Meninges and Dural Venous Sinuses PDF

MENINGES OF THE CNS & DURAL VENOUS SINUSES  A living brain would not do well removed from its housing because it is simply too soft and delicate, unable even to maintain its own shape The protection and help it requires are supplied by the skull, but also by the meningeal la...

MENINGES OF THE CNS & DURAL VENOUS SINUSES  A living brain would not do well removed from its housing because it is simply too soft and delicate, unable even to maintain its own shape The protection and help it requires are supplied by the skull, but also by the meningeal layers and their attachments to the brain and by the cerebrospinal fluid (CSF)  But the protection supplied by the above structures have associated costs The skull and the meningeal layers not only protect and limit the motion of the brain, they also limit any adjustments the brain can make in its position Any neoplasm of neural, glial or meningeal origin, any increased blood volume resulting from hemorrhage, or an increase in the volume of CSF, increase the intracranial pressure and can create a life-threatening situation  There are three layers of meninges – Pia mater – Arachnoid mater – Dura mater  The membrane closest to the nervous system is the pia mater The next membrane is the arachnoid, while the outermost membrane is the dura mater Because of the relative thickness of the meningeal coverings, the dura is often termed the pachymeninx ("pachy" from the Greek for "thick" - think pachyderm) The arachnoid and pia are collectively called the leptomeninges ("lepto-" from the Greek for delicate) In the adult nervous system, the arterial supply to the brain is found in the pia mater, while the venous drainage takes place largely in the dura mater  DURA MATER OF THE BRAIN The dura mater, or simply 'dura', is very strong and fibrous It is actually composed of two layers – The outer periosteal dura is attached to the skull and fused through most of its extent to the inner meningeal dura – In some regions, these dural layers separate and the intervening space forms the dural venous sinuses – At each point of separation, the inner meningeal dural layer plunges deep into the brain, forming a double layer wall or septum Dura Mater of the Brain (continued) Overall, there are four dural inflections, or septa, that divide the intracranial cavity into compartments – Falx cerebri – Tentorium cerebelli – Falx cerebelli – Diaphragma sellae Dura Mater of the Brain (continued) Falx cerebri The largest septum, called the falx cerebri, separates the cerebral hemispheres This sickle-shaped (falx is Latin for "sickle-shaped"), tough dural membrane extends from the crista galli of the ethmoid bone, anteriorly, to the internal occipital protuberance posteriorly The space in which the falx cerebri sits is called the longitudinal fissure Dura Mater of the Brain (continued) Tentorium cerebelli This septum of dura extends at the roof of the cerebellum and supports the occipital lobes inferiorly It is roughly semicircular in shape and extends in the horizontal plane The posterior, inferior edge of the falx cerebri is attached to the midline of the superior tentorial surface The portions of the brain located above the tentorium are referred to as supratentorial space, while those inferior to the tentorium are in the infratentorial space, more commonly referred to as the posterior fossa Dura Mater of the Brain (continued) Falx cerebelli Extends from the midline on the undersurface of the tentorium cerebelli It partially separates the cerebellar hemispheres Dura Mater of the Brain (continued) Diaphragma cella Covers the sella turcica, the "saddle-shaped" depression in the sphenoid bone that contains the hypophysis This dural membrane contains a small hole to accommodate the infundibulum (pituitary stalk), which connects the hypothalamus and the hypophysis (pituitary gland) Diaphragma cella, holding the Dura Mater of the Brain (continued) Blood supply of the cerebral dura mater The dura receives its principal blood supply from the middle meningeal artery, a branch of the maxillary artery, which passes through the foramen spinosum to enter the skull In addition, anterior meningeal branches of the ophthalmic artery and posterior meningeal branches of the occipital and vertebral arteries suppply the rostral and caudal parts of dura, respectively Clinical note A trauma-induced bleed of the middle meningeal artery results in an epidural hematoma, an acute medical emergency As suggested by the name, an epidural hematoma is an accumulation of blood over the dura, or specifically, between the periosteal dura and the bone of the skull Dura Mater of the Brain (continued) Innervation of the cerebral dura mater The supratentorial dura is principally innervated by the trigeminal nerve while the dura in the posterior fossa is supplied by the second and third cervical spinal nerves That is to say, there are no sensory nerve endings within the brain and we get no sensations, including pain sensations, from damage to brain tissue itself Pain we recognize as coming from inside our heads reflects sensory nerve activity from nerve fiber terminals primarily innervating blood vessels and sinuses within the dura  SPINAL DURA MATER The meningeal dural layer covering the brain continues on over the spinal cord; the periosteal layer does not Between the spinal meningeal dura and the vertebral periosteum is an actual (not a potential) epidural space It contains connective tissue, fat and internal vertebral venous plexus It is into this space that anesthesia can be introduced to achieve a "spinal block” (epidural anasthesia in child birth) The spinal dura mater travels along each spinal nerve root until becoming continous with the epineurium  ARACHNOID MATER The arachnoid is composed of white fibrous and elastic tissue that forms a continuous membrane resembling a spider's web (hence the name "arachnoid") Despite this delicate appearance, the arachnoid is actually a strong and reasonably impermeable barrier that protects the brain from contents of the dural extracellular space Arachnoid mater (continued) Although essentially avascular, it does receive sensory nerve supply from the trigeminal nerve The arachnoid is closely apposed to the dural membrane and only separated from the dura by a potential space called the subdural space With a trauma to the head that results in a venous bleed, the subdural space becomes expanded and filled with venous blood (subdural hematoma) Fibrous trabeculae extend from the inner surface of the arachnoid membrane to anchor it to the pia Between the arachnoid and pia is an actual defined space called the subarachnoid space, which contains cerebrospinal fluid CSF  PIA MATER The pia mater is a delicate cellular-fibrillar membrane that, unlike the dura and arachnoid, follows the contours (i.e., gyri and sulci) of the brain and is tightly adherent to CNS tissues The spinal pia mater closely invests the cord surface Between the spinal nerve roots, the pia is thickened on either side, forming triangular-shaped lateral extensions known as denticulate ligaments – These ligaments act to suspend the spinal cord in the middle of the dural sheath and act as shock absorbers – The denticulate ligaments fuse with the arachnoid mater and the inner surface of the dura mater Pia mater (continued) In the spinal cord, the pia ends as the conus medullaris and continues as the filum terminale Because the spinal cord only descends as far as the first or second lumbar vertebra and the arachnoid extend to the second sacral vertebra, there is a large lumbar cistern present which is very useful clinically It may be used to take CSF samples during a lumbar puncture or it may be used to inject a spinal anaesthetic In lumbar puncture, a needle is inserted into the lumbar cistern either above or below the fourth lumbar spine, and a sample of CSF is taken Pia mater (continued) The pia is highly vascularized and contains blood vessels whose branches supply CNS tissues Layers of the pia and arachnoid form a cuff around blood vessels as they penetrate the parenchyma of the brain The pia is innervated by trigeminal nerves (sensory), sympathetic nerves (vasoconstriction), and parasympathetic nerves (vasodilation) Vasodilation of intracranial and extracranial arteries stimulates sensory fibers on the blood vessels that frequently results in pain or headache  DURAL VENOUS SINUSES The dural venous sinuses are spaces which lie between the meningeal and periosteal layers of the dura mater Venous blood that originates from the brain and the cranial cavity drain into these sinuses These are valveless sinuses which are lined by endothelium DURAL VENOUS SINUSES (continued) Within the dura mater, there are several venous sinuses – Superior Sagittal Sinus – Inferior Sagittal Sinus – Straight Sinus – Cavernous Sinus – Superior and Inferior Petrosal Sinuses – Transverse Sinuses – Sigmoid Sinuses A: Sphenoparietal B: Intercavernous C: Sigmoid D: Occipital E: Confluence F: Basilar G: Transverse H: Superior petrosal I: Inferior petrosal J: Cavernous K: Superior sagittal A: Superior sagittal sinus B: Great cerebral vein C: Ophthalmic veins D: Facial vein E: Cavernous sinus F: Inferior petrosal sinus G: Internal jugular vein H: Sigmoid sinus I: Superior petrosal sinus J: Transverse sinus K: Straight sinus L: Inferior sagittal sinus DURAL VENOUS SINUSES (continued) Also, to be considered, is the meeting point of the major venous sinuses, which is known as the confluence of sinuses – Located adjacent to the internal occipital protuberance DURAL VENOUS SINUSES (continued) Superior sagittal sinus Situated along the attached border of the falx cerebri and posteriorly becomes continuous with the right transverse sinus The superior sagittal sinus mainly receives blood from the superior cerebral veins These veins drain the lateral surface of the cerebral hemispheres These veins drain the lateral surface of the cerebral hemispheres Arachnoid villi extending from arachnoid mater converge to form large clusters known as arachnoid granulations – These project into the venous lacunae of the superior sagittal sinus – These are concerned with absorption of cerebrospinal fluid DURAL VENOUS SINUSES (continued) Inferior sagittal sinus Located in the lower border of the falx cerebri This sinus receives veins from the medial aspects of the hemispheres, as well as the falx cerebri Great cerebral vein Drains the deep structures of the forebrain Joins the inferior sagittal sinus and together drain into the straight sinus at the junction of the falx cerebri and tentorium cerebelli DURAL VENOUS SINUSES (continued) Straight sinus Originates at the union of the great cerebral vein with the inferior sagittal sinus It extends posteriorly and drains into one of the transverse sinuses, usually the left DURAL VENOUS SINUSES (continued) Cavernous sinus Lies lateral to the body of the sphenoid bone on either side of the cranium These are so named because of their internal trabeculation (has caverns inside) They are traversed on either side by the internal carotid artery, sympathetic plexus around it, and the abducent nerve In the lateral wall of each sinus, between meningeal dura mater and endothelium, are the oculomotor, trochlear, ophthalmic and maxillary nerves DURAL VENOUS SINUSES (continued) Cavernous sinus (continued) The cavernous sinuses of each side are interconnected by the intercavernous sinuses, which are located in the diaphragma sellae Cavernous sinus drains ophthalmic veins and superficial middle cerebral veins, and the sphenoparietal sinuses Cavernous sinuses drain through three routes – Superior petrosal sinus, which in turn drain into the transverse sinus – Inferior petrosal sinus, which directly drains into the internal jugular vein – Cavernous sinus inferiorly drain into the pterygoid venous plexus located in the infratemporal fossa DURAL VENOUS SINUSES (continued) Confluence of sinuses Located where the superior sagittal sinus unites with the straight sinus From the confluence of the sinuses, the blood flows first to the right and left transverse sinuses, and then to the sigmoid sinus The sigmoid sinuses bend into an S shaped curve and continue into the internal jugular vein through the jugular foramen DURAL VENOUS SINUSES (continued) The dural venous sinuses are connected to extracranial veins via valveless veins called the emissary veins Some of these connect the cavernous sinuses with pterygoid and pharyngeal venous plexuses through the foramina ovale, spinosum, and lacerum of the skull Clinical note Thrombosis of dural sinuses Because of the communication between emissary veins with extracranial vessels, cutaneous infections may cause infective thrombosis of the intracranial sinuses It most frequently involves the cavernous sinuses and often follows facial infections, bacterial meningitis, otitis media or sphenoid sinus infections Signs of this disease include: proptosis (forward displacement of an organ, usually the eye), chemosis (oedema of the conjunctiva), and oculomotor nerve paralysis Isolated sixth-nerve palsy may also be present, as well as hypo- or hyperesthesia of the fifth nerve may be found Obstruction of the venous drainage of the brain leads to cerebral swelling (oedema) and the syndrome of raised intercranial pressure