15.2 The Nerve Impulse Flashcards
What is a nerve impulse
A self-propagating wave of electrical activity that travels along the membrane
What is a nerve impulse a temporary reversal of
The potential difference across the axon membrane
What is the temporary reversal between
The resting potential and action potential
How is the the movement of Sodium and Pottasium ions controlled across the axon membrane
- The phospholipid bilayer prevents Na/K diffusing across it
- Channel proteins span the phospholipid membrane. These proteins have ion channels which pass through them. Some of these channels have gates which can be opened or closed so ions can move through them by facilitated diffusion
- Some carrier proteins transport pottasium into the axon and sodium out of the axon. This is the sodium/pottasium pump
What does the controls of ions across the axon do to the charges of the axon
Makes the inside of the axon negatively charged relative to the outside
What is the resting potential in humans
65mV
What is the axon said to be at 65mV (resting potential)
Polarised
How is a potential difference established
- Sodium ions are transported out of the axon, Pottasium ions are transported into the axon
- The active transport of Na out of the axon is greater than the transport of potassium. 3 Na move out for every 2 K that move in
- Although both Na/K are positive, the outward movement of sodium is more than the inward movement of K. As a result there is more sodium ions in the tissue fluid surrounding the axon than in the cytoplasm, and more K in the cytoplasm than the tissue fluid. This creates an electrochemical gradient
- The sodium ions begin to diffuse back into the axon and the potassium move out of the axon
- However, most of the gated channels that allow K ions to move through are open, while most of the gated channels that allow Sodium to move through are closed
Which way does sodium move through the axon membrane
Sodium moves OUT of the axon
which way does potassium move through the axon membrane
Potassium moves INTO the axon
If a stimulus is of sufficient size and detected by a receptor in the nervous system, what happens
Its energy causes a temporary reversal of the charges either side of the membrane
If it is great enough, the negative charge of -65mV inside the axon is changed to +40mV
This is known as the action potential, and the part of the membrane is said to be depolarised.
What does the charge of the inside of the axon change to when an action potential is occuring
from -65mV to +40mV
Why does depolarisation occur
Because the channels in the membrane change shape, and hence open and close, depending on the membranes voltage
Describe the process of an action potential
- At resting potential, the permanently opened K channels are open, but the Na channels are closed
- The energy of the stimulus causes some Na channels in the membrane to open and therefore sodium enters the axon. Being positively charged, they trigger a temporary reversal in the PD across the membrane
- As the sodium ions diffuse into the axon, more sodium channels open, causing an even greater influx
- Once the action potential of 40mV is reached, the voltage gates on the sodium ion channels close (preventing further sodium ion influx) and the voltage gates of the potassium channels open
- With the potassium voltage gated channels open, the electrochemical gradient preventing outward K movement is reversed. Causing more K channels to open. This means more K ions move out, beginning repolarisation
- The outward diffusion of potassium ions causes a temporary overshoot of the chemical gradient, with the inside of the axon being more negative than usual. The closable K channels close, and the Na/K returns to normal. The resting potential of -65mV is returned
What does action potential mean
That the axon membrane is transmitting a nerve impulse
