26 September 2011

• Lab this week: Four Endocrine Cases– Bring textbook– Optional: Bring laptop with AirTerrier

• Test # 1 =Monday, Oct 3rd.– Test Material Cutoff to be announced Friday.

• Lab next week: Measuring action potential conduction velocity in human ulnar nerve.

1QQ # 9 for 8:30 class1. A) List the four categories of glia cells of the

central nervous system and B) state which category you would choose to eliminate from your CNS if you were forced to do so, being sure to C) give your reasons for your selection.

2. A) Describe the location and purpose of a growth cone and B) how axonal transport is associated with growth cones.

1QQ # 9 for 9:30 class1. A) List the four categories of glia cells of the

central nervous system and B) state which category you would choose to eliminate from your CNS if you were forced to do so, being sure to C) give your reasons for your selection.

2. A) Describe the location and purpose of a growth cone and B) how axonal transport is associated with growth cones.

Virtues of SquidGiantAxon

Big questions:1)How do cells generate a resting membrane potential?2)What causes changes in the membrane potential?3)How do cells use these potentials? i.e. What is their purpose?

Fig. 06.09

Fig. 06.10a

There is a concentration gradient favoring the diffusion of Na+ and K+ through the selectively permeable membrane which has ion channels only for potassium.

At the start, is there an electrical driving force?

Fig. 06.10b

With K+ channels open, K+ diffuses down its concentraiton gradient, leaving behind CL- ions which are not permeable through the membrane. As more and more K+ move to the left, the compartment they leave becomes more and more negatively charged.

Is there an electrical driving force?

Fig. 06.10c

Fig. 06.10d

Soon, the accumulation of negative charges seriously impeded the diffusion of K+ as the electrostatic force builds up in opposition to the concentration driving force.

Fig. 06.10eEquilibrium potential = Nernst potential = diffusion potential

Eventually, the electrostatic force that impedes diffusion of K+ is exactly equal to the driving force favoring diffusion based on a concentration gradient. When these two driving forces are equal and opposite, the membrane potential reaches an equilibrium at which the voltage is called

So which compartment corresponds to intracellular fluid?

E ion+ = 61/Z log ([conc outside]/ [conc inside])

E K+ = 61/1 log (5/150)E K+ = -90 mV

The Nernst Equation

• If the membrane is permeable to ONLY ONE ion species and you know the concentrations on both sides of the membrane, use the Nernst Equation to calculate the membrane potential.

Nernst potential for X = 61/Z log [Outside ] / [Inside]

S 2

Fig. 06.10eEquilibrium potential = Nernst potential = diffusion potential

E ion+ = 61/Z log ([conc outside]/ [conc inside])

E K+ = 61/1 log (5/150)E K+ = -90 mV

150 mM5 mM

K+50 mM

Predict the change in membrane potential if K+ were added to the extracellular fluid.

S 1

What hormone regulates the levels of Na+ and K+ in extracellular fluid?

Fig. 06.11aS 3

Now consider a situation in which only Na+ is permeable.

Fig. 06.11bS 4

Fig. 06.11cS 5

Fig. 06.11dS 6

Fig. 06.11eEquilibrium potential for Na+

E Na+ = 61/1 log (145/15)

E Na + = +60 mV

145 mM 15 mM

Extracellular Intracellular

So, given these concentrations of Na+ and a membrane permeable only to Na+, use Nernst equation to calculate what the membrane potential would be.

At the equilibrium potential, no net movement of Na+ because driving forces (concentration and electrical) are exactly equal and opposite.

S 7

Electrical and concentration gradient driving forces for Sodium and Potassium

How does the membrane potential change if 1) permeability to sodium increases2) Permeability to potassium increases

Why is resting membrane potential closer to EK than ENa?

What would happen to membrane potential if suddenly PNa

became very great?

Size and Direction of Arrows show driving forces!

The G-H-K Equation!S 8

The Goldman Hodgkin Katz Equation

• If you know the concentrations of ALL permeable ions and their relative permeabilities, you can calculate the membrane potential using the GHK Equation.

S 9

At RMP, some Na+ leaks in, some K+ leaks out.

S 10

Na+ K+ ATPase maintains the concentration gradients across

cell membranes

Animation of the Pump

What would happen to membane potentials and concentrations of Na+ and K+ if cells didn’t have this pump?

S 11

Animations of the Origin of Resting Membrane Potential

Animation of Resting Membrane Potential (single ion)

YouTube animation of Na-K-ATPase, Sodium Co-transporter, and K Leak channels

Origin of Resting Membrane Potential and intracellular recording

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S 13

Which ion moving in which direction (into or out of cell) is responsible for depolarization and overshoot? Which ion moving in

which direction (into or out of cell) is responsible for repolarization and hyperpolarization?

Can the membrane potential go more negative than -90 mV?

Increase PK+

Increase PNa+

S 14

Increase PK+

How do ions get across the membrane? Ion channels!

Graded potentials are conducted decrementally for only a few millimeters, die out over distance and time, and are proportional to the size of the stimulus.

Leak ChannelsGated Channels….. Ligand-gated….. Mechanically-gated….. Voltage-gated

Electrogenic Sodium-Potassium ATP-ase maintains concentrations across membrane

2K+

3 Na+

S 15

Open Na+ channels, Na+ goes _____

Open K+ channels, K+ goes _____

S 16

Graded potentials are conducted no more than 2mm

Insect bites foot (stimulus).Sensory neuron produces graded potential in proportion to intensity of the stimulus.How is signal conducted to the brain?

S 17