Spinal cord

Spinal cord

      The spinal cord is a solid cylinder with a mean diameter is 10 mm although the size is not the same throughout its length. Its length is relatively constant in adults and equal to 45 cm. It has two bulges, the
cervical and one lumbosacral portions corresponding to innervating the upper limbs and the lower limbs.
As the growth in length of the spinal cord is in the ontogeny less than that of the spine, are not found in the medullary tissue of lower portion the spinal canal is occupied by the last against spinal roots constituting the "ponytail"
   As the growth in length of the spinal cord is during ontogeny less than that of the spine, are not found in the medullary tissue of part lower spinal canal is occupied by the last against spinal roots constituting the "ponytail".
    Although the bone is anatomically continuous and non-segmented structure, we see that his organization morpho-functional is not fundamentally less segmental. Called segment meta mere  or spinal anatomy overall, but especially functional consists of a "medullary slice", 

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The hippocampal formation

The hippocampal formation.

     The hippocampus, named for its shape reminiscent of the marine animal that name, receives inputs from virtually all regions of the cerebral cortex, which run along a sequence of three successive synapses. As indicated by the anatomical data, the hippocampus works in a loop, in the manner of a rotunda in which one enters and leaves turning. Its role can be summarized as that of a comparison between the state of the world and its emotional value. The first is provided from data from the sensory cortex, the second through two-way connections with the nucleus accumbens.

     The round of nerve impulses in the hippocampal circuit performs rhythmically with periods of 10 to 200 milliseconds. This oscillatory activity plays an important role in the formation of memories, and is particularly strong in the dream phases and suggests a possible link between memory and dreams. Given its interconnections with the cingulate cortex and the mammillary bodies, hippocampus certainly plays a major role in emotional processing and memory.

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The limbic system

 The limbic system

     According to Paul MacLean (1958), theoretician of a brain into three parts according to evolutionary criteria, the limbic system is the pale-mammalian brain, the seat of motives and emotions. It is able to respond to this information by using the memory of past information. Therefore involved in emotional processing, learning and memory.

The limbic system is formed by a complex combination of nerve centers and their communication channels that border (border says Latin limbus) the brain-stem. This system, which is located at the base of the cerebral cortex, is part of a functional neuroanatomy together great limbic lobe appointed by Paul Broca in 1878 (see fig. 1). Two pairs and symmetrical nerve structures, located in the middle regions of the brain are at the heart of emotional processes: the hippo-campus and the amygdala. The first is responsible for land management and relational maps about the world. He is also heavily involved in the formation of memories. The second is buried deep within each temporal lobe. Both tonsils used to recognize emotions including fear on the face of the other, to express and to develop packaging associated with situations that are themselves nothing scary. With these structures, the subject learns to associate the pleasure and pain to an object or situation and estimate the intensity and value (hedonistic or aversive) of a stimulus.

Images of the limbic system:

 

 

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Human brain

     Human brain

    Understanding the organization of brain tissue and its logic is one of the central questions of modern neurobiology. But the human brain is the most complex structure we know alive. This body is not homogeneous and its complexity is expressed by the juxtaposition of different areas whose functions are more or less well specified.

On the basis of anatomical and functional data, we identified two brain regions. The outermost part of the brain, cerebral cortex, which envelops the whole cerebral mass, consists of networks relatively insensitive to genetic regulation. However, it is constantly remodeled by the subject's experience. Neural networks of the cerebral cortex are unstable, malleable, partly innate but largely influenced by epigenetic factors. In contrast, the deeper regions of the brain, internal and basal position relative to the cortex, ie the basal brain react structurally very little to change whether environmental or experimental, these structures are stable, genetically specified and ancient evolutionary origin (see nervous system - Neurogenesis). We called it wrongly brain, here referred to as "basal" reptilian. We will see how its sub-cortical areas, managing all vital functions, ensure the perfect match of a subject in its environment.We now know, thanks to the reverse genetics, that the same genotype can give rise to a large number of phenotypes in the brain, or a single phenotype can adapt continuously by epigenetic modifications. This reciprocity between epigenetic gene and shows how it is essential to take into account the developmental mechanisms that contribute to the construction of neural networks in the adult brain to better understand the overall behavior of adaptive type. Note here that the concept of adaptation of the organism to its environment (individuation) interested, above all, the central nervous system (CNS), which is the only one to integrate and manage information from the outside world. In other words, understanding the individuation seen as the result of cognitive processes (perception, language, memory, consciousness ...) returns to try to understand how the history of a subject is part of the CNS. But this story is recorded in the deep structures of the basal brain involved in the management of personal relationships, and extra-corporeal time of the organism with its environment.To account for the anatomical and functional characteristics of the nervous system, it is necessary to distinguish the two components, one central and one peripheral. The peripheral component includes, on the one hand, sensory neurons, connecting the CNS to sensory receptors and, on the other hand, neurons that innervate the muscles and viscera. Describes easily sensory pathways and motor pathways through a simple division of the nerves, or localized transects performed on the nervous centers that distinguish (as shown Magendie) centripetal sensory pathways and motor pathways (or inhibitory ) centrifugal. The second component to which these pathways are connected is called central because it is formed by the nerve centers: the spinal cord and the brain.

Because of the importance of the cerebral cortex are referred to as basal all structures located in the middle part of the brain that frame and hide the two cerebral hemispheres. They include back and forth and up and down the brain stem (Fig. 1), the encephalopathy, limbic system and basal ganglia which we successively analyze the role. These sub-cortical regions are defined by either stratified as in the cerebral cortex, but grouped into nuclei neurons.Like any classification, this division of the mammalian brain cortex in one hand, and basal regions, on the other hand, has its limits. If well aware of anatomical observations, it does not always correspond to a functional reality. The limbic system, for example, includes both cortical areas and sub-cortical areas. However, we consider separately here the properties of sub-cortical structures with emphasis on the rules governing the organization and major functions they perform in relation to the cortex. In this second part will be devoted to this article. We then consider as imaging can see the brain function before determining the study of brain plasticity.

 The basal brain

The brain-stem

The brain, with its two hemispheres, based on a region called the brainstem, consisting, from front to back, the midbrain, the bridge and the medulla. The latter joined through the foramen magnum, the spinal canal and extends through the spinal cord. It is traversed by descending pathways, which carry the electrical signals from the brain to the motor neurons, and ascending pathways that carry sensory information from the body and the outside world to the brain. Although the volume is relatively low compared to the brain, the brain stem is an essential structure for the survival of an individual. Accidental injury of this structure reflected nervous.

In the brain-stem, several nuclei occupy the space vacated by the fiber bundles. They participate in large vegetative functions and enable the integration of body signals. They consist of sensory and motor nuclei that support many signal processing related to eye function, vestibular functions (those in the inner ear) and hearing, but also the sensitivity and motor control of the face, the mouth, throat, respiratory system and heart. Alongside these nuclei, which are connected to the cranial nerves (see brain-stem), groups of neurons broad spectrum of action are also present. The latter, who receive information from what goes to the brain or down, plan (that is to say communicate their ramifications) diffusely in the brain on the one hand and the spinal cord on the other hand . Their free ends of mineralogical neurotransmitters (catecholamines, serotonin, oxyacetylene) that transmits instructions "modulator" type and broad-spectrum spatio-temporal distribution
 

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Prokaryotic and eukaryotic

    Prokaryotic

      Bacteria are so small (1 micron in length) so they are subject to violent thermal agitation of the medium (Brownian motion). Yet they are able to move towards food sources or avoid adverse environmental conditions, moving to the considerable speed for their size, from 20 to 30 micrometers per second. Most often, they are using flagella, which function as a boat propeller. In the intestinal bacterium Escherichia coil, for example, each flagellum is a rigid filament 14 thousandths of micrometers in diameter and 10 micrometers long, which runs at the incredible speed of 200 revolutions per second with a small rotary engine inserted into the membrane and the cell wall (fig. 2). Flagella having a propeller straight shape, its rotation in the direction of clockwise when viewed from the bacterium to the flagellum, produces a thrust on the medium which causes the movement of the bacterium in the opposite direction positive thrust. Different flagella cooperate to produce a rectilinear movement of the bacterium, forming a single braid compatible with the helix in a straight form flagella and their individual rotation. Such a hard movement of the order of one second, after which the cell changes abruptly, and randomly, by pivoting direction on site, with a reversal of the direction of rotation of the flagella, which is incompatible with maintaining a organization of flagella in a single braid producing traction. A change in the frequency of changes in direction of the bacteria caused by signals from the environment to which it is sensitive allows him to move.
Some bacteria have developed a very different alternative means of moving the swim. These pathogenic bacteria, whose development in the body is intracellular. Thus, the bacterium responsible for listeriosis (Listeria monocytogenes) is able to give its flagella when it enters a cell and use it to his benefit systems molecular assemblies of the host cell to pass very quickly, and invade step by step, the cells in the body, thus escaping immune surveillance. Indeed, with some molecules it synthesizes and presents on its surface, this bacterium can govern the assembly of actin microfilaments, forming a so-called comet tail in its wake structure. This mechanism is now the subject of many studies. It is indeed convenient for understanding the assembly mechanisms at the origin of amoeboid movement type in front of the eukaryotic cell model system: it can be observed in a cell-free extract and interested physicists as well as biologists.

 Eukaryotic 

Numerous unicellular eukaryotic species move through ciliatures or flagellar apparatus of great variety, sometimes consist of several thousand flagella or cilia. However, all eyelashes and all flagella have, with few exceptions, the same basic structure, based primarily on microtubules. Cilia or flagella eukaryotic cells are fundamentally different bacterial flagella in size, at least ten times greater in diameter, and in their operation in that, while projecting outside of the cell body, they remain surrounded by the plasma membrane and intracellular organelles are therefore, unlike the bacterial flagellum (Fig. 2). The distinction between lash and flagella in eukaryotes based on their type of beat and not their structure. The difference between the beating of cilia and flagella explains that flagella are often significantly longer than the eyelashes. Flagella push or pull the cell body through trains symmetrical waves that propagate from one end to the other. Cilia function more like oars, or like the arms of a swimmer, and an asymmetrical movement in which one can distinguish an active support of the surrounding liquid phase and a recovery phase which produces no movement . These ciliary or flagellar beating are not completely stereotyped as protozoa are capable of a wide variety of behaviors depending on environmental conditions. Consider two examples, one in ciliates and flagellates in the other.
Swimming cells occurs through a coordinated beating of cilia or flagella. The movement of the cell in a given direction is often accompanied by a slow rotation of the cell body. This is the case for the paramecium, ovoid cell of more than 100 microns in length which has several thousand lashes evenly distributed in longitudinal rows (along meridians). Beat is synchronous with a row of eyelashes to each other in the transverse direction but has a small phase shift along a row, which causes a métachronale spiral wave at the surface of the cell which results in a slow rotation. Paramecium face an obstacle can change course when reversing, under the control of a calcium signal, the beat of her lashes, which has the effect of reducing. Then the ciliary beat stops temporarily on the entire surface of the cell, except in the buccal cavity, and very rich in eyelashes located in an asymmetric position on the front of the cell. This produces a rotation of the axis of the cell body. When the normal beat resumes, the direction of forward motion forms an angle of tens of degrees with the previous management. Paramecium also has answers for many environmental signals, including the gravity field.
For its part, the unicellular green alga Chlamydomonas biflagellate reinardtii moves forward with an asymmetric beat (ciliary type) from its two flagella, whose bases are oriented at 450 with respect to the axis of the cell body, and the direction of movement. The beating of the two flagella occur symmetrically with respect to the plane of symmetry of the flagellar apparatus. There is however a slight shift between the beats of the two flagella, causing again a slow rotation of the cell body. This rotation allows him to explore the environment, particularly to locate the source of light through a photoreceptor structure in an asymmetric position, the stigma (or eye spot), which can, by means of transduction in the plan a governing of the membrane to calcium influx base flagella, result in modification of one beat only two flagella. As a rower using oars asymmetrically to tack the cell and changes its direction and will seek the light they need to photosynthesize. This alga is also able to retreat, especially if it is subjected to bright light, thanks to a particularly elaborate behavior: the flagellar bases (kinetosomes) normally oriented at 900 with respect to each other, can become parallel with the contraction under control calcium small structures that connect them. Two flagella then adopt a flagellar beating type and produce a rearward thrust comparable to that of a propeller. Other solutions have been selected during the evolution of unicellular algae, which can be four or even eight flagella, to ensure the transition between moving forward and backward movement.
There are many other examples. Thus, some unicellular ciliates have a large ciliature extremely differentiated by region of the surface. Groups of eyelashes can be physically associated with each other, forming tendrils whose beat is up, and can be used not only to swim but also to "walk" on surfaces. Other examples can be found in unicellular algae that use their two flagella to capture their prey.

 

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Cell movement

   The movement is with the reproductive capacity, one of the properties that define the living. At the cellular level, certain movements can be observed with the conventional optical microscope, while others occur at a scale of quasi molecular size and require more sophisticated microscopes. Indeed, specialized structures in the production of movement, such as muscle or myofibril sperm flagellum, are true molecular machines whose dimensions are only slightly higher than those of macromolecules that constitute them.

   The mechanochemically basis of cell movements are all based on the use of a nucleotide, adenine tripolyphosphate (ATP) and guano-sine tripolyphosphate (GTP), which controlled hydrolysis provides the energy needed to move. The connection between two phosphates attached to these molecules represents the universal energy source in living.

In this article, we will limit ourselves to the study accessible to direct observation events. After characterizing the magnitude of movements across the living, the main modes of transport at the cellular level will be examined, that is to say, swimming and crawling. This leads us to wonder about the behavioral problems that locomotion reveals: responses to the signals they have an adaptive advantage that could offset their energy cost compared to moving randomly?

What may seem an advantage from an evolutionary perspective remains to be assessed. In this debate, abnormal cell movement mechanisms provide useful lessons

• the phenomenon of Speed :

The speed range of the movements observed in the living is very wide. Swimming cells in a liquid medium are moving at speeds between 0.1 and 1 millimeter per second. The migrating cells on a surface is much slower, moving from 0.01 micrometer per second for fibroblast 0.1 micrometer per second for the neutrophil leukocyte blood. Amoeba are able to move much faster than 10 micrometers per second.

 

Specialized sub cellular structures in motility may produce much more rapid movement. The spread of the beat wave of an eyelash or a Eucharistic flagellum is one millimeter per second, while a muscle or some primitive contractile structures can contract at speeds of the order from 10 to 100 millimeters per second. It is with such performance at the basic structures of a champion sprint can move at a speed of about 10 meters per second, through the coordination of the muscles provided by the nervous system, c is also the case for selected during the development of their ability to run the animals, like the cheetah to catch its prey, or the horse to escape predators. This principle of coordination of actions which can increase performance when assembling basic structures is valid at all scales, from molecules to organisms.

Intracellular movements also occur on a large scale speed. Slow axon transport of the neurons at a rate of about 0.01 microns per second, and the rapid transport of the order of 1 micrometer per second. The movement of chromosomes during phase has an intermediate speed. Some intracellular movements can be much faster, in specialized cells such as melancholias or erythrocytes through which certain animals, such as fish, can rapidly change color to blend with the environment: the movement of charged particles of pigment can are more than 10 micrometers per second. The same goes for the cytoplasmic currents some giant algal cells, up to a size of a few millimeters to several centimeters. Internal cytoplasmic currents amoeba Phys-arum plasm-odium in its shape even faster and can reach a speed of 1 mm per second.






 

 

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development of the nervous system

• The first week of development of the fertilized egg to the button embryonic.

The human body is the result of a merger between a male sex cell (sperm) and a female sex cell (ovum). Upon fertilization, the divisions cell (or mitosis) successive lead to the formation of an egg is 2, then 4, 8, 16, 32, 64 cells. These will then push to the periphery to form a button
embryo, which derive the embryo. 

• The second week of development: the embryonic disc at first slip.

At this stage, the cells of the embryonic bud keep dividing and begin to differentiate by having two superposed layers, the most superficial give entire future embryo.

•  The third week of development of the neural plate to neural groove.

This first sheet differs in turn three layers, the most superficial epidermal ectoderm give the skin and the entire nervous system. Ectodermal region located on the center line is differentiated into neural plate, while the rest operates to give the epidermis, the outermost layer of the skin. Then, the neural plate thickens unevenly: the thin part will become the spinal cord and the brain give swollen portion. The lateral edges of the neural plate and forming a raised neural groove.

• The fourth week of development of the neural groove to neural tube and neural crest.   

The embryo is the overall shape of the newborn and the future. Both edges of the neural groove meet on line
middle to form the neural tube, which becomes the system CNS. When the neural tube closes, longitudinal strip detaches from each of its two sides, forming the neural crest, which will give the peripheral nervous system.

• Over the coming weeks and months.  

 The neural tube forms the entire Central Nervous System:
The posterior part of the neural tube remains hydrochloric cylindrical, straight, small arms and gives the spinal cord. Its anterior portion swollen, further increases considerably in volume and undergoes many curves and changes which result from the different parts of the brain: the brainstem, cerebellum and brain. Together, the cavity of the neural tube wife changes its shape and ventricular wall cavities filled with cerebrospinal fluid (CSF).
The neural crest forms Peripheral Nervous System:
The neural crest cells give rise to neurons of dorsal root ganglia, which are the primary sensory neurons, as well as Schwann cells (cells myelinating PNS).

• Manufacturing cells Central Nervous System.

The neural tube wall is made of neural stem cells. After proliferation, and these precursors derived differ: to neurons or to cells glia. As for microglial cells, which are mainly derived from blood cells (monocytes), which come from another germ layer. 

 

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Ependymal cells

Ependymal cells (or épendymocytes) are cousins ​​astrocytes. They provide the coating
ventricles of the CNS and thus play a role in exchanges between the CNS and cerebrospinal fluid (CSF)
contained in these cavities.
The ependymal cells in the ventricles are loosely joined together by special intercellular adhesion sites called desmosomes, which enable the cells to form a nearly continuous epithelial sheet over the surface of the ventricles and spinal canal. Because the junctions between the ependymal cells are loose, CSF is able to diffuse from the ventricles into the central nervous system. The cells surrounding the choroid plexus are connected by tight junctions, which prevent the leakage of substances and fluids from the blood vessels into the CSF. This protects against the unregulated entry of potentially harmful substances into the ventricles and ultimately the central nervous system.

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Astrocytes

     The cell body contains the nucleus of astrocytes and is of a star shape - hence the name - because of the many cytoplasmic variously branched who leave. Astrocytes are characterized by abundance in the cytoplasm of a particular protein that identifies them. They also have a stock of sugar in the form of glycogen, which is the main energy reserve of the brain, because sugar is the favorite food neurons.
    Astrocytes by their cytoplasmic extensions and questions intersecting contiguous, ensure the consistency and robustness of the brain structure and their extensions around completely capillaries, they contribute to the nutrition of neurons (which have no direct contact with the hair). Astrocytes are involved with neurons and other glial cells, the functioning of the CNS. They communicate  with each other and exchange information with neurons through many joints communicating. In addition, they allow the selectivity of nerve transmission by preventing the diffusion of neurotransmitters. Astrocytes play a role important in September On the one hand, they work alongside microglia as antigen-presenting cells (specialized cells that are circulating foreign bodies, antigens to incompetent cells, lymphocytes), and secondly, they actively proliferate in the plates and are gradually edify a kind of scar glial who has undoubtedly adversely affect remyelination of axons stripped and regrow axons.

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Neuronal death

     Neurons do not usually die before the death of the whole organism. However, several types of injuries and illnesses can shorten the duration of their existence. Neurons disappear and are rarely replaced because there are still few stem cells capable of differentiate into neurons in the mature nervous system. Like most cells, neurons can die in two ways : By necrosis or apoptosis.   Necrosis occurred by acute trauma, chemical or mechanical. Then the cell swells destroyed (lysis) and its contents dispersed in the extracellular medium. This dispersion causes usually an inflammatory reaction, causing secondary trauma to surrounding tissue. Apoptosis can occur in a variety of situations. The cell nucleus is condensed, the cell fragments without dispersing its contents and fragments are rapidly cleared from the tissue surrounding. This form of cell death often requires the production of proteins by the cellspecialized, which means that the cell takes an active part in its own disposal (called" cell suicide "Or            " programmed cell death") So as not to damage the surrounding tissue.Neuronal death by apoptosis, common during development (see above), occurs also when the neuron detects an anomaly in its own operations and that it has a survival time sufficient to establish specific mechanisms of this type of cell death. By example, apoptosis is initiated by metabolic abnormalities which produce the accumulation of Free radicals in the cell, or with an excess of intracellular calcium due to a tear in the membrane.The organization of the nervous system network has the disadvantage of permitting the propagationdistance from the effects of local injury. This propagation can be done to nerve endings the honeycomb body : The principles implemented in their development, survival many neurons is suspended from the presence of neurotic molecules produced continuously level of their termination by other cells. Injury and death of these cells suppress the source the survival signal and thus cause the death of neurons that are connected, while they themselves have suffered no injury.The effects of a lesion can also spread from the cell body to the terminals : The degenerating neurons are no longer capable of maintaining the segregation of the ions of opposite sides of the membrane. So they become depolarized and then emit a large number of action potentials in their axons. This abnormal electrical activity goes in a number of cases excite (depolarize) the target neurons and cause an influx of calcium, for example by activating the channels permeable calcium. These abnormal levels of calcium will then cause cell degeneration targets. Thus, accidental interruption of blood flow to a region of the brain causesrapid degeneration ( necrosis) of neurons in this region and can cause degeneration excitotoxicity a volume substantially larger than that of the initial lesion nerve tissue.    Understanding the mechanisms of cell death represents a major therapeutic challenge for neurodegenerative diseases, that is to say related to the death of particular types of neurons, such asAlzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis. In conclusion, the analysis of the functions of the nervous system is a major task, which passes including the study of neurons. These can be studied in two different optical : one hand, these are cells in the body subject to the laws of biology, and, secondly, they are elementary operators involved in information processing in the nervous system. The neuron  the brain, the reflex of thought, the field of knowledge extends from day to day, but the adventure scientist is probably just beginning.

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Synaptic plasticity

Memory and learning are essential properties of the nervous system : They allow on the one hand, to distinguish among the new elements familiar elements, and, secondly, integrate spent a decision experiences. Synaptic plasticity probably plays a fundamental role in the processes of memory and learning. It corresponds to the change more or less durable effectiveness of a synapse, that is to say the amplitude of the depolarization or hyperpolarization it produces on its target cell. By changing the relative effectiveness of two synapses, a neuron changes the relative weight given to the information passing through. The same effect is also obtained the connection between two neurons through training or removal of synaptic contacts.Synaptic plasticity offers neuron another way to implement the principle of Convergence of synchronous activities (see supra "Alterations"). Convergence is not done then by the change in the number of synapses, but by changing their efficiency. Instead of forming new connections, the neuron will increase the efficiency of active synapses simultaneously.     Experimentally, it is possible to simulate the synchronous activation of several excitatory synapses onthe depolarizing a neuron. Following the above, the stimulation of a synapse associated with this singledepolarization should lead to increased efficiency. This phenomenon has been observed for examplein the hippo campus, a brain region involved in memory processes. The association Experimental depolarization of a pyramidal neuron of the hippocampus and a high stimulation a frequency synaptic terminal from another pyramidal neuron causes a doubling the efficiency of synaptic transmission between neurons. This change can take several hours, even several days (this is called LTP [ long-term potentiation]). We did not observe if the neuron is hyperpolarized.There is probably no universal mechanism for modulation of synaptic transmission and probably several principles governing the different types of synapse according to the specific needs of nervous system. Changes in synaptic efficacy are considered a basic form learning, which likely contributes to the storage in the nervous system.

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