The Human Brain
WHAT WE KNOW NOW is that this organ we call
"The Brain" is a complex organization of nerve cells. The Human Nervous System develops in an indomitable fashion. Germinal cells of the developing human embryo give rise to two primitive types of nervous system cells: Neuroblasts and Spongioblasts (a
blast being an immature cell).
Neuroblasts develop into the nerve cells which form the functional units of the nervous system.
Spongioblasts develop into glial cells (from
glia, Greek for glue), which provide various types of support functions to nerve cells. These two types of cells, Nerve cells and Glial cells, make up the bulk of the adult human brain.
Human Brain Development
Brain development has two very distinct features:
First, subcomponents of the nervous system are formed from cells whose distribution and function are
predetermined before they migrate from the wall of the ventricles.
Second, development is marked initially by an abundance of cells, branches, and connections, with an important part of the subsequent maturation process which consists of cell death and
pruning of this intital surfeit!
The processes of human nervous system growth and differentiation consist of a series of biological changes which occur at precise times and in relatively rigid sequences. Those biological changes include
1) Cellular Migration,
2) Cellular Differentiation (the formation and growth of Axons, Dendrites, and Synaptic connections) and,
3) Myelination.
Cellular Migration:
The Neuron
As nerve cells are called; Neurons have two special properties that distinguish them from all other cells in the body. First they can conduct bioelectric signals for long distances without any loss of signal strength. Second, they possess specific intracellular connections with other nerve cells and with innervated tissues such as muscles and glands. These connections determine the types of information a neuron can receive and the range of responses it can yield in return.
Nerve cells are the primary functional and anatomical cells that act as individual integrating units of the nervous system. Nerve cell bodies are usually located in groups. When located outside of the brain and spinal cord, they are aggregated into communities called
Ganglia. Within the brain and spinal cord, neurons form groups of various sizes and shapes known as
Nuclei, each of which make special contributions to behavior. Neurons are
malleable: They can change their behavior with experience, they can learn, they remember, and they can forget.
Neurons are formed by the mitotic division of neuroblasts within the inner ventricular lining of the embryonic brain. One neuroblast will give rise to two daughter cells that will either migrate or undergo further mitosis. By the middle the second trimester of gestation, the division of the neuroblasts is complete. The exact mechanisms that control migration and determine the destination of the migrating cells are not fully understood. It is known, however, that specialized filaments provide pathways for the migrating cells to follow. Migration of these cells to their target sites may continue for several months prenatally, or even perhaps postnatally!
After a group of migrating neuroblasts has arrived at the surface of its target site, they begin to sprout and elaborate one or more axonal and dendritic projections; differentiation and maturation begins. Subsequently, a new group of daughter cells migrate from that inner lining through those layers already present to form a new outer layer. Hence, a structure such as the Neocortex matures from its inner to its outer surface.
Once formed, neurons do not regenerate and, unless they suffer lethal damage, must live as long as the person in which they are found. There are however, two exceptions: The first is that during development, far more neurons develop than will ultimately survive, so cell death is a pronounced developmental stage in structuring the cytoarchitecture of the adult brain. And the second is that those neurons which leave the Central Nervous System (CNS), such as sensory and motor neurons, retain their capacity to regenerate.
Cellular Differentiation:
Each neuron is enclosed in a specialized Cell Membrane and consists of aCell Body (or Soma), with multiple short fibers, termed Dendrites (Greek for tree), which receive information from other neurons.
Each neuron also has extending from the cell body a fiber known as an Axon (Greek for axle) which may be either long or short, and little end feet, or Terminals , at the ends of the axon. The axon transmits information from its cell body to other neurons in the form of action potentials, or nerve impulses. Each neuron is believed to both send and receive thousands of electrical impulses or stimuli.
(See 
Figure A )
The Cell Membrane
This is an organell which surrounds the entire cell and is composed almost entirely of double layered phospholipid and cholesterol molecules. One part of which is soluble in water, (
hydrophillic), while the other part is soluble only in fats,(
hydrophobic). The phosphate radical portion of the phospholipid is hydrophillic, and the fatty acid radical portion is fat souble.
A special feature of the lipid bilayer is that it is a
lipid-fluid and not a solid. Hence, portions of the membrane can literally flow from one portion to another in the membrane.
This specialized structure provides the neuron with four basic properties:
1)
It contains channels - specially designed spaces that allow some ions in and out of the cell; since the membrane by its very nature is a major barrior impermeable to the usual water-soluble substances such as ions, glucose, urea, and others.
2) The membrane is the site of the electrical charges that are instrumental in information transmission. Since it seperates two fluid compartments, (intracellular and extracellular - each of which containing many ions), a negative charge results on the inside of the cell in the range of -75mV (millivolts).
3) The membrane contains receptors to which peptides and hormones attatch. These compounds may be transported into the cell and carried to the cell nucleus. The hormones act on chromosomes in the nucleus to initiate such processes as protein synthesis, etc.
4) The membrane contains sites for axon terminals as well as receptors. Transmitter substances from the terminals of one neuron attatch to the receptors of another, where they induce changes in the electrical charge across the cell membrane.
The Cell Body (Soma)
The Soma is that area of the cell which surrounds the nucleus. It contains the different substrates that occcupy the cell, collectively called
Protoplasm (composed mainly of five basic substances: water, electrolytes, proteins, lipids, and carbohydrates), along with mitochondria, microtubules, microfilaments, and most of the ribosomes, lysosomes, and the endoplasmic reticulum of the cell.
It is the site for synthesis of virtually all of the neuronal proteins and membranes. Here newly synthesized macromolecules are assembled into membranous vesicles or multiprotein particles, which are transported to other regions of the neuron.
The Dendrites
Organells that collect information (impusles), which is then integrated at the
Axon Hillock close to the cell body. A summary of the input received by the cell is then passed along the axon and through the terminals to other cells. Dendrites develope in a fashion quite different from axons. Their growth starts only after the cell has reached its final position in the cortex and then only at a relatively slow rate. The growth pattern of dendrites is similar to that of axons; growth and division also occurring at a growth cone. Dendritic growth, however, seems to be timed to intercept the axons that are to innervate them.
Even though dendritic development begins prenatally in humans, it persists for extensive periods postnatallly. Before birth, there are only a few dendritic spines on dendrites, but postnatally they begin to develop and densely cover the maturing dendrites. During development, cells undergo stages in which they have an overabundance of branches and spines which are subsequently lost. The loss of dendrites and spines is also referred to as pruning. The remaining branches may undergo extensive growth and branching.
The Axon
These organells begin sprouting from neurons as they migrate to their respective target sites. Sprouting axons grow in a given direction presumably because the cell body is oriented in a particular direction or because of some other factors. Generally the axon grows at a rate of 7 to 170 µm per hour.
The growing end of the axon, known as the
Growth Cone, is the site also where branching occurs. This highly mobile structure moves along those substrates to which it adheres the tightest. As the growth cone moves outward, the cell body stays stationary and the axon elongates due in part to the polymerization of tubulin into microtubules that give the axon its characteristic rigidity.
Growth cones on different neurons have different and changing cell-surface receptors that enable them to recognize and move along cells and matrix components they encounter as they elongate to their specific targets. The specificity in guidance of any axon seems to be dependent on an inherent set of adhesive molecules on the individual growth cone. More specifically, a growth cone probably moves from one short-range target to another as it guides an axon to its destination. It is possible that the growth cone forms synapses and retains the capacity to renew growth, which may thus underlie the formation of new synapses during the course of learning.
Just what is it that causes the progression and guidance of axonal growth? Some axonal growth takes place because they are towed from their cell bodies by a structure that is growing away from the region, such as when muscles grow away from the spinal cord early in development. Other axons traverse enormous distances and grow because they are able to overcome obstacles such as being moved to another location, having thier cell bodies rotated, or having their targets moved. The forces that guide this homing mechanism are not fully understood. It may be possible however that: Axons may follow an electrical or chemical gradient or a particular physical substrate, or they may send out multiple branches, and when one reaches an appropriate target, the others follow.
See Part-2 of
"Hûman Brain Çyto Architecture"
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