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From the simple to the complex
Anatomy by Level of Organization

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Tutorial 6: The Action Potential Tutorial 10: Temporal and Spatial Summation Action potentials are not binary digits

Neurons with Surprising

Axons Play Unexpected Role in Processing Information

An excitatory potential causes positively charged ions to enter the neuron. The neuron is then said to be “depolarized,” because its membrane potential is less negative than its resting potential (which is about -70 mV).

Conversely, an inhibitory potential causes the membrane potential to become more negative than the resting potential, thus taking it even further from the threshold at which an action potential would be triggered. The neuron is then said to be “hyperpolarized.”

Linked Module: Tutorial 12: Ionotropic Receptors in Postsynaptic Membranes

“Actional potential” is the technical term used to describe a nerve impulse. It consists of a brief, reversible polarization that propagates along an axon. It differs from a receptor potential (synaptic potential) in several respects.

First of all, an action potential does not propagate passively, but actively, by means of special voltage-senstive ion channels in the axon. In addition, mammals have a particular mechanism that accelerates the propagation of the action potential.

This process also requires energy from the neuron, which must maintain the activity of the ion pumps that rebalance the charges on either side of the membrane after an action potential has passed.

Action potentials do not vary in amplitude or intensity. They are ”all or nothing” events. If the intensity of a stimulus falls below the neuron’s excitation threshold, nothing happens. But if the intensity of this stimulus exceeds this threshold, it does not matter whether it does so by a small or a large amount. Either way, an action potential will be triggered, and its amplitude and frequency will always be the same for any given cell.

Consequently, the only way a neuron can transmit information is by varying the frequency of its action potentials–the number of action potentials that it transmits per second.

The following animation shows one possible scenario for communication between neurons. Click on “B” and “C” to see two others. (The graph represents recorded variations in a neuron’s membrane potential over time.)

Lien : Neurons: Animated  Cellular & Molecular Concepts (click on 5. Action Potential)

An action potential is thus a temporary reversal of the electrical potential of the axon’s membrane, lasting scarcely a few milliseconds.

Once the action potential has passed a particular location on the membrane, there is a brief refractory period during which it can no longer be stimulated. This phenomenon prevents the action potential from propagating backward and instead forces it to move forward, like a flame travelling down a trail of gunpowder.



La sclérose en plaques

Glial Cells Too Are Sensitive to the Environment

Without myelin, the spinal cord would have to be several metres in diameter in order to conduct nerve impulses at the speed that it actually does.

The myelination of axons speeds up the conduction of nerve impulses, through an ingenious mechanism that does not require large amounts of additional space or energy.

The nodes of Ranvier, located between the myelinated sections of the axon, are areas of low electrical resistance where almost all of the axon’s sodium channels are concentrated. These nodes are where action potentials can regenerate after the ion currents associated with them have propagated passively down the insulating myelin sheath between one node and the next.

This saltatory propagation lets the neuron preserve its energy, because these narrow nodes are the only places where active excitation is needed to propagate the impulse.

This method of propagation also saves a great deal of space. The speed of conduction of a nerve fibre is proportional to its diameter if that fibre is myelinated, but proportional only to the square root of its diameter if it is not. This means that to conduct a nerve impulse at the same speed as a myelinated fibre with a diameter of 20 micrometres (i.e., 100 m/s), a non-myelinated fibre would need to have a diameter of several centimetres.

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