Nature of a Nerve Impulse
A nerve impulse is an electro-chemical message of neurons, the common functional denominator of all nervous system activity. Despite the unbelievable complexity of the nervous organization of many animals, nerve impulses are essentially alike in all nerve cells and in all creatures. An impetus is an “all-or none” phenomenon; either the fiber is taking an impulse, or it is not. Because all impulses are alike, the only means a nerve fiber can change its signal is by altering the frequency of impulse conduction. Frequency change is the terminology of a nerve fiber. A fiber may conduct no impulses at all or very few per second up to a maximum approaching 1000 per minute. The higher the frequency (or rate) of conduction, the heavier is the degree of innervation.
Resting Membrane Potential
Membranes of neurons, like all cellular membranes, have a special permeability that creates ionic imbalances. The interstitial fluid surrounding neurons contains relatively high concentrations of sodium (Na+) and chloride (Cl−) ions, but a low absorption of potassium ions (K+) and large impermeable anions with a negative billing. Within the neuron, the ratio is reversed: the K+ and impermeable anion concentration is high, but the Na+ and Cl− concentrations are down (Figure 35-4; see also Figure 33-1B,) These differences are judged; there is about 10 times more Na+ outside than in and 25 to 30 times more K+ inside than out.
When at ease, the membrane of a nerve cell is selectively permeable to K+, which can cross the membrane through special potassium channels. The permeability to Na+ is nearly zero because the Na+ channels are shut in a breathing membrane. Potassium ions tend to diffuse outward through the membrane, following the gradient of potassium absorption. Very quickly the positive charge outside reaches a stage that prevents any more K+ from diffusing out of the axon (because, like charges repel each other), and because the large onions cannot pass through the membrane, the positively charged potassium ions are pulled backward into the cubicle. Straight off the remaining membrane is at equilibrium, with an electrical gradient that exactly balances the concentration gradient. This resting membrane voltage is usually −70 MV (millivolts), with the inside of the membrane negative with respect to the extraneous.
A nerve impulse is a rapidly moving changes in electrical membrane potential called an action potential (Figure 35-5). It is a very rapid and brief depolarization of the membrane of the nerve fiber. In more nerve fibers, the action potential does not simply give the membrane voltage to zero, but instead overshoots zero. In other words, the membrane potential reverses for an instant so that the outside becomes negative compared with the inner. Then, every bit the action potential moves ahead, the membrane returns to its normal resting membrane potential, ready to conduct another - impulse. The entire event occupies approximately a millisecond. Possibly the most important attribute of the nerve impulse is that it is self propagating; once set about the impulse moves ahead automatically, much like the burning of a primer.
What makes the reversal of polarity in the cell membrane during passing of an action potential? We have ensured that the resting membrane potential depends on the high membrane permeability (leakiness) to K+, some 50 to 70 times larger than the permeability to Na+. When the action potential arrives at a broken point, Na+ channels suddenly open, allowing a flood of Na+ to diffuse into the axon from the outside, making a motion down the concentration gradient for Na+. In reality only a very minute amount of Na+ moves across the membrane— less than one-millionth of the Na+outside—but this sudden surge of positive ions cancels the local resting membrane voltage. The membrane is depolarized, creating a minute electrical “hole.” K ions, finding their electrical barrier gone, begin to act outside the cubicle. Then, every bit the action potential passes, the membrane quickly regains its resting properties. It goes once again practically impermeable to Na+ and the outward movement of K+ is checked. Therefore, the growing phase of the action potential is linked up with the rapid influx (inward movement) of Na+ (Figure 35-5). When the action potential reaches its peak, Na+ permeability is restored to normal, and K+ permeability briefly increases above the breathing stage. Increased potassium permeability causes the action potential to sink quickly to the resting membrane level, during the repolarization phase. The membrane is immediately quick to transmit another nerve impulse.
Sodium Pump A resting cell membrane causes a very low permeability to Na+. Nevertheless, some Na+ leaks through it, even in the resting condition. When the axon is active, during an action potential, Na+ flows inward with each passing impulse. If not withdrawn, the accumulation of Na+ inside the axon would cause the resting membrane voltage of the fiber to decay. This decomposition is prevented by sodium pumps, each a complex of protein subunits embedded in the plasma membrane of the axon (see Figure 3-19,). Each sodium pump uses energy in ATP to transport sodium from the inside to the outside of the tissue layer. The sodium pump in nerve axons, as in many other cell membranes, also moves K+ into the axon while it is moving Na+ out. Therefore, it is a sodium-potassium exchange pump that assists to regenerate the ion gradients of both Na+ and K+. The astrocytes (mentioned earlier) help to sustain the right proportion of ions surrounding neurons by sweeping away excess potassium produced during neuronal activity.
Although the ionic and electrical events associated with action potentials are much the same throughout the animal kingdom, conduction velocities vary enormously from nerve to nerve and from animal to animal—from as slow as 0.1 m/Sec in sea anemones to as fast as 120 m/soak in some mammalian motor axons. The velocity of conduction is closely associated to the diameter of the axon. Small axons conduct slowly because the internal impedance to current current is high. In most invertebrates, where faster conduction velocities are important for quick reaction, such as in locomotion to capture prey or to avoid capture, axon diameters are larger. The giant axon of squids is nearly 1 mm in diameter and carries impulses 10 times quicker than ordinary characters in the same animal. A squid giant axon innervates the animal’s mantle musculature and is utilized for powering mantle contractions when the animal swims by jet propulsion. Similar giant axons enable earthworms, which are normally dull-acting beasts, to take back almost instantaneously into their burrows when startled.
Some invertebrates, including prawns and insects, also take in fast fibers invested with multiple layers of a myelin-like heart that is broken at intervals much like myelinated fibers of vertebrates. Conduction rates, though not equally fast as vertebrate saltatory conduction, are a lot faster than unmyelinated fibers of the same diameter in other invertebrates.
Vertebrates do not possess giant axons, but they can achieve high conduction velocities in some other direction, by a cooperative relationship between axons and the investing layers of myelin laid down by the Schwann cells or oligodendrocytes described earlier. Insulating myelin sheaths are interrupted at intervals by clients (called nodes of Ranvier) where the surface of the axon is exposed to fluid surrounding the face. In these myelinated fibers the action potential depolarizes the axon membrane only at the nodes because the myelin sheath prevents depolarization elsewhere (Figure 35-6). The ion pumps and canals that move ions across the membrane are concentrated in each client. In one case an action potential starts down an axon, depolarization of the first node initiates an electrical flow that extends away to the neighboring node, having it to depolarize and trigger an action potential. Hence the action potential leaps from node to node, a form of conduction called statutory (L. Salt, to dance, leap). The increase in efficiency as compared with nonmyelinated fibers is impressive. For instance, a frog myelinated axon only 12 µm in diameter conducts nerve impulses at the same velocity as a squid axon 350 µm in diameter.
Source:E plant Science