Ultrasound stimulation of the motor cortex during tonic muscle contraction

Transcranial ultrasound stimulation (tUS) shows potential as a noninvasive brain stimulation (NIBS) technique, offering increased spatial precision compared to other NIBS techniques. However, its reported effects on primary motor cortex (M1) are limited. We aimed to better understand tUS effects in human M1 by performing tUS of the hand area of M1 (M1hand) during tonic muscle contraction of the index finger. Stimulation during muscle contraction was chosen because of the transcranial magnetic stimulation-induced phenomenon known as cortical silent period (cSP), in which transcranial magnetic stimulation (TMS) of M1hand involuntarily suppresses voluntary motor activity. Since cSP is widely considered an inhibitory phenomenon, it presents an ideal parallel for tUS, which has often been proposed to preferentially influence inhibitory interneurons. Recording electromyography (EMG) of the first dorsal interosseous (FDI) muscle, we investigated effects on muscle activity both during and after tUS. We found no change in FDI EMG activity concurrent with tUS stimulation. Using single-pulse TMS, we found no difference in M1 excitability before versus after sparsely repetitive tUS exposure. Using acoustic simulations in models made from structural MRI of the participants that matched the experimental setups, we estimated in-brain pressures and generated an estimate of cumulative tUS exposure experienced by M1hand for each subject. We were unable to find any correlation between cumulative M1hand exposure and M1 excitability change. We also present data that suggest a TMS-induced MEP always preceded a near-threshold cSP.

Prof. Dohrn. - Tonic muscle spasm in a stillborn premature fetus of one eclamptic. Caesar section on the dead. - (Centr. F. Gynaec., No. 19, 1895)

A 39-year-old patient with manifestations of severe eclampsia, at 28-29 weeks of pregnancy, was subjected to Caesar treatment immediately after her death. It was not possible to ascertain the heartbeat of the fetus shortly before death. The fetus represented a special state, namely: it was bent, numb, the mouth was tightly closed, the arms and legs were flexed, the fingers were curved like a bird's claw.

Effects of tonic muscle pain on fusimotor control of human muscle spindles during isometric ankle dorsiflexion

Studies on anesthetized animals have revealed that nociceptors can excite fusimotor neurons and thereby change the sensitivity of muscle spindles to stretch; such nociceptive reflexes have been suggested to underlie the mechanisms that lead to chronic musculoskeletal pain syndromes. However, the validity of the “vicious cycle” hypothesis in humans has yielded results contrasting with those found in animals. Given that spindle firing rates are much lower in humans than in animals, it is possible that some of the discrepancies between human experimental data and those obtained in animals could be explained by differences in background fusimotor drive when the leg muscles are relaxed. We examined the effects of tonic muscle pain during voluntary contractions of the ankle dorsiflexors. Unitary recordings were obtained from 10 fusimotor-driven muscle spindle afferents (6 primary, 4 secondary) supplying the ankle dorsiflexors via a microelectrode inserted percutaneously into the common peroneal nerve. A series of 1-min weak contractions was performed at rest and during 1 h of muscle pain induced by intramuscular infusion of 5% hypertonic saline into the tibialis anterior muscle. We did not observe any statistically significant increases in muscle spindle firing rates of six afferents followed during tonic muscle pain, although discharge variability increased slightly. Furthermore, a participant’s capacity to maintain a constant level of force, while relying on proprioceptive feedback in the absence of visual feedback, was not compromised during pain. We conclude that nociceptive inputs from contracting muscle do not excite fusimotor neurons during voluntary isometric contractions in humans. NEW & NOTEWORTHY Data obtained in the cat have shown that muscle pain causes a marked increase in the firing of muscle spindles, attributed to a nociceptor-driven fusimotor reflex. However, our studies of muscle spindles in relaxed leg muscles failed to find any effect on spindle discharge. Here we showed that experimental muscle pain failed to increase the firing of muscle spindle afferents during weak voluntary contractions, when fusimotor drive sufficient to increase their firing is present.

Approach to DBS in the Vicinity of the Ventral Intermediate Nucleus of the Thalamus DBS

The regional anatomy around the DBS lead in the ventral intermediate nucleus of the thalamus (Vim) determines efficacy and adverse effects. Understanding the regional anatomy allows the programmer to adjust the stimulation to provide optimal benefit and the absence of adverse effects. Vim is the target of therapeutic DBS. The ventrocaudal nucleus of the thalamus (Vc) lies posterior to the Vim. Electrical stimulation of Vc can cause treatment-limiting paresthesias. The corticospinal and cortical bulbar tracts in the internal capsule lie lateral and ventral to the Vim. Electrical stimulation of the internal capsule can cause tonic muscle contractions. There are multiple nomenclatures of the subnuclei of the thalamus. Although the term ventrolateral thalamus (VL) is commonly used in the physiology literature, ventral intermediate thalamus (Vim), is used in the DBS literature. Technically, the VL refers to both regions of the thalamus that receive inputs from GPi and cerebellum, whereas Vim refers to the cerebellar-receiving area of the thalamus and is thus a subdivision of the VL and is the target of DBS for tremor-related disorders.