Session 5.2 Propagation of the action potential Flashcards
how fast can the fastest axon transmit action potential
80-120m/s (sensory fibres from muscle spindles, motor neurons) incomparison to 0.5-2 for slowest (sensory fibres from pain, temp and itch (diffuse)
2nd is sharp pain and temp
3rd is preganglionic nerves of autonomic nervous system
the larger the diameter of the axon
the faster the transmission
how can conduction velocity of axon be measured
nerve placed with stimulating electrodes
crush bottom part of nerve = damaged section
1. stimulate nerve at stimulus point, measure the change in external potential as action potential travels down axon, and measure time for action potential to reach point X
2. repeat at different places further down axon to point X
3. further away from stimulus point, then peak of action potential arrives later so increase in delay
4. increase in delay can be used to calculate conduction velocity (x/time)
what does a nerve fibre consist of
several axons with different diameters
why does more than one peak emerge when stimulating nerve
as multiple diameter axons with different conduction velocities
what enables the action potential to travel down the axon
cylinder with cytoplasmic solution sepertaed by membrane from external solution = axon inject current (positively charged)-> repel any other positively charged ions and attract any negatively charged ions and the charge will spread along the axon to form a local circuit, which will cause an immediate, local depolarisation of the axon which will decrease the further you are from current injection
how do you measure length constant
measure the distance it takes for the potential to fall to 37% of its original value
what variable can affect how far the memrbane potential is spread
length constant, the conduction velocity is thus increased
what is capacitance, C
the ability to store charge, property is given by lipid bilayer
what does the membrane resistance depend on
the number of ion channel open, lower = more channels
what variable can affect how far the voltage spreads along the axon
high resistance = further it travels
what variable can change how volatage changes in response to current injection
high capacitance = volatge changes more slowly
what variable affects axpolasm resistance
increases as radius decreases of axon
how will the voltage change in response to the current pulse
where current injected - voltage not change immediately and will then increase slwoly to Vmax
what variables affect how the voltage changes in response to current
capacitance and resistance of membrane
how is propagation maintained
when voltage above membrane threshold, will be initiated to fire action potential which will depolarise the membrane
this local depolarisation ahead of the action potential maintain propagation by allowing a wave of action potential as it moves along the axon
why does action potential only travel in one direction
after depolarisation from action potential, sodium ion channels behind become inactivated and the number of open sodium channels have reduced (refractory), and increased number of potassium ion channels remain open which will cause membrane potential to be more negative.
even though the action potential is firing, will not reverse due to refractory period of inactivated sodium channels behind it
how to increase length constant of the axon
increase diameter of the axon
myelination (with - length constant increased)
how does myelin work
produces a tight layer of schwann cell/dendricytes tightly wrapped around the axon, except on some parts called nodes of ranvier where sodium channels are
what is the difference in length between two nodes of ranvier
1mm
where are ions distributed in non myelinated axon
equally
how does action potential move in a myelinated axon (**)
saltatory conduction
action potential at node
local circuit theory ensures next node is set above threshold so action potential created in next node
why cant non-myelinated axons use saltatory conduction
the depolarisation at next node wont be able to raise potential above threshold for next node so action potential stops)
what is the purpose of the myelin sheath (**)
acts as a good insulator therby increasing length constant so enable local circuit currents to depolarise the next node above threshold and initiate action potential
why is myelinated faster than non myelinated
action potential jumps from node to node
increases conduction velocity
for myelinated axons what is conduction velocity directly proportional to
diameter
for unmyeinated axons what is conduction velocity proportional to
square root of diamater
how does myelin sheath improve conduction
large increase in membrane resistance
large decrease in membrane capacitance
increase in length constant
decrease in time constant
what diseases are associated with decreased conduction of action potentials
the myelin sheath is attacked causing breakdown - multiple sclerosis which affects all CNS nerve and devic’s disease which affects optic and spinal cord nerves = multiple scarring in different parts of CNS
peripheral nervous system - landry-guillian-barre syndrome and charcot-marie-tooth disease
why does demyelination cause disease
the length constant has decreased so next node unable to reach threshold so action potential ceases - stops saltatory conduction
myelinated axons have
high membrane resistance and low membrane capacitance
