Local Anesthetics Flashcards
Local anesthetics reversibly block afferent nerve transmission to produce analgesia and anesthesia without loss of consciousness. What are the 3 ways in order easiest to hardest.
- Autonomic blockade
- Somatic sensory blockade
- Somatic motor blockade
LAs administered near the site of action.
How are Local Anesthetics used
- They are infiltrated around the nerve,
- applied topically to the skin and mucous membranes, or
- injected into blood vessels that are first exsanguinated in order to provide intravenous regional anesthesia
- They are also injected into the subarachnoid and epidural spaces for diffusion to desired levels in the spinal column
Myelinated Nerve Fiber:
Unmyelinated Nerve Fiber:
A Schwann-cell wraps itself around the axon several times, enveloping the axon in a myelin sheath
A single Schwann cell surrounds several axons
Importance of Myelination.
Myelinated vs Unmyelinated Fibers
Conduction:
Propagation of impulses is similar in both
Unmyelinated fibers: impulses travel along the length of the fiber in a continuous fashion
Myelinated fibers, conduction is “saltatory”so fast (50X) that it appears as if impulses leap from one node of Ranvier (no myelin) to the next
Membrane:
Ion Channels are guarded by a gating mechanism. How does this mechanism operate
opens or closes depending on changing physiologic conditions
The membrane forms a barrier across which there is movement of ions along a concentration gradient between the intracellular and extracellular spaces
The membrane forms a barrier across which there is movement of ions along a concentration gradient between the intracellular and extracellular spaces
EXTRAcellular contains a high concentration of SODIUM and low concentration of potassium; reverse for intracellular
Nerve Fibers:
The velocity an impulse travels is proportional to
The diameter of the fiber: the larger the diameter, the higher the conduction velocity
How are fibers classified
Fibers classified according to diameter, three types: Primary afferent axons
A, B and C fibers
Describe A fibers
(myelinated) 1 to 22 microns Subdivided into: α β γ δ in order of decreasing size
Describe B fibers
(myelinated) 1 to 3 micrometers
Describe C fibers
(unmyelinated fibers) 0.1 to 2.5 micrometers
A-alpha fibers:
MP
motor & proprioception
A-beta fibers:
MTP
motor, touch, pressure
A- gamma fibers:
M
motor/muscle tone (muscle spindle)
A-delta fibers:
PTT
fast pain pain, temperature,touch
B-fibers:
PREganglionic autonomic
C- fibers:
Slow/ dull pain, temperature, touch, POSTganglionic autonomic– NO MYELIN
Nerve Fibers fast and slow pathways.
What is the Slowest of the A fibers?
Slowest nerve fiber?
Myelinated A-delta fibers (slowest of the A fibers)
Unmyelinated C fibers (much slower)
Conduction depends on?
Large fibers have the highest conduction velocity and the lowest threshold for excitability
Clinically…. The sensitivity of a peripheral nerve to LA is inversely related to size. That is why you see autonomic blockade first, sensory second and motor last.
In motor neuron, due to the space between the nodes are larger you’ll need more anesthetic to cover a larger area.
Difference between clinical observation and research theories
anatomic issues (larger nerves found deeper in nerve bundles – harder for the LA to reach) variable activity in different nerves (pain fibers fire at higher frequency)….i.e. frequency dependent blockade variable ion channel mechanisms
Difference between clinical observation and research theories
anatomic issues
(larger nerves found deeper in nerve bundles – harder for the LA to reach)
variable activity in different nerves (pain fibers fire at higher frequency)….i.e. frequency dependent blockade
variable ion channel mechanisms
Autonomic nerves are outside of the bundle of nerve fibers and are readily and easily blocked first
Activity also affects blockade. Autonomic are consistently firing over motor.
Frequency dependent blockade.
Outer surface of a peripheral nerve is known as the
mantle (usually more proximal structures)