TLR4 and subarachnoid hemorrhage: Microglia are involved in the occurrence of brain edema, the processes of neuronal apoptosis, and the blood–brain barrier disruption after subarachnoid hemorrhage (SAH) through the signaling pathways mediated by receptors such as Toll-like receptor 4 (TLR4), calcium-sensing receptor (CaSR), and triggering receptor expressed on myeloid cells-1 (TREM-1), which secrete pro-inflammatory cytokines such as interleukins and tumor necrosis factor α.