Action Potentials Department of Biology, WCU

Introduction Electricity Measured in mV Potential Current neutral, positive, negative Measured in mV Potential Current resistors (plasma membrane) Conductor (axon)

Plasma membrane Proteins as channels Passive vs active Protein gates Respond to: chemicals (synapse) Voltage (along an axon) Electrochemical gradient

Resting membrane potential Measurement -70 mV across plasma membrane

Resting membrane potential Due to diffusion gradient and voltage gradient

Membrane potentials Changes result of: Types of signals 1. Permeability of ions 2. Ion concentration Types of signals Graded potentials Action potentials

Membrane potentials Depolarization Hyperpolarization Inside of cell less negative than RMP Hyperpolarization Inside of cell more negative than RMP

Graded potentials Short distance, short lived Reason for “graded” name Types 1. Receptor potential (light) 2. Postsynaptic potential (EPSP or IPSP)

EPSP’s Influx of Na+ causing a depolarization Never reaches threshold

IPSP’s Increase in K+ or Cl- permeability No effect on Na+ permeability

Summation Neural integration Types temporal spatial

Action potentials Characteristics occurs along the axon stronger stimulus, “All or nothing” no decrease in strength unidirectional

Action potentials Phases 1. Resting membrane potential

Action potentials 2. Depolarization - increase in Na+ influx (5,000 x)

Action potentials 3. Repolarization decrease in Na+ influx increase in K+ efflux (10 x)

Action potentials 4. Hyperpolarization sluggish K+ gates potential restored by Na/K pump

Characteristics 1. Unidirectional AP’s are discrete events propagation of impulse

Propagation of impulse

Propagation of impulse

Absolute refractory period Voltage gated Na+ gates are open

Characteristics 2. Threshold stimulus stimuli must be 15 - 20mV

Characteristics 3. All or none phenomenon 4. Stimulus intensity it either occurs or it does not 4. Stimulus intensity firing at a set intensity (can increase)

Characteristics 5. Refractory period absolute (already discussed) relative period will only fire with a stronger stimulus

Relative refractory period Re-open Na+ gate with a stronger second stimulus

Summary Integration of potentials EPSP/IPSP’s GP’s GP’s AP’s AP’s