I- INTRODUCTION :
Neurons possess fundamentally related properties : l & rsquo; excitability and conduction that allow them to receive, to propagate and transmit information in the form of & rsquo; nerve impulses.
Neurons therefore differ in their ability to communicate with each other quickly, sometimes over long distances very accurately,
II- Axonal TRANSPORT :
– Transport fast anterograde (100-400mm/j): renewal of membrane proteins to the axon, NT synthetic enzymes and precursors NM
– Transport slow antegrade (0. 1-2mm/j) Renewal of the cytoskeleton, brings the & rsquo; axoplasm of growing axons.
– Transport mitochondria :
Renewal of mitochondria and the axon endings. 10-40mm / day -Air retrograde axonal: role & rsquo; waste disposal. 150-200mm/j
III- REST POTENTIAL (PR) :
A- Highlighting : diagram
B- Origin of resting potential :
1- passive phenomena :
a- Differences ionic concentrations : (example of the giant squid axon)
Resting, there is an unequal distribution of ions and from & rsquo; across the membrane (board). The charge separation is resulting in & rsquo; origin & rsquo; a passive movement of ions through "channels flight" selective for each species ionique.Ces passive movements are performed by two gradients :
through "channels flight" selective for each ion species. These passive movements are performed according to two gradients :
– A concentration gradient (osmotic) which tends to equalize the concentrations of the two compartments.
– An electrical gradient due to the ddp (Vm) rest and tends to leave the ions according to their electrical charge.
b- Potential balance : This equation Nernst equation to calculate the equilibrium potential of an ion (It ion) c’that is to say the membrane potential for which this ion is in equilibrium with respect to the electrochemical forces.
Ex = R.T/ZF.Ln Xe/Xi
c- membrane permeability : GOLDMAN equation The membrane is permeable to ions and several ionic fluxes are a function not only of the electrochemical strengths (Em-Eion) but also the respective permeabilities, hence the Goldman equation :
Vm=R.T/F.Ln PK.(K+)+ e + a + PNafNa PCl(Cl-)e / PK.(K+)i + PNa(na +)i+PCl(Cl-)i
2- Phénomèncs assets :
To ensure the stability of the ionic concentrations requires the intervention of an active transport mechanism against electrochemical gradients : It is the pump Na + / K + ATP ase.
– Highlighting : experience marked ions
– Operation : diagram
– This pump transports 2 ions K> against 3 Na + ions thus Generator (participates in contrast to membrane potential).
Local variations in membrane potential occur in two forms :
IV- EFFECT OF STIMULUS subthreshold : LOCAL POTENTIAL (electrotonic)
Local variations of V m that allow the transmission of information over short distances.
These local phenomena are due to passive physical properties of the membrane. We define two constants :
a- local response : time constant.
It is a function of the values of Cm and Rm.
b- response spread : constant & rsquo; space.
This is the distance corresponding to a decrease of 63% of the initial amplitude ; it is based on the values of series resistors (RL) :
- Wholesale fiber diameter RL low -► -► constant high time
- fine fiber -► -► RL consistently high low time
V- EFFECT OF STIMULUS suprathreshold : SPREADING THE POTENTIAL FOR ACTION :
This is the mode of communication of the nervous system over long distances.
1- Highlighting : diagram
2- Characteristics :
Stimulation causes after a latency stereotypical variation of Vm ; there are several phases :
– fast and abrupt depolarization with inversion of Vm (from -70 at +30) and the beginning of a rapid repolarization, its duration is 0,5 my brood juncture. This phase corresponds to the absolute refractory period (FOR).
– Slower repolarisation : Corresponds to the relative refractory period (PRR)
– Post hyperpolarization or excitability subnormal.
3- Ionic bases of the action potential :
– At the time of peak Vm tends to E Na +, indeed :
– In medium without Na + is not obtained of the action potential and any change in the extracellular Na + causes a variation in the same direction of the amplitude of the PA.
– If you block Na + channels Voltage Dependent by tetrodotoxin (TTX) : the cell is inexcitable despite a normal PR.
– The technique of "voltage clamp" court shows that the tip, for g = 1 + K, GNA + = 20
Au total : the action potential results from a sudden increase in Na + g with massive influx of Na + and reverse Vm.
– At the threshold value potential, Na + channels VD "closed activatable" pass to the "open" state from which depolarization and opening of other Na + channels VD (processus regenerative).
– The refractory period is due to inactivation of Na + channels VD :
– PRA in all are inactivables
– in PRR they gradually désinactivent
– Repolarization is due to the inactivation Na + channels VD and especially the "delayed" activation of K + channels VD, where efflux of K + and repolarization.
4- nerve conduction :
The potential for action is ‘propagates without attenuation all along the axon ; two situations :
a- Fibers nonmyelinated :
The induced membrane depolarization "local currents" which depolarize neighboring regions where opening of Na + channels VD and forming a remote action potential.
The nerve impulse can not go back because of the inactivation of Na + channels VD.
The conduction velocity is propor tional to the diameter of the axon (low RL).
b- myelinated :
The action potential formed at the first node moves to the second due to the presence of the myelin sheath (insulating) and the scarcity of Na + channels in VD inter nodal regions : is saltatory conduction.
The presence of the myelin sheath provides two, benefits :
– time saving : high conduction velocity.
– an energy gain : reduced activity of the pump Na + / K + ATPase.
Course of Dr A. CHIKHI – Faculty of Constantine