Which substances are found on cell surfaces and respond to nerve and hormone signals?

The correct answer is receptor molecules.

Explanation:

Receptor molecules are the glycoproteins that binds specifically to a hormone or nerve signal and bring change in the cell activity and function. Receptors can be intracellular or cell surface receptor.

Cell surface receptor are found on the plasma membrane surface and mainly binds to hydrophilic signal molecules or hormone, for example, GPCR receptors which are present in all eukaryotes.  

The intracellular receptors are present in the cytoplasm and nucleus and only binds to lipophilic ligand or hormone because only they can cross the cell membrane.

thrombocytopenia, which is a deficiency in blood platelets, can cause bleeding and bruising in the dermis similar to senile purpura. other possible causes include: exposure to ultraviolet rays and aging skin are common causes of senile purpura. senile purpura may also be a sign of collagen loss in the skin and bones.