WEBVTT

1
00:00:03.000 --> 00:00:10.000
Some of the molecules in and around the axon carry an electrical charge, either positive or negative.

2
00:00:10.000 --> 00:00:16.000
The fluid inside the axon contains protein molecules with a negative charge.

3
00:00:16.000 --> 00:00:21.000
These particular protein molecules are found only in the intracellular fluid.

4
00:00:21.000 --> 00:00:29.000
Other electrically charged ions are present in the fluid both inside and outside the axon.

5
00:00:29.000 --> 00:00:37.000
Sodium ions and potassium ions have a positive charge, while chloride ions have a negative charge.

6
00:00:37.000 --> 00:00:46.000
When the axon is in its resting state, there are more negative particles inside the axon than outside the axon,

7
00:00:46.000 --> 00:00:52.000
largely because of the presence of protein molecules inside but not outside the axon.

8
00:00:52.000 --> 00:00:58.000
This produces an electrical charge (called the resting potential) across the membrane.

9
00:00:58.000 --> 00:01:05.000
The interior of the axon is electrically negative—or “polarized”—compared to the exterior.

10
00:01:05.000 --> 00:01:09.000
It's important to remember that there are several types of positively charged

11
00:01:09.000 --> 00:01:14.000
and negatively charged molecules on both sides of the axon membrane.

12
00:01:13.000 --> 00:01:19.000
But from this point on we will concentrate only on the sodium and potassium ions,

13
00:01:19.000 --> 00:01:23.000
because their movement through the membrane is the central issue.

14
00:01:23.000 --> 00:01:27.000
Although sodium ions are present on both sides of the membrane,

15
00:01:27.000 --> 00:01:31.000
they are more concentrated in the extracellular fluid.

16
00:01:31.000 --> 00:01:36.000
Potassium ions are more concentrated in the intracellular fluid.