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What medium do nerve impulses travel in ?

A nerve impulse must essentially travel within a nerve fiber. The transmission of neuronal signals is neither via electron transmissions as in electrical conductors nor photon based as in glass fibers. A nerve depends on ‘action potentials’. Membrane potential, a prerequisite, is a specific distribution of ions along the cell membrane of a nerve.

The involved ions here are,mainly,positively charged sodium ions and potassium ions  along with negatively charged chloride ions and organic anions. These ions have a specific distribution along the membrane and possess a balance which is upheld by permanently opened potassium channels and  a pump that exchanges sodium and potassium ions for energy, known as Na-K-ATPase. Two forces counteract each other : the accumulation of same electrical charges on one side (similar to a magnet, like charges repel each other) and the concentration of same particles on one side . The more you want to concentrate a particle on one side of a membrane, the more energy you need.

Both of these aspects combine to form the ‘electrochemical potential’. Therefore, during the resting membrane potential, the inside of the cell is electrically negatively charged . The organic anions which can’t escape the cell far outweigh the sum of the charge of the potassium ions, hence the negative charge, while the outside of the membrane is positively charged and filled with sodium ions .

During action potential, voltage gated sodium channels (meaning channels that open when exposed to certain charges) open when exposed to a voltage higher than the opening threshold and let sodium ions cross the membrane because of the electrical and chemical gradient. The inward flow of positive charges inside the cell causes the voltage of the cell to become positive, a process called depolarisation. Neighboring voltage gated sodium channels are then activated from this increase in potential and the signal is transmitted along the membrane in one direction. An action potential is then ended by closing of the sodium channels due to positive voltage and the Na-K-ATPase pumps out all sodium ions that are still within the cell, causing the voltage to become negative ( known as repolarisation).



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