Neuron Function and Action Potentials

an AP

• a wave of electricity that travels down the axon of neuron
• from the cell body to the axon terminals
This wave of electricity is actually a brief change in the resting membrane potential of the neuron
• from -70mv to +35mv
Then the membrane returns to its resting potential of -70mv.

• A small amount of Na+ to diffuse into the cell
• A small amount of K+ to diffuse out of the cell

• Briefly opening more membrane gates for Na+ and then for K+
• Thus making the neuron cell membrane
o More permeable first to Na+
o Then to K+
• This causes a change in the numbers of plus charges inside and outside the cell membrane
o Changing the cell membrane potential

• Regular channels
• Ligand-regulated gates
• Voltage-regulated gates
• Mechanically-regulated gates
Each of these passageways is a protein embedded in the cell membrane

Regular channels are always opened
In the resting neuron, Regular Na+ channels and Regular K+ channels allow
• Na+ to diffuse in
• K+ to diffuse out at a slightly faster rate

These gates are normally closed.
They open when a chemical messenger binds to them.

• The neurotransmitter acetylcholine opens ligand-regulated gates in the muscle cell membrane.
• This same neurotransmitter also has this effect on certain nerve cells.

• These gates are normally closed
• They open in response to changes in the resting membrane potential

• These gates are normally closed
• They open in response to mechanical stretch or pressure on the neuron

• From -70mv
• To a critical point called Threshold
o THRESHOLD = -55mv
• This leads to an action potential and opening of Voltage-activated Na +and K+ gates

• Electrical, chemical, thermal or mechanical stimulus
• Applied to the dendrites of a neuron

Smell (Olfactory) receptors

Touch or pressure receptors (pressure)
Stretch receptors (in the lungs, bladder etc.)
Hearing (auditory) receptors- (pressure from sound waves)

• Ligand-regulated gates
• Voltage-regulated gates or
• Mechanically-regulated gates

Touching a pencil puts pressure on the dendrites of a touch receptor in your finger.
This would stretch and open some mechanically-regulated gates.

A chemical coming from a rose binds to the cell membrane of a smell receptor in a person's nasal cavity.
• This causes the opening of ligand-regulated Na+ gates in the smell receptor
• This causes some extra Na+ to rush into the cell
The movement of Na+, causes the membrane potential to change from -70mv

As more Na+ moves into the cell
• The inside of the cell becomes more positive because it has more Na+
• The outside of the cell becomes more negative because it has less Na+
• The difference in charge between the inside and outside of the membrane becomes less
o This changes from a 70mv difference to a 55mv difference

a LOCAL POTENTIAL
• It can be a small or large reduction in the membrane potential

The change in membrane potential of the dendrites, caused by Na+ is called a LOCAL POTENTIAL
• It can be a small or large reduction in the membrane potential
If enough Na+ gates open, the membrane potential of the dendrites will be reduced to the threshold potential of -55mv or less
• This will cause the rest of the neuron to fire (i.e. an action potential)
If only a few Na+ gates open, the membrane potential may not be reduced enough.
• In this case, the neuron will not fire and the membrane of the will return to its resting potential

The neuron would fire in any situation where the membrane potential was LESS
than -55mv. So it would fire at -45mv and at -50mv.

when the membrane reaches the threshold potential or is made even less negative than threshold
• There is a complete action potential (to +35mv)
If the membrane does not reach -55mv
• There is no action potential at all

Different strength scents will cause differences in • The number of olfactory neurons firing
• The frequency of firing or the number of times that a single olfactory neuron fires in one second
o The maximum times a neuron can fire in one second is 2500 times

• Depolarization
• Repolarization
• Hyperpolarization

During this stage the membrane potential changes from -70mv to +35mv
At rest
• The inside of the neuron is 70mv more negative than the outside
At the start of an action potential
• Na+ voltage-regulated gates open
• Na+ rushes in
This causes
• The inside of the neuron to become more positive
• The outside of the neuron to become less positive
Thus the difference in charge between the inside and the outside of the neuron is reduced
• The membrane becomes less polar or depolarized
• The membrane potential reaches zero
As more Na+ moves out
• The membrane potential reverses
• It moves to +35mv

The membrane is no longer depolarizing.
The membrane potential is actually getting more polar as it moves from 0 to +35mv. It is just polar in the opposite direction.
• Now the inside of the membrane is more positive than the outside.

During Repolarization
• Na+ voltage-regulated gates close
o This is a very fast process. The gates close a fraction of a second after they open
• K+ voltage-regulated gates open
o K+ rushes out
The inside of neuron once again becomes more negative than the outside
• Membrane potential drops back to -70mv

The neuron is not the same as it was at rest.
Now there are
• An excess of Na+ on the inside of the cell instead of the outside
• An excess of K+ on the outside of the cell instead of the inside
These will need to be returned to their original positions by an active transport pump, before the cell can fire again.

After the neuron reaches -70mv, the K+ voltage-regulated gates are still open
• This causes the potential to drop below -70mv, to -80mv
The membrane becomes more polarized than it was at rest
• This is called hyperpolarization
The Na+-K+ pump will return the membrane potential to normal

REFRACTORY PERIODS

• An absolute refractory period and
• A relative refractory period

During this time the neuron cannot generate another action potential
• This is the period when Na+ gates open and close and K+ gates open

During this time, an action potential can only be generated by a stronger than threshold stimulus
• This is the period when K+ gates are closing