The Nerve Impulse.
The Neuron at Rest
• The plasma membrane of neurons contains
many active Na-K-ATPase pumps.
• These pumps shuttle Na+ out of the neuron
and K+ into the neuron when ATP is
hydrolyzed.
• Three Na+ are pumped out of the neuron at
a time and two K+ ions are pumped in
• This creates a concentration gradient for
Na+. As Na+ accumulates on the outside of
the neuron, it tends to leak back in.
• Na+ must pass through proteins channels to
leak back through the hydrophobic plasma
membrane. These channels restrict the
amount of Na+ that can leak back in.
• This maintains a strong positive charge on
the outside of the neuron
• The K+ inside the neuron also tends to follow its
concentration gradient and leak out of the cell.
• The protein channels allow K+ to leak out of the
cell more easily.
• As a result of this movement in Na+ and K+ ions,
a net positive charge builds up outside the neuron
and a net negative charge builds up inside.
• This difference in charge between the
outside and the inside of the neuron is
called the Resting Potential.
• The resting potential in most neurons is
–70 mV.
• When the neuron is at rest, it is polarized
Initiation of the Action Potential
• A change in the environment ( pressure,
heat,sound, light) is detected by the receptor and
changes the shape of the channel proteins in part
of the neuron –usually the dendrites.
• The Na+ channels open completely and Na+ ions
flood into the neuron. The K+ channel close
completely at the same time and K+ ions can no
longer leak out of the neuron in that particular
area.
• The interior of the neuron in that area becomes
positive relative to the outside of the neuron.
• This depolarization causes the electrical potential to
change from –70 mV to + 40 mV
• The Na+ channels remain open for about 0.5
milliseconds then they close as the proteins enter an
inactive state.
• The total change between the resting state (-70 mV)
and the peak positive voltage ( +40mV) is the action
potential ( about 110 mV)
• The spike in voltage causes the K+ pumps to open
completely and K+ ions rush out of the neuron.
The inside becomes negative again. This is
repolarization.
• So many K+ ions get out that the charge goes
below the resting potential. While the neuron is in
this state it cannot react to additional stimuli.
• The Refractory period lasts from 0.5 to 2
milliseconds.
• During this time, the Na-K-ATPase pump
reestablishes the resting potential.
Transmission of the impulse
• The stimulus induces depolarization in a
very small part of the neuron, at the
dendrites.
• The sequence of depolarization and
repolarization generates a small electrical
current in this localized area.
• The current affects the nearby protein
channels for Na+ and causes them to open.
• When the adjacent channels open, Na+ions flood
into that area of the neuron and an action potential
occurs. This in turn will affect the areas next to it
and the impulse passes along the entire neuron.
• The electric current passes outward over the
membrane in all directions BUT the area to one
side is still in the refractory period and is not
sensitive to the current. Therefore the impulse
moves from the dendrites toward the axon.
Threshold stimulus
• Action potentials occur only when the membrane
in stimulated (depolarized) enough so that sodium
channels open completely.
• The minimum stimulus needed to achieve an
action potential is called the threshold stimulus.
• If the membrane potential reaches the threshold
potential (generally 5 - 15 mV less negative than
the resting potential), the voltage-regulated
sodium channels all open. Sodium ions rapidly
diffuse inward, & depolarization occurs.
All-or-None Law
• Action Potentials occur maximally or not at all.
• In other words, there's no such thing as a partial or
weak action potential. Either the threshold
potential is reached and an action potential occurs,
or it isn't reached and no action potential occurs.
• However, different neurons have different
densities of Na+ channels and therefore have
different APs
• The AP remains constant as it travels down
the neuron. Its amplitude is always the
same because it corresponds to wide open
Na+ channels.
• The frequency of the AP can change.
Conduction Velocity
• impulses typically travel along neurons at a speed
of anywhere from 1 to 120 meters per second
• the speed of conduction can be influenced by:
– The diameter of a fiber. Velocity increases as diameter
increases.
– Temperature. As temperature increases, the velocity
increases. Axons of birds and mammals can be very
small because of the high body temperature.
– the presence or absence of myelin.
• Neurons with myelin (or myelinated neurons)
conduct impulses much faster than those without
myelin.
• Because fat (myelin) acts as an insulator,
membrane coated with myelin will not conduct an
impulse.
• So, in a myelinated neuron, action potentials only
occur along the nodes and, therefore, impulses
'jump' over the areas of myelin - going from node
to node in a process called saltatory conduction
(the word saltatory means 'jumping')
Summary
• The Action Potential, or nerve impulse is an
electrochemical event involving the rapid
depolarization and repolarization of the
nerve cell membrane.
• The axon terminals of one neuron do not
touch the dendrites of other neurons. What
happens when the impulse reaches the axon
terminal?