FoundationalConceptsNeuroscience

Another place where BTXs have been identified is in the skin and
feathers of the New Guinea pitohui bird. In this case, the bird is known
to acquire the BTXs from eating a type of beetle from the genus Chore-
sine that contains the toxins. This suggests that the Amazonian frogs
may also acquire their BTXs from ingestion of insects. The ultimate
source of BTXs in these cases is not presently known—perhaps syn-
thesis by insects, or perhaps symbiotic microorganisms.
Local anesthetics provide yet another example of the effects of
altering sodium channels. The word anesthesia means “loss of sensa-
tion.” Two general categories of anesthetics are used in medicine: gen-
eral and local. General anesthetics are chemicals that produce a global
loss of sensation over the entire body. They work by impacting the
brain to produce a change in consciousness that affects all perception
(see Chapter 9). Local anesthetics are chemicals that produce a loss
of sensation locally, that is, only in the region of the body near where
they have been applied. They are local because their action is relatively
weak. When the chemical drifts around, enters the bloodstream, and
reaches other parts of the body, its concentration becomes too low to
have any noticeable effect. Local anesthetics are widely used in mod-
ern medicine: from dental procedures to knee, elbow, and brain surg-
eries to childbirth, local anesthetics facilitate the accomplishment of
various procedures without pain.
The first local anesthetic chemical to be appreciated by modern
medicine was cocaine. It was purified in the mid-nineteenth century
from the coca plant, Erythroxylum coca, a South America plant used by
native peoples of the region for thousands of years for its medicinal
and stimulant properties. In addition to its potent effects on the brain
and autonomic nervous system (discussed in Chapter 9), cocaine has a
weaker local anesthetic effect. Several synthetic chemicals have been
developed that also act as local anesthetics to numb sensation but do