lecture 14- brain development Flashcards
The adult human brain
-Most complex system of which we know
-Human brain and CNS consist of trillions of highly specialised cells that work in synchrony
Chemical signals
Electrical signals
Larger and more
complex than brains
of other mammals
The first and second brains
-Human brain makes up about 2/3 of the neuronal mass of the body and contains almost ¾ of all of our synapses (neural junctions through which cells communicate to each other)
-Enteric nervous system (gut) consists of sheaths of neurons embedded in the walls of the gut
It contains ~100 million neurons (more than the spinal cord or the peripheral NS)
Brain outsources the digestion process to this NS
90% of fibers in the vagus nerve carry information from the gut to the brain
ENS uses more than 30 neurotransmitters
95% of the body’s serotonin is found in the bowels
Neurogastroenterology field
brain growth
The brain grows at an astounding rate early in life
At birth, it weighs 25% of adult weight
By age 2, it weights 75% of adult weight
Between the 7th prenatal month and the 1st birthday, the brain increases in weight by ~1.7 grams per day
“Brain growth spurt” - last 3 months prenatally and first 2 years after birth
Central nervous system (CNS):
Brain and spinal cord
Peripheral nervous system:
Nerves attached to the CNS that lie outside of the CNS
soma
The cell body of a neuron, which contains the nucleus
synapse
A junction between the terminal of the axon and the membrane of another neuron
axon
Long thin cylindrical structure that conveys information from the soma of a neuron to its terminal
multipolar neuron
: A neuron with one axon and many dendrites attached to its soma
neurotransmitter
A chemical released by the nerve terminal that has an excitatory or inhibitory effect on another neuron
types of neurons
Basic unit of the brain Three basic types Sensory neurons Motor neurons Interneurons
all neurons have
Cell body - (soma), which contains all the information needed to keep the cell functioning
Dendrites - fibres that receive information from other cells and conduct that information towards the cell body in the form of electrical impulses
Axon - the fibre, anyway from a few micrometers to over a meter in length, that conducts electrical signals away from the cell body to connections with other neurons
Neurons
Basic units of the brain and central nervous system (CNS)
Receive and transmit neural impulses across the synapses
Product of the neural tube of the developing embryo
Neurons migrate along pathways laid down by a network of guiding cells to form the major parts of the brain
The vast majority of neurons a person will ever have has already been formed by the end of the second trimester of pregnancy.
Scientists have recently established that formations of new neurons occur in the hippocampus (an area of the brain important for learning and memory) and these new neurons occur throughout life.
Neuron specialisation
Neurons assume specialised functions depending on where they migrate
Any neuron has the potential to serve any neural purpose (pluripotency)
If a neuron that would normally migrate to the visual area of the brain is instead transplanted to the hearing area of the brain, it will change to become an auditory neuron instead of a visual neuron.
Glial cells
Glial cells are the brain’s white matter, and they make up nearly half the human brain outnumbering neurons 10 to 1
They perform a variety of critical functions including the formation of the myelin sheath around the axons of the neurons
Glial cells also play a role in communication within the brain by
influencing the formation and strengthening of synapses
communicating among themselves in a network separate from the neural network
It is thought that people with schizophrenia or bipolar disorder may have a defect in a gene that regulates the production of myelin
Glial cell types: Astrocytes
Most common of the glial cells
Mop up excess neurotransmitters emitted from synapses
Feed neurons by supplying nutrients and neurotransmitter precursors
Control where and when neurons will make new synapses
Glial cell types: Microglia
Serve as the immune system in the brain
Oligodendrocytes
Extrude myelin for the sheaths around the axons of the neurons
NG2+ cells
Precursor cells to oligodendrocytes in white matter and possibly astrocytes in grey matter
Neurogenesis
Brain starts to develop in weeks 2 and 3, with the folding and fusion of the ectoderm to form the neural tube
Following on is a sequence of events that are genetically determined, epigenetically directed and environmentally influenced
Progenitor cells in the ventricular zone of the brain produce cells for the brain
Development of brain
Initially, the progenitor cells divide, making new cells that are identical
After 7 weeks post conception, a signal is received so that two different cells are produced as they divide – another progenitor cell and a brain cell
The first brain cells are radial glia. They have their body in the ventricular zone and they extend fibres radially outwards.
These radial glia have cup-like feet in the ventricular zone and the fibres extend to the pia mater, located at the outer surface of the future cerebral cortex
Brain development
The next set of brain cells produced by asymmetrical division are Cajal-Retzius cells
These C-R cells establish themselves in a layer beneath the pia mater
A second set of neurons form a layer beneath the C-R cells, and these neurons constitute the first of 6 layers of the cerebral cortex! This is neurogenesis
brain development continued
The cells leave the ventricular zone, pass the first layers of neurons, and establish themselves just inside the layer of C-R cells
Each successive wave of newborn neurons travels past the neurons that were born previously and establishes the next cortical layer
The radial glial cells guide the newborn neurons. The neurons move along the radial fibres and come up to rest next to the C-R cells
Chemicals secreted by the C-R cells cause the neurons to detach from the radial glia fibres and the neurons settle down
Neuron numbers
-Most neurons are present at birth, except for some granular cells in the cerebellum, the hippocampal dentate gyrus and the cerebral cortex, which form postnatally in response to environmental stimuli
-The number of neurons in the human brain peaks at around the 28th week of gestation at levels 40% greater than in the adult
-The average infant has far more neurons and neural connections than adults
-Neurons that successfully interconnect with other neurons crowd out those that don’t, so that about half the neurons produced early in life also die early
-Surviving neurons form hundreds of synapses, many of which will disappear if the neuron is not properly stimulated
The growth of dendrites of the neurons and arborisation rapidly accelerate around the time of the 28th week of gestation onwards
Synaptogenesis
Synaptogenesis is the formation of synapse connections between neurons
Synaptogenesis proceeds rapidly during the brain growth spurt
The peak of synaptogenesis occurs at the 34th week of gestation in humans at the rate of 40,000 new synapses per second
Synaptogenesis continues in postnatal life, with a balance between an increasing rate of synapse elimination and a declining rate of synapse formation. The net number of synapses begins to decrease at puberty
Apoptosis and synaptic pruning
-If neurons are not used, then they will die via the process known as apoptosis and their connections with other neurons will die – this process is called synaptic pruning. Neurons and their synapses that receive the most stimulation continue to function
-The exact timing and dynamics of apoptosis (programmed cell death) and synaptic elimination processes are not well understood at the moment
-Whether synaptogenesis and the elimination of cells and synapses counterbalance one another between 2 and 7 years of age, or whether the net number of synapses increases through these years is unclear
-Synaptic pruning is not visible directly with neuroimaging as yet
Please note how important the early experiences of the infant are, in terms of development of the brain
Experience and brain development
The important work of Austin Riesen and his colleagues in the 1940s and 1950s highlights the important role of early experience and the development of the brain and CN.
Reisen reared infant chimps in the dark for periods of up to 16 months. He showed that dark-reared chimps experienced atrophy of the retina and the neurons of the optic nerve
If the chimp was exposed to light before 7 months then the atrophy was reversed. But if not, then the chimp’s atrophy was irreversible and often led to total blindness if the deprivation lasted longer than one year
Neurons that are not properly stimulated will die
experience and brain development continued
Animals that are reared with lots of companions and with a wide variety of toys to play with have brains that are heavier and display more extensive networks of neural connections than those of litter-mates raised in standard laboratory conditions
If these animals that are raised in an enriched environment are moved back to the standard lab conditions, then their brains lose some of their complexity
Brain areas develop at different rates
- Not all parts of the brain grow are the same rate
- At birth, the most highly developed areas are the lower (subcortical) brain centres – these areas control states of consciousness, inborn reflexes, vital biological functions such as respiration, digestion of food and elimination of wastes
- Surrounding these structures are the cerebrum (outer grey matter aka cerebral cortex and white matter) and the cerebral cortex (outer grey matter), the areas involved in voluntary movements, perception, higher intellectual activities like learning, thinking, language etc
Growth of different brain areas
The first areas of the cerebrum to mature are
the primary motor areas (controlling things like waving the arms and kicking the feet) and
the primary sensory areas (controlling things like vision, hearing, smelling and taste)
These sensory and motor regions are functioning well at birth
By 6 months of age, the primary motor areas of the cerebral cortex have developed to the point that they control the infant’s movements
The inborn reflexes likes the palmar grasp and the Babinski reflex should hav disappeared by now
Thus the higher cortical areas are assuming proper control over the more primative subcortical areas of the body
myelination
Myelination is the process of insulating the axon of the neuron with white matter (80% lipid and 20% protein)
Some of the glia begin to produce a waxy substance called myelin, which forms a sheath around individual neurons
This sheath acts as an insulator to speed up the transmission of neural impulses, allowing the brain to communicate more efficiently with different parts of the body
Myelination begins at 7 months of gestation in the back of the brain and extends gradually to the frontal lobe by 9 months postnatal age
So it proceeds in a posterior to anterior direction
At birth, the pathways between the sense organs and the brain are reasonably well myelinated. So the neonate’s sensory equipment is working well
myelination continued
As neural pathways between the brain and the skeletal muscles myelinate, the child becomes capable of increasingly complex movement
Myelination proceeds very rapidly over the first few years of life, but some areas of the brain are not completely myelinated until the mid to late teens or early adulthood
The reticular formation and the frontal cortex are not fully myelinated at puberty. These are areas of the brain that allow us to concentrate and make good decisions etc. This may explain the lengthening of attention span as one grows up
Myelination enhances the efficiency of processing between the more primitive emotional subcortical areas of the brain and the more regulatory prefrontal cortical area.
An infant’s ability to process emotional expressions on another person’s face may improve as this myelination occurs
It may also underpin the development of Theory of Mind in the child
Brain lateralisation
The highest brain centre, the cerebrum, consists of two halves (or hemispheres) connected by the corpus callosum, a band of fibres
Each of the hemispheres is covered by a cerebral cortex – an outer layer of grey matter that controls sensory and motor processes, perception and intellect
The left and the right hemispheres serve different functions and control different areas of the body
The left cerebral hemisphere controls the right side of the body. It also contains centres for speech, hearing, verbal memory, decision making, language processing and expression of positive emotions
The right cerebral hemisphere controls the left side of the body, and contains the centres for procesing visual-spatial processing, nonlinguistic sounds such as music, tactile sensations and expressing negative emotions
Brain lateralisation may originate during the prenatal period and be well underway at birth
Summary
Neurogenesis is the development of new neurons
Synaptogenesis is the development of new synapses
Apoptosis is programmed cell death
Myelination is the insulation of axon of neuron with white matter
There are four types of glial cells
The brain is the most complex system we know
