The nerve impulse

Question 1

What is resting potential?

Answer 1

• a nerve impulse may be defined as a self-propagating wave of electrical activity that travels along the axon membrane
• it is a temporary reversal of the electrical potential difference across the axon membrane
• the reversal is between two states, called the resting potential and the action potential

Question 2

How is the movement of ions across the axon membrane controlled?

Answer 2

• the phospholipids bilayer of the axon plasma membrane prevents sodium and potassium ions diffusing across it
• channel proteins that span the phospholipids bilayer have gated ion channels in them which can be opened or closed by facilitated diffusion sometimes so sodium and potassium can pass through
• sodium potassium pumps also controls resting potential via active transport in and out of the axon

Question 3

Which events cause the establishment of this potential difference?

Answer 3

• sodium ions are actively transported out of the axon by the sodium potassium pumps
• potassium ions are actively transported into the axon by the sodium potassium pump
• the active transport of sodium is greater than that of potassium ions so three sodium ions move out for every two potassium ions that move in
• although both sodium and potassium ions are positive the outward movement of sodium ions is greater than that of potassium ions so there is an electrochemical gradient veteran the cytoplasm and the tissue fluid
• so the sodium ions diffuse naturally back to the axon while the potassium ions begin to diffuse back out of the axon
• most of the potassium ion gates are open and most of sodium ion gates are closed

Question 4

What is the action potential?

Answer 4

• when a stimulus of sufficient size is detected by a receptor in the nervous system, its energy causes a temporary reversal of the charges either side of this part of the axon membrane
• this is known as the action potential and in this condition this part of the axon membrane is said to be depolarised
• the energy of the stimulus caused some sodium voltage-gated channels in the axon membrane to open and therefore sodium ions diffuse into the axon through these channels along their electrochemical gradient, being positively charged they trigger a reversal in the potential difference across the membrane
• +40 = depolarised - sodium gates close, potassium gates open
• 65 = polarised

Question 5

When might an action potential may fail to occur?

Answer 5

-the outward diffusion of these potassium ions caused a temporary overshoot of the electrical gradient, with the inside of the axon being more negative relative to the outside than usual = hyperpolarisation

Question 6

What is the difference between resting potential and action potential?

Answer 6

• the terms action potential and resting potential can be misleading because the movement of sodium ions inward during the action potential is purely due to diffusion-which is a passive process-while the resting potential is maintained by active transport - which is an active process
• the term action potential simply means that the axon membrane is transmitting a nerve impulse, whereas resting potential means that it is not