In the terminal region of perforant path fibres in the dentate gyrus, a perforant path volley elicits an initial negative-going synaptically generated population (or field) potential, followed by a positive-going spike reflecting the near-synchronous firing of granule cells ( Figure 1A, B). Field potentials and LTP in the dentate gyrus Then, two years later, Terje Lømo described an increase in synaptically evoked responses in the dentate gyrus of the hippocampal formation that could last for hours following repeated high-frequency stimulation (Lømo, 1966). While the efficiency of each stimulus in firing the target cells increased markedly during the train, the increased efficacy was again too short-lived, lasting only a few minutes, to be regarded as a potential mechanism of memory and learning ( Gloor et al., 1964). This resulted in a rapid increase in the number of target cells that fired action potentials as the train progressed, a phenomenon called ‘frequency potentiation’. One approach was to deliver trains of stimuli at 10 Hz or higher to the axons that project to the hippocampus. Others had been looking for examples of synaptic plasticity in the brain. However, PTP rarely lasted for more than a few minutes (Lloyd, 1949). A favoured model for studying such changes in spinal pathways was post-tetanic potentiation (PTP), a transient increase in synaptic efficacy following tetanic (high-frequency) stimulation of the presynaptic neuron. Hebb’s “neurophysiological postulate” asserted that coincident presynaptic and postsynaptic activity resulted in the strengthening of the synaptic connection between the pre- and postsynaptic cell (Hebb, 1949).Īt the beginning of the second half of the twentieth century neuroscientists with an interest in the neural basis of memory were engaged in a search for examples of long-lasting synaptic plasticity in monosynaptic-or at any rate well-characterized-neural pathways in the central nervous system. Konorski introduced the term “synaptic plasticity” to describe the postulated strengthening of the conditioned pathway in classical conditioning (Konorski, 1948).
This insight was subsequently formalized by Jerzy Konorski and Donald Hebb. Ramón y Cajal himself proposed that synapses were the sites at which memories were stored. Modern ideas about the biological basis of memory began with Santiago Ramón y Cajal, and the identification of the synapse as a discrete entity where one neuron can influence the excitability of another.
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