Membrane potential

Question 1

Like all cells, excitable cells have lipid bilayer membranes,
with embedded proteins.

ICF vs ECF ion concentrations

Answer 1

ICF/ECF
K+ = 150/5 mM
Na+ = 15/`150
Cl- = 9/125
Protein = 45/0.1

Question 2

ICF vs ECF ion concentrations

Answer 2

More in ECF = K and Cl

More in ICF = Na and protein

Question 3

RMP is caused by:

Answer 3

This unequal distribution of ions leads to a slight negative charge inside the membrane – the Resting Membrane Potential (RMP)

Question 4

The RMP is maintained by:

Answer 4

the high permeability of the membrane to K+
the active transport of Na+ across the membrane.

both via transmembrane proteins

Question 5

Ion transporters that create a slight negative charge inside relative to outside

Answer 5

K+ leak channel (allows K+ to move out, down CG - after an AP // acc to its CG) == makes cell membrane slightly positive

Na/K ATPase = 3 Na out, 2 K in

Question 6

The Na+ pump is ___________. it creates a slight positive charge on the outside.

Answer 6

electrogenic.

Question 7

When will K+ stop moving?

Answer 7

K+ will stop moving when the electrical gradient is equal and opposite to the concentration gradient.

The voltage at which this occurs = the Equilibrium Potential (EK).

so Kin = Kout

Question 8

The equilibrium potential (Eion) can be determined:

Answer 8

The Nernst Equation.

Every ion has its own equilibrium potential. The Nernst Equation for other ions gives:

Ek for K+ = -86 mV
Ek for Na+ = +60 mV
Ek for Cl- = -70 mV

Question 9

Nernst equation cannot be used to calculate membrane potential because…….

Answer 9

……..a membrane is permeable to more than one ion, and the membrane’s permeability to various ions differs.

It can be used to calculate equilibrium potential for a specific ion.

Question 10

At RMP

According to the Goldmann Equation, Em = -70 mV at RMP.

The RMP is much closer to EK (-86 mV) than it is to ENa (+60 mV). Why?

Answer 10

The biggest weighting will be given to the ion that is the most permeable - K+.

PK+&raquo_space; PNa+&raquo_space; PCl-&raquo_space; Pprotein (Rest)

Question 11

After STIMULATION

There is a very brief and rapid switch in permeability from K+ to Na+

Answer 11

PNa+&raquo_space; PK+&raquo_space; PCl-&raquo_space; Pprotein.

The membrane potential therefore moves from the RMP (-70mV) towards ENa (+60mV). This voltage change is called the Action potential.

Question 12

What is an action potential?

Answer 12

Threshold potential from -70 to -55. Depolarization from -55 to +30. Repolarization from +30 to -70. Hyperpolarization -70 to -85.

Question 13

Ionic currents cross the membrane through voltage-gated channels:

at depolarization - slow and rapid rising phase

Answer 13

1. the voltage-gated Na+ channel
   = opens and Na+ ions enter the cell, when the membrane potential reaches about -55 mV
   (TP) the voltage-gated Na+ channel opens very rapidly
   Na+ rushes into the cell through the ‘activation’ gate of the channel.
2. the voltage-gated K+ channel
   = opens and K+ ions exit the cell. opens when the membrane is depolarised, but more slowly than the Na+ channel
   closes slowly in response to membrane repolarisation

Question 14

The VGNaC has 3 states:

Answer 14

open —-fast—-> inactivated —-slow—-> closed.

VGKC has 2 states: open and closed. slow progression.

Question 15

Absolute refractory period

Answer 15

Is below graph at AP.

The absolute refractory period:The period in which the membrane cannot generate another A.P no matter how big the stimulus- the Na+ channels are inactivated

Question 16

Relative refractory period

Answer 16

Is above the graph at hyperpolarization.

The relative refractory period:
The period in which the membrane can generate another A.P, but only if the stimulus is bigger than normal
- some Na+ channels are recovered
- some K+ channels are still open.

Question 17

AP travelling through unmyelinated neuron…..

Answer 17

A graded potential above threshold reaches the trigger zone at the Axon Hillock.

VGNaC open and Na+ enters the axon.

Positive charge flows into adjacent sections of the axon by local current flow

Local current flow from the active region causes new sections of the membrane to depolarize.

The refractory period prevents backward conduction.

Question 18

Depolarization in essence has ______ effect backward

Answer 18

no effect

Question 19

How does it propagate down the axon in Myelinated neuron?

Answer 19

The action potential travels via current loops.

Question 20

What happens at a node of Ranvier? Nodes of Ranvier have __________.

Answer 20

NoR have high density of Na+ channels.

Question 21

Saltatory Conduction

Answer 21

current jumps from node to node at NoR’s

Question 22

Is the action potential the only electrical impulse generated in neurons?

Answer 22

There are two types :

Action Potentials - large, uniform depolarisations that travel rapidly for long distances without losing strength
all-or-none response

Graded Potentials - variable-strength signals that travel over short distances and lose strength.
REMEMBER: not all graded potentials generate an action potential

Question 23

Graded Potentials

Answer 23

Occur in dendrites, cell bodies or axon terminals; not in axons

A graded potential is the name of the postsynaptic electrical impulse

!the equivalent of the EPP in muscle cells