Neural Action Potentials

What is a synapse?

the region where an axon terminal communicates with its postsynaptic target cell

Where does the input signal come from in the cell?

the dendrites

Where does the integration center come from in a cell?

the cell body

Where does the output signal come from in a cell?

presynaptic axon terminal

What are action potentials?

-move along the neuronal axon

-all or none events

-generate from Na+ and K+ movements across the cell membrane that result in Vm changes

-each potential is rapid, with the SAME amplitude and timing

What are the Nodes of Ranvier?

a section of unmyelinated axon membrane between two Schwann cells

The myelinated sections of the axons do NOT have

ion channels, action potentials can only occur at the Nodes of Ranvier

A new action potential is generated at EACH

Node of Ranvier

Steps to nerve action potentials

What is the membrane potential at rest?

-70 mV

What is the threshold?

membrane potential necessary to trigger an action potential

Na+ and K+ channels that are sensitive and respond to Vm shape the...

amplitude and shape and duration of the neuron action potential

Na+ channels are OPENED in response to membrane

depolarization (threshold Vm)

Na+ channels close when Vm becomes very

positive (~ENa)

The Na+ channels do NOT reset back to resting state until cell membrane becomes...

hyperpolarized (-100 mV)

A small fraction (0.1%) of K+ channels are...

OPEN at rest

K+ channels are MORE open when the membrane is...

depolarized (~ENa) and continue to STAY open until allowing the cell to hyperpolarize

Na+ channel gating mechanism

An absolute refractory period occurs when an...

The inactivation gate closes

The K+ channels does NOT have...

activation and inactivation gates like Na+ channels do

What happens during the ABSOLUTE refractory period?

no stimulus can trigger another action potential (ball and chain COMPLETELY plugged)

What happens during a relative refractory period?

only a larger-than-normal stimulus can initiate a new action potential since you are further away from the threshold (ball and chain somewhat unplugged)

What does a full action potential look like?

What is hyperkalemia?

increased [K+] in the blood

What is the consequence of a more POSITIVE membrane potential (Vm)?

-easier to trigger an action potential because you are sitting at a value closer to threshold

-problems with resetting Na+ voltage-gated channels - does NOT hyperpolarize

-manifests as HYPERexcitability and cardiac abnormalities - signals slow down to the heart since the gates of Na+ are NOT reset

Membrane potential is ultimately determined by...

K+ (potassium) movement

What is hypokalemia?

decreased in [K+] in the blood

What is the consequence of a more NEGATIVE membrane potential (Vm)?

-brings membrane FURTHER away from threshold for an action potential

-action potential generation requires a STRONGER stimulus

-manifest as muscle weakness and cardiac abnormalities

What is local current flow?

-depolarization results in positive current being spread through the axon's cytoplasm in ALL directions

-charge is DIFFUSED away from its SOURCE

-Na+ flows both forwards and backwards

-on the extracellular surface, local currents flow back to the depolarized section of membrane

The refractory period prevents...

backward conduction

The myelin keeps the _____ in the axonal membrane

Na+ (sodium)

What is saltatory conduction?

the "jumping" of an action potential between the unmyelinated nodes of ranvier

Length constant equation

What does rM in the length constant equation mean?

resistance of neuron membrane

What does rI in the length constant equation mean?

internal/intracellular/axial neuron resistance

The LARGER the diameter of an axon, the...

higher the length constant, the FARTHER the action potential will travel

The SMALLER the diameter of an axon, the...

smaller the length constant, the SHORTER the action potential will travel

What are alpha-motor neurons?

neurons that innervate big muscle groups (ex. quadriceps)

The LARGER the diameter of the axon, the _______ the internal/axial/intracellular resistance

SMALLER

The LARGER the diameter of the axon, the ______ the resistance of neuron membrane

LARGER

Ways to modify rI

change diameter

Ways to modify rm

change diameter

myelination (bigger contributor)

An increase in length constant (λ) means that the axonal membrane...

conducts faster, allowing membrane a significant distance away to attain threshold before charge decays

What do Schwann cells do?

form myelin sheath

Small diameter axonal fibers that are

MORE efficient than myelinated fibers

What does Multiple Sclerosis target?

the myelin sheaths of of the spinal cord and brain

decreases λ

cannot hold Na+ in the axonal membrane

cannot be stimulated

What does Tetradotoxin (TTX) do?

irreversibly binds to and blocks the voltage-gated Na+ channel, thereby preventing the generation of an action potential

isolated from a pufferfish, renders it prey paralyzed

used for research

What does Tetraethylammonium (TEA) do?

irreversibly blocks K+ channels, leading to a prolonged action potential duration and elimination of the hyperpolarization phase of the action potential

can restart another action potential

used for research

What do local anesthetics (ex., novocaine and lidocaine) do?

REVERSIBLY blocks voltage-gated Na+ channels, thereby preventing the generation of an action potential

used clinically

What do cardiac glycosides (ex., oubain and digitalis) do?

inhibits Na+-K+ ATPase

oubain is NOT used clinically - irreversible

digitalis is USED clinically - reversible - used for cardiac patients

What does cyanide do?

compromises ATP production by acting as inhibitor to electron transport chain

NOT used clinically