The neurochemistry of LSD - A technical explanation for nontechnical readers.

The primary effect of LSD occurs at receptors intended to respond to serotonin (kudos to enth for touching on this in the most detail so far). Serotonin (aka 5-HT, 5-hydroxy tryptophan) exists as an evolutionarily new addition to our brains. All the highest systems make heavy use of it, including our frontal lobe (and specifically, the prefrontal cortex) that seems to give us an edge (intelligence-wise) over every other animal on Earth. It also has effects outside the brain, which happens to (partially) explain why LSD causes things like jaw clenching and nystagmus. Believe it or not, LSD has more effect on smooth muscle tissue than it does in the brain.

To get right to the point, LSD acts as a 5-HT2 partial agonist (the "2" just designates a particular type of postsynaptic receptor). This results in a decrease in the amount of serotonin that gets dumped into a synapse, but at the same time LSD produces some degree of effect similar to serotonin itself.

Of course, keep in mind that the previous paragraph only describes the best current theory, in that it most fully accounts for LSD's effects at the local level of an actual synapse. A good site with some alternative theories as to its actual action in a serotonergic synapse lives at "http://www.cs.hmc.edu/~ivl/writing/non_fiction/lsd/". (I apologize for the external link, but I have yet to find a better site discussing the specifics, including competing theories, of the activity of LSD at the synaptic level).

So what does serotonin actually do...

It acts to prevent the postsynaptic cell from firing (ie, it acts as an "inhibitory" neurotransmitter). So, decreasing serotonin release into a synapse increases the rate of firing of the postsynaptic cell. Thus, LSD "speeds up" the outer layer of the brain. This doesn't mean you think faster (and certainly not "better"), just differently.

Now that you have that down...

LSD also acts at noradrenergic synapses, though its effect on such sites has received far less research than its effects on serotonergic receptors. I'll focus on the main system affected here, rather than the specific receptors, for that reason.

The Locus Coeruleus, which communicates primarily using norepinepherine (thus the connection with LSD) acts as the "gate" for letting sensory input get through to the parts of us we consider "us" . most of the time, it filters out the vast majority of information we receive, but LSD forces it open, so we become aware of countless features of our environment we normally don't notice. On the down side, LSD also garbles the transmission, leading to the cross-modal experiences one can sometimes experience while tripping (such as "hearing" a color, a classic effect of LSD that most users describe as not really occuring to such an obvious degree).

To summarize, LSD temporarily modifies the way we consciously process information. At the same time, it gives us conscious access to immensely more information, though not reliably true information, than we normally perceive.