The white matter was briefly mentioned in an earlier post, so this is sort of a continuation of the brain's gross anatomy. The white matter is located underneath the cortical gray matter and composed of fatty myelinated axons. It is an integral part of the central nervous system that transmits messages very rapidly. It basically has 3 types of fiber bundles: the projection fibers, commissural fibers, and association fibers.
- Projection fibers are bi-directional, afferent, and efferent bundles. They appear as radiating bundles in the white matter that exit the cerebral cortex and converge towards the brainstem. One bundle carries visual information through the optic radiation. Near the subcortical nuclei, these axons form a compact band known as the internal capsule with anterior and posterior limbs. Afferent (sensory) fibers: mainly the thalamocortical bundles going to the various region of the cerebral cortex. The efferent fibers of the internal capsule arise from the cerebral cortex. They form various tracts, e.g. corticothalamic, corticobulbar, corticospinal, and corticopontine bundles.
- The axons part of the corpus callosum forms the commissural fibers. At different callosal segment, they have different connections: the rostrum (orbitofrontal), genu (frontal lobe), body (sensorimotor and posterior parietal), and splenium (posterior temporal and occipital). Other commissural fibers are the anterior commissure, connecting the olfactory system bilaterally.
- The association fibers form the bi-directional cortico-cortical bridges connecting areas within the same hemisphere. They can be classified as short and long fasciculus: - Superior longitudinal fasciculus connects frontal and parietal lobes.
- Occipito-frontal fasciculus connects frontal and occipital lobes.
- Arcuate fasciculus connects the frontal with posterior temporal lobes.
- Uncinate fasciculus connects orbitofrontal with anterior temporal lobes.
- Inferior longitudinal fasciculus connects temporal and occipital lobes.
- Extreme capsule fasciculus connects lateral temporal and lateral frontal lobes.
Of interest is the coronal section of the cerebral hemisphere from the insula moving inwards to the thalamus. The external capsule connects the motor cortex to the putamen and is unidirectional. The internal capsule connects specific thalamic nuclei to the specific cortical area and hence it is bidirectional.
The most common way to study the white matter is through MRI which can be observed well on T1-weighted, T2-weighted, and FLAIR sequences. More recently, scientists become more interested in modeling brain development over puberty and brain decline associated with aging. Fun facts: Gray matter volume increases in early childhood but declines after puberty. However, white matter volume progressively increases over time, supporting the concept of neural plasticity.
Cerebral organization
The cerebral cortex is organized into six layers that arise from the time of its development. This is the characteristic of the neocortex. Only the piriform cortex and the hippocampal formation, the oldest cortical structures phylogenetically or paleocortex or allocortex, do not exhibit this six-layer arrangement. The projection fibers are more deep-rooted, while the association and commissural fibers are more superficial. Three principal types of cells found in the cortex include the pyramidal, stellate, and fusiform neurons. Their fibers are arranged either tangentially or radially across layers.
Pyramidal cells, with a shape of a triangle with the top end going up to the surface (apical) and the horizontally running dendrites (basal), constitute the most in various cortical layers. The axons are either going down to the white matter (as projection fibers) or to other cortical areas (as association fibers). The biggest pyramidal cell, the Betz cell, is found only in Layer V of the precentral gyrus or motor cortex. Unlike pyramidal cells, granule or stellate cells are small, polygonal or triangular in shape. They are found in all layers, but especially numerous in Layer IV. Fusiform neurons are spindle-like cells found mostly in the deepest cortical layer, their long axis going vertically upward. Apart from these three types of cells, we encounter others, e.g. horizontal cells found mostly in the superficial layers. The works of Cajal and Golgi are crucial in deepening our understanding on these cells.
In brief, six-layered architecture can be described as follow:
a). Layer I (molecular layer), has few cell bodies, mostly axons, Layer II (external granular layer).
b). Layer III (external pyramidal layer), cells forming mainly association or commissural fibers.
c). Layer IV (internal granular layer), mainly the incoming afferent fibers from the thalamus.
d). Layer V (internal pyramidal layer), mainly efferent projection fibers.
e). Layer VI (multiform, fusiform layer).
What is the relationship between this architecture with the earlier functional lobes? Layer III plays a major role in cortico-cortical connections. Layer IV is predominant in sensory areas in the parietal and temporal lobes, e.g. the postcentral gyrus. These regions are granular. Layer V, on the other hand, is predominant in motor areas, e.g. precentral gyrus.
Principal neurotransmitters
Cognition and the Brain
The study of human cognition and the brain is the heart of a classic science popularly known as neuropsychology. The interests existed since the time of Descartes, Gall, Broca, and so on, who studied the link between a neurological condition (e.g. lesions) and certain behavioral or psychological processes. A classic theory, phrenology, says that the brain is divided into discrete and unique areas responsible for a particular function only. The mastery of certain skills can be deduced by the bigger skeletal landmark of the head. An opposing view at that time held that there is no localization of brain functions and that the functions (what they called "Mind") are distributed across different parts of the brain. With more discoveries, modern neuroscience later thought that the brain is divided into many functional specialization. For example, one may use fMRI to elucidate brain areas associated with some behavioural tasks. One fundamental characteristic of the central nervous system is parallelism, that is, a large number of functions are simultaneously processed along two or more pathways. As a result, the damage of one pathway can allow other pathway to function, and that one brain function can be performed not only strictly by one area.
Cerebral organization
The cerebral cortex is organized into six layers that arise from the time of its development. This is the characteristic of the neocortex. Only the piriform cortex and the hippocampal formation, the oldest cortical structures phylogenetically or paleocortex or allocortex, do not exhibit this six-layer arrangement. The projection fibers are more deep-rooted, while the association and commissural fibers are more superficial. Three principal types of cells found in the cortex include the pyramidal, stellate, and fusiform neurons. Their fibers are arranged either tangentially or radially across layers.
Pyramidal cells, with a shape of a triangle with the top end going up to the surface (apical) and the horizontally running dendrites (basal), constitute the most in various cortical layers. The axons are either going down to the white matter (as projection fibers) or to other cortical areas (as association fibers). The biggest pyramidal cell, the Betz cell, is found only in Layer V of the precentral gyrus or motor cortex. Unlike pyramidal cells, granule or stellate cells are small, polygonal or triangular in shape. They are found in all layers, but especially numerous in Layer IV. Fusiform neurons are spindle-like cells found mostly in the deepest cortical layer, their long axis going vertically upward. Apart from these three types of cells, we encounter others, e.g. horizontal cells found mostly in the superficial layers. The works of Cajal and Golgi are crucial in deepening our understanding on these cells.
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| Fig-1: Six different cortical layers of the cerebral cortex, layer-I being the most superficial. |
a). Layer I (molecular layer), has few cell bodies, mostly axons, Layer II (external granular layer).
b). Layer III (external pyramidal layer), cells forming mainly association or commissural fibers.
c). Layer IV (internal granular layer), mainly the incoming afferent fibers from the thalamus.
d). Layer V (internal pyramidal layer), mainly efferent projection fibers.
e). Layer VI (multiform, fusiform layer).
What is the relationship between this architecture with the earlier functional lobes? Layer III plays a major role in cortico-cortical connections. Layer IV is predominant in sensory areas in the parietal and temporal lobes, e.g. the postcentral gyrus. These regions are granular. Layer V, on the other hand, is predominant in motor areas, e.g. precentral gyrus.
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| Fig-2: The distribution of different cortical composition: (1) Agranular; (2) Granular - frontal (dysgranular); (3) Granular - parietal; (4) Granular - occipital; and (5) Koniocortex. Only cortical motor areas are agranular. |
Principal neurotransmitters
A variety of neurotransmitters is associated with neurons of the cerebral cortex. Among those, we have glutamate, aspartate, and γ-aminobutyric acid (GABA). Pyramidal cells are the main efferent neurons that are predominantly glutaminergic and are excitatory. Most interneurons within the cortex, however, are GABAergic and are inhibitory. They are bridging the afferent and efferent fibers together. Therefore the outputs of the cortex are modulated by a variety of cortical afferents via interneurons.
A variety of neuropeptides or monoamines are also found in the cerebral cortex; they influence not only populations of neurons but also local metabolic activity and vascular smooth muscle. The most important monoamines in the cortex are (1) norepinephrine, which originates from the locus ceruleus of the pons and distributes sparsely to all cortical layers; (2) dopamine, which arises from the substantia nigra–pars compacta and the adjacent ventral tegmental area and is found in moderate amounts in layers I and VI and sparsely in layers II to V; and (3) serotonin, which arises from the raphe nuclei and distributes heavily to all cortical layers.
Cognition and the Brain
The study of human cognition and the brain is the heart of a classic science popularly known as neuropsychology. The interests existed since the time of Descartes, Gall, Broca, and so on, who studied the link between a neurological condition (e.g. lesions) and certain behavioral or psychological processes. A classic theory, phrenology, says that the brain is divided into discrete and unique areas responsible for a particular function only. The mastery of certain skills can be deduced by the bigger skeletal landmark of the head. An opposing view at that time held that there is no localization of brain functions and that the functions (what they called "Mind") are distributed across different parts of the brain. With more discoveries, modern neuroscience later thought that the brain is divided into many functional specialization. For example, one may use fMRI to elucidate brain areas associated with some behavioural tasks. One fundamental characteristic of the central nervous system is parallelism, that is, a large number of functions are simultaneously processed along two or more pathways. As a result, the damage of one pathway can allow other pathway to function, and that one brain function can be performed not only strictly by one area.
Modern neuropsychology enjoys a multidisciplinary collaboration among cognitive scientists, physiologists, neuroscientists, and clinical psychologists. Originally, the field drew strong attention when Paul Broca came into contact with a patient undergoing a progressive speech disorder in 1861, who could only produce "tan". After the patient died, Broca found out that his inferior frontal gyrus (IFG) was damaged. Named after Broca, the type of such behavioral deficit linked to the damage of IFG is then called Broca's aphasia. Note: IFG is rostral to the mouth/orofacial musculature of the cortical motor area.
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| Fig-3: The difference between Broca's and Wernicke's aphasia together with affected areas on the left hemisphere. |
Broca's finding was further developed with the findings of Carl Wernicke. He found that in a certain type of language disorder, the patients were able to produce speech but unable to comprehend the conversation. Called Wernicke's aphasia, the damage is found to be around the posterior part of the superior temporal gyrus (STG). This aphasia is not equal to deafness, for the person with Wernicke's aphasia is able to detect sound but unable to make sense of it. He further hypothesized that there is a link between IFG and STG and this is crucial in language. To be able to converse well, one has to first listen and understand the sentences one hears. Note: STG is near to the primary and secondary auditory cortex.
In the 1870s, John Hughlings Jackson proposed that the cerebral cortex is organized hierarchically and that some cortical areas are for higher-order functions (or cognitive) that are neither fully sensory nor motor. These brain areas are called association areas because they serve to associate sensory inputs to motor response and conduct mental processes related to sensorimotor behavior. The mental processes that Jackson attributed to these areas include interpretation of sensory information, the association of perceptions with previous experience, focusing of attention, and exploration of the environment. Jackson's finding is supported by clinical works. The major helps come from surgical rooms of patients with damage or lesion on the specific are, or people with underlying conditions. Other methods include experimental studies with monkeys and rats and the use of non-invasive brain imaging technology.
Before ending, I wish to mention major associative areas in the human brain important in cognition:
- The posterior association area: the margin of the parietal, temporal, and occipital lobes. It integrates information from several sensory modalities such as vision, space, and body senses. It is also involved in language. Separate studies by Holmes and Luria on wounded soldiers found that bilateral injuries to the posterolateral parietal lobe yield to normal visual acuity but the soldiers were unable to scan visually or reach for an object of interest. They could not process together with the visual information when asked to describe in words what that they saw. This shows that the region is critical for integrating different sensory modalities and for using that integrated information to direct behavior.
- The anterior association area: the prefrontal region, rostral to postcentral gyrus. It is involved in the planning of action, shaping behavior, and judgment; a more popular term is the "Executive function". The most popular case showing how the injured prefrontal region leads to behavioral problems is perhaps of Phineas Gage. A series of clinical tests, e.g. the Tower of London test and the Wisconsin Card Sorting Test (WCST), can be used to diagnose people with neuropsychological disorders who have lost their executive functions, such as schizophrenia. WCST is primarily considered a test of executive functions, particularly abstract reasoning and cognitive flexibility in response to external changes.
- The limbic association area: along the lower medial end of the cerebral hemisphere. It is for emotion, learning, and memory. Its involvement in learning and memory comes from the well-known study on patient H.M. by B. Milner in 1960s after both medial temporal lobes had been removed. She first demonstrated the remarkably selective role of this part of the brain in converting short-term into long-term memory. Studies in monkeys have helped establish that association areas in the medial temporal lobe, including the hippocampal formation, receive information from virtually every other association area. In other words, the hippocampal formation is able to sample the whole stream of ongoing cognitive activity and thereby relate different aspects of a single event so that they can be recalled as a coherent experience.
More recently, cognitive neuroscience is recognized as another separate field, combining neuroscience, neurophysiology, and psychology. Scientists now agree that the three areas (the triad) of executive function are working memory, flexible thinking, and inhibitory control.
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