A mechanism that prevents some substances in the blood from reaching the brain. It is achieved by brain capillaries, which unlike other capillaries elsewhere in the body, are composed of endothelial cells sealed together in continuous tight junctions and surrounded by astrocytes that contribute to the selective passage of substances. Lipid-soluble substances such as alcohol, caffeine, nicotine and most anaesthetics, as well as glucose, oxygen and water, pass rapidly into brain cells, whereas proteins, most antibiotics and ions do not enter or enter very slowly. The mechanism protects brain cells against harmful substances and pathogens. See central nervous system.
The gray matter primarily contains nerve cells and associated processes; the white matter consists predominantly of bundles of myelinated nerve fibers. Compare peripheral nervous system. See also brain, spinal cord
Drugs are carried to their sites of action through the bloodstream.
The process is a fast one: The heart recycles the total blood supply in about a minute, pumping drug molecules to all parts of the body.
The brain gets the largest share of that supply -- about 16 percent of all circulating blood. That means that all drugs (and other substances found in blood) are carried to the brain. But not all can enter it.
That's because the brain is surrounded by a wall of veins and capillaries, known as the blood-brain barrier, which protects brain tissues from impurities that are carried in blood. It's practically leakproof: Unlike most other cells, it has no pores. Only oxygen, food, and fat-soluble chemicals can filter across.
All psychoactive drugs pass the barrier, or they wouldn't be psychoactive. Other drugs -- penicillin, for example -- can't and don't.
Once drug molecules make it across the blood-brain barrier, they move to receptor sites throughout the body.
How long they stay there -- and how long they keep on producing effects -- depends on how quickly the drug is broken down and removed in a process called metabolism
Drugs aBrain Drugs
Neurons communicate by sending and receiving electrochemical messages.
These signals travel as a wave of electrical current down the cell body of the neuron and as chemical transmitters (called neurotransmitters) that are fired across the synapse, or the gap separating one neuron from another.
Receptors in adjacent nerve cells are activated by the transmitter, which generates a fresh electrical signal that triggers a new burst of neurotransmitters to neighboring neurons. The whole process happens in a flash, and sets off thousands or millions of similar reactions along the way.
Receptor sites are highly specialized. They respond only to particular sets of messages carried by specific types of neurotransmitters.
It's these chemical messengers, tiny pellets of neurotransmitter that jump the synapse, that form the basis of everything we think and feel.
We could easily spend the rest of this booklet discussing neurotransmitters. Because increasingly, research is showing that they're more than simple messengers.
In a real way, they are the message -- of consciousness itself -- conveyed through a built-in system of stress-reducers and pain-relievers that acts as the body's own medicine cabinet.
Where do drugs and alcohol fit in?
They change the way neurotransmitters interact with receptors.
How? In lots of ways.
For starters, drug...