What is diffusion potential and what causes it?
Electrical potential caused by a concentration gradient of an ion.
What's the electrical potential of a normal nerve fiber required to stop K diffusion out? Na diffusion in?
-94mv for K and +61mv for Na
Define the Nernst potential
The diffusion potential level across a membrane that exactly opposes the net diffusion of a particular ion through the membrane.
What determines the magnitude of the Nernst potential?
The ratio of the concentrations of that specific ion on the two sides of the membrane
What is the formula for Nernst potential?
EMF= +- 61 x log Ci/Co
What 3 factors determine diffusion potential when a membrane is permeable to several different ions?
- Concentration gradient
-Permeability of the membrane to each ion
-Polarity of the electrical charge of each ion
What equation is used to calc membrane potential on the inside, with 2 univalent + ions and 1 univalent - ion is involved?
Goldman-Hodgkin-Katz equation p58
Which 3 ions are most important in creating membrane potentials in nerve and muscle fibers?
Na, K, Cl
What is the resting membrane potential of large nerve fibers?
-90mv
What is a K leak channel?
Allows K to leak out of a nerve cell even in resting state. They also leak a small amount of Na.
Describe ion movement during depolarization and repolarization.
Membrane suddenly becomes permeable to Na when threshold is reached, allowing a lot of Na inside the axon. The -90mv charge is immediately neutralized = depolarization. Na channels then close and K channels open more than usual, pushing K out and re-estab
At what membrane potential do voltage gated Na channels open?
between -70 and -50 mv
Can Na voltage gated channels open again prior to repolarization?
No, the inactivation gates stay closed until the cell is near resting membrane potential.
Describe the function of voltage gated K channels, and their timing in relation to Na voltage gated channels.
There is a conformational opening in the voltage gated K channels when membrane potential rises towards 0. They are slower though and arent open until Na channels are beginning to close.
Describe the role of Ca ions in relation to membrane potentials.
Similar in function as Na in some cells, and contributes to depolarizing of the cell. Voltage gated Ca channels are slower though and provide more sustained depolarization. (important in cardiac and smooth muscle)
What type of feedback cycle opens Na channels?
Positive feedback
Define the threshold for stimulation of an action potential.
The voltage where an action potential is triggered. -65mv in nerve fibers.
What is the "All or nothing" principle?
Once an action potential is triggered, it either travels across the entire membrane is conditions are right, or doesn't travel at all if conditions aren't right.
How does the cell re-establish the Na/K gradient after an action potential?
Na/K ATPase pumps
What triggers Na/K ATPase pumps?
Excess Na ions inside the cell. There is an exponential increase in its activity when stimulated.
What causes the plateau in heart a fibers action potential?
Prolonged action of slow Ca channels. The fast Na channels cause the spike, Ca channels cause the plateau.
Describe the mechanism responsible for the spontaneous rhythmicity of heart fibers.
Resting membrane potential in the SA node cells are -60 to -70mv, which isn't enough to keep the Na and Ca channels completely closed, which causes increases in membrane potential until threshold is reached.
What keeps the heart from depolarizing immediately after repolarization?
Towards the end of each action potential and a little after, the membrane becomes more permeable to K. The increased outflow of K leaves the interior more negative than normal for nearly a second after the end of an action potential. That state is called
What is saltatory conduction?
The 'jumping' of action potentials between nodes of Ranvier. Electrical potentials flow through the extracellular fluid outside the myelin sheath and through the axoplasm.
What 3 triggers can start an action potential?
mechanical disturbance of the membrane, chemical effects on the membrane, or passage of electricity through the membrane.
What's an acute local potential?
Potential changes in a single cell membrane, they are called acute subthreshold potentials when they fail to elicit an action potential.
How do local anesthetics and/or high Ca concentrations inhibit nerve impulses?
They decrease membrane permeability to Na. Local anesthetics work directly on the activation gates of Na channels, making it difficult for the gates to open.