In the muscular system, calcium is release by the sacroplasmic reticulum where it stimulates the movement of tropomyosin and troponin proteins. These two proteins help the actin and myosin filament move. These filaments are what caused muscles to contract.
In the the skeletal system, calcium makes up the structure of bones. Calcium is apart of the bone matrix. The bone sort of acts like the calcium warehouse that maintains the amount of calcium in the bloodstream. The bone stores calcium in salts such as hydoxyapatite (make up of phosphate and calcium).
In nervous system, calcium stimulates the release of neurotransmitters from the synapse. Action potential of the neuron causes charge to flow down the axon to the synapse. Then the charge stimulates the calcium gates to open on the synapse which allows calcium ions to flow into the synapse. In response, the vesicles, storing the neurotransmitters, release their contents to the pre-synapse of the next neuron. Basically, calcium plays a key role in sending signals in the nervous system.