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chapter 25.docx

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Andre Lecours

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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 matter contains all axons that run to and from the neurons in the cerebral cortex  Is continuous with the internal capsule (links the cortex with the brain stem (esp. hypothalamus)  Corpus callosum is continuous with the cortical white matter and forms an axonal bridge that links cortical neurons of the two cerebral hemispheres  Forebrain Structure-Function Relationship  Is the seat of perceptions, conscious awareness, cognition and voluntary action  Depends on extensive interconnections with the sensory and motor neurons of the brain stem and spinal cord  The most important structure in this area is the cerebral cortex  Neurons in the olfactory bulbs receive information from cells that sense chemicals in the nose and relays this information for further analysis  Info from the eyes, ears and skin is also brought to the cerebral cortex o These senses are relayed from the thalamus  Thalamic neurons send axons to the cortex via the internal capsule. o The axons of each internal capsule carry info to the cortex about the contralateral side of the body  How does the right side know what the left side is doing? Via corpus callosum  Cortical neurons send axons through the internal capsule, back to the brain stem and others through the spinal cord o An important way cortex can command voluntary movement o Another way is by communicating with neurons in the basal ganglia  Although the hypothalamus lies just under the thalamus, it is functionally more closely related to certain telencephalic structures such as the amygdala. o It performs many primitive functions o Controls the visceral (autonomic) nervous system  It can increase heart rate  Play a key role in sex in response to needs  Find food and drink o Differentiation of the Midbrain  Unlike the forebrain, midbrain develops little during brain development  The dorsal surface of the mesencephalic vesicles becomes the tectum  The floor of the midbrain becomes the tegmentum  CSF-filled spaces in between constricts into a narrow channel called the cerebral aqueduct  Connects rostrally with the third ventricle of the diencephalon
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