Influence of superior colliculus on cat neck motoneurons

Synaptic actions of tectofugal pathways on abducens motoneurons in the cat

Organization of pathways between the superior colliculus (CS) and abducens motoneurons (VI-MNs) was studied in cats under pentobarbital anesthesia using intracellular recordings from VI-MNs and adjacent reticular neurons. Latencies of EPSPs elicited by contralateral CS stimulation indicate that a small fraction of the excitatory pathway may be monosynaptic while its major part is disynaptic. As suggested by an analysis of synaptic responses to microstimulation of the paramedian pontine region, excitatory impulses descend in the tectobulbospinal tract after crossing at midbrain levels. An attempt was made to identify interneurons of the excitatory tectoabducens pathway in the region just ventral and rostroventral to the VI-nucleus. About one-quarter of the reticular neurons in this region received monosynaptic excitation specifically from the contralateral CS. They were acceptable as interneurons with regard to other response characteristics too. Axonal projection to, or through, the abducens nucleus was demonstrated for some of them by intranuclear microstimulation or by tracing axons after Procion yellow injections. It is suggested that "premotor" interneurons of the excitatory tectoabducens pathway are concentrated in the vicinity of the abducens nucleic. A similar investigation of inhibitory responses to ipsilateral CS-stimulation indicates that inhibitory pathways are at least disynaptic and, for the most part, contain three or more synapses. In its initial trajectory the inhibitory pathway appears to be identical with the tectobulbospinal tract,but it decussates for the second time at caudal pontine levels to reach ipsilateral VI-MNs.

The effect of passive eye movement on unit discharge in the superior colliculus of the cat

The effects of passive eye rotation on unit discharge in the superior colliculus has been examined in the chloralose anaesthetized cat. Providing that velocity exceeded 50 degrees/sec, passive eye movement led to brief-burst discharges in the superior colliculus. This discharge occurred as the eye traversed a fixed point. The displacement of this point from the primary position has been defined as the displacement threshold for that unit. Displacement thresholds range from 3 to 30 degrees and are consistent over a wide range of velocities but become larger at the velocity threshold for the unit. Conduction times to the superior colliculus ranged from 7 to 108 msec.

The spinal course and distribution of fore and hind limb muscle afferent projections to the superior colliculus of the cat

1. Projections to the superior colliculus from fore and hind limb muscle nerves have been examined. 2. Hind leg nerve stimulation at strengths sufficient to excite Group II and III fibres elicited unit discharge in all layers of the superior colliculus and underlying tegmentum. 3. Forelimb nerve stimulation excited units in all layers of the superior colliculus and underlying tegmentum. Twenty per cent were activated by stimulation below Group II threshold, the remainder by stimulation above Group II threshold. 4. Most units activated by limb afferent nerve stimulation were also activated by visual stimuli. There was considerable convergence from both flexor and extensor nerves and most units were excited by stimulation of nerves in more than one limb. 5. Units fired either with a short or long latency or with a paired discharge. Short latency units were most common in deep collicular layers and the tegmentum, long latency units were most common in superficial layers. 6. The spinal pathway of the afferents is mainly in the dorsolateral quadrant contralateral to the recording site. The anatomical characteristics of the pathway are similar to the lateral funiculus climbing fibrespinocerebellar pathway.