Topics 3-4

How does an unequal distribution of ions produce a resting Em?

The membrane is selectively permeable to certain ions(usually K+).

When recording injury potentials, Bernstein found that:

temperature varied linearly with temperature as predicted in the Nerst equation for K+.

When measuring the membrane potential for the squid giant axon, Em followed the Nernst equation except at low levels of K+. Why?

Due to Na+ leak into the cell

How does the GHK equation differ from the Nernst equation?

GHK considers movement of multiple ions and their permeability

In cells that do not actively pump Cl- across their membranes, Erest= _____.

Erest = E_Cl

How is the unequal distribution of ions maintained?

Active transport of 3 Na+ out, 2K+ in through the sodium potassium pump

What is an electrogenic pump?

A pump that exchanges unequal numbers of ions and allows electrical charge to accumulate across the membrane

Cardiac glycoside drugs ______ Na+/K+ pump activity. Give 3 examples.

inhibit; ouabain, digitoxin, and digoxin

How do cardiac glycoside drugs increase cardiac muscle contractibility?

By inhibiting the Na+/K+ pump, it increases intracellular [Na+], which reverses Na+/Ca++ pump, leaving more Ca2+ inside the cell, increasing force of contractions.

Post-tetanic hyperpolarization(PTH) an result from:

Stimulation of Na-K pump activity by a prolonged, high-frequency burst of action potentials.

In order to contribute to resting membrane potential, an ion channel must be _________.

Mostly open at rest

a type of channel that generates a large fraction of resting potassium “leak”, or “background” current

2P K+/K2P

What K+ channels contribute to RMP?

K2P, M-type, HCN type(Na+/K+ channel)

What Na+ channels contribute to RMP?

HCN type(Na+/K+ channel), NALCN cation channels, V-gated Na+ channels

What are 2 major functions of action potentials?

1. Rapid transmission of information over long distances

2. Control of effector responses (i.e., muscle contraction or release of neurotransmitters).

The translation of neural activity from a graded amplitude code to a spike frequency code

Spike transduction

1 / interval between spikes

Spike frequency

the period after an action potential during which a stimulus cannot elicit a 2nd action potential

Absolute refractory period

The period after an action potential during which the threshold for initiation of a 2nd action potential is increased.

Relative refractory period

Using a Wheatstone bridge to measure the electrical impedance of a giant squid axon, what did Cole and Curtis discover about action potentials?

membrane impedance decreased/conductance increased during an action potential

What happens to sodium permeability during APs?

There is a selective and transient increase in membrane permeability to Na+ ions

The amplitude of the AP recorded in the giant squid axon is proportional to:

Extracellular Na+ ions

What does the feedback amplifier in a voltage clamp do?

Generates a current that is directly proportional to the difference between the measured membrane potential and the command potential

What does a voltage clamp do?

Measures ionic currents flowing across a cell membrane by "clamping" the membrane potential at a set voltage, preventing it from changing in response to ion channel activity

Which 2 different voltage-gated ion currents were revealed by voltage clamping?

an early inward current and a late outward current

The outward movement of channel protein S4 during depolarization

Gating currents

Describe the relationship between sodium current, conductance, and driving force.

increasing GNa, decreasing driving force

Describe the relationship between potassium current, conductance, and driving force.

Increasing GK+, increasing driving force

Early inward current is carried by ____ ions and the late outward current is carried by ____ ions.

Early inward current is carried by Na+ ions and the late outward current is carried by K+ ions.

What does Tetraethylammonium (TEA) do?

Blocks VG K+ channels

What does the enyzme pronase do?

Blocks Na+ channel inactivation

True or false: Na+ channel activation and inactivation are dependent on one another

False, Na+ channel activation and inactivation are separate processes

Is Na+ channel inactivation or activation slower?

Inactivation is slower

How does the K+ negative feedback loop work

Depolarization—>increased K+ membrane permeability—>brings Vm closer to Ek, opposing depolarization

How does the Na+ postitive feedback loop work

Depolarization—>increased Na+ membrane permeability—>brings Vm closer to Ena, further depolarizing the membrane

How can you raise the AP threshold?

By enhancing the K+ mechanism (↑GK) and/or reducing the Na+ mechanism (Na channel inactivation)

What is the ball and chain method of Na+ channel inactivation?

Inactivation gate of Na+ hangs out(ball) of the cytoplasm. Activation creates a receptor for the inactivation gate. The inactivation gate can then bind to the receptor

What is the effect of raising external Ca2+ concentration in nerve and muscle excitability?

Decreases excitability

What does Parathyroid hormone (PTH) do?

Increases the extracellular [Ca2+] in the blood

What does calcitonin do?

Decreases the extracellular [Ca2+] in the blood

Causes and effects of hypocalcemia

Caused by a decrease in extracellular [Ca2+] by hyperthyroidism or hypoparathyroidism. Increases the excitability of nerve and muscle.

Causes and effects of hypercalcemia

Caused by an increase in extracellular [Ca2+] by hypothyroidism or hyperparathyroidism. Decreases excitability of nerve and muscle.

Describe the surface potential theory

The charged surface of membranes attracts Ca2+ ions and create an electric field inside the membrane which adds to that provided by the resting potential

Absolute Refractory Period is mainly driven by _____________.

Na⁺ channel inactivation.

Relative Refractory Period is mainly driven by _____________.

Increased K⁺ conductance

What ion channels are in the node of ranvier?

Na+ and K+