Neuron Physiology

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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 :

Several types
– 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..

Figure 1. Membrane potential before and after penetration of the microelectrode in the neuron.

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 INTRACELLULAR

mM

Extracellular

mM

POTENTIAL

BALANCE

K+ 400 20 -75
na + 50 404 +55
Cl- 52 560 -60
A- 385

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.

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.

Figure. L’sodium influx is blocked by an inhibitor of the synthesis of’ATP. If from him’ATP is injected directly into the’giant axon, l’inflow resumes temporarily, then s’stop as soon as’ATP injected is exhausted. The intensity of the recovery of’influx is proportional to the dose d’ATP injected.

– Operation : diagram

Figure. The cycle of the Na / K pump. Under his form E1, l’ATPase has a strong affinity for Na + Its fixation site being open towards the’inside the cell, it sets three ions Na- intracellular and hydrolyzes a molecule d’ATP which it fixes inorganic phosphate (P). This binding changes the conformation Site, who s’open towards’outside ; simultaneously. l’enryme loses its affinity for Na- which is released into the extracellular medium. She acquires a high affinity for K + ions and two fixed. is unstable, it then resumes its form El, at the same time as’it releases K * and its inorganic phosphate in the intracellular compartment.

Local variations in membrane potential occur in two forms :

IV- EFFECT OF STIMULUS subthreshold : LOCAL POTENTIAL (electrotonic)

– equivalent electrical model of the membrane :

Figure. equivalent electrical circuit of the neuron membrane with the currents , potassium (K) and sodium (Na).

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.

Time constant

– response spread : constant d’space, it is a function of the RL values

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

– A post hyperpolarization


– the action potential results from a sudden increase in Na + g with massive influx of Na + and reverse Vm.

– 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.

– The technique of "voltage clamp" court shows that the tip, for g = 1 + K, GNA + = 20

WE- -nerve conduction : two situations :

– Fibers nonmyelinated :

The induced membrane depolarization "local currents" which depolarize neighboring regions where opening of Na + channels VD and forming a remote action potential.

– myelinated :

conduction saltatoire

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