Calcium is used in many different signaling pathways, ie through G proteins, ion channels.
Every person starts out as a Calcium wave. The wave triggered during fertilization it stops multiple sperm from fertilizing the ovum.
Calcium ions are also commonly known to be used in muscle contraction. It also occurs in other secretary cells including nerve cells. Calcium pump is used to keep the levels in the cytosol low; eukaryotes have a pump on the plasma membrane to do this it uses ATP, to pump calcium out of the cytosol. These are called SERCA and PMCA. Also located on the plasma membrane is the sodium calcium exchanger (Na+/ Ca2+). Mitochondria also have a role to play in keeping the calcium levels of the cytosol low. This is done via secretary vesicles. Calcium is the perfect trigger as it is in low concentrations in the cytosol of resting cells ~ 10 -7M this is low in comparison to the extracellular fluid concentration which is ~10 -3. Concentrations are also high in the sarcoplasmic reticulum (SR) which is located in the muscle. Calcium is released into the sarcoplasmic reticulum membrane via the calcium voltage gated calcium pumps. It then binds to troponin, then an actin-myosin bridge is formed and contraction occurs.
Buffers are also around in the cytosol and if there wasn’t the CICR pathway (Calcium Induced Calcium Release) this allows the amplification of calcium from the IP3 receptor to create a calcium wave hence the signal.