The surface of a cell membrane (including a nonstimulated or resting neuron) is usually electrically charged, or polarized, with respect to the inside. This polarization arises from an unequal distribution of positive and negative ions between sides of the membrane, and it is particularly important in the conduction of muscle and nerve impulses. A characteristic change in neuron membrane polarization and return to the resting state, called an action potential, forms a nerve impulse that is propagated along an axon.
4.1 Distribution of Ions
Because of the active transport of sodium and potassium ions, cells throughout the body have a greater concentration of sodium ions (Na+) outside and a greater concentration of potassium ions (K+) inside. The cytoplasm of these cells has many large, negatively charged particles, including phosphate ions (PO4–3), sulfate ions (SO4–2), and proteins, that cannot diffuse across the cell membranes.
Potassium ions pass through cell membranes much more easily than sodium ions. This makes potassium ions a major contributor to membrane polarization. Calcium ions are less able to cross the resting cell membrane than either sodium ions or potassium ions.
4.2 Resting Potential
Sodium and potassium ions follow the laws of diffusion and show a net movement from high concentration to low concentration as permeabilities permit. Because a resting cell membrane is more permeable to potassium ions than to sodium ions, potassium ions diffuse out of the cell more rapidly than sodium ions can diffuse in. Every millisecond, more positive charges leave the cell by diffusion than enter it. As a result, the outside of the cell membrane gains a slight surplus of positive charges, and the inside is left with a slight surplus of impermeant negative charges.
The difference in electrical charge between two regions is called a potential difference. In a resting nerve cell, the potential difference between the region inside the membrane and the region outside the membrane is called a resting potential (-70mV). As long as a nerve cell membrane is undisturbed, the membrane remains in this polarized state. At the same time, the cell continues to expend energy to drive the Na+/K+ “pumps” that actively transport sodium and potassium ions in opposite directions. The pump maintains the concentration gradients responsible for diffusion of these ions in the first place.
Figure 5- The resting potential. (a) Conditions that lead to the resting potential. (b) In the resting neuron, the inside of the membrane is negative relative to the outside. (c) The Na+ /K+ pump maintains the concentration gradients for Na+ and K+ ions.
4.3 Potential Changes
Nerve cells are excitable; that is, they can respond to changes in their surroundings. Some nerve cells, for example, are specialized to detect changes in temperature, light, or pressure from outside the body. Many neurons respond to neurotransmitters from other neurons. Such changes (or stimuli) usually affect the resting potential in a particular region of a nerve cell membrane. If the membrane’s resting potential decreases (as the inside of the membrane becomes less negative when compared to the outside), the membrane is said to be depolarized.
Local potential changes are graded. This means that the magnitude of change in the resting potential is directly proportional to the intensity of the stimulus. That is, if the membrane is being depolarized, the greater the stimulus, the greater the depolarization. If neurons are depolarized sufficiently, the membrane potential reaches a level called the threshold potential, which is approximately –55 millivolts. If threshold is reached, an action potential results, which is the basis for the nerve impulse.
Figure 6- Action potentials. (a) A subthreshold depolarization will not result in an action potential. (b) Stimulation from multiple presynaptic neurons may cause the postsynaptic neuron to reach threshold, opening voltage-gated channels at the trigger zone.
|Explanation is available on