Transcript AP – All or nothing

AP - Overview
(Click here for animation of the gates)
The passage and speed of
an action potential
The Refractory Period
• There is a time after depolarisation where no
new AP can start – called the refractory
period.
– Time is needed to restore the proteins of voltage
sensitive ion channels to their original resting
conditions.
– Na+ channels cannot be opened, as it can’t be
depolarised again.
WHY?
– AP travel in one direction only.
– Produces discrete impulses.
– Limits the frequency of impulses.
After the action Potential
• During the action potential, the membrane is
depolarised.
• Following the impulse K+ ions move out of the
membrane, this is repolarisation
• The membrane briefly becomes
hyperpolarised (more negative on the inside
than usual)
• The Na+ / K+ channels close
The refractory period
• With the Na+ / K+ channels closed, the
cation pumps can now begin to restore
the balance between the ions
• Na+ is pumped out and K+ pumped in.
• During this time a new action potential
can not be set up until resting potential
is achieved.
Purpose of refractory period
• Ensures action potential move in one
direction (from receptor to effector)
• To distinguish between one action
potential and the next (the greater the
stimulus, the higher the frequency).
Waves of Depolarisation
• After an action potential, some of the
sodium diffuse sideways.
• Causing sodium ion channels in the next
region of the neurone to open.
• Causes impulse to propagate along the
neurone.
AP – All or nothing
• AP only happens if the stimulus reaches a
threshold value.
– Stimulus is strong enough to cause an AP
– It is an ‘all or nothing event’ because once it
starts, it travels to the synapse.
• AP is always the same size
• An AP is the same size all the way along the
axon.
• The transmission of the AP along the axon is
the nerve impulse.
• Bigger stimulus will cause more frequent
action potentials.
Myelination
Unmyelinated Neurones
• Localised electrical currents are set up
and the action potential is propagated
along the neurone.
• The wave travels the whole length of
the neurone.
Myelinated Neurones
• The axons of many neurones are encased in
a fatty myelin sheath (Schwann cells).
• Where the sheath of one Schwann cell
meets the next, the axon is unprotected.
• The voltage-gated sodium channels of
myelinated neurons are confined to these
spots (called nodes of Ranvier).
Na+
Sodium channel
Na+
Nodes of Ranvier
Na+
Myelinated Neurones
• The in rush of sodium ions at one node
creates just enough depolarisation to reach
the threshold of the next.
• In this way, the action potential jumps from
one node to the next (1-3mm) – called
saltatory propagation
• Results in much faster propagation of the
nerve impulse than is possible in
unmyelinated neurons.
Na+
Sodium channel
Na+
Nodes of Ranvier
Na+
Factors Affecting the Speed of
an AP
1. Myelin sheath – electrical insulator –
the AP jumps from one Node of
Ranvier to another = SALTATORY
CONDUCTION.
–
–
Myelinated = 90ms-1
Unmyelinated = 30ms-1
Factors Affecting the Speed of
an AP
2. Diameter of the axon – greater diameter =
faster conductance (due to less leakage).
Factors Affecting the Speed of
an AP
3. Temperature – higher temp = faster
nerve impulse (rate of diffusion is
faster, enzyme activity is faster e.g.
ATPase.
How do we detect the size of a
stimulus?
• The number of impulses in a given time –
the larger the stimulus, the more
impulses generated.
• By having neurones with different
threshold values – the brain interprets
the number and type of neurones and
therby determines its size.