Question

Animal Physiology Question

1. Chemical messages bind to 10 ligand-gated ion channels on the dendrites and cell body of a motor neuron. At the axon hillock, 5 of these ligands produce excitatory graded potentials (i.e. depolarizing, called EPSPs) of 5mV each and the other 5 produce inhibitory graded potentials (i.e. hyperpolarizing, called IPSPs) of 2mV each. The threshold potential of this neuron is -55mV (resting membrane potential is -70mV). Will an action potential be produced? Assume that all impulses get to axon hillock at the same time and there is no decay in signal. To get full points you need to show all the math to justify your answer.

Answer 1

Given,

resting membrane potential (RMP) = -70 mV

threshold potential of neuron (TMP) = -55 mV

So, minimum EPSP required to produce action potential = RMP - TMP = 70 - 55 = 15 mV

Now,

number of ligands that produce EPSP = 5

EPSP of each excitatory ligand = 5 mV

So, total excitatory grade potential produced (GP +ve) = 5 X 5 = 25 mV

Also,

number of ligands that produce IPSP = 5

IPSP of each inhibitory ligand = 2 mV

So, total inhibitory grade potential produced (GP -ve) = 5 X 2 = 10 mV

Hence, net excitatory grade potential produced = (GP +ve) - (GP -ve) = 25 - 10 = 15 mV

Therefore, we can see that net excitatory grade potential produced is equal to the minimum EPSP required to produce an action potential. Hence, action potential will be produced.