The Action Potential

The Resting Potential of the Neuron

results from difference in ion distribution inside and outside of cell (-70mV)

Forces Behind Resting Potential

Selective Permeability- some molecules pass through membrane more freely than others; ion channels

Sodium-Potassium Pump- transports 3 Na out of, 2 K into cell

Result:Concentration GradientElectrical Gradient

Forces Behind Resting Potential

What would you do to generate electrical signal fast?

RP takes work (Na/K Pump). Why do we bother?

Molecular Basis of Action Potential

Sodium channels open once threshold is reached, influx of sodium

Potassium channels open at AP peak; potassium flows out

http://faculty.washington.edu/chudler/ap.html http://www.blackwellscience.com/matthews/channel.html

The All-or-None Law

The size, amplitude, and velocity of an action potential are independent of the intensity of the stimulus that initiated it.

How then is stimulus intensity coded?

Figure 2.20  Saltatory conduction in a myelinated axonAn action potential at the node triggers flow of current to the next

node, where the membrane regenerates the action potential.

The Action Potential

The RefractoryPeriod

Absolute Refractory Period

Sodium gates are firmly closed

Membrane cannot produce AP, regardless of the stimulation.

Relative Refractory Period

Sodium gates in usual state, but the potassium gates remain open.

Stronger than normal stimulus needed for action potential.

Propagation of the Action Potential

From: Axon Hillock

To: Terminal Buttons

AP travels in one direction only. WHY?

The Myelin Sheath and Saltatory Conduction

Myelin Sheaths increase the speed of neural transmission

Nodes of Ranvier-Short area’s of the axon that are unmyelinated

Saltatory Conduction-jumping action of actions potentials from node of Ranvier to node of Ranvier

Signaling Without Action Potentials

Depolarizations and hyperpolarizations of dendrites and cell bodiesSmall Local neurons-produce graded potentials (membrane potentials that vary in magnitude and do not follow the all-or-none law)