Nerve Impulses- The Action Potential

Topic 4

what are the different terms for an action potential?

spike, nerve impulse, discharge

what does an action potential (AP) do?

conveys information over long distances, rapid reversal of the membrane potential, deploarizes then repolarizes

what does the frequency and pattern of an AP do?

form neural code

what does it mean that an action-potential is all-or-none?

it is triggered only when the threshold value of -40mV has been crossed

what does the voltage clamp do?

used to clamp the membrane potential at any given value

what did voltage clamps propose?

existence of sodium-potassium gates in the axonal membrane, these gates are opened above a threshold level by depolarization, propsed to exist 20 years before voltage clamp discovery

what is a voltage-gated sodium channel?

one large polypeptide, 4 transmembrane domains and one ion-selective pore, each domain is composed of 6 alpha-helices(S1-6), S4 helix is the voltage sensor, conformation changes when voltage reaches a threshold value

what does the pore loop between S5 and S6 do in the voltage-gated sodium channel

acts as selectivity filter

why is there a preference for Na+ over K+

water acts as a chaperone for Na+ and K+ ions, water-K+ complex too large to pass through sodium channel

what is the patch-clamp method?

Erwin Neber, used to examine properties of single ion channels (claps membrane potential at desired level), can observe the ion channels flip between conductance states that are interpreted as open and closed

what are the response properties of voltage-gated Na+ channel?

closed channel, channel opened upon depolarization (1ms opened), inactivation when channel blocked, deactivation occurs when pore closes (Vm must approach resting level)

what is the absolute refractory period

another AP is not possible under any circumstances

voltage-gated potassium channels?

open later, four separate polypeptide subunits join together to form a pore, potassium resets membrane potential, opens in response to depolarization like sodium

how is the voltage-gated potassium channel a delayed rectifier?

potassium and sodium current move in opposite directions, potassium out and sodium in

what are the phases of an action potential?

rising phase (Na+ influx), overshoot (K+ efflux), falling phse (K+ efflux, Na+ influx ends), undershoot (K+ efflux continues until voltage-gated K+ channels close and Na+/K+ pump brings Vm back to rest

what is the relative refractory period?

period when relatively greater depolarizing current is needed to generate AP,

how does the intitiation of synaptic transmission-propagation of the AP work?

influx of positive charge depolarizes the segment of axon before it, AP propagates in only one direction because of inactivation of Na+ channels

how many directions do action potentials travel in?

one

what is orthodromic travel?

axon to the axon terminal

what is antidromic travel?

axon terminal to the axon, artificial method

what is the conduction velocity set at for an AP?

10 m/s but can vary

what factors influence conduction velocity?

spread of action potential along membrane (dependent upon axon structure), path of the positive charge (along the inside of the axon, across axonal membrane), axonal diameters (bigger=faster), number of voltage-gated channels (fewer=faster)

what is the role of the myelin sheath in AP conduction?

provide electrical insulation

what is saltatory conduction

positive charge forced to travel along axon

what are the AP initiation sites?

axon hillock, sensory nerve endings

what is tetrodotoxin?

blocks Na+ permeable pore, pufferfish

what is saxitoxin?

Na+ channel-blocking toxin, red tide

what is batratoxin?

blocks inactivation so channels are open longer than usual, channels open at lower voltagees