Initiation or generation of a nerve impulse
- 1. Initiation or Generation of a Nerve Impulse (Action Potential) At rest, the voltage-sensitive sodium and potassium gates in the channels in the plasma membrane of a nerve cell are nearly closed and the Na+/K+ pump moves 3 Na+ ions to the ECF and 2 K+ ions to the ICF. This contributes to the formation of a resting membrane potential of -70 millivolts (mV). Resting Membrane Voltage (RMV) Initiation of the Nerve Impulse Continued: The sodium/potassium pump also contributes to the resting membrane potential by pumping out three sodium ions to the ECF and pumping in two potassium ions to the ICF. The effect of this process is to make the outside of the nerve cell positive compared to the inside of the cell which becomes negative (-70 mV).
- 2. At rest, the ICF of an axon has a voltage of about -70 mV When the membrane of the axon is properly stimulated, Na+ ions begin to leak into the ICF. This causes the voltage to change to a less negative state. When ICF voltage reaches a threshold of about - 55 mV, sodium gates open. As sodium gates open, Na+ flow through sodium channels increases and quickly changes the voltage from a resting level of - 70 mV to + 30mV. This rapid shift from a negative to a positive state is called DEPOLARIZATION. At +30 mV, the sodium gates close. When the sodium gates close at +30 mV, the depolarization process stops. Interestingly, the + 30 mV condition causes the potassium gates to open and allows potassium to flow from the ICF to the ECF. The potassium flow quickly reverses the potential from + 30 mV to about - 70mV. This is called REPOLARIZATION. The rapid depolarization and repolarization process generates a a voltage pulse peak that is called the ACTION POTENTIAL or NERVE IMPULSE. The formation of a nerve impulse stimulates the formation of still another nerve impulse in the next section of the axon membrane
- 4. Overview of Transmission of Nerve Impulse Action potential → synaptic knob → opening of Ca+ channels → neurotransmitter vesicles fuse with membrane → release of neurotransmitter into synaptic cleft → binding of neurotransmitter to protein receptor molecules on receiving neuron membrane → opening of ion channels → triggering of new action potential Neurotransmitter is broken down by enzymes & ion channels close -- effect brief and precise How a synapse works