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Chapter 4

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University of Toronto St. George

Chapter 4: Neural Conduction and Synaptic Transmission  Resting Membrane Potential  Parkinson’s disease  Nerve cells in the substantia nigra are dying. These produce dopamine which is delivered to the striatum which helps control movement. Without dopamine, the striatum can’t do its job  Dopamine isn’t an effective treatment because it doesn’t readily penetrate the blood-brain barrier  However, L-dopa can do so and is turned to dopamine in the brain  Membrane potential = the difference in electrical charge between the inside and outside of a cell, which is -70mV  Factors  Na+  Na ions tend to be driven into neurons by the high concentration outside the membranes  The sodium-potassium pumps maintain a high Na concentration outside the membrane +  K  K ions tend to move out of the neuron because of high internal concentration -  Cl  There is little resistance in the neural membrane to the passage of Cl ions, so they are readily forced out due to negative internal potential  Once they accumulate outside the membrane, they tend to move down their concentration gradient back into the neuron  The Cl ion tendency to move out of the neuron is equal to that of moving in, so the distribution of Cl is kept at equilibrium  Generation and Conduction of Postsynaptic Potentials (both EPSPs and IPSPs are graded responses, meaning that they are proportional to the intensity of the signals that elicit them)  Depolarize = a decrease in the resting membrane potential (from -70mV to -67mV for example)  Called excitatory postsynaptic potentials (ESPs) because they increase the chances of neural firing  The threshold of excitation of action potential to depolarise is about -65mV  Hyperpolarize = an increase in the resting membrane potential (from -70mV to -72mV for example)  Called inhibitory postsynaptic potentials (ISPs) because they decrease the chances of neural firing  Both EPSP and IPSP signals travel rapidly and decrementally, the latter of which means that they decrease in amplitude as they travel. They don’t travel more than a few millimeters before dying out  Action Potentials  The action potential (AP) is massive but momentary and reverses the membrane potential from about -70mV to +50mV  APs are not graded responses, they are all-or-none responses  The threshold of excitation of action potential to depolarise is about -65mV  Integration: the integration of incoming signals over space and time by a neuron  Multipolar neurons add together all the graded excitatory and inhibitory postsynaptic potentials reaching its axon and decides to fire or not to fire on the basis of their sum  Spatial Summation  There are three possible combinations:  Multiple EPSPs are produced simultaneously at DIFFERENT parts of the receptive membrane to produce a greater EPSP  Multiple IPSPs are produced simultaneously at DIFFERENT parts of the receptive membrane to produce a greater IPSP  EPSPs and IPSPs are produced simultaneously at DIFFERENT parts of the receptive membrane to cancel each other out  Temporal Summation  Postsynaptic potentials produced in rapid succession at the SAME synapse sum to form a neuron of greater potential  The reason that stimulations of neurons can add together over time is that the postsynaptic potentials they produce often outlast them. Therefore, any overlap is superimposed and add  Since this happens so quickly, sometimes a neuron will just add the E/IPSPs together as one  All dendritic signals have a similar amplitude when they reach the cell body since those that originate in synapse far from the axon trigger zones have a mechanism to for amplifying the signals  Conduction of Action Potentials  Ionic Basis of Action Potentials  Voltage-activated ion channels (uses only ions close to the membrane)  They pump Na+ ions out because some of the ions pass through (though most can’t pass the membrane) but when the membrane potential reduces to the threshold of excitation, the ion channels open and Na+ pumps in  This makes the membrane potential go from the usual -70mV to +50mV  This causes the opening of the potassium channels, and K+ ions near the membrane are driven out through the channels and close after about 1 millisecond  Once the K+ channels close, repolarisation happens from the efflux of the K+ ions  After repolarisation, the potassium channels slowly close, so
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