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Physiology III
Neuroscience
Ventricles & Meninges

(Haines, Ch. 6,7)

Keywords

Cerebrospinal Fluid (CSF)
produced by the choriod epithelial cells of the lateral, third and fourth ventricles. Composed of water, sodium chloride, magnesium, potassium, calcium, glucose and proteins. The CSF produced by the choriod plexuses passes through the ventricular system to exit the fourth ventricle through the foramina of Luschka and Magendie, and enters the subarachnoid space, circulates around the brain and spinal cord and reenters the vascular system through thearachnoid villi that extend into the superior sagittal sinus. About 330 to 380ml of CSF enters the venous circulation per day. This provides the buoyancy necessary to prevent the weight of the brain from crushing nerve roots and blood vessels against the internal surface of the skull. The movement of CSF is influenced by two major factors, pressure gradient between the points of production (choroid plexuses), and the points of transfer into the venous system (arachnoid villi), and movement by pure mechanical means – the gentle movement of the brain during normal activities, and the pulsations of the numerous arteries found in the subarachnoid space. (Haines p. 93)
Choroid Plexus
found in both lateral ventricles and in the third and fourth ventricles, develops from involuted ependymal cells, along with vessels and a small amount of vascularized connective tissue which enlarge and form elevations called VILLI. The abundant capillaries in the connective tissue core of each villi are surrounded by a basal lamina. The endothelial cells of these capillaries have numerous fenestrations, which allow a free exchange of molecules between blood plasma and the ECF in the connective tissue core. The choroidal cell has microvilli which are attached to its neighbor by continuous tight junctions that seal off the subjacent extracellular space from the ventricular space. This represents the BLOOD-BRAIN BARRIER which is specialized to control the flow of ions and metabolites into the CSF. (Haines p. 92)
Lateral Ventricles
the cavities of the telencephalon, C-shaped, consisting of an anterior horn, a body, and posterior and inferior horns. The anterior horn and body of the lateral ventricle are bordered medially by the septum pellucidum (at rostral levels) and by the fornix ( at caudal levels), and dorsally by the corpus callosum. The floor of the body of the lateral ventricle is made up of the thalamus, and the caudate nucleus is found in the lateral wall of the lateral ventricle throughout its extent. In the temporal lobe, the inferior horn of the lateral ventricle contains the tail of the caudate nucleus in its lateral (dorsolateral) wall, the hippocampus in its ventral wall, and the amygdaloid complex in its anterior end. The openings between lateral and third ventricles, called the interventricular foramina of Monro are located between the fornix and the anterior and medial end of the thalamus.
Anterior Horn

Posterior Horn

Inferior Horn

Third Ventricle
the cavity of the diencephalon, is a narrow, vertically oriented midline space that communicates rostrally with the lateral ventricles and caudally with the cerebral aqueduct. The boundaries of the third ventricle are formed by a variety of structures, most important, the dorsal thalamus and hypothalamus, and by the supraoptic recess, the infundibular recess, the pineal recess, and the suprapineal recess. The rostral wall of the third ventricle is formed by a short segment of the anterior commissure and a thin membrane, the lamina terminalis, which extends from the anterior commissure ventrally to the rostral edge of the optic chiasm. The floor of the third ventricle is formed by the optic chiasm and infundibulum and their corresponding recesses, plus a line extending caudally along the rostral aspect of the midbrain to the cerebral aqueduct. The caudal wall is formed by the posterior commissure and the recesses related to the pineal, and the roof is the tela choriodae from which the choroid plexus is suspended.
It is the cavity of the diencephalon. It is narrow, vertically oriented on the midline. It communicates rostrally with the lateral ventricles and caudally with the cerebral aqueduct (which is connected to the rostral portion of the Fourth ventricle). (Marieb pg. 409) (Haines pg. 89).
Cerebral Aqueduct
the extension of the ventricle through the mesencephalon, communicates rostrally with the third ventricle and caudally with the fourth ventricle. This midline channel is about 1.5mm in diameter in adults and contains no choriod plexus. Its narrow diameter makes it especially susceptible to occlusion. The result is a blockage of CSF and enlargement of the third and lateral ventricles at the expense of surrounding brain tissue. The cerebral aqueduct is surrounded on all sides by a sleeve of gray matter that contains small neurons called the periaqueductal gray or central gray.
A canal like connection between the Fourth (caudal) and Third (rostral) ventricles. The Fourth ventricle closes to create the Cerebral Aqueduct. (Marieb pg. 408 & 409) (Haines pg. 91)
Fourth Ventricle
a pyramid-shaped space that forms the cavity of the metencephalon and myelencephalon. The apex of this ventricle extends dorsally into the base of the cerebellum, and caudally it tapers to a narrow channel that continues into the cervical spinal cord as the central canal. Laterally the fourth ventricle extends over the surface of the medulla as the lateral recesses, to eventually open into the area of the pons-medulla junction through the foramina of Luschka. The irregularly shaped foramen of Magendie is located in the caudal sloping roof of the fourth ventricle. The floor is formed by the pons and medulla. The only openings between the ventricles of the brain and the subarachnoid space surrounding the brain are the foramina of Luschka and Magendie in the fourth ventricle.
Foramina Of Luschka
Haines 91
Foramen Of Magendie

Dura (dura mater)
the outermost portion of the human meninges, adherent to the inner surface of the skull, separated from the vertebrae by the epidural space. Composed of flattened fibroblasts that have sinuous processes forming the dural border cell layer. The extracellular spaces between the flattened cell processes of the dural border cells contain an amorphous substance but no collagen or elastic fibers. Because of this loose arrangement, enlarged extracellular spaces, and lack of extracellular connective tissue fibrils, the dural border cell layer constitutes a plane of structural weakness at the dura-arachnoid junction.
Faix Cerebri
the largest of the infoldings or septa of the dura, attached to the crista galli rostrally, to the midline of the inner surface of the skull dorsally, and to the surface of the tentorium cerebelli caudally. The superior sagittal sinus is found where the falx cerebri attaches to the skull, the straight sinus where it fuses with the tentorium, and the inferior sagittal sinus in its free edge.
Tentorium Cerebelli
second largest of the dural infoldings. Rostrally, it attaches to the clinoid processes, rosrolaterally to the perrous portion of the temporal bone, and caudolaterally to the inner surface of the occipital bone and a small part of the parietal bone. The tent shape of the tentorium divides the cranial cavity into infratentorial and supratentorial compartments. The supratentorial compartment is divided into right and left halves by the falx cerebri. The edges of the right and left tentoria, as they arch from the clinoid processes to join at the straight sinus, form the tentorial notch. The occipital lobe is above the tentorium, the cerebellum below it, and the midbrain passes through the notch.
Diaphragma Sella
the smallest of the dural infoldings, forms the roof of the hypophyseal fossa and encircles the stalk of the pituitary. The cavernous sinuses are found on either side of the sella turcica, and the anterior and posterior intercavernous sinuses are found in their respective edges of the diaphragma sella.
Arachnoid (arachnoid mater)
internal to the dura, the arachnoid is a thin cellular layer that is attached to the overlying dura, but, with the exception of the arachnoid trabeculae, is separated from the pia mater by the subarachnoid space. The arachnoid around the brain is contiguous with the arachnoid lining the inner surface of the spinal dura. The arachnoid is avascular and does not contain nerve fibers. It is regarded as having two parts, the arachnoid barrier cell layer, and the arachnoid trabeculae.
Arachnoid Barrier cell layer is tenuously attached to the dural border cell layer by occasional cell junctions. The cells have closely apposed cell membranes and are joined to each other by numerous "tight" junctions; hence the "barrier" characteristic of the layer. This close apposition of cell membranes excludes any significant extracellular space, serves as a barrier against the movement of fluids or other substances, and also imparts strength to the membrane.
Arachnoid Trabeculae
are composed of flattened, irregularly shaped fibroblasts that bridge the subarachnoid space in a random fashion, anchoring the brain in a delicate fashion. The attachments of the trabecular cells and their framework of collagen fibrils give added strength to the arachnoid mater.
**the dura is adherent to the skull, the arachnoid to the dura, the arachnoid trabeculae to the pia, and the pia to the surface of the brain**
Arachnoid Granulation (arachnoid villi)
small specialized portions of the arachnoid that protrude into the superior sagittal sinus through openings in the dura. Structurally adapted for the transport of CSF from the subarachnoid space villi into the venous circulation via the sinus, through small intercellular channels located between cells, and by way of a vacuole-mediated transport of fluid and other elements through villus cells down the pressure gradient.
Subarachnoid Space
located between the arachnoid barrier cell layer and the pial cells located on the surface of the brain or spinal cord. This space contains CSF, trabecular cells and collagen fibrils, arteries, and veins. These large vessels in the subarachnoid space may rupture, resulting in the spread of blood around the brain: a subarachnoid hemorrhage. It is common to refer to the brain as ‘floating’ in this space, but it is actually suspended within this space as described above.
Pia (pia mater)
located on the surface of the brain and spinal cord and closely follows all their various grooves and elevations. The pia and arachnoid together constitute the leptomeninges. The spinal cord is anchored in the subarachnoid space by three structures: two pial modifications plus a reticulated septum of arachnoid cell processes that attaches to the dorsal midline of the cord. The first of the pial structures, the denticulate ligaments, run longitudinally along each side of the spinal cord about midway between the dorsal and ventral roots and attach to the inner surface of the arachnoid-lined dural sac. Second, extending caudally from the conus medullaris is a tough strand composed primarily of pia; this is the filum terminale. The filum terminale attaches to the caudal end of the dural sac which, in turn, attaches to the coccyx as the coccygeal ligament. Together these structures serve a function analogous to that of the arachnoid trabeculae around the brain.
Blood Brain Barrier
See Choroid Plexus above

 


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