5.3 Flashcards
Once an AP has been set up, it spreads rapidly along the
length of the neurone
As part of the membrane becomes depolarised - sodium ions move into the membrane -
these sodium ions then cause the depolarisation of the membrane immediately ahead
In an unmyelinated neurone, a nerve impulse passes along the neurone as a
wave of depolarisation
myelinated neurones have Schwann cells wrapped around their axons forming a myelin sheath which doesn’t allow
ions to pass through
Nodes of Ranvier between the Schwann cells are
small gaps
Voltage-gated sodium and potassium channels are found in the neurone membrane in the nodes of Ranvier - sodium and potassium channels can enter and leave through these channels so
action potentials can occur here
in a myelinated neurone, sodium ions enter at one node and pass along the next where they cause further depolarisation - known as
saltatory conduction
Saltatory conduction - impulse ‘jumps’ from one node to another, which means that an
AP passes much more quickly than on unmyelinated
A stimulus has to be of a certain intensity to cause an action potential - called a
threshold value
If a stimulus is below the threshold value then
no action potential is generated
The action potential is the same size regardless of the
intensity of the stimulus
A stimulus either causes an action potential or it does not - known as the
‘all-or-nothing rule’
The nervous system distinguishes between larger and smaller stimuli by the
frequency of action potentials
A larger stimulus causes more action potentials to be generated within
a particular time period
Action potentials pass along a particular neurone at a constant speed
they do not speed up or slow down
Speed of transmission varies:
myelinated neurones
wider diameter axons
Myelinated neurones:
transmit impulses more quickly than unmyelinated neurones
Wider diameter axons:
transmit impulses more quickly than narrower diameter axons as they have smaller surface area to volume ratio, so they lose fewer ions by LEAKAGE
Importance of the refractory period:
Keeping impulses flowing in one direction only
-cannot pass both ways along a neurone because the membrane in the region behind the AP is in its refractory period
Also makes sure that APs are separated from each other (discrete) and that there is a maximum number of APs that can pass along a membrane in a particular period of time
