How do Nerve Impulses Start?• Energy from a stimulus causes the sodium channel to

open (they change shape and “gate” opens). Na+ ions move in (down their electrochemical gradient).

• Energy of stimulus causes a temporary reversal of the charges either side of axon membrane

• Negative charge inside becomes positive (depolarised)

• Depolarisation occurs because ion channels in axon membrane change shape (they open or close depending on voltage across membrane)

• These channels are called voltage-gated channels.

Action Potential (AP)• An action potential occurs when a neurone sends

information down an axon.– Is an explosion of electrical activity– The resting membrane potential changes (reversed)

• At resting potential some K+ channels are permanently open but most of the Na+ voltage-gated channels are closed.

• Resting potential is –70mv (inside the axon).

1

AP - Depolarisation• When stimulated, the membrane potential is

briefly depolarised.– Stimulus causes the membrane at one part of the

neurone to increase in permeability to Na+.– Na+ voltage-gated channels open and Na+ enter

the axon down their electrochemical gradient by diffusion.

– This causes resting potential to move towards 0mV.

2

AP - Depolarisation• When depolarisation reaches –30mV

(threshold limit) more Na+ channels open (for 0.5ms).– Causes more Na+ to rush in by diffusion

cell becomes even more positive.– An example of POSITIVE FEEDBACK

Na +

out

in

K

closed(leak)

open

+

-Na

3

AP – The Peak of the action potential

• Once the action potential of around +40mV has been established, the voltage gates on the Na+ channels close.

• This causes K+ voltage gated channels to begin to open (as well as permanently open ones).

out

in

Na

closed(leak)

open

+

-

K +

K

4

AP - Repolarisation• K+ rush out making the inside the cell

more negative.

– Since this restores the original polarity, it is called repolarisation.

out

in

Na

closed(leak)

open

+

-

K +

K

5

Hyperpolarisation• There is a slight ‘overshoot’ in the movement of K+ meaning that the

inside of the axon is more negative than usual.

• This is called hyperpolarisation.

• This overshoot occurs because more K+ channels are open compared to resting potential (BOTH voltage gated and permanently open channels for K+ are open).

• Only the permanently open K+ channels were open at RESTING POTENTIAL

• At hyperpolarisation, the voltage gated K+ channels now start to close.

6

Repolarised to Resting Potential

• The resting potential is restored and maintained by the Na+/K+ ATPase pump.

• 3Na+ pumped out and 2K+ pumped back in. K+ can diffuse back out (via permanently open channels) but very few Na+ can diffuse back in. Again outside becomes positive and inside negative.

• The axon is said to be repolarised.

• More positive outside membrane and more negative inside once again.

7

AP - Overview

(Click here for animation of the gates)

• http://bcs.whfreeman.com/thelifewire/content/chp44/4402s.swf

Passage of Action Potential• Once created, an action potential rapidly “moves” along an axon.

• Essentially what is happening is that the area of depolarisation “jumps” along the axon.

• Where depolarisation occurs (i.e. an action potential is produced) this acts as the trigger to form another action potential downstream of the previous one.

• Depolarisation can only happen at Nodes of Ranvier (no myelin around axon) so this is why action potentials “jump” from region to region. SALTATORY CONDUCTION. Saltus – to leap

• Some neurones are un-myelinated so action potentials flow more like a wave.

Mexican Wave Analogy

• Think of everyone seated as the RESTING POTENTIAL

• A few people in the stadium stand up to make an ACTION POTENTIAL.

• People next to these original few, see what is happening and stand up as well – continuation of ACTION POTENTIAL.

• Original “creators” of wave sit down (REPOLARISED) and return to RESTING POTENTIAL.

• And so on...ACTION POTENTIAL or wave moves around stadium.

Mexican Wave Clip.

http://www.youtube.com/watch?v=r732RCnhbFQ

Why Does Original Action Potential Trigger Others to Form?

• Localised electrical circuits are created by an influx of Na+ ions (during action potential).

• This positive region inside the neuron lies adjacent to a region inside the neuron that is negative inside (at resting potential).

• This difference in charge inside the neurone creates a current (flow of electrons).

• This current opens up the Na+ ion channels in the region of resting potential causing it to become depolarised (action potential created further along).

• Propagation of the Action Potential

Passage of Action Potential• When Na+ channels open up – depolarisation occurs

again.

• As before, when depolarisation happens (+ inside), the K+ ion channels open up, releasing K+ to the outside. Na+ close.

• It then becomes + outside and – inside once again (repolarised).

• The newly repolarised region uses Na+/K+ pumps to achieve resting potential once again.

Electric Current• ELECTRIC CURRENT is the flow of charge.

• Conventionally this is the flow of positive charge.

• However, in a simple circuit such as that illustrated the current in the wire is composed of electrons that flow from the negative pole of the battery (the cathode at the bottom of the battery) and return to the positive pole (the anode at the top of the battery, marked by a +).

• Electrons are negatively charged particle so they moves towards the positive but current flows in the opposite direction to the flow of electrons (that is from positive to negative).

• A region that loses its electrons becomes more positive