30 September 2008 Pick up endocrine quiz from table. Grade in pencil near question # 11. Grades on Quiz 2: 81 (Mean) ± 15 (standard deviation) Max = 100 (n=3) 20 grades in the A range (90 or better) 12 grades below C range. Reminder: Bring Androstenedione paper to lab this week.

Why is resting membrane potential closer to EK than ENa? The Goldman Equation Why is resting membrane potential closer to EK than ENa? Electrical and concentration gradient driving forces for Sodium and Potassium What would happen to membrane potential if suddenly PNa became very great? Size and Direction of Arrows show driving forces! Graded potentials in dendrites and cell body, action potentials in axon because of the types of ion channels found in these membrane domains. How does the membrane potential change if 1)permeability to sodium increases 2) Permeability to potassium increases

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

….. Ligand-gated ….. Mechanically-gated ….. Voltage-gated Leak Channels Gated Channels ….. Ligand-gated ….. Mechanically-gated ….. Voltage-gated 3 Na+ Electrogenic Sodium-Potassium ATP-ase maintains concentrations across membrane 2K+ 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.

Open Na+ channels, Na+ goes _____ Open K+ channels, K+ goes _____

….. Ligand-gated ….. Mechanically-gated ….. Voltage-gated Types and locations of Ion Channels Sensory neuron Leak Channels Gated Channels ….. Ligand-gated ….. Mechanically-gated ….. Voltage-gated Interneurons & Motoneurons

Insect bites foot (stimulus). Sensory neuron produces graded potential in proportion to intensity of the stimulus. How is signal conducted to the brain? Sending electrical signals over long distances Graded potentials are not suitable (no more than 2mm)

To reset from inactivated state to closed state, membrane must repolarize. Open at -55 mV Membrane must repolarize to “reset” Na+ Channels to be capable of opening again. Compare and contrast voltage-gated Na and K channels based on time to open and duration of open time.

Relative permeabilities Duration of AP Refractory periods absolute RP relative RP Rising Phase Falling Phase Why does the peak of the action potential not reach ENa? Properties of V-gated Na and K channels account for the shape of the action potential and the refractory periods.

What accounts for the afterhyperpolarization? Explain the shape of the action potential based on the properties of Voltage-gated sodium and potassium channels (when and how long each type opens and closes.)

Who Cares? Novacaine, lydocaine, xylocaine, All block voltage-gated Na+ channels Prevent action potentials, so stimulus does not result in an action potential in sensory neurons which would convey that information to the brain where person would be conscious of the stimulus!

What happens when the membrane is depolarized by more than about 15 mV? Action potentials are all or nothing. Analogy of shutter release pressure on a camera, either trips shutter or not. How is the intensity of a stimulus encoded by action potential if all action potentials have the same size (amplitude)?

How does an action potential move along the axon? Why doesn’t the amplitude get smaller with distance? Why is the conduction of an action potential unidirectional? What is the absolute refractory period and is going on with voltage gated sodium channels that accounts for the relative refractory period? What is the relative refractory period and what is going on with voltage gated sodium channels that accounts for the relative refractory period? Axon Hillock Axon

In unmyelinated axons, action potential must be generated at each point along the membrane, a relatively slow process that involved influx of Na+ which sets up positive feedback cycle. In myelinated axons, action potential must be generated only at the nodes of Ranvier, which allows AP to be conducted much faster and with fewer ions moving, and thus less energetically expensive.