Recall that general anesthetics have unwanted side effects (hypotension, cardiac disturbances etc.).  Adding skeletal muscle relaxants can allow you to decrease the dose of general anesthetic and therefore minimize the side effects

Also: Paralyze mechanically ventilated patients to maintain controlled ventilation.  (also facilitates intubation procedure)

Succinylcholine causes initial depolarization, but then leads to flaccid paralysis because acetylcholinesterase in the NMJ is unable to cleave it.

Succinylcholine is cleaved by plasma cholinesterase (pseudocholinesterase)

Tubocurarine: it causes a large release of histamine, a key mediator in asthma attacks. 

tubocurarine will make it feel like you're sucking air through a tube/straw

Succinylcholine

This is a depolarizing drug, which means it causes the opening of the Na+/K+ channel at the motor end plate.  This promotes Na+ entry into the cell and concommitant K+ exit from the cell.  Because the drug keeps the channel open, the permeability of the membrane to K+ remains high, so potassium continues to leave the cell.  The extracellular K+ concentration increases, resulting in hyperkalemia.  This can promote cardiac arrhythmias.

Atraconium is broken down to laudanosine, which promotes seizures at high levels. 

Other side effects include bronchospasm and vasodilation as a result of histamine release (though less than tubocurarine) and tachycardia subsequent to the vasodilation...

Tubocurarine, a long-acting non-depolarizing drug, is largely excreted unmetabolized by the kidneys.  Therefore, its plasma concentrations will become elevated in patients with renal failure.  Recall that side effects of high doses include hypotension, tachycardia, histamine release and bronchospasm.

Pancuronium would also be contraindicated

Halogenated (inhalational) general anesthetics.

Also recall the increased risk of malignant hyperthermia when these two drugs are combined.

General anesthetics, Aminoglycosides, Tetracyclines, and Ca++ channel blockers

The benzodiazepines are used to reduce background tone of muscles.  They do not seriously affect the ability of muscles to contract when stimulated.

They are used in cases of spinal chord injury, chronic back pain, and stress-induced muscle tension.

Baclofen, a GABA derivative, can be administered intrathecally.   It is used in cases of spastic cerebral palsy, and to reduce extensor tone/clonus in ALS or MS patients.

Baclofen produces the same spasmolytic effect as benzodiazepines but has less severe sedation.

Intrathecal injections are performed through someone's back, i.e. baclofen

Baclofen is a GABA derivative that acts at presynaptic terminals to reduce transmitter release.  (It is believed to prevent Ca++ influx)

Baclofen can cause increased blood sugar levels, so increase the hypoglycemic drug dose

MAOIs and alcohol are exacerbated by baclofen (all CNS depressants)

TCADs and baclofen may cause muscle hypotonia

Tizanidine is a spasmolytic drug that stimulates alpha-2 receptors (both pre- and post-synaptic)

It is used in cases of MS and ALS

What drug is used in all of the following conditions:

malignant hyperthermia
neuroleptic malignant syndrome
serotonin syndrome

Dantrolene (IV)

Dantrolene blocks Ca++ release from the sarcoplasmic reticulum, thereby inhibiting excessive muscle contraction/spasm.

All three of the above disorders include muscle spasms!

Malignant Hyperthermia can result from administration of inhaled general anesthetics or succinylcholine (Using both increases the risk further).  It is the main cause of death in anesthesia. 

Patients with this predisposition have abnormal ryanodine receptors of the sarcoplasmic reticulum.  When stimulated by certain agents (ex: anesthetics, succinylcholine), there is uncontrollable release of calcium leading to excessive muscle contraction.  This muscle contraction leads to hyperthermia.

IV dantrolene, which blocks ryanodine receptors, is included in the treatment of malignant hyperthermia.