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Lecture 4

Unit 1 Lecture 4 - Neurons.docx

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Dalhousie University
PSYO 1011

PSYO 1011 Neurons 1 Lecture 4: Neurons September 22, 2009 Phineas Gage: • Spike propelled through his face & head • Full recovery, but balance between intellectual faculties & animal propensities destroyed • Mind radically changed o “No longer Gage” Cells Of The Nervous System: • Neurons o Building blocks of the nervous system o Linked together in circuits o Receive, process & send messages o 200+ different neurons based on structure  Billions of neurons in the nervous system • Glia o Outnumber neurons 10:1 o Support neurons & hold them in place o Multiple functions Neuronal Structure: • Dendrites o Increase SAof cell body o Receive input from other neurons  Send these messages to the cell body • Soma (Cell Body) o Contains nucleus & other cell machinery vital to cell life • Axon o Carries nerve signals away from cell to other neurons, muscles or glands o Up to a meter long  Branches out at end to form axon terminals • Terminals o Connect neuron to the next cell  Neuron-Neuron o Release neurotransmitters • Synapses o Where neurons interconnect (but do not touch) • Myelin Sheath o Fatty cells that surround & insulate axon • Nodes of Ranvier o Uninsulated gaps between myelin sheath Blood-Brain Barrier: • Prevents substances from entering the brain PSYO 1011 Neurons 2 o Glial cells offer this protection Nerve Conduction:An Electrochemical Process • Neurons o Generate electricity & release chemicals o Use own chemicals as a source of energy • Neuron Cell Membrane o Protects inner structures o Selectively permeable • Environment Surrounding Neurons (Extracellular Environment) o Salty & liquid + o High concentration of Na  Positively charged  High concentration of positively charged ions • Environment Inside Neurons (Intracellular Environment) o Negatively charged o Produces an electric resting potential  ~ -70 mV (millivolts)  When in resting state, said to be polarized Resting Potential: Substance Symbol Change Resting Location Proteins A- - Intracellular Potassium K+ + Intracellular Sodium Na+ + Extracellular Chloride Cl- - Extracellular • Net Charge ~ -70 mV • Membrane is polarized Action Potential: • Sudden reversal in neural membrane voltage o Depolarization  Shift from negative to positive o Voltage shifts from -70 mV to +40 mV • How does this depolarization occur? o Dendrites or cell body stimulated by axons from other neurons  Causes graded potentials o Graded Potentials  Small shifts in electric potential  Proportional to amount of incoming stimulation  If graded potential large enough to reach action potential threshold, the neuron discharges w/ an action potential o All-or-None Law  Action potential occurs w/ max intensity or not at all PSYO 1011 Neurons 3 • Ion Channels o Structures in cell membrane  Allows specific ions to enter/leave the cell o Vital to functioning of graded potentials Ion Channels & Action Potential: • Membrane reaches threshold for action potential o Specific ion channels activated  Only for a very small period of timey o Na+ shift from closed  open  Na+ attracted to negative electrical force w/i cell • Enter neuron via ion channel  Na+ brings +’ve charge into cell • Cell shifts from negative  positive o This complete action potential causes depolarization The Myelin Sheath: • Fatty, whitish insulation layer o Derived from glial cells during development • Nodes of Ranvier o Interruptions in myelin sheath  Where myelin extremely thin or absent • UnmyelinatedAxons o Action potential travels like a burning fuse • MyelinatedAxons o Electrical conduction skips from node to node o High conduction speeds Neuronal Signaling: • Neuron has 2 jobs o Transmit message along neuron  From dendrites  terminal o Pass message to next neuron  Across synapse • Currents are carried by ions o Electrochemical gradient driving force for ion movement Two Types of Signaling: • Postsynaptic Potentials (PSPs) o AKAgraded or receptor potentials o Starts at synapse or sensory receptor o Excitatory (EPSPs) or inhibitory (IPSPs)  Graded in size o Dissipate over time/distance o Influence probability of action potential PSYO 1011 Neurons 4 • Action Potentials o Starts at axon o One flavour, one size o Necessary for transmitter release Neuronal Signaling (Contd) • Postsynaptic membrane contains receptors o Transmitter will either fit or not fit receptor • Receptors coupled to ion channels o Excitatory = Na or Ca+  i.e.)+gluta-ate o Inhibitory = K or Cl  i.e.) GABA o Channels are ligand gated
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