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Biomedical subjects

K A Nithi

Publications and source records attributed to K A Nithi.

5 recordsLinked to original sources
PubMed2000

Mapping motor cortex projections to single motor units in humans with transcranial magnetic stimulation.

We devised a method to investigate the cortical organization of corticomotoneurons (CMs) to upper limb muscles. A spike-triggering technique was used, in which a tonically discharging single motor unit (SMU) triggered transcranial magnetic stimulation (TMS) of motor cortex, and the probability of producing short-latency discharges (primary excitatory responses [PERs]) was measured. PER probabilities were mapped in 34 SMUs, using a 16 cm(2) scalp grid with the central reference point having a probability of 0.5. Maps showed a single optimum point of scalp stimulation and significant decreases in PER probability with shifts of 2 cm from this point, for all subjects. These findings suggest that the colony of CMs projecting to an individual SMN is contained within a small volume of motor cortex. Changes in PER probability with shifts in stimulation site may reflect the organization of other intracortical neurons mediating TMS activation of these CMs.

Adult↗
PubMed1998

Peripheral and central motor conduction in amyotrophic lateral sclerosis.

Conventional peripheral motor conduction studies and transcranial magnetic stimulation (TMS) studies, to measure central motor conduction time (CMCT), to the first dorsal interosseous muscle (FDI) were performed on 65 patients with amyotrophic lateral sclerosis (ALS). The hands of each patient were classified into one of four groups depending on the presence of physical signs of lower motor neurone (LMN) and/or upper motor neurone (UMN) involvement. Statistical analysis was made of the results from patients compared with previously established normal values and with those from a control group of 53 normal subjects. Results between the four groups of patients were compared in order to assess any correlation between neurophysiological findings and physical signs. A reduction in the amplitude of compound muscle action potentials (CMAP), prolongation of distal motor latency (DML) and F wave latency were found in 36%, 34% and 19% of hands respectively. These abnormalities were more common in hands with LMN signs. In nine hands, prolongation of DML occurred in the absence of muscle wasting or weakness. CMCT abnormalities were present in 17% of patients with ALS but did not appear to correlate with physical signs.

Action Potentials↗
PubMed1997

Corticomotor threshold to magnetic stimulation: normal values and repeatability.

The threshold for corticomotor excitation by magnetic stimuli is a clinically important parameter in the assessment of central motor conduction. Hitherto, arbitrary criteria have been adopted to define threshold and there are few published data using a figure of eight coil. A new method is described in which the highest stimulus intensity evoking responses in the relaxed first dorsal interosseous muscle with a probability of zero [lower threshold (LT)] and the lowest intensity evoking responses with a probability of one [upper threshold (UT)] have been determined. The method, which utilizes a coil located and optimally oriented over hand motor cortex, minimizes the number of stimuli needed. Both LT and UT are normally distributed and are independent of age, gender, and hemisphere. In 102 healthy hands of 55 control subjects 23-84 years of age, mean +/- SD UT was 46.6 +/- 9.4% and LT was 38.0 +/- 8.6% maximum stimulator output. Repeatability estimates indicated an absolute change of more than 13% or 11% in UT or LT respectively in the same individual over 1-3 months was significant at the 5% level.

Adolescent↗
PubMed1997

Corticomotor threshold is reduced in early sporadic amyotrophic lateral sclerosis.

The pathogenesis of idiopathic amyotrophic lateral sclerosis (ALS) remains unknown, but accumulating evidence suggests a neuroexcitotoxic mechanism may have some credence. Glutamate-induced hyperexcitability of cortical or spinal motoneurons may be expected to manifest itself as a reduced threshold for activation of these structures. We have measured corticomotor threshold to the first dorsal interosseous (FDI) muscles of 48 patients with sporadic ALS using magnetic brain stimulation and have correlated the findings with physical signs of upper and/or lower motor neuron degeneration. We find that if FDI in patients with ALS shows no weakness, wasting, or signs of an upper motor neuron lesion, mean corticomotor threshold is significantly lower than in 102 healthy control FDI muscles (P = 0.02). In contrast, FDI muscles showing signs of lower motor neuron degeneration only or mixed upper and lower motor neuron signs are associated with a raised corticomotor threshold (P = 0.008, P < 0.001, respectively). We conclude that early in ALS, at a time when hand muscle function is normal, corticomotor threshold is reduced and suggest that this may be a manifestation of abnormal excitability of cortical or spinal motoneurons to neurotransmitters, whose action will ultimately lead to cell death.

Aged↗