THE ACTION POTENTIAL 30S Biology Crash course part 2 ELECTRICITY In response to a stimulus, an electrical impulse is fired down a neuron to the next. 1 strength, but frequency varies. The nerve impulse is called the Action Potential The body is electrically neutral (positive = negative) Some parts are more positive or more negative Membranes are used to keep the negatives and positives separate b/c they attract. We keep them separate to build Action Potential (potential energy) ELECTRICITY Each neuron is like a little battery with the potential to do work. When needed, the electricity is allowed to flow (in response to a stimulus) In a cell, we call this difference between the negative and positive a Membrane Potential. THE RESTING NEURON More negative inside, positive outside The difference is known as the Resting Membrane Potential Always -70mV Na+ ions outside, K+ ions inside, but bigger negative proteins inside. Overall negative charge (-70mV) Neuron has a negative membrane potential - polarized. SODIUM POTASSIUM PUMP The organization of charges is done by a protein called the sodium potassium pump. They are lots located in the axon membrane. Pump in 2 potassium and lets out 3 sodium This creates the difference in charges called the Electrochemical Gradient. NATURE LIKES BALANCE The neurons are always trying to restore balance Ion channel proteins in the membrane help transport positive and negative ions across to balance out the charges. It doesn’t like be depolarized. There are a few different kinds The movement of ions is key to all electrical events in the neurons. SENDING SIGNALS Graded potential: small impulses that cause only a few channel proteins to open. Causes small change in membrane potential. In order to send longer signals, a larger change is needed called an axon potential. We need to depolarize a resting neuron causing a large change in the membrane potential. ACTION POTENTIAL THRESHOLD At resting state, all the proteins are sitting closed at -70mV. THEN! A stimulus occurs! The stimulus and resulting change have to be strong enough for the true action potential to kick in Threshold = -55mV This is an all-or-nothing event To get to threshold, only the Na-K pumps open. Only the -55mV is reached, the other proteins open, allowing many more Na+ ion to flow in. This makes the whole thing very depolarized to the point where it is positive (+40mV) REPOLARIZATION Once the charge has passed, repolarization begins. Potassium channels open allowing potassium ions to flow out which rebalances the charges. Initially the repolarization goes to far, to -75mV at which point it is hyperpolarized. The Na-K channels then open once again to balance everything – bring it back to rest.(-70mV) When all of this is happening, the neuron cannot respond to any other stimulus. Called the Refractory Period It prevents signals from traveling in both direction at the same time. CONDUCTION VELOCITY The threshold of the action potential does not change. What does change is the frequency of the signals. Weak stimulus triggers less frequent action potentials Stronger signals trigger more frequent action potentials. Conduction Velocity: The speed of axon potential Reflexes need fast AP Intestines and blood vessels have slow AP MYELIN VS. NON-MYELIN NEURONS Myelin axons conduct signals faster then nonmyelin axons. The myelin allows the current to leap from one gap to the next (Nodes of Ranvier). Called Saltatory Conduction.