Neuron Physiology

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I- INTRODUCTION :

Neurons have two properties fundamentally linked : l & rsquo; excitability and conduction that allow them to receive, to propagate and transmit information in the form of & rsquo; 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, brings the & rsquo; axoplasm of growing axons.
– Transport mitochondria :
Renewal of mitochondria of & rsquo; axon and 10-40mm / d endings
– Retrograde axonal transport : role & rsquo; waste disposal. 150-200mm/j

III- REST POTENTIAL (PR) :

A- Highlighting :

Characteristic of all living cells, its value varies from cell to cell & rsquo; autre.Les electrical properties that arise from this are ddp behind 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 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 :

– 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 & 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 :

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 & rsquo; sodium influx is blocked by an inhibitor of the synthesis of & rsquo; ATP. If the & rsquo; ATP was injected directly into the & rsquo; squid giant axon, l & rsquo; influx resumes transiently, and s & rsquo; stop when the & rsquo; injected ATP is depleted. The intensity of the resumption of the & rsquo; influx is proportional to the dose of & rsquo; injected ATP.

– Operation : diagram

Figure. The cycle of the Na / K pump. Under his form E1, l & rsquo; ATPase has high affinity Na + ottoman Its binding site being open to the & rsquo; inside the cell, it sets three ions Na- intracellular hydrolysis and a molecule & rsquo; ATP lay down the inorganic phosphate (P). This binding changes the conformation Site, that s & rsquo; opens to the & rsquo; outside ; simultaneously. l & rsquo; 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 that & rsquo; it releases K + and 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 & rsquo; 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