Local Anesthetics Flashcards
(112 cards)
Local anesthetics definition
Drugs that reversible block conduction of electrical impulses along nerve fibers
Schwann cells
Support and insulate each axon
What type of cells surround each axon
Schwann cells
Unmyelinated nerves and Schwann cells
Small nerves, single Swann cells cover several axons
Myelinated nerves and Schwann cells
Larger nerve, Schwann cell covers only one axon and has several concentric layers of myelin
nodes of ranvier
Periodic segments between Schwann cells along the axon that do not contain myelin
There are VGNa channels in these segments and are the primary site of LA action - action potentials jump from nerve to nerve aka saltatory conduction
What is saltatory conduction
APs jump from nerve to nerve via nodes of ranvier
How many nodes must LAs inhibit VGNa channels in to block impulses
Three successive nerves
What are bundles of axons called
Fasiculi
What are the layers of the connective tissue that cover fasciculi
There are 3
Endoneurium - thin, delicate collagen that embeds the axon in the fascicule
Perineurium - consists of layers of flattened cells that binds groups of fascicules together
Epineurium - surrounds the perineurium and is composed of connective issue that holds fascicles together to form a peripheral nerve
Falyar’s explanation of the neuriums
The endoneuriums surround individual axons
The perineurium binds fasicicles together
The epineurium holds all of those bundles of fascicles together to form a peripheral nerve
RMP of axon
-70mV to -90mV
What physiologic mechanisms help create RMP
Na-K pump in axolemma
Intracellular K ratio of 30:1
Membrane impermeable to other ions
Excess of negatively charged ions in axoplasm
Nernst equation
Expresses the charge created by K+ concentration gradient
What charge puts VGNa into active states (end of depol)
20mV
What restores RMP
NA - K pump
3 Na’s leave for each 2 K’s that enter
LA mechanism of action
Bind to VGNa channels preferentially to those in open, inactive states
They also block K, Ca, and GPCRs to a lesser extent
This blocks transmission of nerve impulses
They DO NOT alter the RMP or threshold potential
Modulated receptor hypothesis of LA action
Preference to attach during active or inactive states
Frequency dependent blockade
Resting nerve is less sensitive to LA than one repeatedly stimulated
AKA = use-dependent or phasic block
“Works better when its doing something”
Are LAs acids or bases? And describe the mechanism of how they enter the cell
All LAs are weak bases
Unionized (unprotonated) base form of the LA diffuses through cell membrane and then becomes re-ionized once inside cell and is able to attach to the inner portion of the VGNa channel
Which type of nerves to LAs preferentially bind to
Smaller, unmyelinated nerves
Aka larger, myelinated nerves are harder to block
Differential blockade order?
Preganglionic (think sympathetic nerves) are blocked first, followed by small C fibers and small A fibers
= loss of pain and temp
Touch and proprioception can still be present
Type A - Alpha fiber characteristics
Proprioception, motor
Diameter = 6-22 um
Heavy myelination
Last to be blocked
Type A beta fiber characteristics
Touch, pressure
Diameter 6-22 um
Heavy myelination
Intermediate time to block