Cerebral Cortex

Original Editor - Lucinda hampton

Top Contributors - Lucinda hampton, Jess Bell, Stacy Schiurring, Rucha Gadgil, Tarina van der Stockt, Kim Jackson and Joao Costa  

Introduction

The cerebrum is the largest anatomical area of the brain. It is a highly developed structure, containing between 14 billion and 16 billion neurons. The major functions of the cerebrum include, but aren't limited to, controlling voluntary muscular movements of the body, sensation, memory, emotions, and executive functioning.

The cerebrum is made up of both gray and white matter. The surface of the cerebrum is the cerebral cortex. This sheet of neural tissue has up to six layers of nerve cells. It is covered by the meninges and is often referred to as grey matter. The cerebral cortex covers the internal white matter.

Cerebral Anatomy

Subcortical structures associated with the cerebral cortex

The cerebrum is composed of two cerebral hemispheres (i.e. the right and left hemispheres), which are connected via subcortical pathways, including the corpus callosum. The corpus callosum is a thick tract of nerve fibres that facilitates communication between each side of the brain. These connections, as well as the connections from the cerebral cortex to the brainstem, spinal cord and subcortical nuclei deep within the cerebral hemisphere, form the white matter of the cerebral hemisphere. The deep nuclei include structures such as the basal ganglia and the thalamus. The external suface of the brain is highly convoluted, with many folds. This distinct, convoluted shape evolved over time, as the volume of our cortex increased more rapidly than our cranial volume. If the cerebral cortex was removed and unfolded, it would cover several yards or metres.

• The convolutions consist of grooves known as sulci. The sulci separate the more elevated regions or ridges, which are called gyri.
• The three main sulci in each hemisphere are the (1) central sulcus, (2) parieto-occipital sulcus and (3) lateral fissure.
• Each hemisphere is divided into four lobes using certain consistently present sulci as landmarks. These lobes are the: frontal, parietal, temporal and occipital lobes. They are named after the overlying cranial bones.

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  Lobes of the brain

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  Cerebral cortex and surface anatomy

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  Note the raised sulci and depressed gyri forming the surface of the cerebrum

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  The area in green denotes the corpus callosum

Cerebral Cortex

The cerebral cortex is the outer layer of the cerebral hemispheres. Functionally, it is possible to divide the cortex into primary areas and association areas:

• primary areas: receive and send information
• association areas: process and interpret information

The cerebral cortex is involved in several functions of the body including:

• determining intelligence
• determining personality
• motor function
• planning and organisation
• processing sensory information
• language processing

Because of its structure, the cerebral cortex also plays the role of messenger between different lobes and hemispheres. This communication takes place via tracts or fasciculi which are organised as commisural fibres (between hemispheres), association fibres (within the hemispheres), and projection fibres (cortex to subcorticular structures).

The cerebral cortex mainly contains:

Primary and association areas of the cerebral cortex

• sensory areas, which receive input from the thalamus and process information related to the senses. They include the visual cortex of the occipital lobe, the auditory cortex of the temporal lobe, the gustatory cortex, and the somatosensory cortex of the parietal lobe. Within the sensory areas are association areas that give meaning to sensations and associate sensations with specific stimuli.
• motor areas: including the primary motor cortex and the premotor cortex. These areas regulate voluntary movement. Motor output from the brain to the body travels along an upper and lower motor neuron. The upper motor neuron originates in the cortex or brainstem and synapses with the lower motor neuron in the brainstem or spinal cord, which then travels down to the target muscle.

Neocortex

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  Cerebral cortex
  The neocortex is the newest part of the cerebral cortex to evolve. The six-layer neocortex is a distinguishing feature of mammals; it has been found in the brains of all mammals, but not in any other animals.
• Neurons in various layers connect vertically to form small microcircuits, called 'columns'.
• In humans, 90% of the cerebral cortex and 76% of the entire brain is neocortex.
• The neocortex is the largest and most powerful area of the human brain. All of its important cognitive functions are made possible by the convergence of two distinct streams of information: a "bottom-up" stream, which represents signals from the environment, and a "top-down" stream, which transmits internally generated information about past experiences and current aims.

Allocortex

• The allocortex (also known as heterogenetic cortex) has fewer cell layers than the neocortex (i.e. fewer than six).
• It is phylogenetically more ancient than the neocortex, and it evolved to handle olfaction and the memory of smells.
• Due to its simpler cellular organisation, the allocortex is unable to form as many complex microcircuits as the neocortex.
• The specific regions of the brain normally described as part of the allocortex are the:
  • archicortex
    • olfactory cortex
    • hippocampus
  • paleocortex (has 3 three to five layers)

Cerebral Lobes

Frontal Lobe

The frontal lobe is anterior to the central sulcus and superior to the lateral fissure. It is located under the frontal bone in the skull. It is divided into four main gyri:

• precentral gyrus: delineates the anterior boundary of the precentral gyrus
• superior frontal gyrus: divides the superior and middle frontal gyri
• middle frontal gyrus
• inferior frontal gyrus: divides the middle and inferior frontal gyri

The frontal lobe is further divided into:

• Primary motor cortex: found within the precentral gyrus. It controls the voluntary movements on the contralateral, or opposite, side of the body. It is organised somatotopically, so the medial part controls the lower extremities, the intermediate part controls the trunk and upper extremities, and the lateral part controls the facial muscles.
• Premotor cortex: lies anterior to the primary motor cortex. It communicates with the primary motor cortex as well as other areas of the brain and spinal cord to influence movement functions, particularly in the selection of movement based on internal and external cues.
• Frontal eye field: a small area anterior to the premotor cortex involved in voluntary control of certain types of eye movements, such as active visual search.
• Prefrontal cortex: responsible for high level human behaviours: executive functions (like planning and meeting goals), decision making, self-control, memory, and personality.
• Broca's area: a small area within the inferior frontal gyrus is responsible for speech output. It is present in the dominant hemisphere, which is the left hemisphere for most individuals. A lesion to Broca’s area results in Broca’s aphasia.

Damage to the motor cortex only affects the upper motor neuron, and as such, it results in symptoms consistent with upper motor neuron syndrome. This includes contralateral weakness; hypertonia, or increased muscle tone; and spasticity.

Clinical pearl: Broca's aphasia

Also known as expressive or non-fluent aphasia. Aphasia is normally considered a cortical sign and its presence suggests dysfunction or damage of the dominant cerebral cortex. Broca's aphasia occurs from damage to a specific area of the frontal lobe.

Signs and symptoms:

• Spontaneous speech output is markedly diminished
  • loss of normal grammatical structure: small linking words, conjunctions and the use of prepositions are lost.
• Patients can exhibit interjectional speech when given enough time, however the words are expressed with much effort.
• The ability to repeat heard phrases is impaired
• Despite impairments, produced words are often intelligible and contextually correct, and comprehension remains intact
• Patients may become frustrated from their difficulty in communicating clearly making some sliding into depression.
• May often present with right hemiparesis/hemiplegia as the frontal lobe is also important for motor movements.

Parietal Lobe

The parietal lobe lies posterior to the central sulcus, anterior to the parieto-occipital sulcus, and above the lateral fissure. This lobe primarily integrates perception and sensation.

Within the parietal lobe, there is:

1. Postcentral gyrus between the central sulcus and postcentral sulcus: The primary somatosensory cortex is found here. This area is responsible for contralateral touch, temperature and pain. It is also arranged somatotopically similar to the primary motor cortex.
2. Superior and inferior parietal lobule, divided by the intraparietal sulcus: resides the somatosensory association cortex and secondary somatosensory cortex which communicate with the primary somatosensory cortex and other areas of the brain to integrate and process the sensory information received.

Occipital Lobe

The occipital lobe is the smallest lobe in the cerebrum, and lies posterior to the parieto-occipital sulcus. It performs the function of receiving and processing visual information. Each occipital lobe receives mixed input from both the contralateral and ipsilateral visual field in each eye.

The occipital lobe consists of:

1. Primary visual cortex: located around the calcarine sulcus on the medial side of the occipital lobe
2. Secondary visual cortex

Temporal Lobe

The temporal lobe lies inferior to the lateral fissure and is responsible for memory, hearing, and language.

It consists of:

1. Primary auditory cortex: lies in the superior temporal gyrus and receives input from the ears, both ipsilaterally and contralaterally.
2. Auditory association area: interprets auditory input
3. Wernicke’s area: A small area in the superior temporal gyrus is responsible for language comprehension found in the dominant hemisphere, which is the left for most individuals. Broca’s area and Wernicke’s area are connected by a fibre tract called the arcuate fasciculus.

Clinical Pearl: Wernicke's aphasia

Also known as receptive or fluent aphasia. The most common cause of Wernicke’s aphasia is an ischemic stroke affecting the dominant hemisphere temporal lobe.

Signs and symptoms:

• Impaired language comprehension
• However, speech may have a normal rate, rhythm, and grammar
  • the individual is able to use complete sentences but they are nonsensical and difficult to understand

Insular Cortex/Lobe (Insula)

• Located within the lateral sulcus
• Makes up about 2% of the total cortical area and is still poorly understood.
• It is involved in a variety of functions including: consciousness, emotion, cognitive functions, sensorimotor processing, taste, auditory and vestibular functioning, as well as pain pathways
• Isolated insular lesions, such as insular strokes, are uncommon but can occur, resulting in wide ranging deficits.

Limbic Lobe

• The limbic lobe is cannot be categorised as a separate lobe, as it crosses portions of the frontal, parietal, and occipital lobes on the medial side of each hemisphere.
• It is involved in motivationally driven and emotional behaviours, memory, homeostasis, and sexual behaviour

Blood supply

In the cerebral cortex, the circle of Willis plays an important role in the blood supply- mainly the posterior cerebral artery, middle cerebral artery and the anterior cerebral artery.

1. Posterior cerebral artery supplies the occipital lobe and parts of the temporal lobe through the temporal branch, the occipital branch, and the parieto-occipital branch
2. Middle cerebral artery supplies the insular cortex and parts of the frontal, parietal, and temporal lobes through the frontal branch, parietal branch, and temporal branch
3. Anterior cerebral artery supplies the frontal and parietal lobes through the frontal branch, orbital branch, and parietal branch