A man has suffered all his life with debilitating cramps and spasms. He went to a muscle specialist who said it could have to do with either his muscle proteins or calcium. The man can’t understand why two such different things could cause his problems.

Identify one way a change in muscle proteins could cause his cramps, and one way calcium could cause them.

What is his basic problem? His muscles contract too much.

What proteins are involved in normal muscle contraction? They’re underlined:

Acetylcholine released from motor neuron attaches to Ach receptor on muscle cell

Ach receptor causes Na+ channels to open

Cell depolarizes

Wave of depolarization runs down T-tubules to myofibrils

Depolarization causes sarcoplasmic reticulum to open its Ca2+ channels and release Ca2+

(the sarcoplasmic reticulum will then use a Ca2+ pump to pick the Ca2+ up again)

Ca2+ attaches to troponin

Troponin and tropomyosin move off the binding site where myosin grabs actin

Myosin grabs actin and pulls

How could these proteins cause his muscles to contract too much?

Ach receptors could be overactive, causing the muscles to fire too much

Na+ channels could be stuck open

Sarcoplasmic reticulum’s Ca2+ channels could be stuck open

Sarcoplasmic reticulum’s Ca2+ pump might be too slow in picking the Ca2+ up again

Troponin and tropomyosin might be absent, or not be blocking myosin’s attachment to actin

Myosin might be attaching to actin in a different spot than the binding side

How could Ca2+ be involved?

Too much Ca2+ in the myofibril would cause too much contraction

Ca2+ OUTSIDE THE CELL, in the blood, also blocks Na+ channels. Too little Ca2+ in the blood would leave the Na+ channels unblocked and increase muscle contraction.