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Transcript of Neural Development
Development of the Neural Tube & Neurons
The neural tube of embryonic chordates is formed by the infolding of ectoderm followed by elongation of the tube:
all species in Phylum Chordata develop a dorsal nerve cord at an early stage in their development
this process is called neurulation
in humans, neurulation occurs during the first month after conception
an area of ectoderm cells on the dorsal surface of the embryo develops into the neural plate
cells of the neural plate change shape, causing it to fold inwards, forming a groove along the back of the embryo - this becomes the neural tube
So we have neurons...now what?
Immature neurons migrate to a final location.
Neurons migrate in a way that is similar to the movement of
Cytoplasm and organelles are moved from the trailing end of the neuron to the leading edge by contractile actin filaments.
Neuronal migration is especially important to brain development, as some neurons are produced in one part of the developing brain, but migrate to their final position.
A developing neuron forms multiple synapses.
the growth of an axon (or dendrite) is directed so that it connects to another neuron (or cell) so they may communicate
a synapse is developed between the two neurons, or a neuron and another cell - this involves development of special structures on each cell's membrane, and in the synaptic cleft between
most neurons develop multiple synapses (minimum 2), and those in the brain may develop hundreds!
The plasticity of the nervous system allows it to change.
connections between neurons can be changed by
growth of axons and dendrites
establishment of new synapses
elimination of synapses
pruning of dendrites, branches of axons, or entire neurons
this continues throughout life, but slows down considerably after age six
plasticity is the basis for forming memories and certain forms of reasoning, and is important to repairing damage to the CNS
Neurons are initially produced by differentiation in the neural tube.
of the billions of neurons in the CNS, most are located in the brain
early in embryonic development, part of the ectoderm develops into neuro-ectodermal cells in the neural plate
the neural plate develops into the neural tube, and proliferation of cells (by mitosis) and differentiation leads to the billions of neurons that make up the nervous system
Incomplete closure of the embryonic neural tube can cause spina bifida.
the vertebrate spine consists of a series of bones called vertebrae
each vertebra has strong centrum and a thinner arch, which encloses and protects the spinal cord
early in embryonic development, the centrum develops on the ventral side of the neural tube
tissue grows up and around the spinal cord, eventually meeting on the dorsal side, forming the arch
improper fusion of the arch, likely caused by the neural tube not closing properly, leaves a gap - this condition is called
An axon grows from each immature neuron in response to chemical stimuli.
immature neurons consist of a cell body with cytoplasm and a nucleus
the axon is a long, narrow outgrowth of the cell body
axon branches and dendrites may develop, depending on the type of neuron - this is directed by chemical stimuli in the developing embryo
Some axons extend beyond the neural tube to reach other parts of the body.
like the roots of plants, axons grow at their tips
within the CNS, axons are short, connecting to other neurons in the CNS
axons extending into the PNS can be very long:
in humans, axons can be more than 1 m long
in larger mammals, axons can be several metres
as long as the cell body is intact, the axon may be able to regrow if severed or damaged
Synapses that are not used, do not persist.
many synapses are formed during fetal development
new synapses can be formed at any time of life
when transmission occurs at a synapse, chemical markers are left behind, causing the synapse to become stronger
inactive synapses do not have these markers, and are eventually eliminated
Neural pruning involves the loss of unused neurons.
the brains of newborn babies have more neurons than adults
some neurons lose dendrites and/or axon branches
unused neurons undergo apoptosis