The Brain III: Meninges and Ventricles
The Meninges
The brain and spinal cord are covered by the Meninges, three fibrous membrane layers which support and protect the brain. The layers are the dura mater, the arachnoid villus, and the pia mater from superficial to deep. All three layers enclose an envelope of cerebrospinal fluid which provides mechanical support, hydraulic protection and a matrix for the distribution of the blood supply. The meninges also serve an important circulatory function. (the meninges are more commonly called the dura, arachnoid, and pia, without using the term mater.)
The dura mater (hard mother) is the outermost and thickest of these membranes and serves to protect the brain from front-line mechanical trauma and guard against dessication (moisture removal); patients can survive severe skull fractures if the bone fragments do not penetrate the dura mater. The portion of the dura mater overlying the cerebral hemispheres and brain stem contains two separate layers of connective tissue that are fused except in certain regions where they separate to form the intracranial venous sinuses: an outer periosteal layer and an inner meningeal layer. The periosteal layer is attached to the inner surface of the skull. Moreover, within the dura mater are large, low-pressure blood vessels that are part of the return path for cerebral venous blood. These vessels are termed the dural sinuses.
Two important partitions arise from the meningeal layer and separate different components of the cerebral hemispheres and brain stem where the dura mater is folded in the cranial cavity to form distinct fibrous barriers; the falx cerebri, located between the two cerebral hemispheres and the tentorium cerebelli which demarcates the superior limit of the posterior fossa. The dura mater that covers the spinal cord is continuous with both the meningeal layer of the cranial dura and the epineurium of peripheral nerves.
The delicate and filamentous arachnoid villus lies beneath the dura mater and appears to be loosely applied to the surface of the brain. Many of the major arteries can be seen on the surface of the brain beneath the arachnoid. The arachnoid villlus adjoins but is not tightly bound to the dura mater, thereby allowing a potential space, the subdural space, to exist between them. This space is important clinically. Because the dura mater contains blood vessels and breakage of one of its vessels due to head trauma can lead to subdural bleeding and to the formation of a hematoma (blood clot.) In this condition the hematoma pushes the arachnoid mater away from the dura mater, fills the subdural space, and compresses underlying neural tissue.
Several spaces are found in association with the meninges. between the bone and the dura mater is the epidural space, and beneath the dura mater is the subdural space. The bone, dura and arachnoid are normally closely applied to one another so that the epidural and subdural spaces are potential spaces in which blood or pus may accumulate. Beneath the arachnoid (above pia mater) is the subarachnoid space, which surrounds the entire brain and spinal cord and is filled with cerebrospinal fluid (CSF). The subarachnoid space communicates with the interior of the brain via the ventricular system. The space between the arachnoid mater and pia mater is the subarachnoid space. The veins and arteries that overlie the surface of the central nervous system are located in the subarachnoid space.
Filaments of arachnoid mater pass through the subarachnoid space and connect to the pia mater, giving this space the appearance of a spider's web. (arachne = spider) After leaving the fourth ventricle, cerebrospinal fluid circulates over the surface of the brain and spinal cord within the subarachnoid space. The innermost meningeal layer, the pia (tender mother) mater, is very thin and delicate and adheres to the surface of the brain and spinal cord. It is composed of a very thin layer of tissue that is so closely attached to the brain surface that it cannot be seen.
The Ventricles and Cerebrospinal Fluid
The ventricular system is located deep within the brain and is lined with a ciliated cuboidal epithelium called the ependyma. Leptomeninges, multi-tufted vascular organs that arise by the ependyma, and blood vessels folding into the ventricles called Choroid Plexuses are found in the lateral, third and fourth ventricles. These structures in combination are rich in the enzymes that are the main source for the production of cerebrospinal fluid (CSF). CSF circulates throughout the ventricles and subarachnoid space.
A lateral ventricle is located in each of the cerebral hemispheres and is divided into an anterior horn located in the frontal lobe, body and atrium located in the parietal lobe, posterior horn in the occipital lobe, and inferior horn in the temporal lobe. The lateral ventricles communicate with each other and the third ventricle of the diencephalon via the interventricular foramina of Monro. The aqueduct of Sylvius leads from the third ventricle to the fourth ventricle, located dorsal to the pons and medulla. The ventricular system communicates with the subarachnoid space in the fourth ventricle via two foramina of Luschka and the foramen of Magendie.
CSF is reabsorbed through the arachnoid granulations in to the superior sagittal sinus. The rate of CSF formation remains relatively constant at approximately 0.35 ml/min (500 ml/day). Most of the CSF is actively secreted into the ventricular system by the choroid plexuses but some is also derived directly from the interstitial fluid of the brain and crosses the ependyma to enter the ventricles.
Circulation of the CSF is pulsatile and promoted by the beating of the cilia of the ependymal cells and the pulsatile changes in the volume of intracranial blood that occurs with cardiac systole and respiratory movements. The to-and-fro movement of CSF results in a directional flow, that is, from the lateral, third, and fourth ventricles to the subarachnoid space, where it then circulates in two major directions. The more important pathway is rostrally and dorsally toward the intracranial venous system where CSF exits through arachnoid villi that project into the superior sagittal sinus. CSF also drains into the spinal subarachnoid space with some resorption through the dural sleeves of the nerve roots.
Cerebral Spinal Fluid (CSF) covers brain and the spinal cord, produced in the choroid plexus, 125-150cc, replaced 4-5 times a day; formed by the choroid plexus within the ventricles and functions to cushion, bathe, nourish, maintain shape, hydraulic fluid.
Cavities within the central nervous system contain cerebrospinal fluid
The central nervous system has a tubular organization. Within it are cavities, collectively termed the ventricular system, that contain cerebrospinal fluid. cerebrospinal fluid is a watery fluid that cushions the central nervous system from physical shocks and is a medium for chemical communication. An intra-ventricular structure, the choroid plexus, secretes most of the cerebrospinal fluid.
The ventricular system consists of ventricles - often with bizarre/irregular shapes - where cerebrospinal fluid accumulates, and narrow communication channels. There are two lateral ventricles, and each is located within one cerebral hemisphere. They are further subdivided into a body and three compartments termed horns: anterior, posterior, and inferior. The confluence of the three horns is termed the atrium. Between the two halves of the diencephalon is the third ventricle, forming a midline cavity.
The fourth ventricle is located between brain stem and cerebellum: the medulla and pons form the floor of the fourth ventricle and the cerebellum, the roof. The ventricles are interconnected by narrow channels: The inteventricular foramina connect each of the lateral ventricles with the third ventricle, and the cerebral aqueduct, in the midbrain, connects the third and fourth ventricles. The ventricular system extends into the spinal cord as the central canal. Cerebrospinal fluid exits the ventricular system through several apertures in the fourth ventricle and bathes the surface of the central nervous system.
**Ventricles - lateral ventricles foramen of monro connect two lateral ventricles to the third ventricle, fourth ventricle aqueduct of sylvius connects the third and fourth ventricles
Cisterna Magna - CSF escapes into the Cisterna Magna through openings in the roof of fourth ventricle; foramina of luschka (2 lateral) foramen of magendie (medial)
The Blood-Brain Barrier
The blood-brain barrier provides selective permeability to chemicals, pathogens and nutrients. (see vascular neuroanatomy)
The brain and spinal cord are covered by the Meninges, three fibrous membrane layers which support and protect the brain. The layers are the dura mater, the arachnoid villus, and the pia mater from superficial to deep. All three layers enclose an envelope of cerebrospinal fluid which provides mechanical support, hydraulic protection and a matrix for the distribution of the blood supply. The meninges also serve an important circulatory function. (the meninges are more commonly called the dura, arachnoid, and pia, without using the term mater.)
The dura mater (hard mother) is the outermost and thickest of these membranes and serves to protect the brain from front-line mechanical trauma and guard against dessication (moisture removal); patients can survive severe skull fractures if the bone fragments do not penetrate the dura mater. The portion of the dura mater overlying the cerebral hemispheres and brain stem contains two separate layers of connective tissue that are fused except in certain regions where they separate to form the intracranial venous sinuses: an outer periosteal layer and an inner meningeal layer. The periosteal layer is attached to the inner surface of the skull. Moreover, within the dura mater are large, low-pressure blood vessels that are part of the return path for cerebral venous blood. These vessels are termed the dural sinuses.
Two important partitions arise from the meningeal layer and separate different components of the cerebral hemispheres and brain stem where the dura mater is folded in the cranial cavity to form distinct fibrous barriers; the falx cerebri, located between the two cerebral hemispheres and the tentorium cerebelli which demarcates the superior limit of the posterior fossa. The dura mater that covers the spinal cord is continuous with both the meningeal layer of the cranial dura and the epineurium of peripheral nerves.
The delicate and filamentous arachnoid villus lies beneath the dura mater and appears to be loosely applied to the surface of the brain. Many of the major arteries can be seen on the surface of the brain beneath the arachnoid. The arachnoid villlus adjoins but is not tightly bound to the dura mater, thereby allowing a potential space, the subdural space, to exist between them. This space is important clinically. Because the dura mater contains blood vessels and breakage of one of its vessels due to head trauma can lead to subdural bleeding and to the formation of a hematoma (blood clot.) In this condition the hematoma pushes the arachnoid mater away from the dura mater, fills the subdural space, and compresses underlying neural tissue.
Several spaces are found in association with the meninges. between the bone and the dura mater is the epidural space, and beneath the dura mater is the subdural space. The bone, dura and arachnoid are normally closely applied to one another so that the epidural and subdural spaces are potential spaces in which blood or pus may accumulate. Beneath the arachnoid (above pia mater) is the subarachnoid space, which surrounds the entire brain and spinal cord and is filled with cerebrospinal fluid (CSF). The subarachnoid space communicates with the interior of the brain via the ventricular system. The space between the arachnoid mater and pia mater is the subarachnoid space. The veins and arteries that overlie the surface of the central nervous system are located in the subarachnoid space.
Filaments of arachnoid mater pass through the subarachnoid space and connect to the pia mater, giving this space the appearance of a spider's web. (arachne = spider) After leaving the fourth ventricle, cerebrospinal fluid circulates over the surface of the brain and spinal cord within the subarachnoid space. The innermost meningeal layer, the pia (tender mother) mater, is very thin and delicate and adheres to the surface of the brain and spinal cord. It is composed of a very thin layer of tissue that is so closely attached to the brain surface that it cannot be seen.
The Ventricles and Cerebrospinal Fluid
The ventricular system is located deep within the brain and is lined with a ciliated cuboidal epithelium called the ependyma. Leptomeninges, multi-tufted vascular organs that arise by the ependyma, and blood vessels folding into the ventricles called Choroid Plexuses are found in the lateral, third and fourth ventricles. These structures in combination are rich in the enzymes that are the main source for the production of cerebrospinal fluid (CSF). CSF circulates throughout the ventricles and subarachnoid space.
A lateral ventricle is located in each of the cerebral hemispheres and is divided into an anterior horn located in the frontal lobe, body and atrium located in the parietal lobe, posterior horn in the occipital lobe, and inferior horn in the temporal lobe. The lateral ventricles communicate with each other and the third ventricle of the diencephalon via the interventricular foramina of Monro. The aqueduct of Sylvius leads from the third ventricle to the fourth ventricle, located dorsal to the pons and medulla. The ventricular system communicates with the subarachnoid space in the fourth ventricle via two foramina of Luschka and the foramen of Magendie.
CSF is reabsorbed through the arachnoid granulations in to the superior sagittal sinus. The rate of CSF formation remains relatively constant at approximately 0.35 ml/min (500 ml/day). Most of the CSF is actively secreted into the ventricular system by the choroid plexuses but some is also derived directly from the interstitial fluid of the brain and crosses the ependyma to enter the ventricles.
Circulation of the CSF is pulsatile and promoted by the beating of the cilia of the ependymal cells and the pulsatile changes in the volume of intracranial blood that occurs with cardiac systole and respiratory movements. The to-and-fro movement of CSF results in a directional flow, that is, from the lateral, third, and fourth ventricles to the subarachnoid space, where it then circulates in two major directions. The more important pathway is rostrally and dorsally toward the intracranial venous system where CSF exits through arachnoid villi that project into the superior sagittal sinus. CSF also drains into the spinal subarachnoid space with some resorption through the dural sleeves of the nerve roots.
Cerebral Spinal Fluid (CSF) covers brain and the spinal cord, produced in the choroid plexus, 125-150cc, replaced 4-5 times a day; formed by the choroid plexus within the ventricles and functions to cushion, bathe, nourish, maintain shape, hydraulic fluid.
Cavities within the central nervous system contain cerebrospinal fluid
The central nervous system has a tubular organization. Within it are cavities, collectively termed the ventricular system, that contain cerebrospinal fluid. cerebrospinal fluid is a watery fluid that cushions the central nervous system from physical shocks and is a medium for chemical communication. An intra-ventricular structure, the choroid plexus, secretes most of the cerebrospinal fluid.
The ventricular system consists of ventricles - often with bizarre/irregular shapes - where cerebrospinal fluid accumulates, and narrow communication channels. There are two lateral ventricles, and each is located within one cerebral hemisphere. They are further subdivided into a body and three compartments termed horns: anterior, posterior, and inferior. The confluence of the three horns is termed the atrium. Between the two halves of the diencephalon is the third ventricle, forming a midline cavity.
The fourth ventricle is located between brain stem and cerebellum: the medulla and pons form the floor of the fourth ventricle and the cerebellum, the roof. The ventricles are interconnected by narrow channels: The inteventricular foramina connect each of the lateral ventricles with the third ventricle, and the cerebral aqueduct, in the midbrain, connects the third and fourth ventricles. The ventricular system extends into the spinal cord as the central canal. Cerebrospinal fluid exits the ventricular system through several apertures in the fourth ventricle and bathes the surface of the central nervous system.
**Ventricles - lateral ventricles foramen of monro connect two lateral ventricles to the third ventricle, fourth ventricle aqueduct of sylvius connects the third and fourth ventricles
Cisterna Magna - CSF escapes into the Cisterna Magna through openings in the roof of fourth ventricle; foramina of luschka (2 lateral) foramen of magendie (medial)
The Blood-Brain Barrier
The blood-brain barrier provides selective permeability to chemicals, pathogens and nutrients. (see vascular neuroanatomy)
Optic Chiasma and the third ventricle.
What large blood vessels normally run beneath the chiasma and surround it?
What structure forms the walls of the third ventricle and is located just lateral to it?
How many hypothalamuses do you have? What structure forms the anterior border of the diencephalon, separating it from the caudal wall of the telencepalon?
Hemi-section of the brain, locate on one hemisphere as much of the ventricular system as possible along the medial wall.
Find the central canal then the fourth ventricle .
With what brain subdivisions is the latter associated?
Next find the aqueduct. Which ventricles does it join?
What large blood vessels normally run beneath the chiasma and surround it?
What structure forms the walls of the third ventricle and is located just lateral to it?
How many hypothalamuses do you have? What structure forms the anterior border of the diencephalon, separating it from the caudal wall of the telencepalon?
Hemi-section of the brain, locate on one hemisphere as much of the ventricular system as possible along the medial wall.
Find the central canal then the fourth ventricle .
With what brain subdivisions is the latter associated?
Next find the aqueduct. Which ventricles does it join?