Action Potential Action Potential The 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 Like the pressure on the trigger of a gun: slight pressure gun does not fire, more pressure = gun fires

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

Synaptic Activity

General Properties Electrical – the Presynaptic & Postsynaptic membranes are locked together at gap junctions, start action potential more quickly Located int CNS & PNS but very rare Few 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