Introduction – Threshold is the membrane potential at which an action potential is generated

• Channel opening and closing Þ D E_m Þ channel opening and closing Þ D E_m
• Parallel Batteries Model = E_M dependent G_Na/G_K ratio
• Need to consider Capacitance = amount of charge stored per unit D V across the membrane
• At given moment E_M = stored charge on membrane; V_m = Q/C Q = charge C = membrane capacitance (same for all membranes)
• Capacitative current = movement of +charge onto the inside membrane surface with corresponding movement of positive charge off the outside surface
• therefore Ý during action potential
• I_k =G_k(V_M-E_K) I_k= open K channels, (V_M-E_k) = Driving Force on K ions

• Depolarization Þ (1) Capacitive current (2) Ionic Current
• at T_o, E_m = E_r Þ I = Capacitative current generates initial depolarization circuit – No ionic current
• at T_f, E_m = E_finalÞ I = Ionic current = depolarizing current – No capacitance current
• D t Þ D V Þ Ý driving force Þ Ý Ionic current
• Note: Time constant = the time required for the capacitor to reach 2/3 of steady state = RC; this is constant
• However- V=IR(1-e^-t/RC) therefore V/Dt is an exponential relationship and DV/Dt=the time constant = t
• The time constant is the rate of change of the E_m during depolarization (slope of voltage curve fig 4)
• t µ C therefore Early depolarization, Ý C = Ý t ; near steady state, ß C =ß t ; t = 0Þ constant V
• Another passive property is K leakage during E₀

• Note that the subthreshold stimuli depolarizes at a constant rate – then passively returns to rest
• Just near threshold but not at, there is a small active response
• At threshold there is an action potential
• More intense stimuli can cause faster action potential with Ý amplitude due to greater recruiting of Na and K channels
• There is a trade off between strength and duration of threshold: they are inversely related
• Rheobase is the minimum current necessary to reach threshold before steady state

• Subthreshold potentials actually decrement in all but smaller axons. Passive movement of D V = electrotonic potentials
• G_M= summed conductance of leakage channels; G_I= conductance in cytoplasm > G_I Þ > the voltage spread
• E_m decays exponentially from point of stimuli. Length constant = (G_I/G_M)^1/2 usually 1mm or less.
• G_I/G_M µ r^1/2 so as axon radius increases, G_I increases more than G_M
• Passive spread important to dendrite – synapse close to cell body more effective

• Action potentials travel along the cell membrane without decrease in amplitude = Propagation
• Propagation and Conduction refer to action potential only; Transmission only refers to synapses
• Mechanism of Propagation
• Open Na Channels at site of action potential inject current into axon Þ Current spreads
• Some of current = Capacitative Þ depolarization of adjacent regions Þ reaches threshold Þ action potential
• action potential travels in only one direction due to absolute refractory period behind the action potential
• Conduction in the normal direction (body Þ synapse) = orthodromic; reverse direction = antidromic
• action potential travels with a finite velocity. Large length constants Þ Ý conduction velocity because ß leak Ý spread
• Therefore large axons conduct more rapidly because length constant µ r^1/2
• How to Ý conduction velocity?
• Mylenation by glial cells acts as insulationÞ ß leakÞ Ý effective length constantÞ Ý conduction velocity
• Nodes of Ranvier contain high-density Na channels