Phrenic nerve afferent activation of neurons in the cat SI cerebral cortex

Abstract

Stimulation of respiratory afferents elicits neural activity in the somatosensory region of the cerebral cortex in humans and animals. Respiratory afferents have been stimulated with mechanical loads applied to breathing and electrical stimulation of respiratory nerves and muscles. It was hypothesized that stimulation of the phrenic nerve myelinated afferents will activate neurons in the 3a and 3b region of the somatosensory cortex. This was investigated in cats with electrical stimulation of the intrathoracic phrenic nerve and C 5 root of the phrenic nerve. The somatosensory cortical response to phrenic afferent stimulation was recorded from the cortical surface, contralateral to the phrenic nerve, ispilateral to the phrenic nerve and with microelectrodes inserted into the cortical site of the surface dipole. Short‐latency, primary cortical evoked potentials (1° CEP) were recorded with stimulation of myelinated afferents of the intrathoracic phrenic nerve in the contralateral post‐cruciate gyrus of all animals (n= 42). The mean onset and peak latencies were 8.5 ± 5.7 ms and 21.8 ± 9.8 ms, respectively. The rostro‐caudal surface location of the 1° CEP was found between the rostral edge of the post‐cruciate dimple (PCD) and the rostral edge of the ansate sulcus, medio‐lateral location was between 2 mm lateral to the sagittal sulcus and the lateral end of the cruciate sulcus. Histological examination revealed that the 1° CEP sites were recorded over areas 3a and 3b of the SI somatosensory cortex. Intracortical activation of 16 neurons with two patterns of neural activity was recorded: (1) short‐latency, short‐duration activation of neurons and (2) long‐latency, long‐duration activation of neurons. Short‐latency neurons had a mean onset latency of 10.4 ± 3.1 ms and mean burst duration of 10.1 ± 3.2 ms. The short‐latency units were recorded at an average depth of 1.7 ± 0.5 mm below the cortical surface. The long‐latency neurons had a mean onset latency of 36.0 ± 4.2 ms and mean burst duration of 32.2 ± 8.4 ms. The long‐latency units were recorded at an average depth of 2.4 ± 0.2 mm below the cortical surface. The results of the study demonstrated that phrenic nerve afferents have a short‐latency central projection to the SI somatosensory cortex. The phrenic afferents activated neurons in lamina III and IV of areas 3a and 3b. The cortical representation of phrenic nerve afferents is medial to the forelimb, lateral to the hindlimb, similar to thoracic loci, hence the phrenic afferent SI site in the cat homunculus is consistent with body position (thoracic region) rather than spinal segment (C 5–C 7). The phrenic afferent activation of the somatosensory cortex is bilateral, with the ipsilateral cortical activation occurring subsequent to the contralateral. These results support the hypothesis that phrenic afferents provide somatosensory information to the cerebral cortex which can be used for diaphragmatic proprioception and somatosensation.