arousal such as sleeping and dreaming, to detect abnormalities (such as deafness and visual
disorders in infants), and to study cognition. Another technology, positron emission
tomography (PET) produces color computer graphics that depend on the amount of
metabolic activity in the imaged brain region. When neurons are active, an automatic
increase in blood flow to the active region of the brain brings more oxygen and glucose nec-
essary for respiration. Blood flow changes are used to create brain images when tracers (such
as radioactively tagged glucose) injected into the blood of the subject emit particles called
positrons, which are converted into signals detected by the PET scanner. Functional MRI
(fMRI)shows the brain at work at higher resolution than the PET scanner. Changes in
oxygen in the blood of an active brain area alters its magnetic qualities, which is recorded
by the fMRI scanner. After further computer processing, a detailed picture of that local
brain activity emerges. With new brain imaging technology, psychologists can explore far
more about our abilities than ever before, from well-known systems like perception to less
understood systems like motivation and emotion.Organization of Your Nervous System
Your patterns of behavior generally involve masses of neural tissue rather than a few neurons.
All of the neurons in your body are organized into your nervous system. Your nervous
system has subdivisions based on location and function. The two major subdivisions are
your central nervous system and your peripheral nervous system. Your central nervous
system consists of your brain and your spinal cord. Your peripheral nervous system
includes two major subdivisions: your somatic nervous system and your autonomic nerv-
ous system. Your peripheral nervous system lies outside the midline portion of your nerv-
ous system carrying sensory information to and motor information away from your central
nervous system via spinal and cranial nerves. Your somatic nervous systemhas motor neu-
rons that stimulate skeletal (voluntary) muscle. Your autonomic nervous system has motor
neurons that stimulate smooth (involuntary) and heart muscle. Your autonomic nervous
system is subdivided into the antagonistic sympathetic nervous systemandparasympa-
thetic nervous system. Sympathetic stimulation results in responses that help your body
deal with stressful events including dilation of your pupils, release of glucose from your
liver, dilation of bronchi, inhibition of digestive functions, acceleration of heart rate, secre-
tion of adrenalin from your adrenal glands, acceleration of breathing rate, and inhibition of
secretion of your tear glands. Parasympathetic stimulation calms your body following sym-
pathetic stimulation by restoring digestive processes (salivation, peristalsis, enzyme secre-
tion), returning pupils to normal pupil size, stimulating tear glands, and restoring normal
bladder contractions. Your spinal cord, protected by membranes called meninges and your
spinal column of bony vertebrae, starts at the base of your back and extends upward to the
base of your skull where it joins your brain. The cord is composed mainly of interneurons
and glial cells, which are all bathed by cerebrospinal fluid produced by your glial cells.The Brain
Your brain, which has the consistency of soft-serve yogurt, is covered by protective
membranes called meninges and is housed in your skull. The evolutionary approach
describes the brain’s evolution from more primitive organisms, reasoning that new types of
behavior developed as each new layer of the brain evolved. According to one evolutionary
model (triune brain), the human brain has three major divisions, overlapping layers
with the most recent neural systems nearest the front and top. The reptilian brain, which
maintains homeostasis and instinctive behaviors, roughly corresponds to the brainstem,
which includes the medulla, pons, and cerebellum. Developmental psychologists call theBiological Bases of Behavior 67