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Airway smooth muscle is primarily innervated by cholinergic parasympathetic nerves, while its adrenergic sympathetic innervation is sparse to non-existent. Specifically, cholinergic parasympathetic signalling increases the airway tone, meaning the airway tone is proportional to the vagal tone. [8] [10]
Regardless of the trigger, the mechanism of syncope is similar in the various vasovagal syncope syndromes. The nucleus tractus solitarii of the brainstem is activated directly or indirectly by the triggering stimulus, resulting in simultaneous enhancement of parasympathetic nervous system tone and withdrawal of sympathetic nervous system tone.
While baseline vagal input is constant, the degree of stimulation it exerts is regulated by a balance of inputs from sympathetic and parasympathetic divisions of the autonomic nervous system, with parasympathetic activity generally being dominant. Vagal tone is frequently used to assess heart function, and is also useful in assessing emotional ...
Parasympathetic innervation of the heart is partially controlled by the vagus nerve and is shared by the thoracic ganglia. Vagal and spinal ganglionic nerves mediate the lowering of the heart rate. The right vagus branch innervates the sinoatrial node. In healthy people, parasympathetic tone from these sources is well-matched to sympathetic tone.
Sympathetic resonance or sympathetic vibration is a harmonic phenomenon wherein a passive string or vibratory body responds to external vibrations to which it has a harmonic likeness. [1] The classic example is demonstrated with two similarly-tuned tuning forks. When one fork is struck and held near the other, vibrations are induced in the ...
Hermann von Helmholtz observed in On the Sensations of Tone that the tone of a string tuned to C on a piano changes more noticeably when the notes of its undertone series (C, F, C, A ♭, F, D, C, etc.) are struck than those of its overtones. Helmholtz argued that sympathetic resonance is at least as active in under partials as in over partials ...
In neurogenic shock, the body loses its ability to activate the SNS so that only parasympathetic tone remains. The resulting loss of sympathetic tone, which plays a major role in other forms of shock, is responsible for the unique and atypical features mentioned above. [7] [9]
However, when vagal tone is removed, there is little inhibition to the pacemaker, and according to polyvagal theory, rapid mobilization ("fight/flight") can be activated in times of stress, but without having to engage the sympathetic-adrenal system, as activation comes at a severe biological cost. [17]