Axonal projection of descending pathways responsible for eliciting forelimb stepping into the cat cervical spinal cord

Summary The descending pathways responsible for eliciting forelimb stepping are located in the lateral funiculus (Yamaguchi 1986). In order to determine into which spinal segments the descending pathways project and to know the projections and functions of the other descending system, the ventral funicular pathways, we placed various lesions in the cervical spinal cord of decerebrate cats with the lower thoracic cord transected and studied their effects on forelimb stepping evoked by stimulation of the midbrain locomotor region. (1) The lateral funiculus was transected on one side. The operation removes descending input to all the segments caudal to the lesion. Experiments with serial transections from the caudal to rostral segment revealed that stepping activity of the limb on the lesioned side is reduced when the lesion is placed at the level between the C6 and C7 segment and then between C5 and C6. A slight reduction of activity was also observed after a lesion placed between C7 and C8. (2) Consistently, bilateral transection of the lateral funiculus at the level between C5 and C6 abolished stepping movements of both forelimbs. (3) The cervical cord was split in the parasagittal plane through the dorsal root entry. The operation removes the descending input to the segment in which the lesion is placed. The parasagittal lesions from the C1 to C6 did not abolish stepping activity, although a lesion placed between C5 and C6 could slightly affect stepping. The results, (1)–(3) suggest that the lateral funicular pathways project into the spinal segments mainly at the C6–C7 level with some rostrocaudal extension into C5 and C8. (4) Complete transections of the medial part of the spinal cord cut extensor bursts short and raised stepping frequency. Nevertheless, if the lesion at C1–C5 spared the ventromedial part of the ventral funiculus, it did not result in such high-frequency stepping or in weakened extensor activity. In the case of segments caudal to C6, medial transections which spared the corresponding region could result in such stepping. It is suggested that the pathways descending through the ventromedial part of the ventral funiculus in the rostral segments provide extensor activity during stepping. They may change their course in the more dorsal part of the ventral funiculus below the C6 and presumably project into the grey matter of more caudal segments.     

Transient aphagia produced following bilateral destruction of the lateral hypothalamic area and quinto-frontal tract of chicks

Six groups of broiler chicks, Gallus domesticus, sustained bilateral lesions to specific neural structures residing in the lateral hypothalamic and thalamic areas. In contrast to past data reported for the albino rat, the pigeon, Columba livia and barbary dove, Streptopelia risoria, bilateral destruction of the chick lateral hypothalamic area (LHy), quinto-frontal tract (QF), and stratum cellulare externum (SCE) resulted in transient aphagia and rapid recovery of lost body weight. Similarly, bilateral destruction of the nucleus reticularis superior (RS) and nucleus intercalatus (ICT) resulted in a temporary 1–3 day period of aphagia with body weight returning to pre-operative levels in approximately 4 days. Bilateral destruction of the ansa lenticularis (AL) resulted in a more prolonged period of aphagia (4 days) and an average 8-day period to recover lost body weight. Additional data suggest that more persistent aphagia can be induced following lesions to the posterior hypothalamus and midbrain. Specifically, bilateral lesions which destroyed the following combination of neural structures resulted in prolonged aphagia: AL, QF and posterior LHy; AL and posterior nucleus of the AL (ALp); and AL, ALp and QF. It is suggested that the AL and ALp contain neurons which are part of a more complex system that modulates or controls motor activity and feeding behavior in birds.     

Colour and brightness signals of parvocellular lateral geniculate neurons

Summary We recorded from single neurons in the parvocellular layers of the lateral geniculate body of anesthetized monkeys. Spectral response curves of parvocellular neurons depended on the luminance ratio between the chromatic stimuli and achromatic background. From response/intensity curves, we determined the relative luminance between a coloured and an achromatic (white) light at which a given cell became non-responsive (critical luminance ratio, CLR). The spectral dependence of the CLRs of narrow (N) and wide band (W) cells with opponent receptor input showed characteristic differences. The activity of W-cells increased with luminance increase of a white light and of a coloured light in the specific spectral region of the cell (yellow-red for the long wave length sensitive WL-, and yellow-green-blue for the short wave length sensitive WS-cells), while N-cells were activated by their specific spectral light (blue for NS-cells, red for NL-cells) and by a luminance decrease of achromatic white. N-cells discriminate best between their characteristic colour and white at luminance ratios below their respective CLR, while W-cells distinguish best between a light of their characteristic colour and white at chromatic/ achromatic luminance ratios above their respective CLR. Yellow sensitive W-cells with a narrow spectral sensitivity peaking around 570 nm and with only a small or no response to white light, could enable distinction between white and yellow of similar luminance. The findings are consistent with the opponency model of spectrally sensitive cells in the LGB. We discuss their implications for colour coding by parvocellular cells. N- and W-cells appear to behave complementary with respect to luminance information (N-cells may be compared to the cat's off-cells, W-cells to on-cells). S- and L-cells are complementary with respect to colour. The yellow sensitive WM-cells are critical for the discrimination of yellow and white, while cells with excitatory cone input from blue and red cones (W-SL-cells) may aid the perception of purple. The fact that, at different relative luminance ratios between a chromatic stimulus and a white background, the whole family of parvocellular cells is involved differently in coding for colour, may explain the different appearance of colours against a white background at different luminance ratios and the perception of induced colours.This work was supported by a NATO collaborative research grant to Dr. Arne Valberg (650/83)     

Reciprocal lateral inhibition of on- and off-center neurones in the lateral geniculate body of the cat

Summary In the lateral geniculate body (LGB), intra- and quasi-intracellular records were done. With small light stimuli shone into different parts of the receptive field, EPSPs and IPSPs could be elicited. Stimulation of the exact center of an on-center cell produced a pure excitatory response, that of an off-center neurone pure inhibition. This response lasted throughout the stimulus. At light off, inhibition was elicited in on-center cells and excitation in off-center cells. A stimulus in the field periphery produced a mixed response with a small and short excitation followed by large inhibition in on-center cells, and a short inhibition followed by postsynaptic depolarization in off-center cells. At light off, on-center cells showed depolarization after a short polarizing phase, and off-center cells a broad polarization which interrupted the initial small excitation. The latencies of both the excitatory and inhibitory center responses at light on and off characteristic for the two types of neurones, were 20–30 msec shorter than the reversed responses elicited by stimulation of the receptive field surround.The findings are compatible with a model in which each geniculate on-center cell gets its major excitatory input from one optic tract on-center fibre and inhibitory input from several off-center fibres with nearby receptive fields. An off-center LGB-cell receives its main excitation essentially from one offcenter fibre and inhibition from several on-center cells. The responses to moving stimuli also agreed with this model. The presence of recurrent inhibition within the LGB could be confirmed by electrical stimulation. But it could not be decided whether the reciprocal inhibition of on- and off-center cells was due to forward or backward inhibition. The spontaneous activity of on- and off-center cells which were simultaneously recorded with one electrode, showed a mutual inhibition 6–8 msec after one cell had fired. Anatomical data relevant to the model are discussed and some functional implications are suggested.     

The effects of reverse monocular deprivation in monkeys II. Electrophysiological and anatomical studies

Summary Monkeys had one eye closed at about 30 days of age for 14, 30, 60, or 90 days, then opened, and the fellow eye closed for another 120 days. The animals then had at least 10 months of binocular visual experience before extensive behavioral training and testing were carried out. In terminal experiments concluded more than 18 months later, microelectrode investigations of the striate cortex demonstrated that there was almost a complete absence of binocular neurons in all animals. The initially deprived eyes (IDEs) dominated the majority of cortical neurons, even when soma size measurements of lateral geniculate neurons indicated that the LGN cells driven by the IDE had not regained their normal size. The monkeys which had significant interocular differences in spatial vision also exhibited abnormalities in the distribution of the metabolic enzyme, cytochrome oxidase (CO), within the striate cortex. These results demonstrate that many of the severe alterations in cortical physiology and eye dominance produced by early monocular form deprivation can be reversed, with recovery of normal cortical function, via the reverse-deprivation procedure.Supported by National Eye Institute grants R01 EY01120, R01 EY03611, R01 EY01139, and EY02520     

Inhibitory interaction in the cat's lateral geniculate nucleus

Summary Spike activities of optic tract fibers and corresponding relay cells were recorded simultaneously in layers A and A₁ of the dorsal lateral geniculate nucleus of the cat. Light stimuli of various diameters were shone into the receptive field center of these unit pairs and their input/output ratios were determined. An increase of the stimulus size leads to an impairment of the input/output ratio in on-center and off-center relay cells. This suppressive effect has approximately the same latency as the excitatory response.Intracellular recordings suggest that the inhibitory effect of the surround is due to a postsynaptic process. Inhibitory postsynaptic potentials occur during and — under certain stimulus conditions —before the excitatory response. The short latency of these IPSPs suggests that they result from the activity of adjacent units with the same RF characteristics as the recorded neuron. This inhibitory input is not restricted to the RF periphery but may also be activated by stimulation within the RF center. Most neurons are also inhibited by units with antagonistic center responses.During the period of this research Ernst Pöppel held a training grant of the Stiftung Volkswagenwerk, Az. 11 1015.     

Indirect hypothalamo-cerebellar pathway? Demonstration of hypothalamic efferents to the lateral reticular nucleus

Summary Hypothalamic efferents to the lateral reticular nucleus (NRL) have been demonstrated in the cat by means of anterograde and retrograde axonal transport of the wheat germ agglutinin — horseradish peroxidase (WGA-HRP) complex. Pressure injections of the WGA-HRP complex into the hypothalamus resulted in anterograde labelling of branching terminal axons both in the NRL and in an adjacent area, presumably the ventrolateral catecholaminergic cell group (A1). After microiontophoretical ejections of the WGA-HRP complex into the NRL from a ventral approach, retrogradely labelled neurons were found in the lateral, dorsal, posterior and anterior hypothalamic areas and in the tubero-mammillary, dorsomedial and periventricular nuclei. The projection is bilateral with a clear ipsilateral predominance and has its main origin in the lateral hypothalamic area. The locations of hypothalamic cells projecting to the NRL are somewhat different from those giving rise to hypothalamo-cerebellar and hypothalamo-spinal connections. The present demonstration of a hypothalamic input to one of the major precerebellar relay nuclei introduces a new possible indirect route through which the cerebellum may be influenced by the hypothalamus. The different indirect and direct hypothalamo-cerebellar pathways and their potential functional importance are discussed.     

Visual and vestibular signals in the lateral mesencephalic tegmental region of the cat

Summary Single unit recordings from two alert cats were used in an attempt to further elucidate the function of the lateral mesencephalic tegmental region (LTR), a part of the mesencephalon forming a link between the superior colliculus and the lower brain stem. A total of 155 units recorded from the LTR were tested with visual, vestibular and acoustic stimuli. Of these, 54 cells (36%) were characterized as either visually (n=33) or vestibularly (n=21) responsive and an additional 13 cells were driven by complex acoustic stimuli. Visually responsive cells typically were directionally selective with large, mainly contralateral receptive fields. Vestibularly responsive cells were modulated by stimulation of either the horizontal canals (yaw stimulation; n = 16) or of both pairs of vertical canals (pitch stimulation; n=5). About half of the cells with activity modulated by rotation about the yaw axis increased discharge during ipsiversive (Type I), the other half during contraversive rotation (Type II). Of the 5 cells with activity modulated by pitch stimulation, 4 preferred the nose-down and only 1 the nose-up direction. Although the discharge of units responsive to yaw stimulation was roughly in phase with head velocity (mean phase lag with respect to head velocity: 10.6 deg), none of the vestibular cells had activity correlated with eye position, eye velocity or movement of visual stimuli. Our observations suggest that the LTR might introduce visual and vestibular signals into the tecto-facial pathway which may be used to adjust the size of pinna movements with respect to the size of ongoing head- or body movements.     

Stability of intracranial self-stimulation following hypophysectomy

The possibility of a hypothalamo-pituitary involvement in the intracranial self-stimulation phenomenon was investigated. Threshold and optimal stimulation currents were identified by means of the method of limits, in rats with bipolar electrodes implanted in the lateral hypothalamus. Total or sham hypophysectomies were performed after the establishment of these current values, and found to have no effect on reinforcing brain stimulation. It was concluded that a hypothalamo-pituitary mechanism does not subserve reinforcing brain stimulation.     

Taste preferences induced by trains of rewarding lateral hypothalamic stimulation of differing durations

The question of whether prolonged stimulation of the lateral hypothalamic area (LH) simply diminishes in rewarding effect or becomes aversive was tested in 32 rats using a taste preference technique. The animals were allowed to drink novel, coffee-flavoured water for 10 min, and then received 60 trains of LH stimulation with differing durations, at an intensity proven to be rewarding at 0.5 sec duration. In a test 24 hours later, those animals that had been stimulated with 1 or 5 sec trains showed a shift in preference from tap water to coffee water, but those stimulated with 10 sec trains did not. The results show that prolonged stimulation not only loses its positive reinforcement effect but becomes negative.