http://en.wikipedia.org/wiki/Public-key_cryptographyThere is a mathematical relationship between the private key and the bitcoin address.
This mathematical relationship allows the transaction creator to use the private key to calcualte a digital signature. This digital signature is provided with the transaction. Because of the mathematical relationship, the solo-miners (or mining pool operators) and all full nodes can use the public key to verify that the digital signature is valid without ever knowing what the private key was.