Slides in this set
The phospholipid bilayer of the axon
plasma membrane prevent sodium and
potassium ions diffusing across. The Due to the existence of many controls the axon is
protein molecules known as the intrinsic negatively charged that the outside this is known as
proteins contain channels some of the resting potential. The resting potential can
which have gates that open and close to range from 50 90 millivolts with the average being
allow sodium or potassium to move 65mV. When the axon is at this condition it is called
through at any one moment. Therefore polarised.
both the potassium and sodium voltage The potential difference ( defined as the difference in
gates cannot be open at the same time. charge inside and outside the axon) is due to several
Both ions have different gates which are events:
displayed on the diagram. Some of the
channels are permanently open allowing
diffusion of the ions.
Some intrinsic proteins actively
transport both sodium in and potassium
out of the axon membrane this process
is known as the sodium potassium
1: Sodium ions actively transport out of the 6: Most gates in that potassium ions to move
axon by sodium potassium pump. through are open while most gates for sodium
ions are closed
2: Potassium ions actively transport in the
axon by sodium potassium pump. 7: Due to this reason axon membrane is 100 time
more permeable to potassium ions therefore
3: Active transport of sodium ions is greater diffuse back out faster than sodium ions diffuse
than of potassium, for every 3 potassium back in. this increases the potential difference
moving out 2 sodium ions move in. between negative inside and positive outside of
4: The existence of a chemical gradient is
due to both sodium and potassium are 8: Electrical gradient also causes the movement of
positively charged, as the outwards of the the potassium and sodium ions. As more
sodium ion is greater than the inward potassium move out the outside of the axon
movement of the potassium ions. There is a become more and more positive, therefore
greater amount of sodium ions in the tissue outward movements of the potassium ion
fluid around the axon than in the cytoplasm become more difficult as being positively charged
and more potassium in the cytoplasm than they are attracted to negative charge inside the
in the axon. axon. They repel the positive surrounding which
prevents them from moving out the axon.
5: Sodium ions begin to diffuse back
naturally into the axon while the potassium 9: Equilibrium is established when the chemical
ions naturally diffuse out the axon. and electrical gradient is established with no net
Potassium ion channels Sodium ions
open and so potassium actively
Plasma membrane of ions diffuse back out of transported out
axon axon of the axon
into the axon…read more
1: resting potential some potassium voltage gate channels are open
Action potential when a stimuli received by (permanently ) sodium voltage gated channels are closed.
the receptor its energy causes a temporary
reversal of charge in the axon of the 2: energy of the stimulus leads to some sodium voltage gated
membrane. Negative charge inside the channels in the axon membrane being open hence sodium diffuse
membrane become positive charged. This into the axon through the channels along the electrochemical
is known as action potential. The gradient. Being positively charged they cause a reversal potential
membranes condition is known as difference across membrane.
depolarised. This occurs when channels
changed shape i.e. open and close 3: as sodium ions diffuse into axon more sodium channels open
depending on the voltage of the causing greater production in sodium ions by diffusion.
membrane therefore they are called
voltage gated channels 4: when action potential around +40mV voltage gate on the sodium
ion channels close preventing further influx in sodium ions.
Resting potential essentially means that the Potassium ion voltage gates begin to open.
axon membrane is not transmitting a nerve
impulse. 5: potassium channels open electrical gradient that was preventing
outwards movement of potassium ion is reversed causing more
Action potential means that the axon potassium ion channels open. This means more potassium ions
membrane is transmitting a nerve impulse. diffuse out causing repolarising of axon.
6: outward diffusion of potassium ions cause temporary overshoot of
electrical gradient with inside of the axon being negatively charged
than the outside(hyperpolarisation). Gates of potassium channels
close and activities of sodium potassium pump again. Sodium ions
are pumped out while potassium ions are pumped in. The resting
potential is re-established at 65 (figures used for textbook). This is
when the axon is repolarised.…read more