The nerve impulse
At rest, there is a negative potential difference (about -60 mV to -90 mV, the resting potential) between the intracellular side of the membrane of the neuron and its extracellular face. This potential difference results from a difference in concentration of ions between the inside and outside the neuron secondary to selective permeability of the plasma membrane and partly to active transmembrane ion currents (eg, the sodium pump potassium-ATP-asique). There is also leakage currents affecting potassium ions into the extracellular environment through ion channels open transiently specific (because of fluctuations in local electro-chemical).
The nerve impulse is characterized by an instantaneous change and localized the permeability of the membrane of the neuron: ions sodium (Na +) enter the cell by passing through ion channels selectively permeable to sodium. The membrane potential then takes a positive value (about 35 mV) close to potential electro-chemical balance of sodium (ENA). This phenomenon is called depolarization. Then, very quickly ions potassium (K +) out of the cell passing through other ion channels, permeable to potassium. The membrane potential decreases to reach a value lower than the value of the resting potential: this is called repolarization then hyperpolaristion.
Then there is a phase of returning to normal thanks to the action of an ion pump ATP-asique potatium sodium-dependent. The local variation, transient and stereotypical transmembrane potential of the axon with depolarization andthe repolarization, called the action potential. It only lasts a few milliseconds. The action potential or nerve impulse is propagated step by step along the axon of a neuron or a node of Ranvier in the next (saltatory conduction).
Synapses
Complete Schematic view of a neuron
There are 1 more than 100 000 synapses per neuron (mean 10 000). Neurons are cells champions of connectivity and interdependence.
The relay ensures that the transmission of nerve impulses is the synapse. There are two kinds of synapse.
- The electrical synapses (gap junctions, also called gap junction), which are mainly found in invertebrates and lower vertebrates, rarely in mammals.
- The chemical synapses, largely predominate in mammals and man. Some brain circuits that require very fast to ensure survival, have retained electrical synapses.
The synapse consists of a presynaptic element, a synaptic cleft and a postsynaptic element.
- The element is either the presynaptic membrane of the terminal button of the axon, or the membrane of a dendrite. This is the site of synthesis and accumulation often neurotransmitter. It ensures the release of the neurotransmitter under the influence of an action potential. It contains the presynaptic vesicles containing the neurotransmitter. There are 4 types of vesicles:
- The vesicles rounded center clear, spherical, diameter 40 to 60 nm. They contain the acetylcholine, the glutamic acid, and substance P
- The vesicles flattened in the center clear, rather oval in shape with a diameter of 50 nm. They contain GABA and glycine, thus neurotransmitter inhibitors.
- Small dense-core vesicles, spherical, and in diameter from 40 to 60 nm. They contain noradrenaline, the dopamine, and serotonin.
- The large dense core vesicles, spherical, 80 to 100 nm in diameter.
- The element may be the postsynaptic membrane of an axon, a soma, a dendrite, a somatic cell (eg muscle cells). According to their effect on different synapses and excitatory synapses inhibitory. There is a thickening of the postsynaptic membrane, which becomes very wide and very dense (this allows the electron microscope, to easily identify the direction of propagation of information)
- The synaptic cleft, which measures about 20 nm wide. It is filled with dense material parallel to the membranes.
Usually, the initial location of the depolarization is the postsynaptic membrane. The nerve impulse then propagates along the membrane of the dendrite and the soma in diminishing gradually. If at the emergence cone, the potential is sufficient (as the all or nothing), action potentials are generated which propagate along the axon without loss. In arriving at the membrane of the terminal button, they trigger the release of microvesicles containing neurotransmitters, which diffuse through the cleft before being picked up by receptors on the postsynaptic membrane.
The propagation of nerve impulses is something that consumes energy, especially to activate pumps that restore ion balance, after re-permeabilization of the membrane to ions (closing of ion channels). This energy is supplied by degradation of the adenosine triphosphate (ATP) to adenosine diphosphate (ADP). The ATP is then regenerated by the mitochondria.
Can be classified topographically different types of chemical synapses.
- Axodendritique, the most frequent
- Axosomatique,
- Axoaxonique for regulating the postsynaptic neuron,
- Dendrodendritique and Dendrosomatique for the lateral transfer of impulses,
- Somatosomatique.