General Topography of the Brain
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Main Anatomy Index
| The spinal cord I
Last updated 30 March 2006
General Topography of the Brain
The Medial Surface
The cerebral hemispheres conceal
most of the rest of an intact brain.
Hemisection reveals many parts of the diencephalon,
brainstem, and cerebellum and additional features of the cerebral
hemispheres.
- The midbrain, which is continuous
with the diencephalon;
- The pons;
- And the medulla, which is continuous
with the spinal cord.
- The 2 cerebral hemispheres are joined by the corpus callosum, a large bundle of fibres.
- It consists of:
- A splenium posteriorly, which is enlarged and rounded;
- A body;
- An anterior, curved genu;
- Which tapers into a ventrally
directed rostrum.
- Portions of the medial surfaces of the diencephalon form the
walls of the narrow,
slit-like third ventricle.
- This opens into the large lateral ventricle of each cerebral hemisphere through an interventricular foramen (of Monro).
- Posteriorly the third ventricle is continuous with the cerebral aqueduct (of Sylvius).
- The aqueduct in turn is continuous with the fourth ventricle of the pons
and medulla.
- The fourth ventricle is continuous with the microscopically tiny central canal of the caudal
medulla and the spinal cord.
Surface Features of the Cerebral
Hemispheres
Click here for medial, lateral, dorsal
and ventral views of the cerebral
hemispheres.
- Each ridge of the cerebral hemisphere is called a gyrus.
- Each groove between ridges is called a sulcus.
- Particularly deep sulci
are often called fissures.
- The appearance of various gyri and sulci varies considerably
from one brain to another.
- Major features are, however, reasonably constant.
Cerebral Lobes
There are 4 prominent sulci:
- The central sulcus (of Rolando);
- The lateral sulcus (of Sylvius);
- The parietooccipital sulcus;
- And the calcarine sulcus.
- These and the preoccipital notch are used to divide
each cerebral hemisphere into 4
lobes.
The Frontal Lobe
The frontal lobe extends from the anterior
tip of the brain (the frontal pole) to the central sulcus.
Inferiorly it ends at the lateral sulcus.
- On the medial surface of the brain, it extends
posteriorly to an imaginary line from the top of the central sulcus to the corpus callosum.
Gyri of the Frontal Lobe
make up the lateral surface of the
frontal lobe
- The precentral gyrus is anterior
to the central sulcus and parallel
to it. It extends to the precentral sulcus;
- The superior;
- The middle;
- And the inferior frontal gyri are oriented parallel to one another. They are roughly
perpendicular to the precentral gyrus.
- The superior frontal gyrus continues onto the medial surface as far as the cingulate
sulcus.
- The inferior frontal gyrus is visibly divided into 3 parts:
- The orbital part, which is most
anterior and is continuous with the inferior (orbital) surface of the frontal lobe;
- The opercular part, which is most
posterior and forms a portion of the frontal operculum;
- And the wedge-shaped triangular part, which lies
between the other two.
- The inferior or orbital surface
of the frontal lobe is mostly occupied by a group of gyri of variable appearance that are collectively called orbital gyri.
- The only named gyrus on this surface is the gyrus rectus.
- This is the most medial and extends
onto the medial surface of the hemisphere.
- Between the gyrus rectus and the orbital
gyri is the olfactory sulcus.
- This contains the olfactory bulb and tract.
- The medial surface of the lobe contains part of the cingulate gyrus, extensions of the superior
frontal gyrus, precentral gyrus, and gyrus rectus.
General Functional Areas of the
Frontal Lobe
The frontal lobe contains 4 general functional areas:
- The primary motor cortex occupies much of the precentral gyrus. It contains many of the cells of origin of descending motor pathways and is involved in the initiation of voluntary movements.
- The premotor area is made up of the remainder
of the precentral gyrus together with adjacent
portions of the superior and middle frontal gyri. It is also functionally
related to the initiation of voluntary movements.
- Broca's area
is in the opercular and triangular parts of the inferior frontal
gyrus of the left hemisphere. It is important
in the production of written
and spoken language.
- The prefrontal cortex is a very
large area comprising the remainder of the frontal lobe. It is involved with what may very generally be
described as personality, insight,
and foresight.
The Parietal Lobe
The parietal lobe extends from the central
sulcus to an imaginary line connecting the top
of the parietooccipital sulcus and the preoccipital
notch.
Inferiorly it is bounded by the lateral sulcus
and the imaginary continuation of this sulcus to the posterior
boundary of the parietal lobe.
- On the medial surface, it is bounded
inferiorly by the corpus callosum and calcarine sulcus, anteriorly
by the frontal lobe, and posteriorly
by the parietooccipital sulcus.
Gyri of the Parietal Lobe
The lateral surface of the parietal lobe is divided into 3 areas:
- The postcentral gyrus;
- The superior;
- And the inferior parietal lobules.
- The postcentral gyrus is posterior
to the central sulcus and parallel
to it.
- It extends to the postcentral sulcus.
- The intraparietal sulcus runs posteriorly
from the postcentral sulcus toward the occipital
lobe.
- It separates the superior and inferior
parietal lobules.
- The inferior parietal lobule is composed of :
- The supramarginal gyrus, which caps
the upturned end of the lateral
sulcus;
- And the angular gyrus, which similarly caps the superior temporal sulcus.
- The angular gyrus is typically broken up by small sulci and may overlap
the supramarginal gyrus.
Medial Surface of the Parietal Lobe
This surface of the parietal lobe contains a portion of
the cingulate gyrus and the medial
extension of the postcentral gyrus.
- It is completed by an area called the precuneus.
- This is bounded by the cingulate gyrus, the parietooccipital sulcus, and the marginal
branch of the cingulate sulcus.
- The extensions of the precentral
and postcentral gyri onto the medial surface of the
hemisphere are referred to together as the paracentral lobule.
- This is partly in the frontal and parietal lobes.
General Functional Areas of the
Parietal Lobe
The parietal lobe is associated with 3 functions:
- The postcentral gyrus more or less coincides with primary somatosensory cortex; it is concerned
with the initial cortical processing of tactile and proprioceptive
information.
- The inferior parietal lobule of the left hemisphere,
together with portions of the temporal lobe, is involved
in the comprehension of language.
- The remainder of the parietal cortex subserves complex aspects of spatial
orientation and perception.
The Temporal Lobe
The temporal lobe extends:
- Superiorly
: to the lateral sulcus and the
line forming the inferior boundary of the parietal lobe.
- Posteriorly
: it extends to the line connecting the top
of the parietooccipital sulcus and the preoccipital
notch.
- Medial surface
: its posterior boundary is an
imaginary line from the preoccipital notch to the splenium of the corpus callosum.
Gyri of the Temporal Lobe
The lateral surface of the temporal lobe is composed of
the superior, middle,
and inferior temporal gyri.
- The superior temporal gyrus continues into the lateral sulcus.
- Here it forms one of its walls.
- Part of the superior temporal gyrus forms the temporal operculum.
- The inferior temporal gyrus continues onto the inferior surface of the lobe.
- The rest of the inferior surface is made up of the broad
and often discontinuous occipitotemporal (fusiform) gyrus and the parahippocampal gyrus.
- These are separated from one another by the collateral sulcus.
- The occipitotemporal gyrus is partly in the occipital lobe and partly in the temporal
lobe.
- The parahippocampal gyrus is continuous
with the cingulate gyrus around the splenium
of the corpus callosum.
- This is by the isthmus of the cingulate gyrus.
- The anterior end of the parahippocampal gyrus turns backward and forms a medially directed
bump called the uncus.
- The superior border of the parahippocampal gyrus is the
hippocampal sulcus.
- Folded into the temporal lobe at the hippocampal sulcus
is the hippocampus.
- This is part of the limbic system.
General Functional Areas of the
Temporal Lobe
The temporal lobe is associated with 3 functions:
- A small area of that portion of the superior temporal gyrus
that lies in the lateral sulcus is the primary
auditory cortex.
- The parahippocampal gyrus and hippocampus
are parts of the limbic system.
- The temporal lobe is involved in complex aspects of learning
and memory.
- The 2nd and 3rd functions overlap to some degree.
The Occipital Lobe
This is bounded:
- These separations do not correspond to precise functional
subdivisions.
Gyri of the Occipital Lobe
The gyri of the lateral surface of the occipital lobe
are usually referred to as lateral occipital gyri.
- On the medial surface, the wedge-shaped
area between the parietooccipital and calcarine sulci is called the cuneus
(L. wedge).
- The gyrus inferior to the calcarine
sulcus is the lingual gyrus.
- The lingual gyrus is adjacent to the posterior
portion of the occipitotemporal gyrus and is separated from it by the collateral
sulcus.
- It is continuous anteriorly with the parahippocampal
gyrus.
- The transition from lingual to parahippocampal gyrus
occurs at the isthmus of the cingulate
gyrus.
General Function of the Occipital
Lobe
This lobe is almost exclusively concerned with visual functions.
The primary visual cortex is contained in the walls of the calcarine sulcus.
- The remainder of the lobe is referred to as visual association cortex and is involved in higher
order processing of visual information.
The Insula
This is an additional area of cerebral cortex that lies
buried in the depths of the lateral
sulcus.
It is concealed from view by portions of the frontal, parietal, and temporal lobes.
- This overlies the site where the telecephalon
and diencephalon fused during embryological
development.
- The portion of a given lobe overlying
the insula is called an operculum (L. lid).
- There are frontal, parietal,
and temporal opercula.
- The circular sulcus outlines the insula and marks its borders with the opercular areas of
cortex.
The Cingulate Gyrus
This is immediately superior to the corpus
callosum.
It can be followed posteriorly to the splenium of the corpus callosum, where it turns
inferiorly as the narrow isthmus of the
cingulate gyrus.
From here it continues as the parahippocampal
gyrus of the temporal lobe.
- These 2 gyri give the appearance
of encircling the diencephalon.
- They, together with the olfactory bulb and tract, and certain other small cortical areas, are often
referred to separately as the limbic lobe (L. limbus,
border).
The Diencephalon
It has 4 divisions:
- The thalamus;
- The hypothalamus;
- The epithalamus;
- And the subthalamus.
- The first 3 can be seen on a hemisected brain, but the subthalamus is an internal structure.
The Thalamus
The thalamus is an ovoid nuclear mass, part of which
borders on the 3rd ventricle.
- It is of major importance in both sensory
and motor systems.
- The line of attachment of the roof
of this ventricle is marked by a horizontally oriented ridge,
the stria medullaris thalami.
- Parts of the medial surface of the 2 thalami fuse in
many brains.
- This area is called the interthalamic adhesion or massa intermedia.
- The massa intermedia is missing in many brains and does
not appear to perform any unique
function.
- Posteriorly, the thalamus protrudes over the most rostral part
of the brainstem.
- Anteriorly, it abuts the interventricular foramen.
- No sensory information (except olfactory information)
reaches the cerebral cortex without prior
processing in the thalami.
- Also, anatomical loops characteristic of motor systems, typically involve the thalami as well.
The Hypothalamus
This is inferior to the thalamus.
It is separated from it by the hypothalamic sulcus in
the wall of the 3rd ventricle.
- It also forms the floor of the ventricle.
- Its inferior surface includes the infundibular stalk
and the mammillary bodies.
- The hypothalamus is the major visceral control centre
of the brain.
- It is involved in the limbic system as well.
The Epithalamus
This comprises of the midline pineal gland and several
small neural structures.
The Brainstem
This plays a major role:
- In cranial nerve functions;
- In conveying information to and from
the cerebrum'
- And also some special functions of its own.
- The brainstem is divided into:
- The midbrain;
- The pons;
- And the medulla.
The Midbrain
The tectum (L. roof) of the midbrain consists of paired lumps, the superior and
inferior colliculi (L. small hills).
This is dorsal to the cerebral
aqueduct.
The Pons
This consists of:
- A protruding basal portion, which is oval
in the sagittal section;
- And the overlying pontine tegmentum, which forms part
of the floor of the 4th ventricle.
The Medulla
This consists of:
- A rostral open portion, which contains part of the 4th ventricle,
- And a caudal closed portion, which is continuous with the spinal cord.
- The point of attachment of most cranial
nerves and additional brainstem structures can be seen in an inferior
view of the brain.