Forty Two

Question 1

What are the 3 types of cerebral cortex? What makes up 90% of the cortex?

Answer 1

The term cortex means the outer covering of the cerebrum. It is highly folded because of

the confined space of the skull during fetal development. The cortex consists of gryi

(convolutions) and sulci (grooves). There are three types of cortex: neo-, paleo-, and
archicortex. The neocortex appears latest in evolution and constitutes about 90% of the

total cerebral cortex. The paleocortex is restricted to the base of the telencephalon and is

associated with the olfactory system, while the archicortex comprises the hippocampal

formation. Since the paleocortex is closely associated with olfaction and the archicortex

is a part of the limbic system, both are considered with those systems.

Question 2

What are the two main neuronal cell types in the cortex? What do they look like? What is their function? Where do they synapse?

Answer 2

The young adult cortex contains billions of neurons consisting of five types of cells. The

two main neuronal cell types are the pyramidal and granule cells. The pyramidal cells

are comprised of a pyramid-shaped cell body which gives off a large apical dendrite

directed toward the surface of the cortex and several large basal dendrites that pass

horizontally from the base of the cell. The axon proceeds from the base of the cell and in

most cases leaves the cortex to reach other cortical areas or subcortical nuclei. The

pyramidal cells are, therefore, the chief cortical efferent or output neurons.

The granule or stellate cells are the main interneurons of the cortex and greatly

outnumber the pyramidal cells. These small cells give off numerous short dendrites in all

directions and a short axon that arborizes on the other neurons in the vicinity. Granule

cells occur in great numbers in all cortical areas and are especially numerous in the

sensory and association areas.

Question 3

How is the cortex arranged anatomically? Name the six layers.

Answer 3

The neurons of the neocortex are arranged in six horizontal layers. Most superficial is

the cell-poor molecular layer and deepest is the multiform layer. In between are

alternating external and internal granular (layers 2 and 4) and pyramidal (layers 3 and 5)

layers.

Question 4

How is the cortex arranged functionally?

Answer 4

Although the neurons of the cortex are arranged in six layers oriented parallel to the

surface, the functional units of cortical activity are organized in groups of neurons

oriented perpendicular to the surface. These vertically oriented functional units are

called cortical columns and appear to average about 300 microns in diameter and to

contain thousands of neurons that are interconnected in the vertical direction. Each

column forms a working unit with a very specific function.

Question 5

What are 4 types of neurons or connection sin the cerebral cortex? Generally, where does each originate and where does it synapse? From which layer does it originate and which layers receive synapses?

Answer 5

The neurons of the cerebral cortex have: 1) intracortical, 2) associational, 3) commissural

and 4) subcortical connections. Intracortical connections are quite short and occur

chiefly through the axons of horizontal cells in layer I and horizontally coursing

collaterals from the axons of the pyramidal neurons in other layers. The association

neurons are pyramidal cells in layer III which give rise to axons that enter the subcortical

white matter and pass to other cortical areas in the same hemisphere. Commissural

neurons are pyramidal cells in layer III whose axons enter the white matter and pass via

the corpus callosum or anterior commissure to cortical areas usually homologous, in the

opposite hemisphere. Subcortical connections occur via projection fibers that arise from

pyramidal neurons chiefly in layer V, although some in layer VI also contribute to this

group. Layer V gives rise to most of the corticofugal projection fibers, i.e., spinal,

nuclear, pontine, rubral, striate, reticular, etc., while layer VI gives rise to the

corticothalamic projection fibers.

As far as the distributions of incoming fibers are concerned, the association and

commissural fibers end chiefly in layers II and III. The specific thalamocortical

projections terminate in layer IV. In general, therefore, layer IV is considered to be

chiefly responsible for receiving input, layers V and in a more limited sense VI as

chiefly responsible for output, and the supragranular layers I, II and III, chiefly

involved in associational activities.

Question 6

What are association fibers? What are two kinds and what are they like? Give some examples.

Answer 6

Connections from gyrus to gyrus and from lobe to lobe in the same hemisphere are

made via the association fibers. The short association fibers (arcuate fibers or loops)

connect adjacent gyri, while the long association fibers form bundles that connect more

distant gyri. The main long association bundles are the superior longitudinal fasciculus

and its temporal component, the arcuate fasciculus, the uncinate fasciculus, the

inferior longitudinal fasciculus and the cingulum.

Question 7

What are commisural fibers? What are two examples? What are they like and where are they located?

Answer 7

Connections between neocortical areas of the two hemispheres occur via the

commissural fibers. Two main groups exist: the corpus callosum and the anterior

commissure. The anterior commissure has several components, the most important of

which interconnects the middle and inferior temporal gyri.

The corpus callosum reaches is greatest development in man. It contains more than 300

million fibers which for the most part interconnect homologous cortical areas.

Question 8

What are projection fibers? What are the two different kinds?

Answer 8

Projection fibers connect the cerebral cortex with subcortical nuclei and are classified as

corticofugal if they carry impulses away from the cortex, or corticopetal if they carry

them toward the cortex. The corticofugal projection fibers are distributed to nuclei at all

levels of the brainstem and spinal cord and most have been described with the motor

system. The corticopetal projection fibers arise chiefly in the thalamus and can be

distributed to specific or to widespread cortical areas. In most cases the connections

between the thalamic nuclei and the cerebral cortex are reciprocal.

Question 9

How is the cerebral cortex divided anatomically? Functionally? What are primary, secondary, and association areas?

Answer 9

Anatomically, the cerebral cortex is described according to lobes (frontal, parietal,

temporal, occipital, etc.), which are subdivided into gyri. Functionally, the cortex is

described according to the numerical areas that were originally demarcated by Brodmann,

not on the basis of function but on cytoarchitecture. In addition, the cortex can also be

subdivided into primary, secondary and association areas. Primary areas have specific

functions that include voluntary movement, somatosensation, vision and audition.

Adjacent to the primary areas are secondary areas that have similar but more complex functions. Association areas correlate information from widespread sources and are related to higher mental functions. Most of the human cerebral cortex consists of association areas, allowing us to put thoughts together.

Question 10

What are 3 general disorders that might result from damage to association areas? Describe them.

Answer 10

Damage to association areas may result in: 1) agnosia, the inability to interpret sensory

stimuli, such as sounds or images; 2) apraxia, the inability to carry out a voluntary

movement in the absence of paralysis, sensory loss and ataxia; and 3) aphasia, the

inability to understand or communicate speech, writing or signs.

Question 11

Where is the primary visual cortex? How is it broken up? What information does it receive/project? When it is stimulated by visual input, what are the two places it projects? Where do these areas project to? What is the function of these last areas of visual info?

Answer 11

The occipital cortex contains the primary visual and visual association areas. The primary visual cortex (Brodmann’s area 17), also called the striate area, receives the optic radiation from the thalamus and is located in the gyri forming the walls of the calcarine fissure. The inferior part of the cuneus forms the upper wall of the very deep calcarine fissure and here is represented the lower half of the contralateral field of vision. The upper half of the contralateral visual field is projected to the lingual gyrus which forms the lower wall of the calcarine fissure.

When stimulated by visual input the striate cortex relays information to the parastriate

(association) visual cortex. These areas consist of area 18, the parastriate cortex, and area

19, the peristriate cortex. These extrastriate areas interpret input from the primary

visual cortex and project to the temporal and parietal association cortices so that the

meaning of a visual image may be interpreted and remembered. Evidence suggests that

the temporal association cortex analyzes the form and color of the visual scene while

those in the parietal cortex analyze the spatial aspects of vision, such as the analysis of

motion and positional relationships between objects in the visual scene. Connections

from the extrastriate areas to the temporal and parietal associations areas are respectively

called the “what?” and “where?” pathways of visual processing.

Question 12

What do unilateral lesions of the primary visual cortex result in? Lesions of the extrastriate cortex? Lesions of where pathway? Lesions of the what pathway?

Answer 12

Unilateral lesions of the primary visual cortex result in contralateral homonymous

hemianopsia with or without macular sparing.

Lesions of the extrastriate cortex, specifically those that involve the dorsal spatial

pathway (“where?”) that projects to the parietal lobe can result in the inability to

appreciate the motion of objects. Lesions involving the ventral object recognition

pathway (“what?”) that project to the temporal lobe can result in the inability to see the

world in color, referred to as cerebral achromatopsia.

Question 13

What functions does the temporal cortex contain? Where is the primary auditory cortex located? Where Wernickes area located? What is its function? What is the homologous area on the non-dominant hemisphere involved with?

Answer 13

The temporal cortex forms almost a fourth of the entire cortex and contains the auditory

area as well as areas associated with emotions and higher mental functions such as

memory and speech. The auditory cortex is situated in the transverse temporal gyri of Heschl which is buried in the wall of the lateral fissure. The primary area, #41, is chiefly in the anterior gyrus although it may extend slightly into the adjacent part of the posterior gyrus.

The posterior part of area 22 in the superior temporal gyrus, usually the left (dominant

hemisphere), comprises the sensory or receptive speech area, called Wernicke’s. This

area contains the mechanisms for the comprehension and formulation of language. The

homologous area on the non-dominant hemisphere is important in the hearing of sounds,

rhythm and music.

Question 14

What do the inferior and medial portions of the temporal lobe comprise? What are they involved with?

Answer 14

The inferior and medial portions of the temporal lobe comprise a major component of the

limbic lobe. These temporal structures include the hippocampus and parahippocampal

gyrus and are involved in establishing new declarative memories. The sensation of

olfaction is mediated through the limbic system as well as is emotional and affective

behavior. Olfactory fibers terminate in the uncus, the most medial edge of the anterior

part of the parahippocampal gyrus.

Question 15

What do the middle and inferior temporal gyri comprise? What is their function? Give examples.

Answer 15

The middle and inferior temporal gyri comprise temporal association cortex. These

areas store auditory and visual information. For example, temporal association cortex on

the dominant hemisphere stores information that links speech sounds with their symbolic

significance while the non-dominant hemisphere stores visually based information.

Question 16

What do unilateral lesions of the auditory cortex result in? What might lesions in the surrounding association areas of the dominant hemisphere result in? The non-dominant? What might lesions in wernickes area result in? The homologous area? What might unilateral lesions of the limbic lobe result in? Bilateral? What can precede the onset of temporal lobe seizures? What will lesions of the temporal association cortices result in?

Answer 16

Unilateral lesions of the auditory cortex result in no significant hearing loss because of

the bilateralism of the central auditory pathways. Lesions which involve the surrounding

association areas can result in difficulty in hearing spoken words (dominant hemisphere)

or difficulty in appreciating rhythm/music (non-dominant hemisphere).

Lesions of superior temporal gyrus on the dominant hemisphere (Wernicke’s area)

result in receptive or fluent aphasia. Lesions in areas of the non-dominant hemisphere

homologous to Wernicke’s area result in the impairment of comprehending the affective

aspects of language.

Unilateral lesions of the limbic lobe result in few if any symptoms. However, bilateral

lesions of the limbic lobe can result in the inability to learn new verbal or non-verbal

information (declarative memory). In addition, aggressive or antisocial behavior may

result if the amygdala and its connections are involved. Olfactory auras, the perception

of odors not actually present, can precede the onset of temporal lobe seizures.

Lesions of the temporal association cortices result in deficits of recognition. Thus,

damage to either temporal lobe can result in agnosias, that is, the inability in recognizing,

identifying, and/or naming different categories of objects despite the fact that sensory

systems are intact. Interestingly, lesions of the inferior occipitotemporal cortex results in

the inability to recognize faces referred to prosopagnosia.

Question 17

What are the functional areas of the parietal cortex? Where are they?

Answer 17

The parietal cortex contains the following functional areas: 1) primary sensory, 2)

secondary sensory, 3) gustatory and 4) association. The primary sensory cortex (SI)

occupies the postcentral gyrus and the adjoining part of the paracentral lobule. It

consists of three longitudinal zones: area 3 which includes the cortical tissue in the floor

and posterior wall of the central sulcus, area 1 in the anterior two-thirds of the convex

surface of the postcentral gyrus, and area 2 in the remaining third of the convex surface

and the adjoining anterior wall of the postcentral sulcus.

The secondary sensory area (S-II) comprises a strip of cortex that extends from the

parietal operculum, which is in the upper wall of the lateral fissure and covers the insula,

into the posterior part of the insula.

The primary gustatory cortex appears to be in area 43 of the postcentral gyrus and

includes the parietal operculum and insula and is adjacent to the tongue regions of the

primary sensory and motor areas.

The parietal association area consists of the superior and inferior parietal lobules. The

superior parietal lobule contains areas 5 and 7. Area 5 receives its input chiefly from

the S-I cortex while area 7 has widespread connections with the visual and motor areas of

the cortex. The inferior parietal lobule includes two gyri: the supramarginal (area 40)

and the angular (area 39). These receive input from the other parts of the parietal lobe

as well as from association areas in the frontal, occipital, temporal and limbic lobes.

Question 18

What is the result of unilateral lesions of the primary somatosensory cortex? Of the primary gustatory cortex? Of parietal association areas? What is contralateral neglect syndrome? Where is the lesion? What is astereognosis? What lesion results in it? What is dressing apraxia? Where is the lesion?

Answer 18

Unilateral lesions of the postcentral gyrus and posterior part of the paracentral lobule

result in impairment of the finer aspects of somatic sensation and deficits in position

sense and movement of the contralateral body parts. However, lesions do not abolish

tactile sensation or pain sensation. Lesions involving the primary gustatory cortex

result in aguesia, the loss of the sense of taste.

Widespread lesions of parietal association areas result in deficits in attention, particularly of the opposite side of the body and visual field. For example, people have

trouble telling where objects are; they have trouble seeing motion and depth, reaching for

things, telling left from right, and seeing complex objects as a whole. Specifically, a

massive lesion of the right inferior parietal lobule and/or surrounding areas can result in

the inability to attend to objects in the left side of the visual field and even one’s own left

half of the body. This is referred to as contralateral neglect syndrome. Lesions

involving the superior parietal lobule result in contralateral astereognosis. Lesions of

the non-dominant parietal association cortex can result in the inability to perform the

relatively complex task of dressing (dressing apraxia).

Question 19

What are the main functional areas of the frontal cortex? Where are they located and what is their function?

Answer 19

The frontal cortex enables us to function as affective and socially appropriate human

beings and contains the following main functional areas: 1) primary motor, 2) premotor,

3) frontal eye field, 4) supplementary motor, 5) motor speech, and 5) prefrontal. The

primary motor area corresponds to Brodmann’s area 4 and occupies the anterior wall

of the central sulcus and the adjoining part of the paracentral lobule.

The premotor cortex, area 6, is located in the anterior part of the precentral gyrus,

although dorsally it enlarges and includes the more posterior part of the superior frontal

gyrus as well. It contains extensive reciprocal connections with the primary motor cortex

and gives rise to some corticobulbar and corticospinal pathways that influence local

circuit and lower motor neurons of the brainstem and spinal cord. Evidence suggests that

neurons of the premotor cortex encode the intention to perform a particular movement

and therefore seem to be involved in the selection of movements from a repertoire of

possible movements.

The frontal eye field, area 8, is located immediately in front of area 6, chiefly in the

middle frontal gyrus, although it extends into the superior frontal gyrus as well.

The supplementary motor area consists of the extensions of areas 6 and 8 onto the

medial aspect of the frontal lobe. Thus, it lies in the medial part of the superior frontal

gyrus just in front of the paracentral lobule. Like the premotor cortex, this area mediates

the selection of movements but appears to specialize in initiating movements specified by

internal rather than external cues. For example, brain scans show that this region is

activated when a subject performs motor sequences from memory.

The motor speech center, Broca’s area, comprises areas 44 and 45 in the opercular

and triangular parts of the inferior frontal gyrus of the hemisphere dominant for

language, usually the left.

The prefrontal cortex is referred to as the frontal association cortex and is divided into

two main regions: orbital and lateral. The orbital region, sometimes called the

orbitofrontal, is located on the inferior surface of the frontal lobe and includes chiefly

the orbital gyri, while the lateral area, frequently called the dorsolateral prefrontal

region, includes the gyri on the convexity of the frontal lobe in front of areas 8 and 45.

Question 20

What do unilateral lesions of the precentral gyrus result in? The premotor cortex? Bilateral lesions of the supplementary cortex? Unilateral lesions of the frontal eye field? Lesions of broca area? Of the non-dominant homologous area?

Answer 20

Unilateral lesions of the precentral gyrus and accompanying paracentral lobule result

in contralateral monoplegia or hemiplegia depending on the extent of damage.

Lesions of the premotor cortex and supplementary motor cortex are often obscure

with little or no deficits. Bilateral lesions of the supplementary motor cortex can result in

incontinence, specifically the loss of inhibition (the uninhibited reflex bladder).

Unilateral lesions of the frontal eye field results in transitory paralysis of conjugate eye

movements to the opposite side.

Lesions of inferior part of the dominant frontal lobe (Broca’s area) result in expressive

or nonfluent aphasia. Lesions on the non-dominant area homologous to Broca’s area

lead to difficulty in expressing the emotional aspects of language.

Question 21

Generally, what do massive lesions of the prefrontal cortex result in? Specifically, what do lesions of the dorsolateral prefrontal region tend to produce? What do lesions of the orbitofrontal region lead to? How might the prefrontal region be related to depression? Schizophrenia?

Answer 21

Massive lesions to the prefrontal cortex result in a wide range of cognitive disabilities

that can change a person’s personality (i.e. collective character, behavioral,

temperamental, emotional, and mental traits). This loss of cognitive function is most

apparent in the inability to select, plan and/or execute appropriate behaviors.

Specifically, lesions of the dorsolateral prefrontal region tend to produce an apathetic

and lifeless state incapable of making decisions and/or acting independently. It’s been

shown that this region functions abnormally in psychotic disorders such as schizophrenia.

Lesions of the orbitofrontal region lead to impulsive, disinhibited behavior and poor

judgment. Recent evidence suggests a link between clinical depression and the prefrontal cortex. Brain scans of individuals suffering from depression show areas of
abnormal activity in the orbital and medial prefrontal cortex.

Question 22

What are the two main areas of speech? What is each responsible for? What do lesions of them result in? What differences are there between them? What associations/connections exist between them? What happens in the connection is damaged?

Answer 22

Speech is also represented in widespread areas, but it appears to be limited to one

hemisphere referred to as the dominant hemisphere. Two main areas exist: Broca’s in

the inferior frontal gyrus and Wernicke’s in the region where the superior temporal gyrus

and the inferior parietal lobule meet. A lesion in Broca’s area is associated with an

expressive or motor aphasia which is characterized as non-fluent because of the slow

and prolonged output of words, poor articulation and short sentences containing only

several words. A lesion in Wernicke’s area results in receptive or sensory aphasia

characterized by the normal production of words so that the individual is fluent but the

use of words is defective. This patient substitute’s one word for another, inserts

meaningless words, or strings together words or phrases of great length but no meaning.

Broca’s area contains the motor programs for the production of language, while

Wernicke’s area contains the mechanisms for the comprehension and formulation of

language. Broca’s area directly influences the areas of the motor cortex that generate
words. Unlike Wernicke’s area, it has nothing to do with comprehension and, therefore,

the motor aphasic is aware of the deficiency whereas the receptive aphasic cannot

comprehend speech and thus is unaware of the defect. Wernicke’s area projects to

Broca’s area via association fibers in the superior longitudinal fasciculus and its temporal

branch, the arcuate fasciculus. A lesion interrupting these fibers produces a conduction

aphasia in which the speech deficiency is similar to a receptive aphasia but

comprehension is intact so that the patient makes repeated attempts to say the right

words.