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
Resting Potential
• 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
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 period:
• This lasts for about 1 msec during which no
impulses can be propagated however intense
the 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.