From resting membrane potential to action potential propagation

Question 1

Nearly all cells, including neurons, maintain differences of electrical potential across their cell membrane. This resting potential is typically? and how can it be measured?

Answer 1

• Between -40 mV and -90 mV, by convention measured inside relative to outside
• It can be measured by sticking a fluid-filled (e.g. 3 M KCl) glass micropipette

Question 2

What is the reference electrode ?

Answer 2

• The reference electrode is often a larger glass pipette filled with extracellular solution and positioned near the cell
• Both pipettes are connected to a voltmeter via Ag-AgCl electrodes
• The voltage of the reference electrode is subtracted in the voltmeter from the measuring electrode to give the readout of the transmembrane resting potential

Question 3

How does the cell’s resting membrane potential come about ?

Answer 3

• Mainly as a consequence of the unequal distribution of permeant ions, particularly K+, across the cell membrane
• This is analogous to a K+-selective artificial membrane that is much more permeable to K+ than to Cl-
• The instant the container is filled with the two different solutions, no electrical potential difference exists across the membrane
• Then K+ flows down its concentration gradient, transferring + charge to the outside, until the force exerted by the + charge is equal and opposite to the force of the concentration gradient
• At this point, electrochemical equilibrium is reached
• This equilibrium is described by the Nernst equation

Question 4

What is the K+ equilibrium potential (Ek) ?

Answer 4

V(in) - V(out) is the K+ equilibrium potential (EK)- it depends on the K+ concentration ratio and the absolute temperature

Question 5

What is the K+ concentration gradient (as well as that for Na+) is actively maintained by?

Answer 5

• The K+ concentration gradient (as well as that for Na+) is actively maintained by the Na+-K+ pump. If a cell dies, the cation concentration gradient disappears
• The resting potential of a cell is usually somewhere near EK, but is less negative
• This is because the cell membrane has a small permeability to other ions, with different equilibrium potentials, mainly Na+ and Cl-

Question 6

What is the membrane potential of the equilibrium potentials ?

Answer 6

The membrane potential is a ‘weighted average’ of the equilibrium potentials of all the permeant ions. As weighting factors we can use the permeabilities, p’, of the membrane for the different permeant ions.

Question 7

What is Vm ? and what is it also known as ?

Answer 7

The membrane potential, Vm, can then be described by the constant field equation, also known as the Goldman-Hodgkin-Katz equation

Question 8

What can we use instead of permeability as the weighting factor ?

Answer 8

Alternatively, using electrical membrane conductance (g) rather than permeability as the weighting factor we obtain the Chord Conductance Equation

Question 9

A small voltage applied to a nerve fibre decays along the fibre because ?

Answer 9

the resulting current doesn’t just flow through the longitudinal resistance RL, but some of it leaks away across the membrane resistance RM

Question 10

The steepness of the decay depends on ?

Answer 10

The space (or length) constant: λ = √(RM/RL)

• increases with the √ of the fibre diameter
• Even for a large myelinated never fibre λ (lambda) is only about 4mm

Question 11

Graded potentials cannot be propagated along nerve fibres by ?

Answer 11

• Passive conduction over more than 1cm or so

- Most nerve fibres are much longer than this, e.g. the axon of a motor neuron to foot muscles is about 1 m long

Question 12

What do Nerve fibres need to regenerate the size of the applied voltage ? and how is this done ?

Answer 12

• Nerve fibres need an amplification mechanism that can regenerate the size of the applied voltage
• This is done by voltage-sensitive ion channels positioned along the axon
• This amplification mechanism only works for applied voltages above a certain threshold value

Question 13

What is Membrane conductance (g) ?

Answer 13

It is the inverse of resistance (R), and is proportional to the number of open ion channels in the cell membrane

Question 14

What can a capacitor store ?

Answer 14

A capacitor can store charge, proportional to the potential across it

Question 15

Explain how action potentials travel along a nerve fibre ?

Answer 15

• Action potentials travel along a nerve fibre by means of ‘local circuit’ currents
• The currents depolarize the membrane ahead of the action potential
• This depolarization opens more Na+ channels, leading to further depolarization
• Once threshold is reached the action potential propagates further along the axon•etc

Question 16

What is Conduction Velocity?

Answer 16

The speed of action potential travel along the axon is the fibre’s conduction velocity

Question 17

The conduction velocity is faster:

Answer 17

• the further the local circuit currents spread – related to the space constant λ
• the faster the local circuit currents spread – related to the time constant τ (tau)

Question 18

The spread of current is slowed down by ?

Answer 18

The charging of the membrane capacitance CM, such that τ = RM x CM

Question 19

For an unmylinated nerve fibre, what is the conduction velocity ?

Answer 19

The conduction velocity is roughly approximated by
λ/(τ + T)
- T is a measure for the time between threshold depolarisation and the peak of the action potential

Question 20

λ increases with the ? However, τ is ?

Answer 20

• √ of the fibre diameter

- τ is independent of fibre diameter

Question 21

The result is that the conduction velocity increases approximately with ?

Answer 21

The √ of the fibre diameter

Question 22

Myelination increases λ (but not τ) by ?

Answer 22

Forcing the local circuit currents to jump from node to node: saltatory conduction

Question 23

The thickness of the myelin sheets is optimised in such a way that ?

Answer 23

The conduction velocity of myelinated axons increases linearly with the axon diameter

Question 24

Action potentials are ‘all or nothing’:

Answer 24

They carry no information about the size of the stimulus that elicited them (The power of a stimulus is not proportional to the power of the action potential)

Question 25

So how do neurons code the intensity of their synaptic input?

Answer 25

• Neurons respond with ‘trains’ of action potentials to incoming synaptic currents
• Their firing frequency goes up as the incoming synaptic activity increases

Question 26

Smaller synaptic currents have a higher threshold potential for action potential generation than ?

Answer 26

Larger currents, due to accommodation (partial inactivation of Na+current during the slower subthreshold depolarisation)

Question 27

Different patch clamp recording configurations: three methods for recording activity in single ion channels (A, B, D) and whole-cell recording (C)

Answer 27

• Whole-cell recording: most often used; summed activity of all ion channels in the cell; original intracellular solution is replaced by solution in the patch pipette
• On-cell patch: single channel activity; preserves original intracellular solution
• Inside-out patch: single channel activity; good for studying modulation of ion channel by second messengers
• Outside-out patch: single channel activity; good for studying ligand-gated ion channels
• Not shown: perforated patch; a variant of whole-cell recording that preserves key molecules (such as second messengers and Ca2+ buffers) in the intracellular solution