Memory.
PKGhatak, MD
Barbara Streisand sings in her famous song the "Memory” --
"Midnight,
not a sound from the pavement
Has the moon lost her memory?
She
is smiling alone
In the lamplight, the withered leaves collect at
my feet
And the wind begins to moan.............
All
alone with the memory
Of my days in the sun
If you touch me,
you'll understand what happiness is
Look, a new day has begun."
https://www.youtube.com/watch?v=MWoQW-b6Ph8
What is memory.
One medical dictionary says– “the mental faculty that enables one to retain and recall previous experienced sensations, impressions, information, and ideas.”
But this definition does say anything about how the mental faculty enables. Intensive research is going on currently to address that question.
It is easier to answer where memory is stored in the brain than how memory is formed. With the wide use of brain imaging tools, MRI has advanced research greatly at the macroscopic level. Functional MRI locates memory centers very preciously and the location is based on the nature of information, emotion associated with it and how long the memory is stored. The Hippocampus, Neocortex, Amygdala, Cerebellum, and prefrontal cortex serve as memory centers. Hippocampus is the initial place where memory is formed. The long term memory is shifted to the neocortex from the hippocampus, and when the process is complete the memory disappears from the Hippocampus. Emotional or fear provoking memories are generated with input from Amygdala. Body movement associated memories are stored in the cerebellum.
Memory formation.
Humans are capable of remembering a vast amount of information. If one has any doubt just watch TV the Jeopardy show, and you will be simply amazed.
You need not be surprised that the vast amount of written literature is accomplished with only 26 letters of the alphabet and a few punctuation marks and simple rules of grammar.
And the computer memory is written by just two inputs 0 and 1. Brain cells use Long Term Potentiation (LTP) and Long Term Depression (LPD) to record memory.
LTP and LTD.
Two types of effects are observed in memory formation. LTP is observed when information is strong and repeated. LTD happens when information is received once or only a few times and the memory lasts maybe a minute only. LTP and LTD are alphabets of memory. Synaptic plasticity and neurons are the punctuation and grammar of memory. Memory is formed from persistent changes in the strength of connections between the neurons. Different groups of neurons are responsible for different information and thoughts
Synaptic Plasticity.
Synaptic plasticity describes persistent changes in the strength of connections between the neurons producing structural changes both in the synapses and neurons and the creation of new synapses. And new neurons are formed in the hippocampus when information is repeated often.
Molecule of Memory.
The molecule that is universally present in all nucleated cells is called Calmodulin.
Calcium ion (Ca++) is only selectively admitted inside the cells when a stimulus arrives at the synaptic junction or neuron. Inside the cells, Ca++ combines with calmodulin to form Calcium / Calmodulin dependent protein kinase (CaMKII).
CaMKII is widely used in many important cell functions. Inside the neurons, the CaMKII exists as isomers; and for memory generation, four isomers of CaMKII are involved. Each isomer functions independently and only in one aspect of memory formation.
Calmodulin acts as a messenger carrying information from the cell cytoplasm to the nucleus of the cell. Kinase is an enzyme that breaks down high energy ATP (adenosine triphosphate) and the released energy that is utilized in chemical reactions.
CaMKII by repeated interaction with the nucleus induces gene variations and results in new mRNA generation. CaMKII activates down the line Kinases for chemical reactions. Many other molecules have also been identified that play various roles in storing and retrieving information from neurons.
Types of memory.
Memories develop in two phases. Short term and Long-lasting memory.
The short term memory-
In the initial phase, the learning of short term memory the brain retains the information only for a brief period of time. According to Meffert of Johns Hopkins, the brain only makes transient changes of the memory molecules already exist and minutes later the molecule is recycled or destroyed.
Long lasting memory.
Meffert thinks the brain cells not only make changes in the molecules already present but also change the gene expressions – turning on and turning off genes. Neurons produce new proteins, increase the rate of production, or shut down lines of proteins.
The strengthening of synaptic connections, forming new connections, and enlargement of neuron partnerships are also part of this process.
Memories are Explicit or Declarative, and Implicit or Non-declarative.
Explicit memories deal with facts, events, people, places and objects. The center is located in Hippocampus. Implicit memories are motor skills and perceptual. The cerebellum is involved. The reflex action in invertebrates is equivalent to the explicit memory of vertebrates.
Taken from Richard F. Thompson and Jeansok J. Kim. PNSA, Nov.26,1996.
Episodic memories.
Telving called another type of memory-the Episodic Memory. Episodic memory allows us to replay old experiences in our own mind resulting in refinement, making associations with similar experiences that result in enhancement of that experience. If emotions are generated during the replay, then the Amygdala stores that memory. In a recent publication, Dr. Reddy from France says in human "Time Cells" which are present in the hippocampus, fire repeatedly and link time, place, person of an event as the hippocampus is developing the memory. That is why we remember not only the event but when and under the circumstance - like "what were you doing when you heard that President Kennedy was shot"
Memory development, retention and recall are subjects of serious study at present. There are many more molecules, enzymes and cell groups involved in many aspects of memory. The increasing incidence of dementia in the elderly has produced a strain on the finances of medical care and personnel. A clear understanding of different aspects of memory is urgently needed so that the fundamental pathology of dementia will be understood and properly addressed.
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