Along with muscle cells, neurons are unique in that they are excitable; that is, they respond to stimuli by generating electrical impulses. Electrical responses of neurons (modifications of the electrical potential across their membranes) may be local (restricted to the place that received the stimulus) or propagated (may travel through the neuron and its axon). Propagated electrical impulses are termed action potentials. Neurons communicate with each other at synapses by a process called synaptic transmission.
The membranes of cells, including nerve cells, are structured so that a difference in electrical potential exists between the inside (negative) and the outside (positive). This results in a resting potential across the cell membrane, which is normally about −70 mV.
The electrical potential across the neuronal cell membrane is the result of its selective permeability to certain charged ions. Cell membranes are highly permeable to most inorganic ions, but they are almost impermeable to proteins and many other organic ions. The difference (gradient) in ion composition inside and outside the cell membrane is maintained by ion pumps in the membrane, which maintain a nearly constant concentration of inorganic ions within the cell (Fig 3–1 and Table 3–1). The pump that maintains Na+ and K+ gradients across the membrane is Na, K-ATPase; this specialized protein molecule extrudes a+ from the intracellular compartment, moving it to the extracellular space, and imports K+ from the extracellular space, carrying it across the membrane into the cell. In carrying out this essential activity, the pump consumes adenosine triphosphate (ATP).
Na+ and K+ flux through the resting nerve cell membrane. Notice that the Na+/K+ pump (Na+/K+-ATPase) tends to extrude Na+ from the interior of the cell, but it carries K+ ions inward. (Reproduced, with permission, from Eccles JC: The Physiology of Nerve Cells. Johns Hopkins University Press, 1957.)
Table 3–1 Concentration of Some Ions Inside and Outside Mammalian Spinal Motor Neurons. |Favorite Table|Download (.pdf)
Table 3–1 Concentration of Some Ions Inside and Outside Mammalian Spinal Motor Neurons.
Concentration (mmol/L H2O)
Equilibrium Potential (mV)
Two types of passive forces maintain an equilibrium of Na+ and K+ across the membrane: A chemical force tends to move Na+ inward and K+ outward, from the compartment containing high concentration to the compartment containing low concentration, ...