Shinobu Toma

Microneurography as a tool in clinical neurophysiology to investigate peripheral neural traffic in humans.

Microneurography is a method using metal microelectrodes to investigate directly identified neural traffic in myelinated as well as unmyelinated efferent and afferent nerves leading to and coming from muscle and skin in human peripheral nerves in situ. The present paper reviews how this technique has been used in clinical neurophysiology to elucidate the neural mechanisms of autonomic regulation, motor control and sensory functions in humans under physiological and pathological conditions. Microneurography is particularly important to investigate efferent and afferent neural traffic in unmyelinated C fibers. The recording of efferent discharges in postganglionic sympathetic C efferent fibers innervating muscle and skin (muscle sympathetic nerve activity; MSNA and skin sympathetic nerve activity; SSNA) provides direct information about neural control of autonomic effector organs including blood vessels and sweat glands. Sympathetic microneurography has become a potent tool to reveal neural functions and dysfunctions concerning blood pressure control and thermoregulation. This recording has been used not only in wake conditions but also in sleep to investigate changes in sympathetic neural traffic during sleep and sleep-related events such as sleep apnea. The same recording was also successfully carried out by astronauts during spaceflight. Recordings of afferent discharges from muscle mechanoreceptors have been used to understand the mechanisms of motor control. Muscle spindle afferent information is particularly important for the control of fine precise movements. It may also play important roles to predict behavior outcomes during learning of a motor task. Recordings of discharges in myelinated afferent fibers from skin mechanoreceptors have provided not only objective information about mechanoreceptive cutaneous sensation but also the roles of these signals in fine motor control. Unmyelinated mechanoreceptive afferent discharges from hairy skin seem to be important to convey cutaneous sensation to the central structures related to emotion. Recordings of afferent discharges in thin myelinated and unmyelinated fibers from nociceptors in muscle and skin have been used to provide information concerning pain. Recordings of afferent discharges of different types of cutaneous C-nociceptors identified by marking method have become an important tool to reveal the neural mechanisms of cutaneous sensations such as an itch. No direct microneurographic evidence has been so far proved regarding the effects of sympathoexcitation on sensitization of muscle and skin sensory receptors at least in healthy humans.

Response characteristics of cutaneous mechanoreceptors to vibratory stimuli in human glabrous skin

We investigated the responsiveness of mechanoreceptors in the glabrous skin of the hand to triangular vibratory stimuli when the frequency and applied pressure to the skin were varied. Using a microneurographic technique, 13 single afferent activities were recorded from 4 kinds of mechanoreceptors. Both fast-adapting (FA) and slow-adapting (SA) units were sensitive to the vibratory stimuli. The relationship between the most sensitive frequency and applied pressure to the skin were analyzed as a tuning curve. FA-type I (FAI) was sensitive to vibratory stimuli at 30-40 Hz and the frequency which entrained one-to-one discharge at lower pressure was between 10 and 80 Hz. FA-type II (FAII) sensitivity was augmented sharply over 60-100 Hz. SA-type I (SAI) and SA-type II (SAII) responsiveness was almost the same and characteristic sensitivity to the vibratory stimuli was revealed under 15 Hz. Functional roles of these units, especially type I units, are discussed in relation to active touch such as texture discrimination with a finger.

Intracerebral source localization of mental process-related potentials elicited prior to mental sweating response in humans

We measured the mental sweating response (MSR) and the skin sympathetic nerve activity (SSNA). Mental arithmetic or recall questions first elicited SSNA and then elicited MSR. MSR was used as the trigger point of time 0 ms to average EEGs. The averaged EEGs contained slow wave fluctuations, which occurred 5 s prior to the MSR onset. The current source locations of the MSR-related potentials were estimated by EEG dipole tracing method in two subjects. Mental stress activated the inferior frontal gyrus 5.5 s prior to the MSR and then 0.5 s later, the lateral part of the hippocampus in a subject, and they activated the medial part of the amygdala 5 s prior to the MSR in another subject. Indirect contact of the brain with the mind associated with mental questions was discussed.

Single unit responses of human cutaneous mechanoreceptors to air-puff stimulation

OBJECTIVE To investigate responsiveness of human cutaneous mechanoreceptor to selective tactile stimuli produced by brief air-puff stimulation. METHODS Using percutaneous microneurography, activities of single sensory units innervating glabrous skin of the hand were recorded, and air-puff stimuli with a short rise time (0.5 ms), generated by a high-speed air control system, were applied to the receptive field. Receptor activation time was estimated as the latency difference between electrically and air-puff evoked responses. RESULTS Thirty units were analyzed: all 4 kinds of mechanoreceptors of human glabrous skin (fast adapting type 1 [FA I, n=7], fast adapting type 2 [FA II, n=4], slowly adapting 1 [SA I, n=5] and slowly adapting 2 [SA II, n=14]) were activated by air-puff stimulation. Estimated receptor activation times were 0.6-6.2 ms (mean 2.2 ms). FA II units occasionally responded twice or more to a single air-puff stimulus. CONCLUSIONS Brief air-puffs can activate all 4 human cutaneous mechanoreceptors, and the receptor transduction time is estimated as approximately 2.0 ms. Properties of human mechanoreceptors can be studied using air-puff stimulation and microneurography.

Activity‐dependent excitability changes in chronic inflammatory demyelinating polyneuropathy: A microneurographic study

The purpose of this study was to investigate activity‐dependent excitability changes in polyneuropathy and their correlation with symptomatology. First, we recorded sensory nerve action potentials (SNAPs) with an intraneural microelectrode during impulse trains in 11 patients with chronic inflammatory demyelinating polyneuropathy. When the stimulus frequency was increased to ≥20 Hz, all patients showed marked decreases in the amplitudes of averaged SNAPs (128 responses) associated with latency increases. The amplitude decreases were much greater than those in patients with axonal neuropathies. In single‐unit recordings, responses showed latency increases, which were small but sufficient to cause decreases in the averaged responses. Clinical sensory impairment was correlated with the degree of preexisting conduction block or axonal loss, but not with the degree of rate‐dependent amplitude decreases. Activity‐dependent changes occur preferentially in demyelinating neuropathy and are a sensitive measure of demyelination. The mechanism responsible for the amplitude decreases could be conduction slowing or block caused by activity‐dependent hyperpolarization.

Inhibitory effect of acupuncture on the vibration-induced finger flexion reflex in man

The effect of acupuncture on the tonic finger flexion reflex caused by mechanical vibration of the index finger was studied in healthy man. The volar side of the index finger was tapped by a vibrator (100 Hz), while flexion forces were recorded. A silver needle inserted into the acupuncture point (Wai-Kuan) inhibited the vibration-induced finger flexion reflex. In this study, the inhibitory effect of acupuncture on the reflex contraction, rather than pain sensation and analgesia, was demonstrated.

Properties of human skin mechanoreceptors in peripheral neuropathy

OBJECTIVES To investigate the properties of mechanoreceptors in patients with peripheral neuropathy. The skin mechanoreceptor is a terminal organ of the primary sensory neuron, which is likely to be affected earlier and more severely than is the nerve trunk by peripheral neuropathies. METHODS Single sensory unit responses to air-puff and electric stimulation were recorded using the microneurographic technique in the glabrous skin of the hand. Receptor transduction time was estimated by a latency difference between electric- and air-puff-induced responses. RESULTS A total of 38 mechanoreceptive units were obtained from 14 normal subjects. All the units responded to air-puff stimuli irrespective of the receptor type, and receptor transduction time was approximately 2 ms. A total of 32 units were recorded from 11 patients with neuropathy of variable causes. Seven (22%) of the 32 neuropathic units did not respond to air-puffs despite their ability to respond to electric stimulation. Compared to normal ones, units from patients with peripheral neuropathy had significantly higher mechanical thresholds, but receptor transduction times did not differ significantly. CONCLUSIONS Changes in receptor properties in human neuropathy are characterized by increased mechanical threshold without prolongation of receptor transduction time, possibly due to a high threshold for generating receptor potentials.

Attenuation of somatosensory evoked potentials by acupuncture and tactile skin stimulation in man

SummaryThe effects of acupuncture and tactile skin stimulation on somatosensory evoked potentials (SEPs), elicited by the median nerve stimulation, were investigated in healthy subjects. Acupuncture needles were inserted into either Hegu plus Shousanli, Hegu plus Waiguan, or Shousanli positions ipsilateral to the median nerve stimulation. Tactile skin stimulation was applied to either the ulnar side of the palm, or the dorsal surface of the hand or forearm ipsilaterally to the nerve stimulation. It was found that acupuncture significantly suppressed the amplitude of P22 and P40, and that the tactile skin stimulation of the ulnar side of the palm significantly suppressed the amplitude of P22 and P40, but that the peak latencies were not affected. Dipole tracing analysis showed that the location and vector direction of P22 were not changed but the vector moment of P22 was changed by both acupuncture and tactile stimulation. Based on these findings the suppressive effect of acupuncture and skin stimulation on P22 was proposed to be due to the afferent inhibition in the somatosensory cortex. Although the suppressive mechanism of P40 by tactile skin stimulation seemed to be similar to that of P22, the suppression of P40 by acupuncture appeared to include different mechanisms.

Increased muscle sympathetic nerve activity after glucagon administration in man

Muscle sympathetic nerve activity (MSNA) after glucagon administration was quantitatively measured in five healthy male volunteers (age: 20-28 years old). After an overnight fasting, 1 mg glucagon dissolved in a 1-ml vehicle was injected i.v. as a bolus. MSNA, ECG and blood pressure were simultaneously recorded 15 min before and after the injection. MSNA (bursts/min), heart rate (HR), and systolic and diastolic blood pressures (dBP and sBP) were significantly increased at 2-3 min after glucagon injection. Their means and standard deviations of control for 15 min vs. 3 min after the injection were in the five subjects; MSNA, 10 +/- 6 vs. 30 +/- 10 bursts/min, HR, 61 +/- 10 vs. 76 +/- 8 beats/min, dBP, 66 +/- 9 vs. 91 +/- 9 mmHg and sBP, 122 +/- 12 vs. 152 +/- 11 mmHg. These results suggest that glucagon has a direct facilitatory effect on the central sympathetic nervous system.

Tactile sensory nerve potentials elicited by air-puff stimulation: a microneurographic study.

Article abstract To investigate the sensory nerve responses to selective touch stimulation, sensory nerve action potentials after brief air-puffs were recorded with a microelectrode. In patients with peripheral neuropathy, those with impairment of tactile sensations had significantly smaller responses than did those without tactile impairment, suggesting receptor activation failure as well as nerve conduction failure. Brief air-puff stimulation, when combined with microneurography, could be used for evaluating the tactile receptor properties in humans.