Memories – Neural Mechanism

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Definition

Memory is the ability to recall past events at the conscious or unconscious level. The ability of human brain to store and retrieve memories is perhaps the most important ability.


Memory stores include: –

  1. Vocabulary.
  2. Knowledge of language.
  3. All facts which person learns.
  4. All people known.
  5. All skills which one learns.
  6. On playing, e.g. walking, talking, swimming, etc.

Brain stores all this diverse information so that it can be easily assessed and used.

Classification


Memory is classified as:

(i) Short-term memory- It includes memories that last for seconds or at most minutes unless they are converted to long-term memories.

(ii) Intermediate long-term memory. It lasts for days to weeks but eventually is lost.


(iii) Long-term memory. Once stored this type of memory can be recalled up to years
or even a lifetime long.

Short Term Memory – Neural Mechanism


Short-term memory lasts for a few seconds, e.g. memory up to 7-10 digits telephone
numbers. Memory only lasts as long as the person continues to think about the numbers.
Suggested mechanisms of short-term memory:

  1. Continual neuronal activity.– This is due to traveling of signal through a circuit of
    reverberating neurons.
  2. Presynaptic facilitation or inhibition- This occurs in the synapses that lie on the presynaptic terminals and not in the subsequent neuron. This causes prolonged facilitation or inhibition for as long as few seconds to a minute.
  1. Synaptic potentiation. When train of impulses passes along the presynaptic
    terminal, the amount of calcium ions in presynaptic terminal increases with each impulse. When it becomes greater, there is prolonged release of neurotransmitter at the synapse.

Intermediate Long-Term Memory – Neural Mechanism


Intermediate long-term memory lasts for many minutes to many weeks. This results
from temporary physical or chemical changes or both at the presynaptic terminal.

One terminal is from primary input sensory neuron and terminates on the surface
of the neuron that is to be stimulated. This is called sensory terminal. The other
terminal lies on the sensory terminal and is called facilitator terminal. Changes
occur as follows:


1. Stimulation of facilitator neuron at the same time so that the sensory neuron
is stimulated. It causes serotonin release at the facilitator synapse on sensory
presynaptic terminals.


2. Serotonin acts on the receptors present in the sensory terminal surface and activates adenyl cyclase which causes formation of cyclic AMP.


3. Cyclic AMP activates protein kinase which causes phosphorylation of a protein
that is the part of potassium channels. This blocks the channel and the blockage
can last for a few minutes to a few weeks.


4. Blockage of potassium channels prolongs the action potential which causes
activation of calcium channels. Massive quantities of calcium enter the sensory
terminal causing greater release of transmitter which facilitates synaptic
transmission.

5. Thus, when sensory neuron and facilitator neuron are simultaneously stimulated, facilitator neuron causes prolonged change in sensory terminal that produces memory trace.

Second suggested mechanism for intermediate long-term memory: Stimuli from
two separate sources acting on a single sensory neuron can cause long-term changes in the membrane properties of the entire postsynaptic neuron under appropriate
conditions.

Long Term Memory – Neural Mechanism


Long-term memory is due to actual structural changes at the synapse that enhance or
suppress signal conduction. There is no real demarcation between prolonged type of
intermediate long-term memory and long-term memory.


Suggested mechanism for long-term memory:

  1. There is increase in total area of the vesicular release site in the presynaptic ter-
    minal during development of long-term memory traces. Increase in vesicular site
    starts within hours after initiating the training session.
  2. There is also an increase in the number of transmitter vesicles in the presynaptic terminal.
  3. In addition, there is also an increase in the number of terminals themselves, i.e. the number of synapses increases. These effects facilitate transmission of signals which is the basis of learning.
  4. It is also possible that there is a change in the number of neurons in the used circuits.
  5. For short-term memory to be converted into either intermediate long-term or long-term memory, it must become consolidated, i.e. memory must in some way initiate the chemical, physical and anatomical changes in the synapses that are responsible for the long-term type of memory. This process requires 5 to 10 minutes for minimum consolidation and an hour or more for maximal consolidation.
  6. This occurs due to the phenomenon of rehearsal, i.e. rehearsal of the same information repeatedly.

Memories placed into longer-term

Memory storehouses are codified into different classes of information. During this process, similar information is recalled from the memory storage bins and is used to help the process of added information. The new and old are compared for similarities and differences.

Thus, during the process of consolidation, the new memories are not stored randomly in the brain but are stored in direct association with other memories of the same type.

Hippocampus is one of the important output pathways of reward and punishment areas of limbic system. All these reward and punishment signals together provide the background mood and motivation of the person. Among these is the drive in the brain to remember either pleasant or unpleasant.

The hippocampus is one of the important sites for storage of long-term memory. Removal of Hippocampus leads to inability of the person to establish new long term memories of those types that are basis of intelligence. This condition is known as Anterograde Amnesia

Wernicke’s area, which is a major locus of intellectual operations, also helps in memory storage. It helps in analysis of memory so that it can be stored in association with similar old memories.