CONDUCTION OF NERVE IMPULSE

Nerve Impulse:

A nerve impulse is a wave of electrochemical changes, which travels along the length of the neuron involving chemical reactions and movement of ions across the cell-membrane.

Electric potential is a measure of the capacity to do electrical work. It represents a type of stored energy which is manifested during separation of charges across a barrier. In the case of neuron, the charges are positive and negative ions, and the charge separating barrier is the plasma membrane. The electrical potential that exists across a cell membrane is known as the membrane potential. A typical neuron at rest is more positive electrically outside than inside the cell membrane. This net difference in charge between the inner and the outer surface of a non-conducting neuron is called the resting membrane potential. The major factors which are involved in resting membrane potential are:

Sodium and potassium ions:

Of the many kinds of ions present in the nerve cells and the surrounding fluid, sodium (Na⁺) and potassium (K⁺) ions are the most important. Sodium ions are tenfold higher in concentration outside than inside the membrane surface, whereas potassium ions are twenty times more concentrated inside than outside. All the neurons have very active sodium and potassium pumps located in their cell membranes. Driven by the splitting of ATP, these pumps transport Na⁺ out and K⁺ into the cell, both against their respective concentration gradients. For every two K⁺ ions that are actively transported, inward, three Na⁺ ions are pumped out. So inside becomes more negative than outside of the cell membrane of neurons. 

Negative organic ions:

The large negative organic ions such as proteins, organic acids, etc. are much more inside the membrane than outside, where they are only in negligible concentration. This makes the inside of neuron membrane more negative.

Leakage of K⁺ ions from neurons:

The cell membrane is virtually impermeable to all ions except K⁺ ions. Since the 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 accounts for more negative charges inside than outside the cell membrane of neuron.

No conduction of nerve impulse:

There is no conduction of nerve impulse.

The membrane potential of -0.07 volts exists.

(a) In the unstimulated state, a neuron has a membrane potential of approximately -70mV. The active concentrations of the principal ions inside and outside the neuron are indicated by the sizes of the chemical symbols (Na⁺ = sodium ion, K⁺ = potassium ion). (b) Two of the major causes that contribute to the negative resting potential are the active exchange of Na⁺ for K⁺ ions, and the outward diffusion of K⁺. The sodium - potassium pump actively transports Na⁺ out and K⁺ into the cell, and is powered by the splitting of ATP by an associated enzyme, ATPase.

Initiation of nerve impulse:

Under normal conditions a nerve impulse is initiated by appropriate stimuli (called threshold stimulus) applied at one end of the neuron and it results in a remarkable localized change in the resting membrane potential. It disappears or a brief instant and is replaced by a new potential called action or active membrane potential which is in the form of impulse. During this state, the inner membrane surface becomes more positive than the outside. This change is so brief (for perhaps a millisecond) that only a portion of the neuron is in the active membrane potential state.

The major factors involved in changing the resting membrane potential to active membrane potential are:

Na⁺ and K⁺ ion movement:

The passage of nerve impulses is associated with an increase in permeability of Na⁺ ions moving inwards upsetting the potential momentarily, making inside more positive than outside. Neurophysiologists believe that the increased permeability is due to the opening of specific pores in the membrane, termed “sodium gates”. When these gates open, sodium ions rush into the neuron by diffusion. Some K⁺ ions move out.

(a) When a neuron is stimulated, the cell membrane at the point of stimulation undergoes momentary reversal in charge (dark color) called an action potential. Perhaps for a millisecond the inside of the membrane becomes positive relative to the outside. (b) Sequence of membrane potential changes associated with an action potential: (1) resting potential (polarized state); (sodium gates open and Na⁺ diffuses into the cell, causing a depolarization of the membrane; (sodium gates close and potassium gates open; (4) K⁺ diffuses out, causing a re-polarization of membrane; (5) sodium - potassium pump restores original ion gradients and resting potential (recovery). Steps (2) - (5) take a mere 2 - 3 milliseconds.

Charges are reversed:

The inner side of the cell membrane has an excess of positive ions on its internal, surface, and the outer surface becomes more negative.

Passage of nerve impulse:

During the active membrane potential, neuron conducts the impulse in the form of nerve impulse.

Membrane potential:

Active membrane of +0.05 volts exists.

These charges occur along the length of neuron till the impulse reaches synapse. Soon after the passage of the impulse, the resting membrane potential is restored by the movement of a small number of ions especially K⁺ moving out. This neuron is ready to conduct another impulse.

It may be added that in myelinated neurons the impulse jumps from node to node (node of Ranvier). This is called Saltatory impulse.

Speed of nerve impulse:

The normal speed of nerve impulses in humans is 100 meters per second, but maximum speed recorded is 120 ms^-1.