How a Nerve Impulse Travels
Impulses travel not only along the length of a nerve cell but also from cell to cell. Within a neuron the impulse is transmitted electrically. However, chemicals are generally involved in moving the impulse from cell to cell.
The Nerve Impulse
Like all cells, neurons have a certain electrical charge on the inside and outside of their cell membranes. The axon of a neuron when the neuron is at its resting potential—that is, when it is not carrying an impulse.
The outside of the axon has about 10 times as many sodium (Na+) ions as the inside. Inside the membrane are negatively charged organic ions and about 30 times as many potassium (K+) ions as outside. The membrane keeps the Na* ions outside and the negatively charged organic ions inside. The K+ ions move in and out of the axon freely. At resting potential, the inside of the cell membrane has a slightly negative charge, and the outside has a slightly positive charge. In this case the cell is said to be polarized.
When the nerve fiber is stimulated, its membrane suddenly becomes permeable to Na+ ions at the place where the stimulation occurs. The negative ions inside the membrane then attract the Na* ions. Some Na+ ions move rapidly to the inside of the cell. The presence of these positively charged ions causes that part of the interior to become more positive than the outside. These electrical changes create an action potential, and the neuron is said to be depolarized.
The membrane remains permeable to Na+ ions for only half a millisecond. However, this brief electrical charge is enough to start the action potential moving down the nerve fiber. How does this movement occur? The positively charged ions inside the cell move toward the negatively charged area next to the region of stimulation. The positive ions cause this area to become depolarized and the membrane to become permeable to Na+. More Na+ ions then rush inside the membrane, causing that section of the interior to become positive. Again, positively charged ions are attracted to the adjoining negatively charged area, and thus the action potential moves along the nerve fiber.
The rapid change from negative to positive charge within the membrane is an electrical wave called a nerve impulse. A nerve impulse can be described as the movement of the action . potential along a neuron.
As soon as an impulse passes a section of nerve fiber, the membrane once again becomes permeable only to K+ ions. The neuron then returns to its resting potential in preparation for the next impulse. The process of returning to resting potential involves an active transport system known as the sodium-potassium pump. The sodium-potassium pump carries Na+ ions to the outside and K+ ions to the inside of the membrane.
In myelinated axons, the myelin sheath acts as an insulator against electrical impulses. Because of this insulation, the exchange of ions across the membrane takes place only at the nodes of Ranvier, where the sheath is interrupted. This periodic, rather than continuous, exchange results in a leaping of the impulse from node to node. As a result, impulses travel along myelinated axons 50 times faster than they do along unmyelinated axons, sometimes as fast as 100 meters per second (224 miles per hour).
Impulses travel from neuron to neuron, but adjoining neurons generally do not touch one another. Therefore, an impulse must cross from the axon of one neuron to the dendrites of another. This junction is called a synapse. An impulse does not “jump” across the space, however. In fact, the original impulse ends when it reaches the end of an axon. At that point, however, the impulse causes the release of chemicals that generate new impulses in the next neuron.
Many axon branches terminate in tiny bulblike structures called synoptic buttons, which contain numerous synoptic vesicles. A synaptic vesicle is a tiny sac that holds chemical substances called neurotransmitters that stimulate nearby dendrites to start new impulses. A neurotransmitter released into the space, called the synaptic cleft, diffuses rapidly to nearby dendrites. There it disturbs the resting potential of the dendrites and so generates new impulses.
An impulse eventually reaches an effector cell, such as a muscle fiber. In this situation, a neurotransmitter is released from motor neurons through motor endplates, which are located at the ends of axons near muscle fibers. The neurotransmitter then causes the muscle to contract.
Starting a Nerve Impulse
To “fire” a neuron—that is, to get a nerve impulse going in the first place—a stimulus must have a certain level of strength called a threshold. If the energy level of a stimulus falls below the threshold, the neuron will not fire. However, a stimulus with an energy level greater than the threshold does not cause a faster or stronger impulse. The neuron either fires or it doesn’t, a phenomenon known as the all-or-none response. The intensity of a sensation depends on the number of neurons stimulated. After an impulse, the neuron must rest for about one-hundredth of a second. A stimulus, no matter how strong, cannot fire the neuron during this time.
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