What is Nerve Impulses in human biology?

A nerve impulse is an electrical signal carried by a nerve cell. Unlike electrical transmission in wires, this impulse is non-decremental. That is, it does not decrease in strength with distance from the initiation site, because the signal is constantly reinforced using energy obtained from ATP. The signal, called an action potential, consists of a change in electrical potential, or voltage, across the nerve cell membrane that lasts for only one or two milliseconds, but is propagated down the axon to other nerve cells or effectors. The ultimate target of a nerve impulse can be either a hormone-producing gland, a neurosecretory cell such as are found in the hypothalamus, or a muscle.

Voltage is a measure of the difference in potential energy between two points that causes electrically charged particles such as electrons or ions, to move from one point to another. The voltage difference across the membrane at rest, about -70 mV, can be measured by inserting a tiny electrode into the nerve cell. During the passage of a an impulse, the membrane potential changes from -70 mV to +40 mV. After the passage of the impulse, the membrane quickly returns to its resting potential.

The difference in potential between the inside and outside of the nerve cell is caused by an abundance of negatively charged macromolecules, such as proteins, inside the cell. These are too large to pass through the pores in the cell membrane. While some positively charged potassium ions move in from the extracellular space, they cannot completely neutralize the charge inside the cell because they must eventually flow against a concentration gradient. Since there is less potassium outside the cell than inside, potassium tends to diffuse out through pores in the membrane. Over time, the attraction of the electrical potential difference is balanced by the difference in concentration, and the resting potential of -70 mV is reached.

The extracellular fluid also contains sodium (Na⁺) ions, but these cannot move freely through the potassium channels, because although the sodium atom is smaller than potassium, the sodium ion tends to become hydrated, with a layer of ordered water surrounding it that greatly increases its effective diameter. Also, pores called ion channels, formed by proteins in the lipid bilayer, tend to be very selective in the type of ion they will allow to pass through the membrane. One of these channels, the sodium-potassium pump uses energy from ATP to continuously transport sodium to the outside of the membrane and potassium to the inside, maintaining the concentration gradient of the two ions.

The nerve cell is excited by neurotransmitters released either by the presynaptic membrane of another neuron, or directly by a sensory receptor cell. These neurotransmitters depolarize the postsynaptic membrane to a threshold level to trigger an action potential. The action potential is a transient reversal of the resting potential, called a depolarization. Sodium channels are opened to allow sodium ions to enter the cell. The impulse is propagated along the axon, down the cell membrane, at speeds up to 120 meters per second. The impulse is an all-or-none response; that is, the extent of depolarization is always the same in a single axon no matter how strong the initiating stimulus. Immediately following the action potential, sodium channels are closed and the neuron is unresponsive to stimuli for a fraction of a millisecond. Following this refractory period, the sodium-potassium pump regenerates the resting potential.

In myelinated nerve cells, the electrical impulse current flows from node to node where the membrane is depolarized because ion exchange cannot occur in myelinated parts of the cell. This type of conduction, called saltatory conduction, is much faster than impulse propagation in an unmyelinated cell. The speed of conduction is particularly important in reflex movements such as pulling away the hand away from a hot object.