Department of Physiology, Northwestern University School of Medicine, Chicago, Illinois 60611
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ABSTRACT |
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Perlmutter, S. I., Y. Iwamoto, L. F. Barke, J. F. Baker, and B. W. Peterson. Relation between axon morphology in C1 spinal cord and spatial properties of medial vestibulospinal tract neurons in the cat. J. Neurophysiol. 79: 285-303, 1998. Twenty-one secondary medial vestibulospinal tract neurons were recorded intraaxonally in the ventromedial funiculi of the C1 spinal cord in decerebrate, paralyzed cats. Antidromic stimulation in C6 and the oculomotor nucleus identified the projection pattern of each neuron. Responses to sinusoidal, whole-body rotations in many planes in three-dimensional space were characterized before injection of horseradish peroxidase or Neurobiotin. The spatial response properties of 19 neurons were described by a maximum activation direction vector (MAD), which defines the axis and direction of rotation that maximally excites the neuron. The other two neurons had spatio-temporal convergent behavior and no MAD was calculated. Collateral morphologies were reconstructed from serial frontal sections to reveal terminal fields in the C1 gray matter. Axons gave off multiple collaterals that terminated ipsilaterally to the stem axon. Collaterals of individual axons rarely overlapped longitudinally but projected to similar regions in the ventral horn when viewed in transverse sections. The number of primary collaterals in C1 was different for vestibulo-collic, vestibulo-oculo-collic, and C6-projecting neurons: on average one every 1.34, 1.72, and 4.25 mm, respectively. The heaviest arborization and most terminal boutons were seen in the ventral horn, in laminae VIII and IX. Varicosities on terminal branches in lamina IX were observed adjacent to large cell bodiesputative neck motoneurons
in counterstained tissue. Some collaterals had branches that extended dorsally to lamina VII. Neurons with different spatial properties had terminal fields in different regions of the ventral horn. Axons with type I responses and MADs near those of a semicircular canal pair had widely distributed collateral branches and numerous terminations in the dorsomedial, ventromedial, and spinal accessory nuclei and in lamina VIII. Axons with type I responses that suggested convergent canal pair input, with type II responses, and with spatio-temporal convergent behavior had smaller terminal fields. Some neurons with these more complex spatial properties projected to the dorsomedial and spinal accessory but not to the ventromedial nuclei. Others had focused projections to dorsolateral regions of the ventral horn with few branches in the motor nuclei.
Spatial coordination of the vestibulocollic reflex is achieved by an appropriately weighted distribution of semicircular canal and otolith inputs to neck motoneurons. As reported in the previous paper (Perlmutter et al. 1998 Axons of 21 secondary vestibulospinal neurons recorded in eight decerebrated, paralyzed cats were labeled and reconstructed. Vestibular responses of these neurons were included in the data set of the preceding paper (Perlmutter et al. 1998
Neuronal recording
Neurons were recorded intraaxonally in the ventromedial funiculi in C1 (Fig. 3A) within the anatomically defined borders of the bilateral medial vestibulospinal tracts (Holstege 1988
Morphological characterization
Well-penetrated axons were injected iontophoretically with HRP or Neurobiotin after recording their responses to three-dimensional rotations. Axons were injected if the measured resting potential was at least
Twenty-one neurons recorded intraaxonally in C1 were injected with HRP or Neurobiotin and labeled sufficiently to reconstruct their axonal morphology (Fig. 1B). These neurons were activated monosynaptically by single-pulse stimulation of one labyrinth, were recorded within the anatomic boundaries of the MVST, and had activity modulated during 0.5-Hz whole-body rotation. Staining was seen in 138 collaterals from five contralaterally projecting vestibulo-oculo-collic (c-VOC) neurons (39 collaterals), eight ipsilaterally and two contralaterally projecting vestibulo-collic(i-VC, c-VC) neurons (82 collaterals), one contralaterally projecting vestibulo-oculo-C6 (c-VO-C6) neuron (2 collaterals), and five ipsilaterally projecting vestibulo-C6 (i-V-C6) neurons (15 collaterals).
Stem axons and primary collaterals
Seven stem axons were injected with HRP and stained for distances of 6.3-12.1 mm (mean ± SD, 8.9 ± 2.3 mm). Fourteen stem axons were injected with Neurobiotin and stained for 9.5-30.7 mm (15.7 ± 6.4 mm). Terminal branches were labeled primarily in C1, and Neurobiotin-injected axons often could be traced into C2 and the caudal medulla (Fig. 2). Stem axons traveled in the dorsal two-thirds of the medial ventral funiculus, usually dorsal to the tip of the ventral horn (Fig. 3A). Their position relative to the gray matter border usually was fixed as the axon descended below rostral C1 (e.g., Fig. 4A). Contralaterally projecting axons tended to lie closer to the midline than those descending ipsilaterally (Fig. 3A), but there was no clear correlation between an axon's location within the ventral funiculus and its spatial properties or projection type (i.e., VOC, VC, V-C6).
Branching patterns within C1 gray matter
Staining in 49 of the 138 collaterals was classified as complete or mostly complete (see METHODS) and their branching patterns were analyzed. The general observations of collateral morphology reported in this section are in good agreement with the more complete anatomic work of Shinoda et al. (1988
Terminal fields and spatial properties
The responses of 19 neurons were well described by the "cosine-tuned gain" model (see Perlmutter et al. 1998 NEURONS WITH TYPE I RESPONSES CONSISTENT WITH SINGLE CANAL PAIR INPUT.
Neurons with type I responses and MADs suggesting input from a single canal pair tended to project to a wide area in the C1 ventral horn (Table 1). The motor nuclei were the primary targets, and most axons terminated in the dorsomedial, ventromedial, and spinal accessory nuclei. Collaterals also branched extensively in lamina VIII, often including regions lateral and dorsal to the motor nuclei. Figures 4-6 show data from neurons with maximal responses in planes near those of the ipsilateral horizontal, anterior, and posterior canals, respectively.
NEURONS WITH MORE COMPLEX SPATIAL PROPERTIES.
Neurons with either type I responses that suggested convergent input from more than one canal pair, type II responses or spatio-temporal convergent (STC) behavior tended to have more focused terminal fields in the C1 gray matter and appeared to project less densely to the motor nuclei. Localized projections to dorsal and/or lateral portions of the ventral horn, rare in the population of type I neurons with single canal pair input, were observed for some axons (Table 1).
We have studied 21 secondary MVST neurons with three different approaches to elucidate the functional organization of the medial vestibulospinal projection. Each of the methods provides independent information on the morphophysiological properties of the cells. First, the projection pattern of each neuron was identified electrophysiologically. Second, their spatial response properties were characterized by applying rotations in many planes in three-dimensional space. Finally, the neurons were labeled with intraaxonal injection of HRP or Neurobiotin to reveal the morphology of C1 collaterals. These experiments suggest a correlation between the spatial properties and anatomic projections of secondary MVST neurons. In the following text, we discuss the validity and limitations of the anatomic method used, compare our morphological observations with those in previous studies, and consider how electrophysiological, functional, and morphological attributes of vestibulospinal neurons are correlated. Finally, we consider possible mechanisms for the spatial transformations of the vestibulocollic reflex based on the morphophysiological characteristics of secondary MVST neurons.
Interpretation of morphological data
Our intraaxonal recordings were maintained while the animal was rotated in many three-dimensional planes before injection of tracer. This undoubtedly caused deterioration of some electrode penetrations before injection, and consequently some staining was not optimal. For example, terminal boutons were not always seen on fine diameter branches of collaterals near the injection site. However, we believe for the following reasons that many collaterals were sufficiently labeled to estimate the extent of the terminal fields of 21 MVST neurons. First, when observed under the light microscope at high magnification, it was clear which collaterals had dense staining that remained deep through repeated levels of ramification, down to fine diameter branches. Collaterals that were stained poorly, like those far from injection sites, had a much different appearance under the microscope. The density of staining was weaker and became fainter as the distance from the stem increased. On this basis, we classified the 49 best-stained collaterals as completely stained (terminal boutons observed) or mostly stained (some branches without terminals). Our conclusions on terminal field distribution were based only on these collaterals. Second, the rostrocaudal extent of branching of the 49 collaterals and those of MVST neurons completely stained with HRP by Shinoda et al. (1988 Morphology of MVST axons
The collateral morphology and overall distribution of terminal branches and boutons of the 21 stained axons were in agreement with data on MVST neurons from previous autoradiographic and HRP labeling studies (Donevan et al. 1990 Correlation of axonal morphology and spatial properties
The present study extends the work of Shinoda et al. (1988 Spatial transformations in the vestibulocollic reflex
The preceding paper (Perlmutter et al. 1998
INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References
), the responses of some medial vestibulospinal tract (MVST) neurons suggest that part of the necessary convergence of afferent signals occurs on second-order cells. We hypothesized that components of the input-output transformation of the reflex may be executed by projections of these neurons, which appear to receive convergent horizontal and vertical semicircular canal pair input, to specific groups of motoneurons. However, additional processing of spatial information must occur outside the vestibular nucleus to account for the maximal vertical responses of most neck muscles to rotations about the pitch axis (Iwamoto et al. 1996
; Kasper et al. 1988
; Wilson et al. 1990
). Bolton et al. (1992)
, who found that the vertical responses of neck muscles were not present on reticulospinal neurons, hypothesized that this convergence of vestibular signals occurs within the spinal cord. Information on the spinal targets of vestibulospinal neurons with identified spatial properties is needed to test both of these hypotheses.
; Perlmutter et al. 1998
) have demonstrated that secondary vestibulospinal neurons with different spatial properties have different gross anatomic projection patterns (e.g., vestibulo-oculo-collic, vestibulo-C6). However, these studies did not provide information on the terminal fields of particular spatial signals within the spinal gray matter. Projections of vestibulospinal neurons to upper cervical interneurons and directly to neck motoneurons both may be involved in executing the spatial transformations of the vestibulocollic reflex.
; Sugiuchi et al. 1995
; Uchino and Isu 1991
; Wilson and Maeda 1974
; Wilson and Yoshida 1969
; Wilson et al. 1977
). Direct recordings of laminae VII and VIII interneurons that probably project to cervical motoneurons also have demonstrated short-latency vestibular inputs (Bolton et al. 1993
; Endo et al. 1994
; Schor et al. 1986
; Sugiuchi et al. 1992
; Wilson et al. 1984
). However, little information is available on the response properties of vestibulospinal pathways that terminate in specific laminae of the cervical cord.
; Donevan et al. 1990
; Holstege 1988
; Isu and Yokota 1983
; Nyberg-Hansen 1964
; Petras 1967
; Shinoda et al. 1986a
). Terminations in laminae II-VI and X also have been reported (Donevan et al. 1990
, 1992b
). The morphological characteristics of single secondary MVST axons in the upper cervical cord were described in detail by Shinoda et al. (1988
, 1992)
. They found that single neurons had terminations in multiple motor nuclei and in laminae VII and VIII. However, the spatial properties of these cells were unknown because their responses to natural vestibular stimuli were not examined.
, 1992)
, the spatial properties of intraaxonally recorded neurons are characterized by their responses to three-dimensional rotations. Tracers then are injected into the same neurons to visualize their axonal morphologies. Results will show extensive divergence of canal pair signals at C1 and a more restricted projection of neurons carrying more highly processed spatial signals.
.
METHODS
Abstract
Introduction
Methods
Results
Discussion
References
), which describes the methods for animal preparation (Fig. 1B of that paper), unit recording, and physiological data analysis.
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FIG. 1.
A: electrophysiological identification of a secondary contralateral projecting vestibulo-oculo-collic (c-VOC) axon recorded in the left medial vestibulospinal tract (MVST; axon 8103). Vertical calibration bar applies to all traces. a: monosynaptic responses to electrical stimulation of the right labyrinth. b: direct activation after stimulation of the oculomotor nucleus at an intensity straddling threshold. c: collision test between spontaneous action potential (used to trigger scope sweeps) and spike evoked with suprathreshold oculomotor stimulation (top). Evoked response is blocked by a preceding spontaneous spike as the stimulus delay is shortened (bottom), verifying that the stimulus-evoked action potential is conducted antidromically from the oculomotor nucleus. B: low magnification view of stained axonal branches in the C1 ventral horn. C: presynaptic boutons of an MVST axon in close proximity to, and possibly making synaptic contact with (right), large motoneuron-like cells in the C1 ventral horn.
; Nyberg-Hansen 1964
). Recordings were made with glass micropipettes of 8-40 M
impedance, filled with 0.5-1.0 M KCl and either 10% horseradish peroxidase (HRP) or 4% Neurobiotin (Vector Laboratories) dissolved in 0.05 M Tris(hydroxymethyl)aminomethane buffer (pH 7.6). Secondary neurons were identified (Fig. 1Aa) by consistent activation within 1.5 ms of single-pulse stimulation of either labyrinth (Wilson and Melvill Jones 1978; see Perlmutter et al. 1998
for details). Antidromic responses to C6 and oculomotor nucleus stimulation (Fig. 1Ab) identified axons as vestibulo-oculo-collic, vestibulo-collic, vestibulo-C6, or vestibulo-oculo-C6 neurons. For most axons, a collision test between stimulus-evoked and spontaneously occurring action potentials was performed to confirm that the former were antidromically conducted spikes and not synaptic responses (Fig. 1Ac). Each axon was identified as ipsilaterally or contralaterally projecting by the relation between the recording side and the labyrinth from which the neuron was monosynaptically activated (Fig. 1Aa).
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FIG. 3.
A: location of stained stem axons in the ventral funiculus at the mid-C1 level, shown in a sketch of a frontal section. , ipsilaterally projecting axons, ×, contralaterally projecting axons. Outline of ventral horn and bottom of spinal cord are drawn; central canal is small circle at upper right; vertical line denotes midline. ···, approximate borders between laminae VII and VIII and between laminae VII and VI. B: distribution of large thionin-stained neurons, probably motoneurons, in the rostral C1 segment of one animal; 5 transverse sections (500 µm) superimposed. Shaded cells appeared to be contacted by small varicosities, presumably synaptic boutons, on fine axonal branches of cell 5605 (Fig. 6B). C and D: general location of neck motor nuclei within the rostral (C) and mid (D) C1 gray matter. Nuclei were defined by columns of large cell bodies, like those in B, in superimposed frontal sections counterstained with thionin or cresyl violet. Spinal accessory nucleus migrates from a central and dorsal position in lamina VIII at rostral C1 (C) to a lateral position in caudal C1 (D) (Brichta et al. 1987
; Rapoport 1978
; Uchino et al. 1990
). VM, ventromedial nucleus; DM, dorsomedial nucleus; SA, spinal accessory nucleus; CCN, central cervical nucleus; VII, VIII, Rexed's laminae.
). Neuronal MADs were compared with those of the semicircular canal pairs. Canal pairs were referenced by listing the canal with excitatory inputs first; for example, type I responses are produced by inputs from: left horizontal-right horizontal canal pair (lhc/rhc); left anterior-right posterior canal pair (lac/rpc); left posterior-right anterior canal pair (lpc/rac; see Fig. 3 of Perlmutter et al. 1998
). The other two cells exhibited strong spatio-temporal convergence with a minimum response ratio >0.2 (see Perlmutter et al. 1998
).
20 mV (DC) or action potential amplitude was
10 mV. Positive pulses of 10-20 nA and 100-ms duration (50% duty cycle) were passed through the pipette for 5-20 min. Intraaxonal DC potential and responses to weak labyrinth shock were monitored continuously during the injection, which was terminated if spike amplitude or resting potential deteriorated severely. Threshold and latency of responses to labyrinth, oculomotor nucleus, and C6 stimulation were reexamined after several minutes of injection. One to six injections were made in each animal. Two to 18 h after the first of these injections, the animal was anesthetized deeply with pentobarbital sodium (50 mg/kg iv), heparinized, and perfused transcardially. Animals in which Neurobiotin had been injected were perfused with 0.9% saline followed by 4% paraformaldehyde in 0.1 M phosphate buffer. Fixative was infused for 30-60 min and frequently followed by infusion of a solution of 30% sucrose in phosphate buffer. In HRP experiments, saline was followed by a perfusate of 0.5% paraformaldehyde and 2.5% glutaraldehyde in 0.1 M phosphate buffer. The spinal cord and brain stem were removed and stored in the sucrose-buffer solution.
). Neurobiotin-labeled tissue was incubated for 3 h in a phosphate buffer solution of an avidin-biotin-HRP conjugate (ABC reagent, Vector Laboratories) and then stained with the same DAB protocol. Sections were mounted, dehydrated, and cleared using standard histological procedures.
; Light and Kavookjian 1985
). For the best stained collaterals, the density of staining remained deep over the entire extent of branching, and terminal boutons were labeled on most or all of the distal branches. These collaterals were classified as "completely stained." Other collaterals remained deeply stained through several levels of ramification until fine branches, with diameters similar to those of the terminal branches in the best stained collaterals, were seen. Few terminal boutons were observed, however. These collaterals were classified as "mostly stained." Stained stem axons and branches were traced at ×200 magnification with the aid of a drawing tube attached to the microscope. Axonal arborizations were reconstructed as composite drawings from serial sections of stained segments (Fig. 1B) aligned at entrance and exit points of adjacent sections. Cell bodies then were counterstained with thionin or cresyl violet in sections with reconstructed collaterals. Forty-nine collaterals were classified as completely or mostly stained and serve as our database for examining branching patterns and terminal field distribution. Collaterals shown in Figs. 4-9 are projected onto a single frontal section of the spinal cord, representing 400- to 1,840-µm-thick blocks oftissue.
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FIG. 4.
Reconstructions of C1 collaterals and responses to 0.5 Hz, whole-body rotations for 2 secondary MVST neurons with maximum activation direction vectors (MADs) aligned with the left horizontal-right horizontal canal pair (lhc/rhc) vector. Average responses to rotations in the horizontal and one vertical plane are shown in bottom inset as cumulative spike histograms for many cycles of rotation. Upward deflection of table (i.e., head) position trace corresponds to rotation to the side ipsilateral to the neuron's cell body for yaw and to ipsilateral ear down rotation for roll (Figs. 4A, 6B, and 8A) or nose up rotation in other vertical planes (other figures). Top inset: MAD normalized to a length of 1. Front view: components of the MAD for yaw and pitch rotations, as shown. Top view: pitch and roll components. Reconstructions show morphology of each collateral projected onto 1 frontal section of the C1 spinal cord. Most rostral collateral is shown top, most caudal at bottom. Border of ventral horn drawn for all collaterals; midline structures drawn for most rostral collateral. Stem axon is short, thick line in the ventral funiculus. A: axon 8412, an ipsilaterally projecting vestibulo-collic (i-VC) neuron activated by horizontal rotations to the ipsilateral side (yaw gain = 2.8 spikes·s 1·deg
1); MAD nearly aligned with that of lhc/rhc vector (see Fig. 3, Perlmutter et al. 1998
for canal MADs). Collaterals j, k, and l (Fig. 2) are shown. Central canal is flattened circle at top right, anterior median fissure is at bottom right. B: axon 8702, a contralaterally projecting vestibulo-collic (c-VC) neuron, with similar spatial properties (yaw gain = 2.7 spikes·s
1·deg
1). Collaterals d, e, and f are shown.
RESULTS
Abstract
Introduction
Methods
Results
Discussion
References
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FIG. 2.
Schematic of collateral distribution and rostrocaudal extent of terminal branches for all stained neurons (format from Shinoda et al. 1986a , 1992
). Each stem axon is represented as a vertical line scaled to show the length over which it was stained. Horizontal lines depict the relative position of all primary collaterals. Short vertical lines correspond to the distance over which branches were stained for each collateral; for many collaterals, the full rostrocaudal extent of branching was probably larger because branches were not completely stained. Large dots show injection sites for each axon. Neuron and collateral labels are referred to in the legends of Figs. 4-9. Axons are grouped by projection pattern (see text for description).
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FIG. 5.
Collateral morphologies and responses of a neuron with MAD aligned with the left anterior-right posterior canal pair (lac/rpc) vector. Same format as in Fig. 4 (turntable position trace, not shown in bottom inset, same as in Fig. 4). Axon 5809, a c-VOC neuron, with response gain in pitch of 6.6 spikes·s 1·deg
1. Collaterals b, c, and d are shown.
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FIG. 7.
Collateral morphologies and responses of a neuron with type I responses and MAD suggesting convergent canal pair input. Same format as in Fig. 5. Axon 8005, an i-VC neuron with gain in yaw of 3.6 spikes·s 1·deg
1 and gain in roll of 2.2 spikes·s
1·deg
1. Collaterals c, d, and e are shown.
one every 4.25 ± 2.19 mm. Intercollateral distance did not vary systematically along the rostrocaudal length of individual axons, but collaterals were not always evenly spaced (Fig. 2). Axons that could be traced through the spino-medullary junction gave off one or two collaterals in the caudal brain stem, except cell 8412, which had many collaterals near the obex (Fig. 2).
, 1992)
and Donevan et al. (1992a)
.
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TABLE 1.
Morphophysiological properties of 21 stained MVST axons
1 from each neuron) extended rostrocaudally between 400 and 1,840 µm (802 ± 304 µm). However, it is possible that the rostrocaudal extent of collaterals classified as mostly stained was slightly underestimated because of incomplete staining of some branches. The rostrocaudal extent of collateral branching was not correlated to projection pattern (Fig. 2).
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FIG. 6.
Collateral morphologies and responses of neurons with MADs aligned with the left posterior-right anterior canalpair (lpc/rac) vector. Same format as in Fig. 5. A: axon 5605, an ipsilaterally projecting vestibulo-C6 (i-V-C6) neuron with maximum response for nose up rotations in a plane close to that of the lpc/rac (gain = 6.8 spikes·s 1·deg
1). Collaterals c, d, and e are shown. B: axon 8706, a c-VOC neuron with maximum response gain of 6.6 spikes·s
1·deg
1. Collaterals e and f are shown projected onto 1 frontal section (intercollateral distance = 500 µm).
) and MADs were calculated. Fifteen of these exhibited only type I yaw and/or roll responses (i.e., activated by rotation in directions that excite ipsilateral canal afferents), and 4 neurons exhibited type II responses (activated by rotation in directions that excite contralateral canal afferents). Twoi-V-C6 neurons exhibited spatio-temporal convergent behavior with no clear null response plane (Baker et al. 1984
; Iwamoto et al. 1996) and MADs were not calculated. The terminal fields of the 49 best-stained collaterals from these axons (
1 for each neuron) were examined for correlations with the neurons' spatial properties.
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FIG. 8.
Neurons with type II yaw responses and few terminals in the ventral horn. Same format as in Fig. 5. A: axon 8003, an i-V-C6 neuron with gain in yaw of 3.7 spikes·s
1·deg
1. Collaterals a and b are shown. Section containing the most proximal part of the primary collateral and the stem axon was lost. B: axon 8006, a c-VO-C6 neuron with gain in yaw of
3.4 spikes·s
1·deg
1 and gain for rotation about the lac/rpc axis of 6.8 spikes·s
1·deg
1. Collateral a shown.
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FIG. 9.
Neuron with type II yaw response and extensive terminals in the ventral horn. Same format as in Fig. 5. Axon 8605, an i-VC neuron with yaw gain of 3.1 spikes·s
1·deg
1 and gain for rotation about the lpc/rac axis of 3.3 spikes·s
1·deg
1. Collaterals e and g are shown; collateral g extended rostrocaudally for 1760 µm and is shown in two frontal reconstructions of 1,040- and 720-µm thickness
branches in middle drawing continue into bottom drawing.
DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References
, 1992)
and Donevan et al. (1992a)
was similar (see further). Third, the general pattern of collateral morphology and terminal distribution for secondary MVST axons reported by Shinoda et al. (1992)
, where terminal boutons were stained for almost all branches, are similar to those described here. Nonetheless, it is possible that the extent of collateral branching and terminal field distribution for neurons that we classified as mostly stained were underestimated slightly because of incomplete staining.
; Vanner and Rose 1984
). However, many boutons appeared to be contacting large, counterstained cell bodies when viewed at high magnification (Figs. 1 and 3). Shinoda et al. (1992)
also have reported axonal terminals of MVST neurons contacting cell bodies and proximal dendrites of presumed neck motoneurons in the upper cervical cord. The time course of excitatory postsynaptic potentials (EPSPs) produced by secondary vestibulospinal neurons in neck motoneurons also suggests that synapses are made on cell bodies and/or proximal dendrites (Isu et al. 1988
; Uchino et al. 1988
, 1990
).
neurons with responses that resemble ipsilateral semicircular canal afferents and those with more complex responses. There is considerable variability of axonal morphology for neurons within these groups probably because each category includes neurons with different roles in the vestibulocollic reflex and perhaps with other vestibular functions. In addition, several cell types described in the previous paper (e.g., C6-projecting neuron with MAD near the roll axis) were not stained. Our conclusions, therefore, are not intended to be comprehensive but rather to indicate a trend relating the physiological and morphological properties of neurons studied to date. Further elaboration of the relationship between spatial properties and terminal field distribution must await morphophysiological characterization of a larger sample of MVST neurons.
, 1992a
; Holstege 1988
; Isu and Yokota 1983
; Nyberg-Hansen 1964
; Petras 1967
; Shinoda et al. 1988
, 1992
). There were no clear differences between the morphology of ipsilaterally and contralaterally projecting axons, consistent with Shinoda et al. (1992)
.
) and for MVST neurons in the upper cervical cord (Donevan et al. 1992a
; Shinoda et al. 1992
). The rostrocaudal extent of branching of individual collaterals (mean = 802 µm for the 49 best-stained collaterals) was also similar to those of secondary MVST axons stained with HRP by Shinoda et al. (1992
; 819 µm) and Donevan et al. (1992a
; 760 µm). Corticospinal and rubrospinal (Shinoda et al. 1982
, 1986b)
neurons, in contrast, have more widely spaced collaterals with more longitudinally expansive arborization. The branches of all but one stained MVST collateral remained ipsilateral to the ventral funiculus in which the stem axon was recorded. Shinoda et al. (1992)
and Donevan et al. (1992a)
also found very few MVST collaterals crossing in the spinal cord. Collaterals that cross the midline in the spinal anterior commissure are more common for lateral vestibulospinal neurons projecting to lower cervical segments (Shinoda et al. 1986a
). In most cases, sibling collaterals from the same stem axon projected to similar regions in the frontal plane of C1, as described by Shinoda et al. (1992)
and Donevan et al. (1992a)
.
; Bakker et al. 1984
; Brichta et al. 1987
; Gordon and Richmond 1991
; Rapoport 1978
; Rose and Keirstead 1988
; Richmond et al. 1978
; Sugiuchi and Shinoda 1991
). Stained MVST axons had boutons in close contact with large cell bodies in these nuclei (Fig. 3D). Monosynaptic projections of MVST neurons to neck motoneurons in the ventromedial (Rapoport et al. 1977
; Uchino et al. 1988
; Wilson and Maeda 1974
; Wilson and Yoshida 1969
), dorsomedial (Isu et al. 1988
), and spinal accessory (Fukushima et al. 1979
) nuclei have been documented electrophysiologically.
; Isu and Yokota 1983
; Shinoda et al. 1986a
) and contacting different species of motoneurons (Shinoda et al. 1992
).
; Sugiuchi et al. 1992
). Lamina VIII appeared to be the primary termination zone of several axons, most with spatial properties that did not reflect excitatory input from a single, ipsilateral semicircular canal.
, 1992a
,b
). Some collaterals described here did become faint as they reached lamina VII, and a few branches projecting dorsal to the ventral horn were probably not detected. However, our results are in good agreement with the HRP studies of Donevan et al. (1992a)
and Shinoda et al. (1992)
in suggesting that secondary neurons do not contribute significantly to the MVST projection to cervical areas dorsal to lamina VII. It seems likely that nonsecondary vestibulospinal neurons, many of which have spatial properties different from those of secondary neurons (Iwamoto et al. 1996
), are the source of these terminations.
, 1992)
by directly correlating the collateral morphologies of individual secondary MVST axons with their gross axonal projection patterns (VOC, VC, etc.) and responses to three-dimensional rotations. This analysis revealed a relationship between morphology and spatial properties, although there was variability in morphology from axon to axon and some neurons had terminal fields that were exceptions.
made similar discriminations between narrow and widespread terminal fields for MVST axon collaterals. In their PHA-L and HRP study on C2-C3 collaterals, they reported that some neurons had "focused projections" to primarily one lamina, whereas others had "broad terminal fields" extending throughout the ventral horn. The present results suggest that these morphological classes of neurons had distinct spatial properties.
). The responses of VOC, VC, and V-C6 neurons suggested different patterns of input from the semicircular canal pairs. In the present study, we found that a neuron's projection pattern was also predictive of its collateral morphology. Neurons that were activated antidromically from C6 had far fewer collaterals in C1 than those that terminated in the upper cervical segments (Fig. 2). In addition, four of six C6-projecting axons had C1 collaterals that terminated primarily in dorsolateral lamina VIII and lamina VII and gave off few branches in the motor nuclei, unlike most VOC and VC neurons. These findings support the suggestion that most C6-projecting neurons do not play a large role in the vestibulocollic reflex (Perlmutter et al. 1998
).
) reported that the relative sensitivity of some MVST neurons to horizontal and vertical rotations is similar to that of particular neck muscles (Banovetz et al. 1995
). We hypothesized that projections to neck motoneurons from two groups of neurons that received convergent input from horizontal and vertical semicircular canal pairs could account for part of the signal that drives these muscles during the vestibulocollic reflex.
). One neuron (6209) with such spatial properties was stained in the present study. This neuron's response was suitable for providing inhibitory input to the occipitoscapularis and rectus capitis posterior major muscles. It projected to dorsolateral lamina VIII, apparently targeting interneurons and not motoneurons. This is not consistent with our hypothesis that neurons with spatial properties like those of neck muscles project directly to their motoneurons. For this neuron, a signal that was appropriate to inhibit extensor muscles during nose up rotations was distributed to spinal interneurons rather than directly to extensor motoneurons.
). These neurons can provide appropriate excitatory input to ventral flexor muscles. Two neurons with such responses were stained in the present study, axons 5811 and 8605 (Fig. 9). Both neurons had widespread terminations to all three C1 motor nuclei and lamina VIII. Although these axons had terminals in the dorsomedial nucleus, where they probably synapsed on flexor motoneurons, their projections to the dorsomedial and spinal accessory nuclei are not consistent with a specialized role in activating flexor muscles.
; Uchino and Hirai 1984
; Uchino and Isu 1991
; Wilson and Maeda 1974
; Wilson and Yoshida 1969
). For example, the widespread terminations in C1 motor nuclei of axons with MADs aligned with the horizontal canal pair axis probably mediate disynaptic excitation (inhibition) of contralateral (ipsilateral) neck muscle motoneurons (Fukushima et al. 1979
; Isu et al. 1991
; Shinoda et al. 1994
). Activation of these neurons by horizontal rotations would generate postsynaptic potentials in C1 motoneurons that contribute to compensatory horizontal head movements. Similarly, neurons with MADs aligned with the ipsilateral anterior/contralateral posterior canal pair vector projected to the ventromedial nucleus, where dorsal neck extensor motoneurons are located (e.g., Fig. 5), and neurons with MADs aligned with the ipsilateral posterior/contralateral anterior canal pair vector projected to the dorsomedial nucleus, where ventral neck flexor motoneurons are located (e.g., Fig. 6). These pathways are consistent with the hypothesis that the maximal vertical responses of many neck muscles to pitch rotation are generated by convergence of bilateral signals onto motoneurons (Iwamoto et al. 1996
; Kasper et al. 1988
; Uchino and Isu 1991
; Wilson et al. 1990
).
), this suggests that these neurons produced EPSPs in neck flexor, as well as extensor, motoneurons. This connection is not consistent with the compensatory vestibulocollic reflex.
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ACKNOWLEDGEMENTS |
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The authors thank K. Powell for help in conducting some of the experiments, C. Polchow for the histological preparation and staining of the tissue, and R. Holmberg, University of Washington, for assistance with the illustrations.
This study was supported by National Institutes of Health Grants NS-22490, NS-17489, and DC-01559.
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FOOTNOTES |
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Present address of Y. Iwamoto: Dept. of Physiology, Institute for Basic Medical Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305, Japan.
Present address and address for reprint requests: S. I. Perlmutter, Regional Primate Research Center, University of Washington, Box 357330, Seattle, WA 98195.
Received 17 June 1996; accepted in final form 4 September 1997.
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REFERENCES |
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