Graded Potentials & Action Potentials Excitatory Post Synaptic Potential (EPSP) -it results from the opening of ligand-gated Na + channels -some positive Na + ions come into the cell making the cytosol slightly more positive (depolarized) -it brings the membrane potential closer to threshold (-55mV)

Inhibitory Post Synaptic Potential (IPSP) -it results from the opening of ligand-gated Cl - or K + channels -Some positive K + ions leak out or some negative Cl - ions leak in making the cytosol more negative (hyperpolarized) -it takes the membrane potential further away from threshold (-55mV)

Typical Neuron = EPSP + IPSP (Graded Potential Summation at Axon Hillock) Axon Hillock

Na + K+K+ Na + /K + ATPase Threshold! Voltage-Gated Sodium Channel Voltage-Gated Potassium Channel OPEN OPENING - SLOWLY Na + K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ FINALLY ssOPEN Na + K+K+ K+K+ K+K+ ATP ADP + P-P- Na + K+K+ K+K+ DepolarizationRepolarization Hyperpolarization Return to RMP (-70mV) Outside Neuron - Extracellular Fluid Inside Neuron - Cytosol

Resting Membrane Potential

Outside of Neuron – Extra Cellular Fluid (ECF) Inside of Neuron - Cytosol Proteins (-) Resting Membrane Potential = The difference in the distribution of charged particles (ions) between the inside and the outside of the cell (mV) In neurons, the resting membrane potential is normally around -70mV - this means that the inside of the neuron is more negative relative to the outside of the neuron. Plasma Membrane Sodium Na + Potassium K+K+ Chloride Cl - Phosphate P-P-

What factors contribute to the resting membrane potential? 1.Unequal distribution of ions 2.Leakage channels and the inability of most anions to leave the cell 3.Na + /K + ATPases

Plasma Membrane 1. Unequal distribution of ions: Outside of Neuron Inside of Neuron The Extracellular fluid (ECF) is naturally rich in Na + and Cl - The cytosol (inside the cell) is rich in K + and anions (negatively charged molecules)

Outside of Neuron Inside of Neuron Plasma Membrane Na + K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ Cl - P-P- P-P- P-P- P-P- P-P- P-P- Proteins (-) This unequal distribution of ions and large, negatively charged molecules naturally makes the inside of the neuron more negative relative to the outside

Plasma Membrane 2. Leakage Channels: passive diffusion Outside of Neuron Inside of Neuron K+K+ Na + K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ Proteins (-) Since there are more Potassium (K + ) channels leaking positive ions out then there are Sodium (Na + ) channels leaking positive ions in, and because the large negatively charged molecules can’t escape, this helps to keep the inside of the neuron more negative relative to the outside.

3. Na + /K + ATPases Outside of Neuron Inside of Neuron Na + /K + ATPase Na + K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ ATP ADP + Na + K+K+ K+K+ ATP ADP + P-P- K+K+ K+K+ P-P-

These three factors: 1.Unequal distribution of ions 2.Leakage channels and the inability of most anions to leave the cell 3.Na + /K + ATPases combine to maintain the neuron’s resting membrane potential at -70 mV, where the inside is negative relative to the outside.

Chemical Synapse

Calcium Ions (Ca 2+) Action potential arrives at terminal end bulb. Voltage-gated Calcium channels open and Ca 2+ rushes in. Post synaptic neuron Sodium Ions (Na +) Synaptic vesicles migrate and fuse with membrane – releasing ACh into synaptic cleft. Acetylcholine (Ach)

Synaptic vesicles migrate and fuse with membrane – releasing ACh into synaptic cleft. Action potential arrives at terminal end bulb. Voltage-gated Calcium channels open and Ca 2+ rushes in. Post synaptic Neuron ACh binds to ligand-gated Na + channels allowing Na + to enter the post synaptic neuron. Calcium Ions (Ca 2+) Sodium Ions (Na +) Acetylcholine (Ach) Graded Potential This generates a graded potential, which travels down the dendrite.