Lecture 9- The action potential Flashcards
3 features of an AP
1) All or nothing (only occurs if threshold is reached) 2) Non-overlapping 3) Unidirectional
AP are different in
different structures
if a stimuli does not depolarise the membrane above the threshold…
the AP will not fire down the hillock of the axon
hillock of axon
The axon hillock is the last site in the soma where membrane potentialspropagated from synaptic inputs are summated before being transmitted to the axon
when cells are depolarised which channels are open
sodium
-moves Vm (membrane potential) closer to ENa
when the cell is polarising the …… are open
potassium
- moves Vm back to Ek
Vm
membrane potential
voltage clamp
enables membrane currents to be measured overtime at a set membrane potential
Time course of conductance changes during an action potential
During the course of an action potential, the permeability to different ions changes considerably.
At the beginning of an AP Na+ conductance is high and K+ conductance is low. Mid way through the AP Na+ conductance considerably reduces and K+ conductance increases. By the end of the AP when the cell is at resting potential, the conductance of both sodium and potassium will be 0.
Summary of sodium and potassium channel activity during AP
- Summary of sodium and potassium channel activity during AP
- NaV (voltage gated) channels open
- Na+ influx into cell
- Positive feedback to trigger opening of other NaV channels
- Membrane depolarisation
- At ENa NaV channels inactivate and KV channels open
- Na+ influx stops
- K+ efflux
- Membrane repolarises
Is Na/K ATPase pump involved in the repolarisation of the AP?
NO
the refractory period is the
Recovery period after AP has fired- the amount of time it takes for an excitable membrane to be ready for a second stimulus.
during the refractory peroid
New AP will not fire
types of refractory period
ARP- absolute refractory period
RRP- relative refractory period
absolute refractory period
Is the period of time during which a second action potential ABSOLUTELY cannot be initiated, no matter how large the applied stimulus is (i.e. during active AP).
Nearly all Na+ channels are in inactivated states
relative refractory period
Is the interval immediately following the Absolute Refractory Period during which initiation of a second action potential is INHIBITED, but not impossible. As voltage-gated potassium channels open to terminate the action potential by repolarizing the membrane, the potassium conductance of the membrane increases and the K+ ions move out of the cell and bring the membrane potential closer to the equilibrium potential for potassium and this can lead to membrane hyperpolarization.
Na+ channels are recovering from inactivations- excitability returns to normal as the number of channels in the inactivated state decreases and as the number of open voltage KV channels closing increases
inacgtivated voltage channels
cannot open
active voltage gated channels
may not be open, but have the potential to
hyperpolarisation
Occurs when the membran potential becomes more negative than the normal resting potential.
Until K+ conductance returns to the resting value, a greater stimulus will be required to reach the threshold for another AP
what marks the end of the relative refractory period
Return of equilibrium resting potential
- Ek
molecular properties of votlage gated Na+ channel
molecular properties of votlage gated K+ channel
different types of cells have
different conduction velocities
the larger the axon the
the lower the cytoplasmic resistance and the higher the conduction velocity
classes of peripheral axons (6)
Aα
Aβ
Aγ
A𝛿
B
C
which class of peripheral axon has the created conduction velocity
Aα- widest
Which class of peripheral axons has the smallest conduction velocity
C- narrowest
what type of fibres are Aα
sensory fibres fron muscle spindles, motor neurone to skeletal muscle
what type of peripheral axon is A𝛿
sensory fibre from pain and temp receptors (sharp localised pain)
what type of peripheral axon is B
preganglionic neurones of the autonomic nervous sstem (myelinated)
what type of peripheral axon is C
sensory fibres from pain, temp and itch receptors (diffuse pain)
local current theory
Injection of current into an axon will cause the resulting charge to spread along the axon and cause an immediate local change in the membrane potential.
A change in MP in one part can spread to adjacent areas of the axon
conduction velocity is determined by
how far along the axon these local currents can spread
name 3 properties of an axon that leads to high conduction velocity
- High membrane resistance
- Low membrane capacitance
- Large axon diameter- leads to low cytoplasmic resistance
capacitance, C, is simply the
ability to store charge
- property of the lipid bilayer
high capacitance
voltage changes more slowly in response to a current injection- decrease in spread of local current
low capacitance
increase conduction velocity
membrane resistance dpends on the number of
ion channels open
low resistance
the more ion channels open and more loss of the local current across the membrane
high resistance
increase conductance- change in voltage spreads further along the axons
what can increase conduction velocity of a neurone
myelination
effect of myelination
reduce capacitance and increase the resistance of the axonal membrane
which types of neurones are myelination - larger or smaller diameter?
larger e.g. motorneurons
what process does myelination faciliate
saltatory conduction
what is the myelin sheath made out of in the peripheral NS
schwanna cells
what is myelin sheath made out of in the CNS
oligodendrocytes
density of sodium channels in myelinated neruones
high density in nodes of ranvier- quick conduction of AP
density of sodium channels along non-myleinted axon
evenly distributed
structure of mylelin sheath
Layers of Schwann cells tightly folded around each other
- Shwanna cells surround the outside of the neurone
saltatory conudction
Myelin sheath acts as a good insulator causing local circuit currents to depolarise the next node above threshold and initiate an action potential
Action potnetial “jumps “from node to node allowing much faster conduction velocity- AP only occurs at nodes.
what occurs when demyelination happens
Action potential threshold cannot reach threshold in regions of demyelination and stop saltatory conduction
what is the most common demylinating disease
multiple sclerosis
multiple sclerosis causes
all CNS nerves to become demyelinated
what is a less common demyelinating disease of the CNS
Devic’s disease
Devic’s disease
demylination of optic and spinal cord nerves only
demylinating disease of the PNS
- Landry-Guillain-Barre syndrome
- Charcot-Marie-Tooth disease
name two widely used anaesthetics
lidocaine and procain
local anaesthetics can be
protonated or unprotonated
- unprotonated can pass the membrane
anaesthetics work by
blocking Na+ channels
Easier when the channel is open and have a higher affinity to inactivated state of Na+ channel
how does lidocaine act as a loacl anaesthetic
Act by binding to and blocking Na+ channels
This stops AP from occurring
Local anaesthetics block conduction in nerve fibres in the following order:
- Small myelinated axons
- Non-myelinated axon
- Large myelinated axon
