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Dr. Chris Doumen Week 7 2401 : Anatomy/Physiology Central Nervous System The B rain TextBook Readings ♦ Pages 431 through 435 and 463 - 467 ♦ Make use of the figures in your textbook ; a picture is worth a thousand words ! ♦ Work the Problems and Questions at the end of the Chapter The brain starts its development from an embryonic neural tube. At the end of the 4th week, the neural tube will have developed enough to show the beginning parts of what will become the brain. The brain starts to develop from 3 regions of the embryo which are called the primary brain vesicles. • Proencephalon • Mesencephalon • Rhombencephalon These develop into secondary brain vesicles and will form the adult mature structures. The adult brain is mushroom shaped and can be divided into 4 principal parts • Proencephalon becomes Cerebrum (Telencephalon) and Diencephalon (containing Thalamus, Hypothalamus and Epithalamus) • Mesencephalon becomes the midbrain portion of The Brain stem • The Cerebrum and Diencephalon are also grouped together under the name "forebrain" Terminology: • One axon : = nerve fiber • A group of nerve fibers = Nerve • A group of nerve fibers in the CNS = tract • A group of nerve cell bodies outside the CNS = ganglia • A group of nerve cell bodies inside the CNS = nuclei • Gray matter = unmyelinated cell bodies and dendrites • White matter = myelinated axons . Collin County Community College District Rhombencephalon becomes Metencephalon and Myelencephalon • Metencephalon develops into pons and Cerebellum • Myelencephalon becomes the Medulla oblongata 2401 : Anatomy/Physiology Page 2 of 6 The figure above shows the secondary vesicles (top left) as seen from a top view, and the same structures from a lateral view in growing human embryo. The Forebra in The cerebral hemispheres make up the bulk of the brain mass and sit like a mushroom cap on the brain stem. The surface ( 2 to 4 mm thick) is composed of gray matter called the ce re bral corte x , containing billions of neuron cell bodies. Beneath the cortex lies the cerebral White matter, consisting out of myelinated axons and dendrites with islands of gray matter situated deep in the white matter e.g. th e su bc ortical nuc lei. During embryonic development, there is a rapid increase in brain size in which the gray matter enlarges faster than the white matter. The result is that the cortex starts to fold over upon itself, creating many convolutions Definitions : • Gyri ( gyrus) = elevated ridges of brain tissue • Sulci ( sulcus) = shallow grooves in-between the ridges • Fissures = deeper grooves, separating large regions of the brain Most prominent fissure is the longi tu dinal fiss ure, separating the two hemispheres. The trans ve rs e fiss ure separates the cerebral hemispheres from the cerebellum below. Each hemisphere is furthermore subdivided in 5 lobes by sulci and most of them named after the cranial bone that overlie them. : F ront al , p ariet al , occipit al and tem poral lobe . The 5th one is buried under the temporal lobe. 2401 : Anatomy/Physiology Page 3 of 6 • Ce nt ral sulc us separates the frontal lobe from the parietal lobe • the gyrus anterior of the central sulcus = pre-ce nt ral gyrus • the gyrus posteriorly of the central sulcus = p os t-c entral g yru s • A p ariet o- occi pit al sulcu s, located on the posterior medial surface, separates the parietal lobe from the occipital lobe below • A deep l at eral sulc us outlines lower laying temporal lobe, separating it from the inferior borders of the frontal and parietal lobe Use this website to learn more about lobes and sulci: http://uta.marymt.edu/~psychol/brain.html The C erebral Ventricles During the embryonic development, the neural tube expands as well to form fluid containing chambers within the cerebellum. These are called the ventricles and the fluid is referred to as CerebroSpinal Fluid There are four such ventricles. Two laterals ventricles are located in each hemisphere. They connect via an inte rve nt ricul ar f orame n with the third ventricle , located in the diencephalons. This connects via the ce re bral aqu educt to the fourth ventricle, located in the brainstem, starting between pons and cerebellum and extending inferiorly to connect with the central canal of the spinal cord. The C ranial M eninges The cerebral hemispheres are protected by the bones of the skull and the cranial meninges. The meninges are fibrous membranes that provide structural and functional support and are located immediately deep to the skull. 2401 : Anatomy/Physiology • Dura m ate r : It is the Outermost layer. It has two subblayers called the endosteal layer( fused to the bone) and the meningeal layer. A large gap between these two layers provide space for fluids such as the large midsaggital blood sinus. • Arach noi d mate r : Provides the arach noi d villi ( see later) and the sub-arachnoid space. This latter area is the space into which Cerebrospinal fluid flows as well as the location of majopr blood vessels feeding the brain. • Pia m ate r : The innermost layer that sticks to the brain itself. Page 4 of 6 CerebroSpina l Fluid The CSF is a fluid that circulates in the subarachnoid space ( between arachnoid and pia mater) around the brain and through cavities within the brain. It nourishes and protects the brain. CSF is produced mainly by structures called the ch oroi d ple xu s in the lateral, third and fourth ventricles. CSF flows from the lateral ventricle to the third ventricle through the interventricular foramen (also called the foramen of Monro). The third ventricle and fourth ventricle are connected to each other by the cerebral aqueduct (also called the Aqueduct of Sylvius). CSF then flows onto the 4th ventricle and into the subarachnoid space through the foramina of Luschka (there are two of these) and the foramen of Magendie (only one of these). CSF circulates up the subarachnoid space of the brain and is eventually reabsorbed into the a blood vascular sinus called the superior sagittal sinus. The absorption occurs throu gh arach noid villi , extension from the arachnoid meninges 2401 : Anatomy/Physiology Page 5 of 6 Choroid Plexus es The ch oroi d ple xu se s are networks of capillaries in the walls of the ventricles. They are capillaries covered by epe ndymal cell s (epithelial cells of the ventricles) that form CSF by filtration of blood plasma and then secreting it into the ventricles. The very tight junctions between the ependymal cells prevents fluid to leak from the blood between the cells into the CSF. So, blood plasma has to pass through the cell. This provides an efficient cellular barrier and protects the brain-spinal cord from potential harmful substances in the blood. It is called the blood-cerebrospinal fluid barrier. The entire CSF is about 150 ml and contributes to the homeostasis of the brain in 3 main ways • provides mechanical protection by serving as a shock absorber • provide chemical protection by maintains an optimal chemical environment • contributes in the circulation and exchange of nutrients, waste products between blood and nervous tissue CSF circulates continuously and is formed and reabsorbed at a rate of 480 ml/day. Blockage of CSF movement results in increased pressure on the brain (= hydrocephalus) The C erebrum The cerebral cortex is the executive suite of the nervous system. It enables us to perceive, communicate, remember, understand, appreciate, initiate voluntary movements. Thus all qualities of the conscious behavior. Consisting out of gray mater, it is thus a collection of cell bodies, dendrites, unmeyelinated axons but no fiber tracts. Although correlation of localization and function can sometimes be pinpointed to certain domains, other function are more difficult to locate and seem to be complex and overlapping. We will examine the functional aspects of the cortex with reference to these functional areas. However, always keep these generalizations about the cerebral cortex in mind. 2401 : Anatomy/Physiology Page 6 of 6 1. No functional area of the cortex acts alone; conscious behavior involves the entire cortex 2. There are 3 kinds of functional areas: • Motor areas : control voluntary motor functions • Sensory areas : provide for conscious awareness and sensation • Association areas: act to integrate the diverse information 3. Each hemisphere is concerned with the sensory and motor function of the opposite side 4. The two hemisphere are not entirely equal in function (lateralization, specialization) White Matter Different parts of the brain are connected and communicate by means of tracts of white fibers. Ass oci ati on fi be rs : connect and transmit impulses between gyri of the same hemisphere; smaller ones are called arc u at e fibers, longer one are referred to as f asciculi Commis su ral fi be rs : connect gyri in one hemisphere to corresponding gyri of the opposite hemisphere (Corpus callosum is the main bridge between right and left side) Projecti on fibers : They form ascending ( from spinal cord up to the brain, and usually are sensory ) and descending tracts ( from brain towards and into the spinal cord, usually motor fibers)