Assume the below for the following question if value not asked for:
Vm = -60 mV Erev = 40 mV ECl = -80 mV
ENa = +40 mV Ek = -70 mV Threshold = -20 mV
2. If the ERev of the channel is determined by Na+ and K+, and its conductivity for Na+ and K+ are equal, i.e., gna = gk for that channel, can you determine ERev? Why or why not? What is ERev for this channel?
Purpose: This question is designed to get students to understand that ERev is governed by the equilibrium potentials of the ions which it is permeable/conductive to.
Answer: ERev is determined by Na+ and K+ in this example. If the conductances are equal, then ERev is in the middle of ENa & EK . If you do the calculations, based on the equilibrium potentials provided above… ENa & EK = 40 + – 70 / 2= -15 mV.
3. An unknown receptor has been identified which binds GABA. It has an Erev of +40 mV. If I apply GABA to this unknown receptor, is the post-synaptic potential (PSP) excitatory or inhibitory? Is the effect a depolarization or a hyperpolarization? Is the current inward or outward? Be ready to explain.
Purpose: This question is designed to get students to dissociate the neurotransmitter from the receptor function. GABA is typically associated with hyperpolarization and inhibition, but it is not always the case. The students need to test their understanding of what governs excitation/inhibition and depolarization/hyperpolarization.
First, calculate current. Using ERev is +40 mV, we can calculate driving force. Vm is given as -60 mV. I = (g)(Vm – ERev) = -60 mV – +40 mV = -100. Remember, we make the assumption that g is 1 (perfectly conductive for that channel), so we can calculate out the current direction and whether its positive/negative. If driving force is -100, this means that the current is negative, and thus inward. Once again, to reiterate the point, we are discussing conventional current flow, which is defined by the flow of + ions.
Second, use current to determine if it is depolarizing or hyperpolarizing. If current is negative and inward, this is making the inside of the neuron more positive, as the GABA bound channel is trying to bring the neuron to its ERev at +40 mV. While the channel is open due to GABA binding, the neuron is pulled towards ERev.
Third, use threshold and compare it to ERev to determine if it is excitatory or inhibitory. If ERev < threshold, it is inhibitory. If ERev > threshold, it is excitatory. In our case, our ERev is +40 mV, and threshold is -20 mV. ERev > threshold, thus excitatory.
4. If the ERev is instead -30 mV, and I apply GABA, is the post-synaptic potential (PSP) excitatory or inhibitory? Is the current inward or outward? Is the effect a depolarization or a hyperpolarization? Be ready to explain.
Purpose: The goal of this is to get students to dissociate depolarization with excitation… and make them understand instances in which it may be inhibitory.
First, calculate current. Using Erev at -30 mV, we calculate driving force. Vm is -60 mV as a given. -60 – – 30 = -60 + 30 = -30 mV. Current is negative, thus inward.
Second, determine depolarization/hyperpolarization. Because it is inward and negative current, it is a depolarization.
Third, determine excitation/inhibition. Once again, use ERev < threshold = inhibition, ERev > threshold = excitation. In this case, ERev is -30 mV, threshold is -20 mV. In this case, ERev < threshold, thus inhibitory. Do not get confused by depolarization/hyperpolarization and make the mistake of absolute association with depolarization = excitation / hyperpolarization = inhibition. There are instances, such as in this example, where depolarization leads to inhibition, because the channel prevents the neuron from ever getting to threshold… If the membrane potential ever gets more positive than ERev, the current will turn positive, with an outward flow of positive ions to try to bring the membrane potential back to ERev. For example, if the membrane potential somehow gets to -25 mV, -25 mV – – 30 = +5… thus current would be outward, positive ions flowing out, making the cell more negative again… and most importantly, pulling it AWAY from threshold.
5. If I patch clamp the channel at +40 mV, then apply GABA to that channel 3 seconds after the patch clamp for an additional 5 seconds while under patch clamp (seconds time scale), what would the current trace look like? Assume Erev is the same as in #5. Draw 10 seconds worth of recording, starting immediately after the patch clamp.
Purpose: The purpose of this question is 2-fold. First, to get them to understand that without ligand binding, the function of the receptor/channel is non-existent,because it is not open. Secondly, to get them to understand the relationship between current and voltage/channels.