Memory

By

Dr. Sumaira Iqbal

Learning Objectives

• To define memory

• To explain its types

• To explain the processes of habituation and facilitation

• To explain the mechanisms involved in various types of memory

Higher Mental Functions

• Also known as Higher intellectual Functions

• Includes learning, thought processing, speech and memory

• Unlimited storage capacity

Memory

Learning

• Acquisition of knowledge or skills as a result of experience, introduction or both resulting in change in behavior based on individual sensory experience

Memory

• Storage of acquired knowledge for later recall

• Acquisition, storage and retrieval of information

Memory

Memory Trace:

• Neural change responsible for retention of knowledge

• A particular sequence of signal transmission through a synapse again and again

Storage/encoding

• Process by which information is converted in to usable forms to be stored

Memory

• Retrieval

–Process of remembering the stored information

• Forgetting

– Inability to retrieve the stored information

Processes

• Encoding : attending to new information

• Consolidation : altering newly stored information

• Storage : at specific sites

• Retrieval : recalling information

Types of Memory

(On basis of duration of storage)

• Sensory: (msec - one sec)

• Working memory: positive & negative memory

• Short term / Primary: (sec to min)

• Intermediate / Secondary: (Days to weeks)

• Long term / Tertiary: (Years/ life time)

Types of Memory

• Declarative memory– WHAT

– Involves awareness and consciousness

– Semantic– memory of words, language and education

– Episodic—memory of events and objects

• Non-Declarative memory– HOW

– Procedural memory

– Does not involve consciousness

– Use of keyboard or cycling

Synaptic plasticity

• Ability of synapse to undergo physical and structural change responding to a sensory input

• Increases synaptic strength

• Calcium plays role in synaptic strength

• Follow use it/ loose it phenomena

• Short term Synaptic plasticity

– Changes at presynaptic terminal

• Long term Synaptic plasticity

– Changes at postsynaptic terminal

Memory

• Positive memory--To be stored

• Increase synaptic strength

• Mechanisms

– Sensitization

–Post tetanic potentiation

– Long term potentiation

• Negative memory –To be discarded

• Decrease synaptic strength

• Mechanisms

–Habituation

– Long term depression

Mechanism for Habituation

• Results at the sensory terminal--progressive closure of calcium channels at the terminal membrane,

• Cause of this calcium channel closure is not fully known.

• Smaller than normal amounts of calcium ions can diffuse into the habituated terminal

• Less sensory terminal transmitter is released

Working Memory

• Information available for a short time while deciding what to do with it

–A central executive (Prefrontal cortex)

–Rehearsal system (retaining verbal bits)

–Visuo- spatial system (retaining visual & spatial aspects of object)

Short term memory

• Reverberating circuit

• Post tetanic potentiation

• Pre synaptic facilitation

Short Term Memory• Reverberating Circuit:

– Continual neuronal activity

– Creates temporary memory trace

• Presynaptic Facilitation or Inhibition:

– Depends on Neurotransmitter released

• Synaptic Potentiation:

– Enhanced synaptic conduction

– Increased Ca accumulation at pre synaptic terminal

– Prolonged release of NT

– Enhanced synaptic strength due to tetanizing stimuli for about up to 60 seconds

Mechanism for Facilitation

1. Stimulus

2. Release of serotonin from facilitator terminal

3. Activation of adenylyl cyclase in sensory terminal

4. Formation of cAMP

5. Activation of protein kinase

6. Blocking/phosphorylation of K channels

7. Rise in intracellular voltage

8. Prolonged opening of Ca channels

9. Enhanced entry of Ca in presynaptic terminal

10.Increase release of neurotransmitter

Intermediate Memory

• Secondary / intermediate memory:

– Anatomical changes in the synapses

– Physical and chemical changes in synaptic terminal

Long term memory

• Long term potentiation

• Physical and chemical changes at synapse

Long term memory

1. Structural changes at synaptic membrane:• Increase in number of vesicle release sites• Increase in number of neurotransmitter vesicles• Increase in number of pre synaptic terminals• Change in structure of dendritic spines and increased size of

dendritic neck2. Number of neurons and their connectivity changes:• Plasticity of nervous system in early life• Formation of new neurons in adult (hippocampus)• Nerve growth factors

Long Term Potentiation

• Prolonged increase in strength of existing synapse by repeated stimuli

• Can last from weeks to days

• Helps in consolidation

• Mechanism– High levels of calcium

– Activation of NDMA and APA receptors

– Activation of CAMKII(Ca+2 calmodulin dependent kinase II)

• Long term depression-- Low levels of calcium

Consolidation of Memory

• Conversion of short term memory to long term memory

• Hippocampus is site of consolidation

• Time required

–Minimum 5-10 minutes

–Maximum 1 hour

Consolidation of Memory

• Factors affecting consolidation

• Increase

– Rehearsal time

• Decrease

– Drowsiness

– Low mood

– Lack of interest

– Disorganized information

Consolidation of Memory

• Codification

– Knowledge is assessed

– New and old knowledge are compared for similarities and differences

– Only differences are stored

– Synchronization takes place

Hippocampus

• Part of limbic system located at medial side of temporal lobe

• Stores new information temporarily

• Consolidates memory

• Removal of hippocampus in treatment of epilepsy leads to anterograde amnesia

• Declarative memory is lost

• Non declarative procedural memory is retained

Amnesia

• Anterograde– Unable to establish new long term memories

– Consolidation is lost

– Hippocampus lesion

– Forgets new things

• Retrograde – Thalamic lesion– helps to search the traces and readout the stored

information

– Forgets old things

THANK YOU