spasm - A sudden, involuntary contraction of a muscle or group of muscles.

fibrillation -

1. The fine, rapid twitching of individual skeletal or smooth muscle fibers with little or no movement of the muscle as a whole.
2. The rapid uncoordinated twitching movements that replace the normal rhythmic contraction of the heart and may cause a lack of circulation and pulse.

rigor mortis - The temporary stiffening of skeletal muscles observed in the period between 4 hours and 48 hours after death; due to the leakage of calcium ions from the sarcoplasmic reticulum into the general sarcoplasm which triggers binding of actin and myosin; the leakage occurs because in death there is no further ATP production and the Ca⁺² pumps in the sarcoplasmic reticulum membranes cease to function.

fibrosis - The formation of excessive dense fibrous tissue ("scar tissue"), in a repair or reactive disease process.

fibromyositis - A chronic inflammation of muscle tissue with an overgrowth = hyperplasia of the connective tissue including scar tissue formation.

muscular dystrophy = MD - Any of a group of hereditary progressive skeletal muscle disorders caused by a defect in one or more genes that control muscle function and characterized by muscular weakness and gradual irreversible wasting of skeletal muscle; e.g., Duchenne's muscular dystrophy.

myasthenia gravis - An autoimmune disease characterized by progressive fatigue and generalized weakness of the skeletal muscles, especially those of the face, neck, arms, and legs, caused by impaired transmission of nerve impulses following an autoantibody attack on acetylcholine receptors at the neuromuscular junctions.

Describe:

8. The cause of rigor mortis?

Rigor mortis is the temporary stiffening of skeletal muscles observed in the period between 4 hours and 48 hours after death.  It is due to the leakage of calcium ions from the sarcoplasmic reticulum into the general sarcoplasm which triggers binding of actin and myosin.  The leakage occurs because in death there is no further ATP production and the Ca⁺² pumps in the sarcoplasmic reticulum membranes cease to function.