Consider a “typical” motor neuron with a resting membrane potential of -70mV and
ID: 300097 • Letter: C
Question
Consider a “typical” motor neuron with a resting membrane potential of -70mV and a threshold potential of -55mV. This neuron receives two signals, X and Y, at the same time. Signal X arrives at the far end of a dendrite, and causes ten neurotransmittergated K+ channels to open. Signal Y arrives at the cell body and causes three neurotransmitter-gated Na+ channels to open. a) How will signal X affect the membrane potential at its reception site? Explain b) How will signal Y affect the membrane potential at its reception site? Explain c) Both of these signals arrive at the axon hillock at exactly the same time. Will the neuron fire an action potential? Yes, no, or maybe? Explain your answer [Assume that the duration of the open state of these Na Voltage gated channels and K+ voltage gated channels is the same; assume that the flow of ions (#ions crossing per sec) in a Na+ voltage gated channel is identical to that of a K+ voltage gated channel).
Explanation / Answer
a)Signal X at the the far end of a dendrite will result in repolarisation.Since K+ channels open, potassium ions will come out of the cell causing repolarisation and the membrane potential will be back to -70mV.
b)Signal Y at its reception site will result in depolarisation.Opening of the sodium gated channels will allow more Na+ ions to move into the cell.The inside of the cell will become more positive and thus depolarised.
c) No, this will not fire an action potential when both the signals reach the axon hillock at the same time.An action potential first results in depolarisation followed by repolarisation and bak to resting potential.When both the Na+ and K+ channels remain open for the same duration with identical flow of ions,the process of firing of action potential will be hindered.