Action Potential Flashcards
whats is the potential difference across
the membrane of all cells
what is the range for resting membrane potential
20 - 90mV
what is more negative - intracellular or extracellular fluid
intracellular fluid is more negative than extracellular fluid
Equal numbers of +ve and –ve charges in ECF and ICF
But, ion/charge distribution is ‘polarised’
what is the intracellular membrane charge
negative
what is the extracellular membrane charge
positive
is the resting membrane potential natural
no
3 key ions in the ICF and ECF
sodium Na+
potassium K+
chloride Cl-
sodium ions in ECF compared to ICF
ECF= 145mM ICF= 15mM
potassium ions in ECF compared to ICF
ECF= 4mM
ICF=150mM
Chloride ions in ECF compared to ICF
ECF=110mM
ICF= 10mM
the resting membrane potential causes which ion to want to move in and which ion to want to move out of the cell
sodium wants to move into the cell (higher conc in ECF)
potassium wants to move out of the cell (high conc in ICF)
what do substances need to be in order to move across the membrane
hydrophillic
so ions cannot diffuse (only small leak)
sodium concentration gradient
Na+ high concentration in ECF flows down concentration gradient to ICF
Into cell
potassium concentration gradient
K+ high concentration in ICF flows down concentration gradient to ECF
Out of cell
what is the ‘resting’ membrane potential impermeable and very permeable to
impermeable to Na+
very permeable to K+
what is the diffusion potential and what ion creates this
Diffusion of K+ leaves excessive positive charge inside cell compared to the ECF
This potential gradient arising from diffusion is the ‘resting membrane potential’
what is the RMP mainly due to
diffusion of K+ from cell interior through K+ channels
The small amount of Na+ that leaks into the cell is expelled by the Na+/K+ pump
The Na+/K+ pump also contributes by
- exchanging unequal numbers of Na+ and K+
The Na+/K+ pump is ‘electrogenic’
- ATP required
action of Na+/K+ pump
The pump moves 3 Na+ outwards and 2 K+ inwards
‘electrogenic’
- ATP needed
what is the process of action potential
Process of bringing from RMP to inverted arrangement and back
what is the threshold for action potential
-55mV
what is the rising phase of action potential due to
Na+ influx through voltage gated Na+ channels
what is the falling phase of action potential due to
K+ efflux
voltage gated K channels
what are ion channels
Transmembrane proteins
Aqueous channel through membrane
Gated opening:
- Ligand e.g. Ca channels usually
- Voltage e.g. Na and K channels usually
what are voltage gated ion channels opened by
Specific voltages open and close the channel
Ion selective (specific):
- Na+ channel
- K+ channel
- Ca++ channel
can an ion channel have more than one gate
yes
e.g. Na channel has 2 gate arrangement
first stage of AP after stimulus applied
lead to depolarisation is sufficient strength
MP moves towards the ‘threshold’
gated ion channels are closed
what happens when the MP reaches the ‘threshold’
cause Na+ voltage gated channels to open (both)
Na+ influx
- more depolarisation
more Na channels recruited throughout influx leads to greater level of depolarisation
K+ channels remain closed
what causes the MP to overshoot 0mV after reached above threshold
all sodium channels are opened
so maximum Na+ influx
what happens when MP reaches +35mV
Na+ channels shut
- inactivation (‘h’ gate closes)
K+ channels open
- K+ efflux begins
reverse of process stop Na movement and start movement of K
what occurs in the AP downstroke and refractory phase
‘recovery’ phase
Na channels shut
- the refractory period
K+ channels open so efflux continues
what happens after the refractory phase when the MP returns to resting state
ion channels return to resting state
- Na+ H gate opens after refractory period so M gate closed
- K+ gate closed
excitability restored
what gate is responsible for the refractory period
Na+ H gate
what is the purpose of the refractory period
ensure signal moves unidirectionally
the neuron cannot generate another AP until the first one has ended
what happens when the -55mV threshold is reached
AP is all or none, cannot add together APs
no need to alter stimulus as will travel through neuron once triggered
what happens at the AP threshold
voltage-gated Na+ channels open
- Na+ diffuse in leads to further depolarisation
Positive feedback involved here
what happens at AP peak
Na+ channels close; voltage-gated K+ channels open;
K+ diffuse out causes repolarisation
Return to resting membrane potential
what is the period of in-excitability post AP called
refractory period
what is the sequence of opening and closing of Na+ gates in AP
M gate closed in first phase
Then both open
Then H gate in second phase (refractory period)
where a sodium gates positioned
they are intracellular
what are 4 consequences of the refractory period
Limits maximum firing frequency of action potentials in axons
Ensures unidirectional propagation of action potentials
Prevents summation of action potentials
Prevents summation of contractions in cardiac muscle – the cardiac AP lasts as long as the ventricular contraction
how does an AP travel through a neuron
AP in one section of axon sets up longitudinal current flow
- This depolarises adjacent ‘resting’ parts of the axon
The AP is regenerated further along the axon
More current flows, and the next region of axon is activated
Action potentials travel along the axon as waves of depolarisation
- Crawling through axon
- Travelling waves of depolarisation
what is the effect on AP diameter on the speed of AP propagation
the speed of AP propagation increases with axon diameter
large axons compared to small axons conduction s speed
large axons conduct impulses more rapidly than small ones
what is the issue with smaller thinner axons and propagating AP
they are more fragile
less membrane for AP to work
why does myelinated axons require more energy
made of 1000s of Schwann cells
- Each need fed and maintained to survive
more efficient to have unmyelinated if don’t need speed
what is the role of myelination
Myelination increases speed of AP propagation for a given diameter of axon
what is a myeline sheath made of on an axon
consists of many layers of cell membranes wrapped round the axon
Myelin laid down by glial cells (Scwann Cells)
what lays down myeline
glial cells (Schwann Cells)
what is the physical role of myelin
forms an insulating layer, reducing leakage of current from axon
- rather like lagging a hot water pipe
what are the interruptions in myeline sheath called
Nodes of Ranvier
what is the role of Nodes of Ranvier
axon membrane is exposed to the ECF, and ion flow can occur
where the AP is
- need space so can jump from one node to next
what type of conduction occurs in myelinated axons
saltatory conduction
how is the propagation speed greater for myelinated axons compared to unmylinated
In myelinated nerve, the passive currents spread further along the axon
There are fewer regeneration steps per unit length of axon
- high concentration of channels at node (unmyelianted has same amount but more spaced)
Thus, the AP propagates more rapidly than in unmyelinated axons
what covers myeline
types of connective tissue membrane
what is the name of the connective tissue which covers the bunch of axons
perineurium
- has BV around it
what surrounds the perineurium (bunch of axons) and BVs
the epineurium
what are the connective tissue membranes that surround axons made of
lipid layers
Structure with high concentration of lipids
- Fat pads
Blood vessels
can nerves have different types of axons in it
yes
- can have myelinated and unmyelinated in one aggregation
myelination of alpha beta nerves
myelinated
myelination of alpha delta nerves
myelination of
myelination of C fibres
unmyelinated
role of alpha beta nerves
mechanoreceptors
role of alpha delta nerves
mechanoreceptors
thermoreceptors (cold)
nociceptors
chemoreceptors
role of C fibres
mechanoreceptors
thermoreceptors (hot and hot)
nociceptors (pain)
autonomic symp system
what happens to myelinated axons as they enter tooth pulp
lose myelination
what is the effect of myelination of LA acting
lack of myelination means LA works faster on axon
