vigilancetxt

1 - Continuous vigilance even during sleep
Autoradiographic measurements of blood flow and
glucose utilization have confirmed many times over
that the auditory system is the most active neural
circuit in the brain. The visual system is not far
behind. Other subcortical structures such as the
vestibular nuclei, mammillary bodies, thalamic nuclei,
caudate and putamen of the subcortical motor system,
and hippocampus are also among the most active
metabolic centers of the brain. The data are shown in
tables 3 and 6.

Like auditory physiology, neuroanatomy is a highly
specialized field. Current understanding of sensory
and motor pathways that connect the vestibular nuclei,
mammillary bodies, and other brainstem nuclei of high
metabolic rate can be found in textbooks like that by
Nolte & Angevine (1995). But neuroanatomy will
remain a “work in progress” until all circuits of the
brain are mapped out. The brief summaries given
here are derived mainly from Nolte & Angevine’s text
and that of Truex & Carpenter (1969), the textbook we
used in my student days.

Blood flow and metabolism are not uniform throughout
the brain. Why? The brainstem nuclei of high
metabolic rate appear to be constantly active in
support of a rank order of essential tasks. Among
these are control of autonomic functions like heartbeat
and breathing. For some not easy to understand
reason the auditory system is most active, but as
already pointed out, hearing is the sense that
continually monitors the environment, even during
sleep.

In situations like hypoxia that compromise metabolic
activity, protective adjustments take place to aid in
preservation of the functions most essential for
survival. Hearing appears to be such an essential
function. The deaf consider themselves part of a
different culture, and do not think of themselves as
handicapped. However, there are situations that are
likely to be especially hazardous for those who are
deaf. Most of us take for granted the ability to hear
danger approaching, and in which direction. Hearing
is a primary means for orienting to environmental
events.

Metabolism in the auditory system remains most active
under an array of insults from oxygen lack to alcohol
intoxication and ingestion of poisonous substances.
Maintenance of high metabolic activity under adverse
conditions has been revealed in most of the research
studies in which the techniques for blood flow and
glucose uptake were used. The somewhat less active
nuclei like the mammillary bodies, caudate, and
hippocampus then become more vulnerable.

Any factor that suddenly and totally blocks aerobic
metabolism will most severely affect the brainstem
nuclei of high metabolic rate. Such situations most
often are lethal, but if resuscitation is possible these
most active nuclei in the brain will be the sites of
damage, and the inferior colliculi in the midbrain
auditory pathway are then most prominently damaged.

Brain Structure	cc/gm/min	Brain System
Inferior colliculus	1.80	auditory
Sensory-motor cortex	1.38
Auditory cortex	1.30
Visual cortex	1.25
Medial geniculate	1.22	auditory
Lateral geniculate	1.21	visual
Superior colliculus	1.15	visual
Caudate	1.10	subcortical motor
Thalamus	1.03
Association cortex	0.88
Cerebellar nuclei	0.87
Cerebellar white matter	0.24
Cerebral white matter	0.23
Spinal cord white matter	0.14

Deoxyglucose uptake in monkey & rat brain

Brain Structure	Monkey	Albino Rat	Brain System
	SD 1-4	SD 2-7
Inferior colliculus	103	197	auditory
Auditory cortex	79	162
Vestibular nucleus	66	128
Medial geniculate	65	131	auditory
Superior olivary nucleus	63	133	auditory
Visual cortex	59	107
Mammillary body	57	121	limbic
Superior colliculus	55	95	auditory
Thalamus, lateral nucleus	54	116
Caudate-putamen	52	110	subcortical motor
Cochlear nucleus	51	113	auditory
Cerebellar nuclei	45	100
Sensorimotor cortex	44	120
Lateral geniculate	39	96	visual
Hippocampus	39	79	limbic
Cerebellar cortex	31	57
Cerebellar white matter	12	37