side Res$ng poten$al -70 mV inside to out: 1. Selec've membrane permeability to ions due to channels • K+-selec've ion channels always open, so 100 x more permeable to K+ than Na+ 2. Diﬀerent ionic concentra'ons on either side of membrane
• Passive diﬀusion of K+ down its concentra*on gradient (150mM -> 5mM) out of the cell 2. Na+/K+–ATPase • 3 Na+ out, 2 K+ in • Net movement of posi*ve charge out of the cell • Only directly contributes -4mV to the res*ng poten*al • However it establishes the chemical gradient of K+ and Na+ Na+ itself contributes li1le to the res3ng poten3al as the membrane is impermeable to it.
produce over a permeable membrane 2 Simpliﬁes at 18ºC for monovalent ions to: 2 is the equilibrium poten1al for a speciﬁc ion, the gas constant (8.314 J.K-1.mol-1), the temperature (K), Faraday's constant (96,500 C.mol-1), and the ionic valency (e.g. +1 for Na+)
5 -90 Cl- 10 125 -70 Na+ 15 150 +60 As the membrane poten.al lies at –70mV, it's clear that another factor other than simple chemical gradient is involved in determining the poten.al diﬀerence – and that is the permeability of the membrane.
of depolarisa.on to the adjacent area of membrane via local currents 2. Regenera.on through the ac.on poten.al as the resultant poten.al diﬀerence is ampliﬁed back to full size Both posi%ve and nega%ve feedback loops are involved: • "All or none" eﬀect is due to the posi3ve feedback loop between membrane depolarisa3on and Na+ permeability • Delayed ac3on voltage–gated K+ channels produce nega3ve feedback, returning membrane to its res3ng poten3al Only a small number of ions ﬂow during channel opening – the change in membrane permeability is the most signiﬁcant eﬀect on poten;al diﬀerence, not the number of ions ﬂowing
1. Closed (res1ng) 2. Open (ac1on poten1al) 3. Inac1vated (refractory) Depolarisa*on causes charged S4 domain to move outwards, opening pore Opens for 0.7ms before inac2va2on Tetrodotoxin block the mouth from the outside LAs (ester & amide) block the channel a6er diﬀusing through the membrane
of propoga#on than passive conduc#on due to the latency between threshold poten#al and full size depolarisa#on Cell membranes behave as capacitors as they are thin and hold charge well Myelin increases the resistance and reduces the capacitance of the membrane, so ac6on poten6als can only occur at nodes of Ranvier Between the nodes, local currents cause depolarisa3on of the next node (passive = faster) Myelin conserves energy due to the reduced ion ﬂux requiring less energy to restore res5ng ionic concentra5ons
to ac1on poten1al threshold by (slow inﬂux of Ca2+) • Phase 0: Depolarisa1on by opening long-las1ng voltage- gated Ca2+ channels and inward movement of Ca2+ ( ). • No Na+ current involved at the SAN • Low peak depolarisa1on of 20mV • Phase 3: Repolarisa1on: • Reduc1on in depolarising currents ( & inac1vated at posi1ve poten1als) • Increase in repolarising currents ( ac1vated by posi1ve poten1als) causing late increase in K+ permeability (funny current) is a high background leak of Ca2+ through Na+ channels.