Biomedical subjects

Timothy S Miles

Publications and source records attributed to Timothy S Miles.

25 records · Page 2Linked to original sources

PubMed2002

Dexterity is not affected by fatigue-induced depression of human motor cortex excitability.

Following a fatiguing muscle contraction, motor evoked potentials (MEPs) evoked by transcranial magnetic brain stimulation remain depressed for many minutes, reflecting a reduction in the excitability of the corticospinal projection. No functional significance has been linked to this observation. We postulated that dexterity would be affected when MEPs are depressed. MEPs were recorded from the first dorsal interosseous muscle of 11 healthy subjects in resting muscles before and after a fatiguing maximum voluntary contraction. This induced significant MEP depression in all subjects. No change in dexterity was seen when MEPs were depressed or at any time point throughout the experiment. We conclude that fatigue-induced MEP depression is not associated with a decline in dexterity.

Adult↗

PubMed2002

Induction of persistent changes in the organisation of the human motor cortex.

Motor learning must involve changes in the organisation of the brain, and it seems axiomatic that afferent signals generated during repeated motor practice contribute to this. In this study, motor-point stimulation of the first dorsal interosseous (FDI) muscle was paired with transcranial magnetic stimulation of the human motor cortex on three successive days to determine whether repeated stimulation sessions result in enduring reorganisation of the motor cortex. This repeated "dual" stimulation induced significant changes in the excitability of the motor cortex together with expansion of the area of scalp from which these responses were elicited. The expansion in muscle representation was accompanied by large movements in the centre of gravity (CoG), suggesting a true reorganisation of the underlying cortical representational zone. The changes persisted for at least 2 days following the last stimulation session. It is concluded that repeated dual stimulation is capable of inducing long-lasting reorganisation within the motor cortex. These changes may be similar in nature to those seen in the motor cortex during motor learning. Moreover, these observations suggest that it may be possible to induce the motor cortex of patients who have suffered strokes to reorganise in a way that improves the voluntary control of the weakened muscles.

Adult↗

PubMed2002

Afferent and cortical control of human masticatory muscles.

Like most other muscles, the human masticatory muscles are controlled by descending signals from the cortex and other supraspinal structures, as well as afferent signals arising in receptors in muscles, skin and other tissues. However, the special functional roles of the masticatory system, and in particular the fact that the muscles on both sides are usually used together, has led to some special adaptations of function.

Humans↗

PubMed2002

A simple and inexpensive system for monitoring jaw movements in ambulatory humans.

A simple and inexpensive method for recording vertical movements of the human mandible relative to the maxilla is presented. Measurements are made from accelerometers and a Hall-effect device temporarily glued to the upper and lower anterior teeth. The accelerometer signals are integrated once to give velocity and a second time to give position. Movements of the mandible relative to the maxilla are obtained by integrating the difference between the two accelerometer signals. The (relative) velocity and position records derived in this way are linear, but subject to drift when the jaw is stationary. Steady mandibular position is obtained from the Hall-effect system, but this signal must be corrected for its inherent non-linearity. This device can record rapid movements of the mandible even when the head is unrestrained, and interferes minimally with normal jaw movements.

Diagnostic Equipment↗

PubMed2002

Stability of maps of human motor cortex made with transcranial magnetic stimulation.

Cortical representation maps derived by transcranial magnetic stimulation (TMS) are often used, inter alia, in studying the plasticity of the brain. Parameters such as map area, map volume, optimal stimulation site and centre of gravity are commonly used to quantify changes in the topography of the motor cortex. However, reports on the stability of these parameters over time has not been conclusive. In the present study, the areas of the scalp from which responses were evoked from corticospinal cells projecting to three intrinsic hand muscles were systematically mapped with TMS at intervals of 24 hours, one week and two weeks from eight normal subjects. The area, "volume" and centre of gravity of these maps did not change significantly over this period. It is concluded that mapping with TMS is suitable for studies which aim to study the effect of various interventions on the cortical representation of individual muscles in human subjects.

Adult↗

PubMed2002

Alterations in corticospinal excitability with imposed vs. voluntary fatigue in human hand muscles.

We aimed to determine whether postexercise depression of motor-evoked potentials (MEPs) could be demonstrated without voluntary muscle activation in humans. Voluntary fatigue was induced with a 2-min maximal voluntary contraction (MVC) of the first dorsal interosseous (FDI) muscle. On another occasion, "electrical fatigue" was induced with trains of shocks delivered for 2 min over the FDI motor point. Five of the twelve subjects also underwent "sequential fatigue" consisting of a 2-min MVC of FDI followed by 20 min of rest and then 2 min of motor point stimulation. Voluntary fatigue induced MEP depression that persisted for at least 20 min. Electrical fatigue induced a transient MEP facilitation that subsided 20 min after the stimulation and became depressed within 30 min. Thus MEP depression can be induced by both voluntary and electrical fatigue. With electrical fatigue, the initial depression is "masked" by transient MEP facilitation, reflecting cortical plasticity induced by the prolonged electrical stimulation. MEP depression probably reflects tonic afferent input from the exercising muscle that alters cortical excitability without altering spinal excitability.

Adolescent↗

PubMedDate not supplied

Reorganization of the human motor cortex by sensory signals: a selective review.

1. The normal human motor cortex can be made to reorganize by repeated stimulation of proprioceptive inputs, with or without concurrent stimulation of the motor cortex by transcranial magnetic nerve stimulation. Appropriate stimulation induces a focal increase in the excitability of corticospinal projections to specific muscles and, possibly, an increase in the area of the cortex projecting to those muscles. 2. We have shown that repeated stimulation on several successive days causes this 'plastic' reorganization to persist for at least several days. We have also used this approach to determine whether increases in the excitability of the motor cortex can be induced in stroke patients (in whom cortical excitability is usually depressed) and whether this is accompanied by functional changes. 3. The results of these studies were mixed but, in patients in whom plastic changes were induced, there were improvements and sometimes marked improvements in both motor function and some electrophysiological parameters. The reasons for the inconsistent results are not clear, but do not appear to relate to the site, size or nature of the lesion.

Electrophysiology↗