Discussion for Week of 10-6

Biology 313: Discussion for Week of 10-6 22 points 1. (4pts) a. application of TTX b. Application of the scorpion toxin charybdotoxin, which blocks Ca++ activated K+ channels c. In a saline in which Ca++ is replaced by Mg++, so that [Ca++]out = 0, you pass enough current through your recording electrode to cross threshold d. In a saline where [Ca++]out is increased. 2. (3pts) As we discussed in class, axons that are myelinated have some physical features that are different from unmyelinated axons. In addition to the presence of myelin and its gaps at the nodes of Ranvier, the myelinated axon has a concentration of voltage gated Na+ channels at the nodes and very few of these between the nodes, under the myelin. However, the distribution of K+ leakage channels is fairly constant both under the myelin and at the nodes. In multiple sclerosis, the immune system for as-yet unknown reasons attacks and destroys myelin in the CNS. For action potentials along an axon, this leads to a slowing of propagation, or a complete failure of propagation so that a neural signal does not reach its destination. Based on the information above and what you have learned about myelin in class: a) Why are action potentials slowed when axons are demyelinated? Explain in terms of myelin’s effect on the time constant. b) Why would action potentials fail to propagate? Explain in terms of myelin’s effects on the length constant c) A new drug called Fampridine-SR is used to treat multiple sclerosis. This drug blocks K+ leakage channels. How would you expect it to help an axon that was demyelinated by multiple sclerosis? 3. (6pts) Heart muscle cells spontaneously fire action potentials with pacemaker properties, but are subject to neural control by the sympathetic and parasympathetic nervous systems. Heart muscle cells are also electrically coupled. For each of the following, describe an experimental manipulation and the predicted result that would show it is true: a. there is a chemical synapse between the vagus nerve and the heart muscle b. heart muscle cells are electrically coupled c. the heart cells’ pacemaker properties are due to a leak of Na+ into the cell 4. (9pts) You have identified a synapse between the H and the S neurons in the brain of the Binghamton Bare Cat, and have determined that the H neuron is presynaptic to the S neuron. You have tested a number of potential neurotransmitters and have found that application of Cationic acid to the S neuron mimics the effects of electrical stimulation of the H neuron. Using the patch clamp recording technique, you can have any concentrations of ions inside or outside the membrane of the recorded neuron you choose. The concentrations you have chosen are given below: ​ ​ ​ ​ inside​ ​ ​ + [K ]= 200mM​​ [Na+]= 40mM​​ [Cl-]= 13mM​ ​ outside [K+]= 10mM [Na+]= 400mM [Cl-]= 170mM a). By voltage clamping the S neuron to different Vm, you find that application of Cationic acid induces membrane responses with a reversal potential of about -3.37mV. Draw out what you would record in a voltage clamp when Vm is held at the -60mV, -20mV, -3.37mV, 0mV, 20mV, 50mV. and Cationic acid is applied (they don’t have to be to scale, and you can put them all on the same plot). Be sure to label your axes! b). Which 2 ions flow through this channel, assuming equal conductances to both (show your work)? c). Now determine the reversal potentials for the two cases where one of these ions was 4X more permeable through the Cationic acid gated channel than the other (do this twice; once for one ion being 4X more permeable than the other and again if it is the other ion). For each case, would Cationic acid produce an epsp or an ipsp? State your reasoning, assuming -20mV as threshold