Nervous system pdf - د. بسام

PHYSIOLOGY

Dr. Basim Mohamad Awan

Lecture 21

MEMORY

Memory is the ability of the brain to store information and

retrieve it at a later time. The storage capacity of the human

brain is limited. So, the information that flow into the brain is

classified into significant one that will stored and insignificant

one that is ignored and not stored. The most important ones (less

than 1%) are selected and stored, but all the rest are neglected

and forgotten.

STORAGE CAPACITY OF THE BRAIN

The information unit is the "bit", which is the simplest form of

sensory experience; i.e. a letter, a line, a color, a tone, a smell...etc.

The capacity of all the sensory system to send information to the

brain is less than 50 bits/s. An average rate of information flow is

about 20 bits/s. Ten neurons are required to store one bit of

information. The total storage capacity of the human brain is

about 3x 10

bits.

TYPES OF MEMORY :

Four different types of memory:

[I] SENSORY MEMORY (Immediate memory).

[II] PRIMARY MEMORY (Short-term memory)

[III] SECONDARY MEMORY (Long-term memory)

[IV] TERTIARY MEMORY (permanent memory)

[I] SENSORY MEMORY (Immediate memory)

Any sensory signal is automatically stored in the sensory memory

for few seconds. This memory can accommodate 15-20 bits.

Forgetting

starts

spontaneously

immediately

after

the

information is acquired. The spontaneous gradual decline in the

amount of stored information is called "fading" of information.

The spontaneous disappearance of information from the memory

is called "extinction" of information. The information in

sensory memory can be transferred to the secondary memory.

[II] PRIMARY MEMORY (Short-term memory)

This is a memory that lasts for few minutes to few hours. The

information enters this memory by verbalization; i.e. through

spoken or written words. The primary memory is not found in

young infants or animals as they would not understand the

meaning of spoken or written words.

The capacity of primary memory is small. Bits of information

are introduced into it in chronological order, one after the other.

The new information replaces the old ones. The latter are then

forgotten.

The access to the primary memory stores is rapid; one can retrieve

the information rapidly.

[III] SECONDARY MEMORY (Long-term memory)

This is a memory that lasts for several hours up to several years.

The information is introduced into this memory from the sensory

and primary memories by two mechanisms:

1. Stimulation of the reward or punishment systems.

2. Repeated practice or rehearsal of the experience.

The capacity of the secondary memory is large. The information

is stored according to its significance. The bits of information of

related significance are stored together. The access to the

secondary memory stores is slow; it takes some time to remember

the wanted information.

Forgetting of information in the secondary memory occurs when

a new information conflicts with an old stored one. In this case,

one information cancels the other either by repulsion or by

replacement. If the 'old information repels the new one; the

process is called "proactive inhibition" and if the new

information replaces the old one, the process is called

"retroactive inhibition".

In this way, the more we have learned, the greater the liability to

forget the new information by proactive inhibition. The old

information inhibits the acquisition of new ones. Therefore, the

blame for most of our forgetting must be placed on what we

have learned before.

[IV] TERTIARY MEMORY (Permanent memory)

This is the permanent memory. The information stored in this

memory are never forgotten; e.g. one's name or the ability to

read and write. Information in the tertiary memory comes from

the secondary memory by years of practice, which strongly

consolidates the memory. The stored information in the tertiary

memory remains available for retrieval even if information in

other memories is erased by brain injury or disease. This is

because information in the tertiary memory occupies large areas

of the brain and more than one "copy" of the information is

stored in different regions of the brain.

The access to the ternary memory is very rapid, e.g. one

immediately remembers his name if he is asked about it.

MECHANISMS OF MEMORY

[I] SENSORY MEMORY

Sensory memory is made by prolonged after discharge in the

neuronal circuits. Three mechanisms are involved:

(a) Activation of reverberating circuits leading to repeated

reactivation of neurons.

(b) Short term synaptic potentiation by multiple successive

stimulation of input neurons.

(

)

Synaptic sensitization by coupling the sensory experience

with intense or noxious stimulus.

[II] PRIMARY MEMORY

The primary memory is made by the formation of temporary

memory traces. A memory trace is a newly developed pathway

for signal transmission resulting from facilitation of new

synapses. This leads to the creation of new circuits in the brain

that keeps the memory of the experience. Activation of these

circuits brings the memory up to one's mind. There are two

possible mechanisms for the formation of the new memory

traces:

(a) Long term potentiation of synapses. During verbalization

the brain catches the new interesting information and rehearses it

several times.

(b) Changes in the physical properties of the postsynaptic

membrane leading to enhanced sensitivity to the chemical

transmitter.

[III] SECONDARY MEMORY

Secondary memory is made by formation of "memory

engrams". A memory engram is a long-lasting memory trace

formed by structural changes in presynaptic terminals. These

changes include:

(a) Increase in the total area of the specific release sites in

the cell membrane. Transmitter vesicles get attached to these

sites before they rupture and release their transmitter into the

synaptic cleft. The increase in the release sites increases the

amount of transmitter released at the presynaptic terminal, thus

facilitating synaptic transmission.

(b) Increase in the number of transmitter vesicles in the

presynaptic terminal.

(c) Increase in the number of presynaptic terminals by

formation of new terminals.

The memory engrams remain for a long time, up to several

years. Formation of memory engrams requires protein synthesis.

Antibiotics which inhibit protein biosynthesis interfere with the

formation of memory engrams. They prevent the consolidation

of memory but do not affect the primary memory.

[IV] TERTIARY MEMORY

Tertiary memory is made by the formation of permanent

memory traces (permanent engrams). These permanent

engrams are made by structural changes in the neurons. The

changes are similar to those in secondary memory. Tertiary

memory may be considered as an advanced stage of secondary

memory.

CONSOLIDATION OF MEMORY

Consolidation of memory means the transfer of information

from the Sensory and primary short-term memories to the

secondary long-term memory. This process takes from 5 minutes

for minimal consolidation to two hours for maximal

consolidation. Consolidation of memory could be interrupted by

deep anesthesia, brain concussion or electroconvulsive

therapy (ECT). Accordingly, if a sensory impression is made,

then followed within 5 minutes by brain concussion or ECT or

anesthesia, the experience gets extinct. This explains why

patients who had brain concussion in accidents cannot remember

what happened at the time of the accident. If the concussion

occurs 2 hours after" the sensory experience, the information

remains unaffected. If it occurs within two hours, the memory is

affected proportionately.

Consolidation occurs to the information which attracts the

attention of the mind. The brain automatically rehearses this

information. A wide awake person consolidates memories far

better than a person with mental fatigue. This is because the

wide awake brain is more attracted by the new information and

is capable of making rapid and more frequent rehearsals that

would consolidate memory more effectively. Normal quiet sleep

consolidates the memory of information received before the

onset of sleep.

ENCODING OF MEMORY

Encoding of memory is the classification, then placing each memory

item with other related items in the proper memory store; i.e.

memory archiving. It is part of the consolidation process. The

hippocampus plays a central role in this function. All bits of

information go first to the hippocampus where they are sorted out

as significant or insignificant. If the information is classified as

significant, signals are sent to the mamillary bodies of the

hypothalamus. From the hypothalamus signals proceed on to the

orbitofrontal cortex, then to the basal forebrain (Fig. 19-1).

From the basal forebrain (nucleus basalis of Meynert) there are

diffuse cholinergic projections to the memory stores which are

found in all parts of the neocortex, the amygdala and the

hippocampus. The amygdala associates the memories formed

through different senses, then through its connections with the

hypothalamus (the amygdalohypothalamic pathways) it is

responsible for the emotional and autonomic responses to

memories.

Figure 21-2:

Brain areas concerned with encoding of long term

memory

Long-term memories are stored in the form of engrams in

different regions of the brain. To retrieve a certain memory item,

one should get an access to the specific engrams of this item and

activates them. This can be done by different associations. For

example the word "Egypt" could be accessed and activated by

other words as "the pyramids", "the Nile" or hearing part of the

opera "Aida" or seeing a photo of a pharaoh with his

characteristic features and head dress. Each of these methods of

access is considered as a specific "key" which "unlocks " and

activates the specific engrams of the stored item.

FACIAL RECOGNITION AREA

The impression of faces is stored in the "facial recognition

area" in the neocortex of the undersurface of the temporal and

occipital lobes (Fig. 21-3). In right-handed persons and some of

the left handed ones, the facial recognition area is much better

developed in the right (representational) hemisphere than in the

left (categorical) hemisphere.

Bilateral lesion in this area leads to "prosopagnosia" which is'

inability to recognize people by their faces. In this case, the

patient can recognize persons by their voices. A prosopagnostic

patient develops emotional and autonomic reactions on seeing a

familiar face, but he wouldn't identify the person until he hears

him speaking.

NAMING OF OBJECTS AREA

This is the function of an area in the most lateral portion of

both the anterior occipital and posterior temporal lobes. In

this area, the names of different objects are stored. Like the

facial recognition area, it is better developed on the right side

than in the left side. Damage of this area leads to inability to

name objects, although the patient is able to know what the

object is, its value or its use. Only the name is lost from memory.

This is probably the brain area which gave Adam the advantage over

angels. According to the holy Qur'an, after the creation of Adam,

angels failed to name objects when Allah asked them to do, Adam by

the command of Allah, preceded and told the names of objects.

Figure 21-3: Basal view of the human facial recognition

area.

AMNESIA

Amnesia (Gr. = forgetfulness) means the inability to remember

past experience. There are several types of amnesia:

1. RETROGRADE AMNESIA

It is the inability to recall events that occurred shortly before the

onset of brain malfunction without affecting memories of the

remote past. Retrograde amnesia occurs with brain concussion

(post-traumatic amnesia), anesthesia, electric shock (therapeutic

ECT or accidentally). In these conditions, a transient brain

malfunctioning erases the memories of events over a long period

before the onset of malfunctioning. During recovery, the length

of the period of amnesia shrinks progressively till it involves only

several minutes.

2. ANTEROGRADE AMNESIA

It is the inability to form new memories. The memories

consolidated before the onset of amnesia are retained. The

sensory and primary memories are functional but cannot be

consolidated.

Anterograde amnesia occurs with bilateral lesions in the

hippocampus or other structures involved in the encoding of

memory.

3. PSYCHOGENIC OR HYSTERICAL AMNESIA

This is a rare condition characterized by sudden loss of memory

for usually all information in the secondary and tertiary

memories. This usually follows a severe psychological trauma or

it may be an unconscious response to internal conflict or an

intolerable life situation. It is a purely functional disorder

without any organic disease. It can be differentiated from

amnesia caused by damage or disease in the brain tissue by three

characteristics:

(I) All the personal data are forgotten, including the patient's

own name.

(ii) The amnesia is not affected by key stimuli; seeing members

of his family, his friends or his personal effects.

(iii) The inability to remember past events persists although the

patient can make new memories.

Complete recovery of memory almost always occurs.

ALZHEIMER DISEASE AND SENILE DEMENTIA

Alzheimer disease is caused by degeneration of the cholinergic

nerve fibers which project from the nucleus basalis of

Meynert to the neocortex, the amygdala and the

hippocampus. The disease is characterized by deterioration of

intellectual abilities as impairment of memory, lack of judgment

and inattentiveness. The disease may occur at any age. In old

age, it is called senile dementia. It is the commonest cause of

dementia in old age (found in 10-15% of the population above

the age of 65). The anticholinesterase drug eserine

(physostigmine) produces some improvement but it does not

stop the progress of the disease.