5- Membrane Potentials

Explain potential, current, and resistance in neurons

• potential - the difference in charge between 2 points (caused by movement between)

• current - flow of electrical charge (ions) from one point to another

• resistance - the opposition to the flow of ions from one point to another (membrane)

How is the resting membrane potential in neurons generated?

• more K inside than outside

• more Na outside than inside

• cell membranes create a barrier

• channels create permeability to allow ion flow, with more K going out than Na going in (creating a comparatively negative inside)

• Na/K pump maintian sconcentration gradient, pumping Na out and K in against their concentration gradient

• movement of ions acorss membrane generates potential

What happens to resting membrane potential when postively charged ions like Na+ flow in?

membrane potential becomes more positive (compared to outside cell)

Which way does sodium want to move and which way does potassium want to move in cells?

• lots of sodium outside cell, so sodium wants to move in

• lots of potassium inside cell, so potassium wants to move out

What happens to resting membrane potential when postively charged ions like Na+ flow out?

gets more negative inside (and more positive outside)

What causes the negative resting potential energy in cells?

cells more permeable to K than Na, so more K moves out than Na moves in, making the inside less positive than the outside

What channels in cells are always open for K/Na and why?

• leaky channels

• allow Na/K to move down concentration gradient

What is depolarization and hyperolorization in regards to membrane potential?

• depolarization = less negative

• hyperpolarization = more negative

What do changes in membrane potential cause in cells?

send signals in/between cells

What happens to membrane potential when you

• increase extracellular potassium?

• increase extracellular sodium?

• decrease extracellular potassium?

• decrease extracellular sodium?

• depolarization (more positive)- less K leaving cell, less negative inside

• depolarization - more Na going into cell, more positive inside

• hyperpolarization (less positive) - more K leaving cell, more negative inside

• hyperpolarization - less Na going into cell, more negative inside

How can you change membrane potential?

• change ion concentrations (due to a pathological condition)

• change permeability

What are chemically-gated channels and when are they used in neurons?

• when a chemical/ligan arrives, channel opens up to allow a specific type of ion through

• opening/closing causes local potentials

What are voltage-gated channels and when are they used in neurons?

• channels open/close to allow ions through in response to changes in membrane potential

• used to generate action potentials and repolarize action potentials

What two types of signals do changes in membrane potential produce?

• local/graded potentials (incoming signals)

• action potentials (outgoing signals)

Describe the rules for local/graded potentials

• caused by an incoming signal from another cell that opens/closes chemically gated ion channel

• occurs in the dendrites/cell body

• can be depolarizing (due to sodium channels opening) or hyperpolarizing (due to potassium channels opening)

• can be weak or strong (due to lots of channels opening or staying open longer)

• strength decreases over distance

• due to the opening or closing of chemically gated ion channels

What does a chemically-gated potassium channel opening cause in the cell body of a neuron? What does a chemically-gated potassium channel opening cause in the cell body of a neuron?

• potassium - more potassium can leave, causing hyperpolarization

• sodium - more sodium can enter cell, causing depolarization

• result in local potential

Describe the rules of action potentials

• caused by depolarization at axon hillock from a local potential that makes membrane potential reach threshold

• occurs down axons

• all or nothing

• self propogating

• due to opening of voltage-gated channels

Describe the changes in permeability of sodium and potassium inside neurons that cause action potentials

1. resting state - activation gates on Na/K voltage gated channels closed

2. depolarization - membrane potential at axon hillock depolarized to threshood, voltage-gated Na channels open with activation gate, causing more depolarization

3. repolarization - inactivation gate closes after a millisecond, stopping Na from entering cell through the; K+ channels open slowly at threshold to repolarize

4. 4. hyperpolarization - K+ channels slow to close, causing hyperpolarization

Describe voltage gated sodium channels

• allows Na+ into cell

• used in action potentials to cause more depolarization once threshold is reached

• has activation gate thats closed at rest and opens with depolirization aboe threshold

• has incativation gate that blocks channel shortly after opening

• must reset after AP

Describe voltage-gated potassium channels

• allow K to flow out of cell to repolarize cell during action potentials

• closed at rest

• opens slowly with depolarization above threshold

• closes slowly after repolarization

• workds through negative feedback; depolarization causes channels to activate, which makes things less positive

What is the absolute refractory period and relativerefractory period in action potentials?

• absolute - no new action potential can occur, voltage-gated Na+ channels already open or blocked by inactivation gate

• relative - membrane potential is hyperpolarized after action potential, need very strong signal to reach threshold

What affects the speed of action potential propagation?

• axon diameter - larger has less resistance and faster action potential

• myelination - allows action potential to skip through myelinated areas to gaps where channels are, speeds up action potentials