Nerve impulses

Define a Nerve impulse:

• a self-propagating wave of depolarisation that travels along the surface of the axon membrane

Explain maintenance of the Resting potential:

• The resting potential is maintained through a balance between Na⁺ and K⁺ inside and outside the axon

• Movement of these ions across the axon membrane is controlled by:

• The phospholipid bilayer is impermeable to charged particles, so the sodum and potassium ions cannot diffuse across it

• Specific, channel proteins embedded through the phospholipid bilayer can act as channels to allow these ions to pass into or out of the axon. Some are ‘gated’ channels, which open and close when needed. There are different gates for potassium and sodium ions. There are also some channels which remain open at all times - Leak channels

• Ions move through the protein channels by facilitated diffusion

• Some proteins actively transport potassium ions into the axon and sodium ion out of the axon - knwown as sodium-potassium pumps. Each pump double up as an ATP hyrolase enzyme, which hydrolyses ATP, providing the energy for active transport

How is a resting potential establish ?

• When a neurone is not conducting an impulse, there is a difference between the electrical charge inside and outside the neurone - resting potential

• There are more positive ions (Na⁺ and K⁺), outside compared to inside. Therefore, the inside of the neurone is comparatievly more negative at -70mV

• The resting potential is maintained by a sodium potassium pump, involving active transport and therefore ATP.

• The pump moves 2 K⁺ ions in and 3 Na⁺ ions out. This creates an electrochemical gradient and results in K⁺ diffusing out and Na diffusing in. - However, because the membrane is more permeable to K⁺, more K⁺ moved out by leak channels

• results in -70mV

Are the gated channels open or closed during resting potential?

Closed

Draw a diagram for resting potential

Draw the graph between voltage and time

What is an action potential?

• A atimulus is recienved by a receptor, its energy causes a temporary reversal of the charges across the axon membrane - the usual resting potential of -65mV inside the axon becomes more positive.

• In this state the axon is known as being depolarised. If the axon is depolarised enough, it can result in the generation of an action potential

The stages which cause an action potential are:

1. The axon is in its resting state, with a resting potential of approximately -70mV

2. When the axon is stimulated, some gated sodium ion channels in the membrane of the axon open, making the membrane more permeable to sodium ions. Na⁺ ions rapidly diffuse intothe axon - cuases depolarisation

3. if sufficiently large, this depolarisation causes voltage-gated Na⁺ channels to open, this causes even more Na⁺ ions to diffuse into the axon

4. This then activates and open even more voltage-gates Na⁺ channels, leading to even greater influx of sodium ions, until the inside of the membrane becomes charged to +40mV - this is known as action potential

Explain repolarisation

• When the inside of the axon becomes charged to +40mV, all of the gated sodium ion channels close, meaning the axon is again almost impermeable to Na⁺ ions.

• At +40mV, as the sodium ion channels close, voltage-gated potassium ion channels open and K+ ions diffuse rapidly out the axon - repolarisation

Explain hyperpolarisation and resting potential steps

1. So many potassium ions diffuse out of the axon that they cause a more negative potential difference than normal, dropping the axon interior down to about -90mV - known as hyperpolarisation and causes the relative refractory period

2. The voltage-gated potassium ion channels close and the sodium potassium pump restores the concentration gradients for Na⁺ and K⁺ - returns the resting potential of the axon to -70mV

How long does the whole process last

2-3 milliseconds

What is the all or nothign principle?

• The events outlines above will only be set into motion if the initial stimulus is larger than a specific threshold value

• If the initial stimulus is not large enough, then the voltage-gated sodium channels will not open and the axon membrane will not become fully depolarised - no action potential will be generated

• If the stimulus is large enough (above the threshold), then an action potential will be generated at a constant size and speed - all or nothing principle

• increasing the initial stimulus will not produce a larger or faster action potential - however increase the frequency of action potential

What id the Refractory period ?

• After an action potential has been generated, the membrane enters a refractory period when it can’t be stimulated, becuase sodium channels are recovering and can’t be open

• This is important for 3 reasons

• it ensures that discrete impulses are produced, meaning that an action potential cannot be generated immediately after another one - makes sure that each is seperate

• It ensures that action potential travels in one direction . This stops the action potential from spreading out in two directions which would prevent a response

• It limits the number of impulse transmissions. This is important to prevent overrection to a stmulus and therefore overwhelming the senses