K. Nakashima

Electric and magnetic stimulation of human motor cortex: surface EMG and single motor unit responses.

1. 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 electromyographic (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 stimulation). Magnetic stimulation 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 (less than 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 0.5, 1‐2, 2.5‐3.5 and 4‐5.5 ms later. Peaks occurring within these intervals are referred to as P0 (the earliest anodal), 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 P0 peak. 5. Anticlockwise magnetic [corrected] stimulation, at threshold, often produced several peaks. These always included a P1 peak, and usually a P3 peak. A P0 peak in the PSTH was never produced by an anticlockwise stimulation [corrected] at intensities which we could explore with the technique. 6. Clockwise magnetic [corrected] 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. 7. On most occasions when more than one peak was observed in a PSTH, the unit 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.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of digital nerve stimuli on responses to electrical or magnetic stimulation of the human brain.

1. Reflexes were elicited in the first dorsal interosseous muscle of seven normal subjects by electrical stimulation of the digital nerves of the index finger at 3 times perceptual threshold while subjects maintained a constant voluntary contraction of the muscle. The average response in the surface‐rectified electromyogram (EMG) consisted of an early inhibitory (I1) component followed by a later excitation (E2). 2. Low intensity anodal electrical or magnetic scalp stimuli were given over the motor cortex in order to elicit muscle responses within the period of the I1 and E2 reflex components. 3. Compared with control responses elicited in the absence of digital nerve stimulation, responses to electrical cortex stimulation were suppressed in the I1 period and facilitated during the E2 period of the reflex. In contrast, responses evoked by magnetic stimulation were suppressed during I1 and also for the first 10 ms or so of the E2 response. Magnetically evoked responses were facilitated during the later part of the E2 reflex. 4. Similar effects were seen when the probability of firing of single motor units was studied. 5. In three subjects, small taps were given to the abducted index finger in order to stretch the first dorsal interosseous muscle and evoke reflexes which were of comparable size to the E2 reflex evoked by digital nerve stimulation. In contrast to the experiments in which digital nerve stimuli were given, responses evoked by magnetic stimulation over motor cortex were facilitated at all times during the course of the reflex evoked when the muscle was stretched. 6. We conclude that single electrical stimuli applied to the digital nerves can reduce for a short period the excitability of motor cortex to magnetic stimulation. This occurs at a time when the same stimulus is evoking an excitatory (E2) reflex in the average surface‐rectified EMG.

An exteroceptive reflex in the sternocleidomastoid muscle produced by electrical stimulation of the supraorbital nerve in normal subjects and patients with spasmodic torticollis

We examined suppression of EMG activity in the contracting sternocleidomastoid muscles, produced by electrical stimulation of the supraorbital nerve in 10 normal subjects and 9 patients with spasmodic torticollis. This exteroceptive reflex in the sternocleidomastoid muscle consisted of 2 or 3 phases: (1) an early, small, and unstable phase of facilitation, followed by (2) a period of suppression beginning 35 msec after the stimulus, lasting for 35 msec with a reduction in EMG activity to approximately 40% of the prestimulus level, and (3) a further phase of facilitation at a latency of 70 msec, with duration 35 msec and an increase in EMG activity to approximately 35% above prestimulus levels. The latency and duration of the suppressive phase of this reflex were similar to the exteroceptive suppression of EMG activity in the masseter muscle after supraorbital nerve stimulation (masseter silent period). In patients with spasmodic torticollis, the depth of this exteroceptive suppression in the sternocleidomastoid muscles was less than that observed in an age-matched cohort of normal subjects, although the latency and duration were normal. In contrast, exteroceptive suppression in the masseter muscle was normal. These findings suggest abnormal function of inhibitory interneuronal networks between the 5th cranial nerve and the motor neurons of the spinal accessory and upper cervical nerves which mediate exteroceptive suppression in the sternocleidomastoid muscle in patients with spasmodic torticollis.

CUTANEOUS EFFECTS ON PRESYNAPTIC INHIBITION OF FLEXOR-IA AFFERENTS IN THE HUMAN FOREARM

1. In six subjects, H reflexes obtained in the flexor muscles in the forearm were inhibited by single motor threshold shocks to the radial nerve in the spiral groove. The first two phases of the inhibitory time course were studied: with intervals between the radial and median nerve shocks of ‐1 to +1 ms, and +5 to +30 ms. These two phases are thought to be due respectively to disynaptic inhibition between radial Ia afferents and flexor alpha‐motoneurones, and to presynaptic inhibition of flexor Ia afferents. 2. Single or short trains (10 ms, 400 Hz) of cutaneous stimuli to the dorsal or palmar aspect of the proximal phalanx of the index finger or to the superficial radial nerve at the wrist, reduced the amount of presynaptic inhibition by 10‐20%, but had no effect on the earlier disynaptic inhibition. Single stimuli to either side of the index finger or trains of stimuli to the ventral side, had no effect on the size of control H reflexes elicited alone. 3. Effects of cutaneous nerve shocks on presynaptic inhibition could be seen with stimuli as small as 1.5 x perceptual threshold. 4. Anaesthesia of the hand in one subject reversibly increased the amount of presynaptic inhibition and decreased the amount of disynaptic inhibition. 5. We conclude that, as in the cat, cutaneous input can modulate transmission in presynaptic inhibitory pathways in man.