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PS263 - Ch 4 Textbook.docx

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Todd Ferretti

PS263 – Neural Conduction & Synaptic Transmission Resting Membrane Potential  Membrane Potential: The difference in electrical charge between the inside and the outside of the cell.  Recording M.P: Position the tip of one electrode inside the neuron & the tip of another outside the extracellular fluid.  Microelectrodes: (intracellular) their tips are less than one-thousandth of a mm in diameter – much to small to be seen with the naked eye.  Resting Potential: The steady membrane potential of about -70mV – with this charge built up across its membrane, a neuron is said to be polarized.  Ion Basis: The salts in neural tissue separate into + and – charged particles called ions – resting potential results from the fact that the ratio of negative to positive charges is greater inside the neuron then outside  Random Motion: (homogenizing) Ions in normal tissue are in constant random motion, particles in random motion tend to become evenly distributed because they are more likely to move down their C.G than up.  Electrostatic Pressure: (homogenizing) any accumulation of charges + or – in one area tends to be dispersed by the repulsion among the like charges in the vicinity and the attraction of opposite charges concentrated elsewhere.  Sodium Potassium Pumps: The transport of Na+ ions out of neurons & the transport of them are not independent processes. Performed by energy- consuming mechanisms in the cell membrane that continually changes 3 Na+ ions inside two K+ ions outside.  Transporters: Mechanisms in the membrane of a cell that actively transport ions or molecules across the membrane. Conduction of Postsynaptic Potentials  When neurons fire they release neurotransmitters from their terminal buttons which diffuse across the synaptic clefts & interact with specialized receptor molecules on receptive membranes of the next neurons in circuit.  Depolarize: NT’s may decrease the resting membrane potential from -70 to -67 mV; Excitatory Postsynaptic Potentials (EPSPs): they increase the likelihood that the neuron will fire. (travel passively)  Hyperpolarize: It increases the resting membrane potential from -70 to -72 mV; Inhibitory Postsynaptic Potentials (IPSPs): decrease the likelihood that the neuron will fire. (travel passively)  Both potentials are graded responses: the amplitudes are proportional to the intensity of the signals that elicit them: Weak signals elicit small & vice. 1. Rapid – instantaneous for most purposes. 2. Decremental – IPSPs & EPSPs decrease in amplitude as they travel through the neuron – most do not travel more than a couple of mm Postsynaptic Potentials & Action Potentials  Threshold of Excitation: If the sum of depolarizations & hyperpolarizations reaching the section of the axon adjacent to the axon hillock at any time is sufficient to depolarize the membrane to a level.  Axon Hillock: Conical structure at the junction between the cell body & the axon) PS263 – Neural Conduction & Synaptic Transmission  Action Potential: Massive, but momentary (1 millisecond) reversal of the membrane potential from -70 to about +50mV.  All-or-None Responses: Can either occur to their full extent or not at all.  Integration: Adding or combining a number of individual signals into one overall signal. Conduction of Action Potentials  Voltage-Activated Ion Channels: Ion channels that open/close in response to change in the level of membrane potential.  Absolute Refractory Period: Brief period of about 1 to 2 milliseconds after the initiation of an action potential during which it is impossible to elicit a second one.  Relative Refractory Period: Period during which it is possible to fire the neuron again but only applying higher-than-normal levels of stimulation.  Refractory Period is responsible for the fact that A.P.’s normally travel along axons in only one direction because the portions of an axon over which an A.P. has just travelled are left momentarily  First, the conduction of A.P along an axon is nondecremental; A.P do not grow weaker as they travel along the axonal membrane.  Second, A.P are conducted more slowly than postsynaptic potentials – these 2 things occur because conduction of EPSPs and IPSPs are passive, whereas the axonal conduction of A.P is largely active.  A.P travels passively along axonal membrane to the adjacent voltage- activated sodium channels, which are yet to open. The arrival opens these channels allowing Na+ to rush into a neuron and generation full A.P.  Antidromic Conduction: If electrical stimulation of sufficient intensity is applied to the terminal end of an axon, an A.P will be generated and travel along the axon back to the cell body.  Orthodromic Conduction: Axonal conduction in the natural direction from cell body to terminal buttons – triggers exocytosis.  Nodes of Ranvier: Gaps between adjacent myelin segments – ions can pass through the axonal membrane only here.  Saltatory Conduction: the transmission of action potentials in myelinated axons. * At what speed are action potentials conducted along an axon? 1) Conduction is faster in large-diameter axons and faster for myelinated. Mammalian motor neurons (that synapse on skeletal muscles) are large & myelinated thus some can conduct at speeds of 100 meters per second where small one conduct at 1 m/sec. - the maximum velocity of conduction in human motor neurons is about 60 meters per second.  Interneurons: conductions in these are typically passive and decremental.  Hodgkin-Huxley Model in Perspective – 1950s based on the study of squid motor neurons, large and easily accessible in PNS (these neurons are LARGE) Due to the simplicity of these neurons it is hard to apply to the mammalian brain. Properties of c
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