2401 : Anatomy/Physiology

Transcription

1 Dr. Chris Doumen Week : Anatomy/Physiology The Brain Central Nervous System TextBook Readings Pages 431 through 435 and 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 Rhombencephalon becomes Metencephalon and Myelencephalon Metencephalon develops into pons and Cerebellum Myelencephalon becomes the Medulla oblongata 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

2 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 Forebrain 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 cerebral cortex, 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. the subcortical nuclei. 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 longitudinal fissure, separating the two hemispheres. The transverse fissure 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. : Frontal, parietal, occipital and temporal lobe. The 5 th one is buried under the temporal lobe.

3 2401 : Anatomy/Physiology Page 3 of 6 Central sulcus separates the frontal lobe from the parietal lobe the gyrus anterior of the central sulcus = pre-central gyrus the gyrus posteriorly of the central sulcus = post-central gyrus A parieto-occipital sulcus, located on the posterior medial surface, separates the parietal lobe from the occipital lobe below A deep lateral sulcus 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: The Cerebral 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 interventricular foramen with the third ventricle, located in the diencephalons. This connects via the cerebral aqueduct 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 Cranial Meninges 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.

4 2401 : Anatomy/Physiology Page 4 of 6 Dura mater : 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. Arachnoid mater : Provides the arachnoid 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 mater : The innermost layer that sticks to the brain itself. CerebroSpinal 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 choroid plexus 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 4 th 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 through arachnoid villi, extension from the arachnoid meninges

5 2401 : Anatomy/Physiology Page 5 of 6 Choroid Plexuses The choroid plexuses are networks of capillaries in the walls of the ventricles. They are capillaries covered by ependymal cells (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 Cerebrum 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.

6 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. Association fibers : connect and transmit impulses between gyri of the same hemisphere; smaller ones are called arcuate fibers, longer one are referred to as fasciculi Commissural fibers : connect gyri in one hemisphere to corresponding gyri of the opposite hemisphere (Corpus callosum is the main bridge between right and left side) Projection 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)