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 & rsquo; is to say, the membrane potential at which this ion is vis-à-vis equilibrium electrochemical strengths.
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 action potential is & lsquo; no attenuation propagates 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