Nerve conduction: clinical examples and experimental evidence

Question 1

How is a membrane potential measured?

Answer 1

Electrodes connected to an amplifier and recording system are placed on either side of the cell membrane. One in cell and other reference electrode

Ions move and allow a current to flow and thus PD.

Use of very small glass pipette to minimise damage to cells

Question 2

Hodgkin and Huxley’s experiment

Answer 2

Measured the absolute magnitude of the electrical potential in a living cell by inserting a 50 micrometer capillary electrode into a squid giant axon and it became -60mV

On stimulation, production of a shock, the PD was 100 mV, showing a switch of charge

Question 3

How was the impermeability of sodium ions at resting potential disproven?

Answer 3

Radioisotope sodium-24 injected and tracked

Question 4

Experiments on the active transport of ions

Answer 4

Radioisotope sodium-24 loaded in giant squid axon. Concentration of extracellular sodium measured every 10 minutes.

Metabolic inhibitor added, DNP, causing the count rate of sodium to drastically drop. Ouabain also inhibits Na/K ATPase

Shows that ATP is required to help maintain the charge balance.

Question 5

Explain voltage clamping

Answer 5

Use: measure the ionic conductance of a nerve membrane

1. two internal electrodes attached- one to monitor the centre of the stretch of axon to be clamped and the other to pass uniform current

Question 6

Neher and Sakmann’s voltage clamp method

Answer 6

Single ion channel removed from a small patch of membrane.

Question 7

Different forms of patch

Answer 7

Whole cell patch- entire piece of membrane maintained in patch

outside out patch- small piece of membrane with circle facing out

inside out patch- small piece of membrane inside

Question 8

How to determine length of refractory periods?

Answer 8

Constantly shock axon and see if action potential is formed.

Question 9

How was saltatory conduction proven?

Answer 9

Tasaki showed that the threshold of stimulation in a myelinated fibre was much lower at the nodes than the internal stretches

Local anaesthetics were more effective at the nodes.

Question 10

How was the ion channel distribution proven?

Answer 10

Used antibodies with fluorescent tags to bind to both potassium and sodium channels. Shown to be concentrated at the nodes