Download Neuroanatomy of the Brain

Neuroanatomy of the Brain
Cerebrum
The cerebrum or telencephalon, together with the diencephalon, constitutes the
forebrain. It is the most anterior or most superior region of the central nervous system.
"Telencephalon" refers to the embryonic structure, from which the mature "cerebrum"
develops. The dorsal telencephalon, or pallium, develops into the cerebral cortex, and the
ventral telencephalon, or subpallium, becomes the basal ganglia. The cerebrum is also
divided into symmetric left and right cerebral hemispheres.
The cerebrum is noteworthy for its convoluted surface. The bumps are called gyri,
and the grooves are called sulci or, if they are especially deep, fissures. The precise
pattern of gyri and sulci can vary considerably from person to person, but many features
are common to all people. The postcentral gyrus lies immediately posterior to the central
sulcus, and the precentral gyrus lies immediately anterior to the central sulcus. The
neurons of the postcentral sulcus are involved in somatic sensation, and those of the
precentral gyrus control voluntary movement. Neurons in the superior temporal gyrus are
involved in hearing.
White matter is one of the two components of the central nervous system and
consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears
pinkish white to the naked eye because myelin is composed largely of lipid tissue veined
with capillaries. Its white color is due to its usual preservation in formaldehyde. White
matter is composed of bundles of myelinated nerve cell processes (or axons), which
connect various grey matter areas (the locations of nerve cell bodies) of the brain to each
other, and carry nerve impulses between neurons. The other main component of the brain
is grey matter (actually pinkish tan due to blood capillaries). The white matter is the
tissue through which messages pass between different areas of gray matter within the
nervous system. Using a computer network as an analogy, the gray matter can be thought
of as the actual computers themselves, whereas the white matter represents the network
cables connecting the computers together. The white matter is white because of the fatty
substance (myelin) that surrounds the nerve fibers (axons). This myelin is found in almost
all long nerve fibers, and acts as an electrical insulation. This is important because it
allows the messages to pass quickly from place to place. The brain in general (and
especially a child's brain) can adapt to white-matter damage by finding alternative routes
that bypass the damaged white-matter areas, and can therefore maintain good connections
between the various areas of gray matter. Unlike gray matter, which peaks in
development in a person's twenties, the white matter continues to develop, and peaks in
middle age.
The corpus striatum is a compound structure consisting of the caudate nucleus
and the lentiform nucleus, which consists of the putamen and the globus pallidus. The
globus pallidus is a sub-cortical structure of the brain. The globus pallidus is divided into
two segments by the medial medullary lamina. The medial segment of the dorsal
pallidum, internal globus pallidus (GPi), and lateral division of the dorsal pallidum,
external globus pallidus (GPe), are thus the two parts of the dorsal pallidum that are two
closed nuclei surrounded everywhere by myelinic walls. The internal capsule is an area
of white matter in the brain that separates the caudate nucleus and the thalamus from the
lenticular nucleus. The internal capsule contains both ascending and descending axons. It
consists of axonal fibres that run between the cerebral cortex and the pyramids of the
medulla. The claustrum, which is suspected to be present in all mammals, is a fairly thin
(fraction of 1 mm to multiple mms) vertical curved sheet of subcortical gray matter
oriented sagittally between the white matter tracts of the external capsule and extreme
capsule. The Claustrum is lateral to the putamen and medial to the insular cortex and is
considered by some sources to be part of the basal ganglia. One of the interesting features
of the claustrum is the uniformity in the types and numbers of cells. The amygdaloid
bodies are almond-shaped groups of nuclei located deep within the medial temporal
lobes. They perform a primary role in the processing and memory of emotional reactions,
the amygdalae are considered part of the limbic system. The substantia nigra is located in
the mesencephalon that plays an important role in reward, addiction, and movement. The
substantia nigra, along with four other nuclei, is part of the basal ganglia. The substantia
nigra lies in the midbrain, dorsal to the cerebral peduncles. Humans have two substantiae
nigrae, one on each side of the midline. The substantia nigra is the largest nucleus in the
midbrain.
Numerous deep grooves in the cerebral cortex, called cerebral fissures, originate
in the extensive folding of the brain’s surface. The main cerebral fissures are the lateral
fissure between the frontal and temporal lobes; the transverse fissure, which divides the
cerebrum from the cerebellum; and the longitudinal fissure, which divides the cerebrum
into two hemispheres.
The cerebral cortex can be divided into four sections, which are known as lobes.
The frontal lobe, parietal lobe, occipital lobe and temporal lobe have been associated with
different functions ranging from reasoning to auditory perception. The frontal lobe is
located at the front of the brain and is associated with reasoning, motor skills, higher
lever cognition, and expressive language. The parietal lobe is located in the middle
section of the brain and is associated with processing tactile sensory information such as
pressure, touch, and pain. A portion of the brain known as the somatosensory cortex is
located in this lobe and is essential to the processing of the body's senses. The temporal
lobe is located on the bottom section of the brain. This lobe is also the location of the
primary auditory cortex, which is important for interpreting sounds and the language we
hear. The hippocampus is also located in the temporal lobe, which is why this portion of
the brain is also heavily associated with the formation of memories. The occipital lobe is
located at the back portion of the brain and is associated with interpreting visual stimuli
and information. The primary visual cortex, which receives and interprets information
from the retinas of the eyes, is located in the occipital lobe. The insula is a cerebral cortex
structure deep within the lateral fissure between the temporal lobe and the frontal lobe.
The insula plays a role in diverse functions usually linked to emotion or the regulation of
the body's homeostasis. These functions include perception, motor control, selfawareness, cognitive functioning, and interpersonal experience.
Major Structures of the Cerebrum
The corpus callosum is a structure of the brain in the longitudinal fissure that
connects the left and right cerebral hemispheres. It facilitates communication between the
two hemispheres. It is the largest white matter structure in the brain, consisting of 200250 million contralateral axonal projections. It is a wide, flat bundle of axons beneath the
cortex. Much of the inter-hemispheric communication in the brain is conducted across the
corpus callosum.
The falx cerebri so named from its sickle-like form, is a strong, arched fold of dura mater
which descends vertically in the longitudinal fissure between the cerebral hemispheres. It
is narrow in front, where it is attached to the crista galli of the ethmoid; and broad behind,
where it is connected with the upper surface of the tentorium cerebelli. Its upper margin
is convex, and attached to the inner surface of the skull in the middle line, as far back as
the internal occipital protuberance; it contains the superior sagittal sinus. Its lower margin
is
free
and
concave,
and
contains
the
inferior
sagittal
sinus.
The central sulcus is a fold in the cerebral cortex. The central sulcus is a
prominent landmark of the brain, separating the parietal lobe from the frontal lobe and
the primary motor cortex from the primary somatosensory cortex.
The precentral gyrus is in front of the postcentral gyrus from which it is separated
by the central sulcus. Its anterior border is the precentral sulcus, while inferiorly it
borders to the lateral fissure.
Only a small part of the parietoccipital sulcus is seen on the lateral surface of the
hemisphere, its chief part being on the medial surface. The lateral part of the
parietoccipital sulcus is situated about 5 cm. in front of the occipital pole of the
hemisphere, and measures about 1.25 cm. in length. The medial part of the parietoccipital
sulcus runs downward and forward as a deep cleft on the medial surface of the
hemisphere, and joins the calcarine fissure below and behind the posterior end of the
corpus callosum.
The pyramidal tract is a collection of axons that travel between the cerebral
cortex of the brain and the spinal cord. The pyramidal tract mostly contains motor axons.
The pyramidal tract originates from pyramidal cells in layer V of the cerebral cortex.
About half of its fibers arise from the primary motor cortex. Extrapyramidal tracts are
chiefly found in the reticular formation of the pons and medulla, and target neurons in the
spinal cord involved in reflexes, locomotion, complex movements, and postural control.
These tracts are in turn modulated by various parts of the central nervous system,
including the nigrostriatal pathway, the cerebellum, the vestibular nuclei, and the basal
ganglia. The basal ganglia are a group of nuclei in the brains of vertebrates, situated at
the base of the forebrain and strongly connected with the cerebral cortex, thalamus and
other areas. The basal ganglia are associated with a variety of functions, including motor
control and learning. All of these regulatory components can be considered part of the
extrapyramidal system, in that they modulate motor activity without directly innervating
motor neurons.
The corpus striatum is a compound structure consisting of the caudate nucleus
and the lentiform nucleus, which consists of the putamen and the globus pallidus. The
globus pallidus is a sub-cortical structure of the brain. The globus pallidus is divided into
two segments by the medial medullary lamina. The medial segment of the dorsal
pallidum, internal globus pallidus (GPi), and lateral division of the dorsal pallidum,
external globus pallidus (GPe), are thus the two parts of the dorsal pallidum that are two
closed nuclei surrounded everywhere by myelinic walls. The internal capsule is an area
of white matter in the brain that separates the caudate nucleus and the thalamus from the
lenticular nucleus. The internal capsule contains both ascending and descending axons. It
consists of axonal fibres that run between the cerebral cortex and the pyramids of the
medulla. The claustrum, which is suspected to be present in all mammals, is a fairly thin
(fraction of 1 mm to multiple mms) vertical curved sheet of subcortical gray matter
oriented sagittally between the white matter tracts of the external capsule and extreme
capsule. The Claustrum is lateral to the putamen and medial to the insular cortex and is
considered by some sources to be part of the basal ganglia. One of the interesting features
of the claustrum is the uniformity in the types and numbers of cells.
The hippocampus is a major component of the brain. It belongs to the limbic
system and plays important roles in long-term memory and spatial navigation. Like the
cerebral cortex, with which it is closely associated, it is a paired structure, with mirrorimage halves in the left and right sides of the brain. In humans and other primates, the
hippocampus is located inside the medial temporal lobe, beneath the cortical surface. The
dentate gyrus is part of the hippocampal formation. It is thought to contribute to new
memories as well as other functional roles.
Cingulate gyrus is a gyrus in the medial brain. It partially wraps around the
corpus callosum and is limited above by the cingulate sulcus.
The Anterior Commissure is a bundle of nerve fibers (white matter), connecting
the two cerebral hemispheres across the midline, and placed in front of the columns of the
fornix.
Ventricles
There are four cerebral ventricles: the paired lateral ventricles, and midline the
third and fourth ventricles. The two lateral ventricles, located within the cerebrum, are
relatively large and C-shaped, roughly wrapping around the dorsal aspects of the basal
ganglia. Each lateral ventricle extends into the frontal, temporal and occipital lobes via
the frontal (anterior), temporal (inferior), and occipital (posterior) horns, respectively.
The "body" and "atrium" are situated between the anterior and posterior horns. The
lateral ventricles both communicate via the interventricular foramina with the third
ventricle, found centrally within the diencephalon. The two ventricles are separated by
the septum pellucidum, a thin, triangular, vertical membrane. It separates the anterior
horn of the left and right lateral ventricles. It runs as a sheet from the corpus callosum
down to the fornix. The fornix is a C-shaped bundle of axons in the brain, and carries
signals from the hippocampus to the mammillary bodies and septal nuclei. The third
ventricle communicates via the cerebral aqueduct, located within the midbrain, with the
fourth ventricle, found within the hindbrain. The three foramina to the subarachnoid
space are found here, permitting cerebrospinal fluid produced in the ventricles to
surround the brainstem, cerebellum, and cerebral cortex. The fourth ventricle is also
continuous with the central canal, allowing CSF to bathe the inside surface of the spinal
cord as well. The cerebral aqueduct, contains cerebrospinal fluid (CSF), is within the
mesencephalon (or midbrain) and connects the third ventricle in the diencephalon to the
fourth ventricle, which is between the pons and cerebellum.
Thalamus
The thalamus is a midline paired symmetrical structure. It is situated between the
cerebral cortex and midbrain, both in terms of location and neurological connections. Its
function includes relaying sensation, special sense and motor signals to the cerebral
cortex, along with the regulation of consciousness, sleep and alertness. The thalamus
surrounds the third ventricle. It is the main product of the embryonic diencephalon.
The medial surface of the thalamus constitutes the upper part of the lateral wall of
the third ventricle, and is connected to the corresponding surface of the opposite thalamus
by a flattened gray band, the intermediate mass.
Hypothalamus
The hypothalamus contains a number of small nuclei with a variety of functions.
One of the most important functions of the hypothalamus is to link the nervous system to
the endocrine system via the pituitary gland (hypophysis).The hypothalamus is located
below the thalamus, just above the brain stem. It forms the ventral part of the
diencephalon. It is roughly the size of an almond.
The mammillary bodies are a pair of small round bodies, located on the
undersurface of the brain, that form part of the limbic system. They are located at the
ends of the anterior arches of the fornix. They, along with the anterior and dorsomedial
nuclei in the thalamus, are involved with the processing of recognition memory. They are
believed to add the element of smell to memories.
The tuber cinereum is a hollow eminence of gray matter situated between the
mammilary bodies and the optic chiasm. The tuber cinereum is part of the hypothalamus.
Laterally it is continuous with the anterior perforated substances and anteriorly with a
thin lamina, the lamina terminalis. The infundibulum, a hollow conical process, projects
from the tuber cinereum. The infundibulum extends forward and down where it is
attached to the posterior lobe of the pituitary gland.
The optic chiasm is the part of the brain where the optic nerves (CN II) partially
cross. The optic chiasm is located at the bottom of the brain immediately below the
hypothalamus.
Midbrain
The mesencephalon (or midbrain) comprises the tectum (or corpora
quadrigemini), tegmentum, the ventricular mesocoelia (or "iter"), and the cerebral
peduncles, as well as several nuclei and fasciculi. Caudally the mesencephalon adjoins
the pons (metencephalon) and rostrally it adjoins the diencephalon (Thalamus,
hypothalamus, et al).
In the brain, the corpora quadrigemina are the four colliculi—two 1, two
superior—located on the tectum the dorsal aspect of the midbrain. The corpora
quadrigemina are reflex centers involving vision and hearing.
The cerebral aqueduct, contains cerebrospinal fluid (CSF), is within the
mesencephalon (or midbrain) and connects the third ventricle in the diencephalon to the
fourth ventricle, which is between the pons and cerebellum.
The red nucleus is a structure in the rostral midbrain involved in motor
coordination. In humans, the red nucleus mainly controls the muscles of the shoulder.
Cerebellum
The cerebellum plays an important role in the integration of sensory perception,
coordination and motor control. In order to coordinate motor control, there are many
neural pathways linking the cerebellum with the cerebral motor cortex (which sends
information to the muscles causing them to move) and the spinocerebellar tract (which
provides proprioceptive feedback on the position of the body in space). The cerebellum
integrates these pathways using the constant feedback to fine-tune motor activity.[1]
The tentorium cerebelli is an extension of the dura mater that separates the
cerebellum from the inferior portion of the occipital lobes. The tentorium cerebelli is an
arched lamina, elevated in the middle, and inclining downward toward the circumference.
It covers the superior surface of the cerebellum, and supports the occipital lobes of the
brain. The cerebellar vermis is a narrow, wormlike structure between the hemispheres of
the cerebellum. It is the site of termination of the spinocerebellar pathways that carry
subconscious proprioception. The falx cerebelli is a small triangular process of dura
mater, received into the posterior cerebellar notch. Its base is attached, above, to the
under and back part of the tentorium cerebelli; its posterior margin, to the lower division
of the vertical crest on the inner surface of the occipital bone. As it descends, it
sometimes divides into two smaller folds, which are lost on the sides of the foramen
magnum.
The cerebellar cortex is the thin gray surface layer of the cerebellum, consisting
of an outer molecular layer or stratum moleculare, a single layer of purkinje cells (the
ganglionic layer), and an inner granular layer or stratum granulosum.
Cerebellar cortex appears very different from cerebral cortex in that it consists of
small, leaflike laminae, referred to as folia. Structurally the cerebellum consists of a
three-layered, gray cellular mantle called the cerebellar cortex and a core of white matter
containing four paired intrinsic (i.e., deep) nuclei, the dentate, globose, emboliform, and
fastigial. Three paired fiber bundles—the superior, middle, and inferior peduncles—
connect the cerebellum with the midbrain, pons, and medulla, respectively. The inferior
cerebellar peduncle carries many types of input and output fibers that are mainly
concerned with integrating proprioceptive sensory input with motor vestibular functions
such as balance and posture maintenance. The middle cerebellar peduncles are composed
entirely of centripetal fibers, which arise from the cells of the nuclei pontis of the
opposite side and end in the cerebellar cortex. The superior cerebellar peduncles, two in
number, emerge from the upper and medial part of the white substance of the
hemispheres and are placed under cover of the upper part of the cerebellum.
Pons
The pons is a structure located on the brain stem. It is cranial to (up from) the
medulla oblongata, caudal to (down from) the midbrain, and ventral to (in front of) the
cerebellum. It is above the medulla, below the midbrain, and anterior to the cerebellum.
The pons regulates relaxation, and is associated with the sense of higher purpose.
Medulla Oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of
neurology and similar contexts where no ambiguity will result, it is often referred to as
simply the medulla. The medulla contains the cardiac, respiratory, vomiting and
vasomotor centers and deals with autonomic functions, such as breathing, heart rate and
blood pressure.
The anterior or ventral portion of the medulla oblongata is named the pyramid and
lies between the anterior median fissure and the antero-lateral sulcus. Its upper end is
slightly constricted, and between it and the pons the fibers of the abducent nerve emerge;
a little below the pons it becomes enlarged and prominent, and finally tapers into the
anterior funiculus of the medulla spinalis, with which, at first sight, it appears to be
directly continuous.
The two pyramids contain the motor fibers that pass from the brain to the medulla
oblongata and medulla spinalis, corticobulbar and corticospinal fibers. When these
pyramidal fibers are traced downward, it is found that some three-fifths or more of them
leave the pyramids in successive bundles, and decussate in the anterior median fissure of
the medulla oblongata, forming what is termed the pyramidal decussation or motor
decussation. Having crossed the middle line, they pass down in the posterior part of the
lateral funiculus as the lateral cerebrospinal fasciculus.