Memory Information Consciousness
The term 'memory' refers to the encoding, storage and retrieval of learned information. (1) It is very complex and can be classed in one of two ways. Firstly the qualitative classification divides memory into declarative memory available to consciousness and non-declarative memory which is not available to consciousness.
The other way, and possibly a more useful way, is by temporal classification where memory is categorised into immediate memory, working memory and long term memory. (1)
Hebb's postulate of 1949 stated: “When an axon of cell A is near enough to excite a cell B and repeatedly and persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased.”(2) This outlines the phenomenon of synaptic plasticity which has been investigated at length for its link with memory formation.
A note on synaptic strengthening
Synaptic strengthening or Long Term Potentiation (LTP) has been strongly associated with memory and is one of many forms of synaptic plasticity. It was first demonstrated by Bliss and Lømo in 1973 (4) where they studied the dentate gyrus, an area of the hippocampus, in a rabbit. “Brief high-frequency stimulation” was applied at the performant path resulting in dentate gyrus LTP. Since then extensive research has been carried out showing that LTP does not only occur in the hippocampus but in many different regions of the brain across many different species (4). LTP is now considered to be the “cellular correlate” (5) of memory i.e. enable memory, not equal it (4).
LTP requires the NMDA receptor (NMDA-R) which allows influx of calcium and “the drive of AMPA receptors into synapses”. (6)
Associative Memory
Memories can also include those “emotionally-fuelled experiences”. (7) In humans, the amygdala work closely with the brain's memory centres to form emotional memories. Various studies have been undertaken using fear memories as an example of associative memory.
Invertebrate Study
An experiment conducted by Eric Kandel and his team at Colombia University involved the use of a mollusc Aplysia californica, which has a small number of neurons in its nervous system, to examine the synaptic circuits involved in two forms of behavioural plasticity :
Habituation
This form is displayed by touching the siphon. Initially the gill “contracts vigorously” thereby withdrawing it but if the siphon is touched repeatedly, this contraction becomes weaker At a cellular level, the number of vesicles available for release between sensory and motor neurons, and therefore synaptic transmission, is decreased. This is known as synaptic depression and results in the decreased magnitude of contraction shown in the graphs above.
Sensitization
A strong electrical (noxious) stimulus is then applied to the tail and paired with touching the siphon (a non-noxious stimulus). The noxious stimulus “sensitises the gill withdrawal reflex” so we see an increase in the magnitude of gill contraction.
If the tail is then shocked without following it with a touch of the siphon the “gill withdrawal reflex remains enhanced for at least an hour”.
Over a course of days the stimulus coupling can be repeated and changes can be seen in the mollusc's behaviour, for example a decline in intensity of the gill withdrawal response, providing evidence for a long term memory.
Again, if the neuronal network is studied additional sensory neurons innervating the tail can be seen to become involved. This recruitment is a evidence of synaptic plasticity.
This is an example of associative memory and “Pavlovian fear conditioning”, referring to the way in which Pavlov conditioned his dogs to respond by ringing a bell before giving them food. (6) This principle can also be applied to humans.
Rat Study
Malinow and colleagues discovered a way in which to “label plastic synapses and prevent synaptic plasticity during fear conditioning” (6) in the lateral nucleus of the amygdala in a rat. Plastic synapses were easy to identify as the GluR1 subunit of the AMPA-R was labelled with green fluorescent protein (GFP). It is worth noting that the GluR1 and GluR5 subunits are vital in plasticity. (8) The “plasticity block” was achieved by truncating GluR1 so only the GluR1 C-terminus was expressed, also labelled with GFP. Shown below are the results of the experiment.
Modifying the AMPA-R renders them inactive and prevents synaptic plasticity. If rats are subjected to this antagonistic effect on the AMPA-R prior to fear conditioning, their short term and long term fear memories are impaired, further demonstrating a link between synaptic plasticity and memory.
Forgetting
Human Study
If the hippocampus is removed the brain loses its ability to create new memories. A very important case study is that of H.M. In an attempt to cure his epilepsy certain parts of his brain were removed in 1953, including the hippocampus. Post-operation H.M. suffered from anterograde amnesia and still suffers to this day. Memories of events prior to the operation remain intact but he “lacked the ability to commit anything new to memory” (7). The hippocampus is an observed site of synaptic plasticity so H.M. is living evidence of a connection between synaptic plasticity and memory formation.
Rat Study - The Morris Water Maze
Further evidence of this can be seen when lesions are made in an animal's hippocampus. An experiment involving the study of spatial memory in rats was conducted (1). A Wild type rat was placed in a circular pool of “opaque water” containing a small platform, with the surrounding environment providing “visual cues”. The rat's pattern of movement was recorded during several trials.
A rat with lesions in its hippocampus underwent the same number of trials in the same pool. The results are shown below: It can be seen that the route taken by wild type rats to locate the platform becomes more direct with more trials, and with this the time taken decreases. On the other hand, mutated rats' route remains very complicated and time taken remains approximately the same. The
conclusion is that wild type rats are able to remember the location of the platform but those rats with hippocampal lesions can not. This is a second piece of evidence that the hippocampus, together with its synaptic plasticity, is essential in memory formation.
Preventing LTP
Eva Pastalkova and colleagues suppressed LTP by inhibiting an integral molecule in maintaining LTP, called PKMζ. If the inhibitor was injected into the hippocampus as many as several days after spatial learning the stored memory was lost, outlining the importance of LTP in memory maintenance. (3)
So far LTP has been covered with respect to memory formation, but as mentioned earlier, despite being a very important form, it is not the only from of synaptic plasticity. Long term depression (LTD) is the “dominant form” of synaptic plasticity in the cerebellum, where motor learning and the associative learning we have seen takes place. (3) It is the weakening or elimination of synaptic connections.
It is possible to induce LTD using low frequency (1Hz) stimulation (3) in Wistar rats but not in Hooded Lister rats. However if Hooded Lister rats are placed in a “novel (non-stressful)” environment then LTD can be induced. It is the same case for the Wistar strain but when placed into their former environment after two weeks LTD “facilitation” is lost (9). It can therefore be concluded that LTD has role in spatial mapping (3).
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