Know how concentration gradients affect diffusion'

1
(No Transcript)
2
Class 2 Membrane potential
Goals

• Know how concentration gradients affect
  diffusion.
• Learn how membrane permeability is regulated by
  selective ion channels.
• Understand how the equilibrium potential of a
  single ion develops.
• Understand how the resting potential of a cell
  is determined by which ions that can flow across
  its membrane, and their concentration gradients.
• Understand how electrical current flows across
  neural membranes and changes over time.

3
Cell membrane
4
Diffusion
5
Selective permeability
6
Potassium equilibrium
-90 mV
7
Balance of diffusion attraction
8
Little change in concentration
9
Sodium equilibrium
10
Nernst equation
Eion is the equilibrium potential for a single
permeant ion Eion (mV) 58 / charge log
(ionout / ionin)
E? (mV) 58 mV 58 / 1 log (10 mM / 100
mM) EK (mV) 81 mV 58 / 1 log (5 mM /
125 mM) ENa (mV) 58 mV 58 / 1 log (150
mM / 15 mM) ECa (mV) 145 mV 58/ 2
log(100mM / 0.001mM) ECl (mV) 58 mV 58 /
1 log ( 100 mM / 10 mM)
11
Steady-state
12
Resting Potential
13
Equivalent circuit of membrane
Rl
14
Passive conductance
length constant, l Vx is the voltage at distance
X V0 is the voltage at the source l is the length
constant.
X (cm)
15
Passive conduction of current
Rmembrane 1/Surface A 1/(2p
radius) Rlongitudinal 1/x-section A
1/(pradius2)
Rm
Rl
radius
Length
l Rm/Rl radius
l
16
Capacitance
17
0
t
100
t RmCm
18
stimulus
Takes time to charge
spread
19
Overall lessons

• The resting potential is based on the fact that
  K is actively pumped into all cells.
• Given the concentration gradients of K, the
  selective permeability of the resting to K, the
  resting potential results by diffusion with no
  energy required.
• Ions pass membrane through protein pores called
  channels.
• Equilibrium is reached when there is a balance
  between tendency to diffuse electrical
  attraction.
• The membrane of cells is permeable to both K
  and Na, so both ions contribute to the resting
  potential.
• Na leakage into cell must be counter-acted
  with a Na/K exchange pump.
• Electrical potential changes as current spreads
  along membranes.
• Electrical properties of membrane determine how
  the potential changes over distance and over time.

20
(No Transcript)