excitable cells 2 Flashcards
(11 cards)
what are the Characteristic Features of an Action Potential
1)Rising Phase-rapid depolarization of the membrane
2)Overshoot-where membrane potential is above Zero, in this case ~+40mv
3)Falling Phase-rapid ‘repolarization’ of the membrane, note that it goes more –ve than the starting resting membrane potential
4)Undershoot or ‘after hyperpolarization’…gradually ‘declines’ so that membrane potential comes back to ‘resting’ levels.
how are action potentials generated
-a stimulus cause channels in the membrane which are permeable to cations to open- usually Na+
-if sufficient depolarization occurs, the neurone reaches
“ THRESHOLD”- this is the potential you need to reach to ‘trigger’ an action potential.
-Once it is triggered, it happens no matter what, hence why referred to as ‘all or none’
properties of action potential
-Transient, rapid and reversible change in membrane potential from –ve to +ve
-Different types of excitable cell may have different types of action potential
-Neuron AP often triggered by Na+ permeability increase
-AP’s or ‘spikes’ generated by a cell
=all of the same size and duration
=do not decrease as conducted down the axon
-size of action potential stays the same which isn’t the case for graded potentials
what are the Changes in membrane permeability that underlie an action potential
-rest= 80mv- mostly permeable to to K+
-depolarisation= Na+ channels open therefore and increase of Na+ into cells (increased permeability) and a more +ve membrane potential
-repolarisation= Na+ ion channels close and some K+ channels open and membrane potential decreases
when do Voltage-Gated Na+ Channels open
-open when membrane is slightly depolarised
-the channel is a portion (pore in membrane) which is very sophisticated with a large number of transmembrane domains
-Concentration of charge near plasma membrane (5nm) affects voltage sensors
what manner do Na+ channels inactivate
-time and voltage manner
-this prevents ions from moving across the channel
-Channel inactivation occurs quickly ~ 1ms (Step 3)
-Channel de-inactivation (Step 4) must occur before channels can be activated again
-‘delayed rectifying channels’ ‘rectify or restore the membrane potential’- potassium channels open slowly
what are some useful poisons
-Tetraethylammonium, TEA. K+ channels
-Lidocaine Na+ channels
-Tetrodotoxin, TTX. Puffer fish (sp.) (fugu). Na+ channels
-Saxitoxins, STX. Dinoflagellates (sp). Na+
whats action potential conduction
-propagation along axon- happens because of diffusion of Na+ (or any +ve charge) across the membrane
-diffusion in both directions down the axon
-when +ve ions move down the axon there is a serge of +ve charge enough to tip the membrane potential over threshold which depolarises the membrane
-axons generate action potentials across entire length of action potential
What are the factors influencing the conduction velocity
-diameter- resistance to current flow is inversely proportional to cross-sectional area of the axon
- myelination- it prevents current loss along the axon by increased Rm and increases the Space Constant
=Space Constant is distance from site of depolarization where it has fallen to 37%
-many unmyelinated small axons
because the space constant proportionate Rm/ Ri so the benefit of a high membrane resistance is reduced by the high internal resistance
=metabolic and volume costs of myelination
what are the Factors influencing spread of depolarization down the axon
-Physical properties of the axon
-Diameter
-Leaky (permeability) of membrane
how is myelin made up
-by wrapping the plasma membrane of glia support cells round the axon
-Schwann cells in peripheral nervous system, oligodendrocites in cns
-There are breaks in the myelin sheeth where ion channels involved in generation of action potentials are concentrated, and where action potentials will therefore be generated- Node of Ranvier
-As depolarization travels down the axons, the action potentials appear to skip from Node to node - ’saltatory conduction’
