. POLA, J., AND D. A. ROBINSON. 1978. Oculomotor signals in medial longitudinal fasciculus of the monkey. J. Neurophysiol. 41: 245-259. RANCK, J. B. 1975. Which elements are excited in electrical stimulation of mammalian central nervous system: a review. Brain Res. 98: 417-440. REMMEL, R. S., J. POLA, ANDR. D. SKINNER. 1978. Pontomedullary reticular projections into the region of the ascending medial longitudinal fasciculus in cat. Exp. Brain Res. conjugate
7.
8. 9. 10. 11.
32: 31-37. 12.
REMMEL, R. S., AND R. D. SKINNER. 1979. Monosynaptic vestibular-nerve input to vestibular and adjacent reticular neurons projecting into the ascending media1 longitudinal fasciculus. Exp. Neural. 66: 200-204.