Action Potential Flashcards by niamh ritchie

whats is the potential difference across

the membrane of all cells

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what is the range for resting membrane potential

20 - 90mV

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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’

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what is the intracellular membrane charge

negative

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what is the extracellular membrane charge

positive

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is the resting membrane potential natural

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3 key ions in the ICF and ECF

sodium Na+

potassium K+

chloride Cl-

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sodium ions in ECF compared to ICF

ECF= 145mM
ICF= 15mM

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potassium ions in ECF compared to ICF

ECF= 4mM

ICF=150mM

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Chloride ions in ECF compared to ICF

ECF=110mM

ICF= 10mM

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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)

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what do substances need to be in order to move across the membrane

hydrophillic

so ions cannot diffuse (only small leak)

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sodium concentration gradient

Na+ high concentration in ECF flows down concentration gradient to ICF

Into cell

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potassium concentration gradient

K+ high concentration in ICF flows down concentration gradient to ECF

Out of cell

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what is the ‘resting’ membrane potential impermeable and very permeable to

impermeable to Na+

very permeable to K+

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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’

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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

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action of Na+/K+ pump

The pump moves 3 Na+ outwards and 2 K+ inwards

‘electrogenic’
- ATP needed

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what is the process of action potential

Process of bringing from RMP to inverted arrangement and back

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what is the threshold for action potential

-55mV

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what is the rising phase of action potential due to

Na+ influx through voltage gated Na+ channels

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what is the falling phase of action potential due to

K+ efflux

voltage gated K channels

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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

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what are voltage gated ion channels opened by

Specific voltages open and close the channel

Ion selective (specific):

Na+ channel
K+ channel
Ca++ channel

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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