Lecture 4 - Electrical Excitability Flashcards
what is the sodium hypothesis?
when the membrane has been depolarised to threshold, voltage gated Na channels open allowing Na influx
influx depolarises the membrane further so more channels open so there is more depolarisation
what happens in repolarisation?
combination of:
Na channel inactivation
opening of voltage gated K channels by depolarisation causing K efflux
what is the all or nothing response?
because the Na channels that cause depolarisation are voltage gated, positive feedback means that once some channels open, all of them will (because the opening of some causes more depolarisation)
what happens in the absolute refractory period?
nearly all Na channels are inactivated so excitability is 0
what happens in the relative refractory period?
Na channels are recovering from inactivation so excitability is returning to normal
what is accommodation?
the longer the stimulus, the larger the depolarisation necessary to initiate an ap because Na channels become inactivated
what are the molecular properties of voltage gated Na and Ca channels?
one peptide four homologous repeats six transmembrane domains one domain is voltage sensitive function requires one subunit
what are the molecular properties of voltage gated K channels?
four peptides six transmembrane domains one domain is voltage sensitive function requires four subunits
how do local anaesthetics work?
bind to and block Na channels, stopping ap generation
higher affinity to inactivated Na channels
in what order do local anaesthetics block conduction in nerve fibres?
small myelinated axons
non myelinated axons
large myelinated axons
how can extracellular recording be used to measure conduction velocity?
electrodes are used to raise membrane potential to threshold to generate an ap
record changes in potential between stimulating and recording electrodes
conduction velocity can be measured from distance and time
what is the local circuit theory?
depolarisation of a small region of membrane produces transmembrane currents in neighbouring regionsthis opens more voltage gated Na channels causing propagation of the apthe further the local current spreads, the faster the conduction velocity of the axon
what properties of an axon lead to high conduction velocity?
high resistance
high diameter
low capacitance
why does high resistance increase conduction velocity?
ohms law says that higher resistance means higher potential difference across the membrane
more voltage means more Na channels are open so easier to reach threshold
why does large diameter increase conduction velocity?
ohms law says that lower resistance (from larger diameter) gives a larger current so the ap will travel firther
