Worksheet Action Potential

Biopsychology Worksheet 1.3 – Action Potential 1. Name the ions involved in resting potential and state where they are mostly located during resting potential. 1) Sodium (Na+) outside the axon 2) Potassium (K+) inside the axon 3) Chloride (Cl-) outside the axon 4) Large protein molecules (A-) inside the axon 2. The following describes how an action potential occurs. Fill in the blanks. 1) The neuron’s voltage is resting at -70 mV. 2) The neuron is stimulated in some way. 3) Some chemically gated sodium (Na+) channels open around the axon hillock of the neuron. 4) The Na+ travels into the cell through the open channels. (This happens because there are two forces acting on Na+: diffusion and electrostatic pressure/concentration gradient.) 5) The incoming Na+ causes the cell voltage to change and the cell’s potential becomes positive. 6) The voltage of the cell becomes more positive as Na+ comes in. If enough Na+ comes in and the cell voltage reaches -55 to -65mV, the voltage-gated Na+ ion channels will open, allowing more Na+ to rush into the cell. 7) This makes the cell become even more positive. 8) Once the cell reaches a voltage of +30 mV, the voltage-gated Na+ channels all close, preventing any more Na+ from coming into the cell. ***TEST QUESTION*** 9) Meanwhile, a few potassium (K+) channels have opened, allowing K+ to leave the cell. 10) The sodium-potassium pump tries to restore balance by pumping Na+ out and K+ in the cell, but K+ is still leaving through the open K+ channels. 11) Because these K+ channels are slow to close, the voltage inside the cell dips below the resting potential voltage (-70mV) in a phenomenon called hyperpolarization, which causes a refractory period (-90 mV). 12) When all of the K+ channels finally close, the cell returns to resting potential. 3. Graph the voltage of a neuron as it goes through the process described above. What event occurs at the peak, when the voltage drastically decreases? At the peak (+30 mV), the sodium channels close, and the potassium channels open. Resting potential (-70 mV) 4. What would happen to the voltage of a neuron if Cl- were let into the cell instead of Na+? Why? The voltage of the cell would be more negative (hyperpolarize) because the protein and chloride are negatively charged, so they can’t “fire” and communicate with each other. 5. Explain the difference between an Excitatory Postsynaptic Potential (EPSP) and an Inhibitory Postsynaptic Potential. What causes each? EPSP: sends positive signal, exciting next neuron; depolarization caused by influx of sodium, making the neuron more likely to produce an action potential Ex: ion X+  EPSP; if K+ or Na+ is there, it will excite the next neuron IPSP: sends a negative signal, inhibiting the next neuron; hyperpolarization caused by efflux of potassium, making the neuron less likely to produce an action potential Ex: ion Y-  IPSP; if Cl- is there, it will inhibit the next neuron 6. Explain what happens when an action potential reaches the end foot. 1. At the terminal buttons, calcium channels open and calcium enters the end feet 2. Calcium binds to calmodulin, called the calcium-calmodulin complex. 3. The calcium-calmodulin complex binds to vesicle and pushes it towards the end foot, and the vesicle/end foot membranes fuse together (exocytosis) 4. This releases neurotransmitters in the synapse, and this binds to the dendrites of the next neuron 7. Explain what each term means as it relates to neural communication. Ions: potassium & sodium ions drive change in membrane potential which fuels neural communication via action potential Potential: electrical charge of the neuron Resting Potential: -70 mV; cell is at rest Action Potential: begins at -55 mV Depolarization: becomes more positive Hyperpolarization: become more negative Saltatory Conduction: charge leaps over myelin; increases speed of action potential Nodes of Ranvier: unmyelinated portion of the axon (found on PNS) Voltage-Gated Ion Channels: gated protein channel that opens and closes only at specific membrane voltages Absolute Refractory Period: state of an axon in the repolarizing period; no other action potentials can occur Relative Refractory Period: state of an axon in the later phase of action potential where a greater electrical stimulation can trigger another action potential