Co-Planar Stereotaxic Atlas of the Human Brain—3-Dimensional Proportional System: An Approach to Cerebral Imaging, J. Talairach, P. Tournoux. Georg Thieme Verlag, New York (1988), 122 pp., 130 figs. DM 268

I have developed "tennis elbow" from lugging this book around the past four weeks, but it is worth the pain, the effort, and the aspirin. It is also worth the (relatively speaking) bargain price. Including appendixes, this book contains 894 pages of text. The entire panorama of the neural sciences is surveyed and examined, and it is comprehensive in its scope, from genomes to social behaviors. The editors explicitly state that the book is designed as "an introductory text for students of biology, behavior, and medicine," but it is hard to imagine any audience, interested in any fragment of neuroscience at any level of sophistication, that would not enjoy this book. The editors have done a masterful job of weaving together the biologic, the behavioral, and the clinical sciences into a single tapestry in which everyone from the molecular biologist to the practicing psychiatrist can find and appreciate his or

Principles of Neural Science

Components of heart rate variability have attracted considerable attention in psychology and medicine and have become important dependent measures in psychophysiology and behavioral medicine. Quantification and interpretation of heart rate variability, however, remain complex issues and are fraught with pitfalls. The present report (a) examines the physiological origins and mechanisms of heart rate variability, (b) considers quantitative approaches to measurement, and (c) highlights important caveats in the interpretation of heart rate variability. Summary guidelines for research in this area are outlined, and suggestions and prospects for future developments are considered.

https://www.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8986.1997.tb02140.x

Heart rate variability: Origins, methods, and interpretive caveats

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for “central” fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal cha...

Spinal and Supraspinal Factors in Human Muscle Fatigue

1. In ten normal volunteers, a transcranial magnetic or electric stimulus that was subthreshold for evoking an EMG response in relaxed muscles was used to condition responses evoked by a later, suprathreshold magnetic or electric test shock. In most experiments the test stimulus was given to the lateral part of the motor strip in order to evoke EMG responses in the first dorsal interosseous muscle (FDI). 2. A magnetic conditioning stimulus over the hand area of cortex could suppress responses produced in the relaxed FDI by a suprathreshold magnetic test stimulus at interstimulus intervals of 1-6 ms. At interstimulus intervals of 10 and 15 ms, the test response was facilitated. 3. Using a focal magnetic stimulus we explored the effects of moving the conditioning stimulus to different scalp locations while maintaining the magnetic test coil at one site. If the conditioning coil was moved anterior or posterior to the motor strip there was less suppression of test responses in the FDI. In contrast, stimulation at the vertex could suppress FDI responses by an amount comparable to that seen with stimulation over the hand area. With the positions of the two coils reversed, conditioning stimuli over the hand area suppressed responses evoked in leg muscles by vertex test shocks. 4. The intensity of both conditioning and test shocks influenced the amount of suppression. Small test responses were more readily suppressed than large responses. The best suppression was seen with small conditioning stimuli (0.7-0.9 times motor threshold in relaxed muscle); increasing the intensity to motor threshold or above resulted in less suppression or even facilitation. 5. Two experiments suggested that the suppression was produced by an action on cortical, rather than spinal excitability. First, a magnetic conditioning stimulus over the hand area failed to produce any suppression of responses evoked in active hand muscles by a small (approximately 200 V, 50 microsecond time constant) anodal electric test shock. Second, a vertex conditioning shock had no effect on forearm flexor H reflexes even though responses in the same muscles produced by magnetic cortical test shocks were readily suppressed at appropriate interstimulus intervals. 6. Small anodal electric conditioning stimuli were much less effective in suppressing magnetic test responses than either magnetic or cathodal electric conditioning shocks.(ABSTRACT TRUNCATED AT 400 WORDS)

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1993.sp019912

Corticocortical inhibition in human motor cortex.

As knowledge of cerebral mechanisms and the role of the direct corticospinal tract has developed, the importance of spinal mechanisms in the control of movement has been underestimated. However this ‘belies the complex interneuronal machinery that resides in the spinal cord … the final movement is only that part of the supraspinally derived programme that the spinal cord circuitry deems appropriate’.1 It is therefore pleasing to read the thought-provoking paper by Ravits and Stack2 on the lower motor neuron (LMN) homunculus. Unlike the cortical homunculus as established by Penfield and Boldrey3 through direct brain stimulation, Ravits and Stack2 did not undertake functional studies to develop the LMN representation. Rather, a pictorial schema was developed based solely around histological anatomy, with what the authors described as aesthetic leeway. And here we have some reservations. In contrast to such an aesthetic approach, having studied the original somatosensory maps derived from animal studies, Penfield and Boldrey3 developed their knowledge of brain network function through direct stimulation during neurosurgical procedures on awake patients, with a combination of simultaneous movement and verbal reports provided by their patients to describe tactile sensations. The patient recordings were combined with these descriptions to establish a comprehensive map of motor and sensory functions in the cerebral cortex of human subjects. This functional approach was not associated with histological analysis, and the authors accepted that they could not produce a ‘functional representation within strict cytoarchitectural boundaries’.3 In the case of establishing a cortical homunculus, data were accumulated from multiple stimulus paradigms for each site, from a cross-section of patients, such that large datasets were established. Using their meticulous approach, Penfield and Boldrey3 were able to determine at first hand that there existed a degree of functional connectivity and overlap between stimulation territories, rather than an orderly sequence. 

The lower motor neuron homunculus: function matters.

Whither needle EMG

WS-10-1 The development of conduction block in human axons

Publisher Summary This chapter focuses on the structure and function of the contractile elements of a muscle. To determine the functional capacity of a muscle, a number of factors must be determined that include the distance from origin to insertion, muscle length, muscle fiber length, and the angle of pennation between muscle fibers and the main force axis. This allows the physiological cross-sectional area (PCSA) of a muscle to be calculated. The final biomechanical factor to consider in the assessment of muscle performance is the mechanical moment arm at each joint crossed by the muscle. Over the usual physiological range, this value can vary systematically, depending on the joint. Irrespective of a motor unit's (MU) types and properties, muscle and joint architecture is neatly adapted to the usual physiological demands placed on the muscle, with some reduction in metabolic cost. There are two small-scale features of a muscle that are being increasingly recognized as altering its function: the first feature considers muscle as a composite material; the muscle fibers are embedded in a complex matrix rather than being parallel elements that generate force only longitudinally, while the second features recognizes that muscle is a tissue with thixotropic properties, which means that its resistance to stretch decreases following lengthening and gradual recovery.

David Burke

Papers

The lower motor neuron homunculus: function matters.

Whither needle EMG

WS-10-1 The development of conduction block in human axons

Chapter 2 Skeletal muscle: structure and function

The Circuitry of the Human Spinal Cord: Plasticity in Spinal and Corticospinal Pathways