Lecture 11: The Brain II - PDF

Main Title The Brain II 0  Embryology of the CNS  In the embryo, the Neural tube forms, consisting of a wall and a lumen:  The wall of the neural tube develops into the nervous tissue of the brain and spinal cord.  The lumen becomes the cavities (ventricles)...

Main Title The Brain II 0  Embryology of the CNS  In the embryo, the Neural tube forms, consisting of a wall and a lumen:  The wall of the neural tube develops into the nervous tissue of the brain and spinal cord.  The lumen becomes the cavities (ventricles) of the CNS. Development of Brain Vesicles  Third Week of Intrauterine Life: The neural tube dilates, forming primary brain vesicles.  Primary Brain Vesicles (Third and Fourth Week): The neural tube differentiates into three primary vesicles: The Prosencephalon (forebrain), Mesencephalon (midbrain), and Rhombencephalon (hindbrain).  Secondary Brain Vesicles (Fifth Week): The primary vesicles differentiate into Five secondary vesicles:  Prosencephalon divides: into Telencephalon and Diencephalon.  Rhombencephalon divides: into Metencephalon and Myelencephalon.  Mesencephalon remains undivided. Ventricular Formation (Adult Structures)  As secondary vesicles develop, they create spaces within the neural tube that form the ventricular system of the brain: (‫)ﻣﮭﻢ‬  Telencephalon → Cerebrum → Forms the lateral ventricles.  Diencephalon → Thalamus and Hypothalamus → Forms the Third ventricle.  Mesencephalon → Midbrain → Forms the Cerebral aqueduct.  Metencephalon → Pons and Cerebellum → Forms part of the Fourth ventricle. Cavi es of the hindbrain forms  Myelencephalon → Medulla Oblongata → Forms part of the Fourth ventricle.  Note: The remaining neural tube does not dilate and forms the spinal cord and its central canal. 1  Ventricular System (Brain Chamber)  The ventricular system is a network of interconnected cavities within the brain.  Components: Two Lateral Ventricles, Third Ventricle, Fourth Ventricle.  Function: The ventricles produce cerebrospinal fluid (CSF) through the Choroid plexuses” Specialized vascular structures within the ventricular walls that produce CSF”. Two Lateral Ventricles  Two large, C-shaped cavities located within the cerebral hemispheres(cerebrum), filled with cerebrospinal fluid (CSF).  Separated by a thin, transparent membrane in the midline, called the Septum pellucidum:  Located directly inferior to the Corpus Callosum (a mass of white matter that connects the two cerebral hemispheres).  Connected to the Third ventricle (in the diencephalon) via the Interventricular foramina. Components of Each Lateral Ventricle:  Body: The largest part, located in the parietal lobe.  Three horns:  Anterior horn: Extends and projects into the Frontal lobe.  Posterior horn: Projects into the Occipital lobe.  Inferior horn: Projects into the Temporal lobe. Third ventricle  Location:  A narrow, slit-like space located at the midline between the two parts of the diencephalon.  Positioned between the two masses of the Thalamus.  It connects with the lateral ventricles through the Interventricular foramina. 2  Boundaries:  Lateral wall: Bounded by the two masses of the Thalamus.  Floor: Formed by the Hypothalamus.  The Third ventricle communicates with the fourth ventricle via the cavity of the midbrain, known as the Cerebral aqueduct. Fourth Ventricle  It is the cavity of the Hindbrain.  Location:  Anteriorly: Located between the Pons and Medulla oblongata.  Posteriorly: Bounded by the Cerebellum.  Foramina (in its roof): :‫الفتحات‬  Two lateral apertures: Foramina of Luschka.  One median aperture: Foramen of Magendie.  These foramina open into the subarachnoid space, allowing the drainage of cerebrospinal fluid (CSF).  CSF also flows downward into the central canal and the subarachnoid space of the spinal cord. Cerebrospinal Fluid (CSF)  De inition: A clear fluid composed mainly of water, along with nutrients and ions, in which the brain "floats."  Source:  Produced by the filtration of blood through the Choroid plexus in the ventricles.  The brain absorbs nutrients from the CSF and releases waste products into it, which then return to the blood.  This process is similar to the exchange that occurs in the capillary beds of tissues but happens via the CSF in the brain. 3  Location of CSF (in and around the brain)  Inside:  Brain ventricles: Two lateral ventricles, third ventricle, and fourth ventricle.  Central canal of the spinal cord and the lumen of the remaining neural tube.  Around: Subarachnoid space surrounding the brain and spinal cord via the apertures of the fourth ventricle.  Since CSF surrounds the brain and spinal cord (components of the CNS), it allows them to "float" in this protective fluid. Functions of CSF  Reduces brain weight from ~1.5 kg to a few grams by buoyant force, as per Archimedes' principle.  Acts as lymphatic drainage for the brain, compensating for the absence of lymphatic vessels in nervous tissue.  The choroid plexus is a membranous structure in the wall of ventricles that produces cerebrospinal fluid (CSF). Flow of CSF  Lateral ventricles → Interventricular foramina → Third ventricle → Cerebral aqueduct → Fourth ventricle → Foramina of Luschka and Magendie → Subarachnoid space. (‫)ﻣﮭم‬  CSF is eventually drained from the subarachnoid space into the venous system via arachnoid villi/granulations projecting into the Superior Sagittal Sinus. (‫)ﻣﮭم‬ Hydrocephalus  Disruption in CSF flow, whether due to blockage, overproduction, or impaired drainage, can cause its accumulation, leading to increased intracranial pressure and Hydrocephalus.  So Hydrocephalus is the accumulation of cerebrospinal fluid (CSF) within the brain.  Excess CSF in the ventricles causes head enlargement in infants due to increased pressure, as their skull bones are not yet fused. 4  Causes of hydrocephalus include overproduction of CSF, obstruction of CSF flow, and interference with CSF absorption at the Arachnoid granulations.  In infants: The head increases in size due to movable skull bones that aren't firmly attached.  In adults: Increased intracranial pressure occurs because the rigid skull cannot expand.  Treatment:  Surgical procedures: Implanting a shunt to drain CSF from the ventricles to another body cavity, typically the peritoneal cavity or heart.  Third Ventriculostomy: Creating a pathway for CSF flow if there's an obstruction in the third ventricle. Arteries of the brain  The main arteries of the brain include:  Two Internal Carotid Arteries (ICA): These supply the Anterior and Middle parts of the brain, primarily the anterior part.  Vertebral Artery: Supplies the posterior part of the brain and brainstem. Internal Carotid Artery (ICA)  The ICA is a branch of the Common Carotid Artery (CCA), arising from the brachiocephalic trunk on the right and directly from the arch of the aorta on the left.  The two CCAs ascend in the neck within the carotid sheath. At the level of C3-C4 (generally C4), which is also the level of the superior border of the thyroid cartilage.  Parts of the Internal Carotid Artery: The Cervical Portion1  The cervical part of the ICA is the longest, located in the neck from the bifurcation of the CCA to its entry into the skull through the carotid canal, with no branches, so it does not supply the neck region. 5  The Petrous Portion2  The petrous portion of the ICA is located inside the carotid canal within the petrous part of the temporal bone.  Running upward, anterior, and medial, passing above the foramen lacerum before emerging into the cranial cavity. (It passes above, not through, the foramen lacerum) The Cavernous Portion3  The cavernous portion of the ICA is S-shaped, located inside the cavernous sinus, and turns backward to become the cerebral portion. The Cerebral Portion4  The cerebral portion of the ICA passes medial to the Anterior Clinoid Process, penetrates the dura mater, and enters the subarachnoid space, where cerebral arteries are located, unlike meningeal arteries which are in the Epidural space. Branches of the ICA Ophthalmic Artery1  Passes through the optic canal to supply the orbit and terminate as the Supraorbital and Supratrochlear arteries.  Supplies the forehead region and the anterior part of the scalp Anterior Cerebral Artery2  Supplies the medial surface of the cerebral hemispheres as far back as the occipital lobe (parieto-occipital sulcus).  Also supplies about 1 inch (2 cm) of the superior lateral surface of the cerebrum.  Connected to the opposite anterior cerebral artery by the Anterior communicating artery. Middle Cerebral Artery3  The largest branch of the ICA and Runs in the lateral sulcus (Sylvian fissure). 6   Supplies most of the lateral surface of the cerebrum, except the superior inch (supplied by the anterior cerebral artery) and the inferior inch (supplied by the posterior cerebral artery). Posterior Communicating Artery4  Connects the ICA to the posterior cerebral artery and its Part of the Circle of Willis. Choroidal Artery5  Supplies branches to the choroid plexus of the lateral and third ventricles. The Vertebral Artery  Originates from the First part of the Subclavian artery.  Ascends through the transverse foramina of the upper six cervical vertebrae, arches over the posterior arch of the atlas.  Pierces the posterior Atlanto-occipital membrane to enter the skull through the foramen magnum.  Joins the other vertebral artery at the Anterior surface of the brainstem, between the medulla and pons, to form the Basilar artery.  The fusion of the two vertebral arteries ascending through the cervical vertebrae to form the Basilar artery is known as the Vertebrobasilar System. Main Branches of the Vertebral Artery:  Posterior Meningeal Artery: Two branches, one from each vertebral artery, supply the posterior meninges and enter the skull through the foramen magnum.  Anterior and Posterior Spinal Arteries: Supply the spinal cord.  Posterior Inferior Cerebellar Artery (PICA): Supplies the cerebellum, along with the superior cerebellar artery and anterior inferior cerebellar artery (both from the basilar artery).  Medullary Arteries: Supply the medulla oblongata. 7  Branches of the Basilar Artery  Anterior Inferior Cerebellar Arteries: Supply the cerebellum anteriorly and inferiorly.  Labyrinthine Arteries: Supply the inner ear, passing through the IAM.Internal Acoustic Meatus  Pontine Arteries: Supply the pons.  Superior Cerebellar Arteries: Supply the superior portion of the cerebellum.  Basilar Artery Termination: Divides into the two posterior cerebral arteries Circle of Willis  Also known as the "cerebral arterial circle" or "circulus arteriosus Cerebri." a cross-connection between adjacent channels, tubes, fibers, or other parts of a network.  Anastomosis between branches of the two ICAs and the two vertebral arteries.  Consists of:(‫)ﺳﻨﻮات‬  Anterior Communicating Artery.  Anterior Cerebral Artery.  Internal Carotid Artery.  Posterior Communicating Artery.  Posterior Cerebral Artery.  Basilar Artery. Why Does the Circle of Willis Exist?  The nervous system is essential for controlling body activities, and a 3-5-minute shortage of oxygen will lead to necrosis (neural cell death).  The Circle of Willis provides alternative routes for blood supply to the brain, ensuring that if one route is blocked, the brain still receives blood from other arteries, preventing damage. 8