Summary Cognitive Neuroscience Readings 300
How Do Drugs Affect Behaviour
Drug Routes in the Nervous System
Weak acid: Drug is easily absorbed across the stomach lining (alcohol).
Weak base: Drug cannot be absorbed until it passes through the stomach and
into the intestine, which might destroy it.
After absorption in the stomach/intestine, the drug needs to enter the blood.
Because blood consists of a lot of water, the drug needs to be hydrophilic. Then,
the drug is diffused in 6 litres of blood. Then, the drug must enter the
extracellular fluid and needs to be small enough to do this, is then diluted by
another 35 litres of water and then can even be destroyed by metabolic
processes. When one barrier is deleted (for example by inhaling/injecting a drug),
the dosage can be divided by 10 and still have the same effect.
Revisiting the Blood-Brain Barrier
Capillary: the smallest blood vessels. In the brain (causing the blood-brain
barrier), the capillaries are much more strict on letting substances in and out
than in the body. Capillary cells are made of endothelial cells. In the body, these
cells are not connected well, so substances can pass through. In the brain, the
endothelial cells form tight junctions, which do not allow substances to pass
through so easily. Besides, astrocyte glial cells are attached to the endothelial
cells, feeding them, removing waste and possibly also maintaining the tight
junctions. HOWEVER, not the whole brain is protected: the pituitary gland and the
hypothalamus do not have tight junctions. The postrema of the lower brain stem
does not have a blood-brain barrier, because this allows for a vomiting response.
The pineal gland also does not have the barrier to allow for hormones to regulate
day-night cycles.
Of course, some molecules do need to enter the brain. They can do this in two
ways:
- Small molecules can pass through when they are soluble in fat (thus not
ionized), for example oxygen;
- Glucose, amino-acids and other food components can enter by active
transport (ATP pumps, proteins).
, Blood-brain barrier functions:
- The electrical activity of the brain depends on extracellular concentrations
of ions; these concentrations should not get mixed up with/distorted by
ions from the body;
- Neurochemicals from the body should be kept out to prevent distortion of
neural communication;
- The barrier protects the brain from hormones, toxins and most diseases.
How the Body Eliminates Drugs
Individual Differences in Response to Psychoactive Drugs
- Size: tall people are less affected.
- Gender: females are more affected (smaller body size + hormones).
- Age: older people are more affected because of less efficient barriers.
Drug Action at Synapses: Agonists and Antagonists
- Agonists: increase effectiveness of neurotransmission.
- Antagonists: decrease effectiveness of neurotransmission.
Acetylcholine: Agonists can work to stimulate the release of ACh (black widow
spider), stimulate receptors (nicotine is similar to ACh receptors), or block
inactivation of ACh. > muscle rigidity.
Antagonists can work to decrease the release of ACh (botulin), or decrease the
workings of ACh receptors (curare). > muscle flaccidity.
IT DEPENDS ON THE DRUG WHETHER IT CAN ALSO CROSS THE BLOOD-BRAIN
BARRIER! E.g. nicotine can cross, curare cannot.
