Cortex

The cortex, more formally known as the cerebral cortex, is the outermost layer of the brain that plays a key role in many high-level cognitive functions, including memory, attention, perceptual awareness, thought, language, and consciousness^[1]. The term "cortex" is derived from the Latin word for "bark", and it was named this way because it appears as a layer that wraps around the rest of the brain, similar to how bark wraps around a tree^[2].

The cortex, specifically in humans, generally has a thickness of about 2-4 millimeters, albeit with some variance depending on the exact area within the brain^[4]. Even though it is relatively thin, the cortex possesses an intricate folding pattern, known as gyri (ridges) and sulci (furrows), which substantially enlarges its surface area. This unique folding is vital for accommodating a larger processing capacity within the limited confines of the skull^[5].

The cerebral cortex is largely composed of grey matter, while the underlying, deeper brain structures consist mostly of white matter^[6]. The division between these two regions is generally where the cortex ends and the deeper brain structures begin^[7]. Grey matter, which gets its colour from nerve cell bodies and their associated capillary blood vessels, dendrites and glial cells, is primarily involved in processing and cognition. The white matter, on the other hand, appears white because it is composed largely of myelinated axons, which are essentially nerve fibres covered in myelin^[8]. This fatty substance gives the white matter its colour and facilitates the fast transmission of signals between different parts of the brain^[9].

The cortex is divided into four main lobes: the frontal lobe, parietal lobe, occipital lobe, and temporal lobe^[11].

1. The frontal lobe is involved in various higher cognitive functions such as decision making, planning, and motor function^[12].

2. The parietal lobe is primarily responsible for sensory integration, including the processing of information related to touch, taste, and spatial positioning^[13].

3. The occipital lobe, located at the back of the brain, is primarily responsible for vision^[14].

4. The temporal lobe is involved in processing auditory information and is also critical for memory^[15].

A fifth region, the insula or insular cortex, is a portion of the cerebral cortex folded deep within the lateral sulcus^[16]. The insula is believed to play a role in consciousness and is involved in diverse functions usually linked to emotion or the regulation of the body's homeostasis^[17]. These functions include perception, motor control, self-awareness, cognitive functioning, and interpersonal experience^[18].

The cerebral cortex itself is composed of six layers, based on the types of neurons that exist and their arrangement from the outermost to the innermost layer ^[20]:

1. Layer I: The Molecular Layer - It mainly contains the dendrites and axons that are traveling to other parts of the cortex.

2. Layer II: The External Granular Layer - It contains small pyramidal neurons and numerous stellate neurons.

3. Layer III: The External Pyramidal Layer - It is home to medium-sized pyramidal neurons, along with non-pyramidal neurons.

4. Layer IV: The Internal Granular Layer - It primarily receives inputs from thalamic afferents.

5. Layer V: The Internal Pyramidal Layer - It houses large pyramidal neurons, such as the Betz cells, and it sends output to subcortical structures.

6. Layer VI: The Polymorphic or Multiform Layer - It contains various types of neurons and sends most of its output back to the thalamus.

Each of these layers has a different composition of neuronal cell types and connections, which contributes to different functions. The specific features of the layers can vary depending on the part of the cortex (e.g., motor cortex vs. sensory cortex).