CB25 Excitable Cells

Question 1

Describe the ionic basis of the action potential

Answer 1

• In response to stimulus, the inward movement of + ions can depolarise the cell
• if sufficient to reach threshold, the stimulus triggers a series of events & full depolarisation of the nerve
• initiates an action potential
• after depolarisation, positive ions move out of the cell - initiates repolarisation
• when the negativity of the cell is greater than the resting potential, the cell is hyperpolarised
• eventually returns to resting potential -70mV

Question 2

Describe the time course of change of sodium & potassium conductance’s

Answer 2

• Increase of na+ conductance is rapid & allows for the influx of Na+ ions that is responsible for depolarisation of the action potential
• increase in k+ conductance is delayed & allows for the efflux of k+ ions that is largely responsible for the repolarisation phase of the action potential
• Na+ conductance decreases sharply after peak - caused by inactivation of na+ channels

Question 3

Explain the characteristics of the action potential

Answer 3

Resting potential = - 70mV
Threshold = - 50mV

Once depolarisation reaches threshold, the action potential will fire

Absolute refractory period
Relative refractory period

Question 4

Describe the effects of myelination on velocity of propagation

Answer 4

Myelination improves the rate of conduction

Question 5

What is saltatory conduction

Answer 5

Rapid propagation of the action potential along myelinated axons from one node of ranvier to the next, signal intense due to high concentration of Na+ channels

Depolarising charge jumps between nodes - saltatory conduction