Characteristics of fast and slow corticobulbar fibre projections to reticulospinal neurones

The aim of this work was to investigate the direct involvement of the fronto-parietal cortex in the control of spinal autonomic centers eliciting electrodermal activity (EDA). This autonomic response, linked with the activity of sweat glands, was recorded as skin potential responses (SPRs) from forepaws in the cat. Animals were paralyzed by gallamine and SPRs were obtained under halothane anaesthesia. For each animal, a transection of the medulla sparing only pyramidal tracts was carried out. SPRs were elicited by direct electrical stimulation of pericruciate and posterior parietal cortical areas before and after such a transection. Results showed that in intact preparations, stimulation of the pericruciate cortex evoked SPRs at lower thresholds than the posterior parietal cortex. After the bulbar transection, only the stimulation of pericruciate areas still elicited SPRs at low intensities. Results are interpreted as indicating that fronto-parietal control of EDA is probably mediated by a double descending system: one involving corticoreticulospinal pathways and a direct corticospinal one. We hypothesized that the somatic motor cortex initiates descending programs to autonomic centers at bulbar and spinal levels, and that these centers are involved in autonomic adjustments to somatomotor movements.

Stimulation of the motor cortex evoked postsynaptic potentials in bulbar reticulo-spinal neurons of anaesthetized cats. In 72.4% of reticulo-spinal neurons they were identified as monosynaptic. According to the time characteristics they were classified into two groups (‘fast’ and ‘slow’). Corticobulbar fibres projecting to reticulospinal neurons could be also differentiated as ‘fast’ (conduction velocities above 20 m/s) and ‘slow’ (condition velocities below 20 m/s). Evidence was obtained that ‘fast’ cortico-fugal excitatory postsynaptic potentials are mediated by ‘fast’ cortico-bulbar fibres, and ‘slow’ excitatory postsynaptic potentials by the ‘slow’ ones. It was found that reticulo-spinal neurons with axon conduction velocities of 10.8–65 m/s can be activated by both ‘fast’ and ‘slow’ cortico-bulbar fibres, and the neurons with conduction velocities above 65 m/s only by the ‘slow’ ones. The background activity of reticulo-spinal neurons with slow conducting axons can be characterized as ‘tonic’ (with relatively uniform interspike interval distribution) while the activity of ‘fast’ neurons as ‘phasic’ (with significant grouping of discharges).

The possible functional role of the differentiated cortico-reticulo-spinal connections is discussed.