Laminar distribution and the response of superior colliculus neurons to stimulation of the periodontal ligament in the rabbit

Connections between the trigeminal mesencephalic nucleus and the superior colliculus in the rat

Retrograde tracing methods are employed here to demonstrate that neurons in the trigeminal mesencephalic nucleus (5me) project to the superior colliculus (SC) in the rat. These neurons, mainly of small size, are situated bilaterally in the caudal part of the nucleus. Anterograde tracing studies demonstrated the existence of SC projections to neurons in 5me. The SC fibers contact 'en passant' small as well as large cell bodies of 5me neurons. These pathways suggested a role of the 5me neurons in oculomotor control and associated oro-facial functions.

Oral nociceptive activity in the rat superior colliculus

Single units were recorded using extracellular glass microelectrodes in all laminae of the superior colliculus of the rat under halothane nitrous oxide anaesthesia. Fifty-one units were encountered which responded to a low intensity mechanical stimulus applied to a contralateral or bilateral field located in the oral sphere (intraoral 11, perioral 16), on the face (29) or on the rest of the body (21). Sixteen units responded to a jaw movement. Sixty-one cells were recorded which were preferentially (10) or only (51) activated (30) or inhibited (21) by noxious stimuli. Contralateral or bilateral mechanoreceptive fields located in intraoral (34) and perioral (35) areas were frequent. There is therefore a high incidence of the nociceptive representation of the mouth in the superior colliculus. The other functional properties of the nociceptive units were similar to those reported in other studies. From the subsequent histological localization of the recorded units, it appeared that the nociceptive projections from the intraoral and perioral regions to the superior colliculus reach the lateral part of the intermediate and deep layers of the superior colliculus.

Organization of the crossed tecto-reticulo-spinal projection in rat--II. Electrophysiological evidence for separate output channels to the periabducens area and caudal medulla

The previous paper (Redgrave et al., Neuroscience 37, 571-584, 1990) presented anatomical evidence indicating there are at least two largely segregated components of the crossed tecto-reticulo-spinal pathway which project to the periabducens area and caudal medulla. An immediate question arising from this finding is whether tectal cells which project either to the periabducens area or to the caudal medulla have different electrophysiological response properties. An answer to this question would be relevant to the issue of whether different components of the tecto-reticulo-spinal system are specialized for the production of different classes of orienting movement. Accordingly, extracellularly recorded units in the superior colliculus of urethane anaesthetized rats were tested for antidromic activity following electrical stimulation of the periabducens area or the caudal medulla. When antidromic potentials were successfully recorded the sensory properties of the units were tested with a range of unimodal visual, somatosensory and auditory stimuli. The following results were obtained. (i) Tectal cells antidromically activated by stimulation of the caudal medulla were preferentially sensitive to somatosensory stimuli from the perioral region, while cells activated from the periabducens area were more frequently responsive to auditory stimuli. (ii) Tectal fibres activated by stimulation of the caudal medulla had significantly higher conduction velocities than the fibres activated by electrodes in the periabducens region. (iii) More than 90% of antidromically activated cells were located in stratum album intermediale or dorsal stratum profundum. These electrophysiological findings confirm and extend previous anatomical observations which indicate that components of the crossed descending projection of the colliculus may be functionally specialized for the production of different classes of orienting movements.

Perioral somatosensory but not visual inputs to the flank of the mouse superior colliculus

An electrophysiological and anatomical study identified the sensory inputs to the "flank" of the mouse's superior colliculus, a large, ventrolateral extension of layer IV (stratum griseum intermediate) that has no overlying visual layers (II and III). Electrophysiological recordings with subsequent histological localization showed that the flank receives predominantly somatosensory projections from the perioral region but not visual input. In its caudal parts, the flank also has limb and trunk somatosensory inputs and auditory inputs. The perioral somatosensory projections to the flank are ordered somatotopically. The flank is considered part of the superior colliculus since the perioral inputs are adjacent to inputs from mystacial vibrissae in the more medial parts of the superior colliculus, hence forming a single continuous map. The finding that no visual responses occur above the flank (due to the absence of superficial layers) or within it is in accord with the concept of an intermodality "spatial register". Since flank neurons have somatosensory receptive fields in non-visible parts of the body and they lack visual responses, the flank may be involved more in tactile-dependent rather than in vision-dependent orienting behaviors. Thus, the superior colliculus may, in parallel, carry out sensorimotor transformations related to (1) shift of gaze and (2) tactile-dependent behaviors not involving vision.