Representation of somatosensory inputs within the cortical autonomic network
The relationship between somatosensory stimulation and the autonomic nervous system has been established with effects on heart rate (HR) and sympathetic tone. However, the involvement of the cortical autonomic network (CAN) during muscle sensory afferent stimulation has not been identified. The main objective of the research in this dissertation was to determine the representation of somatosensory afferents in the CAN and their physiologic impact on cardiovascular control. Somatosensory
... activation was elicited by electrical stimulation of type I and II afferents (sub-motor threshold) and type III and IV afferents (motor threshold), and CAN patterns were assessed using blood-oxygenation level-dependent functional magnetic resonance imaging. Study 1 (Chapter 2) established CAN regions associated with sub-motor stimulation including the ventral medial prefrontal cortex (vMPFC), subgenual anterior cingulate cortex (sACC), and posterior insula, along with a trend towards increased heart rate variability (HRV). Motor threshold stimulation was associated with activation in the posterior insula. Having established the CAN regions affected by sensory afferent input, diffusion tensor imaging was used (Chapter 3) to establish structural connections between the cortical regions associated with functional cardiovascular control. We identified two discrete patterns of white matter connectivity between the anterior insula-sACC and posterior insula-posterior cingulate cortex, suggesting that a structural network may underlie functional roles in autonomic regulation and sensory processing. As somatosensory stimulation had modest impact on cardiovascular control under baseline conditions, Study 3 (Chapter 4) aimed to establish the effects of somatosensory stimulation during baroreceptor unloading iv (lower-body negative pressure, LBNP) on muscle sympathetic nerve activity (MSNA) and cortical activity. Sensory stimulation during LBNP led to an attenuated increase in MSNA burst frequency, as well as absent activity in the right insula and dorsal ACC, supporting the sympatho-excitatory role of these regions. No effect of somatosensory stimulation during chemoreflex-mediated sympatho-excitation was observed on MSNA, while right insular and dorsal ACC activities were maintained. Overall, the results of these studies provide evidence of somatosensory representation within the CAN regions that are anatomically linked, and highlight a role for type I and II sensory afferents in modulating autonomic outflow in a manner that depends upon baroreceptor loading.