In this lecture we will look at how neurons generate electrical signals. To understand these phenomena, it is useful to be familiar with some basic concepts of electronics, in particular, voltage, current, resistance and capacitance. If you haven't studied much physics and aren't up to speed on these concepts, you are strongly advised to review these topics on the page http://howyourbrainworks.net/VIRC before the lecture.
Key Concepts to Remember from this Lesson:
You should be familiar with a few widely used anatomical terms, including: forebrain midbrain, brainstem, cerebellum, white matter, gray matter, neuron, dendrite, axon.
- Remember that neurons are essentially tiny bubbles of fatty (phospholipid) membranes which are filled with organic potassium (K+) ions and are bathed in an extracellular fluid rich in sodium cholride (Na+ Cl-).
- Remember also that neurons contain very few free calcium ions (Ca++), but there are more Ca++ ions in the extracellular space.
- Neurons control the flow of ions through their membrane with channels made of protein. These channels can be selective, allowing only certain types of ions through, and perhaps only at certain times.
- Neurons at rest have many K+ "leakage" channels open, so a little K+ will leak out, leaving organic anions (A-) behind. This causes the membrane of the resting neuron to be electrically polarized to about -70 mV. (By convention the polarity of cells is given as inside relative to outside, and once positive K+ has leaked from the cell, there is more negative charge inside than out, hence the minus 70 mV).
- Neurons become "excited" when other channels open to allow, for example, Na+ to enter the cell, which will make the cell membrane less negatively polarized. (Depolarization = excitation). This allows neurons to encode parameters in the outside world through their membrane potential. For example, a stretch receptor neurons in your skin may get more depolarized if it is more stretched.
- Neurons are poor cables. To send signals over great distances (along their axons) they must refresh electrical currents which are lost due to leakage. They do this with voltage gated sodium channels. But opening of these channels leads to a runaway feed forward process (a nerve impulse, also known as an "action potential" or "spike").
Click on the buttons to see videos of the Semester A 2018 lecture.