The contribution of somatosensory afferent inputs from the neck to autonomic regulation of cardiovascular function in humans
Creators: name: Watanabe, Nobuhiro
Publisher: name: RMIT University
Year: 2024

description: There have been reports that manually-delivered therapeutic interventions to the neck may normalise cardiovascular dysfunctions associated with such conditions as arrhythmia and hypertension. Also some studies suggest that stimulation of neck structures may influence cardiovascular function and cardiac sympathovagal balance, indicating that neck sensory inputs may be capable of influencing cardiovascular function. However, its contribution is still not conclusive. Therefore, the aims of this thesis include; 1) to examine the effects of simulated cervical manipulation on autonomic regulation of cardiovascular function in humans and 2) to examine whether muscle proprioceptors in the neck are related to autonomic regulation of cardiovascular function in humans.<br><br>To achieve these purposes, a series of experiments were performed. Primarily, heart rate and blood pressure (BP) were non-invasively recorded and heart rate variability (HRV), known as an indicator of cardiac autonomic drive, was obtained. Additional parameters were measured in several experiments such as finger (FiBF) and muscle blood flow. To deal with the startle reaction, which significantly influences autonomic function, skin potential change (and FiBF) was employed.<br><br>In order to achieve the first purpose of this thesis, a simulated cervical manipulation was delivered using an Activator® Instrument that delivered a brief mechanical stimulus to the neck without head movement. Thus, any involvement of the vestibular system could be excluded. It was found that the mechanical stimulus to the neck significantly decreased BP and altered HRV parameters. Because BP was recorded from the finger, it was necessary to confirm the observed BP changes were not associated with FiBF changes, which are commonly observed in startle reactions. Further analysis showed FiBF did not influence BP measurement, indicating that cervical manipulation itself may be capable of evoking a genuine cardiovascular response.<br><br>In order to achieve the second purpose of this thesis, muscle conditioning and a vibratory stimulus were employed. Muscle conditioning is a manoeuvre presumed to alter the quantity of muscle proprioceptive inputs to the central nervous system in a systematic way by exploiting the thixotropic property of muscles. A vibratory stimulus is known to preferentially stimulate muscle spindles. Hence, it was hypothesised that measured parameters should exhibit a dependency on the form of muscle conditioning if neck muscle proprioceptive inputs contribute to autonomic regulation of cardiovascular function. Vibration was unilaterally applied to the dorsal neck in the sitting position. Muscle conditioning was applied to the neck muscles in sitting and supine postures. Muscle conditioning induced transient changes in BP and FiBF in both postures whereas the vibratory stimulus induced a small-to-slight reduction in BP and forearm muscle blood flow and a moderate increase in FiBF. In a subsequent experiment, 20° head-up tilt (HUT) was applied following muscle conditioning. In response to HUT, immediate responses of both HR and BP were dependent on the form of the manoeuvre (&lt; 10 seconds). Additionally, HRV parameters exhibited muscle conditioning-dependency for prolonged period (&lt; 5 minutes). These results suggest that muscle proprioceptive inputs from the neck may play a significant role in autonomic regulation of cardiovascular function, especially during orthostasis. <br>