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CEREBRUM

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Structure of the cerebral cortex

grey matter outside, white matter inside

consists of approx. 10 billion neurons

thickness : 1.5 – 4.5mm

surface has been increased by gyri which are separated by sulci

contains a mixture of nerve cells, nerve fibers, neuroglia and blood cells

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Layers of the cerebral cortex

i. Molecular /plexiform layerii. External granular layeriii. External pyramidal layeriv. Internal granular layerv. Internal pyramidal layervi. Multiform layer

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Variations in cortical structure

• The areas in the cortex which contain all the 6 layers are called homotypical

• The areas in which the basic 6 layers cannot be identified are heterotypical

• Heterotypical areas are further described as granular and agranular type

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CORTICAL AREAS

Different areas of the cerebral cortex area functionally specialized

The primary sensory areas (with granular cortex) and the primary motor areas (with agranular cortex) are heterotypical and form only a small part of the total cortical surface

The remaining areas have all 6 layers and are known as homotypical or association areas

Brodmann’s areas

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FRONTAL LOBE

i. PRIMARY MOTOR AREAii. PRE MOTOR AREA / SECONDARY MOTOR AREAiii. SUPPLEMENTARY MOTOR AREAiv. MOTOR SPEECH AREA OF BROCA v. FRONTAL EYE FIELDvi. PRE FRONTAL CORTEX

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i. PRIMARY MOTOR AREA

• Brodmann’s area 4

• produces isolated movements of the opposite side of the body

• Origin of 40% of pyramidal fibers

• Specific regions within the area are responsible for movements in the specific parts of the body

• Only movements are represented in this area and not the muscles

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ii. PRE MOTOR AREA

• Brodmann’s area 6

• Main site for cortical origin of extra pyramidal fibers

• Receives inputs from the sensory cortex , thalamus & basal ganglia

• Function: to store programs of motor activity assembled as a result of past experience

• It programs the intended activity of the primary motor cortex & controls the movements in progress

• It is responsible for voluntary motor activities

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iii. SUPPLEMENTARY MOTOR AREA

• It is located at the medial extension of area 6, onto the midline surface of the hemisphere

• Possible functions: postural stabilization of the body the coordination of both sides of the body such as

during bimanual action the control of movements that are internally

generated rather than triggered by sensory events,

control of sequences of movements

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iv. MOTOR SPEECH AREA OF BROCA

• Brodmann’s area 44 & 45

• Present on the pars opercularis (44) and the pars triangularis (45) of the IFG on the dominant hemisphere

• It is responsible for expressive speech & vocalization

• It brings about formation of words by its connections to adjacent primary motor cortex

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v. FRONTAL EYE FIELD

• Brodmann’s areas 6,8 & 9

• The frontal eye field is reported to be activated during the initiation of eye movements such as voluntary saccades and pursuit eye movements

• It is one the most important brain areas in generation & control of eye movements especially in the direction contralateral to the FEF’s location

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vi. PRE FRONTAL CORTEX

• Brodmann’s area 9,10,11 & 12

• Concerned with planning complex cognitive behavior, personality expression, decision making, and moderating social/moral/ethical behavior, insight, foresight etc.

• Regulates a person’s depth of feeling, concentration, orientation

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PARIETAL LOBE

i. PRIMARY SOMESTHETHIC AREAii. SECONDARY SOMESTHETIC AREAiii. SOMESTHETIC ASSOCIATION AREA

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i. PRIMARY SOMESTHETHIC AREA

• Brodmann’s areas 3,1,2

• These areas receive sensory information from thalamic nerve projections

• They are concerned with the perception of exteroceptive ( pain, touch & temperature) and

proprioceptive (vibration, muscle & joint sense) sensations from the opposite half of the body

• Sensory homunculus

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ii. SECONDARY SOMESTHETIC AREA

• Brodmann's area 43

• Smaller and less important than primary sensory area

• Functional significance is not known (lesions to this area may impair some elements of sensory discrimination)

• Neurons responds to sensory stimuli bilaterally, although with much less precision than the primary cortex

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iii. SOMESTHETIC ASSOCIATION AREA

• Brodmann’s area 5 & 7

• This receives synthesized connections from the primary and secondary sensory cortices

• Main function: to receive & integrate different sensory modalities e.g. stereognosis

• relates to past sensory experiences so that information may be interpreted and recognition

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TEMPORAL LOBE

i. PRIMARY AUDITORY AREAii. SECONDARY AUDITORY AREAiii. SENSORY SPEECH AREA OF WERNICKE

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i. PRIMARY AUDITORY AREA

• Brodmann’s area 41 & 42

• Located in the inferior wall of the lateral sulcus and on the superior surface of the STG

• Anterior part of the primary auditory area is concerned with the reception of sounds of low frequency and posterior part of the area is concerned with high frequency

• Unilateral lesion of the auditory area produces partial deafness in both ears (greater loss is on contralateral side)

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ii. SECONDARY AUDITORY AREA

• Area 22

• situated on the lateral surface of the STG slightly posterior to the primary auditory area

• Receives impulses from primary auditory area & thalamus & correlates with past auditory experiences

• Responsible for interpretation of sounds & for association of auditory input with other sensory information

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iii. WERNICKE’S SENSORY SPEECH AREA

• Brodmann’s area 39 & 40

• Located in the dominant hemisphere

• Occupies the posterior part of the STG of the temporal gyrus and angular (area 39) & supra marginal (area 40) gyri of the parietal lobule

• Permits understanding of the written and spoken language and enables a person to read a sentence, understand it and say it aloud

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OCCIPITAL LOBE

i. PRIMARY VISUAL AREAii. SECONDARY VISUAL AREA

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i. PRIMARY VISUAL AREA

• Brodmann’s area 17

• It contains white stria hence also called of as striate area

• It receives afferents from the temporal half of the ipsilateral retina & nasal half of the contralateral retina

• It is concerned with reception & perception of isolated visual impressions like color, size, form, motion, illumination & transparency

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ii. SECONDARY VISUAL AREAS

• Brodmann’s area 18 & 19

• Receives information from the primary visual area

• relate the visual information to past experiences to enable the individual to identify and appreciate what he/she is seeing

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OTHER CORTICAL AREASTASTE AREA

• situated in lower end of post central gyrus in superior wall of lateral sulcus near the insula

• Brodmann’s area 43

VESTIBULAR AREA• Located opposite the auditory area in the superior

temporal gyrus• This area along with the vestibular apparatus of

the middle ear are concerned with appreciation of the positions and movements of head in space.

• The movements of eyes & muscles of the trunk and limbs are influenced in maintenance of balance

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WHITE MATTER OF THE CEREBRAL HEMISPHERES

• It is composed of myelinated nerve fibers of different diameters supported by the neuroglia

• It lies deep in the greater part of each cerebral hemisphere

• According to their connections they are classified as-Association fibers Projection fibersCommissural fibers

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CEREBRAL DOMINANCE

Dominant hemisphere refers to the side concerned with the perception and production of language/speech

90% of have left hemispherical dominance so consequently over 90% of adult population is right handed

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Implications of damage to the following areas

PRIMARY MOTOR AREA (4): paralysis of the contralateral extremities with the

finer & skilled movements suffering the most

PREMOTOR AREA (6): little loss of strength, more difficulty in performing

skilled movements

PRIMARY MOTOR + PREMOTOR AREA: most complete form of paralysis

Jacksonian seizure – due to irritative lesion of area 4

CLINICAL ASPECTS

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PRIMARY SOMESTHETIC AREA Contralateral sensory disturbances

SECONDARY SOMESTHETIC AREA No recognizable sensory defects

SOMESTHETIC ASSOCIATION AREA Astereognosis

PREFRONTAL CORTEX Personality changes, Euphoric tendencies Loss of initiative & judgment Socially/morally unacceptable behavior

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MOTOR SPEECH AREA OF BROCA (44 & 45) (dominant side) Expressive aphasia/ motor aphasia/ non fluent

aphasia

SENSORY SPEECH AREA OF WERNICKE (39 & 40) (dominant side) Receptive aphasia/ sensory aphasia/ fluent

aphasia

MOTOR + SENSORY SPEECH AREAS (dominant side) Global aphasia

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References:

• Textbook of Clinical neuro-anatomy: edition 6 Richard Snell• Inder bir Singh• BD chaurasia

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Doubts unta?

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• Damage to area 39 & 40 of dominant side?

• Sensory homunculus lies on the.. ?

• How many layers are there in the cortex ?

• Damage to area 44 & 45 of dominant side?

• Motor homunculus lies on the.. ?

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