CHAPTER TWO
OVERVIEW OF MEMORY
From physiological point of view, memory as one of the higher intellectual functions or higher cortical functions of the human nervous system (Kaplan et all., 1984), is defined as the ability of the brain to encode, store , retain and retrieve learned or past information(Robert and Landenson, 2004). Information are stored in the brain by changing the basic sensitivity of synaptic transmission between neurons as a result of previous neural activity.The new or facilitated pathways are called memory traces(Eayers and Taylor, 1951).They are important because once the traces are established ,the can be selectively activated by thinking to reproduce the memories.
Experiments in lower animals have demonstrated, that memory traces can occur in all levels of nervous system .Even spinal cord reflexes can change at least slightly in responds to repetitive cord activation which is part of the memory process (Tamasy et al., 1986b).In synopsis most memory that involves intellectual processes is mainly based on memory traces.
2.1 ANATOMICAL BASES OF MEMORY
The anatomical bases of memory was vividly discovered by Noble laureate, Eric Kandel on snail Aplysia. He discovered that Apysia snails happen to have extra-large neurons which can be manipulated easily.By process of elimination, neuron by neuron, he mapped out the entire neural circuit of a simple behavior in the snail that changes and learn in responds to its environment. Then by removing part of the circuit in a petri dish and subjecting the neuron to electric shocks and different chemicals, he determined many of the anatomical pathways that mediates memory formation. There are two separate presynaptic terminals, one is the primary and ends on the post synaptic neuron as in conventional synapse, thus called the sensory terminal. The other presynaptic terminals ends on the sensory terminal itself thus called facilitator terminal. When the sensory terminal is been stimulated without the facilitator terminal, Habituetion occurs. But when both are stimulated, facilitation occurs (Sperling G et al., 1963)
2.2 PHYSIOLOGICAL BASIS OF MEMORY
Memory is stored in brain by the alteration of synaptic transmission between the neurons involved in memory (Andeson and Brink, 2008). Storage of memory may be Facilitated or Habituated (Aguirre et al., 2009).
FACILITATION: This is the process by which memory storage is enhanced. It involves increase in synaptic transmission and increase in post synaptic activity. This can be called positive memory (Fagan et al., 1988).
HABITUATION: This is the process by which memory storage is attenuated (decreases). It involves the reduction in synaptic transmission and slow stoppage of postsynaptic activity. This can be called negative memory (Fagan et al., 1988).
MOLECULAR BASIS OF MEMORY (MEMORY ENGRAM)
Molecular basis of memory can be explained by memory engram, which is a process by which memory is facilitated and stored in the brain by means of structural, biochemical or biophysical changes in responds to external stimulus (Barr R et al., 2007).
(1) Stimulation of facilitator neuron along with sensory neuron causes the releases of serotonin from the facilitator terminal to the sensory terminal.
(2) Serotonin binds serotonin receptor leading to the formation of serotonin-receptor complexes
(3) This complexes activates adenylyl cyclase enzyme in the terminal membrane.
(4) This causes the formation of cyclic adenosine monophosphate (CAMP) in the sensory terminal
(5) CAMP activates protein kinase
(6) The protein kinase causes phosphorylation of potassium channels, blocking the channels.
(7) Action potential continues for a long time, Causing prolong activation of calcium pores thereby allowing large amount of calcium to enter the sensory terminal.
(8) The calcium ions in turns causes the release of serotonin, there by facilitating synaptic transmission to a great extent leading to memory facilitation.
Habituation is due to passive closure of calcium channels of terminal membrane, hence the releases of transmitter is less and action potential is less in number.
Long term memory can be grounded in molecular changes called long term potentiation (LTP).This changes involves N-METHYL DESPARTATE (NMDA) RECEPTORS, in which its concentration is high in hippocampus. This receptor can be block by magnesium ions. When calcium ions enters the dendrites of the postsynaptic neuron, it activates calmodulin via cyclic AMP that leads to varieties of effects namely:
(1) The neuron produces an increased number of NDMA receptors which makes it more sensitive to glutamate.
(2) Glutamates activate genes leading to synthesis of new protein which causes remodeling of synapse
(3) It releases nitric oxide which diffuses back to presynaptic neuron and trigger a chain of reaction of events there that ultimately increases glutamate release.
2.4 TYPES OF MEMORY
SENSORY MEMORY: Sensory memory is the shortest-term element of memory. It is the ability to retain impressions of sensory information after the original stimuli have ended. It acts as a buffer for stimuli received through the five senses of sight, hearing, smell, taste and touch which are retained accurately but briefly (Miller et al 2007).
SHORT TERM MEMORY: This is the capacity for holding a small amount of in information in mind in an active, readily available state for a short period of time .The duration for short term memory is believed to be in order of seconds. This can be called primary or active memory (Hartshorn et al 1998).
LONG TERM MEMORY: This is the final, semi-permanent stage of memory, it has an infinite ability and information can remain their indefinitely. It can be call remote memory (Hartshorn et al 1998).
PROCEDUAL MEMORY: This is the type of long term memory responsible for knowing how to do things i.e. memory of skill (Henrik and Leo 1992)
EXPLICIT MEMORY: This is the conscious, intentional recollection of previous experiences and information such as remembering the time of appointment or recollecting an event from years ago. This can be called declarative memory (helps in recalling facts and verbal information), (Aloha and Paula 2007).
INPLICIT MEMORY: This is the type of memory in which previous experiences aid in the performance of a task without conscious awareness of these previous experiences (Loftus et al 2009).
EPISODIC MEMORY: This is a category of long-term memory that involves the recollection of specific events, situation, and experiences (example your memory of your first day in school). It also involves your memory of the location and time that the event occurred (Veranda et al 2000).
SEMANTIC MEMORY: This is a portion of long term memory that processes ideas and concepts that are not drawn from personal experience. Example include names of color sounds of letters, and basic facts acquired over a life time (zecux et al 2006).
2.5 MEMORY CONSOLIDATION: This is a category of processes that stabilize a memory trace after its initial acquisition .Memory consolidation involves the conversion of short-term memory into long-term memory that can be recalled weeks or years later. When short term memory is activated repeatedly, will initiate chemical, physical and anatomical changes in the synapses that are responsible for the long term type of memory. This mechanism requires 5 to 10 minutes for minimal consolidation and 1 hour or more for strong consolidation.
Studies have shown that rehearsal of the same information again and again in the mind accelerates and potentiates the degree of memory consolidation due to the natural tendency of the brain to rehearse newfound information, especially newfound information that catches the minds attention. The hippocampus is a brain structure which lies under the medial temporal lobe on each side of the brain, which is responsible for memory consolidation. Experiment has shown that lesion or removal of hippocampus can lead to amnesia (memory lost) , paraphrasing, hippocampus is the memory box of the brain (Zola Morgan et al 1996).
2.6 MEMORY ANORMALY (AMNESIA)
This can be called amnesic syndrome, is a deficit in memory caused by brain damage, disease, psychological trauma, sedatives or hypnotic drugs. Amnesia simply means loss of memory.
2.7 TYPES OF AMNESIA
RETROGRADE AMNESIA: This is inability to retrieve information that was acquired or inability of recalling long term information in the hippocampus.
ANTEROGRADE AMNESIA: This is the inability to transfer new information from short-term to long term memory. People with this type of amnesia cannot remember things for a long period of time.
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