1. Yakovlev PI and Lecours A-R
    (1967) The myelogenetic cycles
    of regional maturation of the
    brain.  In A. Minkowski (Ed.),
    Regional Development of the
    Brain in Early Life (pp. 3-70).  
    Oxford: Blackwell Scientific
  2. Langworthy OR (1933)
    Development of behavior
    patterns and myelinization of
    the nervous system in the
    human fetus and infant.
    Contributions to Embryology,
    no. 139 24:1-57.
  3. Moore JK, Perazzo LM, Braun A
    (1995). Time course of axonal
    myelination in the human
    brainstem auditory pathway.  
    Hearing Research 87:21-31, 91:
  4. VonHungen K, Roberts S, Hill
    DF (1975) Serotonin-sensitive
    adenylate cyclase activity in
    immature rat brain. Brain
    Research 84:257-267.
  5. Kungel M and Friauf E (1995).
    Somatostatin and leu-
    enkephalin in the rat auditory
    brainstem during fetal and
    postnatal development.  
    Anatomy and Embryology, 191,
  6. Friauf E, Aragon C, Lohrke S,
    Westenfelder B, Zafra F.
    Developmental expression of
    the glycine transporter GLYT2
    in the auditory system of rats
    suggests involvement in
    synapse maturation. J Comp
    Neurol. 1999 Sep 13;412(1):17-
  7. Friauf E, Lohmann C.
    Development of auditory
    brainstem circuitry. Activity-
    dependent and activity-
    independent processes. Cell
    Tissue Res. 1999 Aug;297(2):
    187-95. Review.
1 -  Maturation and the auditory system
Neurons in the inferior colliculus become myelinated
earlier than in any other structure in the human brain.  
Yakovlev and Lecours (1967) published dramatic
pictures of this finding in their study of myelin
formation from the fourth fetal month through
adulthood and old age in over 200 human brains (see
10) [1].  Yakovlev and Lecours cited the work of
Langworthy (1933) who had noted the early
myelination of the auditory system more than three
decades earlier (see
table 4) [2].  Moore et al. (1995)
again confirmed that myelin formation takes place in
the auditory system between weeks 26 and 29 of
gestation, and that this is when the human fetus first
responds to environmental sounds [3].

Early maturation of the inferior colliculus means that
damage or impairment will be permanent.  
Regeneration of fully developed neurons does not
happen in the brain or spinal cord of higher
vertebrates.  Perhaps this quirk of evolution protects
the mature memory system from being disrupted
through new growth.

The inferior colliculus and other brainstem auditory
nuclei are fully myelinated and mature before birth.  
The auditory receptive area of the cerebral cortex and
cortical language circuits become myelinated during
the first three to four years after birth.  This means
that language development
begins before the areas
for speech reception and production are fully mature;
and this also indicates that early language
development must be accomplished by analysis of
auditory signals within the brainstem.

A growing body of research is providing evidence that
transient neuro-chemical factors appear, and then
disappear, during development.  Von Hungen et al.
(1975) found serotonin sensitive post-synaptic
receptors in the inferior and superior colliculi that
declined with maturation [4].  Kungel and Friauf
(1995) found neuro-peptides that appear then decline
during early development, and Friauf et al. (1999)
reported transient transport systems for delivery of
amino acids that may likewise guide maturational
processes in an orderly fashion [6, 7].

Exposure to drugs and toxic substances during critical
stages of prenatal development may then disrupt the
well-defined schedule required for successive
appearance of transient neurotransmitters, as can
even the briefest period of asphyxia at birth.  The
early maturing auditory system and all of the higher
cortical areas dependent on its integrity (like the
language association tracts of the temporal and
frontal lobes) may then fail to develop normally.

Early myelination seen at 25
gestational weeks in a human
fetus. (larger images below)

Langworthy's data (below)
Figure 10 - From Yakovlev & Lecours (1967)
showing prominent myelination in the inferior
colliculus (ICOL) at 25 gestational weeks
(below) and 29 gestational weeks (right).
Table 4 - From Langworthy (1933) myelination of the acoustic colliculus at 28 weeks gestation
Figure 10:
Table 4: