In order to increase the efficient spectrum usage and to mitigate the current spectrum scarcity, dynamic spectrum access is considered to be an emerging technique. In this paper, based on the concept of dynamic spectrum access, we propose a Markov process that models the two-way interaction process between a primary and a cognitive radio user for dynamic spectrum usage. Several probabilities and matrices of response, no-response, agreement and no-agreement for a spectrum sharing process are derived. Numerical results are then shown to explore the impact of key systems parameters like users arrival rates on the derived probability of response.