Muscle and nerve cells are excitable cells, meaning that the resting membrane potential changes in response to stimuli that activate gated ion channels. The opening and closing of gated channels can change the permeability characteristics of the cell membrane and hence change the membrane potential.
The channels responsible for the action potential are voltage-gated Na+ and K+ channels. When the cell membrane is at rest, the voltage-gated channels are closed (figure 8.8, step 1). When a stimulus is applied to a muscle cell or nerve cell, following neurotransmitter activation of chemically gated channels, Na+ channels open very briefly, and Na+ diffuses quickly into the cell (figure 8.8, step 2). This movement of Na+, which is called a localcurrent, causes the inside of the cell membrane to become posi-tive, a change called depolarization. This depolarization results in a local potential. If depolarization is not strong enough, the Na+ channels close again, and the local potential disappears without being conducted along the nerve cell membrane. If depolariza-tion is large enough, Na+ enters the cell so that the local potential reaches a threshold value. This threshold depolarization causes voltage-gated Na+ channels to open. Threshold is most often reached at the axon hillock, near the cell body. The opening of these channels causes a massive, 600-fold increase in membrane permeability to Na+. Voltage-gated K+ channels also begin to open. As more Na+ enters the cell, depolarization occurs until a brief reversal of charge takes place across the membrane—the inside of the cell membrane becomes positive relative to the outside of the cell membrane. The charge reversal causes Na+ channels to close and more K+ channels to open. Na+ then stops entering the cell, and K+ leaves the cell (figure 8.8, step 3). This repolarizes the cell membrane to its resting membrane potential. Depolarization and repolarization constitute an action potential (figure 8.9). At the end of repolarization, the charge on the cell membrane briefly becomes more negative than the resting mem-brane potential; this condition is called hyperpolarization. The elevated permeability to K+ lasts only a very short time.
In summary, the resting membrane potential is set by the activity of the leak channels. On stimulation, chemically gated channels are opened and initiate localpotentials. If sufficiently strong, the local potentials activate voltage-gated channels to initi-ate an action potential.
Action potentials occur in an all-or-none fashion. That is, if threshold is reached, an action potential occurs; if the threshold is not reached, no action potential occurs. Action potentials in a cell are all of the same magnitude—in other words, the amount of charge reversal is always the same. Stronger stimuli produce a greater frequency of action potentials but do not increase the size of each action potential. Thus, neural signaling is based on the number of action potentials.
Action potentials are conducted slowly in unmyelinated axons and more rapidly in myelinated axons. In unmyelinated axons, an action potential in one part of a cell membrane stimulates local currents in adjacent parts of the cell membrane. The local cur-rents in the adjacent membrane produce an action potential. By this means, the action potential is conducted along the entire axon cell membrane. This type of action potential conduction is called continuous conduction (figure 8.10).
In myelinated axons, an action potential at one node of Ranvier causes a local current to flow through the surrounding extracellular fluid and through the cytoplasm of the axon to the next node, stimulat-ing an action potential at that node of Ranvier. By this means, action potentials “jump” from one node of Ranvier to the next along the length of the axon. This type of action potential conduction is calledsaltatory (sal′ tă-tōr-ē; to leap) conduction (figure 8.11). Saltatory
conduction greatly increases the conduction velocity because the nodes of Ranvier make it unnecessary for action potentials to travel along the entire cell membrane. Action potential conduc-tion in a myelinated fiber is like a child skipping across the floor, whereas in an unmyelinated axon it is like a child walking heel to toe across the floor.
Medium-diameter, lightly myelinated axons, characteristic of autonomic neurons, conduct action potentials at the rate of about 3–15 meters per second (m/s), whereas large-diameter, heavily myelinated axons conduct action potentials at the rate of 15–120 m/s. These rapidly conducted action potentials, carried by sensory and motor neurons, allow for rapid responses to changes in the external environment. In addition, several hundred times fewer ions cross the cell membrane during conduction in myelinated cells than in unmyelinated cells. Much less energy is therefore required for the sodium-potassium pump to maintain the ion distribution.
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