Biosc 0150 Foundations of Biology I Review 12 Chapter 45 46 Review Chapter 45 Electrical signals on animals 1 What ions are responsible for the depolarization phase The repolarization phase Which way the ions flow relative to the cell Na ions in depolarization K in repolarization Na flows into the cell for depolarization K flows out of the cell for repolarization a With regards of sodium and potassium channels explain why i Positive feedback occurs in the opening of Na channels Na channels open when the cell membrane depolarizes and as more channels open more Na flows into the cell further depolarizing the cell causing more voltage gated Na channels to open ii Na stops flowing across a membrane during an action potential After opening the voltage gated Na channels close and temporarily cannot reopen Also sodium reaches its equilibrium potential so not net force is available to drive Na movement iii K channels start opening K channels open in response to membrane depolarization b Explain why the action potential is an all or none phenomenon Once the threshold level of depolarization is attained the probability that the voltage gated sodium channels will open approaches P 100 But below threshold the massive opening of Na channels does not occur P 0 c Predict what would happen if sodium channels continued to open once membrane depolarization was complete Na would continue to move into the cell as voltage gated K channels opened and potassium began to diffuse out of the cell As a result repolarization would take much longer d Multiple Sclerosis is a disease resulting in damage to myelin sheath Explain why the disease leads to impaired electrical signaling Myelin sheath acts as a type of electrical insulator As charge spreads down an axon myelin sheath prevents ions from leaking back out across the plasma membrane of the neuron In myelin degenerates the transmission of electrical signals slows considerably 2 Why must there be an indirect mechanism that transmits electrical signals from cell to cell across their plasma membranes The cytoplasm of most neurons is not directly connected to the cytoplasm of other neurons a Explain the steps of synaptic transmission An action potential reaches the end of the axon and the depolarization made by the action potential opens voltage gated calcium channels near the synapse An electrochemical gradient is made favoring the inward flow of Ca2 through the channels The increased concentration of calcium inside the axon causes synaptic vesicles to fuse with the membrane and release neurotransmitters into the synaptic cleft Neurotransmitters bind to receptors on the postsynaptic cell leading to changes in the membrane potential which may cause another action potential The process ends as the presynaptic cell takes back the neurotransmitter b Suppose there was a drug that permanently closed the calcium channels in neurons What would be the result Though the depolarization from the action potential may set up an electro chemical gradient because the calcium channels are closed off synaptic vesicles will Adel Anthony Megan Dr Swigonova Page 1 of 5 Biosc 0150 Foundations of Biology I Review 12 Chapter 45 46 not be able to respond by binding to the membrane to release neurotransmitters effectively stopping any signals from occurring c Why do you think it is efficient for neurons to reuptake neurotransmitters rather than releasing them and creating new ones Reuptake is necessary because it allows for the recycling of neurotransmitters and regulates the level of neurotransmitter present in the synapse and controls how long a signal resulting from neurotransmitter release lasts Also neurons are constantly sending signals so if neurons had to keep making new neurotransmitters in order to send a signal it would be a lot harder and longer for signals to actually occur 3 Would you expect the neurotransmitter acetylcholine to bind to the same site on the receptor molecule as dopamine would Probably not because neurotransmitters are ligands that bind to specific sites on receptor molecules So acetylcholine has its own site as does dopamine a Explain what is meant by neurotransmitter signals being transduced from chemical to electric signals When a neurotransmitter binds to a ligand gated ion channel in the postsynaptic membrane the channel opens and admits a flow of ions along an electrochemical gradient so there is a change in the membrane potential of the post synaptic cell b Why might it be that the use of cocaine causes hyper state for the user Cocaine prevents the reuptake of dopamine so it is in the synapse between the pre and postsynaptic cell longer so it can bind to the postsynaptic membrane completely without being taken back again c What are EPSPs and IPSPs Excitatory postsynaptic potentials EPSPs are changes in the postsynaptic cell that make action potentials more likely to happen while inhibitory postsynaptic potentials are the opposite d Explain what is meant by summation in postsynaptic neurons EPSPs and IPSPs cancel each other out however if there are sever EPSPs occurring together they add up and make the neuron much more likely to fire and action potential e Ethanol was once used as an anesthetic before drugs such as morphine and novacane were invented Why is ethanol considered to be an anesthetic Ethanol mimics the response of GABA which is considered an IPSP which is inhibitory Because it is inhibitory if someone is in pain it would be able to prevent the brain from sending signals indicating pain thus it is a sedative 4 What would happen to the membrane potential 65 mV at resting under each of the following conditions a flooding the outside of the cell with Cl the membrane potential would be smaller because the charge on the outside of the membrane would become increasingly negative making the relative potentials between the inside and outside of the membrane closer in charge and decreasing membrane potential b flooding the outside of the cell with Na the membrane potential would be bigger because the charge on the outside of the membrane would become increasingly positive making the Adel Anthony Megan Dr Swigonova Page 2 of 5 Biosc 0150 Foundations of Biology I Review 12 Chapter 45 46 c relative potentials between the inside and outside of the membrane farther apart in charge and increasing membrane potential immediately after depolarization the membrane potential will be much higher as a result of the influx of Na ions allowing for a much