Chapter 4 Neurons, Hormones, and the Brain 149the creation and appreciation of art and music. In most mental
activities, however, the two hemispheres cooperate as partners,
with each making a valuable contribution.The Flexible Brain
• Many synapses have not yet formed at birth. During develop-
ment, axons and dendrites continue to grow as a result of both
physical maturation and experience with the world, and through-
out life, new learning results in new synaptic connections in the
brain. Thus, the brain’s circuits are not fixed and immutable but
are continually changing in response to information, challenges,
and changes in the environment, a phenomenon known as plas-
ticity. In some people who have been blind from an early age,
brain regions usually devoted to vision are activated by sound, a
dramatic example of plasticity.
• Brain scans and other techniques have revealed some average
anatomical and functional differences in the brains of males
and females. Controversy exists, however, about what such dif-
ferences mean in real life. Many behavioral or cognitive “differ-
ences” are stereotypes and in reality are small and insignificant,
or even nonexistent. Brain differences do not account for the
fact that many behavioral differences depend on the situation.
Some findings of brain differences have been widely accepted
but then have failed to replicate. Biological differences do not
necessarily explain behavioral ones. And sex differences in ex-
perience could affect brain organization and function rather than
the other way around.Psychology in the News, revisited
• The existence of brain damage in some athletes who have suf-
fered repeated concussions raises medical, psychological, and
ethical issues. What degree of risk should individuals and so-
ciety tolerate in aggressive contact sports, and should children
and teenagers be prevented from playing them?Taking Psychology With You
• Scholars in the field of neuroethics are addressing the implica-
tions of “cosmetic neurology,” especially questions raised by the
development of drugs that are “neuroenhancers.”central nervous system
(CNS) 120
spinal cord 120
spinal reflexes 120
peripheral nervous system
(PNS) 121
sensory nerves 121
motor nerves 121
somatic nervous
system 121
autonomic nervous
system 121
sympathetic nervous
system 121
parasympathetic nervous
system 121
neuron 122
glia 122
dendrites 123
cell body 123
axon 123
axon terminals 123
myelin sheath 124
nodes 124
nerves 124
neurogenesis 124
stem cells 124
embryonic stem (ES) cells 124
induced pluripotent stem
(iPS) cells 125
synaptic cleft 125
synapse 125
action potential 125
synaptic vesicles 126
neurotransmitter 126
receptor sites 126
excitatory and inhibitory
effects 126
serotonin 126
dopamine 126
acetylcholine 127
norepinephrine 127
GABA 127
glutamate 127
hormones 127
endocrine glands 127
melatonin 128
pineal gland 128
oxytocin 128
vasopressin 128
adrenal hormones 128
cortisol 128
epinephrine 128
norepinephrine 128sex hormones
(androgens, estrogens,
progesterone) 128
neuromodulators 128
serotonin transporter 128
endorphins (endogenous
opioid peptides) 128
lesion method 129
transcranial magnetic
stimulation (TMS) 129
transcranial direct current
stimulation (tDCS) 130
electroencephalogram
(EEG) 130
electrode 130
PET scan (positron-emission
tomography) 130
MRI (magnetic resonance
imaging) 131
functional MRI (fMRI) 131
localization of function 133
brain stem 133
pons 133
medulla 133
reticular activating system
(RAS) 133
cerebellum 134
thalamus 134olfactory bulb 134
hypothalamus 134
pituitary gland 134
amygdala 135
hippocampus 135
cerebrum 135
cerebral hemispheres 135
corpus callosum 135
lateralization 135
cerebral cortex 136
gray and white matter 136
occipital lobes 136
visual cortex 136
parietal lobes 136
somatosensory cortex 136
temporal lobes 136
auditory cortex 136
Wernicke’s area 136
frontal lobes 136
motor cortex 136
Broca’s area 136
association cortex 136
prefrontal cortex 136
split-brain surgery 139
visual field 139
plasticity 142
neuroethics 146Key Terms