Transcranial direct current stimulation (tDCS) injects a weak electric current into the brain via electrodes attached to the scalp to modulate cortical excitability. tDCS is used to rebalance brain activity between affected and unaffected hemispheres in rehabilitation. However, a systematic quantitative evaluation of tDCS montage is not reported for the lower limbs. In this study, we computationally investigated the generated electric field intensity, polarity, and co-stimulation of cortical areas for lower limb targeting using high-resolution head models.Volume conductor models have thus been employed to estimate the electric field in the brain. A total of 18 head models of healthy subjects were used to calculate the group-level electric fields generated from four montages of tDCS for modulation of lower limbs.C1-C2 montage delivered higher electric field intensities while reaching deeper regions of the lower-limb motor area. It produced a uniform polarization on the same hemisphere target with comparable intensities between hemispheres but with higher variability.Proper montage selection allows reaching deeper regions of the lower-limb motor area with uniform polarization.First systematic computational study providing support to tDCS experimental studies using montages for the lower limb while considering polarity factor for balancing brain activity.

This chapter turns to the next period in the history of brain studies. It follows up on the previous chapter’s discussion by pinpointing where exactly in the nervous system that consciousness takes place. This chapter thus takes the reader through the findings of the late 19th century that attempted to understand the inner workings of the cerebral cortex, particularly in three key areas: the motor, the visual, and the somatosensory. In doing so, the chapter shows that the identification of the motor and sensory areas accounted for rather more than half of the cortex. It briefly touches upon the questions raised by this topic—in particular the contribution to consciousness—before discussing other aspects of brain maps, including memory and plasticity.

Abstract This chapter tells the story of the discovery of the reticular activating system. At the same time, the chapter traces various attempts to address the larger question of “waking” the cortex and bringing it to a state of consciousness. It turns to two scientists, Horace Magoun and Giuseppe Moruzzi, both of whom conducted experiments to explore the possible effects on the cerebral cortex of stimulating the brain stem. Since the brain’s reticular formation ended just below the thalamus on either side, it was logical to see if it might alter cortical excitability. The chapter shows how Magoun and Moruzzi came to the conclusion that, through its action on the excitability of the cortex, the reticular formation could control the wakefulness of the brain.

Abstract This chapter returns to the subject of gnostic units discussed in Chapter 9, as well cortical columns, both of which form the building blocks of cortical function. Gnostic units are used to describe a neural assembly having knowledge (information). The chapter first expounds on gnostic units and how they relate to the concept/grandmother cells already discussed previously. It then goes on to consider the type of neural structure, which might correspond to a gnostic unit. At the simplest level, electrophysiological recordings have shown that a single neuron could be regarded as a gnostic unit. From here, the chapter conceives of a hierarchy of analyzers in the form of cortical columns. At the highest level will be the column(s) specific for a particular face or object—these cells will fire, and the face or object will be recognized by the conscious brain.

A method is described that, for the first time, allows instantaneous estimation of the Ia fiber input to human soleus motoneurons following electrical stimulation of the tibial nerve. The basis of the method is to determine the thresholds of the most and least excitable 1a fibers to electrical stimulation, and to treat the intervening thresholds as having a normal distribution about the mean; the validity of this approach is discussed. It was found that, for the same Ia fiber input, the percentage of soleus motoneurons contributing to the H (Hoffmann)-reflex differed considerably among subjects; when the results were pooled, however, there was an approximately linear relationship between Ia input and motoneuron output. Weak extension of the great toe diminished the soleus motoneuron reflex discharge in all but 2 of 16 subjects; the results for weak ankle plantarflexion were less consistent, but overall, there was a reduction in soleus motoneuron output also. The methodology should provide new insights into disorders of movement and tone, especially as it permits estimates of motoneuron depolarization to be made. NEW & NOTEWORTHY Assuming a normal distribution of Ia fiber thresholds to electrical stimulation and using the H-reflex, we determined for the first time an Ia input-α-motoneuron output relationship for the human soleus muscle. The relationship varies greatly among subjects but, overall, is approximately linear. Minimal contraction of a toe muscle alters the relationship dramatically, probably due to presynaptic inhibition of Ia fibers. Drawing on the literature, we can calculate changes in α-motoneuron membrane potential.

Transcranial magnetic stimulation (TMS) was first found to be effective in acute migraine 12 years ago, and subsequent studies have confirmed this benefit in approximately two -thirds of treated patients. High response rate, ease of application, and freedom from adverse effects combine to make TMS a natural front-line treatment for migraine, and its use should therefore be encouraged. In relation to the pathogenesis of migraine, the prompt relief of symptoms often observed with TMS is considered incompatible with an underlying neuroinflammatory process and with spreading depression as a cause of aura. Instead, the available evidence points to hyperexcitability of cortical neurons as the immediate cause of headache and any associated symptoms, although the factor(s) leading up to the hyperexcitability remain unclear.

Presynaptic inhibition is a very powerful inhibitory mechanism and, despite many detailed studies, its purpose is still only partially understood. One accepted function is that, by reducing afferent inflow to the spinal cord and brainstem, the tonic level of presynaptic inhibition prevents sensory systems from being overloaded. A corollary of this function is that much of the incoming sensory data from peripheral receptors must be redundant, and this conclusion is reinforced by observations on patients with sensory neuropathies or congenital obstetric palsy in whom normal sensation may be preserved despite loss of sensory fibers. The modulation of incoming signals by presynaptic inhibition has a further function in operating a “gate” in the dorsal horn, thereby determining whether peripheral stimuli are likely to be perceived as painful. On the motor side, the finding that even minimal voluntary movement of a single toe is associated with widespread inhibition in the lumbosacral cord points to another function for presynaptic inhibition: to prevent reflex perturbations from interfering with motor commands. This last function, together with the normal suppression of muscle and cutaneous reflex activity at rest, is consistent with Hughlings Jackson's concept of evolving neural hierarchies, with each level inhibiting the one below it.

The scintillating zigzag pattern that a migraine patient may see as an illusion before the onset of headache offers a unique investigative approach to visual mechanisms. The likeliest interpretation of these zigzags is that they are the spontaneous discharges of the orientation-selective neurons first described in the striate cortex by Hubel and Wiesel (Hubel DH, Wiesel TN. Receptive fields, binocular interaction and functional architecture in the cat's visual cortex. J Physiol (Lond). 1962 Jan;160:106-54; and Hubel DH, Wiesel TN. Receptive fields and functional architecture of monkey striate cortex. J Physiol (London). 1968 Mar;195(1):215-43). Although these cells appear to lie in rows in V1, as Hubel and Wiesel found, very few angles in the visual field are represented; this, and the coarseness of the representation, makes it unlikely that the cells act as feature detectors. The orientation-selective cells could, however, monitor the amount of light falling on the retina and thereby enable color constancy to be achieved. The cells may also serve as coarse movement detectors. The new model of cell organization in human V1 enables us to determine the approximate sizes of the receptive fields of the orientation-selective cells.

INTRODUCTION:The possibility that impulse cross-talk can occur between myelinated human nerve fibers was explored.METHODS:Instances of impulse conduction without decrement were found, and published recordings of compound action potentials of functionally homogeneous fibers were scrutinized.RESULTS:Both analytical approaches yielded results consistent with cross-talk occurring in some nerves after electrical stimulation.CONCLUSIONS:The possible ionic current paths in and out of neighboring fibers, which could be responsible for the phenomenon, have been considered in the light of seminal work on unmyelinated single axons. Muscle Nerve 54: 361-365, 2016.

New evidence concerning the pathophysiology of migraine has come from the results of therapeutic transcranial magnetic stimulation (tTMS). The instantaneous responses to single pulses applied during the aura or headache phase, together with a number of other observations, make it unlikely that cortical spreading depression is involved in migraine. tTMS is considered to act by abolishing abnormal impulse activity in cortical pyramidal neurons and a suggestion is made as to how this activity could arise.

Muscle & NerveVolume 51, Issue 4 p. 623-623 Letter to the Editor Reflections on motor unit number estimation Alan J. McComas MB, Alan J. McComas MB Department of Medicine, McMaster University, Hamilton, Ontario, CanadaSearch for more papers by this author Alan J. McComas MB, Alan J. McComas MB Department of Medicine, McMaster University, Hamilton, Ontario, CanadaSearch for more papers by this author First published: 23 January 2015 https://doi.org/10.1002/mus.24586Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat No abstract is available for this article. References 1 Gooch CL, Doherty TJ, Chan M, Bromberg MB, Lewis RA, Staschuk DW, et al. Motor unit number estimation: a technology and literature review. Muscle Nerve 2014; 50: 884–893. 2 McComas AJ, Fawcett PR, Campbell MJ, Sica RE. Electrophysiological estimation of the number of motor units within a human muscle. J Neurol Neurosurg Psychiatry 1971; 35: 121–131. 3 Galea V, DeBruin H, Cavasin R, McComas AJ. The numbers and relative sizes of motor units estimated by computer. Muscle Nerve 1991; 14: 1123–1130. 4 McComas AJ, Sica REP. Automated quantitative analysis of the electromyogram in partially denervated distal muscles: comparison with motor unit counting. Can J Neurol Sci 1978; 5: 377–383. 5 McComas AJ, Sica REP, Campbell MJ, Upton ARM. Functional compensation in partially denervated muscles. J Neurol Neurosurg Psychiatry 1971; 34: 453–460. 6 McComas AJ. Motor units: how many, how large, what kind? J Electromyogr Kinesiol 1998; 8: 391–402. 7 McComas AJ. Galvani's spark. The story of the nerve impulse. New York: Oxford University Press; 2011. 8 Lucas K. The “all or none” contraction of the amphibian skeletal muscle fibre. J Physiol 1909; 38: 113–133. Volume51, Issue4April 2015Pages 623-623 ReferencesRelatedInformation

Eccles, having returned to Australia from Oxford, is appointed Director of a pathology institute in Sydney, and sets up neurophysiology laboratories there. To assist him, he first recruits Stephen Kuffler and then Bernard Katz, both of them refugees from Nazi-dominated Europe. The three collaborate in examining transmission between nerve and muscle, concluding that the nerve effects are entirely due to acetylcholine, as Dale had proposed.

A method has been developed for measuring the Ia fibre input/motoneurone output relationship for the soleus H-reflex in healthy human volunteers. The shift in the relationship during weak toe extension, and in some subjects during weak plantar flexion, indicates the imposition of an inhibitory mechanism, presumably presynaptic. From these observations, and others previously made on long-loop reflexes, it is argued that the inhibitory mechanism may have evolved to suppress unwanted information from the periphery, not only during movement but in the resting state, and that this development was a necessary accompaniment of encephalisation.

We present a novel instrumentation system for studying tendon and spinal reflexes using a commercial linear servo-motor as a precisely controlled tendon hammer. The system uses a LabVIEW-based program to both control electrical or mechanical stimuli and record and measure the resulting M and H waves. The hammer can deliver tendon taps with selected velocities, durations, frequencies and excursions. Preliminary results for both soleus and flexor carpi radialis muscles show that impact velocity is an important variable in eliciting tendon reflexes. As expected, the tendon reflex amplitude was also found to be dependent on excursion depth, but not as significantly as hammer velocity. Other stimulus paradigms are also presently being investigated

Publisher Summary This chapter deals with the history of neuromuscular system and its events along with the people involved. The muscle system is held responsible for generating force and moment. Moreover, the chapter emphasizes on the events in the muscles and motoneurons. Herophilus (early third century B.C.) has been the first to recognize the involvement of muscles in producing movements and to distinguish between nerves and tendons, as well as between arteries and veins. It was Galen (129-199 A.D.), however, who took the understanding of muscles and nerves carrying out public dissections of human bodies, he experimented on live animals and African monkeys in particular. Henneman was responsible (with Lome Mendell) for pioneering spike triggered averaging, a technique, which has had many applications, including the demonstration of the size principle in human interosseous muscles by Stein. The smallest motor units were the first to be called upon slowly developing contractions, and in certain reflexes. The histochemical analysis of the hindlimb muscles revealed that a steady exercise of moderate intensity (running) made greatest use of slow twitch-oxidative (type I) fibers and maximal intermittent activity (jumping) depended heavily on fast twitch-glycolytic (type IIB) fibers. In the naturally occurring contraction, there was also a progressive reduction in the rate at which the muscle fibers were excited by the motoneurons and that was optimal for delaying fatigue. In some subjects that either electrical stimulation or a supreme effort added increasing amounts of tension as fatigue, progressed during voluntary contractions of the adductor pollicis and quadriceps muscles. Inactivity causes muscles to atrophy and that an even greater loss of bulk may follow nerve damage or disuse. By combining measurements of muscle enzyme activity with those of messenger RNA, continuous muscle stimulation could elevate enzyme activity so rapidly that an increased translation of messenger RNA is involved, with the effects of altered gene expression appearing later. If the maximal rate of shortening is reduced, as in fatiguing muscle, then power must necessarily decline. Also, at the progressively higher rates of limb movement, the contributions of the fast contracting muscle fibers (type II; FF and FR) to power output become increasingly important. Such studies have shown, that shortening contractions, in which power is developed, are more fatiguable than isometric contractions and that this is reflected in greater biochemical changes in the fibers.