Inhibition of Betz cell activity by thalamic and cortical stimulation

Short-term adaptations in spinal cord circuits evoked by repetitive transcranial magnetic stimulation: possible underlying mechanisms

Repetitive transcranial magnetic stimulation (rTMS) has been shown to induce adaptations in cortical neuronal circuitries. In the present study we investigated whether rTMS, through its effect on corticospinal pathways, also produces adaptations at the spinal level, and what the neuronal mechanisms involved in such changes are. rTMS (15 trains of 20 pulses at 5 Hz) was applied over the leg motor cortical area in ten healthy human subjects. At rest motor evoked potentials (MEPs) in the soleus and tibialis anterior muscles were facilitated by rTMS (at 1.2xMEP threshold). In contrast, the soleus H-reflex was depressed for 1 s at stimulus intensities from 0.92 to 1.2xMEP threshold. rTMS increased the size of the long-latency depression of the soleus H-reflex evoked by common peroneal nerve stimulation and decreased the femoral nerve facilitation of the soleus H-reflex. These observations suggest that the depression of the H-reflex by rTMS can be explained, at least partly, by an increased presynaptic inhibition of soleus Ia afferents. In contrast, rTMS had no effect on disynaptic reciprocal Ia inhibition from ankle dorsiflexors to plantarflexors. We conclude that a train of rTMS may modulate transmission in specific spinal circuitries through changes in corticospinal drive. This may be of relevance for future therapeutic strategies in patients with spasticity.

Neocortical seizures: initiation, development and cessation

Different forms of electrical paroxysms in experimental animals mimic the patterns of absence seizures associated with spike-wave complexes at approximately 3 Hz and of Lennox-Gastaut seizures with spike-wave or polyspike-wave complexes at approximately 1.5-2.5 Hz, intermingled with fast runs at 10-20 Hz. Both these types of electrical seizures are preferentially generated during slow-wave sleep. Here, we challenge the hypothesis of a subcortical pacemaker that would account for suddenly generalized spike-wave seizures as well as the idea of an exclusive role of synaptic excitation in the generation of paroxysmal depolarizing components, and we focus on three points, based on multiple intracellular and field potential recordings in vivo that are corroborated by some clinical studies: (a) the role of neocortical bursting neurons, especially fast-rhythmic-bursting neurons, and of very fast oscillations (ripples, 80-200 Hz) in seizure initiation; (b) the cortical origin of both these types of electrical paroxysms, the synaptic propagation of seizures from one to other, local and distant, cortical sites, finally reaching the thalamus, where the synchronous cortical firing excites thalamic reticular inhibitory neurons and thus leads to steady hyperpolarization and phasic inhibitory postsynaptic potentials in a majority of thalamocortical neurons, which might explain the obliteration of signals from the external world and the unconsciousness during absence seizures; and (c) the cessation of seizures, whose cellular mechanisms have only begun to be investigated and remain an open avenue for research.

The Ferrier Lecture: The nature of central inhibition

I feel greatly honoured by the invitation to give the Ferrier Lecture. I attended the first Ferrier Lecture, given by Sherrington in 1929, and I learned from Sherrington to value and admire the pioneer contributions of David Ferrier to neurology. In choosing the subject of inhibition for my lecture I was prompted by the peculiar challenge that inhibition has presented to physiologists ever since it was first demonstrated by the Weber brothers in 1846 that stimulation of the vagus nerve could stop the heart and by Setchenov in 1863 that stimulation of areas in the brain could slow or prevent reflex responses of frog limbs. It was Sherrington who greatly extended and organized knowledge of inhibition in the central nervous system; first, by a series of remarkable investigations, and finally by a theoretical paper published by the Royal Society in 1925, in which excitation and inhibition were given equivalent status in the synaptic mechanisms controlling neuronal discharge. His interest in central inhibition continued to the end of his scientific life, and was the subject of his Nobel Lecture in 1932. I might mention that both my first scientific paper and my D.Phil. thesis were concerned with inhibition, and that I have continued to be more interested in the problem of synaptic inhibition than in any other aspect of neurophysiology. In recent years progress has been so rapid that our understanding of the nature of central inhibition is in several respects more complete than that of central excitation. This illumination has followed rather rapidly upon a long period of ingenious theorizing which is now only of historical interest

Altered patterns of evoked synaptic activity in cortical pyramidal neurons in feline ganglioside storage disease

Postsynaptic potentials evoked by ventrolateral thalamic stimulation were recorded intracellularly from neurons in the precruciate cortex of GM1 mutants with HRP- or LY-loaded microelectrodes. Ganglioside-laden pyramidal neurons exhibiting somal distention and/or meganeurite formation were found to respond to thalamic stimulation with short duration IPSPs. Evoked EPSPs were recorded from two morphologically characterized large basket intrinsic neurons which deployed extensive intracortical axonal arborizations. These findings point to the preservation of intracortical inhibitory networks in the feline model of GM1 gangliosidosis, and to the possibility of abnormal integration of somadendritic inputs in ganglioside-laden pyramidal neurons.

A qualitative and quantitative electron microscopic study of the neurons in the primate motor and somatic sensory cortices

A study has been made of the neuronal somata in the motor and somatic sensory cortices of the monkey. Pyramidal cells in the motor cortex are very similar to those described previously in sensory and parietal cortical areas. The largest pyramidal cells in area 4, the Betz cells of layer V, are up to 50 μm in transverse diameter. Although basically resembling smaller pyramidal cells, the nucleus of a Betz cell often has a complex indentation and is smaller in relation to the overall size of the cell soma than is that of a smaller pyramid and the cytoplasm of Betz cells contains discrete clumps of endoplasmic reticulum. As with other pyramidal cells, the synapses on to Betz cell somata are all of the symmetrical type. Previous descriptions of stellate cells have been of cells receiving a high density of axosomatic synapses of both the asymmetric and symmetrical type. Cells like this are found in both the motor and somatic sensory cortices and have been termed here large stellate cells. In addition to their high density of axosomatic synapses, they have abundant cytoplasm full of organelles and usually contain stacks of endoplasmic reticulum. Their dendrites similarly receive a high density of asymmetric and symmetrical synapses and contain prominent organelles and have a moderately varicose shape. Large stellate cells occur predominantly in layer IV in area 3 b but in the motor cortex they are also found commonly in the lower part of layer III and the upper part of layer V. A third class of neuron has been described in both the motor and somatic sensory cortices and cells of this type have been termed small stellate cells. These receive a low density of axosomatic synapses, but some of these are of the asymmetric type and they have sparse cytoplasm with few organelles. They have a small rounded or fusiform soma, frequently have a dark nucleus, have no apical dendrite and their axon initial segments are thin and may be directed towards the cortical surface. Most of the rounded small stellate cells occur in layer II whereas those with fusiform somata occur more in the deeper layers of the cortex. A quantitative study was made of the cells in a strip of the same width running through the full depth of the cortex in both cortical areas. The absolute numbers of cells in the strips of the motor and somatic sensory cortices were very similar as were the proportions of each type of neuron, 72 % in each area being pyramidal with 21 % being small stellate and 7 % large stellate in the motor cortex, and 23 % small stellate and 5% large stellate in area 3 b . The quantitative study also provided evidence that large and small stellate cells form two distinct populations rather than being a continuum.

Unit activities in the hypothalamus and midbrain during stimulation of hypothalamic attack sites

Unit activity in the hypothalamus and midbrain of unanesthetized cats was studied during electrical stimulation of the hypothalamus at sites that induced attack and at other comparable sites from which attack was not induced. The changes in the firing of units at distances from 0.7 to 1.9 mm from the site of stimulation were similar to those of units at distances from 2.0 to 5.2 mm. Although stimulation in general affected a majority of units, resulting in increases rather than in decreases of firing rates, and produced similar patterns of unit activity, stimulation at sites that induced attack affected more units, produced a greater change in unit activity and markedly increased firing rates in the lateral hypothalamus and the dorsal part of the midbrain.

An inhibitory process in the cerebral cortex

1. In cats, rabbits and monkeys, single cortical shocks can reduce the excitability of cortical neurones for 100-300 msec; the inhibitory effect is readily demonstrated, even in previously quiescent cells, against a background of activity evoked with small amounts of L-glutamate, released from an extracellular recording micropipette by iontophoresis.2. Other forms of cortical activity are also inhibited in a similar way by direct or indirect cortical stimulation; they include single unit discharges produced by iontophoretic applications of ACh or by a cathodal current, spontaneous discharges, and slow wave activity, both spontaneous and evoked.3. Most stimuli which elicit cortical activity also evoke some inhibition in the cortex, for instance, transcallosal volleys, and thalamic or peripheral shocks. In each case, a characteristic, prolonged depression is produced by single shocks.4. The most effective stimuli are direct cortical shocks, especially when applied within the cortex, below a depth of 0.6 mm; surface cathodal shocks are more effective than anodal shocks. These stimuli do not first excite the cells which are inhibited and they are not strong enough to cause appreciable local injury.5. Because of its long duration, the inhibition is often readily maintained by repetitive stimulation at frequencies of 5-7/sec. A cumulative effect leads to a further silent period after the end of stimulation; this increases with the strength, frequency and duration of the tetanus, so that after stimulation at 50-100/sec, the silent period may last for over 1 min. During this time, a stronger depolarizing stimulus can initiate firing.6. The inhibitory effect is often preceded and followed by phases of increased excitability; these may also show cumulative enhancement during repetitive stimulation, and a high frequency tetanus often leads to a short after-discharge, which is then followed by a long silent period, as above. Comparable changes take place in rabbits during spreading depression.7. The inhibitory effect of a direct shock can spread over an area covering 1 cm of cortical surface, affecting the cells through all cortical layers; but the spread is uneven in different directions, being particularly poor under most sulci.8. This type of inhibition can be elicited in all areas of the neocortex, and it is evident in kittens within a week of birth.9. Antidromic pyramidal stimulation is very much less effective in evoking inhibition of Betz cells, and other cortical neurones, than direct cortical stimulation; the inhibition by direct shocks is therefore not likely to be mediated through pyramidal excitation.