VENTRICLES OF BRAIN
The cerebral ventricular system consists of a series of interconnecting spaces and channels within the brain which are derived from the central lumen of the embryonic neural tube and the cerebral vesicles to which it gives rise. Each cerebral hemisphere contains a large lateral ventricle that communicates near its
rostral end with the third ventricle via the interventricular foramen (foramen of Monro).
The third ventricle is a midline, slit-like cavity lying between the right and left thalamus and hypothalamus. Caudally,
the third ventricle is continuous with the cerebral aqueduct, a narrow tube that passes the length of the midbrain, and which is continuous in turn with the fourth ventricle, a wide cavity lying between the brainstem and cerebellum.
The fourth ventricle communicates with the
subarachnoid space of the cisterna magna through the foramen of Magendie, and with the cerebellopontine angles through the foramina of Luschka; caudally it is continuous with the vestigial central canal of the spinal cord.
The ventricular system contains cerebrospinal fluid (CSF), which is
mostly secreted by the choroid plexuses located within the lateral, third
and fourth ventricles.CSF flows from the lateral to the third ventricle, then through the cerebral aqueduct and into the fourth ventricle. It leaves the fourth ventricle through the foramina of luschka to reach the subarachnoid space surrounding the brain.
TOPOGRAPHY AND RELATIONS OF THE VENTRICULAR SYSTEM
LATERAL VENTRICLE
Viewed from its lateral aspect, the lateral ventricle has a roughly C-shaped profile. The shape is a consequence of the developmental expansion of the frontal, parietal and occipital regions of the hemisphere, which displaces the temporal lobe inferiorly and anteriorly. Both the caudate nucleus and the fornix,
which lie in the wall of the ventricle, have adopted a similar morphology, so that the tail of the caudate nucleus encircles the thalamus in a C shape, and the fornix traces the outline of the ventricle forwards to the interventricular foramen.
ANTETIRIOR VIEW OF LATERAL VENTRICLE
The lateral ventricle is customarily divided into a body and anterior
(frontal), posterior (occipital) and inferior (temporal) horns.
The anterior horn lies within the frontal lobe. The posterior aspect of the genu and the rostrum of the corpus callosum bound it anteriorly, and its roof is formed by the anterior part of the body of the corpus
callosum. The anterior horns of the two ventricles are separated by the septum pellucidum. The coronal profile of the anterior horn is roughly that of a flattened triangle in which the rounded head of the caudate nucleus forms the lateral wall and floor. The anterior horn extends back as far as the interventricular foramen. The body lies within frontal and parietal lobes and extends from the interventricular foramen to the splenium of thecorpus callosum.
The bodies of the lateral ventricles are separated by the septum pellucidum, which contains the columns of the fornices
in its lower edge. The lateral wall of the body of the ventricle is formed by the caudate nucleus superiorly and the thalamus inferiorly. The boundary between the thalamus and caudate nucleus is marked by a groove that is occupied by a fascicle of nerve fibres, the stria terminalis,
and by the superior thalamostriate vein. The inferior limit of the body of the ventricle and its medial wall are formed by the body of the fornix. The fornix is separated from the thalamus by the choroidal fissure. The choroid plexus occludes the choroidal fissure and covers part of the thalamus and fornix. The body of the lateral ventricle widens posteriorly to become continuous with the posterior and inferior horns at the collateral trigone or atrium.
The posterior horn curves posteromedially into the occipital lobe. It is usually diamond-shaped or square in outline, and the two sides are often asymmetrical. Fibres of the tapetum of the corpus callosum separate the ventricle from the optic radiation, and form the roof and lateral wall of the posterior horn. Fibres of the splenium of the corpus callosum (forceps major) pass medially as they sweep back into the occipital lobe, and produce a rounded elevation in the upper medial wall of the posterior horn. Lower down, a second elevation, the calcar avis,
corresponds to the deeply infolded cortex of the anterior part of the calcarine sulcus.
The inferior horn is the largest compartment of the lateral ventricle
and extends forwards into the temporal lobe. It curves round the posterior aspect of the thalamus (pulvinar), passes downwards and posterolaterally and then curves anteriorly to end within 2.5 cm of the temporal pole, near the uncus. Its position relative to the surface of the hemisphere usually corresponds to the superior temporal sulcus.
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The roof of the inferior horn is formed mainly by the tapetum of the corpus callosum, but also by the tail of the caudate nucleus and the stria terminalis, which extend forwards in the roof to terminate in the amygdala at the anterior end of the ventricle. The floor of the inferior horn consists of the hippocampus medially and the collateral eminence, formed by the infolding of the collateral sulcus, laterally. The inferior part of the choroid fissure lies between the fimbria (a distinct bundle of efferent fibres that leaves the hippocampus) and the stria terminalis in the roof of the inferior horn. The temporal extension of the choroid plexus fills this fissure and covers the outer surface of the hippocampus.
https://kareenalohia232406.blogspot.com/2022/03/annelida-and-mollusks-full-and-short.htmlTHIRD VENTRICLE
The third ventricle is a midline, slit-like cavity, which is derived from the primitive forebrain vesicle. The upper part of the lateral wall of the ventricle is formed by the medial surface of the anterior two-thirds of the thalamus, and the lower part is formed by the hypothalamus anteriorly and the subthalamus posteriorly. An indistinct hypothalamic sulcus extends horizontally on the ventricular wall between the interventricular foramen and the cerebral aqueduct, and marks the boundary between the thalamus and hypothalamus. Dorsally, the lateral wall is limited by
a ridge covering the stria medullaris thalami.
The lateral walls of the third ventricle are commonly joined by an interthalamic adhesion, or massa intermedia, a band of grey matter that extends from one thalamus to the other. An interthalamic adhesion is almost always found in
humans, more often in women, in whom it is larger by an average of 53%.
Anteriorly, the third ventricle extends to the lamina terminalis. This thin structure stretches from the optic chiasma to the
rostrum of the corpus callosum and represents the rostral boundary of the embryonic neural tube.
Thelamina terminalis forms the roof of a small virtual cavity, the cistern of the lamina terminalis, which lies immediately below the ventricle and is an extension of the interpeduncular cistern. This is clinically important because it contains the anterior communicating artery; aneurysm development and rupture at this site may cause intraventricular haemorrhage through the thin membrane of the lamina terminalis. Above the lamina terminalis, the anterior wall of the third ventricle is formed by the diverging columns of the fornices and the transversely orientated anterior commissure, which crosses the
midline. The narrow interventricular foramen is located immediately
posterior to the column of the fornix and separates the fornix from the
anterior nucleus of the thalamus.
There is a small, angular, optic recess occurs inconsistently at the base of the
lamina terminalis, just dorsal to and extending into the optic chiasma.
Behind it, the anterior part of the floor of the third ventricle is formed mainly by hypothalamic structures. The thin infundibular recess lies immediately behind the optic chiasma and extends into the pituitary stalk. Behind this recess, the tuber cinereum and the mammillary
bodies form the floor of the ventricle.
The roof of the third ventricle is a thin ependymal layer that extends
from its lateral walls to the choroid plexus, which spans the choroidal
fissure.
The body of the fornix lies above the roof.
The posterior boundary of the ventricle is marked by a suprapineal recess, the habenular commissure, a pineal (epiphysial) recess, which extends into the pineal stalk, and by the posterior commissure. Below the posterior commissure the ventricle is continuous with the cerebral aqueduct of the midbrain.
The third ventricle is larger in infants with trisomy 21 compared with
controls.
CEREBRAL AQUEDUCT
The cerebral aqueduct is a small tube, roughly circular in transverse
section and 1–2 mm in diameter.
The aqueduct extends throughout the dorsal quarter of the midbrain
in the midline and is surrounded by the periaqueductal (central) grey
matter. Rostrally, it commences immediately below the posterior commissure, where it is continuous with the caudal aspect of the third ventricle. Caudally, it is continuous with the lumen
of the fourth ventricle at the junction of the midbrain and pons.
The superior and inferior colliculi are dorsal to the aqueduct and the midbrain tegmentum is ventral.
FOURTH VENTRICLE
The fourth ventricle lies between the brainstem and the cerebellum.Rostrally, it is continuous with the cerebral aqueduct, and caudally with the central canal of the spinal cord. In sagittal section, the fourth ventricle has a characteristic triangular profile, and the apex of its tented roof protrudes into the inferior aspect of the cerebellum. The ventricle is at its widest
at the level of the pontomedullary junction, where a lateral recess on both sides extends to the lateral border of the brainstem.
At this point the lateral apertures of the fourth ventricle (foramina of Luschka) open
into the subarachnoid space at the cerebellopontine angle, behind the
upper roots of the glossopharyngeal nerves.The floor of the fourth ventricle is a shallow diamond-shaped, or rhomboidal, depression (rhomboid fossa) on the dorsal surfaces of the pons and the rostral half of the medulla. It consists largely of grey matter and contains important cranial nerve nuclei (V–XII). The precise location of some nuclei is discernible from surface
features. The superior part of the ventricular floor is triangular in shape
and is limited laterally by the superior cerebellar peduncles as they converge towards the cerebral aqueduct. The inferior part of the ventricular floor is also triangular in shape and is bounded caudally by the gracile and cuneate tubercles, which contain the dorsal column nuclei, and, more rostrally, by the diverging inferior cerebellar peduncles.
A longitudinal median sulcus divides the floor of the fourth ventricle. Each half is itself divided, by an often indistinct sulcus limitans, into a medial region known as the medial eminence and a lateral region known as the vestibular area. The vestibular nuclei lie beneath the
vestibular area. In the superior part, the medial eminence is represented
by the facial colliculus, a small elevation produced by an underlying
loop of efferent fibres from the facial nucleus, which covers the abducens nucleus. Between the facial colliculus and the vestibular area the sulcus limitans widens into a small depression, the superior fovea.
In its upper part, the sulcus limitans constitutes the lateral limit of the
floor of the fourth ventricle. Here, a small region of bluish-grey pigmentation denotes the presence of the subjacent locus coeruleus. Caudal to the facial colliculus, at the level of the lateral recess of the ventricle, a variable group of nerve fibre fascicles, known as the striae medullaris,
runs transversely across the ventricular floor and passes into the median
sulcus. In the inferior part, the medial eminence is represented by the
hypoglossal triangle (trigone), which lies over the hypoglossal nucleus.
Laterally, the sulcus limitans widens to produce an indistinct inferior
fovea. Caudal to the inferior fovea, between the hypoglossal triangle
and the vestibular area, is the vagal triangle (trigone), which covers the
dorsal motor nucleus of the vagus. A narrow translucent ridge, the
funiculus separans, which is separated from the gracile tubercle by the
small area postrema, crosses below the vagal triangle.
The roof of the fourth ventricle is formed by the superior and inferior
medullary veli. The thin superior medullary velum stretches across the
ventricle between the converging superior cerebellar peduncles and is continuous with the cerebellar white matter. Dorsally, it
is covered by the lingula of the superior vermis. The inferior medullary
velum is more complex and is mostly composed of a thin sheet, devoid
of neural tissue, formed by ventricular ependyma and the pia mater of
the tela choroidea. Just inferior to the nodule of the cerebellum, a
median aperture, the foramen of Magendie, opens the roof of the fourth
ventricle into the cisterna magna.
The aperture forms when a membranous structure (Blake’s pouch) perforates into the fourth ventricle at the ninth week of fetal development; persistence of this membrane results in cystic obstruction of the median outlet from the fourth ventricle.
CIRCUMVENTRICULAR ORGANS
The walls of the ventricular system are lined with ependymal cells covering a subependymal layer of glia. At certain midline sites in the ventricular wall, collectively referred to as circumventricular organs, the blood-brain barrier is absent and specialised ependymal cells called tanycytes are present.
The functions of ependyma and tanycytes may include secretion into the CSF; transport of neurochemicals from subjacent neurones, glia or vessels to the CSF; transport of neurochemicals from the CSF to the same subjacent structures; and chemoreception.
Adult mammalian neurogenesis occurs in discrete neurogenic niches
that are best characterized in the ependymal and subependymal glial
cell layers in the subgranular zone of the dentate gyrus and in the subventricular zone; the existence of adult human neurogenic niches is controversial.
The circumventricular organs include the vascular organ (organum vasculosum), subfornical organ, neurohypophysis, median eminence, subcommissural organ, pineal gland and area postrema.
The vascular organ lies in the lamina terminalis between the optic chiasma and the anterior commissure. Its external zone contains a richly fenestrated vascular plexus that covers glia and a network of nerve fibres. The ependymal cells of the vascular organ, like those of other
circumventricular organs, are flattened and have few cilia.
The major inputs appear to come from the subfornical organ, locus coeruleus and
a number of hypothalamic nuclei. The vascular organ projects to the median preoptic and supraoptic nuclei. It is involved in the regulation of fluid balance and may also have neuroendocrine functions.
The subfornical organ lies at the level of the interventricular foramen.
It contains many neurones, glial cells and a dense fenestrated capillary plexus, and is covered by flattened ependyma. It is believed to have widespread hypothalamic interconnections and to function in the regulation of fluid balance and thirst.
The neurohypophysis (posterior pituitary) is the site of termination of neurosecretory projections from the supraoptic and paraventricular nuclei of the hypothalamus. Neurones in these nuclei release vasopressin and oxytocin respectively into the capillary bed of the neurohypophysis, where the hormones gain access to the general circulation.
The median eminence contains the terminations of axons of hypothalamic neurosecretory cells. Peptides released from these axons control the hormonal secretions of the anterior pituitary via the pituitary portal system of vessels.
The subcommissural organ lies ventral to and below the posterior
commissure, near the inferior wall of the pineal recess.
The pineal gland is a small structure, approximately 8 mm in diam-
eter, situated rostro dorsal to the superior colliculus and behind the stria
medullaris.
The area postrema is a bilaterally paired structure, located at the caudal limit of the floor of the fourth ventricle. It is an important chemoreceptive area that triggers vomiting in response to the presence of emetic substances in the blood. The area postrema, along with the
nucleus of the solitary tract and the dorsal motor nucleus of the vagus, makes up the so-called dorsal vagal complex, which is the major termination site of vagal afferent nerve fibres.
CHOROID PLEXUS
The vascular pia mater in the roofs of the third and fourth ventricles, and in the medial wall of the lateral ventricle along the line of the choroid fissure, is closely apposed to the ependymal lining of the ventricles, without any intervening brain tissue. It forms the tela choroidea,
which gives rise to the highly vascularized choroid plexuses from which
CSF is secreted into the lateral, third and fourth ventricles.
The body or stroma of the choroid plexus consists of many capillaries, separated from the ventricles by the pia mater and choroid ependymal cells.
In the lateral ventricle, the choroid plexus extends anteriorly as far as the interventricular foramen, through which it is continuous across the third ventricle with the plexus of the opposite lateral ventricle.
From the interventricular foramen, the plexus passes posteriorly, in contact with the thalamus, curving round its posterior aspect to enter the inferior horn of the ventricle and reach the hippocampus.
Throughout the body of the ventricle, the choroid fissure lies between the fornix superiorly and the thalamus inferiorly.From above, the tela choroidea is triangular with a rounded apex between the interventricular foramina, often indented by the anterior columns of the fornices. Its lateral edges are irregular and contain choroid vascular fringes.
At the posterior basal angles of the tela, these fringes continue and curve on into the inferior horn of the ventricle.
When the tela is removed, a transverse slit (the transverse fissure) is left between the splenium and the junction of the ventricular roof with the tectum. The transverse fissure contains the roots of the choroid plexus of the third ventricle and of the lateral ventricles.
CHOROID PLEXUS OF THE THIRD VENTRICLE
The choroid plexus of the third ventricle is attached to the tela choroidea, which is, in effect, the thin roof of the third ventricle as it develops during fetal life. In coronal sections of the cerebral hemispheres, it can be seen that the choroid plexus of the third and lateral ventricles are continuous.
The choroid plexus of the fourth ventricle is similar in structure to that of the lateral and third ventricles. The roof of the inferior part of the fourth ventricle develops as a thin sheet in which the pia mater is
in direct contact with the ependymal lining of the ventricle. This thin sheet, the tela choroidea of the fourth ventricle, lies between the cerebellum and the inferior part of the roof of the ventricle.
CHOROID PLEXUS OF THE FOURTH VENTRICLE
The choroid plexus of the fourth ventricle is T-shaped, with vertical and horizontal limbs, but the precise form varies widely from a single vertical limb to an elongated ‘T’ that extends out through the foramina of Luschka into the cerebellopontine angle.
The vertical (longitudinal) limb is double, flanks the midline and is adherent to the roof of the ventricle.
The limbs fuse at the superior margin of the median aperture (foramen of Magendie) and are often prolonged on to the ventral aspect of the cerebellar vermis. The horizontal limbs of the plexus project into the lateral recesses of the ventricle.
Small tufts of plexus may pass through the lateral apertures (foramina of Luschka) and emerge, still covered by ependyma, in the subarachnoid space of the cerebellopontine angle.
The mean thickness of the choroid plexus in the fourth ventricle in children is 2.5 mm.
BLOOD SUPPLY OF THE CHOROID PLEXUS
The blood supply of the choroid plexus in the tela choroidea of the lateral and third ventricles is usually via a single vessel from the anterior choroidal branch of the internal carotid artery and several choroidal branches of the posterior cerebral artery; the two sets of vessels anastomose to some extent.
Capillaries drain into a rich venous plexus served by a single choroidal vein. The blood supply of the fourth ventricular choroid plexus is from the inferior cerebellar arteries.
CEREBROSPINAL FLUID
CSF is a clear, colourless liquid. Normal CSF contains small amounts of protein and differs from blood in its electrolyte content. It is not simply an ultrafiltrate of blood but is actively secreted by the choroid plexuses in the lateral, third and fourth ventricles.The choroid plexus epithelium constitutes a blood–CSF barrier.
Choroid plexus epithelial cells have the characteristics of transport and secretory cells; their apical surfaces have micro-
villi, and their basal surfaces exhibit interdigitations and folding. Tight junctions (occluding junctions, zonulae adherentes) at the apical ends of the cells are permeable to small molecules.
Fenestrated capillaries lie just beneath the epithelial cells in the stroma of the choroid plexus. The ependymal lining of the ventricles and the extracellular fluid from the brain parenchyma are additional sources of CSF, but how much each
source contributes to CSF production is unclear.
SUBARACHNOID SPACE
The subarachnoid space lies between the arachnoid and the pia mater. It is continuous with the lumen of the fourth ventricle via the median aperture (foramen of Magendie) and the paired lateral apertures (foramina of Luschka) are located at the end of the lateral recesses of the fourth ventricle, and open into the subarachnoid space at the cerebellopontine angle, behind the upper roots of the glossopharyngeal nerve.
The subarachnoid space contains CSF, the larger arteries and veins that traverse the surface of the brain, and the intracranial or intravertebral portions respectively of the cranial and spinal nerves.
Trabeculae, in the form of sheets or fine filiform structures, each containing a core of collagen, cross the subarachnoid space from the deep layers of the arachnoid mater to the pia mater.
The trabeculae are attached to the large blood vessels within the subarachnoid space and may form compartments, particularly in the perivascular regions, thereby possibly facilitating directional flow of CSF through the space.
A thin layer of leptomeninges, often only one cell thick, coats the trabeculae, vessels and nerves that cross the subarachnoid space; it fuses with the arachnoid mater at the margins of the exit foramina in the skull and vertebral column.
Arachnoid and pia mater are in close apposition over the convexities of the brain, such as the cortical gyri, whereas concavities are followed by the pia but spanned by the arachnoid. This arrangement produces a subarachnoid space of greatly variable depth that is location-dependent.
which are continuous with the general subarachnoid space and crossed by long, filamentous trabecula.
The largest cistern, the cisterna magna (cerebellomedullary cistern) is formed where the arachnoid bridges the interval between the medulla oblongata and the inferior surface of the cerebellum.
The cistern is continuous above with the lumen of the fourth ventricle through its median aperture, the foramen of Magendie, and below with the spinal subarachnoid space. It contains the vertebral arteries and the origins of The ependymal cells on the dorsal aspect of the cerebral aqueduct are tall, columnar and ciliated, with granular basophilic cytoplasm; they may be involved in the secretion of materials into the CSF from adjoining axonal terminals or capillaries.
The largest cistern, the cisterna magna (cerebellomedullary cistern) is formed where the arachnoid bridges the interval between the medulla oblongata and the inferior surface of the cerebellum.
The cistern is continuous above with the lumen of the fourth ventricle through its median aperture, the foramen of Magendie, and below with the spinal subarachnoid space. It contains the vertebral arteries and the origins of the posterior inferior cerebellar arteries, the glossopharyngeal, vagus, accessory and hypoglossal nerves, and the choroid plexus.
On either side, the paired cerebellopontine cisterns (angle cisterns or cerebellopontine angle cisterns) are situated in the lateral angle between the cerebellum and the pons. They are traversed by the trigeminal, facial and vestibulocochlear nerves, the anterior inferior cerebellar arteries and the superior petrosal veins.
The prepontine cistern (pontine cistern) is an extensive space ventral to the pons, which is continuous below with the spinal subarachnoid space, behind and laterally with the cerebellopontine cisterns, and ros-trally with the interpeduncular cistern. The basilar artery runs through the pontine cistern into the interpeduncular cistern, which also contains the origins of the anterior inferior cerebellar and superior cerebellar arteries, and the abducens nerves.
CIRCULATION OF CEREBROSPINAL FLUID
The total CSF volume is approximately 150ml, of which 125 ml is intracranial. The ventricle contains about 25 ml, most lying within the lateral ventricles, and the remaining 100 ml is located in the cranial subarachnoid space. CSF is secreted at a rate of 0.35-0.40 ml per minute, which mean the normally about 50% of the total volume of CSF is replaced every five to six hours. An effective means of removal from the cranial cavity is thus essential. CSF flows from the lateral ventricle to the third ventricle and then through the cerebral aqueduct to the fourth ventricle. Mixing of CSF from different choroid sources occurs and is probably assisted by cilia on the ependymal cells lining the ventricle and by arterial pulsation. CSF leaves the fourth ventricles through the medial and lateral apertures to enter the subarachnoid space of the cisterna magna and the subarachnoid cisternae over the front of the pons respectively.
The movement of CSF in the subarachnoid space is complex and is chartered by a fast flow component and a much slower bulk flow component. During systole, the major arteries lying in the basal cisterns and the other subarachnoid space dilate significantly and exert pressure effects on the CSF, causing rapid CSF flow around the brain and out of the cranial cavity into the upper cervical vertebral canal. The pressure wave which causes this outflow of CSF is dispersed through the spinal CSF space which acts as a capacitance vessel, and is transmitted to the dural venous sinuses through the arachnoid granulations. As the blood within the major arteries passes into the major arteries pases into thebrain in late systole and diaastole, CSF re- enters the skull from the spine. This CSF flow occurs at rapid enters and is repeated during every heartcycle. In audition, there is a slow bulk flow of CSF, with a time course measured in hour, which results in circulation of CSF over the cerebral surface in a superolateral direction. CSF is absorbed into the venous system by active diffusion into cerebral capillaries which occurs as a result of the CSF/ interstitial pressure differential.