Action potential

Question 1

describe action potentials

Answer 1

occur in excitable cells - neuron/muscle/endocrine
nerve impulses
transmission of information reliably and quickly over long distances
in cell to cell communication and activate intracellular processes eg innervation of muscle cell - contract
beta cells - provoke release of insulin

Question 2

ionic basis of action potential

Answer 2

permeability of membrane depends on conformational shape of ion channels - open by depolarisation, inactivated by sustained depolarisation, closed by hyperpolarisation/repolarisation
movement of ions changes membrane potential towards equilibrium potential for that ion
membrane potential because of K movement
NOT to do with Na-K pump - electrogenic. efflux Na out of cell to maintain Na concentration gradient

Question 3

describe resting potential

Answer 3

caused by K efflux and finite number of Na channels open
Pk&raquo_space; PNa
membrane potential closer to K equilibrium potential

Question 4

describe the depolarising stimulus

Answer 4

stimulation 
graded result 
small depolarisation
move to the V that a lot of V gated channels respond - this is the threshold potential 
if over threshold = action potential

Question 5

describe upstroke

Answer 5

many Na channels
the small depolarisation is detected so more V gated Na channels are open
Na enter down electrochem gradient
move to Na equilibrium potential
Na influx - causes more channels to open - snowball effect
less K leaving than Na entering

Question 6

describe repolarisation

Answer 6

PNa decrease - V gated Na channel inactivate rapidly
Na entry stops
Pk increase - K channel open and stay open (sense depolarisation)
K leaves down electrochem gradient
membrane potential towards k equilibrium potential

Question 7

what happens at the start of repolarisation

Answer 7

part of protein channel forms a gate
other part - ball and chain hypothesis
react to change in V
flip into mouth of channel
Na can’t enter even though the gate is open
inactivation rapid - 700us
this is the ABSOLUTE REFRACTORY PERIOD - no strength of stimulus will cause an action potential
depolarisation recognised by k channels - open

Question 8

what happens later in repolarisation

Answer 8

as cell repolarises Na inactivation gate drops away

channel can be activated again at next impulse

Question 9

describe after-hyperpolarisation

Answer 9

K channels open - continue to leave cell
more open than in resting - large population of channels so close slowly = hyperpolarisation
membrane potential closer to K equilibrium potential
some V gated K channels close
membrane potential return to resting potential
Absolute refractory period ends

Question 10

describe relative refractory period

Answer 10

Na channel activation gate closed
Na channel inactivation gate open
stronger than normal stimulus can cause AP
need to overcome larger potential to get to threshold in hyperpolarisation

Question 11

time course of changes in permeability

Answer 11

really short

Question 12

describe the regenerative relationship between Pna and membrane potential

Answer 12

threshold - once reached because Na channels open AP triggered - all/nothing - can't stop formation of AP at full size 
positive feedback (depol - opening more Na channels - increase Na permeability - increase Na influx - depol) until inactivation of channels 
refractory state - unresponsive to threshold depolarisation until Na channels recover

Question 13

describe ion movements in action potential

Answer 13

Na in K out
only small number of ions move - less than 0.01%
over time concentration gradient and so ability to form action potential decreases
electrochemical equilibrium restored after AP by Na-K pumps - slow process, against conc gradient - ATP

Question 14

describe passive propagation

Answer 14

graded response decline over space

internal and membrane resistance alters propagation distance and velocity

Question 15

describe active propagation

Answer 15

local current flow depolarise adjacent region towards threshold - if reaches threshold it will depolarise the next section
nit behind is in refractory period so this is unidirectional

Question 16

describe saltatory conduction

Answer 16

impulse jump to node of ranvier where the Na channels are and intensifies
impulse decays down axon until node

Question 17

describe velocity of action potentials

Answer 17

travels quickly
faster in wide myelinated (120m per second compared to 1m per s in small non-m)
small = more internal resistance
non-m - no saltatory conduction (MS and diptheria = poor conduction to skeletal muscles)
slowed by: cold, anoxia, compression and drugs