Bone matrix acts as a reservoir chiefly for phosphorus and:

1. The mineral is needed for muscle contraction. 2. It also affects nerve impulse transmission. 3. The bone matrix acts as a reservoir for minerals, especially calcium and phosphorus. 4. So the answer is calcium. _Source: OpenStax 'Anatomy and Physiology 2e', Ch 6 'Bone Tissue and the Skeletal System'_