L3: Action Potential and conduction

Structure of ion channels:

• K channels: 4 subunits (transmembrane domain) proteins arranged in a barrel to form a pore htat spans the membr, are selective for K+ cuase of polypeptide sequence lines the pores of the ion

• Sodium Channels: single (only 1) subunit made up of 4 times repeating 6transmembr. segments, have pore-lining region-->> determines selectivity, lets Na+ pass through, contain voltage sensing domain rotate&moves up causing pore to open by conformational change!
  
  

Activation State of ion Channels
3 states of conformational change:

• resting state: closed- no ion flow
• activated state: open-ion flow (Na+ influx)
• inactivated state: closed-no ion flow :

*Refractory period: Na+channels become inactivated as the membr. depolarizes and cannot be activated again until the membr. is repolarized.

membr. hyperpolarized until K+ channels close, so more current required to depolarize membr. potential to more positive

• absolute refractory period: when during AP a 2nd stimulus no matter how strong will not produce a 2nd AP, inactivated closed channels
• relative refractory perod: can last 1-15ms, coincides with after hyperpolarisation, interval during which 2nd AP can be produced ONLY if stimulus strength = greater than treshold.

Action Potential/ Spike/Nerve impulse discharge:

rapid large alterations in membr. potential up to 100mV from -70 to +30mV, inside cell becomes more (+)!
Only EXCITABLE (nerve, muscle, endocrine, immune, reproductive) CELLS produce AP

≠ ALL CELLS can produce local potentials

Threshold: point where all Na+ channels open
Overshoot: always go above OmV~ENa+
All or nothing event: once pass threshold always generate this size of potentia, Always get same height, amplitude in graph every cell same Na channels=>same size eq.,
Refractory period:*
Propagate: act of ion potential
No decrement
undershoot: ~EKa+

Conduction velocity: propagation/A.P. conduction, A.P. travels down axon, no backwards cause absolute refractory period inactivated closed channels, SIGNAL<>> MUSCLES

Myelin: ↑diameter of axon, ↑myelin (stimulating substance, round axon, insulation to speed up flow of AP membr.)=> better, faster AP

Nodes of Ranvier (Saltatory=jumping conduction:

node/gaps in myelin, clusters of Na+ channels

Local Potentials: confined to a small region of cell membr., small response,

size: vary,

dicited. stimuli

dif.names depending on location/function

depolarization/hyperpolarization

graded-size/duration

decay rapidly,

travel small distances

show summation

e.g. endplate, synapric, pacemaker, receptor potential

polarized: outside/inside cell=> dif. net charge

depolarized: potential ↓ (-) than resting potential level (Na+ influx)

overshoot: reversal of membrane potential polarity, inside of cell becomes (+) relative to outside

repolarizing: when a membr. potential that has been depolarized returns toward the resting value (K+ Efflux)

hyperpolarized: when potential is more (-) than restin lvl

undershoot: E~K+ r.m.p.

Graded potentials (cause magnitude of potential change can vary): changes in membrane potential that are confined to a relatively small region of the plasma membr. produced < some specific change in cells environment acting on a specialized region of membr. they are given various names related to location of potential/function perform.