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Dental Anesthesiology I > Lecture 2 > Flashcards

Lecture 2 Flashcards

1
Q

the ___ is the structural unit of the nervous system

A

neuron

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2
Q

describe the two directions that neurons transmit information

A
  • afferent (sensory) - transmits toward the CNS
  • efferent (motor) - transmits away from the CNS
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3
Q

the dendrite/dendritic zone is also called the ___, and is responsible for ___

A
  • peripheral process
  • receiving outside information and transmitting it to the soma (cell body)
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4
Q

the axon is also called the ___, and is responsible for ___

A
  • central process
  • transmitting information away from the soma (cell body)
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5
Q

what does the soma do?

A
  • provides metabolic support for the neuron
  • does not participate in impulse transmission
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6
Q

what does it mean when an axon is myelinated?

A
  • it means that those axons are insulated by specialized cells called schwann cells
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7
Q

what is the function of schwann cells?

A
  • wraps around axon
  • provides insulation
  • outermost layer consists of schwann cell nucleus and cytoplasm
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8
Q

what are nodes of ranvier, and what is their function?

A
  • they are the spaces (nodes) between adjacent schwann cells on an axon
  • they provide saltatory conduction
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9
Q

what is saltatory conduction?

A
  • signal impulse leaps from one node of ranvier to the next, resulting in faster signal conduction
  • even if one node gets blocked, there is usually enough energy for the signal to skip over that node to the next one
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10
Q

do myelinated axons provide faster or slower signal transduction than unmyelinated axons, and why?

A
  • faster, due to saltatory conduction provided by the nodes of ranvier
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11
Q

will smaller diameter fibers have faster or slower signal conduction?

A

slower

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12
Q

which fibers provide motor, proprioception, and deep pressure signals, are myelinated, and provide afferent signals to and efferent signals from muscles and joints?

A

alpha and beta

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13
Q

which fibers provide muscle tone signals, are myelinated, and provide efferent signals to muscle spindles?

A

gamma

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14
Q

which fibers provide pain, temperature, and touch signals, are myelinated, and provide afferent signals to sensory nerves?

A

delta

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15
Q

which fibers provide various autonomic functions, pain, temperature, and touch signals, are unmyelinated, and provide afferent signals to sensory nerves?

A

dGammaC

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16
Q

put the following fibers in order from largest diameter to smallest diameter, and indicate whether they are myelinated or unmyelinated:

delta, gamma, dGammaC, alpha, beta

A
  1. alpha & beta (same diameter) - myelinated
  2. gamma - myelinated
  3. delta - myelinated
  4. dGammaC - unmyelinated
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17
Q

when local anesthetics are administered, what order do they block nerve fibers?

A

they will block small unmyelinated fibers first (responsible for pain), then cold, warm, touch, deep pressure, and motor

when local anesthetics wear off, they will wear off in the reverse order (so motor will come back first, pain last)

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18
Q

why do small unmyelinated fibers get blocked by local anesthetics first?

A

because the impulses travel a much shorter distance, so they are more susceptible to nerve block

they also require lower concentrations of anesthetic compared to motor neurons

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19
Q

describe the process of signal transduction

A
  • signal enters peripheral process (aka dendrite)
  • signal is transmitted via the axon (aka central process) to the end process which synapses with the next nerve fiber in the sequence
  • signal arrives in the CNS where it is interpreted and a signal is sent from the CNS efferent neurons to the corresponding body part
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20
Q

what does the plasma membrane consist of, and what is the purpose of the membrane?

A
  • consists of a phospholipid bilayer, proteins, lipids, and carbohydrates
  • the purpose is selective permeability
    • specialized pores/channels exist in the membrane that allow for the passive flow of ions from one side of the membrane to the other; some are gated channels
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21
Q

what are the two types of gradients that exist in the plasma membrane?

A
  • concentration gradient - difference in ion concentration
  • electrochemical gradient - difference in electrical charge (this difference is known as the membrane potential)
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22
Q

the ___ is the difference in electrical charge from one side of the membrane to the other

A

membrane potential

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23
Q

what is the normal resting membrane potential?

A

-70mV

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24
Q

is the inside or outside of the cell more negatively charged?

A

the inside

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25
Q

what are the 5 phases of nerve conduction?

A
  1. initial phase of slow depolarization
  2. rapid phase of depolarization
  3. absolute/relative refractory period
  4. repolarization
  5. signal propagation
26
Q

what happens during the initial phase of nerve conduction (slow depolarization)?

A
  • membrane becomes more permeable to sodium ions
  • influx of sodium ions mediated by gated ion channels, resulting in an excess of sodium ions inside the cell, and an excess of potassium ions outside the cell
27
Q

what happens during the rapid phase of depolarization of nerve conduction?

A
  • membrane potential becomes slightly less negative (aka more positive; goes from -70mV to -55mV; difference of +15mV))
  • slow depolarization to reach the firing threshold of -55mV
28
Q

what happens during the absolute/relative refractory period of nerve conduction?

A
  • absolute - no new action potentials can occur
  • relative - new action potentials can occur, but firing threshold must be larger
29
Q

what happens during the repolarization phase of nerve conduction?

A
  • membrane is impermeable to sodium ions
  • increased permeability to potassium ions
  • to repolarize the membrane, it must use ATP to pump sodium ions against their concentration gradient outside the cell
30
Q

what happens during the signal propagation phase of nerve conduction?

A
  • signal propagated to adjacent nerve segment
31
Q

the ___ is the magnitude of difference in negative electrical potential needed to initiate an action potential. in most cases, a difference of 15mV is sufficient.

A

firing threshold

32
Q

the ___ is the period of time immediately after a stimulus has initiated an action potential during which the nerve is unable to respond to another stimulus regardless of the strength

A

absolute refractory period

33
Q

the ___ is the period of time follow the absolute refractory period during which a new action potential can be initiated, but only by a larger than normal firing potential (greater than 15mV).

A

relative refractory period

34
Q

what is the ion responsible for initiating an action potential?

A

sodium

35
Q

local anesthetics interfere with the ___ process

A

excitation

  • membrane potential - keep it pretty close to where it is at rest
  • threshold potential - LAs alter this
  • depolarization (slow phase) - decrease rate of depolarization
36
Q

what are the two theories of how local anesthetics work?

A
  • membrane expansion theory
  • specific receptor theory
37
Q

describe the membrane expansion theory

A
  • essentially changes the shape of the membrane
  • LA goes through hydrophobic portion of the membrane to cause disturbance of the membrane structure and ultimately prevents an increase in permeability to sodium ions
38
Q

describe the specific receptor theory

A
  • local anesthetic blocks passage
  • LA binds to receptor sites on sodium channel (they replace the calcium that would normally bind there)
  • density of sodium channels differs between myelinated and unmyelinated nerves
39
Q

of the two theories of how local anesthetics work, which theory is more likely?

A

the specific receptor theory

40
Q

the primary action of local anesthetics is to produce a decrease in ___ permeability

A

sodium (conduction blockade)

41
Q

what is the basic proposed mechanism of action of local anesthetics?

A
  1. displacement of calcium ions
  2. LA molecule binds
  3. sodium channel blockade
  4. decrease in sodium conductance
  5. failure to reach threshold
  6. no action potential is developed

essentially results in no ionic movement

42
Q

describe the difference in sodium channel densities between myelinated and unmyelinated axons

A

myelinated axons have a higher density of sodium channels (up to 20,000 per micrometer more) than unmyelinated axons

43
Q

of the following ions, which ones’ movement are unaffected by local anesthetics?

potassium, sodium, chloride, calcium

A

potassium, calcium, and chloride

in other words, LAs only affect the movement of sodium ions

44
Q

why do local anesthetics have a particular impact on children?

A

LAs are absorbed from the site of administration and transported to the cardiovascular system. children are susceptible to systemic toxicity because they have less blood than adults

45
Q

many ___ anesthetics are not very effective

A

topical

46
Q

what are essential properties of a good local anesthetic?

A
  • non-irritating
  • limited damage
  • low toxicity
  • effective
  • short onset
  • long duration of action
  • free from producing allergic reactions
  • capable of sterilization
  • potent without causing harm at low concentrations
47
Q

what are the two classifications of local anesthetics?

A

amino esters and amino amides

48
Q

describe amino esters

A
  • composed of ester linkages on the intermediate chain
  • esters are readily hydrolyzed in aqueous solutions
49
Q

describe amino amides

A
  • composed of amide linkages on the intermediate chain
  • are resistant to hydrolysis
50
Q

what are local anesthetics composed of?

A
  • lipophilic part - biggest part
  • intermediate chain - link type
  • hydrophilic part - some LAs have them and some don’t
51
Q

LAs with lipophilic and hydrophilic parts are considered ___

A

amphipathic

52
Q

what is the pKa range of local anesthetics?

A

7.5-10

53
Q

are local anesthetics weak acids, weak bases, strong acids, or strong bases?

A

weak bases

54
Q

why are local anesthetics combined with acids?

A

more stability and solubility in H2O or saline

lidocaine HCl 2% is a salt - it has the acid (HCl) added

55
Q

what are the two forms that local anesthetics exist in in solution, and what determines the proportion of each?

A
  • RNH + (positively charged cation)
  • RN and H+ (uncharged molecule aka base)
  • the proportions of each is determined by the pH of the tissue
    • High acidity/ low pH
    • lower acidity/ high pH
56
Q

LAs can only diffuse through the nerve sheath when they are in what form?

A

RN form (base/uncharged)

57
Q

describe this picture

A
  • at pH 7.4: 25% (RN) molecules will diffuse through the nerve sheath and reach the intracellular environment; the other 75% (RNH+) will stay extracellularly and then re-equilibrate
  • Of the 25% in intracellular, 75% will be in RNH+ form and 25% will stay in RN form (math is off in the figure)
  • Both inside and outside will try to re-equilibrate
58
Q

describe this picture

A
  • At pH 6: 99% of the molecules are in their RNH+ form (so more inflamed tissue (aka lower pH) will take more anesthetic to work) – remember, the RN form (in this case, 1% of the total molecules) are the useful ones that cross the nerve sheath; there are fewer molecules available in the free base form to elicit a local anesthetic reaction
  • of the 1% of molecules that cross the nerve sheath, 25% remain in the RN form, while 75% are converted to the RNH+ form
59
Q

the pH of tissues affected by LAs will be reduced to a pH of ___

A

5-6

60
Q

normal tissue pH is ___

A

7.4

61
Q

why is sodium metabisulfite added to LAs?

A

Sodium metabisulfite is a preservative for the vasoconstrictor (protects it from being oxidated), and an added bonus is that it makes solutions more acidic which prolongs effectiveness

62
Q

describe benzocaine

A
  • not readily ionizable
  • not affected by pH
  • mucous membranes have a poor buffering capacity, but benzocaine does not need to be buffered, so it works well as a topical
  • relatively safe

Dental Anesthesiology I (9 decks)

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