A cell membrane is usually polarized as a result of an unequal distribution of ions on either side. Pores and channels in membranes that allow passage of some ions but not others control ion distribution.
1. Distribution of ions a. Membrane pores and channels, formed by proteins, may be always open or sometimes open and sometimes closed.
b. Potassium ions pass more readily through resting neuron cell membranes than do sodium and calcium ions.
2. Resting potential a. A high concentration of sodium ions is on the outside of the membrane, and a high concentration of potassium ions is on the inside.
b. Large numbers of negatively charged ions, which cannot diffuse through the cell membrane, are inside the cell.
c. In a resting cell, more positive ions leave the cell than enter it, so the inside of the cell membrane develops a negative charge with respect to the outside.
3. Local potential changes a. Stimulation of a membrane affects its resting potential in a local region.
b. The membrane is depolarized if it becomes less negative; it is hyperpolarized if it becomes more negative.
c. Local potential changes are graded and subject to summation.
d. Reaching threshold potential triggers an action potential.
4. Action potentials a. At threshold, sodium channels open and sodium ions diffuse inward, depolarizing the membrane.
b. About the same time, potassium channels open and potassium ions diffuse outward, repolarizing the membrane.
c. This rapid change in potential is an action potential.
d. Many action potentials can occur before active transport reestablishes the original resting potential.
e. The propagation of action potentials along a nerve fiber is an impulse.
5. Refractory period a. The refractory period is a brief time following passage of a nerve impulse when the membrane is unresponsive to an ordinary stimulus.
b. During the absolute refractory period, the membrane cannot be stimulated; during the relative refractory period, the membrane can be stimulated with a high-intensity stimulus.
6. All-or-none response a. A nerve impulse is an all-or-none response to a stimulus of threshold intensity applied to an axon.
b. All the impulses conducted on an axon are the same.
7. Impulse conduction a. Unmyelinated axons conduct impulses that travel over their entire surfaces.
b. Myelinated axons conduct impulses that travel from node to node.
c. Impulse conduction is more rapid on myelinated axons with large diameters.
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