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Chapter 7.docx

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Brock University
Ingrid Makus

Chapter 7: The Structure of the Nervous System Gross Organization of the Mammalian Nervous System  NS of all mammals has two division: the CNS and the PNS  Anatomical References o Read Page 169 and 170  The Central Nervous System o Consists of the parts of the nervous system encased in bone: the brain and the spinal cord o Three parts of the brain are common to all mammals: the cerebrum, the cerebellum, and the brain stem o The Cerebrum  Rostral-most and largest part of the brain  It is split down the middle into two hemispheres; separated by the deep sagittal fissure.  The right hemisphere receives information and controls the left side of the body; the left hemisphere receives information and controls the right side of the body o The Cerebellum  Lies behind the cerebrum  It contains as many neurons as both cerebral hemispheres combined  It is primarily a movement control center with extensive connections to the cerebrum and spinal cord  Left side of the cerebrum is concerned with the left side of the body and the same applies to the right portion. o The Brain Stem  The remaining part of the brain  Forms the stalk from which the cerebral hemispheres and the cerebellum sprout  A complex nexus of fibers and cells that in part serves to relay information from the cerebrum to the spinal cord and cerebellum; and vice versa  Also the site where vital functions are regulated (e.g. breathing, consciousness, and the control of body temp.)  Considered the most primitive part of the mammalian brain but also the most important to life o The Spinal Cord  Encased in a bony vertebral column and is attached to the brain stem  Is the major conduit of info from skin, joints, and muscles of the body to the brain and vice versa  It communicates with the body via the spinal nerves (part of PNS).  Each spinal nerve attaches to the spinal cord by means of two branches: the dorsal root and the ventral root  The Peripheral Nervous System o Has two parts: the somatic PNS and the visceral PNS o The Somatic PNS  All the spinal nerves that innervate the skin, the joints, and the muscles that are under voluntary control are part of this PNS  The somatic motor axons (command muscle contraction) derives from motor neurons in the ventral spinal cord  The cell bodies of the motor neurons lie within the CNS, but their axons are mostly in the PNS  The somatic sensory axons enter the spinal cord via the dorsal roots; the cell bodies of these neurons lie outside the spinal cord in clusters called dorsal root ganglia o The Visceral PNS  Also called autonomic nervous system (ANS)  Consists of neurons that innervate the internal organs, blood vessels, and glands  Information such as the pressure and oxygen content of the blood in the arteries are brought to the CNS  Visceral motor fibers command the contraction and relaxation of muscles that form the walls of the intestines and the blood vessels, the rate of cardiac contraction, and the secretory function of various glands o Afferent and Efferent Axons  Afferent means to carry to  Efferent is to carry from  The Cranial Nerves o There are 12 pairs of cranial nerves that arise from the brain stem and innervate (mostly) the head o Some are part of the CNS and others are a part of the two divisions of the PNS o Contain a complex mixture of axons that perform different functions  The Meninges o The CNS is protected by three membranes collectively called Meninges  Dura mater  Outermost covering  Leatherlike  Forms a tough, inelastic bag that surrounds the brain and spinal cord  Arachnoid membrane  Just under the dura mater  Has an appearance and consistency resembling a spider web  Pia Mater  A thin membrane that adheres closely to the surface of the brain  Along it many blood vessels that ultimately dive into the substance of the underlying brain  Separated from the arachnoid by a fluid-filled space  Filled with salty clear liquid called cerebrospinal fluid (CSF)  The Ventricular System o The fluid-filled caverns and canals inside the brain constitute the ventricular system o The fluid that runs in the systems is the CSF  CSF is produced by the choroid plexus, in the ventricles of the cerebral hemispheres  CSF flows from the paired ventricles of the cerebrum to a series of connected, unpaired cavities at the core of the brain stem  CSF exits the ventricular system and enters the subarachnoid space through the small openings located near where the cerebellum attaches to the brain stem o The in the subarachnoid space, CSF is absorbed by the blood vessels at special structures  Imaging the Living Brain o Read Pages 174 to 176 Understanding CNS Structure through Development  The entire CNS is derived from the walls of a fluid-filled tube that is formed at an early stage in embryonic development o The tube becomes the adult ventricular system  Read Table 7.1 and 7.2  Formation of the Neural Tube o Embryo begins as a flat disk with three distinct layers of cells called the endoderm, mesoderm and the ectoderm  The endoderm gives rise to the lining of many of the internal organs (viscera)  The mesoderm arise the bones of the skeleton and the muscles  The nervous system and the skin derive entirely from the ectoderm  The ectoderm gives rise to the nervous system: the neural plate o At about 17 days from conception (in humans), the brain consists only of a flat sheet of cells.  Next is the formation of a grove in the neural plate that runs rostral to caudal neural groove  The walls of the groove are called the neural folds o The move and fuse together to form the neural tube o The entire nervous system develops from the walls of the neural tube  As the neural folds come together, some neural ectoderm is pinched off and comes to lie just lateral to the neural tube  neural crest o o All neurons with cell bodies in the peripheral nervous system derive from the neural crest o The neural crest develops in close association with the underlying mesoderm  The mesoderm at this stage forms prominent bulges on either side of the neural tube called somites  From these somites, 33 individual vertebrae of the spinal column and the related skeletal muscles will develop  The nerves that innervate these skeletal muscles are called somatic motor nerves o The process by which the neural plate becomes the neural tube is called neurulation  Occurs very early in embryonic development (22 days after conception)  The Primary Brain Vesicles o The process by which structures become more complex and functionally specialized during development  differentiation o The first step in differentiation is the development (at the rostral end of the neural tube) of the three swellings called the primary vesicles o The entire brain derives from the three primary vesicles of the neural tube  The rostral most vesicle is called the prosencephalon  the forebrain  Behind the prosencephalon lies the mesencephalon  midbrain  Caudal to this is the rhombencephalon  hindbrain  Connects with the caudal neural tube  gives rise to the spinal cord  Differentiation of the Forebrain o The next important development occur in the forebrain  Secondary Vesicles sprout off on both sides of the prosencephalon.  The secondary vesicles are the optic vesicles and the telencephalic vesicles  The unpaired structures that remain after the secondary vesicles have sprouted is called the diencephalon  Differentiation of the Telencephalon and Diencephalon  The telencephalic vesicles together form the telencephalon (endbrain)  It continues to develop in four ways: o They grow posteriorly so that they lie over and lateral to the diencephalon o Another pair of vesicles sprout off the ventral surfaces of the cerebral hemisphere  giving rise to the olfactory bulbs o The cells of the walls of the telencephalon divide and differentiate into various structures o White matter systems develop, carrying axons to and from the neurons of the telencephalon  See Fig.7.13 on page 184  The fluid-filled spaces within the cerebral hemispheres are called lateral ventricles  The space at the center of the diencephalon is called the third ventricle o Whenever you see paired fluid-filled ventricles in a brain section, you know that the tissue surrounding them is the telencephalon  Neurons form two different types of gray matter form in the telencephalon: the cerebral cortex and the basal telencephalon  The diencephalon differentiates into two structure: the thalamus and the hypothalamus  The neurons of the developing forebrain extend axons to communicate with other parts of the nervous system o These axons bundle together to form three major white matter systems: the cortical white matter, the corpus callosum and the internal capsule  Cortical white mat
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