The Mind - Brutes or Angels: Human Possibility in the Age of Biotechnology - page 231

transmitter molecules into the synaptic cleft. The neurotransmitters dif-
fuse across the cleft and get grabbed up by receptor molecules embedded in
the postsynaptic membrane (fig. 9.3). Properties of diverse types of recep-
tors actually determine the nature of the signals received through a neu-
ron's dendritic tree.
Axon terminals from over ten thousand different neurons may form
synapses with a single postsynaptic neuron. By integrating signals from
hundreds of thousands of receptor mole cules, the postsynaptic cell makes a
decision about whether to send an electrical signal down its axon to a thou-
sand other neurons. Whether a postsynaptic neuron sends a signal down its
axon depends upon the amounts and types of neurotransmitters released
into the synaptic clefts of its dendritic tree, the numbers and types of its
receptors, and how long neurotransmitters remain in the synaptic clefts.
neurotransmitters disappear from the synaptic cleft in three ways: by
re-uptake from the cleft back into the presynaptic cell, by degradation
within the cleft, and by simple diffusion out of the cleft (fig. 9.4). As we
will see, psychoactive drugs modify brain function mainly by affecting the
levels and types of neurotransmitters or neurotransmitter receptors in the
brain.
Psychoactive Drugs and How They Work
most of us have methods to modulate our brain's performance. for alert-
ness and a good mood, we get a good night's sleep, go to the gym, drink
coffee, or inhale nicotine. To relax or escape unhappiness, we may drink
alcohol. such habits and activities can be considered neuroenhancers; so
too are brand-name drugs prescribed for depression (Prozac), ADHD (rita-
lin and Adderall), and social anxiety (nardil). These drugs can also elevate
mood or enhance mental performance in persons without diagnosed dis-
orders. These and new generations of similar chemical enhancers that af-
fect mood or cognitive abilities all act at the level of the chemical synapse.
Knowing how some of these agents work gives insight into challenges faced
by designers of new psychoactive drugs and an appreciation for the role of
synapses in making us who we are.