(physiology)what is the role of Calcium ions in the excitation of nerve fibres? and what are the factors affecting propagation of action potential?

(physiology)what is the role of Calcium ions in the excitation of nerve fibres? and what are the factors affecting propagation of action potential? (biochemistry)define optical activity. what type of compounds possesses this property? and mention examples for the importance and functions of phospholipids. (anatomy) anatomy of hip joint.