Psychology2016

The Biological Perspective 53

Yo u m a y h a v e h e a rd o f t h e s e t o f n e u ro p e p t i d e s c a l l e d endorphins—pain-controlling
chemicals in the body. When a person is hurt, a neurotransmitter that signals pain is
released. When the brain gets this message, it triggers the release of endorphins. The
endorphins bind to receptors that open the ion channels on the axon. This causes the cell
to be unable to fire its pain signal, and the pain sensations eventually lessen. For exam-
ple, you might bump your elbow and experience a lot of pain at first, but the pain will
quickly subside to a much lower level. Athletes may injure themselves during an event
and yet not feel the pain until after the competition is over, when the endorphin levels
go down.
The name endorphin comes from the term endogenous morphine. (Endogenous means
“native to the area”—in this case, native to the body.) Scientists studying the nervous
system found receptor sites that fit morphine molecules perfectly and decided that there
must be a natural substance in the body that has the same effect as morphine. Endorphins
are one reason that heroin and the other drugs derived from opium are so addictive—
when people take morphine or heroin, their bodies neglect to produce endorphins. When
the drug wears off, they are left with no protection against pain at all, and everything
hurts. This pain is one reason most people want more heroin, creating an addictive cycle
of abuse. to Learning Objective 4.11.

If the neurotransmitters are out there in the synaptic gap and in

the receptor sites, what happens to them when they aren’t needed

anymore?

CLEANING UP THE SYNAPSE: REUPTAKE AND ENZYMES The neurotransmitters have
to get out of the receptor sites before the next stimulation can occur. Some just drift
away through the process of diffusion, but most will end up back in the presynap-
tic neuron to be repackaged into the synaptic vesicles in a process called reuptake.
(Think of a little suction tube, sucking the chemicals back into the vesicles.) That
way, the synapse is cleared for the next release of neurotransmitters. Some drugs,
like cocaine, affect the nervous system by blocking the reuptake process, as shown in
Figure 2. 4.

reuptake
process by which neurotransmit-
ters are taken back into the synaptic
Figure 2.4 Neurotransmitters: Reuptake vesicles.

CC

Presynaptic neuron

Dopamine

receptor Postsynaptic (receiving) neuron

Dopamine

Synapse

Cocaine Dopamine

reuptake site

Blocked

reuptake site