The switch closure occurs instantaneously and as a result of the instantaneous closure, instantaneous current flows through the circuit. At some point in time (time 0), the switch is closed. The recording electrode initially measures a potential of -60 mV (the resting potential). The figure at right represents an idealized nerve cell. An analogous situation occurs in nerve cells, when they receive an instantaneous stimulus. In fact, it would change as an exponential function of time. But the temperature will not change instantly. How would the temperature change? It will increase from its initial value of 10 oC to a final value of 100 oC. Place a block of metal at 10 oC on a hotplate at 100 oC. They are intrinsic properties of all biological membranes. They have nothing to do with any pumps or exchangers. Why are these called passive properties? They have nothing to do with any of the voltage-dependent conductances discussed earlier. Why do some axons propagate information very rapidly and others slowly? In order to understand how this process works, it is necessary to consider two so-called passive properties of membranes, the time constant and the space or length constant. In fact, the propagation velocity of the action potentials in nerves can vary from 100 meters per second (580 miles per hour) to less than a tenth of a meter per second (0.22 miles per hour). If a sufficient stimulus (heat) is delivered to the wire, the gunpowder will ignite, generate heat, and the heat will spread along the wire to adjacent regions and cause the gunpowder in the adjacent regions to ignite.Ī great variability is found in the velocity of the propagation of action potentials. You can think of an axon as a piece of wire coated with gunpowder (the gunpowder is analogous to the sodium channels). One way of viewing this process is with a thermal analogue. This charge distribution will then spread to the next region and initiate other "new" action potentials. If it depolarizes sufficiently, as it will, voltage-dependent sodium channels in the adjacent region of the membrane will be opened and a "new" action potential will be initiated. As the charge moves to the adjacent region of the membrane, the adjacent region of the membrane will depolarize. Unlike charges attract, so the positive charge will move to the adjacent region of the membrane. Its peak value now will be about +40 mV inside with respect to the outside. Consider for the moment "freezing" the action potential at its peak value.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |