What is associative memory
associative learning, E.associative learning, Term for the formation of neural connections between a neutral stimulus and a second stimulus that has either positive or negative effects on the organism (in the experiment: reward and punishment). Once the organism has recognized the temporal connection between the two stimuli, it reacts to the first stimulus in anticipation of the second. This so-called conditioning is an important form of learning, and the so-called neural connection (association) is its basis. The reassignment of stimuli and reactions results in drives (conditioned appetite, conditioned action, conditioned aversion, conditioned inhibition). Associative learning is a foundation of memory.
Attempts at associative learning in the sea snail Hermissenda crassicornis were able to provide information about the functioning of this learning method. The animals were conditioned in such a way that they learned to associate an unconditional stimulus (turbulence) with a neutral, conditioned stimulus (flash of light), whereupon they triggered the contraction of their foot (a protective reflex) after the conditioned and without the unconditional stimulus. It was possible to reconstruct the genetically determined circuit diagram of the neurons involved in the reflex and to characterize many of the biochemical processes that enable the associative connection. Further studies in rabbits and rats (conditioning, learning to learn from a labyrinth and learning to differentiate between smells) showed that the investigated associative performance there are largely based on the same processes - with regard to the neurotransmitters (GABA), the ion channels in the cell membranes, their regulation via secondary ones Messengers and the effects of these messenger substances. - Biochemical processes (see fig.): In relation to one another, there is 1) a voltage-dependent calcium influx and 2) an intracellular calcium release induced by GABA. The voltage-dependent calcium influx triggered by the stimulation of the corresponding cells (sodium influx) as a result of the conditioned stimulus (1) leads to the activation of the phosphatidylinositol-specific phospholipase C. This causes the formation of diacylglycerol, arachidonic acid and inositol triphosphate (IP3). The latter leads to the release of calcium from intracellular calcium stores, such as the endoplasmic reticulum. The increased diacylglycerol, arachidonic acid and calcium levels activate protein kinase C, which migrates from the inside of the cell into the cell membrane - a crucial process in the formation of the molecular memory trace - and phosphorylates certain proteins, especially the G proteincp20. This "learning protein" inactivates potassium channels, so that the potassium outflow is reduced, i.e. the depolarization of the nerve cell and thus its excitability increases or remains higher. In the meantime, the post-synaptic GABA receptors have been activated by the unconditional stimulus via GABA (2), which in turn activates phospholipases, diacylglycerol and IP, via G proteins3 release. This leads to a further increase in the depolarization of the cell via protein kinase C and cp20. This makes the neuron more easily excitable: it reacts more sensitively to incoming impulses. Ultimately, this corresponds to the actual learning process. In addition, cp20 inhibits the retrograde transport of substances (axonal transport), increases the mRNA turnover or stimulates protein synthesis and induces structural remodeling of the synapses in the neurons involved in learning. This can take place both in the form of “more solid wiring” of the transmission regions involved on the postsynaptic side and by focusing the cell contacts by breaking down certain unused dendritic branches after conditioning. All of this leads to the fact that ultimately the conditioned stimulus alone is sufficient to cause the cells involved to fire and trigger the reflex, and that this memory trace persists even over long periods of time.
Molecular processes on the cell membrane in the light-sensing cells of type B of the sea snail Hermissenda crassicornis during conditioning. Light falls on rhodopsin in the B cells as a conditioned stimulus, which leads to an increase in the calcium level and thus to depolarization. The subsequent reaction steps are described in more detail in the basic text.
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