Right Brain Left Brain
The cerebrum is divided into two halves: the right and left hemispheres They are joined by a bundle of fibers called the corpus callosum that transmits messages from one side to the other. Each hemisphere controls the opposite side of the body. If a stroke occurs on the right side of the brain, your left arm or leg may be weak or paralyzed.
Not all functions of the hemispheres are shared. In general, the left hemisphere controls speech, comprehension, arithmetic, and writing. The right hemisphere controls creativity, spatial ability, artistic, and musical skills. The left hemisphere is dominant in hand use and language in about 92% of people.
Lobes Of The Cerebrum
The cerebral cortex is classified into four lobes, according to the name of the corresponding cranial bone that approximately overlies each part. Each lobe contains various cortical association areas – where information from different modalities are collated for processing. Together, these areas function to give us a meaningful perceptual interpretation and experience of our surrounding environment.
What Does The Brain’s Cerebral Cortex Do
- B.A., Biology, Emory University
- A.S., Nursing, Chattahoochee Technical College
The cerebral cortex is the thin layer of the brain that covers the outer portion of the cerebrum. It is covered by the meninges and often referred to as gray matter. The cortex is gray because nerves in this area lack the insulation that makes most other parts of the brain appear to be white. The cortex also covers the cerebellum.
The cortex makes up about two-thirds of the brain’s total mass and lies over and around most of the brain’s structures. It consists of folded bulges called gyri that create deep furrows or fissures called sulci. The folds in the brain add to its surface area and increase the amount of gray matter and the quantity of information that can be processed.
The cerebrum is the most highly developed part of the human brain and is responsible for thinking, perceiving, producing, and understanding language. Most information processing occurs in the cerebral cortex. The cerebral cortex is divided into four lobes that each have a specific function. These lobes include the frontal lobes, parietal lobes, temporal lobes, and occipital lobes.
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Functions Of The Cerebral Cortex
The frontal lobe is responsible for thinking, planning, performing actions, voluntary movements, speech production, and emotional control. The anterior portion of this lobe is called the prefrontal cortex and it represents the highest part of the CNS.
This is where the highest forms of thought, emotion, and perception of oneself and the social environment take place.
Temporal lobes are involved in the processes of:
- Music features.
The parietal lobes house the following centers:
- The central part of the somatosensory function which consists of cones for touch, pain, temperature, pressure
- Spatial observations of space and organization of activities in space
- Centers for processes regarding attention, body language, and some math skills.
The occipital lobes are responsible for:
- Visual observation
- Perception of shape, color, movement, and light.
The activities of the cortex are mostly conscious while the activities of the subcortical structures are unconscious.
What Is The Gray Matter And White Matter
Gray and white matter are two different regions of the central nervous system. In the brain, gray matter refers to the darker, outer portion, while white matter describes the lighter, inner section underneath. In the spinal cord, this order is reversed: The white matter is on the outside, and the gray matter sits within.
Gray matter is primarily composed of neuron somas , and white matter is mostly made of axons wrapped in myelin . The different composition of neuron parts is why the two appear as separate shades on certain scans.
Each region serves a different role. Gray matter is primarily responsible for processing and interpreting information, while white matter transmits that information to other parts of the nervous system.
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Evolution Of The Cerebral Cortex
It is now well established that the cerebral cortex forms as a smooth sheet populated by neurons that proliferate at the ventricular surface and migrate outwards along radial glial fibers . Differences in the duration of neurogenesis, which increases more rapidly with brain size for the cerebral cortex than for subcortical areas , lead to a systematic increase in the ratio of the cortical to subcortical regions. Whereas in small brained species the cortical volume expands by virtue of a combined increase in surface area and cortical thickness, the increase of the cortical volume in species with a brain size of more than 34 cm3 is almost entirely due to a disproportionate expansion of the cortical surface area . It is the increase of the cortical surface area beyond that expected for geometrically similar objects of different volumes which creates the need to cortical folding .
Figure 2. Lateral views of the brains of some anthropoid primates showing the evolutionary expansion of the neocortex. Note the diverse configurations and gyral and sulcal patterns. Saimiri sciureus: E = 22 g Macaca mulatta: E = 95 g Pan troglodytes: E = 420 g Homo sapiens: E = 1350 g. Reproduced with permission from Hofman .
What Is The Cerebral Cortex
The cerebral cortex is the outermost layer of the brain that is associated with our highest mental capabilities. The cerebral cortex is primarily constructed of grey matter , with between 14 and 16 billion neurons being found here.
Although the cerebral cortex is only a few millimeters in thickness, it consists of approximately half the weight of the total brain mass.The cerebral cortex has a wrinkled appearance, consisting of bulges, also known as gyri, and deep furrows, known as sulci.
The many folds and wrinkles of the cerebral cortex allow for a wider surface area for an increased number of neurons to live there, permitting large amounts of information to be processed.
The cortex is also divided into two hemispheres, the right and left, which is separated by a large sulcus called the medial longitudinal fissure.
The two hemispheres are connected via bundles of nerve fibers called the corpus callosum, to allow both hemispheres of the cerebral cortex to communicate with each other and for further connections to be made.
A vast array of functions are controlled by the cerebral cortex through the use of the lobes, which are divided based on the location of gyri and sulci. These lobes are called the frontal lobes, temporal lobes, parietal lobes, and occipital lobes.
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Principles Of Brain Evolution
If we assume that biological intelligence in higher organisms is the product of processes of complex sensory information processing and mental faculties, responsible for the planning, execution and evaluation of intelligent behavior, variations among species in intelligence must in principle be observable in the neural substrate. In higher organisms, especially in primates, the complexity of the neural circuitry of the cerebral cortex is considered to be the neural correlate of the brain’s coherence and predictive power, and, thus, a measure of intelligence.
The evolutionary expansion of the cerebral cortex, indeed, is among the most distinctive morphological features of mammalian brains. Particularly in species with large brains, and most notably in great apes and marine mammals, the brain becomes disproportionately composed of this cortical structure . The volume of cortical gray matter, for example, expressed as a percentage of total brain volume increases from about 25% for insectivores to 50% for humans , whereas the relative size of the entire cerebral cortex goes from 40% in mice to about 80% in humans .
Biological Limits To Information Processing
Although the cerebral cortex is not the only brain structure which was selected for in evolution to expand, as a result of growing environmental pressure for more sophisticated cognitive abilities, it has played a key role in the evolution of information processing in the mammalian brain. The primate cortex, as we have seen, has evolved from a set of underlying structures that constrain its size, and the amount of information it can store and process. If the ability of an organism to process information about its environment is a driving force behind evolution, then the more information a system, such as the brain, receives, and the faster it can process this information, the more adequately it will be able to respond to environmental challenges and the better will be its chances of survival . The limit to any intelligent system therefore lies in its abilities to process and integrate large amounts of sensory information and to compare these signals with as many memory states as possible, and all that in a minimum of time. It implies that the functional capacity of a neuronal structure is inherently limited by its neural architecture and signal processing time .
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Myelin Maps In Primate Cortex
Figure 3 shows group average myelin maps for human, chimpanzee, and macaque, displayed on inflated left hemisphere surfaces. These maps are based on the ratio of T1-weighted to T2-weighted structural images, computed for each voxel in the gray matter ribbon, mapped to individual surfaces, and then registered to a group average surface. This qualitative, noninvasive measure correlates strongly with postmortem patterns of myelin staining density and is informative about regional aspects of functional specialization . Red and orange represent heavily myelinated cortex, which includes early areas in somatosensory, motor, auditory, and visual cortex, including the distinctive MT+ complex of visual areas engaged in motion processing and also retrosplenial cortex. Yellow and green represent moderately myelinated cortex and include areas involved in higher stages of visual, auditory, somatosensory, and motor processing. Blue and indigo represent lightly myelinated cortex that extend over much of frontal, parietal, and lateral temporal regions.
Group average cortical myelin maps from human, chimpanzee, and macaque. Black bars indicate relative sizes of the group average inflated surfaces for each species. The MT+ complex refers to heavily myelinated area MT plus the heavily myelinated portions of the neighboring MST complex, also implicated in motion processing .
Ventricles And Cerebrospinal Fluid
Deep in the brain are four open areas with passageways between them. They also open into the central spinal canal and the area beneath arachnoid layer of the meninges.
The ventricles manufacture cerebrospinal fluid, or CSF, a watery fluid that circulates in and around the ventricles and the spinal cord, and between the meninges. CSF surrounds and cushions the spinal cord and brain, washes out waste and impurities, and delivers nutrients.
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The Brain Stem Relays Signals Between The Brain And Spinal Cord And Manages Basic Involuntary Functions
The brain stem connects the spinal cord to the higher-thinking centers of the brain. It consists of three structures: the medulla oblongata, the pons, and the midbrain. The medulla oblongata is continuous with the spinal cord and connects to the pons above. Both the medulla and the pons are considered part of the hindbrain. The midbrain, or mesencephalon, connects the pons to the diencephalon and forebrain. Besides relaying sensory and motor signals, the structures of the brain stem direct involuntary functions. The pons helps control breathing rhythms. The medulla handles respiration, digestion, and circulation, and reflexes such as swallowing, coughing, and sneezing. The midbrain contributes to motor control, vision, and hearing, as well as vision- and hearing-related reflexes.
The Seat Of Consciousness: High Intellectual Functions Occur In The Cerebrum
The cerebrum is the largest brain structure and part of the forebrain . Its prominent outer portion, the cerebral cortex, not only processes sensory and motor information but enables consciousness, our ability to consider ourselves and the outside world. It is what most people think of when they hear the term grey matter. The cortex tissue consists mainly of neuron cell bodies, and its folds and fissures give the cerebrum its trademark rumpled surface. The cerebral cortex has a left and a right hemisphere. Each hemisphere can be divided into four lobes: the frontal lobe, temporal lobe, occipital lobe, and parietal lobe. The lobes are functional segments. They specialize in various areas of thought and memory, of planning and decision making, and of speech and sense perception.
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Principles Of Neural Wiring
Studies in mammals have shown that in species with convoluted brains the mass of interconnective nerve fibers, forming the underlying white matter, is proportional to the 1.28 power of brain volume , meaning that the cortical white matter is a fractal system. As a result, the total cortical surface area, including all gyri and sulci, scales approximately as the 2/3 power of the white matter volume.
In other words, the surface area of the cerebral cortex, and with that the total number of cortical columns, is geometrically similar with the amount of white matter, i.e., with the number and length of the interconnective nerve fibers. In small species with non-convoluted brains a similar relationship was found between the cortical surface area and the mass of myelinated nerve fibers . A fractal dimension of D = 2.70, as found for convoluted brains , suggests a high degree of parallel processing to take place in the cerebral cortex and emphasizes the processing and/or transfer of information across cortical regions in highly corticalized mammals, such as monkeys and apes, rather than within regions. To reach the state of integral parallelism in which each neural component has its own terminal, the length and number of the interconnective axons must be reduced in order to set limits to the axonal mass.
How And Where The Cortex Folds
Much has been written about how the cerebral cortex gets its distinctive folds. Here, we summarize key issues and observations without attempting to be comprehensive. In brief, four main mechanisms have been proposed.
Mechanical tension along the length of axons coursing through the white matter would tend to bring strongly connected regions closer together, thus forming a gyral fold that reduces the wiring length of these connections , whereas sulci would be more likely to form between weakly connected regions as schematized in Figure 2 . This hypothesis has great explanatory power, including the ability to explain consistency of folding in regions dominated by large areas with strong and consistent connections and variability of folding in balkanized mosaics of smaller areas. Also, tension-based folding would naturally lead to compact wiring , which is as important for brains as it is for computer chips. However, there is skepticism in some quarters about tension-based folding space limitations here preclude detailed discussion of these criticisms and our specific counterarguments.
Schematic drawings of the hypothesized complementary roles of intraventricular pressure and axonal tension in early stages of cortical expansion and gyrification. Reproduced, with permission, from Van Essen .
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Mechanisms Of Cortical Folding
Previous hypotheses about cortical folding have emphasized mechanisms intrinsic to the cortical gray matter . Van Essen suggested that extrinsic factors are more important and that tension along axons in the white matter is the primary driving force for cortical folding. By keeping the aggregate length of axonal and dendritic wiring low, tension should contribute to the compactness of neural circuitry throughout the cortex. Despite the many attempts to clarify the mechanical basis of cortical folding the process remains incompletely understood.
Recently, Herculano-Houzel and colleagues have found that connectivity and cortical folding are directly related across species and that a simple model based on a white matter-based mechanism may account for increased cortical folding in the primate cerebral cortex . They argue that the mechanical tension generated by the pattern of connectivity of fiber bundles traveling through white matter may account for the observed pattern of cortical surface convolutions. The authors propose the degree of tension, taken as directly proportional to the morphological characteristics of the fiber bundle , determines how much the cortical surface folds inwards.
Genes Associated With Cortical Disorders
There are a number of genetic mutations that can cause a wide range of genetic disorders of the cerebral cortex, including microcephaly, schizencephaly and types of lissencephaly.Chromosome abnormalities can also result causing a number of neurodevelopmental disorders such as fragile X syndrome and Rett syndrome.
MCPH1 codes for microcephalin, and disorders in this and in ASPM are associated with microcephaly. Mutations in the gene NBS1 that codes for nibrin can cause Nijmegen breakage syndrome, characterised by microcephaly.
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What Is In The Frontal Lobe
The frontal lobe is the most anterior part of the brain. It extends from the area behind the forehead back to the precentral gyrus. As a whole, the frontal lobe is responsible for higher cognitive functions such as memory, emotions, impulse control, problem solving, social interaction, and motor function.
Four Cerebral Cortex Lobes
- Parietal Lobes: These lobes are positioned posteriorly to the frontal lobes and above the occipital lobes. They are involved in receiving and processing of sensory information. The somatosensory cortex is found within the parietal lobes and is essential for processing touch sensations.
- Frontal Lobes: These lobes are positioned at the front-most region of the cerebral cortex. They are involved with movement, decision-making, problem-solving, and planning. The right frontal lobe controls activity on the left side of the body and the left frontal lobe controls activity on the right side.
- Occipital Lobes: Located just below the parietal lobes, the occipital lobes are the main center for visual processing. The visual information is sent to the parietal lobes and temporal lobes for further processing.
- Temporal Lobes: These lobes are located directly below the frontal and parietal lobes. They are involved with memory, emotion, hearing, and language. Structures of the limbic system, including the olfactory cortex, amygdala, and the hippocampus are located within the temporal lobes.
In summary, the cerebral cortex is divided into four lobes that are responsible for processing and interpreting input from various sources and maintaining cognitive function. Sensory functions interpreted by the cerebral cortex include hearing, touch, and vision. Cognitive functions include thinking, perceiving, and understanding language.
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