Action PotentialThe All-or-none Principle

All-or-None

A threshold must be overcome (between -60 mV and -55 mV)

Will open voltage-regulated sodium channels

If less than -62 mV will produce a graded depolarization

The action potential is independent of the relative strength of the depolarizing stimulus

Generation of Action Potential

Generation of Action Potential

Depolarization to Threshold

Activation of Sodium Channels & Rapid Depolarization (this is positive feedback)

Inactivation of Sodium Channels & the Activation of Potassium Channels

Return to Normal Permeability

Action Potential Animations

http://outreach.mcb.harvard.edu/animations/actionpotential_short.swf

http://brainu.org/files/movies/action_potential_cartoon.swf

Propagation of Action Potentials

Continuous Propagation

An action potential is propagated (moved) along an unmyelinated axon

Action potential moves at the same speed along the entire length of the axon

Moves at about 1 meter/second

Similar to dominoes falling

Saltatory Propagation

Myelinated axons allows the action potential to travel faster

Action potential travels only along the nodes

Node to node – skips the internodes

Faster and uses less energy

General Properties

Electrical – the Presynaptic & Postsynaptic membranes are locked together at gap junctions, start action potential more quickly

Located int CNS & PNS but very rareFew areas of the brain, the eye & ciliary ganglia of the PNS

Chemical – Presynaptic & Postsynaptic membranes do not touch, most abundant type

Several types of neurotransmitters

Cholinergic Synapses

Release Acetylcholine (Ach)

Released at neuromuscular junctions

At many synapses in CNS

At all neuron-to-neuron synapses in PNS

At all neuromuscular and neuroglandular junctions within the parasympathetic division of the ANS

Other Neurotransmitters

Norepinephrine – in brain and ANS, has an excitatory, depolarizing effect on the postsynaptic membrane

Dreaming, learning, emotions, sleeping, controls body temp.

Dopamine – a CNS neurotransmitter, may be inhibitory or excitatory

inhibitory – imp. role in our precise control of movements

excitatory – cocaine inhibits the removal of dopamine from synapses in areas of the brain resulting in a “high”

Other Neurotransmitters

Serotonin – inadequate production can have widespread effects on a person’s attention & emotional state & may be responsible for many cases of severe chronic depression

Thought to be involved with regulation of sleep and wake patterns

Gamma aminobutyric (GABA) – inhibitory, in the CNS appears to reduce anxiety

Information Processing by Individual Neurons

Postsynaptic Potentials

Excitatory - a graded depolarization caused by the arrival of a neurotransmitter, affects only the area immediately surrounding the synapse

Inhibitory – a graded hyperpolarization of the postsynaptic membrane, a larger than usual stimuli would be needed to bring the membrane potential to threshold

Postsynaptic Potential

Summation – individual excitatory postsynaptic potentials combine which integrates the effects of all graded potentials

Facilitation – a neuron whose transmembrane potential shifts closer to threshold

the larger the degree of facilitation, the smaller is the additional stimulus needed to trigger an action potential