Cerebral Cortex

Jai Ganesh · 2025-01-24 17:19:09

Cerebral Cortex

Gist

Your cerebral cortex is the outer layer that lies on top of your cerebrum. Your cerebrum is the largest area of your brain. Your cerebrum divides your brain into two halves called hemispheres. The hemispheres are attached by a bundle of nerve fibers called the corpus callosum.

Summary

The cerebral cortex is your brain’s outermost layer. It has a wrinkled appearance due to the grooves and folds that increase its surface area. Here, we look at the anatomy of the cerebral cortex and its functions.

Where Is the Cerebral Cortex?

The cerebral cortex is the outer layer of your brain’s surface, located on top of the cerebrum. The cerebral cortex carries out essential functions of your brain, like memory, thinking, learning, reasoning, problem-solving, emotions, consciousness, and sensory functions.

What Is the Cerebral Cortex?

The cerebral cortex is made up of 14 to 16 billion nerve cells. It's typically only a few millimeters thick but makes up roughly 50% of your total brain mass. It has a wrinkled exterior containing deep indentations and raised projections.

The deep parts in the folds are called the sulci, and the raised portions are called the gyri. The uneven surface of the cerebral cortex holds neurons vital for your brain functions. Your inability to carry out higher mental functions without this organ underlines the importance of the cerebral cortex.

Some interesting facts about the cerebral cortex:

* It has a surface area of roughly 2,500 square centimeters.
* It’s often called gray matter due to its gray color, and the more gray matter you have, the more information your brain can process.
* More than two-thirds of its area is folded.
* The larger surface area allows it to hold more neurons, thus improving brain function.

Cerebral Cortex Anatomy

The cerebral cortex is divided into two equal halves, called the left and right hemispheres. A collection of nerve fibers called the corpus callosum connects the two hemispheres. The two regions send signals and communicate via the corpus callosum. Each hemisphere is further divided into four lobes based on their location. Your cerebral cortex controls several vital functions through these lobes.

* Frontal lobes. These are the largest lobes of the cerebral cortex and are situated at the front of your brain, right behind your forehead.

* Occipital lobes. These are located at the back of your head.

* Parietal lobes. They’re located between the frontal and the occipital lobes on top of the temporal lobes.

* Temporal lobes. These are the second-largest lobes of the cerebral cortex and are located between the frontal and occipital lobes below the parietal lobes.

What Does the Cerebral Cortex Do?

The different lobes combine to complete various functions of the cerebral cortex, all of which are central to controlling your body’s senses, thinking, and other functions.

* Frontal lobes. They are vital for your cognitive abilities, like decision-making and problem-solving. Other functions, like conscious thought and attention to particular tasks, are attributed to these lobes. This region of the cerebral cortex also controls your behavioral patterns, emotions, and ability to generate speech. For example, the frontal lobes come into play when you’re in social situations to ensure appropriate social behavior. They're home to Broca’s area, which manages your language abilities. The frontal lobes define your personality traits and intelligence.

Other important areas in this lobe are the prefrontal cortex and the motor cortex. While the motor cortex controls your body’s movements, the prefrontal cortex is responsible for decision-making and managing other parts of your brain.

* Occipital lobes. Your occipital lobes are at the back of your head and get inputs from the retinas in your eyes. These inputs are then converted into various visual data, like color, movement, and position. The occipital lobes recognize faces and objects and perceive their depth and distance. Research about the capabilities of this region to communicate with other parts of your brain is still ongoing.

* Parietal lobes. The parietal lobes carry out a critical function of combining the sensory information from all parts of your body to create an image of the world around you. They also help in perceiving your body and comprehending touch, temperature, and pressure. The parietal lobes enable you to make sense of your surroundings and manage your movements accordingly in three-dimensional space. For example, they help you move around your house or your city. They're also essential for sensing pain and vibrations.

* Temporal lobes. This region is responsible for integral functions like emotion, memory, and hearing. Your left temporal lobe helps you understand languages, retain spoken information, learn, and frame speech. On the other hand, the right temporal lobe helps you retain nonverbal inputs, identify information, and recognize other people’s facial expressions. The temporal lobes combine inputs from your environment and other parts of your brain. They also help you convert sounds into visual images. When you listen to someone speak, your temporal lobes are activated to help you understand them.

Cerebral Cortex Damage

Damage to any region of the cerebral cortex is typically caused by tumors, autoimmune diseases, bleeding in your brain, or a stroke. Symptoms of possible damage depend on the affected region.

* Frontal lobe. Symptoms include memory issues, challenges with attention, emotional inadequacy, improper social behavior, inability to understand what others say, problems with your speech, and loss of muscle control over one side of the body.

* Parietal lobe. Symptoms include inability to write or identify objects by touching them, challenges with math, numbness, loss of sensation, and lack of hand-eye coordination.

* Temporal lobe. Symptoms include hearing and memory issues, inability to identify faces and objects, and difficulty in understanding languages. Damage to the temporal lobe can also lead to conditions like Alzheimer’s disease, epileptic seizures, and dyslexia.

* Occipital lobe. Symptoms include color blindness, hallucinations, inability to perceive more than one object at a time, and total blindness.

Cerebral Cortex Is Gray Matter

The cerebral cortex contains nerve cells or neurons. The ends of these cells are called dendrites, which transport signals to nearby cells. These dendrites don’t have the typical fatty acid covering called myelin, which gives them a gray appearance. On the other hand, the white matter in your brain contains nerve bundles called axons. The longer parts of these axons are coated with myelin, which gives them a whitish color.

Details

The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. It is the largest site of neural integration in the central nervous system, and plays a key role in attention, perception, awareness, thought, memory, language, and consciousness. The cerebral cortex is the part of the brain responsible for cognition.

The six-layered neocortex makes up approximately 90% of the cortex, with the allocortex making up the remainder. The cortex is divided into left and right parts by the longitudinal fissure, which separates the two cerebral hemispheres that are joined beneath the cortex by the corpus callosum. In most mammals, apart from small mammals that have small brains, the cerebral cortex is folded, providing a greater surface area in the confined volume of the cranium. Apart from minimising brain and cranial volume, cortical folding is crucial for the brain circuitry and its functional organisation. In mammals with small brains, there is no folding and the cortex is smooth.

A fold or ridge in the cortex is termed a gyrus (plural gyri) and a groove is termed a sulcus (plural sulci). These surface convolutions appear during fetal development and continue to mature after birth through the process of gyrification. In the human brain, the majority of the cerebral cortex is not visible from the outside, but buried in the sulci. The major sulci and gyri mark the divisions of the cerebrum into the lobes of the brain. The four major lobes are the frontal, parietal, occipital and temporal lobes. Other lobes are the limbic lobe, and the insular cortex often referred to as the insular lobe.

There are between 14 and 16 billion neurons in the human cerebral cortex. These are organised into horizontal cortical layers, and radially into cortical columns and minicolumns. Cortical areas have specific functions such as movement in the motor cortex, and sight in the visual cortex. The motor cortex is primarily located in the precentral gyrus, and the visual cortex is located in the occipital lobe.

Structure

The cerebral cortex is the outer covering of the surfaces of the cerebral hemispheres and is folded into peaks called gyri, and grooves called sulci. In the human brain, it is between 2 and 3-4 mm. thick, and makes up 40% of the brain's mass. 90% of the cerebral cortex is the six-layered neocortex whilst the other 10% is made up of the three/four-layered allocortex. There are between 14 and 16 billion neurons in the cortex. These cortical neurons are organized radially in cortical columns, and minicolumns, in the horizontally organized layers of the cortex.

The neocortex is separable into different regions of cortex known in the plural as cortices, and include the motor cortex and visual cortex. About two thirds of the cortical surface is buried in the sulci and the insular cortex is completely hidden. The cortex is thickest over the top of a gyrus and thinnest at the bottom of a sulcus.

Folds

The cerebral cortex is folded in a way that allows a large surface area of neural tissue to fit within the confines of the neurocranium. When unfolded in the human, each hemispheric cortex has a total surface area of about 0.12 square metres (1.3 sq ft). The folding is inward away from the surface of the brain, and is also present on the medial surface of each hemisphere within the longitudinal fissure. Most mammals have a cerebral cortex that is convoluted with the peaks known as gyri and the troughs or grooves known as sulci. Some small mammals including some small rodents have smooth cerebral surfaces without gyrification.

Lobes

The larger sulci and gyri mark the divisions of the cortex of the cerebrum into the lobes of the brain. There are four main lobes: the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. The insular cortex is often included as the insular lobe. The limbic lobe is a rim of cortex on the medial side of each hemisphere and is also often included. There are also three lobules of the brain described: the paracentral lobule, the superior parietal lobule, and the inferior parietal lobule.

Thickness

For species of mammals, larger brains (in absolute terms, not just in relation to body size) tend to have thicker cortices. The smallest mammals, such as shrews, have a neocortical thickness of about 0.5 mm; the ones with the largest brains, such as humans and fin whales, have thicknesses of 2–4 mm. There is an approximately logarithmic relationship between brain weight and cortical thickness. Magnetic resonance imaging of the brain (MRI) makes it possible to get a measure for the thickness of the human cerebral cortex and relate it to other measures. The thickness of different cortical areas varies but in general, sensory cortex is thinner than motor cortex. One study has found some positive association between the cortical thickness and intelligence. Another study has found that the somatosensory cortex is thicker in migraine patients, though it is not known if this is the result of migraine attacks, the cause of them or if both are the result of a shared cause. A later study using a larger patient population reports no change in the cortical thickness in patients with migraine. A genetic disorder of the cerebral cortex, whereby decreased folding in certain areas results in a microgyrus, where there are four layers instead of six, is in some instances seen to be related to dyslexia.

Layers of neocortex

The neocortex is formed of six layers, numbered I to VI, from the outermost layer I – near to the pia mater, to the innermost layer VI – near to the underlying white matter. Each cortical layer has a characteristic distribution of different neurons and their connections with other cortical and subcortical regions. There are direct connections between different cortical areas and indirect connections via the thalamus.

One of the clearest examples of cortical layering is the line of Gennari in the primary visual cortex. This is a band of whiter tissue that can be observed with the naked eye in the calcarine sulcus of the occipital lobe. The line of Gennari is composed of axons bringing visual information from the thalamus into layer IV of the visual cortex.

Staining cross-sections of the cortex to reveal the position of neuronal cell bodies and the intracortical axon tracts allowed neuroanatomists in the early 20th century to produce a detailed description of the laminar structure of the cortex in different species. The work of Korbinian Brodmann (1909) established that the mammalian neocortex is consistently divided into six layers.

Layer I

Layer I is the molecular layer, and contains few scattered neurons, including GABAergic rosehip neurons. Layer I consists largely of extensions of apical dendritic tufts of pyramidal neurons and horizontally oriented axons, as well as glial cells. During development, Cajal–Retzius cells and subpial granular layer cells are present in this layer. Also, some spiny stellate cells can be found here. Inputs to the apical tufts are thought to be crucial for the feedback interactions in the cerebral cortex involved in associative learning and attention.

While it was once thought that the input to layer I came from the cortex itself, it is now known that layer I across the cerebral cortex receives substantial input from matrix or M-type thalamus cells, as opposed to core or C-type that go to layer IV.

It is thought that layer I serves as a central hub for collecting and processing widespread information. It integrates ascending sensory inputs with top-down expectations, regulating how sensory perceptions align with anticipated outcomes. Further, layer I sorts, directs, and combines excitatory inputs, integrating them with neuromodulatory signals. Inhibitory interneurons, both within layer I and from other cortical layers, gate these signals. Together, these interactions dynamically calibrate information flow throughout the neocortex, shaping perceptions and experiences.

Layer II

Layer II, the external granular layer, contains small pyramidal neurons and numerous stellate neurons.

Layer III

Layer III, the external pyramidal layer, contains predominantly small and medium-size pyramidal neurons, as well as non-pyramidal neurons with vertically oriented intracortical axons; layers I through III are the main target of commissural corticocortical afferents, and layer III is the principal source of corticocortical efferents.

Layer IV

Layer IV, the internal granular layer, contains different types of stellate and pyramidal cells, and is the main target of thalamocortical afferents from thalamus type C neurons (core-type) as well as intra-hemispheric corticocortical afferents. The layers above layer IV are also referred to as supragranular layers (layers I-III), whereas the layers below are referred to as infragranular layers (layers V and VI). African elephants, cetaceans, and hippopotamus do not have a layer IV with axons which would terminate there going instead to the inner part of layer III.

Layer V

Layer V, the internal pyramidal layer, contains large pyramidal neurons. Axons from these leave the cortex and connect with subcortical structures including the basal ganglia. In the primary motor cortex of the frontal lobe, layer V contains giant pyramidal cells called Betz cells, whose axons travel through the internal capsule, the brain stem, and the spinal cord forming the corticospinal tract, which is the main pathway for voluntary motor control.

Layer VI

Layer VI, the polymorphic layer or multiform layer, contains few large pyramidal neurons and many small spindle-like pyramidal and multiform neurons; layer VI sends efferent fibers to the thalamus, establishing a very precise reciprocal interconnection between the cortex and the thalamus. That is, layer VI neurons from one cortical column connect with thalamus neurons that provide input to the same cortical column. These connections are both excitatory and inhibitory. Neurons send excitatory fibers to neurons in the thalamus and also send collaterals to the thalamic reticular nucleus that inhibit these same thalamus neurons or ones adjacent to them. One theory is that because the inhibitory output is reduced by cholinergic input to the cerebral cortex, this provides the brainstem with adjustable "gain control for the relay of lemniscal inputs".

Columns

The cortical layers are not simply stacked one over the other; there exist characteristic connections between different layers and neuronal types, which span all the thickness of the cortex. These cortical microcircuits are grouped into cortical columns and minicolumns. It has been proposed that the minicolumns are the basic functional units of the cortex. In 1957, Vernon Mountcastle showed that the functional properties of the cortex change abruptly between laterally adjacent points; however, they are continuous in the direction perpendicular to the surface. Later works have provided evidence of the presence of functionally distinct cortical columns in the visual cortex (Hubel and Wiesel, 1959), auditory cortex, and associative cortex.

Cortical areas that lack a layer IV are called agranular. Cortical areas that have only a rudimentary layer IV are called dysgranular. Information processing within each layer is determined by different temporal dynamics with that in layers II/III having a slow 2 Hz oscillation while that in layer V has a fast 10–15 Hz oscillation.

Types of cortex

Based on the differences in laminar organization the cerebral cortex can be classified into two types, the large area of neocortex which has six cell layers, and the much smaller area of allocortex that has three or four layers:

* The neocortex is also known as the isocortex or neopallium and is the part of the mature cerebral cortex with six distinct layers. Examples of neocortical areas include the granular primary motor cortex, and the striate primary visual cortex. The neocortex has two subtypes, the true isocortex and the proisocortex which is a transitional region between the isocortex and the regions of the periallocortex.
* The allocortex is the part of the cerebral cortex with three or four layers, and has three subtypes, the paleocortex with three cortical laminae, the archicortex which has four or five, and a transitional area adjacent to the allocortex, the periallocortex. Examples of allocortex are the olfactory cortex and the hippocampus.

There is a transitional area between the neocortex and the allocortex called the paralimbic cortex, where layers 2, 3 and 4 are merged. This area incorporates the proisocortex of the neocortex and the periallocortex of the allocortex. In addition, the cerebral cortex may be classified into four lobes: the frontal lobe, temporal lobe, the parietal lobe, and the occipital lobe, named from their overlying bones of the skull.

Additional Information

Cerebral cortex is the outermost layer of tissue in the brain. The cerebral cortex, also referred to as gray matter, covers the cerebrum, which is the largest portion of the brain. The cerebral cortex is responsible for integrating sensory impulses, directing motor activity, and controlling higher intellectual functions. In humans, the cerebral cortex is several centimetres thick and has a surface area of about 2,000 square cm (310 square inches); an elaborate series of convolutions known as gyri (bulges) and sulci (grooves) increase its surface area. The extensive development of the cerebral cortex in humans is thought to distinguish the human brain from those of other animals.

The cerebral cortex is made up of billions of neurons and support cells known as glial cells. Functional areas of the cerebral cortex are primarily connected to neurons in white matter, which forms the inner layer of the cerebrum. A key distinction between gray matter and white matter is the absence or presence of myelin, a fatty insulating sheath on neurons that imparts a white appearance; whereas white matter is myelinated, gray matter is unmyelinated.

The gray matter of the cerebral cortex is usually divided into four lobes, roughly defined by their major surface folds. The frontal lobe contains control centres for motor activity and speech, the parietal lobe for somatic senses (touch and position), the temporal lobe for auditory reception and memory, and the occipital lobe for visual reception. Sometimes, the limbic lobe, involved with smell, taste, and emotions, is considered to be a fifth lobe.

The cerebral cortex can be affected by various conditions, including brain injury, developmental problems, and neurodegenerative disease. Traumatic brain injury (damage to the brain from an applied force) can severely limit the functionality of the affected region; for example, injury to the frontal lobe may result in personality disorder, whereas injury to the occipital lobe can affect vision. Disruption of the embryonic or fetal development of the cerebral cortex can result in developmental delays or epilepsy. Likewise, neurodegenerative diseases, which are often associated with aging, can alter the function of the cerebral cortex, resulting in dramatic changes in behaviour and cognition. For example, the loss of neurons and consequent thinning of the cerebral cortex seen in Huntington disease results in irregular and involuntary muscle movements and progressive loss of cognitive ability.

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#1 2025-01-24 17:19:09

Cerebral Cortex

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