brain development summary
1. mammal, primates, human brain follow
2. overall size increases as body size increases
3. blood supply - brain as energy "consumer" &
Outline of brain development: mammals to humans
1. mammal, primates, human brain follow same "plan"
See EHE for overview and more details.
1. same types of cells, genera
EHE, p.110. The sensori-motor "homunculus" mapped out during the 19th and
early 20th century demonstrated a very similar pattern of gross connectivity
and organization. Leyton and (1917) Sherrington mapped out the neocortex
of a gorill
a, orang, and chimp. Later Penfield (1975) found corresponding
patterns in humans.
While brains themselves can vary considerably in size, within a species
and between related species, the cells comprising those brains (glial and
neurons) are very simi
lar in size.
Many of our basic ideas about neurons begain with the famous Spanish
histologist, Santiago Ramon y Cajal
(1852-1934). Cajal discovered that neurons were the basic uni
t of the nervous
system, how they connected into circuits, their overproduction and pruning
in development, and laid out the life history of cells: birth, differentiation
and growth, migration, maturation and death.
size increases as body size increases
The most obvious change is in size -- "encephalization" -- but this covers
up changes in relative organization and interconnectivity. There also is
a problem of interpreting these brain -body size correlations (a
relations): which, if any, is the driving factor in natural selection?
Primates, elephants, and cetaceans all display an increase in encephalization
over ancestral mammals. (Deacon, 1990).
Keep in mind no single factor can explain the va
rious species and individual
differences in brain and consequent behaviors. Also remember there are
costs and benefits for every feature and these can only be assessed with
respect to a given set of conditions, e.g. climate variables, social structures,
1. same numbers of neurons but more space for connections
Even if there were no changes in numbers of neurons, the interconnections
among them can vary greatly. Expansion of brain volume
, even with same
numbers of neurons, can make for increased interconnectivity. Variation
in connectivity during brain development provides a rich opportunity for
emergence of novel behaviors.
(Note that even neurons follow the "form- function" princip
adapted for their various tasks.)
2. "folding" of tissue creates new opportunities
Packing an increasingly large brain into a skull causes a greater surface
area to be compressed, like a wrinkled newspaper, into
that space. This
may also facilitate novel connections, along with different developmental
rates of brain regions.
3. increased energy costs with larger brain
Large brains are not without problems -- birth, sore necks, an
the energy they demand detracts from their presumed benefits in information
processing. Even though our brains may be just 2% of body weight, they
use close to 20% of our calories -- and for children it may be 40%.
In general, all of the above may be seen as consequences of selection for
larger body size. One might also expect to see increased modularization
as distances betwe
en "circuits" increases, making communication more difficult.
This may have be a factor in the evolution of the differential right and
left hemispheric functions in humans. In addition changes in myelinization
may be expected.
(Myelin comes from a typ
e of neural glial cell that wraps around long
axons -- a kind of fatty insulation that in vertebrates enables efficient
transmission and integration of neural signals over "long" distances. Myelin's
importance is evident when it degenerates, resulting in
the symptoms of
3. blood supply - brain as energy "consumer" &
The blood supplies nutrients, oxygen, and serves to cool the brain. Bipedalism
already put an additional burden on bipedal
hominid blood supply just to
pump enough blood up against gravity; increasing brain size compounds the
problem. Temperature regulation within the brain is a crucial function
with little room for error.
4. binocular convergence
of forward facing eyes
(See EHE for drawings.)
1. correlated loss of olfactory structure
As overlapping visual fields became important, olfactory structures lost
out in competition for brain resources; moreover a lar
ge snout may get
2. major brain reorganization
The eye in primate binocular vision sends information to both left and
right visual hemispheres from the central overlapping (foveal) fields.
(See EHE, p.112)
a NAME="Heading12">3. redevelopment of color vision
While many non-mammals have color vision, early primates probably lost
it and then in a shift to diurnal species, regained color vision with subsequent
brain reorganization as well as receptor d
evelopment. Color vision is mediated
by the fovea, rich in cones and necessary for sharp color vision and depth
perception. (See Norby reference).
5. "mosaic" adjustment of brain components in various
species (EHE, p. 123)
While overall size increased, in each species specific adaptive changes
occurred in various parts of the brain. In some cases these adaptive changes
were made possible by correlative changes in brain due to size changes,
i.e. increases in size "pre-adap
ted" the brain to better serve other functions
as opportunity arose.
1. human language regions, olfaction, hypothalamus,
Human brains are specialized for language in the regions around the Sylvian
of the left hemisphere -- in over 90% of humans anyway. Our overly
large cerebellum may also be involved in acquisition and production of
rapid, fine-grained precise movements necessary for language (speech and
Non-human primates show little c
apability for such language movements,
even with great training efforts.
2. cerebral asymmetry in the primate brain
This asymmetry (lateralization or specialization) includes behavioral factors
as well as structural ones.
Thus for over 90% of humans, the left hemisphere
controls most language functions as well as our preferred right hand. (EHE,
121-122). Inspection of the brain reveals that in most humans the left
hemisphere structures are larger than the corresponding ri
ght ones. Lesser
differences have been reported in apes and apes show little of the species
specific hand preference shown by humans.
3. prefrontal cortex expansion
Primates have an expanded prefrontal cortex; humans have
one over twice
that expected for a primate brain of our size --which is already three
times the brain size of a primate with our body size. This prefrontal cortex
is involved in many important primate skills including planning, social
onal regulation, and in humans, language use. (cf. the
lobotomy page and the Phineas Gage story.)
6. Brain circuits are "sculpted" in several ways:
Each individual has its brain structure determined in a number of differe
ways -- some unique, others common to all members of the species or order.
1. direct growth of synapses
Synapses connecting regions of the brain are directed genetically during
2. pruning of neurons and synapses
1. genetic control?
Programmed cell death (apoptosis) and patterns of interconnectivity among
cells is one of the most important and least understood aspects of brain
2. passive loss due to lack of stimulation
Some cells and connections die off when not stimulated by their peripheral
sources of stimulation. Thus while a fetal human and fetal elephant share
many similarities in forelim
b structure and cortical connections, as the
animals develop into their respective structures, feedback from the "fingers"
differs dramatically affecting cortical organization.
Feedback from peripheral stimulation and movement is a very important
or in initial organization of brains as well as reorganization of brains
after injury in adults.
Even if cells do not die from restricted stimulation, they may end
up linking into different circuits as a consequence.
nnections formed in learning
Learning is literally embodied by the formation of new dendritic connections
among neural synapses. This occurs most importantly in the first few years
after birth but substantial reorganization can occur in adulthood eve
though new neurons cannot be formed -- with a few recently claimed exceptions.
(see "monkey hand" mapping overhead).
Darwin himself (1871, p. 146 demonstrated that "the brains of domestic
rabbits are considerably reduced in bulk, in comparison with t
hose of the
wild rabbit..." Research today suggests learning throughout the life span
of many species is manifest in increased synaptic connections reflecting
7. other modes of information and control in brains<
Like the cells involved, the chemistry of mammal brains is quite similar
in terms of basic building blocks. Timing and quantities of essential "humours"
can make considerable difference in impact --both developmentally and in
Neurons --tended by astrocyte glial cells
-- communicate amongst themselves with a complex chemistry that variously
enhances, inhibits, and otherwise modify their functions.
"neurotransmitters", e.g. acetycholine, serotonin, GABA, dopamine,
norepinephrine, nitric oxide, etc. serve specific functions in specific
neurons and can be expected to differ in effect from species to species,
as well as individual to individual.
Psychoactive drugs, e.g. Prozac, amphetamines, have their effects
at this level.)
8. summary Slogans of brain development
1. From lower to higher
The older brainstem develops first and the neocortex (new brain) last!
2. Overproduction then pruning of neurons and synapses
During prenatal and early postnatal development, more neurons and axons
ced than are found in the adult. Infant primates may have up to
twice the adult number of neurons. In a real sense, circuits are sculpted
from the brain by pruning away cells and synapses.
3. Use it or lose it!
ls that are not stimulated by inputs may die off or be recruited
into the different activities of their neighbors.
4. Learning is embodied in new synapses.
At some point the physical structure of the brain is modif
ied when something
is learned. The structure of neurons changes in these processes (and perhaps
even new neurons are produced -- see below.)
5. Early "plasticity" of the CNS (but always some!)
Brain structure and f
unction is most flexible early in the life of the
organism. The most astonishing example of this comes from epileptic infants
who have had one of their entire hemispheres removed (hemispherectomy)
and develop nearly normally in terms of function.
6. Myelination speeds information flow.
Adult primates with large brains require fast flowing information. Myelin
coated axons make this possible.
7. Communication in the CNS is by slow (ho
and fast acting chemicals (neurotransmitters).
Two different chemical communication channels exist, hormones operating
in time scales of hours to months, while neurotransmitters operate in fractions
8. Sexual differentiation via hormones begins prenatally.
Hormone levels controlled by M & F genes begin changing body and brain
structures weeks after conception.
9. Species brain differences reflect differentia
regional growth rates, differential apoptosis, and feedback directed pruning
These processes are barely understood as yet!
9. Research questions: development and rehabilitation
stions have always been at the forefront of brain unknowns.
Recently these have been linked to questions of brain repair and rehabilitation.
Here are several questions about primate brains that will probably be answered
in the near future.
Under what conditions do neuron precursor cells become specific types cells?
Can precursor cells be "transplanted" and shaped somehow to serve needed
recovery after brain injury?
The phenomena of "phantom limbs" had led to the suspicion that after trauma,
the certain regions of the nervous system undergo reorganization. Thus
demonstrations of stimulating a lost "phantom" limb by tickling a cheek
as thought to show how some of the limbs cortical receptor neurons had
synapsed with nearby facial receptors. (e.g. Ramachandran, 1998)
Somewhat similarly there was speculation that unused regions of sensory
cortex might organize themselves with nearb
y regions, changing the normal
cortical organization of deaf or blind individuals. (See a report on this
in Nature, 1/14/99)
Experimental work with monkeys has shown explicitly how this reorganization
can occur. Cells surgically losing their nerve inp
uts from a finger acquired
new inputs from nearby nerves as the monkey was forced to use the injured
forelimb in experimental tasks. (e.g. Kass, 1995 in Gazzaniga, M. ed.)
under what conditions can new neurons form in adults?<
Until the last year or so, it was widely accepted that no new neurons formed
in adult mammal brains. Birds had been shown to form new neurons in the
process of learning new songs, but there had be no suggestion that anything
similar happened in mamma
ls and in particular primates. Yet recently
it has been found that in limited circumstances new neurons may be formed
in the hippocampus -- an area involved in primate learning and memory.
Thus an increase in brain weight du
e to learning may not only be from new
synapses but new neurons as well.
(see more on brain