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Physiology/Pathophysiology of the Nervous System System 49.728 Physiology/Pathophysiology for Nurses

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Agenda Organization of the Nervous System Central Nervous System Structure Peripheral Nervous System Cells of the Nervous System Neurons Glial cells Signalling Mechanisms Equilibrium potentials Graded potentials Action potentials

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Organization of the Nervous System: CNS Three divisions of brain: Forebrain cerebral hemispheres Midbrain Corpora quadrigemini, tegmentum, cerebral peduncles Hindbrain Cerebellum, pons, medulla Brainstem: Midbrain, medulla, pons Connects cerebrum, cerebeluum, spinal cord

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Organization of the Nervous System: Reticular Activating System Key Regulatory Functions: CV, respiratory systems Wakefulness Clinical Link: Disturbances in the RAS are linked to sleep-wake disturbances Reticular Formation Ascending Sensory Tracts Thalamus Radiation Fibres Visual Inputs

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Peripheral Nervous System Autonomic, Motor, Sensory Divisions

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Sensory Motor Sympathetic Parasympathetic Some Basic Plumbing

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Types of Cells: Neurons: Multipolar Unipolar Bipolar Glial Cells: Schwann cells Oligodendrocytes Astrocytes Fibrocytes

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So, what are some roles of glial cells?

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Some Neuron Basics: Soma Dendrites Axon Axon Terminals Initial Segment Functional Significance?

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Equilibrium Potential Membrane Potential at which there is NON NET FLUX of a specific ion Equilibrium potential for K+ is close to that of the resting membrane potential Why?

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Equilibrium Potential Time = 0 K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ - - - - - - -- - - - - - - +

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Equilibrium Potential Time = 1 K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ - - - - - - -- - - - - - - +

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Equilibrium Potential Time = 2 K+K+ K+K+ K+K+ K+K+ K+K+ - - - - - - -- - - - - - - + K+K+

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Equilibrium Potential Time = 4 K+K+ K+K+ K+K+ K+K+ - - - - - - -- - - - - - - + K+K+ K+K+

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At the Equilibrium Potential Net Flux = 0 Equilibrium Potential (Ev) is Different for each ion Why? E v for K + is close to, but not the same as, the resting membrane potential (E m ) Why?

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We can describe this relationship mathematically via the Nernst Equation: Ep = 0.058 log [K + ] outside [K + ] inside So what?

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Neuronal Signaling: Graded Potentials and Action Potential

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Graded Potentials Occur over most of neuron Graded Can summate ++ + + - - - - - - - - - - --- -- + - - - - - - - Excitatory Synapse Inhibitory Synapse Em Time Em Time

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Action Potentials Can start only at initial segment Why? All or None not graded Do not summate

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Action Potentials Star With Current Flow to the Initial Segment Excitatory synapse (green) generates positive influx Positive charge flow throughout soma Some positive charge reaches initial segment, where the membrane becomes depolarized + + + + + + - - - - - - - - - - --- -- + - -

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What Occurs at the Initial Segment When Membrane is at Rest? EFFLUX (K + ) >> INFLUX K+K+ K+K+

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What Happens at the Initial Segment When Threshold is NOT Reached? INFLUX (Na + ) > EFFLUX (K + ) Na + K+K+ K+K+

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What Happens at the Initial Segment When Threshold is Reached? INFLUX (Na + ) >> EFFLUX (K + ) Na + K+K+ K+K+

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Ionic Basis for the Action Potential -70 +25 Membrane Potential (mV) Membrane Permeability Time (ms) P Na + PK+PK+

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So why does permeability change? Voltage Gate Activation Gate When the membrane is polarized, The voltage gate is closed, and the activation gate is open. In Out

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So why does permeability change? Voltage Gate Activation Gate When the membrane is depolarized, The voltage gate opens, and Na + enters down its chemical gradient. In Out Na+

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So why does permeability change? Voltage Gate Activation Gate After a short interval, the activation gate Closes, preventing Na + entry. In Out

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So why does permeability change? Voltage Gate Activation Gate When the membrane repolarizes, the volatge gate closes, resetting the activation gate. In Out

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Three States of the V-Gated Na+ Channel Na+ Closed OpenInactivated

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Whats Happening During the Action Potential -70 +25 Membrane Potential (mV) Membrane Permeability Time (ms) P Na +

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What About K + ? Works like Na +, only slower -70 +25 Membrane Potential (mV) Membrane Permeability Time (ms) PK+PK+

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Some Clinical Points Related to the Action Potential What is the effect of high extracellular K+? Tetrodotoxin is a blocker of v-gated Na+m channels. What is the effect on action potentials? On movement? What is the effect of ischemia on neouronal resting membrane potential? On neurotransmitter release?