Anatomical evidence for ipsilateral collicular projections to the spinal cord in the cat.

Injections of WGA-HRP were made within the C1 segment of spinal cord in cats with a midsagittal section of the midbrain. A small number of labelled cells were found in the latero-caudal part of the deeper layers of the superior colliclus (SC) ipsilateral to the injection sites. Because of the complete section of the dorsal tegmental decussation, these results definitively demonstrate the existence of an ipsilateral tecto-spinal pathway projecting to upper cervical segments in the cat. Ipsilaterally projecting tecto-reticulo-spinal neurons represent about 5% of the total population of tecto-spinal neurons. They were exclusively located in the deeper collicular layers and most of them were found in the latero-caudal part of the SC. Comparison with our previous studies suggests that more ipsilateral tecto-spinal projections that found after the section of the dorsal tegmental decussation probably exist. They may arise from tecto-reticulo-spinal neurons recrossing the midline in the brainstem or in the rostral part of C1. By analogy with the cortico-spinal tract, we suggest that the existence of an ipsilateral tecto-spinal pathway can be regarded as evidence for a substantial development of the cat tecto-spinal system as compared with other mammals

Post-spike facilitation of neck EMG by cat tectoreticulospinal neurones during orienting movements.

1. The activity of fourteen tectoreticulospinal neurones (TRSNs) was recorded intraaxonally in the caudal pons of alert cats during orienting movements towards visual stimuli. TRSN spikes were used to compute the spike-triggered average (STA) of rectified EMG of dorsal neck muscles. 2. Eight TRSNs for which 400-2532 spikes were available were analysed with the STA technique. When the STA was computed from all spikes, significant post-spike facilitation (PSF) was obtained for six of eighteen cell-muscle pairs investigated (5 TRSNs). The mean relative amplitude of PSFs was 7.4% (S.D. 3.7). The onset latencies ranged from 1.1 to 5.0 ms and mean duration was 11.4 +/- 3.1 ms (mean +/- S.D.). 3. Interspike interval distributions were unimodal, with modes between 2.7 and 12.7 ms. Spike trains of TRSNs that produced significant PSFs contained 5-13% of the interspike intervals < or = 5 ms and 22-37% of the intervals < or = 10 ms. To evaluate the contribution of short intervals to PSF, STAs were computed separately for spikes preceded by 'short' (< or = 5 or < or = 10 ms) and 'long' (> 5 or > 10 ms) intervals. 4. When computed from spikes preceded by 'long' intervals, PSF amplitudes were small (mean +/- S.D., 5.3 +/- 2.7%) and onset latencies measured by cusum ranged between 2.4 and 5.4 ms. This is longer than the estimated minimal latency of monosynaptic facilitatory effect on neck EMG (1.9-2.1 ms). 5. Relative amplitudes of PSF obtained with spikes preceded by 'short' intervals were much larger (mean +/- S.D., 14.8 +/- 7.4%), but cusums indicated negative latencies for four of six PSFs. The unrealistically short onset latencies could be accounted for by the summation of facilitation from the trigger spike with that of the preceding spikes. In four of five TRSNs a large increase of PSF amplitude (from 3.2 to 7.2 times the amplitude obtained from 'long' intervals) suggests the presence of frequency-dependent potentiation of synaptic transmission. 6. This study unequivocally demonstrates that some TRSNs produce significant post-spike facilitation of neck motoneurones. This facilitation could be mediated by monosynaptic tectomotoneuronal connections although a contribution by disynaptic connections cannot be definitively ruled out. The high instantaneous firing rates of TRSNs produce a potentiation of the otherwise weak facilitatory action of TRSNs that presumably contributes to a rapid recruitment of motoneurones during initiation of head orienting movements