Neuronal specification

Neuronal specification
Neuronal specification refers to the processes o that generate, shape and reshape the nervous system from the earliest stage of the embryogenesis to the final year of life
There are href="http://chempapy.blogspot.com/">eight stages through which neurons develop. These are is neurogsenic. This is associated with induction and pattering e.g. neuron forming region to be referred on formation of such region
During early embrionic development the ectoderm become specified to give rise to epidermis(skin) and neural plate. The conversion of the un differentiated ectoderm to neural ectoderm require signals from the mesoderm. At the onset of gastration presumptive mesoderm cell move through the dorsal bastopore lip and form a layer in between the endoderm and the ectoderm. These mesodermal cells that migrate around the dorsal midline give rise to structure called notochord. Ectodermal cells over lie ther notochord develop into neural plate in response to signal produced by notochord.

Neuronal Types
The human brain consists of over 1011 neurons associated with over 1012 glial cells.Those cells that remain integral components of the neural tube lining become ependymal cells.These cells can give rise to the precursors of neurons and glial cells. As we have seen above, it is
thought that the differentiation of these precursor cells is largely determined by the environment that they enter (Rakic and Goldman 1982) and that, at least in some cases, a given ependymal cell can form both neurons and glia (Turner and Cepko 1987).The brain contains a wide variety of neuronal and glial types (as is evident from a comparison of the relatively small granule cell with the enormous Purkinje neuron). The fine
extensions of the neuron that are used to pick up electrical impulses from other cells are called dendrites (Figure 12.22). Some neurons develop only a few dendrites, whereas other cells (such as the Purkinje neurons) develop extensive dendritic trees. Very few dendrites can be found on
cortical neurons at birth, but one of the amazing things about the first year of human life is the increase in the number of these receptive regions. During this year, each cortical neuron develops enough dendritic surface to accommodate as many as 100,000 synapses with other neurons. The average cortical neuron connects with 10,000 other neural cells. This pattern of synapses enables the human cortex to function as the center for learning,reasoning, and memory, to develop the capacity for symbolic expression, and to produce voluntary responses to interpreted
stimuli.Another important feature of a developing neuron is its axon (sometimes called a neurite). Whereas dendrites
are often numerous and do not extend far from the neuronal cell body, or soma, axons may extend for several feet. The pain receptors
on your big toe, for example, must transmit their messages all the way to the spinal cord. One of the fundamental concepts of neurobiology is
that the axon is a continuous extension of the nerve cell body. At the turn of the twentieth century, there were many competing theories of axon formation.Theodor Schwann, one of the founders of the cell theory, believed that numerous neural
cells linked themselves together in a chain to form an axon. Viktor Hensen, the discoverer of the embryonic node, thought that the axon formed around preexisting cytoplasmic threads betweenthe cells. Wilhelm His (1886) and Santiago Ramón y Cajal (1890) postulated that the axon was an
outgrowth (albeit an extremely large one) of the neuronal soma. In 1907, Ross Harrison
demonstrated the validity of the outgrowth theory in an elegant experiment that founded both the science of developmental neurobiology and the technique of tissue culture. Harrison isolated a portion of the neural tube from a 3-mm frog tadpole. (At this stage, shortly after the closure of the neural tube, there is no visible differentiation of axons.) He placed this neuroblast-containing
tissue in a drop of frog lymph on a coverslip and inverted the coverslip over a depression slide so
he could watch what was happening within this "hanging drop." What Harrison saw was the
emergence of the axons as outgrowths from the neuroblasts, elongating at about 56 μm/hr.
Such nerve outgrowth is led by the tip of the axon, called the growth cone (see Figure
12.23). The growth cone does not proceed in a straight line, but rather "feels" its way along the
substrate. The growth cone moves by the elongation and contraction of pointed filopodia called
microspikes. These microspikes contain microfilaments,which are oriented parallel to the long axis of the axon. (This
mechanism is similar to the one seen in the filopodial microfilaments of secondary mesenchyme cells in
echinoderms; see Chapter 8) Treating neurons with cytochalasin B destroys the actin microspikes, inhibiting
their further advance (Yamada et al. 1971; Forscher and Smith 1988). Within the axon itself, structural support is
provided by microtubules, and the axon will retract if the neuron is placed in a solution of colchicine. Thus, the
developing neuron retains the same mechanisms that we have already noted in the dorsolateral hinge points of the
neural tube; namely, elongation by microtubules and apical shape changes by microfilaments. As in most migrating cells,
the exploratory filopodia of the growth cone attach to the substrate and exert a force that pulls the rest of the cell
forward. Axons will not grow if the growth cone fails to advance (Lamoureux et al. 1989). In addition to their
structural role in axonal migration, the microspikes also have
a sensory function. Fanning out in front of the growth cone,each microspike samples the microenvironment and sends
signals back to the cell body (Davenport et al. 1993). As we will see in Chapter 13, the microspikes are the fundamental
organelles involved in neuronal pathfinding.

Birth and migration of neuron and glia
neuronal migration is the method by which neuron travel from their origin or birth place to their final position in the brain
The specification of cell fate
The first decision is whether a given cell is to become a neuron or epidermis, if the cell is to become a neuron next decision is what type of neuron it will be, whether it is to become a motor-neuron, sensory neuron,a commissural neuron or other type.
The guidance of axonal growth cones to specific targets
Among the many extra ordinary feature of nervous system development. One the most fascinating is the ability of growing axons to navigate through a complex cellar embryonic terrain to find appropriate synaptic partners that may be millimeter or centimeters.
The formation of synaptic connection
This involve neuron-muscular junction formation. It connect the nervous system to the muscular system via synapses between efferent nerve fibres and muscle fibres also known ass muscle cells.binding to trophic factor for survival and diffentiation
The survival of neuron is regulated by survival factor called trophic factor . These factor produced by a number of sources contribute to neuronal survival. These factor are as follow.
Nerve growth facto( NGF)
Ciliary neurotrophic factor ( CNTF) is another protein that act as survival factor for motor neurons.
Glial derived neurotrophic factor is a member of the TGF6 family of proteins and is a potent trophic factor for striate neurons.
Synaptic elimination
This results due to competitive re-arrangement of function synapsis (synaptic elimination). Several motor neuron compete for each neuronmuscular junction , but only one survive untill adulthood Continued synaptic plasticity during the organism life time
The elongation of the axon formed leads to the.Is the ability of asynapse betweentwo neuron to change in strength over time and is caused by synaptic potential and the receptors used to relay chemical signals . Synaptic plasticity plays a large role in learning and memory in a the brain
NEURAL SPECIFICATION
Neurons are specification hierrachia manner . First decision is whether a given cell is ti become a neurone or epidemis if the cell is to become a neurone, next decision is what type of neurone it will be . Whether it is to become a motor neuron a sensory neuron or a commissural neuron . After this fate is determined.
AXONAL SPECIFICATION
In both cases the specificity of axonal connections ia seen to to unfold in three steps which are.
Pathway selection: The axon travel along route that to leads them to a particular region of the ambry.
Target selection . The axons once they reach the correct area , recognize and bond toa set cell with which they may form stable connections.
Address selection . The initial patterns are repined such each axon binds to a small subset of its possible target.First two process are indepent of neuronal activity ,The third process involve interactions between several active neuron and converts the overlapping projections into a fine turned of connection



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