MEMBRANE POTENTIAL.
voltage across membranes
RESTING POTENTIAL: potential when a neuron is not transmitting an impulse, is at rest. Na+ and Cl- concentrations are higher outside than inside the cell. K+is higher inside the cell, together with another negativelly charged molecules as amino acids. In mammals
Cytoplasm (more negative)
Extracellular fluid (more positive)
Na+
15mM
150mM
K+
150 mM
15mM
Cl-
10 mM
120mM
A-
100mM
- The Na+ K+ gradient are maintained by the SODIUM/POTASSIUM PUMP (active transport)
animation
3 Na+ out 2 K+ in
-During the resting potential together with the pumps many K+ channels are opened and also a relatively small amount of Na+ channels are opened
Net flow of ions: currents of ions are equal and opposite. Membrane is more permeable to potassium than to sodium. UNGATED CHANNELS (always Opened)
ACTION POTENTIAL -Voltage gated channels: open and close when the membrane potential changes -Ion movement down its electrochemical concentration -TRESHOLD LEVEL HAS TO BE REACHED! -(a certain membrane potential needed for the action potential to be triggered) (all or none effect)
During action potentials both types of channels are opened (independently and sequentially response) Na+ channels open before K+ channels (gated)
1) RESTING POTENTIAL - Voltage gated channels are closed - Ungated channels are opened - Na+/K+ pumps are working
2) DEPOLARIZATION A stimulus opens the Na+ voltage gated channels, so there is an influx of Na+. If depolarization reaches the treshold AN ACTION POTENTIAL IS TRIGGERED A positive feedback cycle happens: more Na+ channels will be opened while the K+ remained closed
Inside is more positive than outside at the end
3)REPOLARIZATION
-K+ voltage gated channels opened, so that K+ is going out (inside of the cells negative) -Na+ voltage gated channels closed
4) Final phase of action potential is UNDERSHOOT OR HYPERPOLARIZATION Membrane permeability to K+ is higher than at rest All Na+ volt. gat.channels are closed Some K+ are opened, but eventually closed, so that the neuron returns back to rest. During the REFRACTORY PERIOD no impulse can be transmitted at that point of the neuron (intervals to frequency of action potentials)
Nerve impulse
Synapse animation
Transmission along synapse animation