Summary
Highlights
The video introduces the concept of action potential (AP) in a single nerve fiber, explaining that it's an electrical message propagated along the fiber when stimulated. This message is recorded as an electrical signal on an oscilloscope, and its value can be measured with a voltmeter. The AP arises from the potential difference between the intra- and extracellular environments, originating in a specific region called the 'trigger zone' and propagating along the axon.
The standard experimental setup involves placing a giant axon from a marine animal (like a squid) in a physiological solution that mimics its natural environment. A stimulator with stimulating electrodes (S) applies an electrical stimulus to the axon. If the stimulus is sufficient, it generates an action potential that propagates along the axon. A receiving electrode (R) at the end of the axon detects this message and displays it on an oscilloscope.
The lecture then details the distinct phases of an action potential. The stimulation causes membrane destabilization and a change in its polarity. If the membrane potential reaches -50 mV (the depolarization threshold), an AP can propagate. Voltage-gated channels for Na+ and K+ exist in the membrane; Na+ channels open rapidly, leading to a massive influx of Na+ ions into the cell. This causes the potential to rise from -50 mV to +30-40 mV, a phase called depolarization. Following this, Na+ channels close, and slower K+ channels open, allowing K+ ions to exit the cell. This outflow of positive charges causes the potential to drop back towards -70 mV, known as repolarization.
Due to the slow closing of K+ channels, the membrane potential briefly drops below the resting potential, reaching -80 mV or -85 mV, a phase called hyperpolarization. Finally, the Na+/K+ pump restores the initial ion distribution by expelling Na+ and bringing in K+, returning the membrane to its resting potential of -70 mV. This entire process defines the phases of an action potential.
The video emphasizes the importance of understanding the graph of an action potential, which shows membrane potential (mV) over time (ms). It starts at the resting potential (-70 mV), rises to the depolarization threshold (-50 mV), then undergoes depolarization (up to +30-40 mV), followed by repolarization back to -70 mV, and then hyperpolarization before returning to the resting state. The action potential is defined as a modification of the electrical state of the nerve fiber's plasma membrane, always of constant amplitude (about 100 mV from -70 mV to +30 mV) and duration (about 2 milliseconds). A local, sub-threshold depolarization that does not reach -50 mV is called a stimulation artifact.