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From the simple to the complex
Anatomy by Level of Organization

Help Action potential Membrane potential Tutorial 5: Sodium and Potassium Gradients at Rest
Animation: Channel Gating during an action potential Animation : Electrical Signaling in neurons Animation : Channel Gating during an action potential

Ion channels in neuronal membranes can be classified into two major categories: channels that are always open and that help to establish the neuron’s resting potential, and channels that open and close in response to specific stimuli such as neurotransmitters, second messengers, and fluctuations in the membrane’s electrical potential (voltage).

Action potentials (nerve impulses) are accompanied by the opening of ion channels that are termed “voltage-dependent” because they react to changes in membrane potential. These channels are found only on axons.



The underlying basis of nerve conduction consists essentially of electrical and chemical forces that cause ions to move.

All ions are subject to two very different forces: 1) the force of their chemical concentration gradient,which tends to make them move from areas where they are more concentrated to areas where they are less so; and 2) the force of each ion’s electrostatic charge, which repels it from similarly charged ions and attracts it to oppositely charged ones.

The following diagram shows how these two forces come into play as a neuron conducts a nerve impulse. (Click on numbers 2, 3, and 4 to see the corresponding steps in this process.)

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


Synaptic communication Tutorial 13: Drug Effects on the Synapse Nerve agents and the acetylcholine synapse Neurotransmission

The main steps in synaptic transmission–synthesis, secretion, binding, and inactivation–are well known for several “classical” neurotransmitters that have been studied for quite some time, such as dopamine. On this page we describe these steps using the example of acetylcholine, a neurotransmitter that is very common in the central and peripheral nervous systems as well as in all neuromuscular junctions.

Credit: Dr. Fred E. Hossler et Dr. Roger C. Wagner, University of Delaware

In this photo: the three black arrows identify synaptic vesicles where neurotransmitters are stored in the terminal button of an axon, the letters PC identify two post-synaptic neurons, the black triangle indicates the thickening of the post-synaptic neuronal membrane, and the white triangle that of the pre-synaptic neuronal membrane.

1. Synthesis

“Classical” neurotransmitters are small molecules that are synthesized locally in the terminal button of the axon. The precursors of these molecules are converted into active neurotransmitters by means of one or more enzymes present in the axon.

To produce acetylcholine, for example, the enzyme choline acetyltransferase combines choline with acetyl coenzyme A. Other neurotransmitters, such as neuropeptides (a category that includes endogenous opioids) are far bigger molecules, so they must be synthesized in the neuron’s cell body, where the organelles needed to assemble the amino acids are located.

2. Excretion

Most of the synaptic vesicles where neurotransmitters are stored are attached to cytoskeleton components in the axon’s terminal button, near the active zones where these vesicles will fuse with the button’s membrane. When an action potential reaches the axon’s terminal button, it is accompanied by a massive influx of calcium ions. These generate a cascade of reactions that cause the vesicles to detach from the cytoskeleton components and migrate rapidly to these active zones.

In a process called exocytosis, the vesicles then merge briefly with the terminal button membrane, release their neurotransmitters into the synapse, then close up again and retreat inward, ready to be filled with neurotransmitters again.

Lien : Neurons: Animated  Cellular & Molecular Concepts (click on 6. Neaurotransmitter Release)

3. Binding

If the neurotransmitter released into the synapse is acetylcholine, for example, it then binds to acetylcholine-specific receptors on sodium channels in the post-synaptic neuronal membrane, causing these channels to open and let sodium ions enter this neuron.

The neuron’s membrane then becomes depolarized, causing the neuron to become excited. In contrast, when a neurotransmitter such as GABA or glycine binds to receptors on chloride channels in the postsynaptic membrane, it becomes hyperpolarized, and the neuron is inhibited.

Lien : Neurons: Animated  Cellular & Molecular Concepts (click on 7. Postsynaptic Mechanisms)

4. Inactivation

In this final step, the neurotransmitters break their bond with the receptors and return into the synaptic gap, where they must be inactivated for their effects to cease. Neurotransmitters can be inactivated by one or a combination of the following processes: 1) they can simply diffuse out of the synaptic gap; 2) they can be broken down by enzymes present in the gap, such as acetylcholinesterase, which breaks acetylcholine down into choline and acetate; 3) they can be reabsorbed by the terminal button of the presynaptic neuron (as happens with dopamine, serotonin, and norepinephrine); 4) they can be removed from the gap by glial cells known as astrocytes.

Lien : Neurons: Animated  Cellular & Molecular Concepts (click on 8. Removal of Neurotransmitter)

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