Exam 3: Electrical Potentials & Depolarization

Electrophysiology

The branch of physiology that studies how cells produce electrical potentials and currents

______ is the basis for neural communication and muscle contraction

Electrophysiology

Electrical Potential

the difference in electrical charge across a membrane

Cells have more ____ particles on the inside of the membrane than the outside.

negative

What is the resting membrane potential for a neuron?

-70 mV

Electrical Current

a flow of charged particles from one point to another

What are currents in the body?

the movement of ions, such as Na+ or K+ through channels in the plasma membrane

Why does the resting membrane potential exist?

because of the unequal distribution of electrolytes between the extracellular fluid and the intracellular fluid

The resting membrane potential results from the combined effect of three factors. What are they?

• ions diffuse down their concentration gradient through the membrane

• the plasma membrane is selectively permeable and allows some ions to pass easier than others

• the electrical attraction of cations and anions to each other

What ion has the greatest influence on the resting membrane potential?

potassium (K+)

Why does potassium have the greatest influence on RMP?

because the plasma membrane is more permeable to K+ than any other ion and K+ ions diffuse out of the cell, making the inside more negative.

K+ is about ____ times more concentrated in the ICF than in the ECF?

40 times

Why can’t cytoplasmic anions, such as phosphate, sulfates, ATP, proteins, etc, escape the plasma membrane?

because they are large and negatively charged

The cellular membrane is not very permeable to ____ and RMP is slightly influenced by it.

sodium (Na+)

Sodium is about ____ times more concentrated in the ECF than in the ICF?

12 times

How does sodium influence the resting membrane potential?

• Some Na+ leak into the cell, diffusing down its concentration gradient

• This Na+ leakage makes RMP slightly less negative than it would be if RMP were determined solely by K+

What does the sodium potassium pump do?

it moves 3 Na+ out of the cell for every 2 K+ it brings into the cell

Why does the sodium potassium pump work continuously?

to compensate for Na+ and K+ leakage

The sodium potassium pump requires a LOT of ____.

ATP

____ percent of energy required from the nervous system is used by the sodium potassium pump.

70%

Local potentials are (4)…

1. graded

2. decremental

3. reversable

4. excitatory or inhibitory

Graded

the size of the local potential varies, depending on the stimulus strength (the stronger the stimulus, the more Na+ gates open)

Decremental

the local potential gets weaker the further it gets from the point of stimulation (the shift in voltage caused by Na+ inflow diminishes with distance)

Reversible

if stimulation ceases, the cell quickly returns to its normal resting potential

Explain how local potentials can be either excitatory or inhibitory.

some neurotransmitters make the membrane potential more negative, so it becomes less likely to produce an action potential; while others make it more positive, increasing the likelihood of firing an action potential.

What are local potentials?

• changes in membrane potential of a neuron occurring at and nearby the part of the cell that is stimulated

• short range changes in voltage across a cell

Different neurons require different things in order to be stimulated. What are different ways that neurons can be stimulated?

by chemicals, light, heat, or mechanical disturbance (such as pressure or stretch)

How might a stimuli influence a local potential?

• A chemical stimulant will bind to a receptor on the neuron

• This opens Na+ gates and allows Na+ to enter the cell

• This causes the ICF to become less negative and can lead to depolarization

• Na+ entry results in a current that travels toward the cell’s trigger zone and can initiate an action potential if the threshold is reached.

Depolarization

a change in membrane potential toward zero mV

Current travels down an ____ , toward the…

axon; dendrites

Action Potential

a dramatic change in the electrical potential of a cell membrane, that occurs when a stimulus triggers the opening of voltage-gated ion channels

Action potentials only occur when…

there is a high enough density of voltage-regulated gates

Soma

cell body

Trigger Zone

located at the axon hillock, this is where action potentials are generated

Describe the steps in which an Action Potential is produced (8).

1. A current arrives at the axon hillock and depolarizes the membrane

2. Depolarization must reach threshold (the critical voltage) to open voltage-regulated gates

3. Voltage gates Na+ channels open, Na+ enters and depolarizes the cell, which opes more channels

4. This results in a rapid positive feedback cycle as voltage rises

5. As membrane potential rises above 0 mV, Na+ channels are inactivated and close; voltage peaks at about +35 mV

6. Slow K+ channels open and outflow of K+ repolarizes the cell

7. K+ channels remain open for a time so that membrane birefly becomes hyperpolarized

8. RMP is restored as Na+ leaks in and extracellular K+ is removed by astrocytes

Hyperpolarized

becomes more negative than the RMP

What are the characteristics of action potentials (unlike local potentials)?

• follows an all or none-law (if threshold is reached, neuron fires at its max voltage; if threshold is not reached, it does not fire)

• Non-decremental: does not get weaker with distance

• Irreversible: once started, goes to completion and cannot be stopped

What is the refractory period of an action potential?

the period of resistance to stimulation

What are the two phases of the refractory period?

• absolute refractory period

• relative refractory period

Absolute Refractory Period

• no stimulation of any strength will trigger action potential

• lasts as long as Na+ gates are open, then inactivated

Relative Refractory Period

• only especially strong stimulus will trigger action potential

• K+ gates are still open and any effect of incoming Na+ is opposed by the outgoing K+

• Generally lasts until hyperpolarization ends