Descending pathways from the superior colliculus: An autoradiographic analysis in the rhesus monkey (Macaca mulatta)

Abstract

The autoradiographic tracing method has been used to identify the various descending tectofugal pathways and their targets in the rhesus monkey (Macaca mulatta). The present data reveal that the majority of descending tectofugal axons arise from collicular laminae which lie ventral to the stratum opticum (layer 3). Such descending axons can be grouped into two major bundles or tracts, i.e., the ipsilateral tectopontine-tectobulbar tract and the crossed tectospinal tract (or the predorsal bundle). There is, in addition to these two major pathways, a smaller, commissural projection. The ipsilateral pathway courses laterally and ventrocaudally to terminate within the parabigeminal nucleus, the mesencephalic reticular formation, the dorsal lateral pontine gray (in several discrete patches), the dorsal lateral wing of the nucleus reticularis tegmenti pontis, and within the nucleus reticularis pontis oralis. Other ipsilateral targets of the deep tectal layers are the cuneiform nucleus and the external nucleus of the inferior colliculus. In several experiments transported protein is also apparent within the substantia nigra. Axons which comprise the tectospinal tract, or the predorsal bundle, cross within the dorsal tegmental decussation and descend within the brainstem in a position slightly lateral to the midline. The most rostral and quite extensive target of the predorsal bundle is the nucleus reticularis tegmenti pontis. As the predorsal bundle courses caudally within the pontine tegmentum, labeled axons enter the dorsal and medial regions of both the oral and the caudal divisions of the nucleus reticularis pontis. At caudal medullary levels, the mojority of the labeled axons comprising the predorsal bundle pass ventrally to end quite profusely with the subnucleus b of the medial accessory nucleus of the inferior olivary complex. Caudal to this only a few scattered, labeled axons can be followed into the cervical spinal cord. Labeled axons also pass to the opposite, or contralateral colliculus via the tectal commissure. Such axons appear to arise and end primarily within the deeper tectal layers. In one experiment, the injection invaded the mesencephalic nucleus of the trigeminal nerve. Labeled axons were apparent within the motor nucleus, the chief sensory nucleus (quite profusely) and within the spinal or descending nucleus of the trigeminal nerve.

Arm Movements Induced by Electrical Microstimulation in the Superior Colliculus of the Macaque Monkey

“…It was further identified as a structure that is involved in the generation of patterned locomotion (Shik and Orlovsky, 1976). Because the SC projects to the nucleus cuneiformis (Harting, 1977), electrical microstimulation in the intermediate and deep layers could activate the above-mentioned movement patterns. This possibility is especially interesting because, in the present study, defense-like behavior was sometimes but rarely observed after electrical microstimulation (but see DesJardin et al, 2013).…”

Complex Spike Activity in the Oculomotor Vermis of the Cerebellum: A Vectorial Error Signal for Saccade Motor Learning?

“…The intermediate layer of the SC projects to the b subnucleus of the caudal medial accessory olive (Frankfurter et al 1976;Harting 1977;Huerta and Harting 1984), which, in turn, projects to lobule VII of the vermis (Kralj-Hans et al 2007). Quasi-visual (QV) neurons in the SC (Mays and Sparks 1980), which start discharging 80 -90 ms after a target step and cease firing only when the saccade is executed (and the error is eliminated), could be the source of this visual error signal.…”

“…The midline cerebellum, which is part of a parallel route for oculomotor information from the superior colliculus (SC) to reach the premotor brain stem generator for saccades (Harting 1977;Kralj-Hans et al 2007;Noda et al 1990;Yamada and Noda 1987), has been implicated as a structure where signals are adapted to reduce saccade dysmetria. It consists of the oculomotor vermis (lobules VIc and VII) and the caudal fastigial nucleus to which they project.…”

“…CS activity to the oculomotor vermis originates in the b nucleus of the medial accessory olive (Kralj-Hans et al 2007;Yamada and Noda 1987), which in turn, receives a strong input from the intermediate layer of the SC (Harting 1977;Huerta and Harting 1984). The intermediate layer of the SC contains neurons that have both visual receptive and motor movement fields, i.e., they discharge maximally for stimuli falling on a particular area of the contralateral hemifield and for contralateral saccades of a certain direction and size (for review, see Sparks and Hartwich-Young 1989).…”

Discharge of Monkey Nucleus Reticularis Tegmenti Pontis Neurons Changes During Saccade Adaptation

“…However, saccade adaptation might influence neurons in the NRTP, which receives direct inputs from the SC (Harting 1977;Kawamura et al 1974;Scudder et al 1996), and, in turn, projects directly to the oculomotor vermis (Brodal 1980;Shinnar et al 1975;Thielert and Thier 1993;Yamada and Noda 1987) and to the CFN (Gonzalo-Ruiz and Leichnetz 1990;Noda et al 1990). Therefore, we investigated whether activity in the NRTP is changed during saccade adaptation.…”