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Unit 2 - Central Nervous System.docx

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BIOL 373
Heidi Engelhardt

Unit 2 – The Central Nervous System Evolution of Nervous Systems - First multicellular animals to develop neurons were Cnidaria, the jellyfish and sea anemones o Nervous system consists of a nerve net composed of sensory neurons, connective interneurons and motor neurons o No identifiable control centre - Flatworms have a rudimentary brain o The distinction between central nervous system and peripheral nervous system is not clear o “Brain” consists of a cluster of nerve cell bodies concentrated in the head (cephalic) region o Nerve cords come off the “brain” and lead to a nerve network that innervates distal regions - Segmented worms (annelids) have an advanced nervous system o Cluster of cell bodies no longer restricted to cephalic region o Cell bodies also occur in fused pairs, called ganglia o Each segment of the worm contains a ganglion, simple reflexes can be integrated without input from the brain - Advantages of cephalic brains o Head is the part of the body that first contacts the environment o As brains evolved, they became more associated with cephalic receptors such as eyes, ears and taste - In higher arthropods, such as insects and octopus, specific regions of the brain are associated with particular functions o More complex brain  more complex behaviours Evolution of Brains in Vertebrates - Most dramatic change is seen in the forebrain region, which includes the cerebrum - Cerebrum is the part of the brain that makes humans, human o Responsible for reasoning and cognition Development of the Vertebrate CNS 1) Cells that will become the nervous system lie in a flattened region called the neural plate a. Convergence of the neural plate occurs, a neural tube forms which will eventually form the CNS b. Outer layer of the neural tube will become neurons and the glia of the CNS c. Neural crest cells will become the sensory and motor neurons of the PNS 2) At week 4, the anterior portion (cephalic region) of the neural tube specializes into the forebrain, midbrain and hindbrain 3) At week 6, the CNS has formed 7 major divisions that present at birth a. Cerebrum b. Diencephalon c. Midbrain d. Cerebelum e. Pons f. Medulla oblongata g. Spinal cord h. The lumen of the neural tube enlarges to form the hollow ventricles of the brain 4) At week 11, the growth of the cerebrum is noticeably more rapid than that of the other divisions of the brain a. Cerebrum surrounds diencephalon, midbrain, pons, leaving only the cerebellum and medulla oblongata visible below it. Convoluted surface of cerebrum caused to rapid growth in confined space Organization of the Adult CNS - Brain encased in bony skull (cranium) - Spinal cord runs through a canal in the vertebral column - Body segmentation can be seen in bony vertebrae which are stacked on top of one another and separated by disks of connective tissue - Three layers of membrane, called the meninges lie between the bones and tissues of the CNS – they help stabilize neural tissue and protect it from bruising against bones of the skeleton o Membranes are: o Dura matter – thickest of the three membranes, associated with veins that drain blood from the brain through vessels or sinuses o Arachnoid membrane – loosely tied to inner membrane, leaving a subarachnoid space between 2 layers o Pia matter – the most inner membrane and adheres to the surface of the brain and spinal cord, arteries that supply the brain are associated with this layer Cerebrospinal Fluid - Solution continuously secreted by the choroid plexus, a specialized region on the walls of ventricles - It is similar to kidney tissue - Choroid plexus pump sodium and other solutes from plasma into ventricles o ventricles within brain, hollow central canal within spinal cord o two lateral ventricles, two descending ventricles that extend through in brain stem o CSF in ventricles continuous with fluid in central canal of spinal cord - CSF then flows into subarachnoid space, surrounding the entire brain and spinal cord in fluid Roles of CSF - Provide physical protection o Buoyancy of CSF reduces the weight of the brain 30 fold, thus resulting in less pressure on blood vessels and nerves o CSF helps to cushion the brain - Provide Chemical protection o CSF creates a closely regulated extracellular environment for neurons o Choroid plexus is selective about which substances it transports into the ventricles - A spinal tap, or lumbar puncture can be done to analyze the components of CSF. Blood cells or proteins in the CSF suggests an infection - Compared to blood, has lower K+, Ca2+, HCO3-, glucose, pH, similar Na+ levels as blood - Choroid plexus produces about 500mL of CSF daily, enough to replace the CSF 4 times a day - CSF is reabsorbed into venous blood by arachnoid villi - Choroid plexus is composed of ependymal cells Circulation of CSF 1) Lateral ventricles 2) Third ventricle 3) Fourth ventricle 4) Subarachnoid space, central canal of spinal cord 5) Arachnoid villi 6) Superior sagittal sinus 7) Venous return to heart Blood-Brain Barrier - Functional barrier between interstitial fluid and the blood - Protects the brain against harmful substances and harmful bacteria - Endothelial cells form tight junctions with one another that prevent solute movement between cells - Astrocytes cause tight junction formation - Lipid soluble molecules cross readily , water soluble molecules cannot cross - Hydrophilic substances (ions, amino acids, peptides, proteins) will only cross if specific transporters/carriers are present on endothelial cells of capillaries within CNS Quirks of Neural Tissue - Neurons are obligate aerobes – unable to switch to anaerobic metabolism - Oxygen readily crosses blood brain barrier - Capillaries of CNS express high levels of glucose transporters to provide adequate levels of glucose - Brain responsible for about half of body’s glucose consumption - About 15% of cardiac output received by brain o Critically dependant on adequate oxygen and glucose The Spinal Cord - Major path for information flow between CNS and skin, joints, muscles - Contains neural networks involved in locomotion - Divided into 4 regions  each of which is divided into segments o Each segment gives rise to pair of spinal nerves o Just before a spinal nerve joins the spinal cord, it divides into two branches called roots - Dorsal Root of spinal nerve o Specialized to carry incoming sensory information o The dorsal root ganglia, swellings found on the dorsal roots just before they enter the cord, contain cell bodies of sensory neurons - Ventral Root of spinal nerve o Carries information from the CNS to muscles and glands - Cross section of spinal cord o Has a butterfly shaped core of grey matter and surrounding rim of white matter  White matter  myelinated axons, white colour occurs due to fatty layer. Bundles of axons connecting CNS regions  Tracts  Grey matter  unmyelinated axons, mostly cell bodies and dendrites.  Cluster of cell bodies within CNS  nuclei  Cluster of cell bodies outside CNS  ganglia - Sensory fibres from the dorsal roots synapse with interneurons in the
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