Sfm/cell Physiology/action Potentials

Front 1

Name five uses of APs.

Back 1

Neural stimulation of muscle cells, transmission of sensory input from the periphery to the brain, generation of contraction within muscle, processing of information in the brain, and central control of peripheral nerves and organs.

Front 2

What are excitable cells? What are some common examples?

Back 2

Cells that can generate and conduct APs; nerve cells (including some sensory receptor cells) and muscle cells.

Front 3

When ions enter a cell, what properties determine whether they will stay in or leave?

Back 3

Cable properties--the ratio of membrane to cytosolic volume, how easy it is for ions to diffuse within the cytosol, and how leaky the membrane is (how many channels per square micron of membrane).

Front 4

What happens when most ions enter a cell?

Back 4

Since ions can leak out as well as into the cell, a change in membrane potential induced by entry of ions is typically undone at points away from the site of entry.

Front 5

What's an AP?

Back 5

An all-or-none, propagating, self-regenerating change in membrane potential that is typically initiated by local potentials.

Front 6

In an AP, what reinforces the local potential that depolarizes the cell?

Back 6

V-gated Na channels. In the heart, V-gated Ca channels.

Front 7

What are the three phases in an AP?

Back 7

Depolarization, repolarization, and hyperpolarization.

Front 8

What does depolarization beyond threshold do to V-gated Na channels?

Back 8

It first opens them, leading to depolarization. It then closes them, helping to repolarize the cell.

Front 9

What does depolarization do to V-gated K channels? What are these channels called?

Back 9

It opens them, but more slowly than it opens Na channels. They're called delayed rectifier channels for this reason. Along with the closing of the Na channels, K channels help repolarize the cell.

Front 10

What provides the undershoot in hyperpolarization?

Back 10

Open K channels allow K to leave the cell. The cell is approaching Nernst equilibrium for K.

Front 11

Which ion are cells most permeable to?

Back 11

K, but they're permeable to others as well. Thus, while the resting potential of a cell is close to Nernst for K, it's slightly more positive than EK.

Front 12

What does threshold mean?

Back 12

The level of depolarization required to open enough Na channels to develop an AP. In other words, it's the level of depolarization required to get the local circuit currents flowing.

Front 13

What's a subthreshold potential?

Back 13

A local potential that is too small to generate an AP.

Front 14

What are local circuit currents?

Back 14

Loops of membrane current flowing during the propagation of an AP.

Front 15

What kind of feedback do local circuit currents give?

Back 15

Positive feedback--this ensures that APs are regenerative.

Front 16

Does reaching threshold rely on V-gated Na channels alone?

Back 16

No. It relies on local circuit currents as well, which are the result of several ions.

Front 17

How does Bactine work?

Back 17

It blocks Na channels and raises the threshold. Therefore, pain-sensing neurons (nocioceptors) can't fire APs as effectively.

Front 18

Parathyroid and kidney problems can both cause problems regulating levels of extracellular ____.

Back 18

Ca++.

Front 19

What happens as a result of hypocalcemia?

Back 19

Since extracellular Ca is low, it can't bind to ion channels, which opens those channels. As a result, threshold is lowered. This leads to increased excitability:hyper-reflexia, spontaneous twitching, muscle cramps, tingling, numbness and in extreme cases low calcium tetany.

Front 20

What happens as a result of hypercalcemia?

Back 20

More Na channels than normal are inhibited by extracellular Ca, raising threshold. Symptoms are muscle weakness, paralysis, and coma.

Front 21

What do both increases and decreases in extracellular Ca do to the heart? To the CNS? Why?

Back 21

They cause arrhythmias, memory loss, and confusion. They don't really change the membrane potentials of cells; they change how ion channels sense and respond to membrane potential.

Front 22

What's the absolute refractory period?

Back 22

The period of Na channel inactivation when it is impossible to evoke another AP.

Front 23

What is accommodation?

Back 23

When Na channels become progressively inactivated as a result of a membrane being slowly, slightly, but continuously depolarized. In extreme cases APs cannot occur. This happens in ischemia of the heart and many other pathological conditions.

Front 24

What's the relative refractory period?

Back 24

When Na channels are no longer inactivated, but K channels are still open.

Front 25

What effects does hyperkalemia (elevated extracellular K) have?

Back 25

At first it produces a depolarization that leads to excitability, but if sustained it can lead to accommodation.

Front 26

The symptoms from extended hyperkalemia appear the same as symptoms of ____.

Back 26

Hypokalemia.

Front 27

What effects does hypokalemia have?

Back 27

It gives a hyperpolarization that depresses excitability.

Front 28

What causes afterhyperpolarizations and where are they found?

Back 28

Found in many CNS neurons. Result from the slow-opening of Ca-activated K channels after the opening of delayed rectifier K channels. The Ca-activated K channels open because they are located near voltage-dependent Ca channels, which let in enough Ca to activate them but not enough to produce a significant Ca current or voltage change. AHPs reduce firing by increasing the relative refractory period.

Front 29

Why do heart APs have a long plateau phase?

Back 29

Depolarization causes Ca channels to open--Ca entry through these channels is used in contraction. Some smooth muscle cells have similar APs.

Front 30

What is an HCN?

Back 30

A hyperpolarization-activated, cyclic nucleotide-regulated channel.

Front 31

How is pacemaker potential maintained?

Back 31

Direct binding of cAMP to the HCN channel makes the HCN more sensitive to hyperpolarization. A large number of open K channels cause hyperpolarization, which triggers the opening of HCN, resulting in a net influx of cations which cause the membrane voltage to depolarize. The depolarization opens Ca-selective ion channels which produce a Ca AP. This AP causes a delayed opening of additional K channels, repolarizing the membrane again and causing closure of the Ca channels, which leads to further hyperpolarization. This hyperpolarization reactivates HCN and the cycle starts again.

Front 32

How does epinephrine increase heart rate?

Back 32

It increases cAMP in the heart, making HCN more sensitive. It also stimulates a kinase that phosphorylates Ca channels, making it easier for them to open as well.