Nerve impulse facts for kids

When an action potential arrives at the end of the pre-synaptic axon (yellow), it causes the release of neurotransmitter molecules that open ion channels in the post-synaptic neuron (green). The combined potentials of the inputs can begin a new action potential in the post-synaptic neuron.

Approximate plot of a typical action potential

A nerve impulse is the way nerve cells (neurons) communicate with one another. Nerve impulses are mostly electrical signals along the dendrites to produce a nerve impulse or action potential.

The action potential is the result of ions moving in and out of the cell. Specifically, it involves potassium (K⁺) and sodium (Na⁺) ions. The ions are moved in and out of the cell by potassium channels, sodium channels and the sodium-potassium pump.

Axons of neurons are wrapped by several myelin sheaths, which shield the axon from extracellular fluid. There are short gaps between the myelin sheaths known as nodes of Ranvier where the axon is directly exposed to the surrounding extracellular fluid.

Special faster connections

Electrical synapses between excitable cells are much faster than chemical synapses

Faster electrical synapses are used in escape reflexes, the retina of vertebrates, and the heart. They are faster because they do not need the slow diffusion of neurotransmitters across the synaptic gap. Therefore, electrical synapses are used whenever fast response and coordination of timing are crucial.

These synapses connect the presynaptic and postsynaptic cells directly together. When an action potential reaches such a synapse, the ionic currents cross the two cell membranes and enter the postsynaptic cell through pores known as connexons. Thus, presynaptic action potential directly stimulates the postsynaptic cell.

Images for kids

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  As an action potential (nerve impulse) travels down an axon there is a change in electric polarity across the membrane of the axon. In response to a signal from another neuron, sodium- (Na+) and potassium- (K+) gated ion channels open and close as the membrane reaches its threshold potential. Na+ channels open at the beginning of the action potential, and Na+ moves into the axon, causing depolarization. Repolarization occurs when the K+ channels open and K+ moves out of the axon, creating a change in electric polarity between the outside of the cell and the inside. The impulse travels down the axon in one direction only, to the axon terminal where it signals other neurons.

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  In saltatory conduction, an action potential at one node of Ranvier causes inwards currents that depolarize the membrane at the next node, provoking a new action potential there; the action potential appears to "hop" from node to node.

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  Comparison of the conduction velocities of myelinated and unmyelinated axons in the cat. The conduction velocity v of myelinated neurons varies roughly linearly with axon diameter d (that is, v ∝ d), whereas the speed of unmyelinated neurons varies roughly as the square root (v ∝√d). The red and blue curves are fits of experimental data, whereas the dotted lines are their theoretical extrapolations.

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  Giant axons of the longfin inshore squid (Doryteuthis pealeii) were crucial for scientists to understand the action potential.

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  As revealed by a patch clamp electrode, an ion channel has two states: open (high conductance) and closed (low conductance).

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  Tetrodotoxin is a lethal toxin found in pufferfish that inhibits the voltage-sensitive sodium channel, halting action potentials.

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  Image of two Purkinje cells (labeled as A) drawn by Santiago Ramón y Cajal in 1899. Large trees of dendrites feed into the soma, from which a single axon emerges and moves generally downwards with a few branch points. The smaller cells labeled B are granule cells.

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  Ribbon diagram of the sodium–potassium pump in its E2-Pi state. The estimated boundaries of the lipid bilayer are shown as blue (intracellular) and red (extracellular) planes.