HBF-III LEC 03 Meninges Ventricular System Dural Venous Sinuses Notes PDF 2025

Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. Course Learning Objectives I. Describe the normal anatomy of the meninges of the cranial cavity and exiting cranial nerves. Session Learning Objectives Describe the normal anatomy of the meningeal layers of...

Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. Course Learning Objectives I. Describe the normal anatomy of the meninges of the cranial cavity and exiting cranial nerves. Session Learning Objectives Describe the normal anatomy of the meningeal layers of the cranial cavity. Describe the anatomy of the dura mater compartments, spaces, and transition areas. Describe the anatomy of the spaces defined by the meningeal layers. Describe the development of ventricular system. Describe the anatomy of arterial supply to the brain and cranial cavity. Describe the anatomy of the ventricular system and their relationship to brain structures. II. Relate the anatomy of each structure of the meninges of the cranial cavity to its function(s). Relate the anatomy of the dural sinuses to the bony impressions within the cranial cavity. Relate the anatomy of the meningeal layers to the compartments of the cranial cavity. Relate the anatomy of vessels supplying the cranial cavity to structures in the neck and cranial cavity. Relate the composition and function of CSF III. Apply anatomical knowledge of the meninges of the cranial cavity to evaluate clinically relevant problems. Apply anatomy to evaluate effects of tissue damage/pathology. Apply the structure of the meningeal spaces and their clinical significance in traumatic brain injury and hemorrhagic stroke. Apply anatomy to understand the venous routes that can lead to the spread of infection into the cranial cavity. Apply anatomy to evaluate carotid artery stenosis using radiological images. Apply the anatomy of the meningeal spaces and their clinical significance in traumatic brain injury and hemorrhagic stroke. Apply the uses of the CSF as a diagnostic tool, cerebral perfusion pressure management, and drug delivery. Apply the anatomy and physiology of hydrocephalus. 1 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. Session Content Outline I. The meninges of the cranial cavity A. Dura matter of the cranial cavity 1. Layers of the dura matter. a. Periosteal layer of the dura mater b. Meningeal layer of the dura mater c. Formation of the dural sinuses 2. Dura mater partitions a. Falx cerebri b. Tentorium cerebelli c. Falx cerebelli d. Diaphragm sallae 3. Blood supply and innervation of the dura mater a. Blood supply to the dura mater b. Sensory innervation to the dura mater i. Sensory to the anterior and middle cranial fossa ii. Sensory innervation to the posterior cranial fossa II. Arachnoid mater and pia mater of the cranial cavity A. Arachnoid mater B. Pia mater III. Meningeal spaces A. Epidural space B. Subdural space C. Subarachnoid space D. Major Cistern of the Subarachnoid space E. Dural sinuses 1. General information 2. Naming & descriptions of the dural sinuses a. Superior sagittal sinus b. Inferior sagittal sinus c. Straight sinus d. Occipital sinus e. Confluences of the sinuses f. Transverse sinuses g. Sigmoid sinuses h. Cavernous sinuses i. Basilar sinuses IV. Venous drainage into the dural sinuses A. Cerebral veins 1. Diploic veins 2. Emissary veins 3. Great cerebral v V. Arterial supply to the brain 2 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. A. Internal Carotid Arteries 1. Carotid sinus 2. Carotid body 3. Regions of the internal carotid artery a. Cervical part b. Petrous part c. Cavernous part d. Cerebral part B. Vertebral Arteries 1. Branches of the vertebral arteries 2. Basilar artery a. Branches of the basilar artery i. Rt. & L. Anterior inferior cerebellar arteries ii. Pontine arteries iii. Rt. & L. Superior cerebellar arteries iv. Rt. & L. Posterior cerebral arteries v. Rt. & L. Posterior communicating arteries C. Circle of Willis and its major arterial branches 1. Rt. & L. Posterior cerebral arteries 2. Rt. & L. Posterior communicating arteries 3. Rt. & L. Anterior cerebral arteries 4. Anterior communicating artery. VI. Development of the 5 regions of the brain and the ventricular system A. Derivatives of the developing brain 1. Prosencephalon (Forebrain): a. Telencephalon. b. Diencephalon 2. Mesencephalon (Midbrain) 3. Rhombencephalon a. Metencephalon (Pons and Cerebellum) b. Myelencephalon (Medulla) B. Development of cerebral cortex and the differentiation of the rt & left lateral ventricles 1. C-shaped growth of the telencephalon shapes the lateral ventricles. C. Anatomy of the ventricles and relationships to brain structures 1. General information 2. Lateral Ventricles a. Frontal horn b. Body c. Trigone d. Occipital (posterior) horn e. Temporal (inferior) horn 3. Interventricular foramina (of Monro) 4. Third Ventricle 5. Cerebral Aqueduct 6. Fourth Ventricle 7. Central canal of the spinal cord VII. Understand the basic mechanisms of cerebrospinal fluid (CSF) production, flow and absorption. 3 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. A. General information regarding CSF B. CSF production 1. CSF generated by the Choroid plexus 2. Anatomy of the Choroid plexus 3. Locations of the choroid plexus within the ventricle system 4. Tela choroidea suspension of the choroid plexus 5. Rate of CSF production 6. Turnover rate of CSF C. CSF Flow pathway 1. Pathway through the ventricle system 2. Pathway of CSF from the ventricle system into the subarachnoid space D. CSF absorption 1. Arachnoid granulations 2. Role of the arachnoid granulations in transporting CSF into the superior sagittal sinus E. CSF function 1. Buoyancy 2. Cushions the brain and spinal cord 3. Homeostasis a. Clears metabolic wastes b. Delivers vitamins, nutrients & hormones c. Maintains ionic balances 4. CSF composition F. Diagnostic tool for evaluating CSF composition 1. Subarachnoid hemorrhage 2. Meningitis 3. Multiple sclerosis 4. TB and Polio 5. Intracranial pressure (meningitis) VIII. Clinical Significance A. Epidural hematoma resting from rupture of the middle meningeal artery B. Other types of intracranial hematomas 1. Subdural hematoma 2. Subarachnoid hemorrhage C. Meningioma D. Brain Herniation E. Hydrocephalus 1. Causes 2. Classifications a. Obstructive hydrocephalus b. Aqueductal stenosis c. Communicating hydrocephalus d. Hydrocephalus ex Vacuo e. Idiopathic Intracranial hypertension f. Normal pressure hydrocephalus 3. Characteristic symptoms of hydrocephalus a. Enlargement of the ventricles or sub arachnoid space b. Headaches c. Brain damage 4 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. d. Skull expansion in children 4. Cerebral perfusion pressure management a. Ventriculostomy F. Spread of infection and cancer into the cranial cavity G. Carotid Artery Stenosis 5 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. I. The meninges of the cranial cavity: The meninges in the cranial cavity consists of three distinct layers, the dura mater (outermost layer), the arachnoid mater (intermediate layer) and the pia mater (innermost layer). They contain the brain and are all continuous with the corresponding meningeal layers incasing the spinal cord (See MSK Unit notes: Vertebral column, meninges and spinal cord, Anatomy notes). A. The dura mater of the cranial cavity: 1. Layers of the dura mater: The dura mater consists of two dense fibrous layers, the periosteal layer (outermost layer), which is intimately associated with bony-surfaces of the cranium and a meningeal layer (innermost layer), which is internally positioned to the periosteal layer. a. Periosteal layer of the dura mater: The periosteal layer of the dura mater is unique to the cranium (and is absent from the dura matter that contains the spinal cord). In the cranial cavity, the periosteal dura lines all of the bony surfaces within the cranial cavity (Figures 1 & 3). Figure 1: N101a b. The meningeal layer of the dura mater (Figures 1-4), forms several specialized dural partitions within the cranial cavity. These include the falx cerebri (which divides the right and left cerebral hemispheres), tentorium cerebelli (which partitions the cerebral cortex (superiorly) from the cerebellar cortex (inferiorly)), the falx cerebelli (which divides the right and left cerebellar hemispheres), and the diaphragma sellae (which covers the hypophyseal fossa, housing the pituitary gland (inferiorly), from the hypothalamus of the brain (superiorly): See Figure 2 on the next page. 6 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. Figure 2: Gray’s 8.32 c. Formation of dural sinuses: Separation of the two dural layers can be noted in the dural sinuses. Here, the periosteal dura is again associated with the bony surface whereas; the meningeal dura forms the portion of the dural sinus that is suspended off of the bony surface. Examples include the superior sagittal sinus (shown below in coronal plane, Figure 3), transverse sinus (paired), sigmoid sinus (paired) and cavernous sinus (paired). Details of all are described in section I.B. Figure 3: N103a 7 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. 2. Dural partitions within the cranium a. The falx cerebri is a crescent shaped, double layered sheet of meningeal dura matter that courses in the mid-sagittal plane from its attachments to the crista galli of the ethmoid bone and the frontal crest of the occipital bone, to the frontal and occipital plates, and terminates at its posterior attachment to the tentorium cerebelli. The falx cerebri, subdivides the right and left hemispheres of the cerebral cortex and is associated with three dural sinuses, the superior sagittal sinus (its positioned along the superior edge of the falx cerebri) the inferior sagittal sinus (positioned within the falx cerebri along its free margin) and the straight sinus, which is positioned at its base attachment to the tentorium cerebelli (See Figure 2 on previous page and Figure 5 on the next page). b. The tentorium cerebelli (See Figure 2 on previous page) is a broad horizontal partition that attaches to the right and left petrous ridge of the temporal bone and extends its attachment, in a horizontal plane, along the occipital bone, until it meets its partner at midline. The tentorium cerebelli contains the superior petrosal sinuses (paired, and positioned along the right and left petrous ridges) the transverse sinus, the confluence of sinuses and the straight sinus (Details of the sinuses are in section III of these notes). Note that the cerebral cortex is positioned superiorly, and the cerebellum is located inferior to the tentorium cerebelli. Also note that that the tentorium cerebelli provides a midline opening (called the tentorial notch) along its free edge. Note that, the tentorium cerebelli accommodates the passage of the brainstem into the posterior cranial compartment (See Figure 2). Figure 4: N104 8 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. c. The falx cerebelli partitions the right and left cerebellar hemispheres. It attaches to the internal occipital protuberance and extends in a midsagittal plane from the tentorium cerebelli and runs inferiorly to the foramen magnum. At its base, it encloses the occipital sinus, which empties into the confluence of the sinuses at its juncture with the tentorium cerebelli (Figure 4). d. The diaphragma sallae covers the hypophyseal fossa (which houses the pituitary gland). It contains a midline opening to allow the infundibular stalk of the pituitary to pass from the hypothalamus to the pituitary gland (See Figure 2 on page 2 of my notes). 3. Blood supply and innervation of the dura mater a. Arterial blood supply to dura matter is provided by multiple sources, with the major contribution coming from middle meningeal artery (see Figure 5 below). Note that all of meningeal branches, including the middle meningeal artery, course between the two dural layers. Figure 5: N102a b. Innervation of the dura mater i. The sensory innervation to the dura located within the anterior and middle cranial fossa including the falx cerebri, the diaphragm sellae, the superior surface of tentorium cerebelli, and all of the dura lining the bony surfaces of this region of the cranial cavity provided by all three divisions of the trigeminal nerve [CN V] and from cervical spinal nerves C2-C3. See Figure 6 on the next page. The divisions of CN V and region of coverage are listed below. 9 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. a. Ophthalmic division of cranial nerve V (V1) provides innervation to the falx cerebri, the tentorium cerebelli, diaphragm sellae and the dura lining of the anterior cranial fossa. b. Maxillary division of cranial nerve V (V2) provides innervation to the medial portion of the middle cranial fossa. c. Mandibular division of V (V3) provides innervation to the lateral half of the middle cranial fossa. d. Innervation of the dura located in the posterior cranial fossa is provided by dorsal roots coming from cervical nerves C2 and C3 (Figure 6). Figure 6: Gray’s 8.34 II. Arachnoid mater (middle layer) and pia mater (innermost layer) A. Arachnoid mater is a thin avascular membrane that lines the dural within the cranial cavity. It is continuous with the arachnoid mater in the spinal canal. Its function is to retain (acting as a barrier) cerebral spinal fluid in a space, called the subarachnoid space, between the arachnoid mater and the pia mater covering the brain. The arachnoid mater gives rise to arachnoid granulations that project into the superior sagittal sinus. These facilitate the exit of cerebral spinal fluid (CSF) from the subarachnoid space into the superior sagittal sinus (See Section VI. D., and Figure 21 of these note for details). Look for these in your cadaver after removing the calvaria and opening up the superior sagittal sinus. Often, the arachnoid granulations will produce small cavities within the bony calvaria that are called granular foveolae. 10 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. B. Pia mater is the innermost meningeal layer that physical invests the surface of the brain (Cerebral cortex, cerebellum and the brainstem). The pia mater of the brain and brain- stem is continuous with the pia mater covering the spinal cord. Note, the pia matter cannot be separated from the surface of the brain, or the spinal cord (See Figure 1). III. Meningeal spaces A. Epidural space: In the cranial cavity is defined as a potential space between the periosteal layer of the dura and bony skull. Recall that in the spinal cord, the dura lacks a periosteal layer (Figure 7), and is replaced by lose connective tissue in the epidural space. This is exploited by anesthesiologist to provide a spinal block (e.g. epidural anesthesia). B. Subdural space: Is defined as a potential space between the meningeal layer of the dura matter and arachnoid matter (Figure 7). C. Subarachnoid space: Is defined as the space between the arachnoid matter and the pia matter, which surrounds the brain/brainstem and spinal cord. This space is maintained by arachnoid trabeculae, which traverse the subarachnoid space from the membranous portion of the arachnoid (portion in contact with the dura matter) to the pia covering of the brain/spinal cord. The subarachnoid space contains cerebral spinal fluid (~ 140 ml total) and is usually 2-3 mm in depth (pia to arachnoid membrane) (Figure 7). Epidural space Subarachnoid space Figure 7 11 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. D. Major Cisterns of Subarachnoid Space: A. Cisterns (containing cerebral spinal fluid) of the subarachnoid space in the cranium are radiological landmarks that are associated with defined regions of the brainstem and spinal cord. Examples include: 1. Cerebellomedullary Cistern (also known as the cisterna magna, (See #5 in Figure 8) 2. Quadrigeminal Cistern: (positioned above the superior & inferior colliculi), (See #7 in Figure 8). 3. Prepontine Cistern: (See #6 in Figure 8) 4. Interpeduncular Cistern: (See #9 in Figure 8) 5. Cisterna Magna: (See #5 in Figure 8) Th MB Figure 8: Midsagittal T1 weighted MRI: Cisterns are underlined below. Key to Figure 8: cc: callosum, M: Medalla, MB: Midbrain, Th: Thalamus, S: Spinal cord, #1-Lat. ventricle, #2- 3rd ventricle, #3- Cerebral aqueduct, #4- 4th ventricle, #5- Cisterna magna, # 6-Prepontine cistern, #7- Quadrigeminal cistern, #8- Subarachnoid space, #9- Interpeduncular cistern. 12 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. E. The dural sinuses 1. General information regarding the dural sinuses a. The majority of the dural sinuses are formed from the periosteal and meningeal layers of the dura matter, except for the inferior sagittal and the straight sinuses, which are formed by the meningeal dura layer only. Key point to remember is, if the sinus is associated with the bony cranial cavity, then they are derived from both periosteal and meningeal dural layers. b. All of the dural sinuses are lined by the same endothelium that lines the vasculature. 2. Naming and descriptions of the individual dural Sinuses a. Superior sagittal sinus is located within the superior border of the falx cerebri. Cerebral, diploic, emissary veins, arachnoid granulations (which drain CSF: more on this later in these notes) empty into it. The superior sagittal sinus drains directly into the confluence of the sinuses in the occipital region (Figure 9). b. Inferior sagittal sinus is located in the free-margin of the falx cerebri and receives cerebral veins from neighboring areas of the brain. The inferior sagittal sinus drains into the straight sinus (Figure 9). c. Straight sinus is a horizontally orientated sinus that is positioned in a midsagittal plane between the base of the falx cerebri and the tentorium cerebelli. It receives venous return from the inferior sagittal sinus and the great cerebral vein and empties into the confluence of the sinuses (See Figure 9 and Figure 10). Figure 9: N104 13 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. Figure 10: N105a d. Occipital sinus is located within the base attachment of the falx cerebelli and receives venous input from the vertebral venous plexus (servicing distal regions of the brainstem and cerebellum). The occipital sinus drains directly into the confluence of the sinuses (See Figure 9 on previous page). e. Confluences of the sinuses is located at midline within the tentorium cerebelli at its attachment to the occipital bone (just inferior to the internal occipital crest). It receives input for the superior sagittal sinus, the straight sinus and the occipital sinus and drains into the right and left transverse sinuses (See Figures 9 & 10). f. Transverse sinuses (paired) are contained within the attachment of the tentorium cerebelli (in the grove of the transverse sinus of the skull) to the occipital/parietal/temporal bony plates. It routes venous return from the confluences of the sinuses into the right or left sigmoid sinuses (Figures 9 & 10). g. Sigmoid Sinuses (paired) resides in an “S-shaped” groove within the temporal bone, which leads to the jugular foramen. They receive venous return from the transverse sinuses and the superior petrosal sinuses (described below) and exit the cranium, via the jugular foramen, and empty into the jugular bulb (Figure 9 & 10). 14 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. h. Cavernous Sinuses (Paired) are located along the lateral wall of the body of the sphenoid bone. It provides dura-lined sheaths that allow the passage of the ophthalmic division of the trigeminal nerve (V1), the oculomotor nerve (III), the trochlear nerve (IV) and the abducent nerve (VI) (in route to the superior orbital fissure) and provides the passage of the cavernous part of the internal carotid artery. The cavernous sinus receives venous input from the ophthalmic veins, emissary veins, the pterygoid plexus and the basilar sinus; and empties into proximal end of the sigmoid sinus (See Figure 9 on previous page, and Figure 11). Figure 11: N105b i. Basilar sinus (also referred to as the Basilar venous plexus) is located on the clivus of the occipital bone and communicates with the inferior petrosal sinus, cavernous sinus and the intercavenous sinuses (See Figure 10). IV. Venous drainage into the dural sinuses of the cranial cavity A. All venous return from the brain (cerebral cortex, cerebellum, thalamus and brainstem), empties into the dural sinus system through cerebral veins (You will not be held responsible for identifying their regional naming). It should be noted here, that cerebral veins, emissary and diploic veins (described below), as well as all of the dural sinuses, lack valves. Thus, venous flow can be bidirectional. This can have serious implications with respect to the spread of infection or cancer throughout the brain, the cranial cavity. 1. Diploic veins reside within the spongy bone of the cranial plates and empty directly into the dural sinuses. Note, diploic veins have direct communication with veins from the scalp. This can serve as a route for the spread of infection from the scalp, into the spongy bone of the skull, and into the dual sinus (Figure 12). 15 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. 2. Emissary veins (See Figure 12a) provide direct venous commination between the scalp and the dural sinuses. Examples are the parietal emissary veins, which pass through the parietal foramen (Figure 12b). 3. The great cerebral vein (of Galen) is the largest of all the cerebral veins and is identified as a midsagittal vein which empties directly into the proximal part of the straight sinus (Figures 9 & 10). 4. Cerebral veins Figure 12a: N101a Figure 12b: N9a V. Arterial supply to the brain For this unit you will be held responsible for understanding the course of the paired vertebral arteries from their origin into the cranial cavity and their formation of the basilar artery, as well as to understand the course of the paired internal carotids and their subdivisions from their origin, their path through the carotid canal, the cavernous sinus and into cranial cavity (Figure 11). Lastly, you need to know that the internal carotid and vertebral arteries do communicate with each other in a ring of arteries that make up the circle of Willis. A. The Internal carotid arteries branch off of the common carotid artery at the level of fourth cervical vertebrae and ascend to the base of the cranium, posterior to the external carotid artery. No major branches are given off in route to the cranium. 1. The initial segment of the internal carotid artery is dilated and is called the carotid sinus (Figure 13) and contains baroreceptors that monitor blood pressure. These receptors relay blood pressure information back to the brainstem through branches of the glossopharyngeal and vagal nerves. 16 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. Figure 13: N137 2. Located in a region between the bifurcation of the internal and external carotid artery is a small mass of nervous tissue call the carotid body (Shown in figure 13, but not labeled). This relays chemosensory information from receptors located in the lumen of the artery to monitor O2 and CO2 levels in the blood. This information is also relayed back to the brainstem via CNs IX & X. 3. Anatomically, the internal carotid artery is subdivided into 4 regions (See Figure 14). a. Cervical part: The portion of the internal carotid artery to is located in the neck. b. Petrous part: The portion of the internal carotid artery that runs within the carotid canal. c. Cavernous part: The portion of the internal carotid artery that runs through the cavernous sinus. d. Cerebral part: The portion of the internal carotid artery that resides between the cavernous sinus and at its termination with the formation of the circle of Willis and the middle cerebral artery. 4. The rt & left internal carotid arteries terminate with each giving rise to Rt & L middle cerebral A’s and an anterior cerebral artery and a respective (Rt or L) ophthalmic A. (See Figure 15). 17 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. Figure 14: N138 B. Vertebral arteries: The right and left vertebral arteries ascend in the neck within the transverse foramen of cervical vertebrae C6-C1 and pierce the posterior dura matter between CV1 and foramen magnum of the skull and the arachnoid matter, entering into the subarachnoid space. 1. Once in the subarachnoid space, each vertebral artery will give off a posterior inferior cerebellar artery (PICA), and arterial branches that will give rise to the anterior spinal artery and the rt and left posterior spinal arteries in the cervical region (Figure 15). 2. At the junction between ventral medulla and the pons of the brainstem, the two vertebral arteries merge to form the basilar artery (Figures 15 & 16). a. Branches of the basilar artery: (Figures 15 &16) i. Rt & L Anterior Inferior cerebellar arteries ii. Pontine arteries (not labeled in Figure 15) iii. Rt & L. Superior cerebellar arteries iv. Rt. & L. Posterior cerebral arteries v. Rt & L. Posterior communicating arteries 18  Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. Figure 15 C. Circle of Willis and its major arterial branches: The circle of Willis is an anastomotic arterial loop that links the Rt & L. Internal carotid arteries with blood supply from the Rt & L Vertebral arterys (via the Basilar artery). The circle of Willis encircles the optic chiasm and infundibulum of the pituitary gland (Figure 15), and is formed by the following arteries (See Figures 15 & 16). 1. Rt & L. Posterior cerebral arteries (Supplied by the vertebral arteries via the Basilar A.). 2. Rt and L. Posterior communicating arteries (connects blood supply between the basilar/vertebral arteries with Rt & L. the internal carotid arteries). 3. Rt. & L. Anterior Cerebral arteries (Supplied by the Rt & L. internal carotid arteries) 4. Anterior communicating artery (Forms an anastomotic connection between the Rt. and L. anterior cerebral arteries). 19 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. Figure 16 Pontine Arteries VI. Development of the 5 regions of the brain and the ventricular system A. Early in development, the brain and ventricular system forms a 3- vesicle tube by the 3rd week that consists of the Telencephalon, Mesencephalon and Rhombencephalon (See Figure 17A). This tube rapidly differentiates into a 5-Vesicle Stage (now consisting of the Telencephalon, Diencephalon, Mesencephalon, Metencephalon and Myelencephalon) by the 6th week (See Figure 17 C & D). Along with this development, the ventricular system contained within each division, follows suit, and transitions from a 3 to a 5 vesical system (See Figure 17). Subdivisions of the prosencephalon, mesencephalon and rhombencephalon are listed below. 1. Prosencephalon (Forebrain) gives rise to: a. Telencephalon gives rise to the cerebral cortex, basal ganglia, hippocampus, amygdala) and surrounds lateral ventricles. b. Diencephalon gives rise to the thalamus, hypothalamus, neurohypophysis, & pineal gland, and surrounds 3rd ventricle. 2. Mesencephalon (Midbrain) surrounds cerebral aqueduct (Midbrain and Tectum) 3. Rhombencephalon (Hindbrain) is associated with the 4th ventricle: a. Metencephalon (Pons and Cerebellum) b. Myelencephalon (Medulla) 20 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. B. Development of cerebral cortex and the differentiation of the rt & left lateral ventricles 1. C-shaped growth of the telencephalon shapes the lateral ventricles. Between weeks 4 & 6 the cerebral cortex (Telencephelon) undergoes rapid growth, with each hemisphere expanding anteriorly, posteriorly, laterally and inferiorly (see Figure 17B, D & E). With this growth, the lateral ventricle follows, and creates a “C-shaped” right and left lateral ventricle system (see Figure 18). A B D C E Figure 17: Early Brain Development. Top: 3 Vesicle Stage- 4-week embryo (A &B). Middle: 5 Vesicle Stage-6- week embryo (C &D). Bottom: Lateral view -8-week embryo (E). Arrows- growth directions of cerebral cortex which is anterior, posterior and C-shaped (Carpenter). The term brain vesicle includes the developing neural tissue of the neural tube and the neural canal. 21 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. C. Anatomy of the Ventricles & Relationships to Brain Structures 1. Ventricular system: Is filled with 15-40 ml of cerebral spinal fluid (CSF) and is lined by ependymal cells. 2. Subdivision of the Lateral Ventricles: (See Figure 18) a. Frontal (anterior) horn: Location: frontal cortical lobe, Boundaries: roof: corpus callosum, medial boundary: septum pellucidum. b. Body: (Central part) associated mainly with parietal lobe, roof: corpus callosum c. Trigone: (atrium)-juncture of occipital & temporal horns d. Occipital (posterior) horn: Located within occipital cortical lobe. e. Temporal (inferior) horn: temporal cortical lobe 3. Interventricular foramina (of Monro Rt & L) defines the connection between the lateral ventricle and the 3rd ventricle. The choroid plexus (a landmark for identifying the opening of foramen Monro) passes through from the lateral ventricle into the 3 rd ventricle through the rt & left foramen of Monro. 4. Third Ventricle: a single slit-like midline structure, positioned between the right and left thalamus (derived from the diencephalon). 5. Cerebral Aqueduct (of Sylvius): a single midline narrow channel passing through the midbrain that connects the 3rd ventricle to the 4th ventricle. Figure 18: Gross anatomy of the cerebral ventricular system (superimposed from left to right within the brain). Image from Netter 109A. Central canal (in spinal cord) 6. Fourth Ventricle: a single tent-shaped space bounded by a) the roof (derived from the cerebellum, superior & inferior cerebellum peduncles, and the anterior and posterior medullary velum) and b) the floor (formed from the pons & medulla). See Figure 18. 22 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. 6. Central Canal: Located within the spinal cord and also lined by ependymal cells. It extends the full length of the spinal cord (cervical - sacral level). VII. Understand the basic mechanics of cerebrospinal fluid (CSF) production, flow and absorption. A. The volume of CSF contained in the ventricles of the brain and in the subarachnoid spaces of the cranium and surrounding the spinal cord is approximately 140 ml. It is estimated that ~15-40 ml are contained within the ventricle system and a volume ranging from 100-125 ml contained by the subarachnoid spaces. CSF is constantly being generated, primarily by the choroid plexus, which is located in the rt & left lateral ventricles*, and in the roof of the 3rd & 4th ventricles (Figure 19). Figure 19: Flow of CSF from subarachnoid space into the superior sagittal sinus via arachnoid granulations 23 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. B. Choroid plexus and cerebral spinal fluid (CSF) production 1. The choroid plexus consists of pia mater, blood vessels and ependymal cells. 2. The choroid plexus is located in the body & temporal horn of the lateral ventricles, and exit the rt & left interventricular foramina and extend on the roof of the 3rd ventricle (See Figure 19). Choroid plexus is also located in the roof of the 4th ventricle and exits out into the subarachnoid space on through the rt and left foreman of Luschka (See Figure 19). 3. The choroid plexus is supported by folded pia layers that are defined as the tela choroidea. 4. Rate of CSF production: 20-30 ml/hr. Estimated daily production averages 500-700 ml/day. 6. Turnover rate: ~3.5 times/day. C. CSF Flow pathway (See Figures 19 & 20) 1. Flow Pattern: lateral ventricles → interventricular foramina → 3 rd ventricle → cerebral aqueduct → 4th ventricle → subarachnoid space via the median (foramen Magendie) & lateral apertures (rt & left foramen of Luschka).. Blockage of flow→↑pressure within the brain, a condition defined as hydrocephalus Figure 19: Ventricle system (Lateral to medial view) 2. CSF enters subarachnoid space through 3 openings in 4 th ventricle: These are defined as the: foramen of Magendie: (a single midline opening), and a pair of 24 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. lateral apertures (Rt & L foramina of Luschka). Note, these foramina are associated with the exit of the rt & left Glossopharyngeal nerves (CN-IX). D. CSF Absorption - Arachnoid Granulations 1. CSF produced within the subarachnoid space exits mostly through the arachnoid granulations, which are derived from the arachnoid mater, These are found projecting into the superior sagittal venous sinus (See Figure 20). Limited exchange also occurs around the spinal roots. Figure 20: CSF exchange into Superior sagittal sinus via arachnoid granulations. 2. Exit of CSF into the superior sagittal sinus through the arachnoid granulations is due to a pressure gradient (whereby, intracranial pressure > venous pressure). Note, under normal conditions, flow of CSF from the subarachnoid space into the superior sagittal sinus, is a one-way path (See figure 21). E. CSF Function 1. Buoyancy: brain: wet weight ~1400 g→ buoyancy weight in CSF ~46 g. 2. Cushions the brain and spinal cord. 3. Homeostasis: a. Clears substances from brain - metabolic wastes b. Delivers vitamins, nutrients, & hormones c. Helps maintain ionic balance 25 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. 4. Composition: Clear, Colorless; Specific Gravity =1.007 a. Higher concentrations of sodium, chloride and magnesium. b. Lower concentrations of potassium, calcium, glucose, and proteins, and albumin, than levels found in blood serum. c. CSF is mostly devoid of white blood cells (0-3 lymphocytes/ml). CSF pressure: ranges from 50- 200 mm water. F. Diagnostic tool for evaluating CSF composition 1. Bloody: subarachnoid hemorrhage 2. ↑WBC: possible meningitis (viral or bacterial infections) 3. ↑Antibodies: Multiple sclerosis 4. ↑Protein: TB or polio 5. ↑Cranial pressure: meningitis, edema, tumors or hemorrhaging. ***Note: List above provides examples of possible foreign substances a in a CSF sample. VIII. Clinical significance A. Rupturing of the middle meningeal artery: Note, the course of the middle meningeal artery passes through the pterion (the thinnest bony region of the cranium. See Figure 3 on page 3 of my notes). A sharp impact resulting in the fracturing of the pterion of the skull can rupture periosteal layer of dura containing the middle meningeal artery in this region. The hemorrhaging of the artery usually forms a hematoma between the bony skull and the periosteal layer of the dura, causing the dura mater to be dissected away from the bone. This is referred to as an epidural hematoma. Imaging an epidural hematoma usually appears as a lenticular separation (lens-shaped), that forms between the bony skull and the separated periosteal layer of the dura matter. If left unchecked, the increase in volume of the hematoma, will result in the compression/displacement of the dural sac against the brain. This is life threatening! See Figure 22a, and CT scan Figure 22b, on the next page, and Figure 23A on the next page). 26 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. Figure 22a: Gray’s 8.46 Figure 22b: Gray’s 8.47 B. Other types of intracranial hematomas 1. Subdural hematoma: Defined as blood accumulation between dura and arachnoid matter. A subdural hematoma usually results from a tear in a bridging vein (surface cerebral veins on the cerebral cortex that puncture the meningeal dura to empty in venous sinuses). Hematomas usually appear “crescent-shaped” in MR/CT imaging (if acute) and have a high mortality rate (See Figure 23B). Figure 4A: Epidural hemorrhage/hematoma ma Figure 4B: Subdural hemorrhage/hematoma Figure 23 27 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. 2. Subarachnoid hemorrhage: Blood accumulates between arachnoid matter and pia matter (mixing in with CSF). This usually results from ruptured cerebral aneurysm or torn cerebral artery/vein resulting from a TBI episode (See Figure 23). Note, bleeding within the subarachnoid space will “clog-up” the arachnoid granulations and restrict CSF flow into the superior sagittal sinus. This will result in a rise of inter- cranial pressure upon the brain, and become life-threatening. C. Meningioma: A meningioma is a slow growing benign (non-metastatic) mass that is derived from arachnoid cells at points where blood vessels and cranial nerves traverse the dura. Note, these grow external to the brain tissue. Meningioma growths are primarily found in the cranial cavity, where the arachnoid is associated with the meningeal dura. Common sites include the falx cerebri the tentorium cerebelli (See Figures 25a &25b below). A B Figure 25A: MRI showing an arachnoid meningioma (associated with the meningeal dura mater layer of the frontal bone) impinging on frontal lobe of the cerebral cortex. Figure 25B: MRI showing meningioma associated with the tentorium cerebelli and impinging on the temporal lobe of the cortex in the supratentorial compartment and a smaller growth impinging on the cerebellum in the infratentorial compartment. (Images from Haines, Fundamental Neuroscience for basic and clinical applications,4 th Ed., 2013). 28 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. D. Brain Herniation: Because the bony cranial cavity is resistant to expansion, the internal volume of the cavity is fixed. Therefore, any pathological increase of intracranial volume (due to a hematoma, meningeal growth (cancer or meningioma) or hydrocephalus) will cause compression/displacement of the brain within the cranial cavity (Figure 26). Figure 26: Type of brain herniation: #1 Uncal herniation (impingement of the cortex upon the midbrain, in the region of the tentorial notch), #3 Cingulate herniation (displacement a cortical hemisphere to the opposite side), #4 Transcalvarial (brain escaping the cranial cavity due to a trauma induce fracture), #6 Tonsillar herniation (cerebellum tonsils escaping the cranial cavity through foramen magnum and impinging upon the medulla/spinal cord). E. Hydrocephalus 1. Causes of hydrocephalus a. Physical blockage of CSF flow out of the ventricles b. Failure of CSF to be re-absorbed (e.g., blockage of the arachnoid granulations) 2. Classifications of hydrocephalus a. Obstructive Hydrocephalus: Blockage of flow in the ventricular system, resulting from cysts, tumors, clots, etc. b. Aqueductal Stenosis: Is a form of obstructive hydrocephalus caused by blockage of the cerebral aqueduct (located in the midbrain). This results in the backup of CSF into the 3 rd and lateral ventricles (causing them to enlarge (See Figure 27 on the next page). 29 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. Figure 27: MRI images in midsagittal (A) and coronal (C) planes comparing Normal (images on the left) with hydrocephalus (shown on the right). Note, the enlarged lateral ventricles as well as the enlarged third ventricle in the hydrocephalic brain. Images taken from Haines 4th Ed. c. Communicating Hydrocephalus: Normal flow though the ventricular system however, ↓ CSF absorption through arachnoid granulations becomes impaired causing ↑ CSF pressure. Usually results from a subarachnoid hemorrhage. d. Hydrocephalus ex Vacuo: Results from the brain undergoing atrophy following a stroke or TBI. This leads to ↑ventricle size and CSF volume, but usually, there is no increase in CSF pressure. e. Idiopathic Intracranial Hypertension: ↑ ICP without an increase in the size of the ventricles: Common causes: Obesity, renal failure or Vitamin A toxicity. f. Normal Pressure Hydrocephalus (NPH): Most commonly associated with the elderly, where ICP increases during night. NPH causes a triad of symptoms: apraxia (motor impairment causing difficulty in forming speech), incontinence and dementia. 30 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. 3. Characteristic symptoms of hydrocephalus a. Enlargement of ventricles or subarachnoid space b. Headache c. Decreased cognition d. Skull expansion in children 4. Cerebral perfusion pressure (CPP) management a. Ventriculostomy (inserting catheter into ventricle, Figure 28) is routinely done in patients displaying symptoms of increase intracranial pressure resulting from a severe brain injury, particularly those that have resulted in subarachnoid hemorrhaging. Excessive hemorrhaging in the subarachnoid space will impede the flow of CSF through the arachnoid granulations that will result in a sustained ↑CSF pressure. Figure 28: Ventriculostomy (inserting catheter in to the lateral ventricle) a. Used to prevent brain damage from ↑pressure b. Intracranial pressure monitoring (ICP) – measured using indwelling catheter positioned within the lateral ventricle (see Figure 28). F. Spread of infection and cancer into the cranial cavity: As mentioned earlier, infections from the scalp can gain access to the dural sinuses through emissary and diploid veins (See Figure 12a). In addition, infections from the face, nasal, orbital and oral cavities can gain access to the cranial cavity through venous tributaries leading to the cavernous sinus. The lack of venous valves, allows infections to spread from the cavernous sinus throughout the other sinuses and veins supplying the brain. Arrows in Figure 29 (on the next page) indicate the direction of infections from the face, oral and orbital cavities. 31 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. Figure 29: N87c G. Carotid Artery Stenosis: Eighty-five percent of all strokes are related to ischemia and the most common causative is due to embolic blockage (usually due to a calcified atherosclerotic plaque) within the carotid blub and/or in the initial segment (Cervical part) of the internal carotid artery. Figure 30A (on the next page) shows an angiogram/CT image shows significant narrowing (stenosis) of lumen in the proximal part (Cervical-part) of the internal carotid artery (See arrow). Case courtesy of Dr Bruno Di Muzio, Radiopaedia.org, rID: 31740. Figure 30B (on the next page) shows 3D-modeling of both right and left carotid arteries, with each showing significant stenosis of the lumen in the initial portion of the internal carotid artery (white arrows). Surgical removal of the plaque and the surrounding endothelial lining (procedure called “Carotid Endarterectomy”) are routinely preformed to improve perfusion to the brain. 32 Meninges, Ventricular System, Dural Venous Sinuses Dennis J. Goebel, Ph.D. Figure 30: Abbreviations: CC-common carotid, IC-internal carotid, EC-external carotid. (Case courtesy of Dr Bruno Di Muzio, Radiopaedia.org, rID: 28786). Material and Figure sources: Atlas of Human Anatomy, 6th Ed, Frank H. Netter, Saunders Elsevier, © 2014. Gray’s Anatomy for Students, 3rd Ed., Drake, Vogl & Mitchell, Churchill Livingstone Elsevier, © 2015. Fundamental Neuroscience for Basic and Clinical Applications, Duane Haines, 4th Ed., Saunders Elsevier, © 2013. 33