Conduction of the action potential

A typical neuron consists of four functional regions:

• Axon terminal

The cell body, with its dendrites, which are projections from the cell body that greatly increase the surface area, is the site of communication and input from other neurons. These inputs result in generation of graded potentials that travel a short distance to the axon hillock, the region of the cell body from which the axon arises. Following sufficient stimulation of the neuron, an action potential is generated at the axon hillock. This action potential must then be propagated or regenerated along the third portion of the neuron, the axon. The axon, or nerve fiber, is an elongated projection that transmits the action potential away from the cell body toward other cells. The final component of the neuron is the axon terminal, where the neuron communicates with another cell or cells by way of this action potential. Conduction of the action potential along the length of the axon is the subject of this section; communication between a neuron and another cell is discussed in the following chapter.

The action potential is initiated at the axon hillock (see Figure 4.3). This region is particularly excitable due to an abundance of voltage-gated Na+ channels. As the axon hillock is stimulated by excitatory inputs, there is a marked influx of Na+ ions and this region of the cell membrane becomes positive inside, resulting in an action potential. The rest of the axon is still at its resting membrane potential and is negative inside. As with graded potentials, this electrical signal also travels by local current flow (see Figure 4.3). The (+) charges in the region of the action potential are attracted to the negative charges in the immediately adjacent region of the axonal membrane.

This current flow depolarizes the new region, causing an increase in the permeability of the cell membrane to Na+ ions through voltage-gated ion channels. The subsequent influx of Na+ ions further depolarizes the membrane so that it reaches threshold and a new action potential is generated in this region. At the same time, the original site of action potential generation at the axon hillock repolarizes due to the efflux of K+ ions. This process of generating new action potentials sequentially along the membrane enables the signal to maintain its strength as it travels the distance to the axon terminal.

Another mechanism of conduction of an action potential along the length of a neuron is the saltatory conduction that occurs in myelinated axons (see Figure 4.4). Myelin is a lipid sheath wrapped around the axon at regular intervals. The myelin is not actually part of the axon, but instead comes from other cells. In the central nervous system (brain and spinal cord), the mye-lin-forming cell is the oligodendrocyte, one of several types of support cells for centrally located neurons. In the peripheral nervous system (all neurons that lie outside the central nervous system and communicate with various body parts), myelin is formed by the Schwann cells. The lipid of the myelin in each case comes from multiple layers of the plasma membrane of these cells as they wrap around the axon. This lipid provides good insulation, preventing the movement of current across the cell membrane.

Without ion flux, action potentials cannot be generated in the regions covered with myelin. Instead, they occur only at breaks in the myelin sheath

Figure 4.3 Conduction of the action potential along an axon by local current flow. Upper panel: action potentials are generated at the axon hillock. When stimulated to threshold, this region of the membrane becomes positive (+30 mV) inside relative to the outside due to the influx of Na+ ions. The remainder of the axon is at its resting membrane potential (-70 mV). Middle panel: because opposite charges attract, the (+) charges in the stimulated area are attracted to the (-) charges in the adjacent region of the membrane. This movement of (+) charges, or local current flow, depolarizes this adjacent region. Lower panel: the depolarization of the adjacent region causes activation of voltage-gated Na+ channels and generation of a new action potential. Meanwhile, the original area of stimulation has repo-larized back to the resting membrane potential. This unidirectional process continues along the length of the axon.

Figure 4.3 Conduction of the action potential along an axon by local current flow. Upper panel: action potentials are generated at the axon hillock. When stimulated to threshold, this region of the membrane becomes positive (+30 mV) inside relative to the outside due to the influx of Na+ ions. The remainder of the axon is at its resting membrane potential (-70 mV). Middle panel: because opposite charges attract, the (+) charges in the stimulated area are attracted to the (-) charges in the adjacent region of the membrane. This movement of (+) charges, or local current flow, depolarizes this adjacent region. Lower panel: the depolarization of the adjacent region causes activation of voltage-gated Na+ channels and generation of a new action potential. Meanwhile, the original area of stimulation has repo-larized back to the resting membrane potential. This unidirectional process continues along the length of the axon.

referred to as the nodes of Ranvier. These nodes are located about 1 to 2 mm apart. The flow of current from an active node "skips" down the axon to the adjacent node to cause depolarization and generation of a new action potential. This transmission of the impulse from node to node is referred to as saltatory conduction, from the Latin word saltare, meaning "to leap."

Saltatory conduction results in a significant increase in the velocity of conduction of the nerve impulse down the axon compared to that of local current flow in an unmyelinated axon (see Table 4.2). The speed of conduction is

Figure 4.4 Saltatory conduction. Transmission of electrical impulses in a myelinated axon occurs by way of saltatory conduction. Composed primarily of lipid, the myelin sheath insulates the axon and prevents generation of membrane potentials. Membrane potentials occur only at gaps in the myelin sheath, referred to as the nodes of Ranvier. Therefore, transmission of the impulse, or generation of action potentials, occurs only at the nodes.

Figure 4.4 Saltatory conduction. Transmission of electrical impulses in a myelinated axon occurs by way of saltatory conduction. Composed primarily of lipid, the myelin sheath insulates the axon and prevents generation of membrane potentials. Membrane potentials occur only at gaps in the myelin sheath, referred to as the nodes of Ranvier. Therefore, transmission of the impulse, or generation of action potentials, occurs only at the nodes.

directly correlated to the urgency of the information conveyed by a given neuron. Nerve fibers carrying less important information, such as those regulating slow digestive processes, are unmyelinated. An example of a nerve fiber with myelin is one that innervates skeletal muscle so that movements can be executed rapidly.

The functional significance of myelin is revealed by the neurological deficits observed in patients with multiple sclerosis. This disorder is caused by the demyelination of neurons in the brain, spinal cord, and optic nerve. The loss of myelin disrupts the normal conduction of impulses along the axons of these neurons and results in weakness, numbness, loss of bladder control, and visual disturbances.

Another advantage of the presence of myelin along an axon is that impulse conduction is energetically more efficient. Because action potentials occur only at the nodes of Ranvier, fewer Na+ and K+ ions move in and out of the cell. Therefore, less metabolic energy is required to return these ions to their original positions along the cell membrane and to maintain the proper concentration gradients. In unmyelinated axons, action potentials, and therefore ion flux, occur along the entire length of the axon. These neurons expend more energy returning these ions to their original positions.

A second factor that influences the velocity of action potential conduction is the diameter of the axon. The greater the diameter is then, the lower the resistance to current flow along the axon. Therefore, the impulse is

Table 4.2 Factors Affecting Velocity of Conduction

Factor

Velocity of conduction

Myelination of axon (saltatory conduction) T Diameter of axon

conducted along large nerve fibers more rapidly. Large myelinated nerve fibers, such as those innervating skeletal muscle, exhibit the highest conduction velocity. Small unmyelinated fibers, such as those of the autonomic nervous system innervating the heart; smooth muscle of the blood vessels and gastrointestinal tract; and glands, conduct nerve impulses more slowly.

Conduction of the action potential along the axon is unidirectional. In other words, the nerve impulse travels away from the cell body and the axon hillock toward the axon terminal only. As the current flows from the initial area of activity to the adjacent region of the axon, the new region becomes depolarized and generates an action potential. Simultaneously, the initial area has entered its absolute refractory period due to inactivation of voltage-gated Na+ channels. As a result, as current flows away from the second active area, it has no effect on the original site of activity. Instead, the current continues forward and depolarizes the next adjacent region of the axon. By the time the original site has recovered from the refractory period and is capable of being restimulated, the action potential has traveled too far along the axon to affect this site by way of local current flow. This unidirectional conduction ensures that the signal reaches the axon terminal where it can influence the activity of the innervated cell as opposed to traveling back and forth along the axon ineffectively.

Pharmacy application: local anesthetics

Pain is a protective mechanism that alerts an individual to the occurrence of tissue damage. Stimulation of nociceptors (pain receptors) alters membrane permeability to ions, the predominant effect of which is the influx of Na+ ions down their electrical and chemical gradients. Sufficient Na+ ion influx results in generation of an action potential that is then propagated along the afferent neuron to the CNS, where the painful stimulus is perceived. Local anesthetics, such as lidocaine and procaine (also known as no-vocaine), prevent or relieve the perception of pain by interrupting conduction of the nervous impulse. These drugs bind to a specific receptor site on the voltage-gated Na+ channels and block ion movement through them. Without Na+ ion influx, an action potential cannot be generated in the afferent neuron and the signal fails to reach the CNS. In general, the action of these drugs is restricted to the site of application and becomes less effective upon diffusion of the drug away from the site of action in the nerve.

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Essentials of Human Physiology

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