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When people write about acid and the other classic hallucinogens, they often only use the term psychedelic to describe the effects. In the original work done in experimental psychotherapy with hallucinogens there were two separate states that could be aimed for by the therapist -- psychedelic and psycholytic. "Psychedelic" has a literal meaning of "mind manifesting," and psychedelic therapy sought to cause awe-inspiring, powerful, even religious revelations for the patient. "Psycholytic," however, has a meaning more along the lines of "loosening the action of the mind," and psycholytic doses were used in therapy to recreate past experiences and give the patient a new outlook on his problems.
Of course, we all know now that a great deal of psychotherapy was inane pseudoscience -- Freud was wrong, and Freud is dead. Their work did show something interesting to modern psychology, though, that there was an extreme difference between response to hallucinogen doses under and over a certain threshold. That is, it's not just our imagination that the giggly, colorful, euphoric, high you get from a hit or two of 'cid is dramatically different from the stunning, deep, potentially overwhelming trip caused by five to ten hits. This schism is also seen with psilocybin, DOM, and the rest, so when I mention LSD from now on, I mean any of the classic hallucinogens.
In the brain, LSD seems to give much of its effect by playing havoc with the serotonin system. Specifically, it is taken up by serotonin neurons and released in a manner no different from normal serotonin. Its effect as a drug comes from its interaction with the 5-HT receptors on other neurons, receptors that are meant to pick up transmissions from the serotonin neuron. LSD fits in the receptors pretty well, but isn't as effective as serotonin -- it produces about 25% of the neural response that a serotonin molecule would. LSD also stays stuck in the receptors longer than serotonin, making those receptors unable to pick up further serotonin neurotransmission.
What makes acid so interesting is that it has a much higher affinity for the receptors; whereas a low concentration of serotonin around a receptor doesn't stand a chance of making it fire, a low concentration of LSD probably will. In other words, not only are neurons that are supposed to be sensitive to a given serotonin neuron fired, a bunch of other, less-related neurons are too. This pseudo-random firing, combined with the receptors' temporary inability to receive further transmission, radically disturbs the brain's normal function, and therefore disturbs the user's consciousness.
So, with low (psycholytic, 50 to 200 micrograms) doses of acid, the transmission is effected more than the reception; the drug works primarily as an agonist on 5-HT receptors. There isn't enough LSD in the system to block all of the receptors, so near-normal neurotransmission can happen, even though a few extra neurons are fired in the process. With a high (psychedelic, 300 to 600 micrograms or more) dosage, the transmission doesn't matter as much because the reception isn't there to pick it up; at this dosage the drug's an antagonist. Receptors pick up all of that ambient LSD until they're all blocked, and any form of normal neurotransmission is shredded, entheogenically removing the person's consciousness from consensus reality.
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