Cells of origin of corticospinal projections to phrenic and thoracic respiratory motoneurones in the cat as shown by retrograde transport of HRP

Abstract

A combined electrophysiological and histological approach was employed to identify neurones within the motor cortex which project to the vicinity of spinal respiratory motoneurones, and which may be involved in the alteration of the pattern of breathing under certain conditions. Recording of respiratory phased activity from phrenic, or from thoracic motoneurones within either the upper (T3-4) or lower (T8-9) segments, was followed by the iontophoretic injection of HRP at these recording sites. After injections within the cervical or thoracic ventral horn, 219 cells were retrogradely labelled in 14 experiments. The majority of these cells (88%) were labelled contralateral to the injection site. Following the injection of HRP into the phrenic nucleus, labelling was observed at two major sites within the anterior sigmoid gyrus (ASG), one along the anterolateral edge of the cruciate sulcus, and the other along the ventrolateral border of the ASG. In contrast, cells labelled after injections into the thoracic ventral grey matter were located more medially within the ASG and the posterior sigmoid gyrus (PSG). The populations of cells labelled following phrenic and thoracic injections overlapped, primarily at the lateral edge of the cruciate sulcus. The somas of labelled cells were pyramidal, round or oval. The mean diameters of cortical cells labelled after injections into the lower or upper thoracic segments were 30.5 ± 6.2 and 31.5 ± 5.6 respectively, which were not significantly different in size. However, they were significantly larger than the mean diameter of the cells labelled from injections into the phrenic nucleus (22.7 ± 4.2 μm). In 2 control experiments the entire grey matter of one side of the fifth cervical segment was infiltrated with HRP and a total of 1555 cells were labelled in the motor cortex. Similarly, following a large injection into the upper thoracic spinal cord, 176 cortical cells were labelled. The pattern of labelling in the motor cortex observed in these control experiments differed from that obtained after the more localized injections in that much more extensive areas of the cortex were involved.