Q: When V r of central neurons equals E Cl , synaptic actions that increase g Cl

Question

Q: When Vr of central neurons equals ECl, synaptic actions that increase gCl do not change the postsynaptic membrane potential; that is, IIPSP = (gIPSP)(Vm–EIPSP) = 0 pA. Even though an increase in gCl does not cause an IPSP, it can have an inhibitory effect by increasing the level of excitatory input needed to depolarize Vm to threshold. Activation of an excitatory synapse while the Cl– channels are open causes an influx of Cl– due to an increase in the net driving force as Vm moves away from ECl.

a.     Two cells (A and B) synapse onto a motor neuron, as shown above. The synapse for cell A on the dendrite is excitatory and has an EEPSP of 0 mV. Cell B synapses on the soma near the trigger zone and opens chloride channels that cause an IPSP (EIPSP = ECl) in the motor neuron. Vr of the motor neuron is –65 mV. On the graph below, draw the change in membrane potential (Vm) you would expect to see when you activate: (1) cell A alone (use the post-synaptic potential shown as a reference); (2) cell B alone; and (3) cells A and B together. Consider three situations for which ECl is –65 mV, –70 mV or –90 mV. [2 pts]

b.     When ECl is –55 mV, would you expect to see any change in Vm when you stimulated cell B alone? If so, how and why? (Hint: How would the magnitude compare with the signals you drew above?) If not, why not? [0.5 pts]

c.     When ECl is –55 mV, would you expect to see any change in Vm when you stimulated cell B and cell A together? If so, how and why? (Hint: How would the magnitude compare to the signals you drew above?) If not, why not? [0.5 pts]

Explanation / Answer

Answers

a) 1. Stimulate cell A alone... There will be no change in graph because it is not in the trigger zone.

2. Stimulate cell B alone.... There will be more opening of Cl- channels and as a result more excitory potential will be needed to cross threshold potential and thus to generate action potential.

3 Stimulate cell A and B both.....Then also there will be more Cl- channel opens so one has to increase excitatory potential via cell A to cross threshold and to generate action potential.

b. If ECl is-55 mV, and when we stimulatescell B Alone, there will be easy for action potential to generate because of decrease in threshold of Cl- channel.

c). When ECl- is -55 mV, then mV will be-55 because excitatory potential is zero. So action potential will equals inhibitory potential which is -55 mV.

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