Activation of thalamic ventroposteriolateral neurons by phrenic nerve afferents in cats and rats

Abstract

It has been demonstrated that phrenic nerve afferents project to somatosensory cortex, yet the sensory pathways are still poorly understood. This study investigated the neural responses in the thalamic ventroposteriolateral (VPL) nucleus after phrenic afferent stimulation in cats and rats. Activation of VPL neurons was observed after electrical stimulation of the contralateral phrenic nerve. Direct mechanical stimulation of the diaphragm also elicited increased activity in the same VPL neurons that were activated by electrical stimulation of the phrenic nerve. Some VPL neurons responded to both phrenic afferent stimulation and shoulder probing. In rats, VPL neurons activated by inspiratory occlusion also responded to stimulation on phrenic afferents. These results demonstrate that phrenic afferents can reach the VPL thalamus under physiological conditions and support the hypothesis that the thalamic VPL nucleus functions as a relay for the conduction of proprioceptive information from the diaphragm to the contralateral somatosensory cortex.

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“…elicited by PnA found in this study is consistent with the previous studies of limb muscle afferents (Yates et al, 1985). These results are consistent with PnA entering the cervical spinal cord, projecting to the dorsal horn, which then relays PnA to the brainstem, and projects phrenic afferent information to the somatosensory cortex via a thalamocortical pathway (Zifko et al, 1995;Zhang et al, 2003). The function of this putative pathway could be related to the proprioceptive control of respiratory muscles and respiratory movement control originating in the motor cortex (Frazier et al, 1991).…”

“…and the projections of PnA to the central nervous system (Straus et al, 1997). Although the PnA projection pathways to the cerebral cortex remain unknown, the present study supports a dorsal column mediated pathway that likely involves a multisynaptic thalamic relayed projection (Zhang et al, 2003). It has been shown in the previous studies (Frazier et al, 1991) that C-fiber afferents have much higher threshold and longer latency for PnA stimulation than group Ia, Ib and group II afferents.…”

“…Electrophysiological studies have confirmed that Group I and Group II phrenic nerve afferents project to lateral reticular nucleus , external cuneate nucleus , and both ventral and dorsal respiratory-related areas of brainstem (Macron et al, 1986;Speck et al, 1987). Moreover, in anesthetized cats, short-latency responses have been recorded in the thalamus (Zhang et al, 2003) and somatosensory cortex after electrical stimulation of the phrenic nerve afferents and mechanical probing of the diaphragm. If diaphragmatic proprioceptors project to higher somatosensory brain centers via pathways similar to Group Ia and Group Ib receptors (Landford and Schmidt, 1983), then phrenic afferents should enter the spinal cord ipsilaterally through the dorsal roots of the cervical spinal segments, terminate on dorsal horn neurons, dorsal horn neurons should then project centrally via the dorsal columns to the brainstem and then project to the somatosensory cortex and other supraspinal structures.…”

“…Cortical projection of PnA has been shown in both cortical evoked potentials (Zhang et al, 2003;Davenport et al, 1985) and retrograde fluorescent (Yates et al, 1987) studies. Activation of the somatosensory cortex in cats after electrical stimulation of the contralateral phrenic nerve (Davenport et al, 1986) and intercostals muscles (Davenport et al, 1993) has been reported by this laboratory.…”

“…However, electrical and mechanical stimulation of phrenic afferents are reported to activate thalamic neurons (Zhang et al, 2003) and elicit neural activity in the cat somatosensory cortex . In addition, Zechman et al (1985) reported a correlation of transdiaphragmatic pressure (Pdi) and the perception of inspiratory loads.…”