Introduction to Sympathetic Microneurography

Fig. 1.1

Muscle sympathetic nerve activity (MSNA) and blood pressure. From the top to the bottom, original multi-fiber burst of MSNA, microneurographically recorded from the tibial nerve in a healthy subject, full-wave rectified and integrated MSNA, and systemic blood pressure recorded continuously by Finapres. MSNA discharges spontaneously responding to spontaneous fluctuation of blood pressure, discharging when blood pressure falls, while being suppressed when blood pressure rises, thus controlling blood pressure homeostasis (Figure from Published Paper [21])

Enhanced by maneuvers increasing intrathoracic pressure such as Valsalva’s maneuver

(b)

SSNA:

1.

Spontaneous arrhythmic efferent burst discharges recorded from the skin nerve fascicle
2.

Followed by peripheral vasoconstriction or perspiration
3.

Elicited with almost constant latency by mental stress and sensory stimuli (sound, pain, electrical stimulation of the peripheral nerve trunk, etc.)

Identification of the nerve fibers discharging with the above-mentioned characteristics as belonging to the efferent C-fiber group is based on the following findings: (1) The discharges are not modified by local anesthetic infiltration distal to the recording site, blocking simultaneously recorded afferent somatosensory impulses, while they are abolished rapidly by local anesthetic infiltration proximal to the recording site, with an increase in peripheral skin blood flow in the case of vasoconstrictor activity without changing somatosensory afferent activities [17]. (2) Double recording from two different sites in the same nerve clearly shows that the discharge recorded in the proximal site always precedes the discharge recorded in the distal site. The conduction velocity measured as inter-electrode distance divided by the interval of two discharges shows the value of the C-fiber range to be around 1–2 m/s [18]. (3) Vasoconstrictor and sudomotor components of SSNA are identified by simultaneous recordings of skin blood flow and sweating. Vasoconstrictor bursts precede skin vasoconstriction, while sudomotor bursts precede sweating. Combined bursts of vasoconstrictor and sudomotor components precede both vasoconstriction and sweating (Fig. 1.2).

Fig. 1.2

Skin sympathetic nerve activity (SSNA), skin blood flow, and sweating. From the top to the bottom, skin blood flow measured by laser Doppler flowmetry, sweat rate measured by ventilated capsule method, and full-wave rectified and integrated multi-fiber SSNA bursts, microneurographically recorded from the peroneal nerve in a healthy subject. SSNA discharges spontaneously preceding vasoconstriction (downward arrow, VC vasoconstrictor burst), sweat rate (upward arrow, SM sudomotor burst), or both vasoconstriction + sweat rate (VC + SM vasoconstrictor + sudomotor burst), thus controlling skin blood flow and sweating (Figure from Published Paper [21])

1.2.4 Applications of Sympathetic Microneurography

Sympathetic microneurography has become a potent tool in clinical autonomic testing [19, Chaps. 2, 3, 4, 5, and 6 in this book]. We can record relatively easily spontaneously discharging sympathetic neural traffic in abundant fibers grouped in muscle and skin nerve fascicles as multi-fiber burst activities. Interindividual comparison of MSNA can be done as counting, for example, burst numbers per minute. The burst numbers of MSNA are reproducible in the same individuals when the recordings are made at different intervals, but increase with advancing the age [20]. Recordings of MSNA and SSNA have been applied to elucidate autonomic neural functions under various physiological and pathological conditions. MSNA is important to analyze neural mechanisms involved in the cardiovascular system, while SSNA is essential to understand neural functions related to thermoregulation [21].

We can use sympathetic microneurography to analyze neural functions not only at rest but also when exposed to various environmental stresses [22]. This method has been used to clarify physiological autonomic mechanisms as well as pathological mechanisms, not only in awake state but also during sleep [23, 24]. We used sympathetic microneurography not only in ground-based laboratories but also in various fields such as in high altitude, in cold or hot environment, during water immersion, in airplane, when loading acceleration, in space, and so on. Autonomic neural functions in space have been evaluated using various indirect methods until 1998, when microneurography was first applied during spaceflight to clarify how microgravity influences MSNA in humans. For this research, three American and one Japanese astronauts mastered perfectly the technique of sympathetic microneurography. Two of them aboard Space Shuttle Columbia could measure successfully MSNA from the peroneal nerve of their fellow astronauts. It was revealed that MSNA was rather enhanced during the 12th and 13th days of spaceflight and just after returning to Earth [25–28].

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