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Published byJeremy Wilcox Modified over 8 years ago
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Nervous Systems Neuron… basic unit – Human brain contains roughly 10 11 interconnected neurons that communicate with each other to store memories and effect thought
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Complexity of Nervous Systems The complexity of the organism is directly related to the complexity of the nervous system – Sponge… only animal without a nervous system – Hydra… nerve net (no centralized processing of information) – Flatworms and annelids (cephalization) – The size of the brain or amount of cephalization increases with increasing complexity
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Organization of the Nervous System Central Nervous System (CNS)… brain and spinal cord – Peripheral Nervous System (PNS)… nerves and ganglia Sensory neurons… bring information form the sensor to the CNS Interneurons… transmit information from CNS to the motor neurons – Analyze and interpret based on current context and past experience Motor neurons… send information out from the CNS to the effector Reflex arcs can bypass the input of interneurons
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Neuron Structure Cell body… contains the nucleus Dendrite… receives signals from the effector (sensor) or other neurons Axon… moves the nerve impulse away from the nerve body – Many wrapped in a myelin sheath Insulates the axon and increases conduction speed Synaptic Terminal… site of communication – Presynaptic to postsynaptic cell through neurotransmitters Interneurons generally have far more synapses than motor neurons
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Supporting Cells Glia… supporting cells that provide structural and nutritional support. – Astrocytes… structure and regulation of ions Cause vessel dilatation in areas of active neurons facilitating oxygen and glucose transport – Radial Glia… active in embryo formation in the development of the CNS Act as stem cells along with astrocytes during formation – Oligodendrites & Schwann Cells… form the myelin sheath Primarily of lipid material that is non- polar and provides insulation during conduction of the nerve impulse
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Membrane Potential Resting membrane potential is established across the plasma membrane through maintenance of ion gradients – -60 to -80mV in most mammals – Na +, K +, & Cl - are the primary ions with some charged amino acids in the cytosol and Ca 2+ contributing to potential Nernst Equation – E ion = 62mV(log ([ion] outside/[ion]inside) Maintained through ion channels by the Na/K pumps
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Gated Ion Channels Stretch-gated channels – Open when a membrane is deformed… signal after being touched Ligand-gated channels – Open when a specific chemical is encountered… often found near synapses Voltage-gated channels – Found on axons and dendrites and are used to transmit a nerve impulse Causes either hyperpolarization, depolarization, or reestablishment of resting potential
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Action Potentials An action potential is an all-or- nothing response to depolarization once a threshold has been met – The threshold is a voltage created by a depolarization event strong enough to start an action potential – The action potential is graded… the amount of action potential is determined by the strength of depolarization or hyperpolarizing stimulus – Once started the action potential are carried along the length of axons and dendrites Propagated by voltage-gated channels
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Conduction of Action Potentials Na+ influx after stimulation depolarizes cell – Creates a stimulus for neighboring cell once threshold has been reached – K+ ion gates opens to repolarize membrane Prevents action potential from traveling backward Speed is increased through saltatory conduction across the nodes of Ranvier – Impulse jumps to the gaps between Schwann cells Transmission from toes to spinal cord ~1-2ms Extracellular fluid is only in contact at the nodes
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