The brain falls into three subdivisions:
Forebrain
A.
Midbrain
B.
Hindbrain
C.
The forebrain is composed of : telencephalon & diencephalon.
The Telencephalon has 3 parts:
Cerebrum
1.
White Matter Core
2.
Basal Ganglia
3.
The Cerebrum
The cerebrum has 2 hemispheres : Right & Left
These paired cerebral hemispheres make the largest part of the brain.
They are separated by a deep cleft, the longitudinal fissure ( inter-hemispheric fissure) , and
connected partially by a big white matter bundle called the corpus callosum.
Below the corpus callosum ,both hemispheres are fused with the centrally located
diencephalon to be in continuity with the brainstem and the spinal cord.
Above and around the corpus callosum ,they are separated by a deep cleft: the longitudinal
fissure.
The longitudinal fissure contains a sickle-shaped extension of dura, the falx cerebri.
Each hemisphere has:
Six lobes: frontal, parietal, temporal, occipital, limbic and the insula.
A.
Three poles: frontal, temporal, and occipital.
B.
Three surfaces: lateral, medial, and basal.
C.
Three borders: superior, inferior, and medial.
D.
Sulci & Gyri and the Lateral Sulcus ( = Sylvian Fissure, Lateral Fissure )
The cerebral cortex is a highly convoluted surface. These convolutions increase the total surface area
to three times. The surface of the cerebral hemisphere shows a complex pattern of convolutions, or gyri,
which are separated by furrows of varying depth known as fissures, or sulci.
The area of the adult cerebral cortex is approximately 2200 cm2: its convoluted form increases the
cortical volume to three times greater than it would be if the surface were smooth.
Two prominent furrows, the lateral (Sylvian) fissure and the central sulcus, are the main features on the
lateral cerebral surface, and they determine its surface divisions. The lateral fissure is a deep cleft on the
lateral and inferior surfaces. It separates the frontal and parietal lobes above from the temporal lobe
below and accommodates the middle cerebral vessels. It commences inferiorly at the anterior
perforated substance, extending laterally between the orbital surface of the frontal lobe and the
anterior pole of the temporal lobe and accommodating the sphenoparietal venous sinus. Reaching the
lateral surface of the hemisphere it gives rise to an ascending ramus and a horizontal anterior ramus
which runs into the inferior frontal gyrus. The larger posterior ramus runs posteriorly and slightly
upwards, across the lateral surface of the hemisphere to end in the parietal lobe.
The insula lies in the depths of the lateral fissure.
The central sulcus is the boundary between the frontal and parietal lobes and demarcates the primary
motor and somatosensory areas of the cortex, located in the precentral and postcentral gyri,
respectively. It starts in or near the superior border of the hemisphere, a little behind the midpoint
between the frontal and occipital poles. It runs sinuously downwards and forwards to end a little above
the posterior ramus of the lateral sulcus.
Cerebral Hemisphere Lobes
Frontal Lobe
The frontal lobe includes not only the motor cortex but also frontal association areas
responsible for initiative, judgment, abstract reasoning, creativity, and socially appropriate
behavior (inhibition of socially inappropriate behavior).
The frontal lobe extends from the frontal pole to the central sulcus and the lateral fissure. The
precentral sulcus lies anterior to the precentral gyrus and parallel to the central sulcus. The
superior and inferior frontal sulci extend forward and downward from the precentral sulcus,
dividing the lateral surface of the frontal lobe into three parallel gyri: the superior, middle, and
inferior frontal gyri. The inferior frontal gyrus is divided into three parts: the orbital part lies
rostral to the anterior horizontal ramus; the triangular, wedge-shaped portion lies between the
anterior horizontal and anterior ascending rami; and the opercular part is between the
ascending ramus and precentral sulcus.
The orbital sulci and gyri are irregular in contour. The olfactory sulcus lies beneath the olfactory
tract on the orbital surface; lying medial to it is the straight gyrus (gyrus rectus).
The orbital gyri
are divided by the roughly H-shaped orbital sulcus into the anterior, medial, posterior, and lateral
orbital groups. The anterior orbital gyri are situated between the anterior pole and the transverse limb
of the H. The posterior orbital gyri extend posteriorly from the transverse part of the H to the anterior
margin of the anterior perforated substance. The lateral orbital gyri are positioned lateral to the lateral
vertical limb of the H. The medial orbital gyri are situated between the medial vertical limb of the H and
the olfactory sulcus.
Parietal Lobe
The parietal lobe extends from the central sulcus to the parieto-occipital fissure; laterally, it
extends to the level of the lateral cerebral fissure. The postcentral sulcus lies behind the
postcentral gyrus. The intraparietal sulcus is a horizontal groove that sometimes unites with
the postcentral sulcus. The superior parietal lobule lies above the horizontal portion of the
intraparietal sulcus and the inferior parietal lobule lies below it.
The supramarginal gyrus is the portion of the inferior parietal lobule that arches above the
ascending end of the posterior ramus of the lateral cerebral fissure. The angular gyrus arches
above the end of the superior temporal sulcus and becomes continuous with the middle
temporal gyrus. The precuneus is the posterior portion of the medial surface between the
parieto-occipital fissure and the ascending end of the cingulate sulcus.
Occipital Lobe
The occipital lobe-which most notably houses the primary visual cortex-is situated behind the
parieto-occipital fissure. The calcarine fissure divides the medial surface of the occipital lobe
into the cuneus and the lingual gyrus. The cortex on the banks of the calcarine fissure is the site
of termination of visual afferents from the lateral geniculate body; this region of cortex thus
functions as the primary visual cortex. The wedge-shaped cuneus lies between the calcarine
and parieto-occipital fissures, and the lingual (lateral occipitotemporal) gyrus is between the
calcarine fissure and the posterior part of the collateral fissure.
Temporal Lobe
The temporal lobe lies below the lateral cerebral fissure and extends back to the level of the
parieto-occipital fissure on the medial surface of the hemisphere. The lateral surface of the
temporal lobe is divided into the parallel superior, middle, and inferior temporal gyri, which are
separated by the superior and middle temporal sulci. The inferior temporal sulcus extends
along the lower surface of the temporal lobe from the temporal pole to the occipital lobe. The
transverse temporal gyrus occupies the posterior part of the superior temporal surface. The
fusiform gyrus is medial and the inferior temporal gyrus lateral to the inferior temporal sulcus
on the basal aspect of the temporal lobe. The hippocampal fissure extends along the
inferomedian aspect of the lobe from the area of the splenium of the corpus callosum to the
uncus. The parahippocampal gyrus lies between the hippocampal fissure and the anterior part
of the collateral fissure. Its anterior part, the most medial portion of the temporal lobe, curves
in the form of a hook; it is known as the uncus.
The fusiform (medial occipitotemporal) gyrus is on the basal surface of the temporal lobe.
Insula
The insula is a sunken portion of the cerebral cortex (see the figure below). It lies at the bottom
of a deep fold within the lateral cerebral fissure and can be exposed by separating the upper
and lower lips (opercula) of the lateral fissure. The central sulcus of insula divides insular cortex
into anterior and posterior insular lobes. The anterior lobe is divided by few short sulci while
the posterior one is divided by long sulci. The whole tringular area of the insular cortex is
encircled by the circular sulcus.
Limbic Lobe
The cortical components of the limbic system include the cingulate, parahippocampal, and subcallosal
gyri as well as the hippocampal formation.
The limbic lobe is so named because this cortical complex forms a limbus (border) between the
diencephalon and the more lateral neocortex of the telencephalic hemispheres. This limbic lobe
consists of a ring of cortex outside the corpus callosum, largely made up of the subcallosal and cingulate
gyri as well as the parahippocampal gyrus. More recent authorities revised the concept of the limbic
lobe and refer to the limbic system, which includes the functionally interrelated limbic lobe
(parahippocampal, cingulate, and subcallosal gyri), the amygdala, and the hippocampal formation and
associated structures. The hippocampal formation (a more primitive cortical complex) is situated even
closer to the diencephalon and is folded and rolled inward so that it is submerged below the
parahippocampal gyrus. The hippocampal formation consists of the hippocampus (Ammon's horn); the
dentate gyrus; the supracallosal gyrus (also termed the indusium griseum), which is the gray matter on
top of the corpus callosum; the fornix; and a primitive precomrnissural area known as the septal area.
The cingulate gyrus is the crescent-shaped, or arched, convolution on the medial surface
between the cingulate sulcus and the corpus callosum. The paracentral lobule is on the medial
surface of the hemisphere and is the continuation of the precentral and postcentral gyri.
The Cerebrum: Cerebral Cortex & Hemispheric Surfaces
Wednesday, February 22, 2017
6:09 AM