Chapter 48 (3)

Wha does resting potential result from?

However, when neurons are active, __ and __ change. The changes occur beacause neurons contain __, ion channels that open or close in response to stimuli. This gating of ion channels forms the basis of nearly all elecctrical signaling hte NS.

The opening or closing of ion channels alters the membrane's permeability to particular ions, which in turn alters the __.

Opening more K channels increases the membrane's permeability to K+, increasing hte net diffusion of K+ out of the neuron. In other words, the inside of hte membrane becomes more __. As the membrane potential approaches E_K (-90 mV at 37^oC) the separation of charge, or __ increasies.

Thus, the increase in the magnitude of the membrane potential is called a __. In general, __ results from any stimulus that increases either the outflow of + ions or the inflow of - ions.

Although opening K channels causes __, opening some other types of ions channels has an opposite effect, making the inside of themembrane less negative. This reduction in the magnitude of the membrane potential is called __.

__ in neurons often involves gated sodium channels. If the gated Na channels open, the membrane's permeabiity to Na+ ncreases, causing a __ as the membrane potential shifts toward E_Na (+67 nV at 37^oC)

The types of __ and __ we have considered are called __ because the magnitude of the change in membrane potential varies with the strength of the stimulus. A largger stimulus causes a greater change in permeability and thus a greater change in the membrane potential. __ are not the actual nerve signals that travel along axons, but they have a major effect on the generation of nerve signals.

Many of the gated ion channels in nuerons are __; they open or close in response to a chang ein the membrane potential.
- If a __ opens a votage gated sodium channel, the resulting flow of Na+ into the neuron results in further __. Becuase the sodium channels are voltage gated, an increased __ in turn cuauses more sodium channels to open, leading to an even greater flow of current. The result is a very rapid opening of all the voltage-gated sodium channels. Such a series of events triggers a massive chang ein membrane voltage called a __.

__ are the nerve impulses, or signals, that carry info along an axon.

__ occur whenever a __ increases the membrane voltage to a part value, called a __.
- Once initiated, the __ has a magnitude that is independent of hte strength of the triggering stimulus.

Because __ occur fully or not at all, they represent an __response to stimuli.
- This __ property reflects the fact that __ opens voltage-gated sodium channels, and the opening of sodium chanels causes furter __. This __ feedback loop of __ and channel opening triggers an __ whenver the membrane potential reaches the __.

In most neurons, an __ potential lasts only 1-2 msec. Because __ arre so brief, a neruron can produce hundreds of them per second. Furthermore, the frequency with which a neuron generates _ can vary in response to input. Such differences in __ frequency convey info about signal strength.

Membrane __ opens both types of channels, but they respond infependently and sequentially. Sodium channels open first, initiationg hte action potential. As the action potential proceeds, the sodium channels become inactivated: A loop of the channel protein moves, blocking ion flow through the opening. Sodium channels remain inactivated until after hte membrane returns to the __ and the channels close. K channels open more slowly than sodium channels, but remain open and functional throughout the action potential.

1- At the __, most volatage-gated sodium channels are closed. Some K channels are open, but most voltage-gated K channels are closed.
2- When a stimulus deplarizes the membrane, some gated sodium channels open, allowing more Na+ to diffuse into teh cell. THe Na+ inflow causes further __, which opens stillmore gated sodium chanels, alowing even more Na+ to diffuse into the cell.
3- Once the __ is crossed, this __ feedback cycle rapidly brings the membrane ptential close to E_Na. This stage is called the __.
4- However, two events prevent the membrane potential from actually reaching E_Na: Voltage gated sodium channels inactivate soon after opening, halting Na+ inflow; and most voltage-gated potassium channels open, causing a rapid outflow of K+. Both events quickly bring the membrane potential back toward E_K. This stage is called the __.
5- In the final phase of an action potential, called the __, the membrane's permeability to K+ is higher than at rest, so the membrane potential is closer to E_K than it is at the resting potential. The gated potassium channels eventually close, and the __ returns to __.

The sodium channels remain inactivated during the __ and the early part of hte __. As a result, if a second __ stimulus occurs during htis peridod, it will be unable to trigger an action potential. THe "downtime" following an action potential when a second action potential cannot be initiated is called the __. This interval sets a limit on the max frequency at which action potentials can be generated. The __ also ensures that all signals in an axon travel in one direction, from teh cell body to the axon terminals.

The __ is due to the inactivation of sodium channels, nt to a change in the ion grdients across the plasma membrane. The flow of charged particles during an action potential involvs far too few ions to change the concentration on either side of hte plasma membrane.

An action potential functions as a long distance signal by regenrating itself as it travels from the cell body to the __ terminals.
At the site where an action potential is initiated, Na+ inflow during hte __ creates an electrical current that depolarizes the neighboring region of hte axon membrane.

The __ in the neighboring region is large enough to reach the __, causing hte action potential to be reinitiated there. This process is repeated over and over again as the action potential travels the length of the axon. At each position along the axon, the process is identical, such that the shape and magnitude of hte action potential remain constant.

Immediately behind the traveling zone of __ due to Na+ inflow is a zone of __ due to K+ outflow.
-In the __ zone, the sodium channels remain inactivate. Consequently, the inward current that depolarizes the axon membrane ahead of hte action potential cannot produce another action potential behind it.
o THis prevents action potentials from traveling back toward hte cell b ody. Thus, the action potential that starts at the __ moves in only one direction-toward the synaptic terminals.

Several factors affect hte speed at which action potentials are conducted.
-- One is __: Wider axons conduct action potentials more rapidly than narrow ones because resistance to electrical current flow is inversely proportional the cross-sectional area of a conductor.
o A wide axon provides less resistance to the current associated with an action potential than a narrow axon does. Therefore, the resulting __ can spread farther along the interior of a wide axon, bringing more distant regions of the membrane to the threshold sooner.

Vertebrate axons have narrow diameters but can still conduct action potentials at high speed. Th adaptation that enables fast onduction in anrrow axons is a __, a layer of electrical insulation that surrounds vertebrate axons.

__ are produced by two types of glia- __ in the CNS and __ in the PNS. During development, these specialized glia wrap axons in many layers of membrane. the membranes forming these layers are mostly __, which is a poor conductor of electrical current.
- THus the __ provides electrical insulation for the axon.

The insulation provided by the __ has the same effect as increasing the axons diameter: It caueses the __ current assoited with an action potential to spread farther along hte interior of the axon, bringing more distant regions of the membrane to the threshold sooner.

The great advatage of __ is its space efficiency. A myelinated axon 20 um in diammter has a conduction speed faster than tat of a squid giant axon that has a diameter 40 times greater. Furhter more than 2000 of htose myelinated axons can be packed into the space occupied by just one giant axon.

In a myelinated axon, voltage-gated sodium channels are restricted to gaps in the myelin sheath called __. THe extracellular fluid is in contact with the axon membrane only at the nodes. As a result, action potentials are not generated int eh regions between the nodes.

Rather, the inward current produced during the __ of the action potential at a node travels all the way to the next node, whre it depolarizes the membrane and regenrates the action potential.
- This mechanism is called __ because the action potential appears to jump along the axon from node to node.