The Nerve Impulse Sketch a plasma membrane on

The Nerve Impulse • Sketch a plasma membrane on your whiteboard. • Label it. • How do the following substances move across the plasma membrane? • Oxygen • Sodium ions • Glucose

Nerve impulse A nerve impulse - self propagating wave of electrical disturbance that travels along the surface of the axon membrane. It is not electrical current it is a temporary reversal of the electrical potential difference across the membrane. This reversal is between 2 states called the resting potential and the action potential.

What is going on? What is a resting potential?

Resting Membrane Potential What is it? ‘Ready for action potential transmission’ It is not inactive – it requires lots of energy to maintain it!

The resting potential The movement of Na+ and K+ ions across the axon membrane is controlled • They cannot cross the phospholipid bilayer • Channel proteins can allow facilitated diffusion of ions but can be gated (opened or closed at different times) • Carrier proteins can actively transport ions in or out of the axon – sodium potassium pump

Resting potential The inside of an axon is negatively charged related to the outside. This is known as the resting potential and ranges from 50 -90 m. V, but is usually -65 m. V. In this condition the axon is said to be polarised.

Establishing a Resting potential 1. Na+ ions are actively transported out of the axon by the Na-K pump. 2. K ions are actively transported into the axon by the Na-K pump. 3. 3 Na+ move out for every 2 K+ ions that move in. 4. Na and K ions are both positive but outward movement of Na+ is greater than inward movement of K+

Resting potential 5. Na ions diffuse back into the axon whilst the K ions diffuse out of the axon. 6. But most of the gates in the channels that allow the K ions to move through are open, while most of the gates in the channels that allow Na ions to move through are closed. 7. The axon membrane is more permeable to K ions, which diffuse back out of the axon faster than the Na ions back in. This increases the potential difference (difference in charge).

The Action Potential • When a stimulus of sufficient size is detected by a receptor its energy causes a temporary reversal of the membrane's potential difference • If stimulus is great enough it causes the potential difference to change to +40 mv • This is an action potential – the membrane is depolarised

The Action Potential 1. At resting potential voltage gated K+ channels are open but Na+ voltage gated channels are closed. 2. The energy of the stimulus causes some Na+ voltage gated channels to open. Na+ ions move into the axon along their electrochemical gradient. This causes the potential difference across the membrane to be reversed. 3. As Na+ ions diffuse into axon they cause even more Na+ voltage gated channels to open. 4. Once the action potential of around +40 mv is reached the Na+ voltage gated channels close and prevent more Na+ ions from entering. K+ gated channels open. 5. K+ ions leave the axon, starting the repolarisation of the axon. 6. This causes a temporary hyperpolarisation where the inside of the axon is more negative than at resting potential. The closable gates on the K+ channels now close and the sodium potassium pump reestablishes the resting potential. The axon is repolarised.

Task • Use plasticene models to demonstrate how an action potential is established. • Be ready to describe and explain it!