dendritic spines express glutamate receptors (e.g. ampa receptor and nmda receptor) on their surface. the trkb receptor for bdnf is also expressed on the spine surface, and is believed to play a role in spine survival. the tip of the spine contains an electron-dense region referred to as the "postsynaptic density" (psd). the psd directly apposes the active zone of its synapsing axon and comprises ~10% of the spine's membrane surface area; neurotransmitters released from the active zone bind receptors in the postsynaptic density of the spine. half of the synapsing axons and dendritic spines are physically tethered by calcium-dependent cadherin, which forms cell-to-cell adherent junctions between two neurons.