Transcript AP – All or nothing

The passage and speed of
an action potential
Learning Objectives:
• How does an action potential pass along
an unmyelinated axon?
• How does an action potential pass along a
myelinated axon?
• What factors affect the speed of
conductance of an action potential?
• What is the refractory period?
• What is meant by the “all or nothing”
principle?
Unmyelinated Neurones
• Localised electrical currents are set up
and the action potential is propagated
along the neurone.
Task:
• Complete the cut and stick activity to
show transmission in an unmyelinated
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+
Task:
• AQA A2 Biology pg 170 Q 1 and 2.
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.
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.
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.
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.
Task:
• AQA A2 Biology pg 173 Q1-5