Skeletal muscle relaxants

Types of skeletal muscle relaxants:2 groups

Neuromuscular blockers

• Relax normal muscles (surgery and assistance of ventilation)

• Interfere with transmission at the motor end plate

• No central nervous system activity.

• Used primarily as a part of general anesthesia

Spasmolytics

• Reduce spasticity

• Centrally acting (except dantrolene which act on the skeletal muscle)

• Used in a variety of neurologic conditions

Neuromuscular Blocking Drugs1- Transmission at the neuromuscular junction

1) Motor neuron depolarization causes action potential to travel down the nerve fiber to the neuromuscular junction.

2) Depolarization of the axon terminal causes an influx of Ca2+

3) Calcium influx triggers fusion of the synaptic vesicles with the membrane of the neuron

4) Release of neurotransmitter (Acetylcholine; ACh)

5) ACh diffuses across the synaptic cleft and binds to post-synaptic nicotinic receptor (NM) located on the muscle fiber at the motor end-plate .

Binding of 2 molecules of ACh to the receptor opens the membrane channels causing an influx of Na and outflux of K leading to depolarization of the end plate membrane.

This change in voltage is termed the motor end plate potential. If the potential is small, the permeability and the end plate potential return to normal without an impulse being propagated from the end plate region to the rest of the muscle membrane.

6) If the end plate potential is large, the adjacent muscle membrane is depolarized, and an action potential will be propagated along the entire muscle fiber and ultimately causes the release of Ca2+ from the sarcoplasmic reticulum causing CONTRACTION.

7) Unbound ACh in synaptic cleft defuses away or is hydrolyzed (inactivated) by acetylcholinesterase (AChE).

The depolarization-repolarization cycle of the motor end-plate and muscle membrane.

Neuromuscular blockers

Definition:

Drugs which relax skeletal muscles by acting at the neuromuscular junction.o All of the available neuromuscular blocking drugs are similar in structure to

acetylcholine (e.g., succinylcholine is two acetylcholine molecules linked end-to-end)

o All have one or two quaternary nitrogens, which makes them poorly lipid-soluble (not absorbed orally, must be given parenterally and have limited entry into the CNS).

Types:

1- Antagonist (nondepolarizing) neuromuscular blocking drugs prevent access of acetylcholine to its NM receptor and prevent depolarization of the motor end plate (d-tubocurarine)

2- Agonist (depolarizing) neuromuscular blocking drugs produce excessive depolarization of the motor end plate by causing excessive stimulation of the NM receptor (Succinylcholine)

Non-depolarizing neuromuscular blockers (prototype is tubocurarine)

DurationDrugElimination

Short acting(10-20 min)

1. Mivacurium(moderate histamine

release)

Plasma Ch E-Duration prolonged in impaired renal function ( Ch –E)

Intermediate acting(20-35 min)

1. Atracurium(slight histamine

release)Rocuronium

Spontaneous

Mainly liver

-Metabolite crosses blood-brain barrier and may cause seizures

-onset of action after 60-120 seconds

Long acting(> 35 min)

1. Pancuronium2. Pipecuronium

Mainly kidney

Depolarizing neuromuscular blockers

DrugDurationElimination

Succinylcholine5-10 min Plasma Ch E (metabolizes succinylcholine more rapid than mivacurium).

Only a small amount reaches the neuro-muscular junction

Succinylcholine – induced blockade of the neuro-muscular junction is terminated by diffusion (!)

Mechanism of action of non-depolarizing neuromuscular blockers

Na Ca

K

ACh

ACh

Normal transmission

1- resting

2- active

Non depolarizing neuromuscular blockadeLow doses: •competitive antagonist of Ach•Action reversed by Ach esterase inhibitors

Large doses:•Ion channel is blocked•More weakness of neuromuscular transmission•Action could not be reversed by Ach esterase inhibitors

Other actions:Can block pre-junctional sodium channels and interfere with mobilization of Ach at nerve endings

Mechanism of action of depolarizing neuromuscular blockers

Phase I block:Succinylcholine causes opening of the channels by :1- reacting with the nicotinic receptors (NM)2- Entering the channel and increasing ionic conductanceThis causes depolarization of the motor end plate which causes contraction. Because succinylcholine is not metabolized at the synapse, depolarization persists and the depolarized membranes remain unresponsive to subsequent impulses Phase II block (desensitization):With continued exposure to succinylcholine, the membrane becomes depolarized and cannot be repolarized again (desensitized)

Neuromuscular block

Non-depolarizingNon-depolarizingDepolarizingDepolarizing

Actionmotor weakness, followed by skeletal muscle flaccidity and inexcitability to electrical stimulation

fasciculations especially over the chest and abdomen followed by complete paralysis

Order of muscle involvement

1- smaller muscles (eg,facial, foot, hand)2- larger muscles (eg, abdominal, trunk)3- diaphragm

1- arm, neck, and leg muscles 2- facial and pharyngeal muscles. 3- respiratorymuscle weakness follows rapidly, usually within 60 seconds

Recoveryin reverse order, with the diaphragmregaining function firs

in reverse order

Adverse effects

Non-depolarizingDepolarizing (succinylcholine)

Histamine release (hypotension and bronchospasm)

Slight: atracuriumModerate: mivacurium

Slight

Other cholinergic receptors

Pancuronium: moderate increase in heart rate (inhibit vagus)

Small doses: negative inotropic and chronotropic actions (prevented by atropine) due to:1- Stimulation of NN receptors at sympathetic and parasympathetic ganglia2- Stimulates M2 receptors in heartLarge doses cause positive inotropic and chronotropic actions

Other adverse effects of succinylcholine

EffectCause

Cardiac arrestHyperkalemia due to release of K to blood in cases of burns and trauma

increased intraocular pressure

contraction of myofibrils or dilation of choroidal blood vessel

Increased Intragastric Pressure (causing emesis)

Muscle fasciculations

Muscle Painunsynchronized contractions of adjacent muscle fibers just before the onset of paralysis.

Malignant hyperthermia

Reversal of neuromuscular blockade

NondepolarisingDepolarizing (succinylcholine)

cholinesterase inhibitors as neostigmine and pyridostigmine by:1- increasing the availability of acetylcholine at the motor end plate2- to a lesser extent, neostigmine and pyridostigmine increase release of transmitter from the motor nerve terminal.

Plasma

Interactions with other drugs

• Inhaled anesthetics: augment the neuromuscular blockade produced by nondepolarizing muscle relaxants because

1) CNS depression 2) peripheral vasodilatation which allows a

larger fraction of the injected muscle relaxant to reach the neuromuscular junction

3) decreased sensitivity of the postjunctional membrane to depolarization.

• Aminoglycosides: augment the neuromuscular blockade produced by nondepolarizing muscle relaxants by decreasing release of Ach

• Local Anesthetics enhance the neuromuscular block produced by both nondepolarizing and depolarizing

Indications

1. Muscle relaxation during surgery

2. Control of ventilation in critically ill patients with ventilatory failure

3. Treatment of Convulsions

Spasmolytic drugs

Definition of muscle spasm (spasticity):

1. Increased muscle tone

2. together with muscle weakness

It is often associated with cerebral palsy, multiple sclerosis, and stroke.

Centrally acting spasmolytic drugs

DrugMechanism

1- DiazepamGABA receptor

2- BaclophenGABA receptors causing hyperpolarization by increasing potassium conductanceAverse effects: drowsiness and increased seizure activity

3- Tizanidineα2 adrenoreceptor agonist

4- Gabapentin

Peripheraly acting spasmolytic drugs

Dantrolene:

MOA:

Dantrolene reduces skeletal muscle strength by interfering with excitation-contraction coupling in the muscle fibers

Normal contraction involves release of calcium from its stores in the sarcoplasmic reticulum through a calcium channel

• Dantrolene interferes with the release of calcium through this sarcoplasmic reticulum calcium channel.

• Cardiac muscle and smooth muscle are depressed only slightly, perhaps because the release of calcium from their sarcoplasmic reticulum involves a different process.

Indications:

1- Muscle spasticity

2- Malignant hyperthermia:o Patients at risk for this condition have a hereditary impairment in the

ability of the sarcoplasmic reticulum to sequester calcium.o Following administration of one of the triggering agents (general

anesthetics or succinylcholine) there is a sudden and prolonged release of calcium, with massive muscle contraction, lactic acid production, and increased body temperature.

Treatment of malignant hyperthermia:

1. control acidosis and body temperature

2. Reduce calcium release with intravenous dantrolene