Quantitative input-output relationships between human soleus muscle spindle afferents and motoneurons.

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.

Can cross-talk occur in human myelinated nerve fibers?

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.

Hypothesis: Hughlings Jackson and presynaptic inhibition: is there a big picture?

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.

Transcranial Magnetic Stimulation in Migraine: A New Therapy and New Insights into Pathogenesis.

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.

The Migraine Aura: A Problem for Vision Theory?

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.

Cortical spreading depression in migraine-time to reconsider? / Depressão alastrante cortical na enxaqueca – hora de reconsiderar?

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.

Novas evidências referentes à fisiopatologia da enxaqueca são o resultado de estimulação magnética transcraniana terapêutica (tTMS). As respostas imediatas a pulsos simples aplicados durante as fases de aura ou de cefaleia, em associação a diversas outras observações, tornam improvável a ideia de que a depressão alastrante esteja envolvida na enxaqueca. Considera-se que tTMS tenha sua ação abolindo atividade anormal de impulsos em neurônios corticais piramidais, sugerindo que esta atividade tenha um papel desencadeante.

Cortical spreading depression in migraine-time to reconsider?

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.

Speculations surrounding a spinal reflex.

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.

Tendon reflexes elicited using a computer controlled linear motor tendon hammer.

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.

Evaluation of therapeutic electrical stimulation to improve muscle strength and function in children with types II/III spinal muscular atrophy.

The study aimed to evaluate the effect of low-intensity night-time therapeutic electrical stimulation (TES) on arm strength and function in children with intermediate type spinal muscular atrophy (SMA). The design was a randomized controlled trial with a 6-month baseline control period. Children were evaluated at baseline, 6, and 12 months. TES was applied from 6 to 12 months to the deltoid and biceps muscle, of a randomly selected arm with the opposite arm receiving a placebo stimulator. Thirteen individuals with SMA between 5 to 19 years of age were recruited into the study and eight completed the 12-month assessment. No statistically significant differences between the treatment and control arm were found at baseline, 6, and 12 months for elbow flexors, or shoulder abductors on quantitative myometry or manual muscle testing. There was no significant change in excitable muscle mass assessed by M-wave amplitudes, nor function on the Pediatric Evaluation of Disability Inventory (self-care domain). Therefore, in this study there was no evidence that TES improved strength in children with SMA.