ACTION POTENTIAL, CONDUCTION (Chapter 4)
Neurons are the functional units of the nervous system.
What is the property that allows them to interact with each other?
Neurons are capable of signaling
Neurons communicate by sending electrical signals called
Action Potentials: produced by the movement of ions in and out of the neuron, through the cell membrane.
Ions are charged particles: Positive charges: cations
Negative charges: anions
The cell mebrane is a lipid bilayer which does not allow the passage of ions
Membrane has protein channels that allow the passage of ions
protein channels are very selective
What are the forces that move the ions across the cell membrane?
Ions move along gradients of potential energy. What is potential energy?
In the neuron, ions are
moved by two forces (potential energy):
1.- Concentration gradient (Fig. 4.1)
Due to Concentration gradient between inside and oustside the membrane
K+, Na+, A-, Cl- inons tend to go:
K+: OUT
A- : OUT (large ions:proteins, RNA, DNA, etc, cannot leave the cell)
Na+: IN
Cl- : IN
2. Electrical gradient
Model of neuron: what happens if K+ channels open?
Movement of K+ along a CONCENTRATION gradient
creates an ELECTRICAL gradient (FIG. 4.2).
RESTING POTENTIAL: -70 mV (inside negative with respect to outside).
What happens to Na+? (FIG. 4.2)
CONCENTRATION & ELECTRICAL GRADIENTS PUSH NA+ IN !!
What happens if Na+ channels open?
Momentary reversal of potential: positive inside, negative outside
Na+ cannels
closed Na+ channels open
Outside ++++++++++ - - - - - - - - - - -
_____________________________________________________
Inside - - - - - - -
- - - - ++++++++++++
Resting Potential Action
potential
(-70 mV inside) (+50 mV inside)
Why ion channels open or close: they are GATED by several stimuli:
-electrical stimuli: differences in voltage: voltage gated.
-chemical stimuli: chemical transmitters, (in synapses).
-mechanical: for instance, the tap in the knee that produces the knee jerk reflex.
CHARACTERISTICS OF THE ACTION
POTENTIAL
Na+ and K+ channels in axon are voltage gated.
Threshold potential: 10 mV
(from -70 mV to -60 mV)
Size of action potential: 120 mV: from -70 mV to + 50 mV (all or nothing)
No action potentials in soma or dendrites (but new data suggest otherwise)
The first ionic event in the generation of an action potential is the opening of Na+ channels (Fig. 4.6).
Duration of Action Potential: about
1 msec (Fig. 4.6).
The action potential ends because (Fig. 4.6):
-The gate for Na+ closes,
-The gates for K+ opens: outflow of K+, accumulates + charges outside, bringing the potential inside back to -70 mV.
-Inflow of Cl- attracted by the + charges inside (gates for Cl- are always open).
Immediately after an action potential there is an
Absolute Refractory period,
followed by a Relative Refractory
Period.
Absolute: 1- 2 msec, no response, no matter how big the stimulus
-limits the maximum frequency of firing (500/sec)
-it does not permit the action potential to reverse direction.
Relative, the cell reponds only to strong stimuli.
All-or-nothing. The size of an action potential (110 mV) does not depend on the stimulus.
Similar to firing a gun: when trigger reaches threshold, the bullet is fired with the same speed no matter how strongly the trigger is pulled.
Propagation of Action Potential
Passive membrane
properties
The propagation of action potential is mediated by voltage-gated channels.
A potential at one place triggers the neighboring place (domino effect)
Homology with the burning of a flame down a wick. Heat-gated
channel.
A flame, like the action potential, cannot go back.
Speed of conduction:
Role of Myelin
Saltatory conduction: By isolating a segment of the axon, myelin forces the action potential to jump from one node of Ranvier to the next, thus increasing the speed of propagation of the action potential.
Na+ channels accumulate in the nodes of Ranvier
Another factor that influences speed of action potential propagation is the diameter of the axon. Conduction in large diameter axons is faster than in small diameter axons because the internal resistance is smaller in thick axons (ions can move faster).
Therefore, speed of action potential propagation will be faster in large diameter, myelinated axons, as compared to small diameter, unmyelinated axons.
In large diameter, myelinated axons, the conduction can be as much as 100 m/sec, or 224 miles per hour.
Myelosclerosis, multiple sclerosis:
slow down or stop conduction
Effect of action potentials on
the concentration of ions inside the cell:
Very small.
There is a Na+-K+ pump that kicks Na+ out and brings K+ in to maintain the concentration gradient between outside and inside the neuron at normal values. This pump requires metabolic energy (ATP).
After blockade of the Na+-K+ pump (e.g., with DNP, dinitrophenol; cyanide), the concentration gradient gradually diminishes until action potential are no longer possible.
Mechanisms of action of local and general anesthetics, and venoms:
Local anesthetics (Novocain, xylocaine) attach to Na+ channels, preventing Na+ inflow
General anesthetics (ether, chloroform) Open K+ channels: clamp potential
Scorpion Venom: Keeps Na+ channels open and K+ channels closed
Tetrodotoxin (TTX, from puffer fish) blocks Na+ channels
Cyanide blocks ATP-dependent Na+-K+ pump
