3. Membrane & Action Potentials

Question 1

How would you measure membrane potential?

Answer 1

Use a MICROELECTRODE - a fine micropipette filled with conducting solution (KCl).

The microelectrode penetrates the membrane and is connected to another electrode outside of the cell, via a voltmeter.

Question 2

What is gating? Give two types.

Answer 2

Gating is when channel proteins undergo a conformational change in response to a stimulus and open or close. This alters the selective permeability if a membrane.

The stimulus depends on the type of gating mechanism:

LIGAND GATING - binding of intracellular messenger or extracellular transmitter.

VOLTAGE GATING - change in membrane potential.

Question 3

How is resting potential achieved?

Answer 3

Resting Potential is largely dependent on K+. The membrane is selectively permeable to K+ as VOLTAGE INSENSITIVE CHANNELS are open.

K+ EQUILIBRIUM is achieved as K+ is extruded along its concentration gradient until it is opposed by the membrane potential (which is kept negative by intracellular anions which remain in the cell).

The electrical gradient is balanced with the concentration gradient.

Other ions are able to leak across their channels this the resting potential is rather less negative than Ek.

Question 4

What are the TWO types of synaptic transmission?

Answer 4

1. FAST SYNAPTIC TRANSMISSION (Receptor is a ligand gated ion channel)
2. SLOW SYNAPTIC TRANSMISSION (G Protein Coupled Receptor)
   a) Direct G Protein Gating (G Protein interacts with channel)
   b) Gating via Intracellular Messenger (signal cascade opens channel)

Question 5

What are action potentials and describe their characteristics?

Answer 5

Action Potentials are a change in voltage across the membrane which only occur when the threshold is reached. They are generated by an exponential increase in Na+ permeability which shifts MP toward ENa.

• Depend on ionic gradients and relative permeability
• All or nothing
• Propagated along axon without loss of amplitude

Question 6

How would you measure the currents (i.e. flow of Na+ or K+) across the membrane at specific membrane potentials?

Answer 6

VOLTAGE CLAMP can control membrane potential at certain values so that the movement of Na+ and K+ ions can be measured. This gives an indication of the opening of channels.

We learn from this that K+ channels open after a delay and stay open longer compared with Na+ channels.

Question 7

What causes depolarisation and repolarisation?

Answer 7

DEPOLARISATION

• Na+ channels open in response to depolarisation to threshold
• After threshold, positive feedback loop occurs - more and more channels open thus more depolarisation.

REPOLARISATION

• Delayed K+ Channels opening in response to depolarisation.
• Na+ Channel Inactivation (require hyperpolarisation to recover)

Question 8

What is difference between absolute and refractory period?

Answer 8

Absolute refractory period - initially Na+ channels are open but soon afterwards NEARLY ALL of the Na+ channels are inactivated.

Relative refractory period - Na+ channels are recovering from inactivation (excitability gradually returns to normal)

Question 9

Define the membrane potential.

Answer 9

Membrane potential is the electrical potential difference across a cells plasma membrane.

Question 10

Voltage gated Na+, Ca2+ and K+ channels have similar overall structures. Describe their similarities and differences.

Answer 10

Na+ and Ca2+ channels are similar. These consist of an alpha subunit made of 4 repeating units each with 6 transmembrane domains. The 4th transmembrane domain in each unit is VOLTAGE SENSING.

K+ channels have similar overall structure but is made of 4 alpha units each a quarter that of the Na+ or Ca2+ channel.

Na+ channels have an INACTIVATION PARTICLE.

All cation channels allow other cations through but K+ is MOST SELECTIVE.

Question 11

How do local anaesthetics (e.g. Procaine) work?

Answer 11

Local anaesthetics bind to and block Na+ channels thus preventing action potentials.

1. Local anaesthetics are weak bases which cross membrane in an UNIONISED form.
2. React with a H+
3. Block the Na+ channel

Question 12

In what order do local anaesthetics act on different sorts of axons?

Answer 12

1. Small Myelinated axons
2. Non-Myelinated axons
3. Large Myelinated axons

Question 13

How would you measure action potential conduction?

Answer 13

1. ELECTRICAL STIMULATION - cathode (negative electrode) stimulates axon
2. EXTRACELLULAR RECORDING
   a) DIPHASIC (axon intact)
   b) MONOPHASIC (partially damaged axon)
3. CONDUCTION VELOCITY - distance / time lapse between stimulating and recording electrodes.

Question 14

How is an action potential propagated along the axon?

Answer 14

Change in membrane potential spreads to adjacent sections of the axon via LOCAL CURRENTS. When this causes depolarisation to THRESHOLD, can action potential fires.

Question 15

Define the length constant with regards to action potentials. How can it be prolonged?

Answer 15

LENGTH CONSTANT is the distance it takes for the relative membrane potential to fall to 37% of its original value.

It can be maximised by:
a) HIGH MEMBRANE RESISTANCE

b) LOW MEMBRANE CAPACITANCE
c) LARGE AXON DIAMETER

Question 16

Why are action potentials propagated in only one direction?

Answer 16

Action potentials are UNIDIRECTIONAL because the section of membrane which has just fired an action potential is REFRACTORY so Na+ channels need to recover.

Question 17

Which axons are myelinated and what does it achieve?

Answer 17

Large diameter (motor) axons are myelinated.
Small diameter (C fibre sensory) axons are not myelinated.

Myelination increases conduction velocity in axons of diameters > 1um because myelination allows saltatory conduction.

SALTATORY CONDUCTION reduces capacitance and increases resistance in internodal membrane.

Question 18

What are the consequences of demyelinating conditions?

Answer 18

Demyelinating conditions (e.g. Multiple Sclerosis) cause AUTOIMMUNE destruction of myelin or proteins involved in myelin structure.

This causes initially a block of action potentials (as the internodal regions lack Na+ channels).

Eventually the action potentials return but are slower (as Na+ channels redistribute).

Question 19

What is a Nicotinic Acetylcholine Receptor and how does it initiate an action potential?

Answer 19

Nicotinic Acetylcholine Receptor is a ligand gated ion channel consisting of 5 subunits, each which 4 transmembrane domains. One transmembrane domain in each subunit lines the pore.

nAChR allows Na+ and K+ ions through its pore after binding of an Acetylcholine molecule at each of its 2 alpha subunits.

When the pore is open, the membrane permeability to Na+ is increased significantly thus a REVERSAL POTENTIAL is initiated.

This means the membrane potential shifts towards ENa (~ -10mV).

As the threshold is reached an ACTION POTENTIAL at the motor end plate is fired.

Question 20

What are the TWO types of nAChR blockers?

Answer 20

COMPETITIVE blockers (e.g. Tubocurarine)
- Bind at ACh site without opening pore.

DEPOLARISING blockers (e.g. Succinylcholine)

• Bind to nAChR and open pore but not broken down by acetylcholinesterase therefore cause MAINTAINED DEPOLARISATION.
• Adjacent Na+ channels INACTIVATED
• nAChR DESENSITISED and closed.

Question 21

What is Myasthenia Gravis?

Answer 21

Myasthenia Gravis is an AUTOIMMUNE CONDITION. Antibodies attack nAChRs on motor end plates thus cause reduced amplitude end plate potentials.

Symptoms:

• Profound weakness (exacerbated by exercise)
• Sudden falling
• Ptosis

Question 22

What are Miniature End Plate Potentials?

Answer 22

Miniature End Plate Potentials are induced normally by the SPONTANEOUS release of small numbers of vesicles.

These are of reduced amplitude in myasthenic muscle.