Keywordsresting membrane potential, Na + /K + exchange pump, electrical and chemical gradient, voltage-gated Na + channels, voltage-gated K + channels, action potential, conduction velocity, refractory period
A Lipid/Protein Membrane Separates Intracellular and Extracellular Fluids, 38
The Resting Membrane Potential of Typical Neurons Is Heavily Influenced, but Not Completely Determined, by the Potassium Concentration Gradient, 38
Concentration Gradients Are Maintained by Membrane Proteins That Pump Ions, 39
Inputs to Neurons Cause Slow, Local Potential Changes, 40
Membrane Capacitance and Resistance Determine the Speed and Extent of the Response to a Current Pulse, 40
Action Potentials Convey Information Over Long Distances, 40
Opening and Closing of Voltage-Gated Sodium and Potassium Channels Underlies the Action Potential, 40
Action Potentials Are Followed by Brief Refractory Periods, 41
Action Potentials Are Propagated Without Decrement Along Axons, 41
Action Potentials Can Be Altered by Medications, 42
Medications Act at Voltage-Gated Sodium and Potassium Channels to Decrease Neuronal Activity, 42
Neurons share many properties with other cells, including their complement of organelles, an electrical potential across their surface membranes, and an ability to secrete various substances. What distinguishes neurons is the ways in which they have adapted these common properties for their roles as information-processing and information-conveying devices. For example, neurons have specialized configurations of organelles to support their extended anatomy (see Chapter 1 ). Similarly, they have adapted secretory processes to communicate with each other rapidly and precisely (see Chapter 8 ), and individual neurons use alterations in their membrane potentials to convey information between their various parts (this chapter).
This chapter provides an introductory explanation of how (1) membrane potentials develop and are maintained, (2) neurons use relatively slow potential changes ( graded potentials ) for computational purposes and to convey information over short distances, and (3) neurons use larger, briefer action potentials to convey information over longer distances.
A Lipid/Protein Membrane Separates Intracellular and Extracellular Fluids
The lipid component of the membrane is a diffusion barrier.
Membrane proteins regulate the movement of solutes across the membrane.
Ions diffuse across the membrane through ion channels—protein molecules with pores.
The number and selectivity of ion channels determines the membrane potential.
Changes in membrane potential over periods of milliseconds are produced by changes in current flow across the membrane. This is accomplished by changes in the conformation of some ion channels. Most or all channels can switch back and forth between states in which ions can pass through them easily (“open”) and states in which they cannot (“closed”). Some channels can be induced to spend more time in one state or the other by changes in membrane potential ( voltage-gated channels ), others by the binding of some chemical, or ligand ( ligand-gated channels ).
The Resting Membrane Potential of Typical Neurons Is Heavily Influenced, but Not Completely Determined, by the Potassium Concentration Gradient
The K + concentration inside neurons is much higher than that outside (because of ion pumps described a little later), and their surface membranes contain numerous K + channels that are usually in the open state. If these were the only ion channels in the membrane, the following scenario would develop. The concentration gradient would drive K + ions outward through the channels, creating a deficit of positive charges inside the cell. Opposite charges attract each other, so after a very small number of K + ions had left, the resulting intracellular negativity would pull K + ions back into the cell. At some point the concentration gradient and the electrical gradient would exactly counterbalance each other, and K + ions would enter and leave at equal rates ( Fig. 7.1 ). The system would be in equilibrium , with no net movement of K + in either direction and no energy requirement to stay that way. The transmembrane potential at which this occurs, the potassium equilibrium potential or V K , is a logarithmic function of the concentration gradient and is specified by the Nernst equation . At body temperature, a tenfold change in the K + concentration on one side of the membrane causes a 62 mV change in V K .
In reality, however, the K + channels are not 100% selective for K + , and not all the channels in a resting membrane are K + channels. As a result, Na + ions are also able to flow across the membrane. The Na + concentration outside of neurons is much higher than that inside, so Na + moves into the cell because of both the concentration gradient and the intracellular negativity. The result is competing ion flows ( Fig. 7.2 )—inward Na + flow trying to move the membrane potential to V Na and outward K + flow trying to move the membrane potential to V K . A steady state is reached at a potential somewhere between V Na and V K . Just where the steady state is reached is determined by the ion to which the membrane is more permeable. Hence, V Na and V K are boundary values for the membrane potential, and changes in membrane permeability to Na + or K + will cause the membrane potential to move closer to one or the other; this is the usual basis for electrical signaling by neurons. The membranes of most neurons most of the time are much more permeable to K + than to Na + , so the resting membrane potential is close to V K .
Concentration Gradients Are Maintained by Membrane Proteins That Pump Ions
A major difference between the theoretical K + -selective membrane and the real-life membrane that is permeable to both K + and Na + is that energy is required to maintain the concentration gradients across real-life membranes. If K + continually flowed out and Na + continually flowed in, the ionic concentration gradients across the membrane would fade away and the membrane potential would decline toward zero. This is averted by another class of membrane-spanning proteins, ion pumps, that use the energy liberated by hydrolysis of ATP to pump ions in the opposite direction and so maintain concentration gradients. The best-known example is the Na + /K + exchange pump that pumps Na + out and K + in, compensating for the steady-state leakage through channels.