I- INTRODUCTION :
Neurons have two properties fundamentally linked : l’excitability and conduction that allow them to receive, to propagate and transmit information in the form of’nerve impulses.
II- Axonal TRANSPORT :
– Transport fast anterograde (100-400mm/j):renouvollement membrane proteins of the axon, NT synthetic enzymes and precursors NM -Transportation slow antegrade (0.1-2mm/j):Renewal of the cytoskeleton, bring the’axoplasm of growing axons.
– Transport mitochondria :
Renewal of the mitochondria of’axon and terminations 10-40mm / d
– Retrograde axonal transport : role of’waste disposal. 150-200mm/j
III- REST POTENTIAL (PR) :
A- Highlighting :
Characteristic of all living cells, its value varies from cell to cell’The electrical properties which follow from this ddp are at the origin of the functioning of neurons..
B- Origin :
1- passive phenomena :
a- Differences ionic concentrations :
Resting, there is an unequal distribution of ions on the part and d’other of the membrane (board). The resulting charge separation is at l’origin’a passive movement of ions through selective "escape channels" for each ionic species. These passive movements are carried out according to two gradients :
– A concentration gradient (osmotic).
– An electrical gradient due to the ddp (Vm) rest.
ion concentrations : (example of the giant squid axon)
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 :
In reality, the membrane is permeable to ions and several ionic fluxes are a function not only of the electrochemical strengths (Em-Eion) but also permeabilities or conductances "g" respective, hence the Goldman equation.
2- active phenomena :
To ensure the stability of the ionic concentrations requires the intervention of an active transport process in reverse (against electrochemical gradients) : It is the Na + pump -k + ATP ase.
Local variations in membrane potential occur in two forms :
IV- EFFECT OF STIMULUS subthreshold : LOCAL POTENTIAL (electrotonic)
– equivalent electrical model of the membrane :
These local phenomena are due to passive physical properties of the membrane.
The insulating lipid bilayer is the equivalent of a capacitor ( Cm) ; the conductive proteins offer resistance Rm to the current Im passing through the membrane.
The membrane can therefore be compared to the juxtaposition of elementary circuits connected by series resistors (RL) the intracellular medium.
We define two constants :
– local response : time constant. It is a function of the values of Cm and Rm.
V- EFFECT OF STIMULUS suprathreshold : SPREADING THE POTENTIAL FOR ACTION (PA) :
This is the mode of communication of the nervous system over long distances.
– Characteristics : 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 at 1 m sec and corresponds to the absolute refractory period (FOR).
– Slower repolarisation : Corresponds to the relative refractory period (PRR)
– A post depolarization
– Repolarization is due to inactivation of Na + channels VD and especially activation "delayed" K + channels VD.
– The return to the rest balance is provided by the pump Na + K + ATP ase.
– Ionic bases of the action potential :
– At the time of peak Vm tends to E Na +, indeed :
– If you block Na + channels Voltage Dependent by tetrodotoxin (TTX) : the cell is inexcitable despite a normal PR.
WE- -nerve conduction : two situations :
– Fibers nonmyelinated :
The action potential formed at the first node moves to the second due to the presence of the myelin sheath (insulating) : is saltatory conduction .
Course of Dr. R. Riri – Faculty of Constantine