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.

6 – Peripheral Motor System

Like muscle afferents, cutaneous afferents are not homogeneous, but the reflex pathways fed by the different types of cutaneous afferent have been documented less well than those fed by muscle afferents. Cutaneous afferents are responsible for a wide range of sensations, but most are also capable of modulating motor behaviour through spinal, supraspinal and transcortical pathways. There is a tendency for clinicians to group all cutaneous afferents together, and this creates confusion, leads to the usage of different terms for the same function and the same term for different functions, and makes the systems appear more complex than necessary. There is heterogeneity in: (i) the type of receptor (e.g. mechanoreceptor, thermoreceptor, nociceptor), (ii) the peripheral afferents (which range from large myelinated Aβ afferents to slow unmyelinated C afferents), (iii) the spinal pathways fed by the afferents (few or many interneurones), (iv) their central projection (spinal, supraspinal, transcortical), (v) the importance of the cutaneous contribution (predominantly ‘private’ vs. shared pathways), and (vi) their functional role (contributing to the normal usage of the limb or responsible for withdrawal from a noxious agent). This heterogeneity is reflected in the terminology: ‘flexor’ or ‘withdrawal’ reflexes are considered nociceptive responses (though mechanoreceptors may play a role in their generation), whereas ‘cutaneomuscular’ reflexes refer to responses involved in the control of normal movement. A thesis of this book, addressed in many chapters, is that cutaneous afferents may have a proprioceptive role, perhaps as important for some movements as that played by muscle spindle afferents, whether that influence is mediated by a primarily ‘private’ pathway or by modulating the activity of some other system.

Cutaneomuscular, withdrawal and flexor reflex afferent responses

The Circuitry of the Human Spinal Cord: Monosynaptic Ia excitation and post-activation depression

The stability of the maximal muscle response (Mmax) is critical to H reflex methodology. It has previously been reported that the amplitude of Mmax declines over time. If reproducible, this finding would have implications for all experimental studies that normalise the output of the motoneurone pool against the M wave. We investigated the effect of time on changes in Mmax and the maximal H reflex (Hmax) evoked at 4-s intervals over 60 min. To identify an influence of homosynaptic depression, we extended the interstimulus interval to 10 s and the time to 100 min. Two recording montages over soleus were used to ensure that interelectrode distance was not a critical factor. The soleus Mmax and H reflex were evoked by stimulation of the tibial nerve in the popliteal fossa in 7 subjects who sat with the knee flexed to 30° and the ankle plantar flexed by ~30°. We found no change in the pooled data for Mmax, Hmax, a reflex 50% of maximal, or the current required to produce it. However, one subject had a statistically significant increase in Mmax and a concurrent decrease in Hmax regardless of the interstimulus interval. On average, there was no change in the Hmax/Mmax ratio over time. While both Mmax and Hmax may change in response to many factors, these results suggest that, typically, time is not one of them.

David Burke

Papers

6 – Peripheral Motor System

Cutaneomuscular, withdrawal and flexor reflex afferent responses

The Circuitry of the Human Spinal Cord: Monosynaptic Ia excitation and post-activation depression

The stability of M max and H max amplitude over time

Heat sensitivity of sensory fibers in carpal tunnel syndrome.