L4+5 Graded Potentials & Action Potentials

What are end plate potentials (EPPs)? What causes them?

-a graded response at the motor end plate of the neuromusuclar junction -caused by opening of mixed cation channels that carry both Na+ and K+ -generally very similar to EPSPs

What magnitude is needed for a second stimulus to elicit a second potential during the relative refractory period?

Threshold is higher than for the first action potential, but decreases with time until the end of the relative refractory period when it has returned to the initial value

What kind of work are electrochemical gradients used for?

excitable cells (neurons) use graded potentials and action potentials (types of changes in membrane potential) to generate and transmit signals (info)

What is hyperpolarization?

membrane potential becomes more negative, e.g. -70 to -80 mV -inside of cell becomes more negative

What is threshold (potential)?

point at which stimulus is just adequate to initiate an action potential 50% of the time; graded potentials do NOT have a threshold

Why are membrane potentials important in neurophysiology?

separation of electrical charges requires work; when charges are separated across the cell membrane, they have the potential for doing work

What is the nature of amplitudes for action potentials?

they are all or nothing; once a stimulus is of sufficient strength, an explosive relatively large (approx 100 mV) response of standard size (action potential), occurs

What are the characteristics of action potentials that differentiate them from graded potentials?

-amplitudes are all or nothing -threshold -conduction is non-decremental -refractory period

What is the positive feedback system that results in a rapid depolarization towards ENa?

-at resting potential, many K+ channels are open but very few Na+ channels are open -if membrane depolarization sufficient to reach threshold, some Na+ channels open -opening of Na+ channels causes Na+ to rush into cell, depolarizing cell further -cause of rapid upstroke of AP

What are the characteristics of graded potentials?

-can be either depolarizing or hyperpolarizing, depending on the currents involved (which channels are open + the ionic concentrations inside and outside) -vary in size (graded amplitude; magnitude of response varies with magnitude of stimulus -decremental conduction: magnitude of potential change declines the further you get away from the stimulus site -summation: magnitudes of superimposed potentials are additive

What determines speed of conduction?

-diameter of fiber -presence of myelin

Why doesn't the membrane potential reach ENa?

-driving force for Ina declines as membrane potential approaches ENa -INa decreases due to inactivation of the Na+ channels -Ik is slowly starting to build

What is the net result of saltatory conduction?

-fasted conduction -energy efficient

How does conduction by local current flow occur?

-firing of first AP causes depolarization of cell, so inside is positive in this region of axon -outside of the cell membrane is negative in this region -in areas immediately adjacent, cell is still at rest with the inside negative and outside positive -current flows locally, since these opposite changes attract

What is the restoration of the resting potential (repolarization) following a graded potential due to?

-same forces that originally determined the rmp i.e. permeabilities of the membrane for K+ and Na+ and the driving forces for K+ and Na+

What happens in rare cases of opening of Cl- or K+ channels?

-stabilizes the cell at the resting membrane potential instead of causing hyperpolarization, so the membrane stays at rmp -larger background current caused by opening of more background channels makes subsequent depolarization to threshold more difficult, even though no change in membrane potential occurred -this stabilization effect is also inhibitory because it still makes reaching threshold more difficult

Are there any changes in the membrane potential with increasing strength of depolarizing stimulus?

-when membrane potential reaches threshold, action potentials are generated -increasing stimulus strength above threshold level does NOT cause larger action potentials

How is conduction non-decremental for action potentials?

APs don't decrease in amplitude over distance, so they are propagated (i.e. travel long distances without a loss of amplitude); effective over much longer distances

How does sodium current increase?

Increases rapidly upon stepping beyond threshold; in spite of voltage step being maintained, it doesn't stay high but spontaneously decays (inactivation)

What is the upstroke (depolarizing phase) of the action potential caused by?

a positive feedback mechanism, whereby opening of some Na+ channels causes depolarization, which causes opening of more Na+ channels due to very large driving force for Na+ entry near the resting potential, significant Na+ influx occurs once even a relatively few Na+ channels open

What types of graded potentials are there?

can be either excitatory (depolarizing) or inhibitory (hyperpolarizing or stabilizing)

What happens during a command potential (depolarizing step) to -40 mV?

causes a small inward (depolarizing) Na+ current and a small outward (repolarizing) K+ current

What happens when the membrane depolarizes?

causes sodium channels to rapidly open, then spontaneously undergo inactivation; at the same time, the potassium channels slowly begin to open both channels return to closed state when the membrane repolarizes for a more negative voltage

How does the cell use action potentials to signal inputs of varying intensity?

cell varies frequency of AP firing

Why does the opening of Cl- or K+ channels result in hyperpolarization?

considered inhibitory because it takes the cell further away from the threshold

What causes repolarization?

inactivation of Ina (decreased inward positive current) and simultaneous activation of Ik (increased outward positive current

What is the myelin sheath?

insulating wrapper made up of layers of membranes that covers axons of certain nerves; there are gaps between the myelin along the length of the axon

Why does the opening of mixed cation channels carrying Na+ and K+ result in depolarization?

it is considered excitatory because it brings the membrane close to the threshold for firing action potentials

What causes the threshold to exist?

largely due to voltage-dependence of opening of Na+ channels

What do larger depolarizing steps (to 0mV and +40mV) cause?

larger inward (Na+) and outward K+ currents

How does the diameter of the fiber determine speed of conduction?

larger the fiber, faster it conducts; like electrical cables, the larger the cable (axon), lower the resistance

What is the role of Na+-K+ ATPase in excitability?

maintains RMP by pumping back out extremely small amounts of Na+ entering and pumping back in the K+ leaving at rest and opposing small changes in concentration gradients generated by each action potential; maintains concentration gradients necessary for both RMP and AP

What happens during afterhyperpolarization?

mechanisms responsible for repolarization cause a transient hyper polarization, i.e. membrane potential temporarily becomes more negative than the normal resting potential -occurs because Na+ channels haven't immediately recovered from inactivation and voltage gated K+ channels are slow to close upon repolarization -Na+ permeability is even less than at normal resting potential, while K+ permeability is quite high

What is depolarization?

membrane potential becomes less negative (e.g. from -70 mV to -60 mV) -inside of cell becomes more positive compared to what it was previously -can happen beyond 0 mV and become positive relative to the outside of the cell (overshoot)

What happens instead of reaching ENa?

membrane potential repolarizes

What is repolarization?

membrane potential returns toward resting membrane potential following a hyperpolarization or depolarization

How does the presence of myelin determine the speed of conduction?

myelinated fibers conduct much faster than unmyelinated fibers of the same size

What happens during an absolute refractory period?

no stimulus, however great, will produce a second action potential immediately following a stimulus that initiates an action potential

How is the first action potential initiated?

normally initiated by a graded potential; site of first one is normally axon hillock

How are action potentials propagated?

once first AP is generated, impulse is conducted down the length of the axon by either: -conduction by local current flow -saltatory conduction

What is an absolute refractory period?

period during which an action potential cannot be elicited no matter how strong the stimulus

What is the relative refractory period?

period during which another action potential can be elicited only by a greater than normal stimulus

What is the refractory period?

period of time following the onset of an action potential during which its difficult or impossible to generate another AP

What are excitatory postsynaptic potentials (EPSPs)?

postsynaptic response to an excitatory neurotransmitter opens mixed (non-specific) cation channels which carry both Na+ and K+, resulting in depolarization

What are inhibitory postsynaptic potentials (IPSPs)?

postsynaptic response to inhibitory neurotransmitter opens up Cl- or K+ channels, causing a hyperpolarization

How is saltatory conduction more energy efficient?

process of continually generating APs requires less energy in myelinated nerves

How is saltatory conduction faster?

process of depolarizing a node further away is father than depolarizing adjacent areas and progressing down the axon since there are fewer intermediate steps

What are action potentials?

rapid, large changes in the membrane potential, which have several characteristics that distinguish them from graded potentials, which are normally the stimulus for the generation (firing) of these potentials

What is overshoot?

reversal of polarity of the membrane potential -inside of cell becomes more positive compared to the outside

What is saltatory conduction?

specialized conduction mechanism in some vertebrate neurons in which entire axon doesn't fire action potentials and much of the axon is covered with a layer of myelin -in myelinated nerves, only the regions at the Nodes of Ranvier have fast Na+ channels, and thus are the only regions that can generate action potentials -AP in one node depolarizes the next node to threshold; areas in between don't fire action potentials

What is the shape of an action potential largely determined by?

time-dependent changes in Na+ and K+ currents

How does potassium current increase?

turns on more slowly but is maintained during the voltage pulse; K+ channels don't exhibit inactivation; slower onset and doesn't inactivate as long as the command potential is held constant