On Monday, we talked about the uneven distribution of ions between the inside and outside of the cell, and focused in particular on Na and K ions. This difference in ion concentration can be measured in voltage—(potential for work) and it is measured at -70 mV on the inside of the cell with respect to the outside. This potential difference is due in large part to the high concentration of Na ions on the outside of the cell.
Why are Na+ ions in such high concentration outside the cell?
2 reasons:
Na channels (the protein pores the span the membrane) are closed at rest. At -70mV, Na+ should want to enter the cell down it’s concentration gradient and electrostatic pressure. But, it can’t go anywhere, so it stays outside.
Na/K pump: However, some Na ions manage to sneak in, but whatever Na sneaks in gets pumped out by the Na/K ion pump embedded in the cell membrane, which pumps out 3 Na ions and brings in 2 K ions. Thus, for every operation of the pump, an extra buildup of positive charges results on the outside. The body expends a great deal of energy to keep these gradients (otherwise they will run down and equilibrium will result..as we’ll see, it’s the strong driving force on Na, due to it’s uneven distribution, that allows neural signaling to take place).
So, both of these properties of the cell membrane leave the cell highly polarized at rest…
Consequence? Na poised to rush in, given the opportunity; K fairly content where it is.
That’s “Rest”
Now, what happens when a neuron sends a signal? Small depolarizations (caused ultimately by the events at the synapse) cause the sodium ion channels (that are sensitive to the charge difference—they are “voltage-gated”) to open at the axon hillock…concept of threshold (10-15mV depolarization…when membrane potential changes from -70mV to -55mV)…causes Na to rush in, positive feedback, more Na channels open, causes membrane potential to rise from -70 to +40 (more positives on the inside than outside). At this point, the voltage-sensitive Na channels “deactivate”…they no longer conduct Na ions, until they reset (thus, they prevent backward conduction of electrical signals).
Now what about K? By the time interior membrane potential is positive, K is no longer attracted to the inside…by this time, all of its ion channels are open (it too has voltage-gated channels, they just take longer to open fully). So much K leaves the cell that the interior of the cell becomes HYPERPOLARIZED—even more uneven than at rest (with even more positively charged ions on the outside than the inside).
These events, the ion exchange and resultant temporary reversal of membrane potential…is called the action potential.
Propogation of the action potential down the length of the axon—the action potential “travels” down the surface of the membrane—literally, the Na+ ions that entered at the axon hillock both depolarize the local patch of membrane, but also are attracted downstream—so the ions that enter during an action potential also depolarize adjacent patches of membrane, thus regenerating the action potential at successive points along the surface of the membrane. In this manner, the action potential does not lose strength as it travels down the axon to the axon terminal—the reversal of membrane potential is just as large (from -70 to about +40) at the axon terminal as it is at the axon hillock.
Myelination (causing the action potential to jump from node to node, since the ion exchange can’t take place through the myelinated segments) increases the speed of transmission from one end of an axon to another. Similarly, axon diameter also affects speed—larger diameter axons have less resistance to ion flow so they reach adjacent patches of axon sooner (despite the larger surface area…remember that the action potential really only happens across the membrane’s interior and exterior—it does not involve the entire diameter of the axon).
Events at the synapse.
Structural specializations of the synapse:
Axon terminal: containing synaptic vesicles, which contain neurotransmitter, Calcium ions (Ca++) are in high concentration outside the cell, channels (located here) are voltage-gated and closed at rest (open in response to depolarization).
Dendritic membrane (of postsynaptic cell): contains receptor proteins whose 3-D structure “fits” that of the neurotransmitter.