Sensory characteristics of the cortical input of the hippocampus, the dentate fascia]

Investigation of sensory reactions of the neurones of the dentate fascia was performed in unanaesthetized rabbits. Spontaneous activity usually consisted of long aperiodic high-frequency bursts with silent periods between them. Multimodal convergence was observed in 76% of records. Responses to sensory stimuli constituted three nearly equal groups: 1 -- tonic inhibitory reactions; 2--phasic (equal to stimulus duration) activatory responses; 3--simple "specific" responses of on-type, sometimes with secondary phasic components. In one microelectrode track, perpendicular to the double lamina of granular cells, all neurones usually responded in the same fashion.     

Trajectory encoding in the hippocampus and entorhinal cortex

We recorded from single neurons in the hippocampus and entorhinal cortex (EC) of rats to investigate the role of these structures in navigation and memory representation. Our results revealed two novel phenomena: first, many cells in CA1 and the EC fired at significantly different rates when the animal was in the same position depending on where the animal had come from or where it was going. Second, cells in deep layers of the EC, the targets of hippocampal outputs, appeared to represent the similarities between locations on spatially distinct trajectories through the environment. Our findings suggest that the hippocampus represents the animal's position in the context of a trajectory through space and that the EC represents regularities across different trajectories that could allow for generalization across experiences.     

Phase precession in the human hippocampus and entorhinal cortex

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Theta oscillations decrease spike synchrony in the hippocampus and entorhinal cortex

Oscillations and synchrony are often used synonymously. However, oscillatory mechanisms involving both excitation and inhibition can generate non-synchronous yet coordinated firing patterns. Using simultaneous recordings from multiple layers of the entorhinal–hippocampal loop, we found that coactivation of principal cell pairs (synchrony) was lowest during exploration and rapid-eye-movement (REM) sleep, associated with theta oscillations, and highest in slow wave sleep. Individual principal neurons had a wide range of theta phase preference. Thus, while theta oscillations reduce population synchrony, they nevertheless coordinate the phase (temporal) distribution of neurons. As a result, multiple cell assemblies can nest within the period of the theta cycle.     

Comparison of the effects of serotonin in the hippocampus and the entorhinal cortex

Among the molecular, cellular, and systemic events that have been proposed to modulate the function of the hippocampus and the entorhinal cortex (EC), one of the most frequently cited possibilities is the activation of the serotonergic system. Neurons in the hippocampus and in the EC receive a strong serotonergic projection from the raphe nuclei and express serotonin (5-HT) receptors at high density. Here we review the various effects of 5-HT on intrinsic and synaptic properties of neurons in the hippocampus and the EC. Although similar membrane-potential changes following 5-HT application have been reported for neurons of the entorhinal cortex and the hippocampus, the effects of serotonin on synaptic transmission are contrary in both areas. Serotonin mainly depresses fast and slow inhibition of the principal output cells of the hippocampus, whereas it selectively suppresses the excitation in the entorhinal cortex. On the basis of these data, we discuss the possible role of serotonin under physiological and pathophysiological circumstances.     

Sensory input to neurons of the motor projection of biceps in the cat cortex

Correlation of cortical unit activity in the motor area for the biceps muscle was studied in chronic experiments on cats. In a group of neurons whose activity correlated with movement 68.1% of units had no sensory input from the working limb whereas in a group of neurons not correlating with movement there were 97.6% such units. In 24.2% of group I neurons cutaneous receptive fields of activation type were discovered on the distal part of the dorsal surface of the working limb. Five neurons responding to sensory input from the joints of the working limb were studied in this group.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 9, No. 6, pp. 563–569, November–December, 1977.     

Medial entorhinal cortex commissural input regulates the activity of spatially and object-tuned cells contributing to episodic memory

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Impaired spatial representation in CA1 after lesion of direct input from entorhinal cortex

Place-specific firing in the hippocampus is determined by path integration-based spatial representations in the grid-cell network of the medial entorhinal cortex. Output from this network is conveyed directly to CA1 of the hippocampus by projections from principal neurons in layer III, but also indirectly by axons from layer II to the dentate gyrus and CA3. The direct pathway is sufficient for spatial firing in CA1, but it is not known whether similar firing can also be supported by the input from CA3. To test this possibility, we made selective lesions in layer III of medial entorhinal cortex by local infusion of the neurotoxin gamma-acetylenic GABA. Firing fields in CA1 became larger and more dispersed after cell loss in layer III, whereas CA3 cells, which receive layer II input, still had sharp firing fields. Thus, the direct projection is necessary for precise spatial firing in the CA1 place cell population.     

The long-term potentiation of the late NMDA-dependent components of the responses of the cat motor cortex neurons to stimulation of the direct cortical input of area 5 in the parietal cortex]

Activity of neurons in the motor cortex was recorded in anesthetized cats with glass micropipettes filled with bicuculline solution (bicuculline methiodide, 10 mM in 1 M NaCl). Under these conditions, the minimal (near-threshold) electrical stimulation of the area 5 of the parietal cortex evoked the late neuronal discharges (in 30-200-ms poststimulus interval) in the motor cortex. Such discharges resembled the late NMDA-dependent discharges recorded in the motor cortex of awake cats in response to stimulation of the parietal cortex, which produced the preliminary elaborated conditioned forepaw placing. Under the same conditions, tetanic stimulation of the parietal cortex (100 Hz, 10-20s) led to the long-term potentiation of the late response, which manifested itself as response amplitude augmentation and latency shortening.     

Dynamics of changes in somatosensory input to the motor cortex following lesion of somatosensory cortical areas in dogs

The pattern of change produced in somatosensory evoked potential (EP) in the forelimb projection area within the motor cortex (MI) following lesion of the projection area of the same limb in the somatosensory cortex (SI) or in parietal cortex area 5 was investigated during chronic experiments on waking dogs. Amplitude of the initial positive — negative wave of EP declined to 28–63% of preoperational level in all cases. No significant recovery of EP was noted for three weeks. Thus, a correlation between change in EP and spontaneous recuperation of the precision motor response occurring within two weeks after lesion of the SI did not exist. Nor was EP reinstated in the MI after ablation of area 5, despite complete but gradual reinstatement of EP (after an initial decline to 53%) in the nearby SI region. This protracted depression of EP seems to have been associated with breakdown of somatotopic sensory input from the SI or from area 5 to the MI, since EP in the motor cortex of the intact hemisphere and the hindlimb projection area within the MI on the lesioned side either remained unchanged or recovered within a week or two.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 22, No. 1, pp. 61–68, January–February, 1990.     

Sensory neuroscience: from skin to object in the somatosensory cortex

Humans can perceive the shape of objects by touch alone, by extracting geometric features such as edges. Recently recorded responses of single neurons in the secondary somatosensory cortex of monkeys suggest how the brain integrates tactile shape information across different regions of skin and builds up a representation of tactile objects.     

Cholecystokinin from the entorhinal cortex enables neural plasticity in the auditory cortex

Patients with damage to the medial temporal lobe show deficits in forming new declarative memories but can still recall older memories, suggesting that the medial temporal lobe is necessary for encoding memories in the neocortex. Here, we found that cortical projection neurons in the perirhinal and entorhinal cortices were mostly immunopositive for cholecystokinin (CCK). Local infusion of CCK in the auditory cortex of anesthetized rats induced plastic changes that enabled cortical neurons to potentiate their responses or to start responding to an auditory stimulus that was paired with a tone that robustly triggered action potentials. CCK infusion also enabled auditory neurons to start responding to a light stimulus that was paired with a noise burst. In vivo intracellular recordings in the auditory cortex showed that synaptic strength was potentiated after two pairings of presynaptic and postsynaptic activity in the presence of CCK. Infusion of a CCK_B antagonist in the auditory cortex prevented the formation of a visuo-auditory association in awake rats. Finally, activation of the entorhinal cortex potentiated neuronal responses in the auditory cortex, which was suppressed by infusion of a CCK_B antagonist. Together, these findings suggest that the medial temporal lobe influences neocortical plasticity via CCK-positive cortical projection neurons in the entorhinal cortex.     

The excitation wave returning to the hippocampus through the entorhinal cortex can reactivate the populations of "trained" CA1 neurons during deep sleep

It is suggested that the information about a new stimulus from the neocortex is transferred to the hippocampus and forms there a transient trace in the form of a distributed pattern of modified synapses. During sleep, the neuronal populations which store this trace are reactivated and return to the neocortex the information necessary for consolidation of the permanent memory trace. A possible mechanism of the reactivation of the "learned" hippocampal neurons during memory consolidation is the reverberation of excitation in the neuronal circuits connecting the hippocampus and the entorhinal cortex. In rats, we recorded responses in hippocampal field CA1 to stimulation of the Schaffer collaterals with potentiated synapses during wakefulness and sleep. We showed that in the periods of deep sleep, after the discharge of CA1 neurons, the wave of excitation passes through the entorhinal cortex and via the perforant path fibers enters the hippocampus and the dentate gyrus, causing in the latter the discharge of neurons. The repeated discharge of the CA1 neurons develops as the result of interaction of the early wave which is returned directly via the perforant path fibers and the late wave which is returned via the Schaffer collaterals, but not through the dentate gyrus and hippocampal field CA3 (trisynaptic pathway), but, probably, through the field CA2.     

Quantitative estimations of the entorhinal cortex in Alzheimer's disease

OBJECTIVE: To detect and quantify structural parameters in the entorhinal cortex (EC) of potential use in Alzheimer's disease (AD). STUDY DESIGN: We estimated by stereologic tools the total volume of the EC and subfields EI and ER, the number of neurons and the volume-weighted mean soma volume of layer II neurons. EC morphometric parameters were also assessed in both control and AD cases. RESULTS: In AD, EC volume decreased by 35%, while total number of neurons reached 51%. Also, neuron density had a significant decrease mainly due to change in the EI subfield (31% decrease). The EC showed a decrease in size and a morphology more elliptic and irregular. Moreover, layer II neurons soma size (volume, area, and 1-dimensional parameters) were more rounded. Thus the EC decreases in size and neuron number in AD and minor changes in number per volume were noted. CONCLUSION: These quantitative data can be of value in volumetric MRI studies in AD patients.