Nerve Impulse

A nerve impulse is the way a neuron transmits information. The nature of a nerve impulse has been studied by using excited axons and an instrument called voltameter. Voltage (mV = millivolts) is a measure of the electrical potential differences between two points, which in this case are inside and outside the axon. The change in voltage is displayed on an oscilloscope, an instrument with a screen that shows a trace, or pattern, indicating a change in voltage with time. Nerve impulse can be studied as (a) resting potential (b) action potential

Resting Potential 

(a) Resting Potential

Electrical potential or the voltage is a measure of the capacity to do electric work. The electrical potential that exists across a cell membrane is known as membrane potential, which is equal to about 70 mV indicating that the inside of the neuron is more negative than the outside. This is called resting potential because the axon is not conducting an impulse. The major factors of resting potential are:

Na and K Ions: When the nerve is at rest, there is relatively greater concentration of sodium ions (Na+) outside the membrane and a relatively greater concentration of potassium ions (K+) inside the membrane. The unequal distribution of these ions is in part due to the action of sodium potassium pump. A cell membrane is very permeable to potassium ions and only slightly permeable to sodium ions. Potassium ions tend to diffuse freely through the membrane to the outside, and sodium ions diffuse inward more slowly. At the same time the membrane expends energy to actively. transport these ions in opposite directions, which prevents them from reaching equilibrium by diffusion. Therefore, sodium ions are actively transported outward, and potassium ions are actively transported inward, i.e. the sodium potassium pump is an active transport system in the plasma membrane that pumps three sodium ions out and two potassium ions into the axon. Since the membrane is more permeable to potassium ions than the sodium ions, there are always more positive ions outside i.e. the outside of all membrane becomes positively charged with respect to the inside, which is negatively charged. As long as a nerve cell membrane is undisturbed, the membrane remains in this polarized state.

Negative Organic Ions: In the cytoplasm of the resting potential state of cells, there are large numbers of negatively charged ions, including those of phosphate, sulphate, and protein, that cannot diffuse through cell membranes, this makes inside of the nerve cell membrane more negative.

Leakage Of K+ Ions: The cell membrane is slightly permeable to K, some of it leaks out of the cell. The loss of this positive ion from the neuron by diffusion makes the inside of the nerve cell membrane more negatively charged.

Potential Changes: Nerve cells are excited, that is they can respond to stimuli. The stimuli usually affect the resting potential in a particular region of a nerve cell membrane, and if the membrane's resting potential becomes decreased, the membrane is said to be depolarizing. Changes that occur in the resting potential of membrane are graded. This means the amount of change in potential is directly related to the intensity of stimulation received. Further more, if additional stimulation is received before the effect of some previous stimulation subsides, the change in potential is still greater. This additive phenomena is called summation and as a result of summated potentials, a level called threshold potential may be reached, and once threshold is achieved, an action potential occurs.

The oscilloscope photograph records a resting potential of -70 mV due to the presence of large organic ions inside a fiber. Note also the unequal distribution of Na+ and K+ across the membrane due to the work of the sodium-potassium pump.

The resting potential indicates that the inside of a fiber is negative as compared to the outside. Because of the sodium-potassium pump, there is a concentration of Na+ outside a fiber and K+ inside a fiber.

Action Potential 

(b) Action Potential

An action potential requires two types of special protein lined channels. There is a channel that allows sodium (Na+) to pass through the membrane and another that allows potassium (K+) to pass through the membrane-each of these type of channels has a gate, the sodium channel has a gate called sodium gate, and the potassium channel has a gate called potassium gate. As sodium ions diffuse inward, the membrane loses its electrical charge and becomes depolarized. At the same time potassium ions diffuse outward, the membrane becomes re-polarized, and it remains in this state until it is stimulated again. Active membrane potential (threshold potentials 0.05 volts (50 mV).

The graph records electrical events over time (in milliseconds) at that particular place (1) The graph starts out at -70 mV the membrane’s resting potential (2) The stimulus is applied, at time 0, and in 2-3 milliseconds, the voltage rises from -70 mV to what is called the threshold potential (-50 mV, in this case). The difference between the threshold potential and the resting potential is the minimum change in membrane’s voltage that must occur to generate the action potential. (3) The threshold potential triggers the action potential, the steep upswing (red) on the graph, which reaches a peak of about +35 mV. The entire change from -70 mV to +35 mV occurs within 3-4 milliseconds of stimulation. (4) The voltage then drops back down, undershoots the resting potential, and finally returns to it.

The graph records electrical events over time (in milliseconds) at that particular place (1) The graph starts out at -70 mV the membrane’s resting potential (2) The stimulus is applied, at time 0, and in 2-3 milliseconds, the voltage rises from -70 mV to what is called the threshold potential (-50 mV, in this case). The difference between the threshold potential and the resting potential is the minimum change in membrane’s voltage that must occur to generate the action potential. (3) The threshold potential triggers the action potential, the steep upswing (red) on the graph, which reaches a peak of about +35 mV. The entire change from -70 mV to +35 mV occurs within 3-4 milliseconds of stimulation. (4) The voltage then drops back down, undershoots the resting potential, and finally returns to it.

The numbered parts of the figure show the changes that occur in part of an axon at three successive times, as a nerve signal passes from left to right. (1) When the region of axon (pink) has its Na+ channels open, Na+ rushes inward (pink arrows) and an action potential is generated. (2) When that same region has its K+ channels open, K+   diffuses out of the axon (blue arrows) at this time its Na+ channels are closed and inactivated, and the action potential is subsiding. (3) A short time later, no signs of an action potential would be seen at this (far-left) spot, because the axon membrane here has returned to its resting potential.

While the resting potentials being restored an axon cannot conduct another stimulus. This interval time is called refractory period.

At the nodes of Ranvier, the fiber membrane can become especially permeable to sodium and potassium ions, and a nerve impulse traveling along a myelinated fiber appears to jump from node to node called saltatory impulse. Conduction is all or none- response, i.e. if a nerve fiber responds at all, it responds completely.

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