Intracellular synaptic potentials of primate motor cortex neurons during voluntary movement

Classification of Cortical Neurons by Spike Shape and the Identification of Pyramidal Neurons

Many investigators who make extracellular recordings from populations of cortical neurons are now using spike shape parameters, and particularly spike duration, as a means of classifying different neuronal sub-types. Because of the nature of the experimental approach, particularly that involving nonhuman primates, it is very difficult to validate directly which spike characteristics belong to particular types of pyramidal neurons and interneurons, as defined by modern histological approaches. This commentary looks at the way antidromic identification of pyramidal cells projecting to different targets, and in particular, pyramidal tract neurons (PTN), can inform the utility of spike width classification. Spike duration may provide clues to a diversity of function across the pyramidal cell population, and also highlights important differences that exist across species. Our studies suggest that further electrophysiological and optogenetic approaches are needed to validate spike duration as a means of cell classification and to relate this to well-established histological differences in neocortical cell types.

Intracellular neuronal recording in awake nonhuman primates

Intracellular neuronal recordings from the brain of awake nonhuman primates have remained difficult to obtain because of several formidable technical challenges, such as poor recording stability and difficulties in maintaining long-term recording conditions. We have developed a technique to record neuronal activity by using a coaxial guide tube and sharp electrode assembly, which allows researchers to repeatedly and reliably perform intracellular recordings in the cortex of awake marmosets. Recordings from individual neurons last from several minutes to more than an hour. A key advantage of this approach is that it does not require dura removal, permitting recordings over weeks and months in a single animal. This protocol describes the step-by-step procedures for construction of a custom-made marmoset chair, head-cap implantation, preparation of the sharp electrode and guide tube, neuronal recording and data analysis. As the technique is practical and easy to adapt, we anticipate that it can also be applied to other mammalian models, including larger-size nonhuman primates.

Space-Time Dynamics of Membrane Currents Evolve to Shape Excitation, Spiking, and Inhibition in the Cortex at Small and Large Scales

In the cerebral cortex, membrane currents, i.e., action potentials and other membrane currents, express many forms of space-time dynamics. In the spontaneous asynchronous irregular state, their space-time dynamics are local non-propagating fluctuations and sparse spiking appearing at unpredictable positions. After transition to active spiking states, larger structured zones with active spiking neurons appear, propagating through the cortical network, driving it into various forms of widespread excitation, and engaging the network from microscopic scales to whole cortical areas. At each engaged cortical site, the amount of excitation in the network, after a delay, becomes matched by an equal amount of space-time fine-tuned inhibition that might be instrumental in driving the dynamics toward perception and action.

Expression of Kv3.1b potassium channel is widespread in macaque motor cortex pyramidal cells: A histological comparison between rat and macaque

There are substantial differences across species in the organization and function of the motor pathways. These differences extend to basic electrophysiological properties. Thus, in rat motor cortex, pyramidal cells have long duration action potentials, while in the macaque, some pyramidal neurons exhibit short duration “thin” spikes. These differences may be related to the expression of the fast potassium channel Kv3.1b, which in rat interneurons is associated with generation of thin spikes. Rat pyramidal cells typically lack these channels, while there are reports that they are present in macaque pyramids. Here we made a systematic, quantitative comparison of the Kv3.1b expression in sections from macaque and rat motor cortex, using two different antibodies (NeuroMab, Millipore). As our standard reference, we examined, in the same sections, Kv3.1b staining in parvalbumin‐positive interneurons, which show strong Kv3.1b immunoreactivity. In macaque motor cortex, a large sample of pyramidal neurons were nearly all found to express Kv3.1b in their soma membranes. These labeled neurons were identified as pyramidal based either by expression of SMI32 (a pyramidal marker), or by their shape and size, and lack of expression of parvalbumin (a marker for some classes of interneuron). Large (Betz cells), medium, and small pyramidal neurons all expressed Kv3.1b. In rat motor cortex, SMI32‐postive pyramidal neurons expressing Kv3.1b were very rare and weakly stained. Thus, there is a marked species difference in the immunoreactivity of Kv3.1b in pyramidal neurons, and this may be one of the factors explaining the pronounced electrophysiological differences between rat and macaque pyramidal neurons.

Sensory stimulation shifts visual cortex from synchronous to asynchronous states

In the mammalian cerebral cortex, neural responses are highly variable during spontaneous activity and sensory stimulation. To explain this variability, the cortex of alert animals has been proposed to be in an asynchronous high-conductance state in which irregular spiking arises from the convergence of large numbers of uncorrelated excitatory and inhibitory inputs onto individual neurons. Signatures of this state are that a neuron's membrane potential (Vm) hovers just below spike threshold, and its aggregate synaptic input is nearly Gaussian, arising from many uncorrelated inputs. Alternatively, irregular spiking could arise from infrequent correlated input events that elicit large fluctuations in Vm (refs 5, 6). To distinguish between these hypotheses, we developed a technique to perform whole-cell Vm measurements from the cortex of behaving monkeys, focusing on primary visual cortex (V1) of monkeys performing a visual fixation task. Here we show that, contrary to the predictions of an asynchronous state, mean Vm during fixation was far from threshold (14 mV) and spiking was triggered by occasional large spontaneous fluctuations. Distributions of Vm values were skewed beyond that expected for a range of Gaussian input, but were consistent with synaptic input arising from infrequent correlated events. Furthermore, spontaneous fluctuations in Vm were correlated with the surrounding network activity, as reflected in simultaneously recorded nearby local field potential. Visual stimulation, however, led to responses more consistent with an asynchronous state: mean Vm approached threshold, fluctuations became more Gaussian, and correlations between single neurons and the surrounding network were disrupted. These observations show that sensory drive can shift a common cortical circuitry from a synchronous to an asynchronous state.

Properties of slow oscillation during slow-wave sleep and anesthesia in cats

Deep anesthesia is commonly used as a model of slow-wave sleep (SWS). Ketamine-xylazine anesthesia reproduces the main features of sleep slow oscillation: slow, large-amplitude waves in field potential, which are generated by the alternation of hyperpolarized and depolarized states of cortical neurons. However, direct quantitative comparison of field potential and membrane potential fluctuations during natural sleep and anesthesia is lacking, so it remains unclear how well the properties of sleep slow oscillation are reproduced by the ketamine-xylazine anesthesia model. Here, we used field potential and intracellular recordings in different cortical areas in the cat to directly compare properties of slow oscillation during natural sleep and ketamine-xylazine anesthesia. During SWS cortical activity showed higher power in the slow/delta (0.1-4 Hz) and spindle (8-14 Hz) frequency range, whereas under anesthesia the power in the gamma band (30-100 Hz) was higher. During anesthesia, slow waves were more rhythmic and more synchronous across the cortex. Intracellular recordings revealed that silent states were longer and the amplitude of membrane potential around transition between active and silent states was bigger under anesthesia. Slow waves were mostly uniform across cortical areas under anesthesia, but in SWS, they were most pronounced in associative and visual areas but smaller and less regular in somatosensory and motor cortices. We conclude that, although the main features of the slow oscillation in sleep and anesthesia appear similar, multiple cellular and network features are differently expressed during natural SWS compared with ketamine-xylazine anesthesia.

The pre-movement component of motor cortical local field potentials reflects the level of expectancy

Cortical local field potentials (LFPs) are modulated in parallel with single neuron discharge, but the information they carry is often unclear. Multi-electrode recordings of both LFPs and single neuron activities were made in motor cortex as monkeys performed a delayed pointing task in which the probability of the moment of signal occurrence, and thus movement execution, was manipulated. A large positive LFP component (P1) appeared immediately preceding movement onset only under conditions of low probability, that is, when a response signal was weakly expected. The amplitude of P1 was much smaller when probability of signal occurrence was high, or when the same movement was self-paced. Although P1 has been described as being linked to the descending motor signal, we found that it was more closely associated with the processing of movement-related information than with the ultimate motor command. Its timing did not bear a fixed relationship with movement onset and its frequency of occurrence in each monkey varied in parallel with each animal's overall performance and the percentage of context-related "pre-processing" neurons encountered.

Existing motor state is favored at the expense of new movement during 13-35 Hz oscillatory synchrony in the human corticospinal system

Oscillations in local field potentials in the beta-frequency band (13-35 Hz) are a pervasive feature of human and nonhuman primate motor cortical areas. However, the function of such synchronous activity across populations of neurons remains unknown. Here, we test the hypothesis that beta activity may promote existing motor set and posture while compromising processing related to new movements. Three experiments were performed. First, healthy subjects were instructed to make reaction time movements of the outstretched index finger in response to imperative cues triggered by transient increases in corticospinal synchrony, as evidenced by phasic elevations of beta-frequency band microtremor and intermuscular synchrony. Second, healthy subjects were instructed to resist a stretch to the index finger triggered in the same way. Finger acceleration in the reaction time task and transcortical components of the stretch reflex were measured and compared with those elicited by random cue or stretch presentation. Finally, we sought a correlation between finger acceleration in the reaction time task and cortical synchrony directly measured from the electrocorticogram in two patients undergoing functional neurosurgery. We demonstrate that movements are slowed and transcortical responses to stretch are potentiated during periods of elevated beta-band cortical synchrony. The results suggest that physiological periods of beta synchrony are associated with a cortical state in which postural set is reinforced, but the speed of new movements impaired. The findings are of relevance to Parkinson's disease, in which subcortical and cortical beta-band synchronization is exaggerated in the setting of increased tone and slowed movements.

Characteristic membrane potential trajectories in primate sensorimotor cortex neurons recorded in vivo

We examined the membrane potentials and firing properties of motor cortical neurons recorded intracellularly in awake, behaving primates. Three classes of neuron were distinguished by 1) the width of their spikes, 2) the shape of the afterhyperpolarization (AHP), and 3) the distribution of interspike intervals. Type I neurons had wide spikes, exhibited scoop-shaped AHPs, and fired irregularly. Type II neurons had narrower spikes, showed brief postspike afterdepolarizations before the AHP, and sometimes fired high-frequency doublets. Type III neurons had the narrowest spikes, showed a distinct post-AHP depolarization, or "rebound AHP" (rAHP), lasting nearly 30 ms, and tended to fire at 25-35 Hz. The evidence suggests that an intrinsic rAHP may confer on these neurons a tendency to fire at a preferred frequency governed by the duration of the rAHP and may contribute to a "pacemaking" role in generating cortical oscillations.

Synaptic interactions between primate precentral cortex neurons revealed by spike-triggered averaging of intracellular membrane potentials in vivo

To document synaptic interactions between neurons in the precentral cortex of macaque monkeys, we recorded in vivo the intracellular (IC) membrane potentials of cortical neurons simultaneously with extracellular (EC) action potentials of neighboring cells. The synaptic potentials correlated with EC spikes were obtained by spike-triggered averages (STA) of the IC membrane potentials for 373 cell pairs recorded in anesthetized and awake behaving monkeys. Sixty-three STAs (17%) showed excitatory postsynaptic potentials (EPSPs), beginning after the trigger spike. Pure EPSPs had onset latencies of 0.9 +/- 0.7 msec (mean +/- SD) and amplitudes of 226 +/- 130 microV. Sixteen STAs (4%) showed postspike inhibitory postsynaptic potentials (IPSPs), with onset latencies of 0.4 +/- 0.4 msec and amplitudes of -274 +/- 188 microV. The most common waveform, observed in 82% of the STAs with features, was a broad depolarization straddling the trigger spikes, reflecting synchronized synaptic input to both IC and EC neurons. These average synchronous excitation potentials (ASEPs) began 14.3 +/- 6.6 msec before the trigger spike and had amplitudes of 1064 +/- 867 microV. Twenty-three STAs (6%) showed an average synchronous inhibitory potential (ASIP): a hyperpolarization beginning before the trigger spike and reflecting IPSPs produced by a group of local inhibitory cells synchronized with the trigger cell. ASIPs had an onset latency of -5.5 +/- 2.7 msec and amplitude of -589 +/- 502 microV. Combinations of synchronous and postspike potentials were also observed. Successive recordings provided examples of convergent and divergent connections between EC and IC cells. Neuron pairs with depolarizing postsynaptic potentials (PSPs) in the STA yielded peaks in the cross-correlograms of the IC and EC action potentials; the peak area was proportional to the amplitude of the PSP. These data suggest that a significantly larger proportion of cortical neurons interact through synchronous activity than through simple serial interactions; moreover, synchronous excitation affected more widely separated cell pairs than EPSPs and IPSPs, which were seen most often among the closest cells.

Relationships between axonal diameter, soma size, and axonal conduction velocity of HRP-filled, pyramidal tract cells of awake cats

Relationships between axonal diameter, soma size, and axonal conduction velocity were examined in intracellularly recorded pyramidal tract (PT) cells of conscious cats using pressure injection of horseradish peroxidase. Positive linear correlations were found between axonal conduction velocities and axonal diameters as well as between axonal conduction velocities and soma sizes. All PT cells had somata located in layer V. Slow PT cells had high densities of dendritic spines in layer III; however, so did some fast PT cells, making this morphologic feature unacceptable for distinguishing between slow and fast conducting PT neurons.

Sustained excitatory synaptic input to motor cortex neurons in awake animals revealed by intracellular recording of membrane potentials

1. Most of the intracellular electrophysiological data on cortical neurons has been obtained in anesthetized or reduced preparations, and differs from observations in awake, intact animals. To determine whether these differences are due to experimental techniques or physiological factors, we recorded membrane potentials intracellularly from motor cortex neurons in chronically prepared cats and monkeys under Nembutal-anesthetized, Halothane-anesthetized, and unanesthetized conditions, or during transitions between anesthetized and awake conditions. 2. Resting membrane potentials were found to depend on the anesthetic state of the animal. Membrane potentials of neurons recorded in awake animals were more depolarized than those recorded in the anesthetized state. In the awake state membrane potentials were all less than -65 mV. 3. The input resistance of neurons recorded in awake animals were significantly smaller than those measured in the anesthetized state. Action potentials recorded in awake animals typically showed an undershoot (i.e. negative values at peak), implying that voltage-dependent conductances may be altered. Undershoot of the action potential was more prominent in pyramidal tract neurons (PTNs) than non-PTNs. 4. These data suggested that in awake animals motor cortex neurons, especially PTNs, receive sustained excitatory synaptic input or neuro-modulatory activities.

Effects of behaviorally rewarding hypothalamic electrical stimulation on intracellularly recorded neuronal activity in the motor cortex of awake monkeys

Effects of hypothalamic stimulation (HS) were studied in intracellular recordings obtained from 125 neurons of the motor cortex (MC). HS that was effective in reinforcing bar-press behavior, i.e. satisfactory for intracranial self-stimulation (ICSS), evoked short-latency (less than 3 ms) activation of these cortical neurons more frequently (42% of cells tested) than did HS that was ineffective in reinforcing bar-press behavior (7% of cells tested). Longer latency activation (greater than 3 ms) and inhibition (of variable onset) also occurred, but their incidence was not significantly different when HS was effective or ineffective in producing ICSS. Effects of HS that was effective in producing ICSS were also examined in 23 cells in which the spikes were followed by afterhyperpolarization (AHP) of 1.4-10 mV amplitude and 1.7-54 ms duration. The amplitudes of AHPs of greater than 8 ms duration were reduced after presentations of HSs that were effective as a reinforcer for ICSS. These results suggest that: (1) MC neurons receive reward-related hypothalamic information through pathways sufficiently direct to produce short-latency activation; and (2) a modulation of spike afterhyperpolarization can be observed in conjunction with reception of this information.

Long-term increases in excitability of facial motoneurons and other neurons in and near the facial nuclei after presentations of stimuli leading to acquisition of a Pavlovian conditioned facial movement

Levels of neuronal excitability to injected current were measured intracellularly in facial motoneurons and other neurons in and near the facial nuclei of three groups of awake cats: a "Conditioned" group consisting of animals that had previously received sufficient numbers of paired presentations of click CSs and glabella tap USs to produce eyeblink CRs; a "US-only" group that had received presentations of the USs only; and a "Naive" group that had received neither of these stimuli. Thresholds of intracellularly applied, depolarizing pulse currents required to elicit repeatable spike activity were significantly lower in the "Conditioned" and "US-only" groups than in the "Naive" group. The increased levels of neuronal excitability were correlated with increases in neuronal input resistance. Levels of neuronal excitability remained elevated when measured more than a month after presentations of both CSs and USs, whereas the increases in neuronal excitability decayed within a few weeks in animals given USs only. The increases in neuronal excitability and input resistance following repetitive presentations of glabella tap USs alone appeared to support a latent facilitation of motor performance reflected by an absence of a blink CR to click CS after such presentations but an increased rate of acquisition of subsequent eyeblink conditioning using paired click CS and tap US. The rate of eyeblink conditioning was found to be accelerated in a group of cats given repetitive presentations of tap USs seven days prior to conditioning with paired CSs and USs, compared to a group that was not given USs or CSs before similar conditioning. These findings provide direct, in vivo evidence that increases in the excitability and input resistance of neurons in and near the facial nucleus can occur in cats following presentations of the stimuli used for conditioning.

Cross correlation studies in primate motor cortex: synaptic interaction and shared input

Awake, unrestrained monkeys were trained to reach out with the forelimb and touch a button. Extracellular spike trains were recorded from pairs of neurons in contralateral precentral cortex with the same or separate microelectrodes. The neurons were located in the same or different functional columns as defined by intracortical microstimulation and passive sensory stimulation. Cross correlation analysis showed patterns consistent with synaptic excitation and/or inhibition between members of the cell pairs during the voluntary movement. The strength of correlation was inversely related to distance between columns, with the strongest correlations found between cells within the same column. Inhibitory correlations were virtually restricted to cell pairs within a single column. Temporal analysis showed that direct synaptic interaction and shared input patterns could be clearly distinguished in this physiologic setting. Spatial analysis indicated that shared input was concentrated among columns in the same and adjacent joint controlling zones as well as within a single column. No directional preference of shared input was present, a finding which was consistent with the observed nested organization of the forelimb area.