Neuroanatomy Lesson 10 PDF

NEUROANATOMY Lesson 10: Vasculature of the Nervous System: Arteries, venous sinuses and Meninges Overview of the vasculature of the nervous system The central nervous system, requires constant oxygenation and nourishment. The brain has a particularly high oxygen demand, at rest it...

NEUROANATOMY Lesson 10: Vasculature of the Nervous System: Arteries, venous sinuses and Meninges Overview of the vasculature of the nervous system The central nervous system, requires constant oxygenation and nourishment. The brain has a particularly high oxygen demand, at rest it represents one fifth of the body’s total oxygen consumption. It is also very sensitive to oxygen deprivation, with ischemic cell death resulting within minutes.  Arterial Supply to the Brain There are two paired arteries which are responsible for the blood supply to the brain; the vertebral arteries, and the internal carotid arteries. These arteries arise in the neck, and ascend to the cranium. Within the cranial vault, the terminal branches of these arteries form an anastomotic circle, called the Circle of Willis. From this circle, branches arise which supply the majority of the cerebrum. Other parts of the CNS, such as the pons and spinal cord, are supplied by smaller branches from the vertebral arteries. Arterial Supply to the Brain: Internal Carotid Arteries The internal carotid arteries (ICA) originate at the bifurcation of the left and right common carotid arteries, at the level of the fourth cervical vertebrae (C4). They move superiorly within the carotid sheath, and enter the brain via the carotid canal of the temporal bone. They do not supply any branches to the face or neck. Arterial Supply to the Brain: Internal Carotid Arteries In the cranial cavity, the internal carotids pass anteriorly through the cavernous sinus. Distal to the cavernous sinus, each ICA gives rise to: Ophthalmic artery: supplies the structures of the orbit. Posterior communicating artery: acts as an anastomotic ‘connecting vessel’ in the Circle of Willis. Anterior choroidal artery: supplies structures in the brain important for motor control and vision. Anterior cerebral artery: supplies part of the cerebrum. The internal carotids then continue as the middle cerebral artery, which supplies the lateral portions of the cerebrum. Arterial Supply to the Brain: Vertebral Arteries The right and left vertebral arteries arise from the subclavian arteries, medial to the anterior scalene muscle. They then ascend the posterior aspect of the neck, through holes in the transverse processes of the cervical vertebrae, known as foramen transversarium. After this, the two vertebral arteries converge to form the basilar artery. Several branches from the basilar artery originate here, and go onto supply the cerebellum and pons. The basilar artery terminates by bifurcating into the posterior cerebral arteries. Arterial Supply to the Brain: Vertebral Arteries The vertebral arteries enter the cranial cavity via the foramen magnum. Within the cranial vault, some branches are given off: Meningeal branch: supplies the falx cerebelli, a sheet of dura mater. Anterior and posterior spinal arteries: supplies the spinal cord, spanning its entire length. Posterior inferior cerebellar artery: supplies the cerebellum.  Arterial Supply to the Brain: Arterial Circle of Willis The terminal branches of the vertebral and internal carotid arteries all anastomose to form a circular blood vessel, called the circle of Willis. There are three main (paired) constituents of the circle of Willis: Anterior cerebral arteries: terminal branches of the internal carotid arteries. Internal carotid arteries: located immediately proximal to the origin of the middle cerebral arteries. Posterior cerebral arteries: terminal branches of the basilar artery Arterial Supply to the Brain: Arterial Circle of Willis To complete the circle, two ‘connecting vessels’ are also present: Anterior communicating artery: connects the two anterior cerebral arteries. Posterior communicating artery: branch of the internal carotid, this artery connects the ICA to the posterior cerebral artery. Regional Blood: Supply to the Cerebrum There are three cerebral arteries; anterior, middle and posterior. They each supply a different portion of the cerebrum. The anterior cerebral arteries supply the anteromedial portion of the cerebrum. The middle cerebral arteries are situated laterally, supplying the majority of the lateral part of the brain. The posterior cerebral arteries supply both the medial and lateral parts of the posterior cerebrum. Arterial Supply to the Spinal Cord The spinal cord is primarily supplied by three longitudinal arteries, as it descends from the brainstem to the conus medullaris. These are: Anterior spinal artery: formed from branches of the vertebral arteries, travelling in the anterior median fissure. Gives rise to the sulcal arteries, which enter the spinal cord. Two posterior spinal arteries: originate from the vertebral artery or the posteroinferior cerebellar artery, anastomosing with one another in the pia mater. Arterial Supply to the Spinal Cord Below the cervical level, there is support via anastomosis with the anterior and posterior segmental medullary arteries and radicular arteries. The great anterior segmental artery of Adamkiewicz reinforces circulation to the inferior 2/3 of the spinal cord, and is found on the left in the majority of individuals. The radicular arteries supply (and follow the path of) the anterior and posterior nerve roots. Some radicular arteries may also contribute to supplying the spinal cord. Clinical Relevance: Stroke The brain is particularly sensitive to oxygen starvation. A stroke is an acute development of a neurological deficit, due to a disturbance in the blood supply of the brain. There are four main causes of a cerebrovascular accident: Thrombosis: obstruction of a blood vessel by a locally forming clot. Embolism: obstruction of a blood vessel by an embolus formed elsewhere. Hypoperfusion: lack of blood supply to the brain, due to systemically low blood pressure (e.g shock). Haemorrhage: an accumulation of blood within the cranial cavity. Out of these four, the most common cause is embolism. In many patients, the atherosclerotic embolus arises from the vessels of the neck. Clinical Relevance: Intracerebral Aneurysms An aneurysm is a dilation of an artery, which is greater than 50% of the normal diameter. They are most likely to occur in the vessels contributing to the Circle of Willis. They are particularly dangerous producing few symptoms until they rupture. Upon rupture, blood typically accumulates in the subarachnoid space, with a subsequent increase in intracranial pressure. Once the artery wall has ruptured, it is a medical emergency, and the patient is likely to die unless treated swiftly. Treatment of an intracerebral aneurysm is surgical. Dural Venous Sinuses The dural venous sinuses lie between the periosteal and meningeal layers of the dura mater. They are best thought of as collecting pools of blood, which drain the central nervous system, the face, and the scalp. All the dural venous sinuses ultimately drain into the internal jugular vein. Unlike most veins of the body, the dural venous sinuses do not have valves. There are eleven venous sinuses in total. The straight, superior, and inferior sagittal sinuses are found in the falx cerebri of the dura mater. They converge at the confluence of sinuses (overlying the internal occipital protuberance). The straight sinus is a continuation of the great cerebral vein and the inferior sagittal sinus. Dural Venous Sinuses From the confluence, the transverse sinus continues bilaterally and curves into the sigmoid sinus to meet the opening of the internal jugular vein. The cavernous sinus drains the ophthalmic veins and can be found on either side of the sella turcica. From here, the blood returns to the internal jugular vein via the superior or inferior petrosal sinuses. Superficial System of Veins for draining the cortex: The superficial system of veins is largely responsible for draining the cerebral cortex: Superior cerebral veins: Drain the superior surface, carrying blood to the superior sagittal sinus. Superficial middle cerebral vein: Drains the lateral surface of each hemisphere, carrying blood to the cavernous or sphenopalatine sinuses. Inferior cerebral veins: Drain the inferior aspect of each cerebral hemisphere, depositing blood into cavernous and transverse sinuses. Superior anastamotic vein (Trolard): Connects the superficial middle cerebral vein to the superior sagittal sinus. Inferior anastamotic vein (Labbé): Connects the superficial middle cerebral vein to the transverse sinus.  Deep System of Veins for draining the cortex: Subependymal veins: There are numerous subependymal veins, which will not be described here in detail. These receive blood from the medullary veins and carry it to the dural venous sinuses. The great cerebral vein (vein of Galen) is worthy of a mention; it is formed by the union of two of the deep veins, and drains into the straight sinus. Medullary veins : Originate 1 – 2 cm below the cortical grey matter, and drain into subependymal veins. These drain the deep areas of the brain. Other Structures in the Central Nervous System: the Other Structures in the Central Nervous System The spinal cord is supplied by three anterior and three posterior spinal veins. These veins are valveless, and form an anastamotic network along the surface of the spinal cord. They also receive venous blood from the radicular veins. The spinal veins drain into the internal and external vertebral plexuses, which in turn empty into the systemic segmental veins. The internal vertebral plexus also empties into the dural venous sinuses superiorly. Other Structures in the Central Nervous System: Cerebellum There are two main veins responsible for the venous drainage of the cerebellum, the superior and inferior cerebellar veins. They empty into the superior petrosal, transverse and straight dural venous sinuses. Brainstem Venous drainage of the brainstem is carried out by numerous vessels. The cavernous sinuses: Anatomical Location and Borders The cavernous sinuses are located within the middle cranial fossa, on either side of the sella turcica of the sphenoid bone (which contains the pituitary gland). They are enclosed by the endosteal and meningeal layers of the dura mater.  The cavernous sinuses: Anatomical Location and Borders The borders of the cavernous sinus are as follows: Anterior: superior orbital fissure. Posterior: petrous part of the temporal bone. Medial: body of the sphenoid bone. Lateral: meningeal layer of the dura mater running from the roof to the floor of the middle cranial fossa. Roof: meningeal layer of the dura mater that attaches to the anterior and middle clinoid processes of the sphenoid bone. Floor: endosteal layer of dura mater that overlies the base of the greater wing of the sphenoid bone. The cavernous sinuses: Anatomical Location and Borders Several important structures pass through the cavernous sinus to enter the orbit. They can be sub-classified by whether they travel through the sinus itself, or through its lateral wall: The cavernous sinuses: Contents Several important structures pass through the cavernous sinus to enter the orbit. The cavernous sinus is the only site in the body where an artery (internal carotid) passes completely through a venous structure. This is thought to allow for heat exchange between the warm arterial blood and cooler venous circulation. Dural Venous Sinus System Each cavernous sinus receives venous drainage from: Ophthalmic veins (superior and inferior) – these enter the cavernous sinus via the superior orbital fissure. Central vein of the retina – drains into the superior ophthalmic vein, or directly into the cavernous sinus. Sphenoparietal sinus – empties into the anterior aspect of the cavernous sinus. Superficial middle cerebral vein – contributes to the venous drainage of the cerebrum Pterygoid plexus – located within the infratemporal fossa. Dural Venous Sinus System It is important to note that the superior ophthalmic vein forms an anastomosis with the facial vein. Therefore, the ophthalmic veins represent a potential route by which infection can spread from an extracranial to an intracranial site. The cavernous sinuses empty into the superior and inferior petrosal sinuses, and ultimately, into the internal jugular vein. The left and right cavernous sinuses are connected in the midline by the anterior and posterior intercavernous sinuses. They travel through the sella turcica of the sphenoid bone. a) Filling defects bilaterally in the cavernous sinus (arrows) and fluid in the sphenoid sinus. b) Filling defect in the right ophthalmic vein (arrow). The findings are indicative of cavernous sinus thrombosis and thrombophlebitis. Overview of the meninges The meninges refer to the membranous coverings of the brain and spinal cord. There are three layers of meninges, known as the dura mater, arachnoid mater and pia mater. These coverings have two major functions: Provide a supportive framework for the cerebral and cranial vasculature. Acting with cerebrospinal fluid to protect the CNS from mechanical damage. The meninges are often involved cerebral pathology, as a common site of infection (meningitis), and intracranial bleeds. The meninges: the Dura Mater The dura mater is the outermost layer of the meninges and is located directly underneath the bones of the skull and vertebral column. It is thick, tough, and inextensible. The dura mater consists of two layered sheets of connective tissue: Periosteal layer: lines the inner surface of the bones of the cranium. Meningeal layer: located deep to the periosteal layer. It is continuous with the dura mater of the spinal cord. The meninges: the Dura Mater The dural venous sinuses are located between the two layers of dura mater. They are responsible for the venous drainage of the cranium and empty into the internal jugular veins. The dura mater receives its own vascular supply, primarily from the middle meningeal artery and vein. It is innervated by the trigeminal nerve (V1, V2 and V3). The meninges: Dural Reflections The meningeal layer of dura mater folds inwards upon itself to form four dural reflections. These reflections project into the cranial cavity, dividing it into several compartments, each of which houses a subdivision of the brain.  The meninges: Dural Reflections The four dural reflections are: Falx cerebri: projects downwards to separate the right and left cerebral hemispheres. Tentorium cerebelli: separates the occipital lobes from the cerebellum. It contains a space anteromedially for passage of the midbrain, the tentorial notch. Falx cerebelli: separates the right and left cerebellar hemispheres. Diaphagma sellae: covers the hypophysial fossa of the sphenoid bone. It contains a small opening for passage of the stalk of the pituitary gland. CT scan of a massive extradural haematoma  The meninges: Arachnoid Mater The arachnoid mater is the middle layer of the meninges, lying directly underneath the dura mater. It consists of layers of connective tissue, is avascular, and does not receive any innervation. Underneath the arachnoid is a space known as the sub-arachnoid space. It contains cerebrospinal fluid, which acts to cushion the brain. Small projections of arachnoid mater into the dura (known as arachnoid granulations) allow CSF to re-enter the circulation via the dural venous sinuses. The meninges: Arachnoid Mater The meninges: Pia Mater The pia mater is located underneath the sub-arachnoid space. It is very thin, and tightly adhered to the surface of the brain and spinal cord. It is the only covering to follow the contours of the brain (the gyri and fissures). Like the dura mater, it is highly vascularised, with blood vessels perforating through the membrane to supply the underlying neural tissue.  Autopsy of a patient with meningitis. The dura mater is being retracted to show a grossly swollen cerebrum with pus accumulation.