![]() ![]() Lynch and colleagues found that a rise in postsynaptic calcium was necessary for LTP induction. In the same year Dingledine described that NMDA channels permeate calcium ions, and G. Collingridge and colleagues showed that blocking another glutamate receptor, the NMDA receptor, prevented LTP induction. The requirement for coincident presynaptic activity explains associativity. This requirement for postsynaptic depolarization of sufficient strength to evoke postsynaptic spike activity explains cooperativity. In the mid-1980s several groups showed that LTP induction requires coincident spike activity of the presynaptic terminal and of the target postsynaptic neuron that is, they demonstrated the existence of Hebb synapses. Thus, LTP can form an associative connection between a weak input and a strong one or, alternatively seen, an association between a weak input and the response elicited by the strong one. LTP induction was also found to be associative: A brief activation of a weak synaptic input that did not induce LTP by itself did so when occurring in close temporal contiguity with brief activation of a separate strong synaptic input to the same target neurons. This dependence on the co-activation of many presynaptic fibers (cooperativity), and the restriction of LTP to the synapses of the activated fibers (input specificity), were demonstrated by several laboratories in the late 1970s. Gardner-Medwin reported that LTP could last for weeks and that its induction appeared to depend on the number of activated presynaptic fibers, or rather the number of activated synapses. Lo/mo reported in a short note in 1966 that a few seconds of repetitive synaptic activation (10 to 15 hertz) led to a prolonged potentiation (LTP) of excitatory action on granule cells in the dentate gyrus of the hippocampus. Excitatory action in these synapses is mediated by glutamate acting on AMPA receptors located postsynaptically, and LTP is seen as an increase of this AMPA receptor-mediated transmission. These synapses are typically spine synapses that is, located on small protuberances (spines) on the dendrite. It has mostly been found for excitatory synapses in principal cells. Scientists first described LTP in the rabbit hippocampal formation, and later in the neocortex and a variety of other regions in the vertebrate (including human) nervous system. Because of its longevity and associative induction properties, scientists regard LTP as the prime neuronal model for learning and memory (see Figure 1). This requirement is similar to that proposed on theoretical grounds by Donald Hebb in 1949 for a synaptic modification involved in learning and memory, known as the Hebb synapse. LTP in many regions has associative induction properties based on a requirement for coincident presynaptic and postsynaptic activity. Long-term potentiation (LTP) is the collective name for synaptic plasticity processes in which brief (less than one second) episodes of intense synaptic activity lead to an enhancement of synaptic efficacy lasting hours to weeks, or longer. Overview: Cooperativity and Associativity ![]()
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