Lecture 7: Excitable Cells: Muscle and Nerve

Question 0

Electrical signals are change in membrane potential

Answer 0

Neurons/Muscles have a separation of charge at the membrane which results in a membrane potential known as Vm

Ion channels allow ions to cross the cell membrane rapidly

Current (movement of ions) change the membrane potential

Membrane potential changes are the basis of neuronal/ muscle signaling

Question 1

Lipid bilayer

Answer 1

Impermeable to ions

Cell membrane does not allow ions to cross

Question 2

What are two key contributions to Vm?

Answer 2

1) ions are not distributed equally across the cell membrane
2) membranes are unequally permeable to various ions.

Question 3

Unequal ion concentrations

Answer 3

Distributions vary by animal species and neuron type, but relative distributions similar

Distribution depends on ATP driven pumps

Question 4

Case 1:

Answer 4

Permeable to all ions equally

Ions move down concentration gradients

No electrical potential

Question 5

Unequal permeability to various ions

Answer 5

At rest: more permeable to K+ than Na+

Permeable- a surface that allows another substance to pass through it

Question 6

Case 2:

Answer 6

Membrane permeable to k+ only

K+ moves down concentration gradient

Electrical potential develops

K+ moves back inside cell due to electrostatic attraction

Question 7

A simple cell model: two opposing forces

Answer 7

Concentration gradient and electrical potential

Question 8

What is the equilibrium potential (Ek)?

Answer 8

The equilibrium potential is the membrane potential at which the concentration gradient and electrical potential forces are equal and opposite.

At Ek k+ is at equilibrium (no net movement of k across the membrane)

Question 9

A common misconception

Answer 9

Despite movement of ions, concentration of ions does not significantly change!

A very small percent of ions move to generate a sufficient Vm to balance the diffusion force.

Question 10

What is the Nernst equation?

Answer 10

Ex= (58/z) log (Xo/Xi)

Ex is the mV
X is mM
Z is the charge in the ion (valence)

Na+ and K+ z=1
Cl- z=-1

Question 11

Electrical signals: Nernst equation

Answer 11

Describes the membrane potential that a single ion would produce if the membrane were permeable to only that ion:
Eion= (61/z) log(ion out)/ion in)

Membrane potential is influenced by:
Concentration gradient of ions
Membrane permeability to those ions

Question 12

Nernst equation

Answer 12

Ex= (58/z) log(Xo/Xi)

The concentration gradient and charge of an ion determines the membrane potential at which it is at equilibrium

If no concentration gradient then Xo=Xi and Ex=0

Question 13

Membrane permeable to multiple ions

Answer 13

Vm is the cell membrane at which the net flux of ALL ions together equals zero.

Vm is a steady state, but not an equilibrium.

Question 14

Vm depends on Ex and permeability of ions

Answer 14

If a cell is permeable to multiple ions Vm is a weighted average of the Ex for each permeable ion

Given by Goldman-Hodgkin-Katz equation

Question 15

Goldman-Hodgkin-Katz equation

Answer 15

Predicts membrane potential that results from the contribution of all ions that can cross the membrane.

Vm= 58 log (PkKo + PnaNao + PclCli)/PkKi + PnaNai + PclClo)

Vm is somewhere between the largest Ex and smallest Ex.

Like a mathematical average

Question 16

Goldman-Hodgkin-Katz equation

Answer 16

Ions with greater permeability have greater influence on Vm. The large the influence of an ion X, the closer Vm will be to Ex.

At rest mostly permeable to k+ so Vm close to Ek.

If only permeable to 1 ion (for example K) P for all others is 0 and the equation reduces down to the Nernst equation.

Key concept:

The GHK equation shows us that you can change the membrane potential of a neuron Vm by changing the permeability of different ions.

Question 17

Resting membrane potential

Answer 17

Vm depends upon Ex of permeable ions

K+ tends to leave the cell at Vm

Na+ tends to enter the cell at Vm
The concentration gradient AND the membrane potential are pushing Na+ ions into the cell

Question 18

The Na-K pump

Answer 18

At Vm only the NET movement of all ions considered together is 0 ( ie steady state).

The concentration gradients of ions will run down over a period of hours to days

The Na-K pump uses ATP to maintain the ion concentrations at constant levels

The ions are pumped against their concentration gradient:
Move Na out of the cell

Question 19

Changes in membrane potential

Answer 19

Terminology associated with changes in membrane potential
Depolarization
Repolarization
Hyper polarization

Question 20

Synaptic potential

Answer 20

Binding of neurotransmitter causes receptors to open which increases the permeability of the ions that the receptor prefers

GHK: if a neurotransmitter receptor opens and increases permeability to Na+ Vm will increase towards Ena

Question 21

Synaptic Potentials

Answer 21

Depolarization of the membrane potential is more positive than Vm ( excitatory EPSP)

Hyper polarization if membrane potential more negative than Vm
(Inhibitory IPSP)

Question 22

Passive properties-Grades potentials

Answer 22

Graded potentials decrease in strength as they spread out from the point of origin. Like synaptic potentials

Question 23

Propagation of synaptic potentials

Answer 23

Synaptic potentials travel from synapse in all directions

Synaptic potentials get smaller and slower over distance

Due to passive membrane properties (like an electrical cable).

Question 24

Integration of synaptic potentials

Answer 24

Soma receives lots of EPSPs and IPSPs

The neurons decision maker is at the base of the axon = axon hillock

If membrane potential depolarizes beyond a certain level (threshold) an action potential is triggered

Neuron constantly receiving IPSPs and EPSPs (has thousands of synapses).

These changes are localized in some and time. When two signals run into each other they are linearly added together.

Question 25

Temporal summation

Answer 25

A synapse receives 2 EPSPs separated in time. No summation

2 EPSPs at nearly the same time. Summation.