Histological structure and meninges
The microscopic structure of the cortex is heterogeneous. Depending on the evolutionary age and organizational complexity, three main types of cortex are distinguished:
- Isocortex (isocortex, or neocortex): the youngest part, containing 6 cellular layers. These layers are well visualized with Nissl staining.
- Allocortex (allocortex, old cortex): a more primitive structure consisting of only 3 layers.
- Mesocortex (mesocortex): an intermediate variant between the iso- and allocortex.
The numbering of cortical layers always goes from the superficial to the deeper ones. Glia plays a crucial role in cortical function. There are at least as many glial cells here as there are neurons. They not only provide a supportive framework but also actively interact with synapses, neurons, and the dense capillary network to support metabolism.
The surface of the cortex is covered by the glia limitans (glia limitans). It is formed by a continuous layer of superficial astrocyte end-feet and a basement membrane. The pia mater (pia mater) closely adheres to this structure, forming a potential subpial space between them.
Macroscopic anatomy and lobes
The surface of the cortex is folded. The protruding areas are called gyri (gyri), and the depressions between them are called sulci (sulci).
Historically, brain lobes were named after the adjacent cranial bones. In 1854, the anatomist Gratiolet improved this scheme by adding precise anatomical boundaries. It is important to understand that the division into lobes is a conventional system, convenient for topography, but having practically no ontogenetic significance. Traditionally, 6 lobes are distinguished in each hemisphere:
- Frontal (frontal)
- Parietal (parietal)
- Occipital (occipital)
- Temporal (temporal)
- Insular (insula)
- Limbic (limbic)
Major sulci and fissures are used for orientation. The longitudinal cerebral fissure divides the brain into two halves. The Sylvian (lateral) sulcus separates the temporal lobe. The central sulcus serves as a vital landmark: it separates the precentral gyrus (where the motor cortex is located) and the postcentral gyrus.
MRI topography: frontal lobe and speech
When analyzing MRI scans of the lateral surface of the brain, special attention is paid to the inferior frontal gyrus. It has a characteristic M-shape. The anterior ascending and horizontal rami of the Sylvian fissure divide it into three parts:
- Orbital (pars orbitalis)
- Triangular (pars triangularis)
- Opercular (pars opercularis)
It is here, in the triangular and opercular parts of the dominant hemisphere (usually the left), that Broca's area — the motor speech center — is located.
The sensory (receptive) speech center — Wernicke's area — is typically centered on the supramarginal gyrus (gyrus supramarginalis). On sagittal MRI slices, the supramarginal and adjacent angular (gyrus angularis) gyri resemble an inverted horseshoe. An infarction in Wernicke's area clinically presents as receptive dysphasia and is clearly visible on MRI as a focal signal abnormality.
Insula and primary auditory cortex
Some cortical structures are hidden from direct view or have a specific shape. The insular cortex (insula) is located deep within the Sylvian fissure, has a triangular shape, and is well visualized on parasagittal slices.
The primary auditory cortex is located in Heschl's gyrus (gyrus Heschl). Anatomically, it is a constant convolution originating from the superior surface of the temporal lobe.
Briefly: answers to topical questions
What 6 cellular layers make up the isocortex?
The isocortex (neocortex) comprises six cellular layers differing in structure and function.
- Molecular layer (plexiform layer / layer I) — the most superficial layer beneath the pia mater, containing few neurons and terminal arborizations of processes.
- External granular layer (layer II) — consists of numerous small grain-like cells.
- External pyramidal layer (layer III) — contains small and medium-sized pyramidal nerve cells.
- Internal granular layer (layer IV) — formed by small granule cells (stellate neurons).
- Internal pyramidal layer (layer V) — includes large pyramidal cells whose axons form efferent pathways.
- Multiform layer (layer VI) — the deepest layer, bordering the white matter.
What is the function of the postcentral gyrus?
The postcentral gyrus functions as the primary somatosensory cortex, responsible for the precise discrimination of sensory information. Fine discriminatory analysis of stimuli occurs in this region, allowing the determination of their localization, intensity, and type. The periphery of the body is somatotopically projected onto it, ensuring precise localization of the stimulation site on the body. Conscious perception of vibration and position sense (proprioception) is impossible without the participation of this cortical area.
Which sulci bound the insular lobe?
The base of the insular lobe is bounded by the circular sulcus of the insula (sulcus circularis insulae). This sulcus surrounds the insula peripherally, although it gradually disappears on its inferior surface. The insula itself lies deep at the bottom of the lateral fossa of the cerebrum (fossa lateralis cerebri). Additionally, the central sulcus of the insula (sulcus centralis insulae) is located on the surface of the lobe itself, but it does not bound the lobe; rather, it divides it into anterior and posterior parts.
To remember the three parts of the inferior frontal gyrus from anterior to posterior, recall the acronym 'OTO': O — Orbitalis, T — Triangularis, O — Opercularis.
Class and exam questions
What is the difference between the isocortex and the allocortex?
Isocortex (neocortex) is an evolutionarily new cortex consisting of 6 layers. Allocortex is the old cortex, having only 3 cellular layers.
What does the central sulcus separate?
It separates the frontal lobe from the parietal lobe, specifically dividing the precentral gyrus (motor cortex) from the postcentral gyrus.
What does the supramarginal gyrus look like on a sagittal MRI slice?
The supramarginal gyrus (like the angular gyrus) resembles an inverted horseshoe on sagittal slices.
What else to study on this topic
- Features of identifying the 6 layers of the neocortex using Nissl staining.
- Metabolic interaction of glia with synapses and cortical capillaries.
- Identification of the anterior ascending (AAL) and horizontal (AHL) rami of the Sylvian fissure on MRI.
- MRI presentation of Wernicke's area infarction and clinical manifestations of receptive dysphasia.
- Topography of the insular lobe on parasagittal slices.
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