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neurons of the antagonist muscles through connecting neurons, muscles, so as to produce organized movement of the limb to
called inhibitory neurons, within the spinal cord. a particular place in space.
20
MOVEMENT. The stretch reflex
Sensory neuron (above) occurs when a doctor taps
a muscle tendon to test your
reflexes. This sends a barrage of
Alpha motor neuron impulses into the spinal cord
Extensor muscles activated
along muscle spindle sensory
Muscle fibers and activates motor neu-
spindle
rons to the stretched muscle to
Inhibitory neuron
cause contraction (stretch reflex).
The same sensory stimulus
Flexor muscles inhibited
causes inactivation, or inhibition,
of the motor neurons to the antag-
Stimulus
onist muscles through connection
Response
neurons, called inhibitory neu-
Efferent nerves
rons, within the spinal cord.
Afferent nerves
AH"erent nerves carry messages
from sense organs to the spinal
Inhibitory neurons cord; eH"erent nerves carry motor
commands from the spinal cord to
Excitatory neurons
muscles. Flexion withdrawal
Sensory neuron
(below) can occur when your bare
foot encounters a sharp object.
Motor neurons
Your leg is immediately lifted
Extensor
muscles
(flexion) from the source of poten-
inhibited
-
Extensor muscles activated
tial injury, but the opposite leg
Motor
neurons
responds with increased exten-
Flexor sion in order to maintain your bal-
muscles
ance. The latter event is called the
activated
crossed extension reflex. These
Flexor
muscles
responses occur very rapidly and
inhibited
without your attention because
they are built into systems of neu-
Right leg extends to
balance body
rons located within the spinal
cord itself.
Stimulus
In addition to the motor cortex, movement control also The cerebellum is critically involved in the control of all
involves the interaction of many other brain regions, including skilled movements. Loss of cerebellar function leads to poor
the basal ganglia and thalamus, the cerebellum and a large coordination of muscle control and disorders of balance. The
number of neuron groups located within the midbrain and cerebellum receives direct and powerful sensory information
brainstem regions that connect cerebral hemispheres with the from the muscle receptors, and the sense organs of the inner
spinal cord. ear, which signal head position and movement, as well as sig-
Scientists know that the basal ganglia and thalamus have nals from the cerebral cortex. It apparently acts to integrate all
widespread connections with sensory and motor areas of the this information to ensure smooth coordination of muscle
cerebral cortex. Loss of regulation of the basal ganglia by action, enabling us to perform skilled movements more or less
dopamine depletion can cause serious movement disorders, automatically. There is evidence that, as we learn to walk, speak
such as Parkinson s disease. Loss of dopamine neurons in the or play a musical instrument, the necessary detailed control
substantia nigra on the midbrain, which connects with the basal information is stored within the cerebellum where it can be
ganglia, is a major factor in Parkinson s. called upon by commands from the cerebral cortex.
21
Sleep
leep remains one of the great mysteries of mod- the promise for devising new treatments to allow millions of
ern neuroscience. We spend nearly one-third people to get a good night s sleep.
of our lives asleep, but the function of sleep still
The stuH" of sleep
is not known. Fortunately, over the last few
years researchers have made great headway in Sleep appears to be a passive and restful time when the brain is
Sunderstanding some of the brain circuitry that less active. In fact, this state actually involves a highly active
S
controls wake-sleep states. and well-scripted interplay of brain circuits to produce the
Scientists now recognize that sleep consists of several stages of sleeping.
di"erent stages; that the choreography of a night s sleep The stages of sleep were discovered in the 1950s in experi-
involves the interplay of these stages, a process that depends ments examining the human brain waves or electroencephalo-
upon a complex switching mechanism; and that the sleep stages gram (EEG) during sleep. Researchers also measured move-
are accompanied by daily rhythms in bodily hormones, body ments of the eyes and the limbs during sleep. They found that
temperature and other functions. over the course of the first hour or so of sleep each night, the
Sleep disorders are among the nation s most common brain progresses through a series of stages during which the
health problems, a"ecting up to 70 million people, most of brain waves progressively slow down. The period of slow wave
whom are undiagnosed and untreated. These disorders are one sleep is accompanied by relaxation of the muscles and the eyes.
of the least recognized sources of disease, disability and even Heart rate, blood pressure and body temperature all fall. If
death, costing an estimated $100 billion annually in lost pro- awakened at this time, most people recall only a feeling or
ductivity, medical bills and industrial accidents. Research holds image, not an active dream.
SLEEP PATTERNS. During a night of sleep, the brain waves of a young adult recorded by the electroencephalogram (EEG) gradually slow down and
become larger as the individual passes into deeper stages of slow wave sleep. After about an hour, the brain re-emerges through the same series of
stages, and there is usually a brief period of REM sleep (on dark areas of graph), during which the EEG is similar to wakefulness. The body is com-
pletely relaxed, the person is deeply unresponsive and usually is dreaming. The cycle repeats over the course of the night, with more REM sleep,
and less time spent in the deeper stages of slow wave sleep as the night progresses.
Awake
Awake
Stage 1
Stage 1
Stage 2
Stage 2
Stage 3
Stage 3
Stage 4
Stage 4
Hours
1 2 3 4 6 7
22
THE WAKING AND SLEEPING
BRAIN. Wakefulness is main-
Cerebral
cortex
tained by activity in two systems
of brainstem neurons. Nerve
cells that make the neurotrans-
mitter acetylcholine stimulate
the thalamus, which activates
the cerebral cortex (red path-
way). Full wakefulness also
requires cortical activation by
other neurons that make
monoamine neurotransmitters
such as norepinephrine, sero-
tonin and histamine (blue path-
way). During slow wave sleep,
when the brain becomes less
active, neuron activity in both
pathways slows down. During
Thalamus
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