Action Potentials

What is the affect of Nodes of Ranvier?

- exposed neuron membrane and increased Na+/K+ channels causes saltatory conduction due to high internodal resistance. - AP travels faster than squid, although size of axons smaller.

What is the cells normal resting membrane potential?

-90 mV (approaches the equilibrium potential of K+)

2 ways to increase AP conduction velocity?

1. Increase diameter (size) of axon: decreases internal resistance, increases speed of impulse propagation 2. Increase myelination of axon: increases resistance and decreases capacitance of membrane, causing current to flow down interior of axon where resistance is low.

In order for a potential difference to be present across a membrane lipid bilayer, what two conditions must be met?

1. There must be an unequal distribution of ions of one or more species across the membrane (ie, a concentration gradient). 2. The membrane must be permeable to one or more of these ion species. The permeability is provided by the existence of channels or pores in the bilayer; these channels are usually permeable to a single species of ions. Active transport of ions across membrane restores potential.

Nodes of Ranvier

AP jumps from one node to the next, where myelin sheath is absent

What is taking place during an action potential?

Action Potential is a rapid change in membrane potential due to changing conductances (open closing of channels). Depending on which ion has the greatest membrane permeability, that will drive membrane potential towards the Equilibrium Potential for that ion.

Threshold potential

Action Potentials in nerve fibers are an all-or-none event. Once threshold is reached, the size of the impulse is independent of the stimulus - a greater stimulus does not give a greater Action Potential, although it increases rate for reaching AP. Localized graded potentials may sum together to achieve threshold potential.

What is an action potential?

An action potential (AP) is a transient electrical impulse in which the membrane potential rapidly rises to a peak that is approximately 100 mV more positive than the normal resting potential (10mV, depolarization).

Resting membrane potential (Vm) of a nerve cell

At rest, conductance of K+ ions is greater than the conductance of Na+ ions. Membrane potential (Vm) will approach the equilibrium potential of the ion with greatest permeability. Thus, resting membrane potential approaches equilibrium potential of K+ ions (-90mV).

What is the Relative refractory Period?

Begins at the end of the absolute PR and overlaps with the after-hyperpolarization. An action potential can be elicited but a larger than normal stimulus is required to bring the cell to threshold. Due to some Na channels remaining in inactive state and K channels still open

Tetraethylammonmium (TEA)

Blocks potassium current, reveals sodium current Decreased rate of depolarization and reduction in afterhyperpolarization of spikes.

Tetrodotoxin (pufferfish) (TTX)

Blocks sodium current, reveal potassium current. Inhibits AP, but does not block oscillations in sub threshold membrane potential.

Terminal Boutons

Chemical connections with the effector cells, presynaptic membrane

Depolarization

Depolarization is a reduction in the magnitude of the membrane potential. For example, depolarization occurs if gated Na+ channels open and Na+ diffuses into the cell

Hyperkalemia

Elevated K+ in blood brings membrane potential closer to threshold. Lower stimulus required to trigger AP. Severe Hyperkalemia can cause membrane potential to reach positive voltage that Inactivation gates of Na+ gates remain activated leaving cells in an unexcitable state - they can never recover resting Vm.

Why do cells have a slightly negative resting membrane potential?

For most cells, the permeability of the membrane to K+ is greater than to Na+, meaning K+ ions would diffuse out of the cell faster than Na+ ions could diffuse into the cell. A negative Vm is generated secondary to using the Sodium Potassium pumps to slow down the movement of K+ ions out of the cell and increase the diffusion of Na+ ions into the cell.

What initiates an action potential?

Gated Na+ channels open and Na+ ions diffuse across the membrane into the cell and initiate the upstroke of the action potential.

Graded Potentials

In non-excitable cells (muscle - skeletal - post synaptic dendrites), both hyperpolarizing and depolarizing electrical stimulation result in graded potentials which are changes in polarization where the magnitude of the change varies with the strength of the stimulus.

Other Action Potentials

In other excitable cells, the upstroke of the AP results from voltage gated Ca 2+ channel activation

What prevents the Action Potential from propagating backwards?

Inactivated Na+ channels behind the zone of depolarization prevent the action potential from traveling backwards. Action potentials travel in only one direction: toward the synaptic terminals.

Hypokalemia

Increases concentration gradient - requires greater stimulus to reach AP or can even make cells unexciteable.

What is the Hodgkin Cycle?

It is the Positive Feedback Cycle for the rising phase of the action potential. After initial depolarization by opening of Na+ gated channels, the entry of Na+ causes the opening of additional Na+ gated channels and further depolarization.

What are small local currents?

Leading edge currents that change the membrane potential, thereby creating a new AP ahead of the impulse.

Myelin Sheath

Myelin sheath is the insulation provided by glial cells to axon processes. CNS - oligodendrocytes PNS - Schwann Cells

What type of cells have the potential to generate action potentials?

Nerves, muscle cells, and glandular cells share excitability, which in response to stimuli, rapid changes in membrane potential (action potential) occur.

Resting Potential = Threshold Potential = Peak Active Potential =

Resting Potential = -70 mV Threshold Potential = -50 mV Peak AP = +30 mV

Sodium Pump

Sodium pump uses energy derived from ATP hydrolysis to pump 3 Na+ ions out of the cell and 2 K+ ions into the cell. This results in a low intracellular [Na+] / high intracellular [K+] - sets the stage for movement of Na+ and K+ ions through their respective channels or leak pathways down their concentration gradients.

Sodium Potassium ATPase (pump)

Sodium pumps use energy from ATP hydrolysis to pump 3 Na+ ions out of the cell and 2K+ ions into the cell. This results in a low intracellular Na+ concentration and high K+ concentration and sets the stage for movement of Na+ and K+ through their respective channels or leak pathways down their concentration gradients.

What is the Nernst Equilibrium Potential

The membrane potential at which point the chemical potential is equal to the electric potential.

Principle of Bulk Electroneutrality

The number of positive charges in a solution must be the same as the number of negative charges. Electrical gradient needs to be set up equal to chemical gradient. Desire to have equal charge outside/inside

What is represented by resting membrane potential?

The resting membrane potential represents an equilibrium situation at which the driving force for the membrane-permeant ions down their concentration gradients across the membrane is equal and opposite to the driving force for these ions down their electrical gradients.

What is necessary for an AP to be generated?

Threshold potential must be reach by stimulus.

What is the Absolute Refractory Period?

Time period following peak Action Potential when cell excitability is zero due to complete closure of the inactivation gate (H gate) of sodium gated channels.

Where is an action potential generated in a cell?

Typically in the soma, but an AP can be generated anywhere that an impulse is initiated.

Cardiac Action Potential

Upstroke is very fast - phase O - carried by same class of sodium channels

What is Ohms Law?

V=IR. The current between two points is directly proportional to the potential difference across the two points and inversely proportional to the resistance.

If a cell membrane were only permeable to a single ion, what would the cell's membrane potential default to?

Vm (membrane potential) would default to that single ion's Veq (equilibrium potential).

What is a membrane potential?

Voltage (separation of charge) across the plasma membrane caused by electrochemical gradients. Electrical impulses are transmitted along axons as transient changes in membrane potential.

Neurons contain what type of channel proteins?

Voltage gated ion channels that open or close in response to electrical stimuli. Membrane potential changes in response to opening or closing of channel proteins (change in permeability)..

What is thereupon for hyperpolarization?

Voltage-gated K+ channels are slow to close, resulting in short-period of negative feedback. Once hyperpolarization occurs, the closing of the K+ channels starting closing and K+ conductance go back to baseline.

Multiple Scleroris

WBCs break down myelin sheath. nerve signals slowed or blocked.

Hyperpolarization

When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative. There is an increase in magnitude of the membrane potential. Further away from depolarization.

Where is [Na+] and [K+] highest?

[Na+] - highest outside the cell [K+] - highest inside the cell

Depending on which ion has the greatest permeability, ______.

that will drive the membrane potential towards the Equilibrium potential for that ion.