LSD mainly activates serotonin 5-HT2A receptors in the cortex. This shifts how cortical pyramidal cells fire and boosts glutamate signaling, loosening the brain’s usual filtering of signals. It also interacts with other receptors that may add subtle changes in arousal and mood. For a compact explainer of how it works, see this Q&A.
At the network level, coherence within the default mode network often decreases, while connectivity between normally separate networks increases. Thalamic filtering of sensory and internal signals seems looser, and visual cortex activity often rises, matching stronger colors, moving patterns, and shape distortions.
People commonly report intensified senses, altered time and space, more associative thinking and sometimes synesthesia-like crossovers. Emotions can range from wonder to anxiety. At higher intensities the sense of self may feel thinner or less fixed. Bodily signs can include dilated pupils, alertness, slight heart rate increase, tension or tingling, and the experience tends to last for many hours in phases.
In short, LSD temporarily reconfigures receptor signaling and brain networks, shifting perception, emotion, and self-experience.