Behavior of neurons in the abducens nucleus of the alert cat--II. Internuclear neurons

Effects of target depletion on adult mammalian central neurons: morphological correlates

The morphological sequelae induced by target removal were studied on adult cat abducens internuclear neurons at both the somata and terminal axon arborization levels. The neuronal target--the medial rectus motoneurons of the oculomotor nucleus--was selectively destroyed by the injection of toxic ricin into the medial rectus muscle. Retrograde labeling with horseradish peroxidase demonstrated the survival of the entire population of abducens internuclear neurons up to one year after target removal. However, soma size was reduced by about 20% three months postlesion and maintained for one year. At the ultrastructural level, a considerable deafferentation of abducens internuclear neurons was observed at short intervals (i.e. 10 days after lesion). Large regions of the plasmalemma appeared devoid of presynaptic boutons but were covered instead by glial processes. The detachment of synaptic endings was selective on abducens internuclear neurons since nearby motoneurons always showed a normal synaptic coverage. By one month, abducens internuclear neurons recovered a normal density of receiving axosomatic synapses. Anterogradely biocytin-labeled axon terminals of abducens internuclear neurons remained in place after the lesion of medial rectus motoneurons, although with a progressive decrease in density. Ultrastructural examination of the oculomotor nucleus 10 days after the lesion revealed numerous empty spaces left by the dead motoneurons. Targetless boutons were observed surrounded by large extracellular gaps, still apposed to remnants of the postsynaptic membrane or, finally, ensheathed by glial processes. At longer intervals (> one month), the ultrastructure of the oculomotor nucleus was re-established and labeled boutons were observed contacting either unidentified dendrites within the neuropil or the soma and proximal dendrites of the oculomotor internuclear neurons, that project to the abducens nucleus. Labeled boutons were never found contacting with the oculomotor internuclear neurons either in control tissue or at short periods after ricin injection. These results indicate that the availability of undamaged neurons close to the lost target motoneurons might support the long-term survival of abducens internuclear neurons. Specifically, the oculomotor internuclear neurons, which likely suffer a partial deafferentation after medial rectus motoneuron loss, constitute a potential new target for the abducens internuclear neurons. The reinnervation of a new target might explain the recovery of synaptic and firing properties of abducens internuclear neurons after medial rectus motoneuron lesion, which occurred with a similar time course, as described in the accompanying paper [de la Cruz R. R. et al. (1994) Neuroscience 58, 81-97.].

Long-term effects of selective target removal on brainstem premotor neurons in the adult cat

The electrical activity of antidromically identified abducens internuclear neurons selectively deprived of their target motoneurons was recorded in chronic alert cats. Target motoneurons were killed by the injection of the cytotoxic lectin of Ricinus communis into the medial rectus muscle. Following target removal, the discharge pattern of abducens internuclear neurons showed an overall decrease in firing rate, a significant reduction in their sensitivity to eye position and velocity, and the presence of anomalous responses such as bursts of spikes associated with off-directed saccades. The decreased excitability of abducens internuclear neurons correlated well with a marked reduction in the synaptic efficacy of their inputs. Thus, both excitatory and inhibitory synaptic potentials of vestibular origin showed a noticeable decrease in amplitude. The alterations in firing properties and synaptic transmission were only observed during an initial period of 3 weeks following ricin injection. Within 1 month the electrophysiological parameters returned to control values and remained unaltered for 1 year. Retrograde labelling of abducens internuclear neurons revealed that no cell death occurred after target loss. The anterograde axonal labelling of these neurons showed a progressive decrease in the density of their axonal terminals, and no sign of redistribution to other areas was found. These findings indicate that abducens internuclear neurons are not dependent on the presence of their natural target cells, either for the survival or for the maintenance of appropriate physiological signals.

A physiological study of vestibular and prepositus hypoglossi neurones projecting to the abducens nucleus in the alert cat

1. Vestibular and prepositus hypoglossi (PH) neurones projecting to the abducens (ABD) nucleus were recorded in the alert cat. Their discharge characteristics were analysed to ascertain the origin of the horizontal eye position signal present in ABD neurones. 2. Neurones were classified according to: their location with respect to the ABD nucleus; their antidromic activation from the ABD nucleus; the synaptic field potential they induced in the ABD nucleus with the spike-triggered averaging technique; and their activity during spontaneous and vestibularly induced eye movements. 3. Vestibular neurones projecting to the ABD nucleus were located in the rostral medial vestibular nucleus. They were excitatory on the contralateral and inhibitory on the ipsilateral ABD neurones. Both types of premotor vestibular neurone showed a firing rate weakly related to eye position, increasing for eye fixations in the contralateral on-direction, and decreasing with ipsilateral fixation. Position sensitivity during eye fixations was (means +/- S.D.) 1.8 +/- 0.9 spikes s-1 deg-1 for excitatory neurones and 2.2 +/- 1.3 spikes s-1 deg-1 for inhibitory neurones. Firing rate exhibited a high variability during eye fixations. Their responses during saccades in the off-direction were characterized by a pause that, although less defined, was occasionally present during saccades in the on-direction. Eye velocity sensitivity during spontaneous saccades in the on-direction was 0.17 +/- 0.15 spikes s-1 deg-1 s-1 for excitatory neurones and 0.15 +/- 0.07 spikes s-1 deg-1 s-1 for inhibitory vestibular neurones. During sinusoidal head stimulation at 0.2 Hz, vestibular neurones showed a type I discharge rate with a phase lead over eye position of 86.0 +/- 14.1 deg for excitatory and 80.2 +/- 12.5 deg for inhibitory neurones. Position sensitivity during vestibular stimulation did not differ significantly from values obtained for spontaneous eye movements. However, the velocity sensitivity of premotor vestibular neurones during head rotation was significantly higher (1.6 +/- 0.2 spikes s-1 deg-1 s-1 for excitatory and 1.5 +/- 0.3 spikes s-1 deg-1 s-1 for inhibitory neurones) than during spontaneous eye movements. 4. PH neurones projecting to the ABD nucleus were located in the rostral one-third of the nucleus. These neurones were excitatory on the ipsilateral and inhibitory on the contralateral ABD nucleus. Their firing rates were correlated mainly with eye position, increasing for abducting eye positions of the ipsilateral eye and decreasing with adduction movements.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of GABA in the extraocular motor nuclei of the cat: a postembedding immunocytochemical study

The GABAergic innervation of the extraocular motor nuclei in the cat was evaluated using postembedding immunocytochemical techniques. The characterization of GABA-immunoreactive terminals in the oculomotor nucleus was carried out at the light and electron microscopic levels. GABA-immunopositive puncta suggestive of boutons were abundant in semithin sections throughout the oculomotor nucleus, and were found in close apposition to somata and dendrites. Ultrathin sections revealed an extensive and dense distribution of GABA-immunoreactive synaptic endings that established contacts with the perikarya and proximal dendrites of motoneurons and were also abundant in the surrounding neuropil. GABAergic boutons were characterized by the presence of numerous mitochondria, pleiomorphic vesicles and multiple small symmetrical synaptic contacts. The trochlear nucleus exhibited the highest density of GABAergic terminations. In contrast, scarce GABA immunostaining was associated with the motoneurons and internuclear neurons of the abducens nucleus. In order to further elucidate the role of this neurotransmitter in the oculomotor system, retrograde tracing of horseradish peroxidase was used in combination with the GABA immunostaining. First, medial rectus motoneurons were identified following horseradish peroxidase injection into the corresponding muscle. This was carried out because of the peculiar afferent organization of medial rectus motoneurons that contrasts with the remaining extraocular motoneurons, especially their lack of direct vestibular inhibition. Semithin sections of the oculomotor nucleus containing retrogradely labeled medial rectus motoneurons and immunostained for GABA revealed numerous immunoreactive puncta in close apposition to horseradish peroxidase-labeled somata and in the surrounding neuropil. At the ultrastructural level, GABAergic terminals established synaptic contacts with the somata and proximal dendrites of medial rectus motoneurons. Their features and density were similar to those found in the remaining motoneuronal subgroups of the oculomotor nucleus. Second, oculomotor internuclear neurons were identified following the injection of horseradish peroxidase into the abducens nucleus to determine whether they could give rise to GABAergic terminations in the abducens nucleus. About 20% of the oculomotor internuclear neurons were doubly labeled by retrograde horseradish peroxidase and GABA immunostaining. A high percentage (80%) of the oculomotor internuclear neurons projecting to the abducens nucleus showed immunonegative perikarya. It was concluded that the oculomotor internuclear pathway to the abducens nucleus comprises both GABAergic and non-GABAergic neurons and, at least in part, the GABA input to the abducens nucleus originates from this source. It is suggested that this pathway might carry excitatory and inhibitory influences on abducens neurons arising bilaterally.

Afferents to the oculomotor nucleus in the goldfish (Carassius auratus) as revealed by retrograde labeling with horseradish peroxidase

The goal of this work was to compare the distribution and morphology of neurons projecting to the oculomotor nucleus in goldfish with those previously described in other vertebrate groups. Afferent neurons were revealed by retrograde labeling with horseradish peroxidase. The tracer was electrophoretically injected into the oculomotor nucleus. The location of the injection site was determined by the antidromic field potential elicited in the oculomotor nucleus by electrical stimulation of the oculomotor nerve. Labeled axons whose trajectories could be reconstructed were restricted to the medial longitudinal fasciculus. In order of quantitative importance, the afferent areas to the oculomotor nucleus were: (1) the ipsilateral anterior nucleus and the contralateral tangential and descending nuclei of the octaval column. Furthermore, a few labeled cells were found dorsomedially to the caudal pole of the unlabeled anterior octaval nucleus; (2) the contralateral abducens nucleus. The labeled internuclear neurons were arranged in two groups within and 500 microns behind the caudal subdivision of the abducens nucleus; (3) a few labeled cells were observed in the rhombencephalic reticular formation near the abducens nucleus, most of which were contralateral to the injection site. Specifically, stained cells were found in the caudal pole of the superior reticular nucleus, throughout the medial reticular nucleus and in the rostral area of the inferior reticular nucleus; (4) eurydendroid cells of the cerebellum, located close to the contralateral eminentia granularis pars lateralis, were also labeled; and (5) a small and primarily ipsilateral group of labeled cells was located at the mesencephalic nucleus of the medial longitudinal fasciculus. The similarity in the structures projecting to the oculomotor nucleus in goldfish to those in other vertebrates suggests that the neural network involved in the oculomotor system is quite conservative throughout phylogeny. Nevertheless, in goldfish these projections appeared with some specific peculiarities, such as the cerebellar and mesencephalic afferents to the oculomotor nucleus.

A morphological study of abducens nucleus motoneurons and internuclear neurons in the goldfish (Carassius auratus)

The location and distribution of abducens (ABD) nucleus motoneurons (Mn) and internuclear neurons (Int) were determined in the goldfish (Carassius auratus) by means of horseradish peroxidase and fluorochrome retrograde labeling. ABD Mn were labeled following tracer injection into the ipsilateral lateral rectus muscle. These Mn were arranged in two ventrolateral clusters along the rostro-caudal axis of the posterior brainstem. Both groups of neurons showed a similar number of cells, and their axons ran ventrally to their respective nerve roots. ABD Int were labeled following the injection of the tracer into the contralateral oculomotor nucleus. They also formed two distinct groups in the rostro-caudal axis. The rostral group of Int formed a dorso-lateral cap around the caudal motoneuronal pool, with little if any intermingling. The caudal group of Int was located at the same position in the dorso-ventral and medio-lateral axis as the rostral group, but 500 microns behind it. Both groups of ABD Int had a similar number of neurons. Int axons ascended dorso-medially, then crossed the midline through the internal arcuate fibers, and entered the contralateral medial longitudinal fasciculus. The soma diameters of both ABD Mn and Int were not significantly different. The relative location of both types of neurons is discussed.

Identification of the Purkinje cell/climbing fiber zone and its target neurons responsible for eye-movement control by the cerebellar flocculus

We identified 3 Purkinje cell/climbing fiber zones in the cat cerebellar flocculus. The zones were perpendicular to the long axes of the crooked floccular folia, forming the crooked zones. Each zone was different in axonal projection areas of its target neurons. From the neuronal networks it is theoretically expected that activity changes of a particular zone control eye movement in a particular plane: (1) the rostral and caudal zones on one side control movement in the anterior canal plane on the side of the activity changes and those on both sides control movement in all vertical planes from sagittal to transverse planes; and (2) the middle zone controls movement in the horizontal plane by reciprocal activity changes on both sides. The zone-specific climbing fiber input to a particular zone may contribute to activity changes of the zone in response to mossy fiber input spreading across several zones. Electrical stimulation of each zone evoked the same pattern of eye movement as that theoretically expected from the neuronal networks. This is the first indication that there are indeed functional differences between the Purkinje cell zones in the cerebellum. Our findings support Oscarsson's proposal that each Purkinje cell/climbing fiber zone plus its target neurons may be an operational unit for control of a given motor function.

Neural correlates of horizontal vestibulo-ocular reflex cancellation during rapid eye movements in the cat

1. The aim of the present study is to describe the behaviour of identified second-order vestibular neurones in the alert cat during eye saccades. A selection of neurones which are involved in horizontal eye movements has been made. The activity has been compared with a selected sample of abducens motoneurones recorded in the same animals. 2. Alert head-fixed cats were used for this study. Eye movements were recorded by the scleral search coil technique. Abducens motoneurones were identified by antidromic stimulation from the VIth nerve with chronically implanted electrodes. They were recorded extracellularly. 3. Second-order vestibular neurones were identified by orthodromic stimulation from the vestibular organs. They were recorded intra-axonally and injected with horseradish peroxidase after recording of their physiological characteristics. Their morphology was reconstructed from frozen sections. 4. All the recorded vestibular neurones showed various amounts of eye position sensitivity. The firing rate (F) - horizontal eye position (H) characteristics are compared for abducens and vestibular neurones. The population average values are F = 33 + 4 H for motoneurones and F = 51 + 2.4 H for vestibular neurones. 5. All recorded vestibular neurones showed an increase of discharge rate during contralateral horizontal saccades and a strong decrease or pause during ipsilateral saccades. Firing rate - horizontal eye velocity sensitivity has been calculated. 6. Results suggest a strong inhibitory input on vestibular neurones from the saccadic generator. This mechanism underlies the suppression of the vestibulo-ocular reflex during saccades. Our results suggest that in the cat, for saccades of amplitude smaller than 20 deg, there is a variable degree of suppression which is provided by a projection of excitatory bursters (EBNs) on second-order vestibular neurones through inhibitory type II neurones. 7. We also conclude from this study that the eye position sensitivity of vestibular second-order neurones is in fact a motor signal indicating a motor error, i.e. the amount of head or eye movement which remains to be done in order to align gaze on target with the eyes centred in the orbit.

The abducens motor and internuclear neurons in the rabbit: retrograde horseradish peroxidase and double fluorescent labeling

Horseradish peroxidase and the fluorochromes Fast blue and propidium iodide were injected into the lateral rectus and retractor bulbi muscles and/or the oculomotor nucleus of the rabbit to determine the locations and basic morphology of motoneurons and internuclear neurons in the abducens nucleus. The 1000-1100 motoneurons found were distributed throughout the nucleus except in the rostral and caudal tips, but were most densely clustered in the dorsomedial area, especially in the middle third of the nucleus, where 60% of these cells were found. The rostral and caudal tips were composed of internuclear neurons, 25% of which lay in the rostral third of the nucleus, 35% in the middle third and 40% in the caudal third. In the middle third, interneurons occupied the ventral and lateral areas of the nucleus (where they mingled with motoneurons); in the rostral and caudal thirds they were more widely distributed. At the level of the caudal half of the nucleus it was impossible to distinguish clearly between the most lateral abducens interneurons and the most rostromedial labeled vestibular neurons. The abducens interneurons of the rabbit (320-380) thus differ in interesting respects from those described previously in either lateral eyed or frontal eyed mammals.

Behaviour of Medial Rectus Motoneurons in the Alert Cat

The activity of identified medial rectus motoneurons was recorded in alert cats during spontaneous and vestibular induced eye movements. Medial rectus motoneurons fired a burst of spikes slightly preceding adducting saccades and increased their discharge rate linearly with successive eye positions in the adducting direction. Conduction velocity (21.3 - 98.2 m/s), eye position sensitivity (ks, 7.1 +/- 1.5 spikes/s/deg), and eye velocity sensitivity (rs, 1 +/- 0.2 spikes/s/deg/s) during spontaneous eye movements, and time constants calculated from phase lead analysis (To, 135 +/- 36 ms) showed values similar to those described previously for cat abducens motoneurons. The firing rate during repeated fixation of the same eye position was affected significantly by the direction of the preceding saccade and by the animal's level of alertness. Eye velocity sensitivity was not significantly affected by changes in the animal's level of alertness. A weak negative relationship (coefficient of correlation=-0.56) was observed between eye velocity sensitivity (rv) and sinusoidal rotational frequency, with no change in eye position sensitivity (kv) with stimulus frequency. The subsequent changes in the time constant (Tv) calculated as Tv=rv/kv in relation to stimulus frequency suggests that the oculomotor system deviates from a (linear) first-order model.

A neurophysiological study of prepositus hypoglossi neurons projecting to oculomotor and preoculomotor nuclei in the alert cat

The activity of 62 antidromically identified prepositus hypoglossi neurons was recorded in 10 alert cats during spontaneous, vestibular or visually induced eye movements. Neurons were antidromically activated from stimulating electrodes implanted in the ipsilateral medial longitudinal fasciculus (n = 24), the ipsilateral interstitial nucleus of Cajal (n = 6), the ipsilateral parabigeminal nucleus (n = 2), the contralateral superior colliculus (n = 6) and the contralateral cerebellar posterior peduncle (n = 24). Neurons were identified as eye-movement-related when their rate-position and/or rate-velocity plots showed correlation coefficients greater than or equal to 0.6. They were further classified as "position", "position-velocity" and "velocity-position" according to their relative eye position and velocity coefficients. However, they seemed to be distributed as a continuum in which a progressive decrease of eye velocity sensitivity was accompanied by a proportional increase in eye position sensitivity. "Position-velocity" neurons (n = 9) were mainly horizontal type II neurons projecting to the vicinity of the oculomotor complex; two of these neurons with vertical sensitivity were also activated from the interstitial nucleus of Cajal. Mean position and velocity sensitivity of these neurons were 5.2 spikes/s per degree and 0.62 spikes/s per degree per second, respectively. Pure "position" neurons (n = 7) also showed activation during ipsilateral eye fixations; their mean position gain was 7.3 spikes/s per degree and they projected to the ipsilateral oculomotor and Cajal nuclei, and to the contralateral superior colliculus. "Velocity-position" neurons (n = 18) were type I or II neurons with rather irregular tonic firing rates and a mean velocity gain of 0.75 spikes/s per degree per second. Type II "velocity-position" neurons projected mainly to the oculomotor area, while type I neurons projected preferentially to the cerebellum. A special type of "pause" neuron (n = 5), with very low firing rate and pausing mainly for contralateral saccades, was activated exclusively from the contralateral posterior peduncle. Many neurons with weak eye movement sensitivity (n = 22) were activated mainly (73%) from the cerebellum. It can be concluded that the prepositus hyperglossi nucleus distributes specific eye movement related signals to motor and premotor brainstem and cerebellar structures. The variability of interspike intervals of representative prepositus hypoglossi neurons of each class was compared to the discharge variability of identified abducens motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Location of motoneurons and internuclear neurons within the rat abducens nucleus by means of horseradish peroxidase and fluorescent double labeling

The distribution of abducens motoneurons and internuclear neurons was determined in the rat by injections of horseradish peroxidase or fluorochromes into the ipsilateral lateral rectus muscle and the contralateral oculomotor nucleus either separately or simultaneously. The labeled somata of abducens internuclear neurons were intermingled with the labeled motoneurons at the medial third of the nucleus, but they were more segregated at the rostral third, where the labeled interneurons were more numerous. Internuclear neurons were preferentially located around and ventral to the central part of the facial genu, while motoneurons were located more dorsomedially, closer to the midline than in other species of mammals. The evolutionary trend of the location of both populations of neurons is also discussed.

Target neurons of floccular middle zone inhibition in medial vestibular nucleus

Unitary activities of 288 neurons were recorded extracellularly in the medial vestibular nucleus (MV) in anesthetized cats. In 19 neurons, located in the rostral part of the MV adjacent to the stria acustica, floccular middle zone stimulation resulted in cessation of spontaneous discharges. Systematic microstimulation in the brainstem during recording of 16 of 19 target neurons of floccular middle zone inhibition revealed that the target neurons projected to the ipsilateral abducens nucleus (ABN), and not to the contralateral ABN nor the oculomotor nucleus. The conjugate ipsilateral horizontal eye movement elicited by middle zone stimulation may be mediated by this pathway to motoneurons and internuclear neurons in the ipsilateral ABN. In additional experiments, the MV neurons responding antidromically to ipsilateral ABN stimulation and orthodromically to ipsilateral 8 nerve stimulation were recorded extracellularly. In only 7 of 36 recorded neurons, middle zone stimulation depressed the orthodromic and spontaneous activities. Many neurons were free of floccular inhibition. As to the route of floccular inhibitory control over the vestibulo-ocular reflex (VOR) during visual-vestibular stimulation, we propose that the interaction of target and VOR relay neurons takes place at the ipsilateral ABN and modulates the VOR, in addition to well known Ito's proposal that the interaction of the floccular output and the VOR takes place at secondary vestibular neurons and modulates the VOR.

Motor output to lateral rectus in cats during the vestibulo-ocular reflex in three-dimensional space

The motor output to the lateral rectus eye muscle was studied in decerebrate cats with electromyographic recordings and in alert cats with multi-unit and single neuron recordings from abducens nucleus. The axis of rotation that produced maximal excitation of the lateral rectus was calculated from responses to rotations in many different stimulus orientations, and was found to lie near the axis of the horizontal semicircular canals, but pitched slightly nose down from the canal axis (4.6 degrees). The results from decerebrate and alert cats were in agreement. The dynamics of lateral rectus activation were quite similar in all planes. Responses at high frequencies were in phase with rotation velocity and responses lagged toward position phase as frequency and velocity were decreased. Differences in decerebrate cat low frequency responses to rotations with and without a sinusoidal gravitational stimulus implicated an otolith input to lateral rectus.

Behavioral and morphological effects of oculomotor nucleus lesion on abducens internuclear neurons in the cat

The activity of identified control and injured abducens internuclear neurons was recorded during spontaneous eye movements in the alert cat. From 2 days following the electrocoagulation of the contralateral oculomotor nucleus, abducens internuclear neurons showed a quick fatigability during eye fixations not observed in controls. Discharge rate during saccades started after the beginning of the eye movement and showed a fast exponential-like decay. These abnormal responses were not further recorded from 20 days following the lesion. Morphological studies indicated that 90% of the abducens internuclear neuron population disappeared 2 months following the lesion and showed no sign of recovery one year later.

Behavior of neurons in the abducens nucleus of the alert cat--III. Axotomized motoneurons

The effects of peripheral and central VIth nerve axotomy on abducens nucleus synaptic potentials of vestibular origin and the ultrastructure of intracellularly labeled abducens motoneurons were examined in the anesthetized cat. Subsequent experiments explored the activity of identified abducens motoneurons during spontaneous and vestibular induced eye movements in alert cats prepared for chronic recordings of eye movements, single units and field potentials. Following axotomy the typical disynaptic inhibition of abducens motoneurons induced by electrical stimulation of the ipsilateral vestibular nerve either disappeared or was reduced for 5-30 days. Disynaptic activation produced by contralateral VIIIth nerve stimulation was apparently not affected. These changes were accompanied at the ultrastructural level by a decrease of axosomatic pleiomorphic synaptic endings. No changes were observed in either the number or distribution of synaptic endings on proximal and distal dendrites. Although not expected by results obtained in acute experiments, axotomized motoneurons showed a decreased excitability in the behavioral paradigm. Amplitude of the abducens antidromic field potential was significantly reduced 4-6 days following axotomy and frequent failures were observed in the antidromic somadendritic invasion of single motoneurons. Somatic invasion was obtained by the simultaneous presentation of appropriate visual and/or vestibular synaptic activity. Chronic recordings of field potentials showed their amplitude to recover in 30-40 days. The spontaneous and vestibular induced activity of identified axotomized motoneurons during this period of time differed in several aspects from controls. Motoneurons could not maintain tonic activity during eye fixations and they showed short, low frequency, bursts of activity that followed, rather than preceded, on-directed saccades. In some cases axotomized motoneurons fired during horizontal off-directed and vertical saccades. Position and velocity gains of axotomized motoneurons were lower than control values. The effects of central axotomy were always larger and of longer duration than those following peripheral axotomy. Structural and functional properties influenced by axotomy seemed to recover in 2-3 months, but with independent time courses. The present results differ in many aspects from those described after axotomy in spinal and hypoglossal motoneurons. In addition, they point out that behavior or axotomized neurons in chronic preparations are not predictable on the basis of those described in acute experiments.