MECHANISMS OF THE CEREBRAL CORTEX
- Extensive research in recent years has resulted in a vast increase in our
knowledge of the connections of the neurons of the cerebral cortex.
- This information combined with new methods of studying the functions of
the human ce rebral cortex in the living using electroencephalograms
(EEG), positron emission tomography (PET), and magnetic resonance
imaging (MRI) have led to a new understanding of the functions of the
different areas and the different layers of the cerebral cortex.
- Much of the new information, however, is still me rely factual data and
cannot be used in the clinical setting.
- The cerebral cortex is organized into vertical units or columns of
functional activity measuring about 300 to 600 μm wide.
- Each unit possesses afferent fibe rs, internuncial neurons, and efferent
fibers.
- An afferent fibe r may synapse directly with an efferent neuron or may
involve vertical chains of internuncial neurons.
- The spread of incoming information serving one sensory modality
laterally from one column to an adjacent column, or to columns some
distance away, may permit the individual to start the process of
understanding the nature of the sensory input
- Extensive research in recent years has resulted in a vast increase in our
knowledge of the connections of the neurons of the cerebral cortex.
- This information combined with new methods of studying the functions of
the human ce rebral cortex in the living using electroencephalograms
(EEG), positron emission tomography (PET), and magnetic resonance
imaging (MRI) have led to a new understanding of the functions of the
different areas and the different layers of the cerebral cortex.
- Much of the new information, however, is still me rely factual data and
cannot be used in the clinical setting.
- The cerebral cortex is organized into vertical units or columns of
functional activity measuring about 300 to 600 μm wide.
- Each unit possesses afferent fibe rs, internuncial neurons, and efferent
fibers.
- An afferent fibe r may synapse directly with an efferent neuron or may
involve vertical chains of internuncial neurons.
- The spread of incoming information serving one sensory modality
laterally from one column to an adjacent column, or to columns some
distance away, may permit the individual to start the process of
understanding the nature of the sensory input
Cortical areas:
- Over the past century, evidence have been produced that different areas
of the cerebral cortex are functionally specialized.
- However, the precise division of the cortex into different areas of
specialization, as described by Brodmann, oversimplifies and misleads the
reader.
- The simple division of cortical areas into motor and sensory is erroneous,
for many of the sensory areas are far more extensive than originally
described, and it is known that motor responses can be obtained by
stimulation of sensory areas.
- Until a satisfactory terminology has been devised to describe the various
cortical areas, the main cortical areas will be named by their anatomical
location
Areas in the frontal lobe:
The precentral area:
- Is situated in the precentral gyrus and includes the anterior wall of the
central sulcus and the posterior parts of the frontal gyri; it extends over
the supe ro-medial border of the hemisphere into the paracentral lobule
- Histologically, the characteristic feature of this area is the almost
complete absence of the granular layers and the prominence of the
pyramidal nerve cells.
- The great majority of the corticospinal and corticobulbar fibe rs originate
from the small pyramidal cells in this area.
The precentral area may be divided into two regions:
1-The posterior region, the motor, primary motor area, or Brodmann area 4:
- occupies the precentral gyrus extending over the superior border into the
paracentral lobule
- If electrically stimulated, produces isolated movements on the opposite
side of the body as well as contraction of muscle groups concerned with
the performance of a specific movement.
- Movements of the extraocular muscles, the muscles of the uppe r part of
the face, the tongue, the mandible, the larynx and the pharynx do occur
BILATERALLY
2- The anterior region, the premotor, secondary motor area, or Brodmann area
6, and parts of areas 8, 44, and 45:
- It occupies the anterior part of the precentral gyrus and the posterior parts of
the supe rior, middle, and inferior frontal gyri.
- Electrical stimulation of the premotor area produces muscular
movements similar to those obtained by stimulation of the primary motor
area; however, stronger stimulation is necessary to produce the same
degree of movement.
- The movement areas of the body are represented in inverted form in the
precentral gyrus (the motor homunculus)
- Starting from below and passing supe riorly are structures involved in
swallowing and the tongue, jaw, lips, larynx, eyelid, and brow. The next
area is an extensive region for movements of the fingers, especially the
thumb, hand, wrist, elbow, shoulder, and trunk. The movements of the
hip, knee, and ankle are represented in the highest areas of the precentral
gyrus; the movements of the toes are situated on the medial surface of the
- Over the past century, evidence have been produced that different areas
of the cerebral cortex are functionally specialized.
- However, the precise division of the cortex into different areas of
specialization, as described by Brodmann, oversimplifies and misleads the
reader.
- The simple division of cortical areas into motor and sensory is erroneous,
for many of the sensory areas are far more extensive than originally
described, and it is known that motor responses can be obtained by
stimulation of sensory areas.
- Until a satisfactory terminology has been devised to describe the various
cortical areas, the main cortical areas will be named by their anatomical
location
Areas in the frontal lobe:
The precentral area:
- Is situated in the precentral gyrus and includes the anterior wall of the
central sulcus and the posterior parts of the frontal gyri; it extends over
the supe ro-medial border of the hemisphere into the paracentral lobule
- Histologically, the characteristic feature of this area is the almost
complete absence of the granular layers and the prominence of the
pyramidal nerve cells.
- The great majority of the corticospinal and corticobulbar fibe rs originate
from the small pyramidal cells in this area.
The precentral area may be divided into two regions:
1-The posterior region, the motor, primary motor area, or Brodmann area 4:
- occupies the precentral gyrus extending over the superior border into the
paracentral lobule
- If electrically stimulated, produces isolated movements on the opposite
side of the body as well as contraction of muscle groups concerned with
the performance of a specific movement.
- Movements of the extraocular muscles, the muscles of the uppe r part of
the face, the tongue, the mandible, the larynx and the pharynx do occur
BILATERALLY
2- The anterior region, the premotor, secondary motor area, or Brodmann area
6, and parts of areas 8, 44, and 45:
- It occupies the anterior part of the precentral gyrus and the posterior parts of
the supe rior, middle, and inferior frontal gyri.
- Electrical stimulation of the premotor area produces muscular
movements similar to those obtained by stimulation of the primary motor
area; however, stronger stimulation is necessary to produce the same
degree of movement.
- The movement areas of the body are represented in inverted form in the
precentral gyrus (the motor homunculus)
- Starting from below and passing supe riorly are structures involved in
swallowing and the tongue, jaw, lips, larynx, eyelid, and brow. The next
area is an extensive region for movements of the fingers, especially the
thumb, hand, wrist, elbow, shoulder, and trunk. The movements of the
hip, knee, and ankle are represented in the highest areas of the precentral
gyrus; the movements of the toes are situated on the medial surface of the
cerebral hemisphere in the paracentral lobule. The movements of the anal
and vesical sphincters are also located in the paracentral lobule.
- The area of cortex controlling a particular movement is proportional to
the skill involved in performing the movement and is unrelated to the
mass of muscle participating in the movement.
* The function of the primary motor area is to carry out the individual
movements of different parts of the body
* In order to assist in this function, it receives nume rous afferent fibers from the
premotor area, the sensory cortex, the thalamus, the cerebellum, and the basal
ganglia.
* The primary motor cortex is not responsible for the design of the pattern of
movement but is the final station for conversion of the design into execution of
the movement.
* The function of the secondary motor area is to store programs of motor activity
assembled as the result of past experience
* It is particularly involved in controlling coarse postural movements through its
connections with the basal ganglia.
* In order to do this function, the premotor area receives numerous inputs from
the sensory cortex, the thalamus, and the basal ganglia.
The supplementary motor area:
- Is situated in the medial frontal gyrus on the medial surface of the
hemisphere and anterior to the paracentral lobule
- Stimulation of this area results in movements of the contralate ral limbs,
but a stronger stimulus is necessary than when the primary motor area is
stimulated
- Removal of the supplementary motor area produces no pe rmanent loss of
movement.
The frontal eye field:
- Extends forward from the facial area of the precentral gyrus into the
middle frontal gyrus (parts of Brodmann areas 6, 8, and 9).
- Electrical stimulation of this region causes conjugate movements of the
eyes, especially toward the opposite side.
- The exact pathway taken by nerve fibers from this area is not known, but
they are thought to pass to the supe rior colliculus of the midbrain.
- The frontal eye field is considered to control voluntary scanning
movements of the eye and is independent of visual stimuli.
The motor speech area of Broca:
- Located in the inferior frontal gyrus between the anterior and ascending
rami and the ascending and posterior rami of the lateral fissure
(Brodmann areas 44 and 45).
- This area is important on the dominant hemisphe re (commonly the left),
and ablation will result in paralysis of speech.
- The ablation of this region in the nondominant hemisphere has no effect
on speech.
- The Broca speech area brings about the formation of words by its
connections with the adjacent primary motor areas; the muscles of the
larynx, mouth, tongue, soft palate, and the respiratory muscles are
appropriately stimulated.
The prefrontal cortex:
and vesical sphincters are also located in the paracentral lobule.
- The area of cortex controlling a particular movement is proportional to
the skill involved in performing the movement and is unrelated to the
mass of muscle participating in the movement.
* The function of the primary motor area is to carry out the individual
movements of different parts of the body
* In order to assist in this function, it receives nume rous afferent fibers from the
premotor area, the sensory cortex, the thalamus, the cerebellum, and the basal
ganglia.
* The primary motor cortex is not responsible for the design of the pattern of
movement but is the final station for conversion of the design into execution of
the movement.
* The function of the secondary motor area is to store programs of motor activity
assembled as the result of past experience
* It is particularly involved in controlling coarse postural movements through its
connections with the basal ganglia.
* In order to do this function, the premotor area receives numerous inputs from
the sensory cortex, the thalamus, and the basal ganglia.
The supplementary motor area:
- Is situated in the medial frontal gyrus on the medial surface of the
hemisphere and anterior to the paracentral lobule
- Stimulation of this area results in movements of the contralate ral limbs,
but a stronger stimulus is necessary than when the primary motor area is
stimulated
- Removal of the supplementary motor area produces no pe rmanent loss of
movement.
The frontal eye field:
- Extends forward from the facial area of the precentral gyrus into the
middle frontal gyrus (parts of Brodmann areas 6, 8, and 9).
- Electrical stimulation of this region causes conjugate movements of the
eyes, especially toward the opposite side.
- The exact pathway taken by nerve fibers from this area is not known, but
they are thought to pass to the supe rior colliculus of the midbrain.
- The frontal eye field is considered to control voluntary scanning
movements of the eye and is independent of visual stimuli.
The motor speech area of Broca:
- Located in the inferior frontal gyrus between the anterior and ascending
rami and the ascending and posterior rami of the lateral fissure
(Brodmann areas 44 and 45).
- This area is important on the dominant hemisphe re (commonly the left),
and ablation will result in paralysis of speech.
- The ablation of this region in the nondominant hemisphere has no effect
on speech.
- The Broca speech area brings about the formation of words by its
connections with the adjacent primary motor areas; the muscles of the
larynx, mouth, tongue, soft palate, and the respiratory muscles are
appropriately stimulated.
The prefrontal cortex:
- An extensive area that lies anterior to the precentral area. It includes the
greater parts of the superior, middle, and infe rior frontal gyri; the orbital
gyri; most of the medial frontal gyrus; and the anterior half of the
cingulate gyrus (Brodmann areas 9, 10, 11, and 12).
- Large numbers of affe rent and efferent pathways connect the prefrontal
area with other areas of the cerebral cortex, the thalamus, the
hypothalamus, corpus striatum, & ce rebellum
- The commissural fibers of the forceps minor and genu of the corpus
callosum unite these areas in both cerebral hemisphe res.
- The prefrontal area is concerned with :
1- Makeup of the individual's personality.
2- Regulation of the person's depth of feeling.
3- Determining the initiative and judgment of an individual.
Areas in the parietal lobe:
The primary somesthetic area (primary somatic sensory cortex):
- Occupies the postcentral gyrus on the late ral surface of the hemisphere
and the posterior part of the paracentral lobule on the medial surface
(Brodmann areas 3, 1, and 2).
- The primary somesthetic areas of the cerebral cortex receive projection
fibers from the ventral posterior lateral and ventral posterior medial
nuclei of the thalamus.
- The opposite half of the body is represented as inverted.
- The apportioning of the cortex for a particular part of the body is related
to its functional importance rathe r than to its size. The face, lips, thumb,
and index finger have particularly large areas assigned to them.
- In fact, the size of the cortical area allocated to each part of the body is
directly proportional to the numbe r of sensory receptors present in that
part of the body.
- Although most sensations reach the cortex from the contralateral side of
the body, some from the oral region go to the same side, and those from
the pharynx, larynx, and pe rineum go to both sides.
- On entering the cortex, the afferent fibe rs excite the neurons in layer IV,
from this layer, large numbers of axons leave the cortex and pass to lower
sensory relay stations of the thalamus, medulla oblongata, and the spinal
cord, providing feedback.
- This sensory feedback is largely inhibitory and serves to modulate the
intensity of the sensory input.
- The anterior part of the postcentral gyrus situated in the central sulcus
receives a large number of afferent fibe rs from muscle spindles, tendon
organs, and joint receptors. This sensory information is analyzed & then
passed forward beneath the central sulcus to the primary motor cortex,
whe re it greatly influences the control of skeletal muscle activity.
The secondary somesthetic area (secondary somatic sensory cortex) :
- Is in the superior lip of the posterior limb of the lateral fissure
- It is much smaller and less important than the primary sensory area.
- The face area lies most anterior, and the leg area is posterior.
- The body is bilate rally represented with the contralateral side dominant.
- The detailed connections of this area are unknown & the functional
significance of this area is not understood.
greater parts of the superior, middle, and infe rior frontal gyri; the orbital
gyri; most of the medial frontal gyrus; and the anterior half of the
cingulate gyrus (Brodmann areas 9, 10, 11, and 12).
- Large numbers of affe rent and efferent pathways connect the prefrontal
area with other areas of the cerebral cortex, the thalamus, the
hypothalamus, corpus striatum, & ce rebellum
- The commissural fibers of the forceps minor and genu of the corpus
callosum unite these areas in both cerebral hemisphe res.
- The prefrontal area is concerned with :
1- Makeup of the individual's personality.
2- Regulation of the person's depth of feeling.
3- Determining the initiative and judgment of an individual.
Areas in the parietal lobe:
The primary somesthetic area (primary somatic sensory cortex):
- Occupies the postcentral gyrus on the late ral surface of the hemisphere
and the posterior part of the paracentral lobule on the medial surface
(Brodmann areas 3, 1, and 2).
- The primary somesthetic areas of the cerebral cortex receive projection
fibers from the ventral posterior lateral and ventral posterior medial
nuclei of the thalamus.
- The opposite half of the body is represented as inverted.
- The apportioning of the cortex for a particular part of the body is related
to its functional importance rathe r than to its size. The face, lips, thumb,
and index finger have particularly large areas assigned to them.
- In fact, the size of the cortical area allocated to each part of the body is
directly proportional to the numbe r of sensory receptors present in that
part of the body.
- Although most sensations reach the cortex from the contralateral side of
the body, some from the oral region go to the same side, and those from
the pharynx, larynx, and pe rineum go to both sides.
- On entering the cortex, the afferent fibe rs excite the neurons in layer IV,
from this layer, large numbers of axons leave the cortex and pass to lower
sensory relay stations of the thalamus, medulla oblongata, and the spinal
cord, providing feedback.
- This sensory feedback is largely inhibitory and serves to modulate the
intensity of the sensory input.
- The anterior part of the postcentral gyrus situated in the central sulcus
receives a large number of afferent fibe rs from muscle spindles, tendon
organs, and joint receptors. This sensory information is analyzed & then
passed forward beneath the central sulcus to the primary motor cortex,
whe re it greatly influences the control of skeletal muscle activity.
The secondary somesthetic area (secondary somatic sensory cortex) :
- Is in the superior lip of the posterior limb of the lateral fissure
- It is much smaller and less important than the primary sensory area.
- The face area lies most anterior, and the leg area is posterior.
- The body is bilate rally represented with the contralateral side dominant.
- The detailed connections of this area are unknown & the functional
significance of this area is not understood.
- It has been shown that the neurons respond particularly to transient
cutaneous stimuli, such as brush strokes or tapping of the skin
The somesthetic association area:
- Occupies the superior parietal lobule extending onto the medial surface of
the hemisphere (Brodmann areas 5 and 7).
- This area has many connections with othe r sensory areas of the cortex.
- It is believed that its main function is to receive and integrate different
sensory modalities. For example, it enables one to recognize objects
placed in the hand without the help of vision.
- In othe r words, it not only receives information concerning the size and
shape of an object but also relates this to past sensory experiences; thus,
the information may be interpreted, and recognition may occur.
Areas in the occipital lobe:
The primary visual area (Brodmann area 17):
- Situated in the walls of the posterior part of the calcarine sulcus and
occasionally extends around the occipital pole onto the lateral surface of
the hemisphere
- Macroscopically, this area can be recognized by the thinness of the cortex
and the visual stria
- From the late ral geniculate body, fibers first pass forward in the white
matter of the temporal lobe and then turn back to the primary visual
cortex in the occipital lobe.
- The visual cortex receives fibers from the opposite field of vision (right
cortex receives from left field & vice versa)
- The superior retinal quadrants (inferior field of vision) pass to the
superior wall of the calcarine sulcus, while the infe rior retinal quadrants
(supe rior field of vision) pass to the inferior wall of the calcarine sulcus.
- The macula lutea, is represented on the cortex in the posterior part of
area 17
The secondary visual area (Brodmann areas 18 and 19):
- Surrounds the primary visual area on the medial and lateral surfaces of
the hemisphere
- This area receives afferent fibers from area 17 and other cortical areas as
well as from the thalamus.
- The function of the secondary visual area is to relate the visual
information received by the primary visual area to past visual experiences
enabling the individual to recognize objects in the scene
The occipital eye field:
This field is thought to exist in the secondary visual area in humans
The function of this eye field is believed to be reflex movements of the eye when it
is following an object.
Areas in the temporal lobe:
The primary auditory area (Brodmann areas 41 and 42):
- Is situated in the inferior wall of the lateral sulcus
- Area 41 is a granular type of cortex; area 42 is homotypical and is mainly
an auditory association area.
- Projection fibers arise principally in the medial geniculate body and form
the auditory radiation of the internal capsule .
cutaneous stimuli, such as brush strokes or tapping of the skin
The somesthetic association area:
- Occupies the superior parietal lobule extending onto the medial surface of
the hemisphere (Brodmann areas 5 and 7).
- This area has many connections with othe r sensory areas of the cortex.
- It is believed that its main function is to receive and integrate different
sensory modalities. For example, it enables one to recognize objects
placed in the hand without the help of vision.
- In othe r words, it not only receives information concerning the size and
shape of an object but also relates this to past sensory experiences; thus,
the information may be interpreted, and recognition may occur.
Areas in the occipital lobe:
The primary visual area (Brodmann area 17):
- Situated in the walls of the posterior part of the calcarine sulcus and
occasionally extends around the occipital pole onto the lateral surface of
the hemisphere
- Macroscopically, this area can be recognized by the thinness of the cortex
and the visual stria
- From the late ral geniculate body, fibers first pass forward in the white
matter of the temporal lobe and then turn back to the primary visual
cortex in the occipital lobe.
- The visual cortex receives fibers from the opposite field of vision (right
cortex receives from left field & vice versa)
- The superior retinal quadrants (inferior field of vision) pass to the
superior wall of the calcarine sulcus, while the infe rior retinal quadrants
(supe rior field of vision) pass to the inferior wall of the calcarine sulcus.
- The macula lutea, is represented on the cortex in the posterior part of
area 17
The secondary visual area (Brodmann areas 18 and 19):
- Surrounds the primary visual area on the medial and lateral surfaces of
the hemisphere
- This area receives afferent fibers from area 17 and other cortical areas as
well as from the thalamus.
- The function of the secondary visual area is to relate the visual
information received by the primary visual area to past visual experiences
enabling the individual to recognize objects in the scene
The occipital eye field:
This field is thought to exist in the secondary visual area in humans
The function of this eye field is believed to be reflex movements of the eye when it
is following an object.
Areas in the temporal lobe:
The primary auditory area (Brodmann areas 41 and 42):
- Is situated in the inferior wall of the lateral sulcus
- Area 41 is a granular type of cortex; area 42 is homotypical and is mainly
an auditory association area.
- Projection fibers arise principally in the medial geniculate body and form
the auditory radiation of the internal capsule .
- The anterior part of the primary auditory area is conce rned with the
reception of sounds of low frequency, and the posterior part of the area is
concerned with the sounds of high frequency.
- A unilateral lesion of the auditory area produces partial deafness in both
ears, the greater loss being in the contralateral ear. This can be explained
on the basis that the medial geniculate body receives fibers mainly from
the organ of Corti of the opposite side as well as some fibers from the
same side.
The secondary auditory area (auditory association cortex):
- Is situated posterior to the primary auditory area in the lateral sulcus and
in the superior temporal gyrus (Brodmann area 22).
- It receives impulses from the primary auditory area and from the
thalamus.
- It is thought to be necessary for the interpretation of sounds and
association of the auditory input with other sensory information.
The sensory speech area of Wernicke:
- Is localized in the left dominant hemisphere, mainly in the superior
temporal gyrus, with extensions around the posterior end of the lateral
sulcus into the parietal region.
- The Wernicke area is connected to the Broca area by a bundle of nerve
fibers called the arcuate fasciculus .
- It receives fibers from the visual cortex in the occipital lobe and the
auditory cortex in the superior temporal gyrus.
- The Wernicke area pe rmits the understanding of the written and spoken
language and enables a person to read a sentence, understand it, and say
it out loud
Other cortical areas:
The taste area:
- Is situated at the lowe r end of the postcentral gyrus in the superior wall of
the lateral sulcus and in the adjoining area of the insula (Brodmann area
43).
- Ascending fibers from the nucleus solitarius probably ascend to the
ventral posteromedial nucleus of the thalamus, where they synapse on
neurons that send fibers to the cortex.
The vestibular area:
- Is believed to be situated near the part of the postcentral gyrus conce rned
with sensations of the face.
- Its location lies opposite the auditory area in the superior temporal gyrus.
- This area with the vestibular part of the inner ear are concerned with
appreciation of the positions and movements of the head in space.
- Through its ne rve connections, the movements of the eyes and the muscles
of the trunk and limbs are influenced in the maintenance of posture.
The insula:
- Is an area of the cortex that is buried within the lateral sulcus and forms
its floor
- It can be examined only when the lips of the lateral sulcus are separated
widely.
- Its fibe r connections are incompletely known.
- It is believed that this area is important for planning or coordinating the
articulatory movements necessary for speech
reception of sounds of low frequency, and the posterior part of the area is
concerned with the sounds of high frequency.
- A unilateral lesion of the auditory area produces partial deafness in both
ears, the greater loss being in the contralateral ear. This can be explained
on the basis that the medial geniculate body receives fibers mainly from
the organ of Corti of the opposite side as well as some fibers from the
same side.
The secondary auditory area (auditory association cortex):
- Is situated posterior to the primary auditory area in the lateral sulcus and
in the superior temporal gyrus (Brodmann area 22).
- It receives impulses from the primary auditory area and from the
thalamus.
- It is thought to be necessary for the interpretation of sounds and
association of the auditory input with other sensory information.
The sensory speech area of Wernicke:
- Is localized in the left dominant hemisphere, mainly in the superior
temporal gyrus, with extensions around the posterior end of the lateral
sulcus into the parietal region.
- The Wernicke area is connected to the Broca area by a bundle of nerve
fibers called the arcuate fasciculus .
- It receives fibers from the visual cortex in the occipital lobe and the
auditory cortex in the superior temporal gyrus.
- The Wernicke area pe rmits the understanding of the written and spoken
language and enables a person to read a sentence, understand it, and say
it out loud
Other cortical areas:
The taste area:
- Is situated at the lowe r end of the postcentral gyrus in the superior wall of
the lateral sulcus and in the adjoining area of the insula (Brodmann area
43).
- Ascending fibers from the nucleus solitarius probably ascend to the
ventral posteromedial nucleus of the thalamus, where they synapse on
neurons that send fibers to the cortex.
The vestibular area:
- Is believed to be situated near the part of the postcentral gyrus conce rned
with sensations of the face.
- Its location lies opposite the auditory area in the superior temporal gyrus.
- This area with the vestibular part of the inner ear are concerned with
appreciation of the positions and movements of the head in space.
- Through its ne rve connections, the movements of the eyes and the muscles
of the trunk and limbs are influenced in the maintenance of posture.
The insula:
- Is an area of the cortex that is buried within the lateral sulcus and forms
its floor
- It can be examined only when the lips of the lateral sulcus are separated
widely.
- Its fibe r connections are incompletely known.
- It is believed that this area is important for planning or coordinating the
articulatory movements necessary for speech
Cerebral Dominance:
- An anatomical examination of the two cerebral hemispheres shows that
the cortical gyri and fissures are almost identical.
- Nervous pathways projecting to the cortex do so largely contralaterally
and equally to identical cortical areas.
- Cerebral commissures provide a pathway for information that is received
in one hemisphere to be transferred to the other.
- Nevertheless, certain nervous activity is predominantly performed by one
of the two cerebral hemispheres.
- Handedness, perception of language, and speech are functional areas of
behavior that in most individuals are controlled by the dominant
hemisphere.
- By contrast, spatial perception, recognition of faces, and music are
interpreted by the nondominant hemisphere
- The left hemisphere is dominant in 90% of right handed & 64% of left
handed individuals
- The right hemisphe re is dominant in 10% of right handed & 20% of left
handed individuals
- Both hemispheres are dominant in the remaining 16% of left handed
individuals
- Ambidextrous individuals usually have leftr dominant hemisphe re
- Workers shown that there is more crossed fibers from the left to right
pyramids, others shown that differences in the size of Brocas area exist!
- It is believed that the two hemispheres of the newborn have equipotential
capabilities.
- During childhood, one hemisphe re slowly comes to dominate the other,
and it is only afte r the first decade that the dominance becomes fixed.
- This would explain why a 5-year-old child with damage to the dominant
hemisphere can easily learn to become left-handed and speak well,
whe reas in the adult this is almost impossible.
Clinical Notes:
General considerations:
- The cerebral cortex should be regarded as the last receiving station
involved along a line of stations receiving information from the eyes, ears
and organs of general sensation.
- The function of the cortex is, in simple terms, to discriminate, and it
relates the received information to past memories.
- The enriched sensory input is then presumably discarded, stored, or
translated into action.
- In this whole process, there is interplay between the cortex and basal
nuclei provided by the many cortical and subcortical nervous
connections.
Lesions of the cerebral cortex:
- In humans, the effect of destruction of different areas of the cerebral
cortex has been studied by:
1- Examining patients with lesions resulting from cerebral tumors, vascular
accidents, surgery, or head injuries.
2- Electrical recordings from different areas of the cortex when stimulating
different parts of the cortex in the conscious patient.
- An anatomical examination of the two cerebral hemispheres shows that
the cortical gyri and fissures are almost identical.
- Nervous pathways projecting to the cortex do so largely contralaterally
and equally to identical cortical areas.
- Cerebral commissures provide a pathway for information that is received
in one hemisphere to be transferred to the other.
- Nevertheless, certain nervous activity is predominantly performed by one
of the two cerebral hemispheres.
- Handedness, perception of language, and speech are functional areas of
behavior that in most individuals are controlled by the dominant
hemisphere.
- By contrast, spatial perception, recognition of faces, and music are
interpreted by the nondominant hemisphere
- The left hemisphere is dominant in 90% of right handed & 64% of left
handed individuals
- The right hemisphe re is dominant in 10% of right handed & 20% of left
handed individuals
- Both hemispheres are dominant in the remaining 16% of left handed
individuals
- Ambidextrous individuals usually have leftr dominant hemisphe re
- Workers shown that there is more crossed fibers from the left to right
pyramids, others shown that differences in the size of Brocas area exist!
- It is believed that the two hemispheres of the newborn have equipotential
capabilities.
- During childhood, one hemisphe re slowly comes to dominate the other,
and it is only afte r the first decade that the dominance becomes fixed.
- This would explain why a 5-year-old child with damage to the dominant
hemisphere can easily learn to become left-handed and speak well,
whe reas in the adult this is almost impossible.
Clinical Notes:
General considerations:
- The cerebral cortex should be regarded as the last receiving station
involved along a line of stations receiving information from the eyes, ears
and organs of general sensation.
- The function of the cortex is, in simple terms, to discriminate, and it
relates the received information to past memories.
- The enriched sensory input is then presumably discarded, stored, or
translated into action.
- In this whole process, there is interplay between the cortex and basal
nuclei provided by the many cortical and subcortical nervous
connections.
Lesions of the cerebral cortex:
- In humans, the effect of destruction of different areas of the cerebral
cortex has been studied by:
1- Examining patients with lesions resulting from cerebral tumors, vascular
accidents, surgery, or head injuries.
2- Electrical recordings from different areas of the cortex when stimulating
different parts of the cortex in the conscious patient.
- One thing that has emerged from these studies is that the human cerebral
cortex possesses, in a remarkable degree, the ability to reorganize the
remaining intact cortex so that a certain amount of cerebral recovery is
possible after brain lesions.
The motor cortex:
- Lesions of the primary motor cortex in one hemisphere result in paralysis
of the contralateral extremities, the more skilled movements suffering
most.
- Destruction of both primary & secondary areas produces the most
complete form of contralateral paralysis.
- Lesions of the secondary motor area alone produce difficulty in the
performance of skilled movements, with little loss of strength.
- The jacksonian epileptic seizure begins in the part of the body
represented in the primary motor area that is being irritated, the
convulsive movement may be restricted to one part of the body or it may
spread to involve many regions
The motor speech area:
- Destructive lesions in the left inferior frontal gyrus result in the loss of
ability to produce speech )expressive aphasia)
- The patients, however, retain the ability to think the words they wish to
say, they can write the words, and they can understand their meaning
when they see or hear them.
The sensory speech area:
- Lesions in this area produces loss of ability to understand the spoken and
written word (receptive aphasia)
- Since the Broca area is unaffected, speech is unimpaired, and the patient
can produce fluent speech. However, the patient is unaware of the
meaning of the words he or she uses and uses incorrect words or even
nonexistent words.
Both speech areas:
Destructive lesions involving both the Broca and Wernicke speech areas result in
loss of the production of speech and the understanding of the spoken and written
word (global aphasia).
The dominant angular gyrus:
- This part is often considered a part of the Wernicke area
- Lesion here results in the patient being unable to read (alexia) or write
(agraphia).
The prefrontal cortex:
- Tumors or traumatic destruction of the prefrontal cortex result in the
person's losing initiative and judgment.
- Emotional changes that occur include a tendency to euphoria.
- The patient no longer conforms to the accepted mode of social behavior
and becomes careless of dress and appearance.
- Schizophrenia, which include important disorders of thought, commonly
associated with pathology in this area
- Frontal leukotomy (cutting the fiber tracts of the frontal lobe) and frontal
lobectomy (removal of the frontal lobe) are surgical procedures that have
been used to reduce the emotional responsiveness of patients with
obsessive emotional states and intractable pain.
The sensory cortex:
cortex possesses, in a remarkable degree, the ability to reorganize the
remaining intact cortex so that a certain amount of cerebral recovery is
possible after brain lesions.
The motor cortex:
- Lesions of the primary motor cortex in one hemisphere result in paralysis
of the contralateral extremities, the more skilled movements suffering
most.
- Destruction of both primary & secondary areas produces the most
complete form of contralateral paralysis.
- Lesions of the secondary motor area alone produce difficulty in the
performance of skilled movements, with little loss of strength.
- The jacksonian epileptic seizure begins in the part of the body
represented in the primary motor area that is being irritated, the
convulsive movement may be restricted to one part of the body or it may
spread to involve many regions
The motor speech area:
- Destructive lesions in the left inferior frontal gyrus result in the loss of
ability to produce speech )expressive aphasia)
- The patients, however, retain the ability to think the words they wish to
say, they can write the words, and they can understand their meaning
when they see or hear them.
The sensory speech area:
- Lesions in this area produces loss of ability to understand the spoken and
written word (receptive aphasia)
- Since the Broca area is unaffected, speech is unimpaired, and the patient
can produce fluent speech. However, the patient is unaware of the
meaning of the words he or she uses and uses incorrect words or even
nonexistent words.
Both speech areas:
Destructive lesions involving both the Broca and Wernicke speech areas result in
loss of the production of speech and the understanding of the spoken and written
word (global aphasia).
The dominant angular gyrus:
- This part is often considered a part of the Wernicke area
- Lesion here results in the patient being unable to read (alexia) or write
(agraphia).
The prefrontal cortex:
- Tumors or traumatic destruction of the prefrontal cortex result in the
person's losing initiative and judgment.
- Emotional changes that occur include a tendency to euphoria.
- The patient no longer conforms to the accepted mode of social behavior
and becomes careless of dress and appearance.
- Schizophrenia, which include important disorders of thought, commonly
associated with pathology in this area
- Frontal leukotomy (cutting the fiber tracts of the frontal lobe) and frontal
lobectomy (removal of the frontal lobe) are surgical procedures that have
been used to reduce the emotional responsiveness of patients with
obsessive emotional states and intractable pain.
The sensory cortex:
- Lesions of the primary somesthetic area of the cortex result in
contralateral sensory disturbances, which are most severe in the distal
parts of the limbs.
- Crude painful, tactile, and the rmal stimuli often return, but this is
believed to be due to the function of the thalamus.
- The patient remains unable to judge degrees of warmth, unable to localize
tactile stimuli accurately, and unable to judge weights of objects.
- Lesions of the secondary somesthetic area of the cortex do not cause
recognizable sensory defects.
- Lesions of the sensory association area inte rfere with the patient's ability
to combine touch, pressure, and proprioceptive impulses, hence, inability
to appreciate texture, size, and form (astereognosis)
- Destruction of the posterior part of the parietal lobe, which integrates
somatic and visual sensations, will interfere with the appreciation of body
image on the opposite side of the body!
The primary visual area:
- Lesions involving the walls of the posterior part of one calcarine sulcus
result in a loss of sight in the opposite visual field (crossed homonymous
hemianopia)
- Lesions of the occipital pole produce central scotomas.
The secondary visual area:
Lesions of the secondary visual area result in a loss of ability to recognize objects
seen in the opposite field of vision. The reason for this is that the area of cortex
that stores past visual experiences has been lost.
The primary auditory area:
- A lesion of one cortical area will produce slight bilateral loss of hearing,
but the loss will be greater in the opposite ear.
- The main defect noted is a loss of ability to locate the source of the sound.
- Bilateral destruction of the primary auditory areas causes complete
deafness.
The secondary auditory area:
Lesions in this area result in an inability to inte rpret sounds
contralateral sensory disturbances, which are most severe in the distal
parts of the limbs.
- Crude painful, tactile, and the rmal stimuli often return, but this is
believed to be due to the function of the thalamus.
- The patient remains unable to judge degrees of warmth, unable to localize
tactile stimuli accurately, and unable to judge weights of objects.
- Lesions of the secondary somesthetic area of the cortex do not cause
recognizable sensory defects.
- Lesions of the sensory association area inte rfere with the patient's ability
to combine touch, pressure, and proprioceptive impulses, hence, inability
to appreciate texture, size, and form (astereognosis)
- Destruction of the posterior part of the parietal lobe, which integrates
somatic and visual sensations, will interfere with the appreciation of body
image on the opposite side of the body!
The primary visual area:
- Lesions involving the walls of the posterior part of one calcarine sulcus
result in a loss of sight in the opposite visual field (crossed homonymous
hemianopia)
- Lesions of the occipital pole produce central scotomas.
The secondary visual area:
Lesions of the secondary visual area result in a loss of ability to recognize objects
seen in the opposite field of vision. The reason for this is that the area of cortex
that stores past visual experiences has been lost.
The primary auditory area:
- A lesion of one cortical area will produce slight bilateral loss of hearing,
but the loss will be greater in the opposite ear.
- The main defect noted is a loss of ability to locate the source of the sound.
- Bilateral destruction of the primary auditory areas causes complete
deafness.
The secondary auditory area:
Lesions in this area result in an inability to inte rpret sounds
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