The Central Nervous System
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.
any time, speciﬁc neurons and neuronal pools are involved
both in localized activities (such as motor control) and in
It is holistic and totally interconnected.
“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.
Except during sleep, unconsciousness is always a signal that
brain function is impaired. A brief loss of consciousness is
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-
Signiﬁcant unresponsiveness to sensory stimuli for an ex-
tended period is called
. Coma is
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.
, formerly called
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
, 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
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.
Describe consciousness clinically.
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
(which proceeds to
, and (4)
. Alertness is the highest state of
consciousness and cortical activity, and coma the most depressed.
—awake but relaxed
—common in children
(b) Brain waves shown in EEGs fall into four general
(a) Scalp electrodes are used to record brain wave activity.
Electroencephalography (EEG) and brain waves.