Susan Pockett

Long-term potentiation and depression in the ventral horn of rat spinal cord in vitro.

The aim of this study was to see whether long-term potentiation (LTP) and/or long-term depression (LTD) of synaptic efficacy could be induced in the ventral horn of the rat spinal cord. Transverse slices were cut from neonatal rat spinal cords and maintained in vitro. Field potentials were recorded in the ventral horn in response to stimulation of the dorsal horn-intermediate nucleus region of the slice. Tetanic stimulation at 100 Hz (6 bursts of 50 pulses with 10 s between bursts) resulted in long-term potentiation in 25% of slices, long-term depression in 33% of slices and no long-lasting change of field potential amplitude in the rest. The long-term changes lasted at least 2.5 hours. The principal conclusion of the work is that LTP and LTD can be elicited in the ventral horn.

Long-term depression at synapses in slices of rat hippocampus can be induced by bursts of postsynaptic activity

SummaryIn slices of rat hippocampus, a train of conditioning pulses that would produce long-term potentiation (LTP) if applied to afferent inputs was found to produce a long-lasting depression of Schaffer collateral/ commissural synapses on CA1 cells when instead it was applied to the CA1 axons. The depression lasted undiminished for up to 2 h (the maximum duration of recording). Intracellular recording showed that long-term depression (LTD) of e.p.s.p. amplitude occurred in 66% of cells when this antidromic conditioning stimulation was delivered in normal medium, and in 100% of cells when the antidromic stimulation was delivered in medium containing sufficient Mg++ to block all synaptic transmission. We infer that the difference is because conditioning stimuli sometimes activated test synapses in normal Mg++ but could not in high Mg++. The fact that LTD could be induced in high Mg++ eliminates enhanced inhibitory feedback as a possible mechanism of the long lasting synaptic depression and demonstrates that the mechanism is probably postsynaptic. Resting membrane potential and cell input resistance were the same before and after conditioning, so persisting changes in these postsynaptic parameters can not be the explanation for LTD. LTD of the sort described in this paper could have significant implications for models of learning and memory.

Motor neurotrophic factor in denervated adult skeletal muscle

The presence in a muscle of denervated muscle fibers causes intact nerve terminals to sprout 4. A number of studies have provided indirect evidence that this sprouting is caused by a humoral factor released from inactive muscle fibers 4. However, the effective diffusional range of the sprouting signal within a muscle is only of the order of one muscle fiber diameter or less 3& Thus a viable alternative possibility for the sprouting mechanism is that surface changes on the denervated muscle fibers somehow permit sprouting of intact nerve terminals on adjacent innervated fibers a. If a soluble motor nerve sprout factor does exist, it would be expected by analogy with N G F also to facilitate survival of embryonic motor neurons (as N G F both facilitates survival of embryonic cells 10 and causes sprouting of adult 2 adrenergic neurons). We report here preliminary evidence that denervated adult rat muscle (where sprouting occurs) does contain a factor that permits survival of embryonic motor neurons. Innervated adult muscle (where sprouting does not occur) does not contain such a factor. We used a bioassay for motor neurotrophic factor, similar to that reported by Bennett et a13. A brief description of the method is given here. Under sterile condition 5/zl of a 30 ~ solution of horseradish peroxidase (HRP, Sigma type VI) was injected into the right thigh muscles of 6-7 day chick embryos in ovo. A 50/zm diameter glass microsyringe connected to a 25 #1 Hamilton syringe was used. The right lumbo-sacral hemi cords were removed from embryos which survived for 5 h after injection. Care was taken to remove dorsal root ganglia and meninges. Hemi cords from between 1 and 4 embryos were

Terminal sprouting of motoneurones is a local response to a local stimulus

Terminal sprouting was visualized in silver stained whole mounts of rat sternocostal muscles after resecting one segmental nerve. Sprouts were only found near denervated muscle fibres. The effective diffusion range of the sprout inducing factor released by denervated fibres was 50--100 micrometers. Individual terminals of the same motoneurone sprout independently of each other and only when they intercept the sprout inducing factor.

A method to study global spatial patterns related to sensory perception in scalp EEG

Prior studies of multichannel ECoG from animals showed that beta and gamma oscillations carried perceptual information in both local and global spatial patterns of amplitude modulation, when the subjects were trained to discriminate conditioned stimuli (CS). Here the hypothesis was tested that similar patterns could be found in the scalp EEG human subjects trained to discriminate simultaneous visual-auditory CS. Signals were continuously recorded from 64 equispaced scalp electrodes and band-pass filtered. The Hilbert transform gave the analytic phase, which segmented the EEG into temporal frames, and the analytic amplitude, which expressed the pattern in each frame as a feature vector. Methods applied to the ECoG were adapted to the EEG for systematic search of the beta-gamma spectrum, the time period after CS onset, and the scalp surface to locate patterns that could be classified with respect to type of CS. Spatial patterns of EEG amplitude modulation were found from all subjects that could be classified with respect to stimulus combination type significantly above chance levels. The patterns were found in the beta range (15-22 Hz) but not in the gamma range. They occurred in three short bursts following CS onset. They were non-local, occupying the entire array. Our results suggest that the scalp EEG can yield information about the timing of episodically synchronized brain activity in higher cognitive function, so that future studies in brain-computer interfacing can be better focused. Our methods may be most valuable for analyzing data from dense arrays with very high spatial and temporal sampling rates.

Anesthesia and the Electrophysiology of Auditory Consciousness

Empirical work is reviewed which correlates the presence or absence of various parts of the auditory evoked potential with the disappearance and reemergence of auditory sensation during induction of and recovery from anesthesia. As a result, the hypothesis is generated that the electrophysiological correlate of auditory sensation is whatever neural activity generates the middle latency waves of the auditory evoked potential. This activity occurs from 20 to 80 ms poststimulus in the primary and secondary areas of the auditory cortex. Evidence is presented suggesting that earlier or later waves in the auditory evoked potential do not covary with auditory sensation (as opposed to auditory perception) and it is therefore suggested that they are possibly not the electrophysiological correlates of sensation.

Dopamine changes the shape of action potentials in hippocampal pyramidal cells

Dopamine was found to have two electrophysiological effects on CA1 pyramidal cells in rat hippocampal slices. It increased the slow afterhyperpolarisation caused by a slow Ca2+-activated K+ conductance and it had an effect on action potentials that is postulated to be due to an increase in a fast Ca2+-activated K+ conductance. A given CA1 cell showed either one or both of the two responses to dopamine, or no response.

Membrane properties of deep dorsal horn neurons from neonatal rat spinal cord in vitro

Whole-cell patch-clamp recordings were undertaken to characterize and compare the membrane properties of deep dorsal horn neurons in transverse slices of rat lumbar spinal cord in two age groups, postnatal days (P) 3-6 and 9-16. In both age groups, significant correlations were observed between membrane time constant and cell resistance and between action potential height and its duration at half-maximal amplitude. Cell resistance and action potential half-width values were lower in the P9-16 age group. Neurons were divided into four categories based on their firing properties in response to intracellular current injection: single spike, phasic firing, repetitive firing, and delayed firing. The distribution of neurons within these categories was similar in both age groups which suggests that the firing properties of deep dorsal horn neurons are functionally differentiated at an early postnatal age.

The rotating spot method of timing subjective events

The rotating spot method of timing subjective events involves the subjects watching a rotating spot on a computer and reporting the position of the spot at the instant when the subjective event of interest occurs. We conducted an experiment to investigate factors that may impact on the results produced by this method, using the subjects perception of when they made a simple finger movement as the subjective event to be timed. Seven aspects of the rotating spot method were investigated, using a factorial experiment. Four of these aspects altered the physical characteristics of the computer generated spot or clock face and the remaining three altered the instructions given to the participant. We found compelling evidence that one factor, whether the subject was instructed to report the instant when the finger movement was initiated or the instant when it was completed, resulted in a systematic shift in the response. Evidence that three other factors affect the observed variability in the response was also found. In addition, we observed that there are substantial systematic differences in the responses made by different subjects. We discuss the implications of our findings and make recommendations about the optimal way of conducting future experiments using the rotating spot method. Our overall conclusion is that our results strongly validate the rotating spot method of timing at least the studied variety of subjective event.

Topography, independent component analysis and dipole source analysis of movement related potentials

The objective of this study was to test, in single subjects, the hypothesis that the signs of voluntary movement-related neural activity would first appear in the prefrontal region, then move to both the medial frontal and posterior parietal regions, progress to the medial primary motor area, lateralize to the contralateral primary motor area and finally involve the cerebellum (where feedback-initiated error signals are computed). Six subjects performed voluntary finger movements while DC coupled EEG was recorded from 64 scalp electrodes. Event-related potentials (ERPs) averaged on the movements were analysed both before and after independent component analysis (ICA) combined with dipole source analysis (DSA) of the independent components. Both a simple topographic analysis of undecomposed ERPs and the ICA/DSA analysis suggested that the original hypothesis was inadequate. The major departure from its predictions was that, while activity over many brain regions did appear at the expected times, it also appeared at unexpected times. Overall, the results suggest that the neuroscientific ‘standard model’, in which neural activity occurs sequentially in a series of discrete local areas each specialized for a particular function, may reflect the true situation less well than models in which large areas of brain shift simultaneously into and out of common activity states.