Chapter 12
The Central Nervous System
453
12
Consciousness is difficult to define. And to be frank, reduc-
ing our response to a Key West sunset to a series of interactions
between dendrites, axons, and neurotransmitters does not cap-
ture what makes that event so special. A sleeping person obvi-
ously lacks something that he or she has when awake, and we
call this “something” consciousness.
Current suppositions about consciousness are as follows:
Consciousness involves simultaneous activity of large areas
of the cerebral cortex.
 
It is superimposed on other types of neural activity.
At
any time, specific neurons and neuronal pools are involved
both in localized activities (such as motor control) and in
cognition.
It is holistic and totally interconnected.
Information for
“thought” can be claimed from many locations in the cer-
ebrum simultaneously. For example, retrieval of a specific
memory can be triggered by several routes—a smell, a place,
a particular person, and so on.
Homeostatic Imbalance
12.6
Except during sleep, unconsciousness is always a signal that
brain function is impaired. A brief loss of consciousness is
called
fainting
or
syncope
(sing
9
ko-pe; “cut short”). Most o±en,
syncope indicates inadequate cerebral blood flow due to low
blood pressure, as might follow hemorrhage or sudden emo-
tional stress.
Significant unresponsiveness to sensory stimuli for an ex-
tended period is called
coma
. Coma is
not
deep sleep. During
sleep, the brain remains active and oxygen consumption resem-
bles that of the waking state. In coma patients, oxygen use is
always below normal resting levels.
Factors that can induce coma include: (1) blows to the head
that cause widespread cerebral or brain stem trauma, (2) tumors
consciousness disappears. Tese are typically seen in young
children and usually disappear by age 10.
Tonic-clonic seizures
, formerly called
grand mal,
are the most
severe, convulsive form of epileptic seizures. Te person
loses consciousness, o±en breaking bones during the intense
convulsions, showing the incredible strength of these muscle
contractions. Loss of bowel and bladder control and severe
biting of the tongue are common. Te seizure lasts for a few
minutes, then the muscles relax and the person awakens but
remains disoriented for several minutes.
Many seizure sufferers experience a sensory hallucination,
such as a taste, smell, or flashes of light, just before the seizure
begins. Tis phenomenon, called an
aura
, is helpful because it
gives the person time to lie down and avoid falling to the floor.
Epilepsy can usually be controlled by anticonvulsive drugs.
If drugs fail to control the seizures, a
vagus nerve stimulator
or
deep brain stimulator
can be implanted. Tese devices deliver
pulses to the vagus nerve or directly to the brain at predeter-
mined intervals to stabilize the brain’s electrical activity. A cur-
rent line of research seeks to implant electrodes in the brain to
detect and prevent oncoming seizures.
Consciousness
Describe consciousness clinically.
Consciousness
encompasses conscious perception of sensations,
voluntary initiation and control of movement, and capabilities
associated with higher mental processing (memory, logic, judg-
ment, perseverance, and so on). Clinically, consciousness is de-
fined on a continuum that grades behavior in response to stimuli
as (1)
alertness
, (2)
drowsiness
or
lethargy
(which proceeds to
sleep), (3)
stupor
, and (4)
coma
. Alertness is the highest state of
consciousness and cortical activity, and coma the most depressed.
Alpha waves
—awake but relaxed
Beta waves
—awake, alert
Theta waves
—common in children
Delta waves
—deep sleep
(b) Brain waves shown in EEGs fall into four general
classes.
(a) Scalp electrodes are used to record brain wave activity.
1-second interval
Figure 12.18
Electroencephalography (EEG) and brain waves.
previous page 487 Human Anatomy and Physiology (9th ed ) 2012 read online next page 489 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off