Propagation of an a.potential
- A graded potential above the threshold potential reaches the firing zone
- Na+ voltage-dependent channels OPEN and Na+ enters the axon, that area of the membrane become electropositive.
- Na+ entry depolarizes the membrane, which opens more Na+ channels
- The positive charges flow to adjacent areas of the axon by local current flow
This creates 3 areas:
-one that has already been depolarised
-one that is depolarising
-one that still has to be depolarised
The area that has already been depolarised is in refractory period: K+ voltage-dependent channels are open and Na+ voltage dependent channels, closed.
K+ exiting to the cytoplasm repolarizes the membrane.
Direction of the propagation
The action potential can travel in both directions along a nerve fiber, moving away from the stimulus.
The refractory period prevents the action potential from going backwards.
However, nerve cells usually have a direction of conduction.
-Sensory neurons carry information towards the CNS.
-Motor neurons carry information from the CNS.
-The conduction pathways within the CNS also travel in a certain direction.
Orthrodomic conduction
From dendrites and the soma to the axon
It depends on the:
- Place of stimulation (generally dendrites and soma)
- Place of generation of the action potential (generally in axonic cone)
- Refractory period.
Conduction across unmyelinated axons
Continuos conduction
The action potentials are conducted creating local currents in adjacent areas to the action potential, depolarising all areas of the axon.
Conduction across myelinated axons
Saltatory conduction
The myelin sheath avoids ion flow, but in the Ranvier nodes (interruption of myelin sheath), there is ionic flow.
Local currents can only be produced in Ranvier nodes, so they “jump” from one node to the next.
Effects of demyelination
Myelin loses its isolating properties.
The depolarisation from one node to the next is difficulted as there are current leaks, so the ionic flux decreases, making it more complicated to depolarise the next node.
Molecular architecture of the myelin sheath
MBP: myelin basic protein
PLP: proteolipid protein (in CND)
MOG: myelin oligodendrocyte glycoprotein (CNS)
MAG: myelin-associated glycoprotein (SNC and PNS)
P0: in PNS
Factors that affect the conduction speed of the nerve action potential
The transmission speed of nerve fibers is very variable (between 0.5 and 120 m/s):
– The information to maintain balance in rapid movements must be very fast.
– Other impulses do not require as much transmission speed
Rm/Rin –> the higher the ratio, the more efficient the signal transmission along the fibre is
- MYELINISATION
*Higher conduction speed: every time channels open due to local currents, a larger area is traveled.
* Lower energy expenditure: less channels are opened, so less Na+ enters and K+ exits and less ATP is needed - DIAMETER OF THE AXON
Membrane resistance is increased by covering it with myelin
The internal resistance is decreased with an enlargement of the axons diameter.
In conclusion; the smaller the internal resistance and the larger the membrane resistance is –> bigger velocity
3.TEMPERATURE
* For each 1C increase, the velocity increases 0.5 - 1 m/s
* There is a lineal increase between physiological limits (~ 25 – 40oC)
* CNS keeps temperature constant
* PNS is exposed to environmental temperature
