17. Central and peripheral myorelaxant drugs

Question 1

Use of myorelaxant drugs

Answer 1

• During surgical anaesthesia to obtain skeletal muscle relaxation
• Assist in intubation
• Corneal/retinal surgery
• Therapy of spastic disorders

Question 2

Competitive neuromuscular blocking agents: Function

Answer 2

Selectively antagonise nicotinic receptors by preventing endogenous ACh binding & subsequent muscle cell depolarisation

Question 3

Give the classes of neuromuscular blocking agent

Answer 3

• Competitive (non-depolarising)
• Depolarising

Question 4

Competetive neuromuscular blocking agents:

To produce a complete block…% of receptors must be blocked

Answer 4

90%

Question 5

Competitive neuromuscular blocking agents: properties

Answer 5

• Ionised and water-soluble
• Onset is slow & short duration

Question 6

Competitive neuromuscular blocking agents: Metabolism & excretion

Answer 6

• Metabolised in liver
• Excreted by kidenys

Question 7

Competitive neuromuscular blocking agents are antagonised by…

Answer 7

Cholinesterase inhibitors (Neostigmine)

Question 8

Competitive neuromuscular blocking agents: Effect

Answer 8

1. Motor weakness →
2. Flaccid paralysis of skeletal muscles

• ↓ Blood pressure
• ↓ Coagulability of blood because of heparin release from mast cells

Question 9

Competitive neuromuscular blocking agents: Examples

Answer 9

• Curare (Histamine release)
• Gallamine (Tachycardia; long-acting)
• Atracurium (Safe in liver & kidney disease; most used; intermediate action)
• Cisatracurium (intermediate-acting)
• Rocuronium (intermediate-acting)
• Vecuronium (intermediate-acting)

Question 10

Central muscle relaxants

Answer 10

Relative specific depressant action in CNS

• Causing ↓ motor activity/paralysis of voluntary muscles without LOC
• Do not inhibit other CNS functions
• Can potentiate the effect of anaesthetics and combinations

Question 11

Central muscle relaxants: Examples

Answer 11

• Guaiphenesin
• Baclophen
• Carisoprodol
• Methocarbamol

Question 12

Guaiphenesin

Answer 12

• Spinal interneuronal blocking agent
• Used to adjunct anaesthesia
• Blocks polysynaptic and not monosynaptic reflexes
• Effective against:
  
  • Convulsions
  • Tetanus

Question 13

Baclophen

Answer 13

• GABA_B receptor agonist in CNS
• Causes hyperpolarisation of neurons due to ↑ K⁺ ion conductance
• Large therapeutic index

Question 14

Carisoprodol

Answer 14

Useful for pain - Due to analgesic-sparing effect on opioid analgesics

Question 15

Methocarbamol

Answer 15

• Acts on internuncial neurones of the spinal cord
• ↓ Skeletal muscle hyperactivity

Question 16

Depolarising neuromuscular blocking agents: Function

Answer 16

1. Stimulates opening of nicotinic ACh receptor channel
2. Produces depolarisation of cell membrane
3. Succinylcholine persists at neuroeffector junction, activating nicotinic receptor channels continuously
4. Inactivation of voltage-gated Na⁺ channels → Cannot support further APs

Question 17

Depolarising neuromuscular blocking agents: Properties

Answer 17

Rapid onset; short duration

Question 18

Neuromuscular blocking drugs do not produce…

Answer 18

Analgesia

Question 19

Metabolism of depolarising neuromuscular blocking agents

Answer 19

Metabolised by plasma pseudocholinesterase & liver

Question 20

Depolarising neuromuscular blocking agents: Example

Answer 20

Succinylcholine

Question 21

Succinylcholine

Answer 21

• Dog, cattle, sheep are sensitive
• Weakness & paralysis of skeletal muscles
• ↑ Blood pressure

Question 22

Succinylcholine: Side effects

Answer 22

• Bradycardia
• Hyperkalaemia
• ↑ intraoccular & intragastric pressure
• Anaphylaxis

Question 23

Succinylcholine: Phase I block

Answer 23

• Depolarising block of motor end-plate

Question 24

Succinylcholine: Phase II block

Answer 24

• Competetive block of motor end-plate/partially susceptible to reversal by cholinesterase inhibitors

Question 25

"Muscle relaxants" refers to which 2 therapeutic groups?

Answer 25

* Neuromuscular blockers * Spasmolytics

Question 26

Spasmolytics: Use

Answer 26

Alleviate musculoskeletal pain and spasms * Specifically depress the CNS → ↓ Motor activity/ paralysis * No LOC * May cause drowsiness/sedation

Question 27

Neuromuscular blocking drugs: Aims

Answer 27

* Relax skeletal muscles for surgery * Reduction of dislocations and fractures

Question 28

Neuromuscular blocking drugs: Function

Answer 28

Interferes with transmission at NMJ → Relaxation/paralysis

Question 29

Spasmolytics-smooth muscle relaxants

Answer 29

Parasympatholytics, Sympathomimetics, direct acting substances

Question 30

Spasmolytics-smooth muscle relaxants: Examples

Answer 30

* Papaverine * Drotaverine

Question 31

Papaverine: Effect

Answer 31

* Reduces spasm of visceral smooth muscle * Vasodilation * Bronchodilation * ↓ Heart excitability

Question 32

Papaverine: Indications

Answer 32

* Excessive smooth muscle contraction * Urethral spasm * Colic spasm * Bronchial spasm * Relax uterus (drotaverine)