Download Speed of the nerve impulse

Speed of the nerve impulse
Aim: To recap work done last
lesson!
Nerve impulses – key facts
• A nerve impulse is the movement of an action
potential
• Nerve impulses are always a fixed size and
speed in a given neurone
• Speed 0.5ms-1 – 120ms-1
Factors affecting speed of action
potential
• Presence of myelin sheath
• Diameter:
• Temperature:
Presence of myelin sheath
• Impulse faster with in a myelinated
axon than a unmyelinated axon
• What term is used to describe
conduction in a myelinated axon?
saltatory conduction!
Diameter
• Larger diameter, faster impulse
Why?
Larger diameter relatively less leakage of ions therefore
membrane potentials easier to maintain.
Narrower = greater resistance
In thinner axons the localised circuits form in a much
smaller area therefore conduction is slower.
Temperature
• The higher the temperature the faster the
impulse … Up to a point!
Why?
Higher temperature gives a faster rate of diffusion
Energy for active transport from respiration, which is
dependant on enzymes therefore higher temp faster rate
of reaction.
Too high a temp … enzymes and plasma membrane proteins
denature … No more impulses!
Refractory period
• After impulse sodium voltage gated channels
remain closed for a period of time.
• Therefore its impossible to generate another
action potential
Purpose of refractory period
• Only propagated in one direction. The action
potential cannot be propagated in a region
that is refractory so it can only move
forwards
• Ensures action potentials separated. This
limits the number of action potentials in a
given time
Extra info
Actually there are 2 refractory periods!
Absolute refractory period – Lasts 1ms and no
impulses can pass
Relative refractory period – Lasts 5ms. New
impulse only if the stimulus is more intense
than the normal threshold
All or nothing principle
Impulse only triggered at a certain level of
stimulus, this is called the threshold value
The impulse is always the same size and speed
How do organisms determine the
size of the stimulus?
• Larger stimulus: More impulses in a given time
• Different Neurones with different threshold
values