Transmission of Nerve Impulses

WALT

Neurones transmit impulses as a series of electrical signals

A neurone has a resting potential of – 70 mV

Depolarisation causes an action potential to be transmitted along the axon

Resting Potential

• Experiments have been carried out using Giant Squid axons

• These are large enough to have microelectodes inserted into then to measure changes in electrical charge.

• One electrode is inserted into the axon and one is placed on the outside of the cell membrane

• The difference between the two potential charges is called the resting potential

• The membrane of a neuron is negatively charged internally with respect to outside

• This generates a potential difference of around - 50 - 90 mV (resting potential)

Resting Potential

Resting Potential

Maintaining the Resting Potential

• Cation pumps (Na pumps) maintain active transport of K+ ions in and Na+ out of the neurone

• 3 Na + ions are pumped out at the same time 2 K+ ions are pumped in

• This is done by the Sodium Potassium ATPase pump

Sodium Potassium Pump

Diffusion back• Also within the membrane are channel

proteins that allow both Na+ and K+ ions to diffuse back down their concentration gradient

• However there are many more K+ channels so K+ ions diffuse back much faster than the Na+ ions

• The net result is that the outside of the axon is positively charged compared to inside

An Action Potential

Action Potential

• An action potential is produced when membrane of neuron stimulated, the charge is reversed:

• The inside of the axon was -70 mV and this changes to +40 mV and membrane is said to be depolarized

An Action Potential

• A nerve impulse can be initiated by mechanical, chemical, thermal or electrical stimulation

• Experiment show that when a small electrical current is applied to the axon the resting potential changes from – 70 mV to + 40 mV

• This change in potential is called the action potential

An Action Potential

• An Action Potential is produced due to a sudden increase in the permeability of the membrane to Na+:

• Na+ ions rush into neuron through the Na+ channels to depolarize the membrane, and then further increases its permeability to Na+

• This leads to greater influx & further depolarization --- positive feedback

The Action Potential

• The Na+ ions move into the axon causing the charge to change to +40mV

• This reversal of charge causes the action potential

The Action Potential

• When inside becomes sufficiently positively charged, permeability to Na+ ions start to decrease.

• At the same time as Na+ begins to move inward, K+ begins to move in the opposite direction along a diffusion gradient slowly until the membrane is repolarized.

An Action Potential

• Within about 2 milliseconds, the same portion of the membrane returns to resting potential of -70 mV inside this is called repolarisation

• Provided the stimulus exceeds a certain value (the threshold value), an action potential results.

All or none response

• Above the threshold value, the size of the Action Potential ( A P ) remains constant, regardless of the size of the stimulus

• The size of the A P does not decrease as it is transmitted along the neuron but always remains the same

Progression of The impulse

• When a nerve impulse reaches any point on the axon an action potential is generated.

• Small local circuits exist at the leading edge of the action potential.

• Sodium ions move towards the negatively charged regions.

• This excites the next part of the axon and so the action potential progresses

The Refractory Period

Absolute refractory periodAbsolute refractory period: : • This lasts for about 1 msec during which no This lasts for about 1 msec during which no

impulses can be propagated however intense impulses can be propagated however intense the stimulusthe stimulus

Relative refractory period: • This lasts for about 5 msec during which new

impulses can only be generated if the stimulus is more intense than the normal threshold

The refractory Period

• The refractory period ensures that:

• Impulses can flow in only one direction as the region behind the impulse cannot be depolarised

• It limits the frequency at which successive impulses can pass along an axon.