Inside the neuron than outside concentration

This preview shows page 210 - 211 out of 406 pages.

inside the neuron than outside. Concentration gradients in the body often involve a difference in concentration of sodium and potassium. The Action Potential is simply a shift in the neuron’s membrane potential (from negative to positive) that occurs when ions start flowing in or out of the cell through “gates,” known as ion channels. The gates depend on voltage, and will open and close accordingly in response to the potential of the cell. Before any signal is received, the cell sits at a constant potential called the Resting Potential (experimentally measured to be -70 mV). Under typical situations, the Action Potential propagates as follows: 1. A signal is received from a cell, and the cell becomes depolarized, meaning positive charge flows from the outside of the cell to the inside of the cell. Once a certain potential is reached (experimentally measured to be -55mV), an Action Potential will begin to fire, and a gate will activate letting a larger amount of positive charge flow in (Sodium) 2. When another positive threshold is reached, the first gate is closed and another gate will open causing positive charge (Potassium) to flow out of the cell in a process called polarization 3. After another threshold, the final gate will close and the cell will regress to its Resting Potential Again It is important to note that the Action Potential only fires when the specific thresholds are met, known as the "All or Nothing" principle. Resource: Hodgkin and Huxley Alan Hodgkin and Andrew Huxley, two British born scientists, au- thored a series of five papers describing nonlinear ordinary differential equations that model how action potentials can be initiated and propagated through an axon. The papers detailed their research into the squid giant axon, which has an abnormally large axon (1mm in diameter) big enough to conduct experiments on. This ground-breaking work has a wide range of applications on many organisms and won Hodgkin and Huxley the Nobel Prize in Physiology in 1963. Resource: 1.0.3 Setup In order to describe the way Action Potentials propagate, Hodgkin and Huxley related the mem- brane of the neuron to a circuit (Figure 2). The membrane has a capacitance C m , and accounts for all of the gates that were described previously. The Sodium and Potassium gates are voltage dependent, and are represented in terms of their conductances. To find the total current of ions flowing from inside to outside as a function of time, Kirchhoff’s rules and the Ohm’s Law were used. The capacitance of the membrane is a constant, but by virtue of ions moving, the voltage is not. Using the relationship of capacitance and charge, the membrane current can be solved as such: Q = C m V m dQ dt = C m dV dt Where V m is the intracellular voltage For the gates, Ohm’s Law describes how the conductance (which is 1 R ) and voltage relate to

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture