Action Potential PDF

Nervous System Nerve Impulses Conduction of Nerve Impulses (Electrical Event) A nerve impulse or action potential has both a chemical and electrical component (hence the term electrochemical impulse) Four stages of a nerve impulse:...

Nervous System Nerve Impulses Conduction of Nerve Impulses (Electrical Event) A nerve impulse or action potential has both a chemical and electrical component (hence the term electrochemical impulse) Four stages of a nerve impulse: 1) Polarized/resting state 2) Depolarization 3) Repolarization 4) Refractory Period 1) Resting State The difference in charge across the membrane of a resting neuron is called resting membrane potential. The inside of a neuron has a slight negative charge at rest, whereas the outside has a slight positive charge This results in a resting potential of -70 mV Resting State & the Sodium-Potassium Pump i. Inside the resting cell, there exists a higher concentration of potassium (K+) than sodium (Na+); outside the cell, the opposite is true ii. Potassium ions are able to passively diffuse out of the cell more easily than sodium ions can diffuse into the cell to counteract this concentration gradient (K+ = “leaky” ion channels) Resting State & the Sodium-Potassium Pump 3. In an attempt to balance this difference, the sodium-potassium pump along the membrane surface will exchange three sodium ions from inside the cell for two potassium ions from outside the cell 4. As a result, as excess of positive charge accumulates outside of the cell membrane. A constant membrane potential of -70 mV is maintained 2) Depolarization Action potentials occur when a neuron is stimulated by an electrical impulse. a. An impulse causes sodium gates to fully open, thus allowing Na+ to diffuse freely across the membrane b. Na+ rushes into the cell (down their concentration gradient), leading to a slight positive charge on the inside relative to the outside 2) Depolarization c. This reverses the membrane potential from -70 mV to +40 mV; the membrane is now said to be depolarized d. Once Na+ reaches equilibrium, the membrane once again becomes impermeable to them, and the gates close Depolarization & Threshold Potential An action potential is considered to be an “all-or-none” event because any stimulus that fails to achieve a membrane potential of at least -55 mV will have no effect Note that increasing the stimulus strength does not increase the impulse strength; a neuron will either fire or not fire The intensity of a stimulus is instead experienced as an increased frequency (number) of nerve impulses  Stimuli Stronger depolarizing stimulus +50 +50 Action Membrane potential (mV) Membrane potential (mV) potential 0 0 Threshold –50 –50 Threshold Resting Resting Depolarizations potential potential –100 –100 0 1 2 3 4 5 6 0 1 2 3 4 5 Time (msec) Time (msec) (c) Action potential triggered by a depolarization that reaches the threshold. 3) Repolarization Once an action potential has peaked, Na+ gates close, and K+ gates open so K+ rushes out of the axon. This restores the positive charge outside the membrane; however, Na+ and K+ concentrations are briefly reversed compared to at the resting state (Na+ is higher inside the membrane) The sodium-potassium pumps kicks in and exchanges Na+ for K+, restoring the initial resting potential of -70 mV. This process is called repolarization. Occurs in a wavelike motion down an axon  4) Refractory Period (Hyperpolarization) The sodium-potassium pump often overshoots -70 mV and the cell becomes hyperpolarized (-90 mV). Until the resting potential of the neuron has been properly restored, a second action potential cannot be conducted along the axon; this is referred to as the refractory period During this time, the membrane cannot be made permeable to Na+, so a second wave of depolarization cannot occur The stronger the impulse, the longer it takes for the nerve to recover Re-cap: Action Potentials