SUMMARY1. The effects of different forms of brain stimulation on the discharge pattern of single motor units were examined using the post-stimulus time histogram (PSTH) technique and by recording the compound surface clectromyographic (EMG) responses in the first dorsal interosseous (FDI) muscle. Electrical and magnetic methods were used to stimulate the brain through the intact scalp of seven normal subjects. Electrical stimuli were applied either with the anode over the lateral central scalp and the cathode at the vertex (anodal stimulation) or with the anode at the vertex and the cathode lateral (cathodal stimnulation). Magnetic stiinulation used a 9 cm diameter coil centred at the vertex; current in the coil flowed either clockwise or anticlockwise when viewed from above.2. Supramotor threshold stimuli produced one or more narrow (< 2 ms) peaks of increased firing in the PSTH of all thirty-two units studied. Anodal stimulation always produced an early peak. The latencies of the peaks produced by other forms of stimulation, or by high intensities of anodal stimulation, were grouped into four time bands relative to this early peak, at intervals of -0 5 to 0U5, 1-2, 2-5-3-5 and 4-5-5 ms later. Peaks occurring within these intervals are referred to as P0 (the earliest an6dal), P1, P2 and P3 respectively.3. At threshold, anodal stimulation evoked only the P0 peak; at higher intensities, the P2 or more commonly the P3 peak also was recruited. The size of the P0 peak appeared to saturate at high intensities.4. In five of six subjects, cathodal stimulation behaved like anodal stimulation, except that there was a lower threshold for recruitment of the P2 or P3 peak relative to that of the P0 peak. In the other subject, the P3 peak was recruited before the PO peak.5. Clockwise magnetic stimulation, at threshold, often produced several peaks. These always included a PI peak, and usually a P3 peak. A P0 peak in the PSTH was never produced by a clockwise stimulation at intensities which we could explore with the technique.6. Anticlockwise magnetic stimulation never recruited a P1 peak; in most subjects a P3 peak was recruited first and at higher intensities was accompanied by P0 or P2 peaks. 15PHY 412 B. L. DA ' AND) OTHERS 7. On most occasions when more than one peak was observed in a PSTH, the uniit fired in only one of the preferred intervals after each shock. However, double firing was seen in five units when high intensities of stimulation were used. The intervals between the two discharges was the same as the intervals between peaks in the PSTH.8. Surface EMG responses in the FDI muscle behaved in a way predietable from the behaviour of the single motor units which had been studied.9. These results are discussed in terms of the D and I wave hypothesis proposed for responses of pyramidal tract neurones to surface anodal stimulation of the exposed motor cortex in primates.
Application of a small (around 1 mA), constant electric current between the mastoid processes (galvanic stimulation) of a standing subject produces enhanced body sway in the approximate direction of the ear behind which the anode is placed. We examined the electromyographic (EMG) responses evoked by such stimulation in the soleus and in the triceps brachii muscles. For soleus, subjects stood erect, with their eyes closed, leaning slightly forward. The head was turned approximately 90 degrees to the right or left relative to the feet. In averaged records (n = 40), current pulses of 25 ms or longer modulated the EMG in a biphasic manner: a small early component (latency 62 +/- 2.4 ms, mean +/- SEM) was followed by a larger late component (latency 115 +/- 5.2 ms) of opposite sign, which was appropriate to produce the observed body sway. The early component produced no measurable body movement. Lengthening the duration of the stimulus pulse from 25 to 400 ms prolonged the late component of the response but had little effect on the early component. Short- and long-latency EMG responses were also evoked in the triceps brachii muscle if subjects stood on a transversely pivoted platform and had to use the muscle to maintain their balance in the anteroposterior plane by holding a fixed handle placed by the side of their hip. The latency of the early component was 41 +/- 2.6 ms; the latency of the late component was 138 +/- 4.3 ms and was again of appropriate sign for producing the observed body sway. Galvanic stimulation evoked no comparable responses in either triceps brachii or soleus muscles if these muscles were not being used posturally.(ABSTRACT TRUNCATED AT 250 WORDS)
The aim of this study was to determine whether prolonged, repetitive mixed nerve stimulation (duty cycle 1 s, 500 ms on-500 ms off, 10 Hz) of the ulnar nerve leads to a change in excitability of primary motor cortex in normal human subjects. Motor-evoked potentials (MEPs) generated in three intrinsic hand muscles [abductor digiti minimi (ADM), first dorsal interosseous (FDI) and abductor pollicis brevis (APB)] by focal transcranial magnetic stimulation were recorded during complete relaxation before and after a period of prolonged repetitive ulnar nerve stimulation at the wrist. Transcranial magnetic stimuli were applied at seven scalp sites separated by 1 cm: the optimal scalp site for eliciting MEPs in the target muscle (FDI), three sites medial to the optimal site and three sites lateral to the optimal stimulation site. The area of the MEPs evoked in the ulnar-(FDI, ADM) but not the median-innervated (APB) muscles was increased after prolonged ulnar nerve stimulation. Centre of gravity measures demonstrated that there was no significant difference in the distribution of cortical excitability after the peripheral stimulation. F-wave responses in the intrinsic hand muscles were not altered after prolonged ulnar nerve stimulation, suggesting that the changes in MEP areas were not the result of stimulus-induced increases in the excitability of spinal motoneurones. Control experiments employing transcranial electric stimulation provided no evidence for a spinal origin for the excitability changes. These results demonstrate that in normal human subjects the excitability of the cortical projection to hand muscles can be altered in a manner determined by the peripheral stimulus applied.
The latency and pattern of muscle recruitment in the startle response elicited by unexpected auditory stimulation was determined in 12 healthy subjects. Reflex EMG activity was recorded first in orbicularis oculi. This was of similar latency to the normal auditory blink reflex and, unlike the generalized startle response, persisted despite the frequent presentation of the test stimulus. It is argued that this early latency activity in orbicularis oculi represents a normal auditory blink reflex and is not part of the generalized auditory startle reflex. With the exception of this early latency activity in orbicularis oculi, the relative latencies of both cranial and distal muscles in the auditory startle response increased with the distance of their respective segmental innervations from the caudal brainstem. Thus the earliest EMG activity was recorded in sternocleidomastoid. The recruitment of caudal muscles was relatively slow and the latencies of the intrinsic hand muscles were disproportionately long. The pattern of recruitment of cranial muscles suggests a brainstem origin for the normal startle response. Studies on the auditory startle reflex in animals are reviewed in the light of this finding.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2025 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.