Cerebral cortex

Objectives
• Enumerate the functions of different association areas of cerebral
cortex.
• Define the terms categorical hemisphere and representational
hemisphere and summarize the difference between these
hemispheres.
• Summarize the differences between fluent and non fluent
aphasia, and explain each type on the basis of its
pathophysiology.

• Most incoming specific sensory signals from the
body terminate in cortical layer IV.
• Most of the output signals leave the cortex
through neurons located in layers V and VI;
• The very large fibers to the brain stem and cord
arise generally in layer V; and the tremendous
numbers of fibers to the thalamus arise in layer
VI.
• Layers I, II, and III perform most of the
intracortical association functions, with
especially large numbers of neurons in layers II
and III making short horizontal connections with
adjacent cortical areas.

Anatomical and Functional Relations of the Cerebral
Cortex to the Thalamus and Other Lower Centers
• The thalamus and the cortex together are
sometimes called the thalamocorticalsystem.
• Almost all pathways from the sensory receptors
and sensory organs to the cortex pass through
the thalamus, with the principal exception of
some sensory pathways of olfaction.

The secondary areas make sense out of the
signals in the primary areas.
For instance, the supplementary and premotor
areas function along with the primary motor
cortex and basal ganglia to provide "patterns" of
motor activity.

On the sensory side, the secondary sensory
areas, begin to analyze the meanings of the
specific sensory signals, such as
(1) interpretation of the shape or texture of an
object in one's hand;
(2) interpretation of color, light intensity, directions
of lines and angles, and other aspects of vision;
and
(3) interpretations of the meanings of sound tones
and sequence of tones in the auditory signals

Parieto-occipitotemporal Association Area
• Analysis of the Spatial Coordinates of the Body
• Wernicke's Area Is Important for Language Comprehension
• Angular Gyrus Area Is Needed for Initial Processing of Visual
Language (Reading)
• Area for Naming Objects

Wernicke's area
• The somatic, visual, and auditory association areas all meet one
another in the posterior part of the superior temporal lobe,
• This area of confluence of the different sensory interpretative
areas plays the greatest single role in intelligence.

• Its an area that has almost global importance
• Also called the general interpretative area, the
gnostic area, the knowing area, the tertiary
association area, and so forth.
• It is best known as Wernicke's area*

• After severe damage in Wernicke's
area, a person might hear perfectly well
and even recognize different words but
still be unable to arrange these words
into a coherent thought.
• Likewise, the person may be able to
read words from the printed page but
be unable to recognize the thought that
is conveyed.

Prefrontal Association Area
• Plans complex patterns and sequences of motor
movements.
• Is connected through a massive subcortical
bundle of nerve fibers connecting the
parietooccipitotemporal association area with
the prefrontal association area.
• Output passes through the caudate portion of
the basal ganglia-thalamic feedback circuit for
motor planning
• Important for elaboration of thoughts
• Broca's Area Provides the Neural Circuitry for
Word Formation

Limbic Association Area
• Concerned primarily with behavior, emotions, and motivation.
• The limbic system includes a complex set of neuronal structures
in the mid basal regions of the brain.
• This limbic system provides most of the emotional drives for
activating other areas of the brain and even provides motivational
drive for the process of learning itself.

• On the medial undersides of both occipital lobes
and along the medioventral surfaces of the
temporal lobes.
• The occipital portion of this facial recognition
area is contiguous with the visual cortex, and the
temporal portion is closely associated with the
limbic system that has to do with emotions, brain
activation, and control of one's behavioral
response to the environment.
• Lesion results in Prosopagnosia (inability to
recognize faces).

Concept of cerebral dominance
The general interpretative functions of
Wernicke's area and the angular gyrus, as well
as the functions of the speech and motor
control areas, are usually much more highly
developed in one cerebral hemisphere than in
the other. Therefore, this hemisphere is called
the dominant hemisphere.

Concept of cerebral dominance
• Old term
• Two hemispheres appear to have complementary functions so the
term cerebral lateralization is better.
• Left hemisphere is specialized for language and analytical ability,
and is also called categorical hemisphere.
• The right hemisphere specialized for visuospatial ability is the
representational hemisphere —is true for 97% of all people.

Hemispheric specialization is related to
handedness.
Handedness appears to be genetically
determined.
In 96% of right-handed individuals, who
constitute 91% of the human population, the
left hemisphere is the dominant or categorical
hemisphere.

• Lesions in the categorical hemisphere produce language disorders.
• Lesions in the representational hemisphere produce
astereognosis—the inability to identify objects by feeling them—
and other agnosias.
• Agnosia is the general term used for the inability to recognize
objects by a particular sensory modality even though the sensory
modality itself is intact.
• Lesions producing these defects are generally in the parietal lobe.
• Especially when they are in the representational hemisphere*,
cause unilateral inattention and neglect.

• Individuals with such lesions do not have any apparent primary
visual, auditory, or somatesthetic defects, but they ignore stimuli
from the contralateral portion of their bodies or the space
around these portions.
• This leads to failure to care for half of their bodies and, in
extreme cases, to situations in which individuals shave half their
faces, dress half their bodies, or read half of each page.

• Patients with lesions in the categorical hemisphere are disturbed
about their disability and often depressed, whereas patients with
lesions in the representational hemisphere are sometimes
unconcerned and even euphoric.
• Lesions of different parts of the categorical hemisphere produce
fluent, nonfluent, and global aphasias .

LANGUAGE
Language is one of the fundamental bases of human
intelligence and a key part of human culture.
The primary brain areas concerned with language are arrayed
along and near the sylvian fissure (lateral cerebral sulcus) of the
categorical hemisphere.

• The process of speech involves two principal stages of
mentation:
(1) Formation in the mind of thoughts to be expressed, as well
as choice of words to be used, and then
(2) Motor control of vocalization and the actual act of
vocalization itself.

• Aphasias are abnormalities of language
functions that are not due to defects of vision
or hearing or to motor paralysis. They are
caused by lesions in the categorical
hemisphere.
• The most common cause is embolism or
thrombosis of a cerebral blood vessel.
• Types:
• Fluent, Nonfluent, and Global aphasias.

• In nonfluent aphasia, the lesion is in Broca's
area.
• Speech is slow, and words are hard to come
by.
• Patients with severe damage to this area are
limited to two or three words with which to
express the whole range of meaning and
emotion.

Fluent aphasia(Wernick’s aphasia)
• The lesion is in Wernicke's area in the posterior superior temporal
gyrus in the dominant hemisphere is damaged or destroyed .
• Capable of understanding either the spoken word or the written word
but are unable to interpret the thought that is expressed
• In this condition, speech itself is normal and sometimes the patients
talk excessively. However, what they say makes little sense.
• The patient also fails to comprehend the meaning of spoken or
written words.

Global Aphasia
• When the lesion in Wernicke's area is widespread and extends
• (1) backward into the angular gyrus region,
• (2) inferiorly into the lower areas of the temporal lobe, and
• (3) superiorly into the superior border of the sylvian fissure, the
person is likely to be almost totally demented for language
understanding or communication and therefore is said to have
global aphasia.

Dyslexia
The angular gyrus is the most inferior portion of the posterior parietal lobe,
lying immediately behind Wernicke's area and fusing posteriorly into the
visual areas of the occipital lobe as well.
If this region is destroyed the stream of visual experiences passing into
Wernicke's area from the visual cortex is mainly blocked.
Therefore, the person may be able to see words and even know that they
are words but not be able to interpret their meanings. This is the
condition called dyslexia, or word blindness.

Learning and Memory
• Learning is acquisition of the information and
memory is the retention and storage of that information.

Common classification of memories
(1) short-term memory, which includes memories that last for
seconds or at most minutes unless they are converted into longer-
term memories;
(2) intermediate long-term memories, which last for days to weeks
but then fade away; and
(3) long-term memory, which, once stored, can be recalled up to
years or even a lifetime later.

Explicit or declarative memory
• Is associated with consciousness—or at least awareness—and is
dependent on the hippocampus and other parts of the medial
temporal lobes of the brain for its retention.
• Events and facts

Implicit or non declarative memory
• Does not involve awareness, and its retention
does not usually involve processing in the
hippocampus.
• Such as all the skills developed for hitting a tennis
ball, including automatic memories to
(1) sight the ball,
(2) calculate the relationship and speed of the ball
to the racquet, and
(3) deduce rapidly the motions of the body, the
arms, and the racquet required to hit the ball as
desired-
all of these are activated instantly based on
previous learning of the game of tennis.

Positive and Negative Memory
"Sensitization" or "Habituation" of Synaptic Transmission
The brain has the capability to learn to ignore information that
is of no consequence.
This results from inhibition of the synaptic pathways for this
type of information; the resulting effect is called habituation.
This is a type of negative memory.

• For incoming information that causes
important consequences such as pain or
pleasure, the brain has a different automatic
capability of enhancing and storing the
memory traces. This is positive memory.
• It results from facilitation of the synaptic
pathways, and the process is called memory
sensitization.

There are special areas in the basal limbic
regions of the brain which determine whether
information is important or unimportant and
make the subconscious decision whether to
store the thought as a sensitized memory
trace or to suppress it.

Physical and chemical changes take place when short term
memory is transformed into long term memory.

Structural Changes Occur in Synapses During the Development
of Long-Term Memory
• Increase in vesicle release sites for secretion of transmitter
substance
• Increase in number of transmitter vesicles released
• Increase in number of presynaptic terminals
• Changes in structures of the dendritic spines that permit
transmission of stronger signals

Number of Neurons and Their Connectivities
Often Change Significantly During Learning
• Soon after birth, there is a principle of "use it or
lose it" that governs the final number of neurons
and their connectivities in respective parts of the
human nervous system. This is a type of learning.
• For example, if one eye of a newborn animal is
covered for many weeks after birth, neurons in
alternate stripes of the cerebral visual cortex-
neurons normally connected to the covered eye-
will degenerate, and the covered eye will remain
either partially or totally blind for the remainder
of life.

Consolidation of Memory
The short-term memory if activated repeatedly will initiate
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 minimal consolidation
and 1 hour or more for strong consolidation.
Rehearsal Enhances the Transference of Short-Term Memory
into Long-Term Memory

• Long-term potentiation (LTP) is a rapidly developing persistent
enhancement of the postsynaptic potential response to presynaptic
stimulation after a brief period of rapidly repeated stimulation of the
presynaptic neuron.
• It occurs in many parts of the nervous system but has been studied
in greatest detail in the hippocampus.

New Memories Are Codified During
Consolidation
• Similar types of information are pulled from the memory storage bins
and used to help process the new information.
• The new and old are compared for similarities and differences, and part
of the storage process is to store the information about these similarities
and differences, rather than to store the new information unprocessed.
• new memories are are stored in direct association with other memories
of the same type. This is necessary if one is to be able to "search" the
memory store at a later date to find the required information.

Hippocampus & Medial Temporal Lobe
• For storing verbal and symbolic types of memories (declarative
types of memory) in long-term memory, or even in
intermediate memory lasting longer than a few minutes.
• Therefore, with lesion of these patients are unable to establish
new long-term memories of those types of information that
are the basis of intelligence. This is called anterograde
amnesia.

déjà vu phenomenon,
• An inappropriate feeling of familiarity with new events or in new
surroundings is known clinically as the déjà vu phenomenon, from
the French words meaning "already seen."
• The phenomenon occurs from time to time in normal individuals, but
it also may occur as an aura (a sensation immediately preceding a
seizure) in patients with temporal lobe epilepsy.

Alzheimer Disease & Senile Dementia
• Is the most common age-related neurodegenerative disorder.
• Memory decline initially manifests as a loss of episodic
memory, which impedes recollection of recent events.
• Loss of short-term memory is followed by general loss of
cognitive and other brain functions, the need for constant care,
and, eventually, death.

• Most cases are sporadic, but some are familial.
• Senile dementia can be caused by vascular disease and other
disorders, but Alzheimer disease is the most common cause,
accounting for 50–60% of the cases.
• It is present in about 17% of the population aged 65–69, but its
incidence increases steadily with age, and in those who are 95 and
older, the incidence is 40–50%.

• The cytopathologic hallmarks of Alzheimer disease are
intracellular neurofibrillary tangles
• and extracellular senile plaques, which have a core of Beta -
amyloid peptides surrounded by altered nerve fibers and
reactive glial cells.

• There is increased amounts of beta-amyloid peptide in the brains
of patients with Alzheimer's disease.
• The peptide accumulates in amyloid plaques, which range in
diameter from 10 micrometers to several hundred micrometers
• Alzheimer's disease appears to be a metabolic degenerative
disease.

• A key role for excess accumulation of beta-amyloid peptide in the
pathogenesis of Alzheimer's disease is suggested by the following
observations:
(1) all currently known mutations associated with Alzheimer's
disease increase the production of beta-amyloid peptide;
(2) patients with trisomy 21 (Down syndrome) have three copies of
the gene for amyloid precursor protein and develop neurological
characteristics of Alzheimer's disease by midlife;

(3) patients who have abnormality of a gene that controls
apolipoprotein E, a blood protein that transports cholesterol to the
tissues, have accelerated deposition of amyloid and greatly increased
risk for Alzheimer's disease;
(4) transgenic mice that overproduce the human amyloid precursor
protein have learning and memory deficits in association with the
accumulation of amyloid plaques; and
(5) generation of anti-amyloid antibodies in humans with Alzheimer's
disease appears to attenuate the disease process.

Vascular Disorders May Contribute to Progression of
Alzheimer's Disease
• There is also accumulating evidence that cerebrovascular disease
caused by hypertension and atherosclerosis may play a role in
Alzheimer's disease.
• Cerebrovascular disease is the second most common cause of
acquired cognitive impairment and dementia and likely contributes
to cognitive decline in Alzheimer's disease.
• In fact, many of the common risk factors for cerebrovascular
disease, such as hypertension, diabetes, and hyperlipidemia, are
also recognized to greatly increase the risk for developing
Alzheimer's disease.

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