The scattered nature and active topology of wireless sensor networks (WSN) have some particular requirements -reduced energy consumption and extended network's lifetime. The paper provides a brief introduction about the wireless sensor network including the widely adopted architecture of wireless sensor network and wireless sensor node. The paper also focuses in critical issues of wsn that includes energy per packet, lower energy consumption, average packet delay, energy spent per round, packet size, distance, time until first node dies. The paper focuses on hierarchical routing protocols which are based on network structure scheme and explains how neural networks are helpful in providing energy efficiency to wireless sensor networks.