Nervous system.

The nervous system is the organ systems in animals consist of nerve fibers composed of nerve cells interconnected and essential for sensory sensory perception, motor activity of voluntary and involuntary organ or tissue, and homeostasis of various physiological processes of the body. The nervous system is a network of the most complex and important because it consists of millions of nerve cells (neurons) are interconnected and vital to the development of language, thought and memory. The main working unit in the nervous system is the neuron that is bound by glial cells.

The nervous system in vertebrates are generally divided into two, namely the central nervous system (CNS) and the peripheral nervous system (SST). CNS consists of the brain and spinal cord. SST consists primarily of the nerve, which is a long fiber that connects the SSP to every part of the body. SST includes the motor nerve, mediates the movement of voluntary movement (unconscious), the autonomic nervous system, including the sympathetic nervous system and the parasympathetic nervous system and regulatory functions (settings) involuntary (unconscious) and the enteric nervous system (digestive), a part of the semi-free of the nervous system whose function is to control the digestive system.
At the cellular level, the nervous system is defined by the presence of specific types of cells, called neurons, which are also known as nerve cells. Neurons have special structures that allow neurons to transmit signals quickly and precision to another cell. Neurons transmit signals in the form of electrochemical waves that run along thin fibers called axons, which will cause chemicals called neurotransmitters are released in the engagement is called the synapse. A cell that receives a synaptic signal of a neuron may be excited, inhibited, or modulated. The relationship between neurons form neural circuits that is generated perception of the world organism and determine behavior. Along with neurons, the nervous system contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support.

The nervous system is found in most multicellular animals, but vary in complexity. Multicellular animals that do not have a nervous system altogether are sponges, placozoa and mesozoa, which has the body design is very simple. Ctenophora nervous system and cnidarians (eg anemones, hydra, coral and jellyfish) consists of a diffuse neural network. All kinds of other animals, with the exception of a few types of worms, have a nervous system that includes the brain, a central cord (or two cords running parallel), and nerves that radiate from the brain and central cord. The size of the nervous system vary from a few hundred cells in the simplest worms, to the extent of 100 trillion cells in humans.
At the simplest level, the function of the nervous system is to transmit signals from one cell to another, or from one part of the body to another body. The nervous system is prone to malfunction in various ways, as a result of genetic defects, physical damage due to trauma or toxins, infection, or simply aging. Specificity of medical research in the field of neurology to study the cause of a malfunction of the nervous system, and look for interventions that can prevent or fix it. In the peripheral nervous system / edge (SST), the most common problem is a failure of nerve conduction, which can be caused by a variety of causes including diabetic neuropathy and demyelinating disorders such as multiple sclerosis and amiotrofik lateral sclerosis.
Focusing science research / study of the nervous system is neuroscience.

Structure.

Nervous system name is derived from "nerves", which is a cylindrical bundle of fiber coming out of the brain and central cord, and branches to innervate each body part. Nerves are large enough to be recognized by the Egyptians, Greeks and Ancient Rome, but its internal structure is not understandable to the possibility of testing through the microscope. A microscopic examination showed that the main nerves are comprised of axons of neurons, along with various membrane (sheath) that wrap around nerves and separating them into fasikel. Neurons are nerve evokes not be entirely within the nerve itself; their cell bodies in the brain, central cord, or peripheral ganglia (edge).
All animals of a higher order than the sponges have a nervous system. However, even sponges, animal unicellular, and non-animals such as slime molds have mechanisms in cell signaling to the cell that is the precursor neurons. In animal radially symmetrical as jellyfish and hydra, the nervous system is composed of a network of diffuse isolated cells. In bilaterians animals, which consists of most of the majority of species, the nervous system has a common structure that is originating the beginning of the Cambrian period, more than 500 million years ago.

Cells.

The nervous system has two categories or types of cells: neurons and glial cells.
Neuron.
Nerve cells are defined by the presence of a particularly neuronal cell types (sometimes called "neurone" or "neurons"). Neurons can be distinguished from other cells in a number of ways, but the most fundamental properties is that they can communicate with other cells via the synapse, the junction membrane-to-membrane containing molecular machines and allow fast signal transmission, either electrically or chemically. Each neuron consists of a cell body in which there are cytoplasm and the cell nucleus. From the cell body out two types of nerve fibers, the dendrites and axons. Dendrites function sends impulses to the nerve cell body, whereas the function of axons transmit impulses from the cell body to another nerve cell or to another network. Axons are usually very long. Conversely, short dendrites. Each neuron has only one axon and at least one dendrite. Both of these nerve fibers containing plasma cells. On the outside of the axon there is a layer of fat called myelin formed by Schwann cells attached to the axon. Schwann cells are the primary glial cells in the peripheral nervous system that serves to form the myelin sheath. Myelin function is to protect and nourish the axon. Part of that is not wrapped myelinated axons are called nodes of Ranvier, which can accelerate the delivery of impulses.
Even in the nervous system of a single species such as humans, there are hundreds of different types of neurons, the shape, morphology, and functions vary. Variety includes sensory neurons that transmute physical stimuli such as light and sound into nerve signals, and motor neuron activation transmuting neural signals into muscle or gland; however in most species most neurons receive all of their input from other neurons and send their output to other neurons.

Glia cells.

Glia cells (derived from the Greek word meaning "glue") is a non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system. In the human brain, it is estimated that the total number of glia ballpark nearly equivalent to the number of neurons, although the ratio varies in different brain regions. Among the most important function of glial cells is to support neurons and hold them in place; to provide nutrients to neurons; for electrically insulating neurons; to destroy pathogens and eliminate dead neurons; and to provide guidance directing axons of neurons to target a type of glial cells is important (oligodendrocyte in the central nervous system, and Schwann cells in the peripheral nervous system) menggenerasikan layer of a fatty substance called myelin that wraps axons and providing insulation electrical allow them to transmit action potential faster and more efficiently.
Various neuroglia them are astrocyte, oligodendrocyte, microglia, and makroglia.

Anatomy in vertebrates.

The nervous system of vertebrates (including humans) is divided into the central nervous system (CNS) and the peripheral nervous system (SST).
The central nervous system (CNS) is the largest part, and including the brain and spinal cord. Spinal cavity contains the spinal cord, while the head contains the brain. CNS covered and protected by the meninges, a 3-layer membrane system, including a strong outer layer of skin, called the dura mater. The brain is also protected by the skull, and spinal cord by the vertebrae (spine).
The peripheral nervous system (SST) is a term / collective term for the structure of the nervous system that is not within the CNS. Most majority of bundles of axons are called nerves that consideration into the SST, even when the cell bodies of neurons in the brain or spinal cord. SST is divided into sections somatic and visceral. Part consists of somatic nerves innervating the skin, joints and muscles. Somatic sensory neuron cell bodies located in the 'dorsal root ganglia of the spinal cord. Visceral part, also known as the autonomic nervous system, contains neurons that innervate the internal organs, blood vessels, and glands. The autonomic nervous system consists of two parts of the sympathetic nervous system and the parasympathetic nervous system. Some authors also enter the cell bodies of sensory neurons in the peripheral (for senses such as hearing) as the chart of the SST; but others ignore it.

Vertebrate nervous system can also be divided into areas called gray matter ("gray matter" in American spelling) and white matter. Gray matter (the only gray when stored, and pink (pink) or light brown in living tissue) containing a high proportion of neuron cell bodies. White matter is the main composition of myelinated axons, and take the color of myelin. White matter includes all the nerves and most of the parts of the brain and spinal cord. Grey matter is found in clusters of neurons in the brain and spinal cord, and in cortical layers that line the surface of them. There is agreement that the anatomical cluster of neurons in the brain or spinal cord called the nucleus, while a cluster of neurons in the peripheral called a ganglion. However there are some exceptions to this rule, the recorded part of the brain called the basal ganglia front.

Comparative anatomy and evolution.

Predecessor nerve in sponges.

Sponge does not have a cell related to each other by synaptic junction, ie, no neurons, and therefore there is no nervous system. However, they have a homolog of many genes that play important roles in synaptic function. Recent research has shown that sponges cells express a group of proteins grouped together to form a structure similar to a postsynaptic density (part synapse which receives the signal). However, the function of these structures is still unclear. Although sponges cells showed no synaptic transmission, they communicate with one another through the waves of calcium and other impulses, which mediates some simple actions such as contraction throughout the body.

Radiata.

Jellyfish, comb jelly, and other related animals have a diffuse neural network rather than a central nervous system. In most jellyfish, a neural network is spread more or less evenly across the body; the comb jelly neural networks are concentrated close to the mouth. Neural network consists of sensory neurons, which pick up chemical signals, tactile, and visual; motor neurons, which can activate the body wall contractions; and intermediate neurons, which detects patterns of activity in sensory neurons, and in response, sends signals to the motor neuron groups. In some cases, groups of neurons that are being grouped into different ganglia.
The development of the nervous system in a relatively unstructured radiata. Unlike bilaterians, radiata only have 2 primordial cell layers, endoderm and ectoderm. Neuron is generated from a special cell of ectodermal precursor cells, which also acts as a precursor to any other ectodermal cell types.

Bilateria.

The nervous system of an animal bilaterian, in the form of a nerve cord with segmental enlargement, and a "brain" on the front.
Most animals that there is bilaterians, which means that animals with left and right sides are more or less symmetrical. All bilaterians expected to be derived from a common ancestor as a worm that appeared in the Cambrian period, 550-600 million years ago. Bilaterians basic body shape is a tube with a gut cavity running from the mouth to the anus, and a nerve cord with a magnification (a "ganglion") for each body segment, with a specialty in front of a large ganglion, called the "brain".

The surface area of ​​the human body are innervated by each spinal cord.
Even mammals, including humans, show bilaterians segmented body plan at the level of the nervous system. The spinal cord contains a series of segmental ganglia, which each generate sensory and motor nerves innervating the surface of the body and the muscles which she is employed. In the limbs, the layout of the complex innervation pattern, but in this section appears a series of narrow band. The top three segments owned by the brain, raise the forebrain, midbrain, and hindbrain.
Bilateria can be divided, based on events that can occur very early in embryonic development, into 2 groups (superfila) called Protostome and Deuterostome. Deuterostome includes vertebrates as echinoderms, hemichordate, and xenoturbella. Protostome, more diverse group, including arthropods, mollusks, and various types of worms. There are fundamental differences between the 2 groups in the placement of the nervous system in the body: Protostome have a nerve cord on the ventral side (typically below), while in Deuterostome nerve cord normally exist in the dorsal side (usually the top). In fact, many aspects of the body upside down in both groups, including some gene expression pattern shows the gradient of the dorsal-to-ventral. Most anatomical now consider protostomes and deuterostomes agency "upside down" with each other, a hypothesis first proposed by Geoffroy Saint-Hilaire for insects in comparison with vertebrates. So insects, for example, has a nerve cord that runs along the ventral midline of the body, while the entire vertebrates have spinal cord that runs along the dorsal midline.

Arthropods.

Arthropods, such as insects and crustaceans, have a nervous system is made of a series of ganglia, connected by a ventral nerve cord which consists of 2 parallel connections along the belly .. In general, each body segment has one ganglion on each side, though some ganglia function form brain and other large ganglia. Segment head contains the brain, also known as supraesophageal ganglion. In the insect nervous system, the brain is anatomically divided into protocerebrum, deutocerebrum, and tritocerebrum. Directly behind the brain is subesophageal ganglion, which is made of three pairs of ganglia are fused. This controls the mouth, salivary glands and certain muscles. Many arthropods have well-developed sensory organs, including eyes for vision and antennae to smell odors and pheromones. Sensory information from these organs is processed by the brain. In insects, many neurons have cell bodies located in the brain end and electrically passive - the cell body tasked only to provide metabolic support and did not participate in signaling. A fiber protoplasmik from the cell body and branched, with some parts of the signal transmitting and receiving signals other parts. Therefore, most part of the insect brain have passive cell bodies are arranged along the peripheral cells, while the neural signal processing takes place in a protoplasmik fibers called neuropil, on the inside.

Neurons "unidentified".

A neuron is called identifiable if it has properties that distinguish it from any other neurons in the same animal characteristics such as location, neurotransmitter, gene expression patterns, and connectedness - and if each individual organism derived from the same species has only one neuron with a set the same properties. In very few vertebrate nervous system neurons are "identified" in this sense - in humans, no - but in the nervous system that is simpler, some or all of the neurons may be ultimately unique. In the round worm C. elegans nervous system is the most widely depicted, each neuron in the body is uniquely identified, at the same location and the same connection in each individual worm. One consequence of this fact is recorded that form the nervous system of C. elegans as a whole are specified by the genome, in the absence of plasisitas depending on experience.
The brain of most mollusks and insects also contains a substantial number of neurons identified. In vertebrates, neurons identified the best known are fish Mauthner cell. Each fish has two Mauthner cells, located in the lower part of the brainstem, one on the left and one on the right side. Each Mauthner cells have axons that cross, innervating neurons in the brain the same level and then goes down along the spinal cord, forming various connections along its path. Synapse is generated by a Mauthner cell were so strong that a single action potential can generate major behavioral responses: within milliseconds fish mengkurvakan his body into a C shape, then straightened up, therefore slid quickly to the front. Functionally this is the rapid escape response, triggered most easily by a strong sound waves or pressure waves that suppresses the lateral line organ (side) fish. Mauthner cells is not the only neurons identified in fish - there are more than 20 species, including couples' analog Mauthner cell "in each core spinal segmental. Although the cell is able to evoke a response Mauthner fled individually, in the context of the usual behavior of other cell types usually contribute in shaping the amplitude and direction of the response.
Mauthner cells has been described as a command neurons. A neuron giving the order is a special type of neurons identified, defined as a neuron is capable of controlling a specific behavior individually. Such neurons seem to be most common in the system to escape from a variety of species - the giant squid axons and synapses giant squid, which is used for experiments in neurophysiology because the size is very large, to participate in the fast escape circuit. However, the concept of a neuron giving the order is still controversial because studies have shown that some of the neurons that initially seem to fit that description was only able to induce a response in limited circumstances.

Function.

At the most basic level, the function of the nervous system is to transmit signals from one cell to another, or from one part of the body to another body. There are different ways a cell can send signals to other cells. One way is by releasing chemicals called hormones into the internal circulation, so that they can diffuse away places. Contrary circuitry signaling mode "broadcasting", the nervous system provides a signal from place to place-neuron axons projecting them into a specific target area and form synaptic connections with specific target cells. Therefore, neural signaling specificity were much higher level than hormonal signaling. It is also faster: the fastest nerve signals running at speeds exceeding 100 meters per second.
At the level of a more integrated, the primary function of the nervous system is to control the body. This is done by taking information from the environment using sensory receptors, sending a signal that encodes this information into the central nervous system, processes the information to determine an appropriate response sebuath, and sends output signals to the muscles or glands to activate a response. The evolution of a complex nervous system has enabled a wide range of animal species to have a perception of more advanced capabilities such as outlook, social interactions complex, rapid coordination organ systems, and signal processing integrated sustainable manner. In humans, the sophistication of the nervous system makes it possible to have a language, the concept of an abstract representation, cultural transmission, and many social features which can not exist without the human brain.

Neurons and synapses.

Most neurons send signals through the axon, although some species are able to communicate dendrite to dendrite. (in fact, the types of neurons called cell amakrin have no axons, and communicates only through dendrites them.) Signals of neural propagates along an axon in the waveform electrochemical called action potentials, which generates a signal cell to cell in the axon terminals form synaptic contacts with other cells.
Synapse can be either electrical or chemical. Electrical synapses making direct electrical connections between neurons, but chemical synapses are more common, and more diverse in function. In a chemical synapse, the cell sends a signal called the presynaptic and the cell that receives a signal called the postsynaptic. Both presynaptic and postsynaptic filled with molecular machines that carry the signal. Presynaptic regions containing a large number of very small spherical vessel called synaptic vesicles, filled with neurotransmitter chemicals. When the presynaptic terminal electrically stimulated, an arrangement of molecules attached to the membrane activated, causing the contents of the vesicles are released into the narrow gap between the presynaptic and postsynaptic membrane, called the synaptic cleft (the synaptic cleft). Neurotransmitters then bind to receptors attached to the postsynaptic membrane, causing neurotransmitters into the activated state. Depending on the type of receptor, the resulting effect on the postsynaptic cell may excitation, inhibition, or modulation in many ways more complicated. For example, the release of the neurotransmitter acetylcholine at the synaptic contacts between motor neurons and muscle cells induces a rapid contraction of the muscle cells. The whole process of synaptic transmission requires only a fraction of a millisecond, although the effect on the postsynaptic cell may take longer (not even limited, in the case when the signal sipatik refers to information of a memory trace).
Literally there are hundreds of types of synapse. In fact, there are more than one hundred neurotransmitters that are known, and many of them have multiple receptor types. Many synapses using more than one neurotransmitter-a common arrangement for a synapse is using a small molecule neurotransmitters such as glutamate or fast-acting GABA, in accordance with one or more peptide neurotransmitters that play a role modulatoris slower. Neurologist molecular receptors usually divide into two major groups: chemical gated ion channels (chemically gated ion channels) and second messenger systems (second messenger system). When a chemically gated ion channel activated, the channel will form a place to be passed which allow certain types of specific ions to flow through the membrane. Depending on the type of ions, effects on target cells may excitation or inhibition. When a second messenger system is activated, the system will start a cascade of molecular interactions inside the target cell, which in turn will produce a wide range of complex effects / complex, such as an increase or decrease in the sensitivity of cells to stimuli, or even change gene transcription.
According to the law the so-called principle of Dale, which only has a few exceptions, a neuron releases neurotransmitters are the same in all sinapsnya. However, it does not mean that the neurons secrete the same effect on all targets, because the synapse effect depends not only on neurotransmitters, but the receptors in activation. Due to different target can (and usually does) use different types of receptors, it is possible to have the effect of excitatory neurons in the first set of target cells, an inhibitory effect on the others, and modulation effects complicated / complex on the other. However, two of the most commonly used neurotransmitter, glutamate and GABA, each has a consistent effect. Glutamate has several common types of receptors that exist, but everything is excitatory or modulatori. In the same way, GABA has a general type of receptor is there, but everything is inhibitory. Because of this consistency, the cell glutamanergik often referred to as "excitatory neurons", and GABAergic cells as "inhibitory neurons". This is a deviation terminology - receptors which are excitatory and inhibitory, not neurons - but it is commonly seen even in scientific publications.
One subset of synapses most importantly capable of forming memory traces by means of changes in the strength of synaptic dependent activity lasting. The memory neural most known is a process called long-term potentiation (long-term potentiation, abbreviated LTP), which operates at synapses that use the neurotransmitter glutamate acting on a type of specialized receptors known as NMDA receptors. The NMDA receptor has the property of "association": if two cells involved in synapse activated both at approximately the same time, an open channel that allows calcium to flow toward the target cells. Influx of calcium triggers a second messenger cascade that ultimately leads to increased number of glutamate receptors in the target cells, thereby increasing the effective strength of the synapse. The force changes can take several weeks or longer. Since the discovery of LTP in 1973, many types of memory traces synaptic found, including an increase or decrease in the strength of synaptic induced by a variety of conditions, and take place in various periods diverse. Learning reward (reward learning), for example, depends on the shape variation of LTP were conditioned on an extra input from signaling pathways reward (reward-signaling pathway) using dopamine as a neurotransmitter. All forms of synaptic modification is, collectively, give rise to neuroplasticity, the ability of the nervous system to adapt to variations in the environment.

Systems and neural circuits.

The basic function of neuronal transmit signals to other cells include the ability of neurons to change the signal to the others. A network formed by the interconnected groups of neurons capable of running a variety of functions, including feature detection, pattern generation, and timing. In fact, it is difficult to determine the limits of the type of information that can be carried out by neural networks: Warren McCulloch and Walter Pitts showed in 1943 that even the artificial neural network is formed from a highly simplified mathematical abstraction capable of universal computation. Taking into account the fact that individual neurons are able to make the generation of complex temporal activity patterns freely, so there may be a range of capabilities even for a small group of neurons in the sense of existing outside.

In history, for many years the main view in the functioning of the nervous system is a stimulus-response link. In this concept, the neural process begins with sensory stimuli that activate neurons, generating a signal that propagates through a series of connections in the spinal cord and brain, activates the motor neurons and then generate a response such as muscle contraction. Descartes believes that all animal behavior, and most human behavior can be explained in terms of stimulus-response circuit, although he also believes that the higher cognitive functions such as language was not able to be explained mechanically. Charles Sherrington, in his book in 1906 entitled The Integrative Action of the Nervous System, developed the concept of stimulus-response mechanism in a more detail, and behaviorism, the school which dominated psychology throughout the middle of the 20th century, trying to explain every aspect of behavior human behavior in the context of stimulus-response.
However, electrophysiological studies that began in the early 20th century and reached its productivity in 1940 shows that the nervous system contains various mechanisms to produce activity patterns are intrinsically, without requiring an external stimulus. Neurons are found capable of producing a series of regular action potential, or a series of explosions (sequences of bursts), even in full isolation. When the active neurons are intrinsically connected with each other in complex circuits, the possibility of a temporary income more complicated pattern becomes much greater. The modern concept looked nervous system functions in part within the framework of a series of stimulus-response, and partly within the framework of the activity patterns generated intrinsically - both types of activity to interact with others for generations repetitive behavior.

Excitatory reflex circuit and other stimuli.

Type simplest neural circuits are curved reflex (reflex arc), which starts from sensory input and motor output ends with, passing through a series of neurons in the middle. For example, consider "withdrawal reflex" which causes the hand pulled back after touching a hot stove. The circuit begins with sensory receptors in the skin that is activated by heat levels that endanger: a special type of molecular structure attached to the membrane causes heat to change the electric field across the membrane. If a change in the potential ekletrik large enough, it will generate an action potential, which is transmitted along axons of receptor cells, towards the spinal cord. There will axons make excitatory synaptic contact with other cells, some of which are projecting (send axonal output) to the same region of the spinal cord, and the other projecting into the brain. One target is a series of spinal interneurons projecting into motor neurons to control the arm muscles. Interneurons excite the motor neurons, and if the excitation is strong enough, some of the motor neurons generate action potentials, which runs along the axon to the point where they made contact with the excitatory synaptic muscle cells. Excitatory signals trigger the contraction of muscle cells, which causes the joints in the arm angle change, pull the arm away.
In fact, this scheme relates to a variety of complications. Although for the most simple reflex neural pathways short of sensory neurons to motor neurons, neurons that close there is also participating in the circuit and modulates the response. Furthermore, there is a projection from the brain to the spinal cord that is capable of enhancing or inhibiting reflex.
Although the simplest reflex is mediated by the circuit may be entirely within the spinal cord, the response is more complex / complicated to rely on signal processing in the brain. Consider, for example, what happens when an object moves in the peripheral visual area, and someone saw him. Early sensory response, the retina of the eye, and the final motor response, the oculomotor nucleus of the brainstem, everything is not different from all in a simple reflex, but in stages between completely different. Not only 1 or 2 steps processing circuits, probably a dozen visual signal passes through integration phase, involving the thalamus, cerebral cortex, basal ganglia, the superior colliculus, the cerebellum and some brain stem nucleus). These areas form the signal processing functions that include the detection features, perception analysis, redial memory, decision-making, and motor planning.
Detection feature is the ability to extract biologically relevant information from a combination of sensory signals. In the visual system, for example, sensory receptors in the retina of the eye is only able to detect "points of light" in the outside world individually. The second level of visual neurons receive input from groups of primary receptors, neurons higher receive input from groups of neurons second level, and so on, forming a hierarchical process level.At each stage, an important information extracted from the signals collected and disposed of unimportant information. At the end of the process, the input signal represent "points of light" has been transformed into neural representation of objects in the surrounding world and nature. The most sophisticated sensory processing occurs in the brain, but the extraction of complex features also occur in the spinal cord and peripheral sensory organs such as the retina.

Income intrinsic pattern.

Although the stimulus-response mechanism is most easily understood, the nervous system can also control the body in various ways that do not require external stimulus, through the rhythm of activity generated from inside. Because of various ion channels sensitive to voltage that can be embedded in the membranes of neurons, different types of neurons are able, even in isolation, the sequence of generations rhythm action potential, or a change in the rhythm of the explosion and the high level of the quiet period. When neurons are intrinsically connected with the rhythm of the other by synapses eksitatoris response or inhibition, the resulting network is able to produce a diverse dynamic behavior, including the dynamics of withdrawal (attractor), periodicity, and even chaos. A network of neurons that use its internal structure to produce a structured output on a temporary basis, without the need for a structured stimulus that corresponded temporarily called central pattern generators.

The generation of internal patterns operate within a wide range by the time scale, from milliseconds to hours or even longer. One of the important types is the temporal pattern of circadian rhythms - that is, the rhythm with a period of approximately 24 hours. All the animals that have been studied show circadian fluctuations in neural activity, which controls the circadian changes in behaviors such as sleep-wake cycle. Research from the 1990s has shown that the circadian rhythm is generated by a "genetic clock" which consists of a group of specific genes whose expression level increases and decreases throughout the day. Animals as diverse as insects and vertebrates have the same genetic clock system. The circadian clock is influenced by light but continues to work even when light levels are kept constant and there is no indication when the other externally available. These clock genes are expressed in various parts of the nervous system as well as many peripheral organs, but in mammals the entire "network hours" is maintained in synchronization by the signal coming out of a main time keeper in a small part of the brain called the suprachiasmatic nucleus.

Delivery stimuli.

All cells in the human body has an electrical charge that is polarized, in other words the potential difference between the outside and the inside of a cell membrane, not least of nerve cells (neurons). The potential difference between the outside and inside of the membrane is called membrane potential. Information received by Indra will be forwarded by the nerves in the form of impulses. In the form of electrical impulses voltage. Impuls will take the path along the axon of a neuron before it is delivered to other neurons via synapses and would like it continue until it reaches the brain, where it will be processed impulse. Then the brain sends impulses to the organ or sense intended to produce the desired effect through the same mechanism of impulse transmission.
Animal membranes have a resting potential of about -50 mV s / d -90 mV, the resting potential is retained by the membrane potential as long as no stimulation to the cell.
The arrival of the stimulus will cause depolarization and hyperpolarization in the cell membrane, it causes the working potential. Potential employment is a sudden change in membrane potential due to the advent of stimuli. At the time of employment potential occurs, the membrane potential depolarization from the resting potential (-70 mV) turned into a +40 mV. Axons vertebrates generally have a myelin sheath. The myelin sheath is composed of 80% lipids and 20% protein, making it is dielectric or inhibiting the flow of electricity, and this causes the job potential can not be formed in the myelin sheath; but part of the axon called nodes of Ranvier are not covered by myelin.
Excitatory conduction in myelinated axons done saltatori delivery mechanism, namely the potential work to be conducted by "hopping" from one node to another node until it reaches synapses.
At the end of the neurons are the meeting point between neurons called synapses, excitatory neurons that transmit so-called pre-synaptic neuron and which will receive the so-called excitatory post-synaptic neuron. The tip of each neuron axons form a bulge which there are mitochondria to provide ATP for the delivery process and the excitatory synaptic vesicles containing neurotransmitters such as acetylcholine generally (ACh), adrenaline and noradrenaline.
When arriving at excitatory synapses, axon tip of the pre-synaptic neuron will make synaptic vesicles closer and fused to the membrane. Neurotransmitters are then released through exocytosis. At the end of the post-synaptic neuron axon, neurotransmitter receptor proteins bind to molecules and respond by opening ion channels in the membrane of axons which then converts the membrane potential (depolarization or hyperpolarization) and a potential cause of work on the post-synaptic neuron.
When impulses from the pre-synaptic neuron neurotransmitter existing stops will be degraded. The degraded molecules and then goes back to the end of the pre-synaptic neuron axon by endocytosis.

Development.

In vertebrates, it is important in the development of neural embryonic include birth and differentiation of neurons from stem cells, migrating neurons immature from their birthplace in the embryo to their final position, the growth of axons of neurons and directing the growth cone motility through embryo toward fellow postsynaptic, income synapses between these axons and their postsynaptic colleagues, and eventually lifelong changes in synapses that allegedly underlie learning and memory.
All animals bilaterians at early stages of development to form a gastrula polarized, with a tip of the so-called polar animals and other vegetal pole. Gastrula has a disc shape with 3 layers of cells, the innermost layer is called endoderm, which evoke the basis of most internal organs, a middle layer called the mesoderm, which evokes the bones and muscles, and the outermost layer is called the ectoderm, which evokes the skin and nervous system.

In vertebrates, the first sign of the emergence of the nervous system is the emergence of thin cells along the middle of the back called disc nerve (neural plate. The inside of the disc nerve (along the midline) is intended to be a central nervous system (CNS), and the outside of the peripheral nervous system (SST). As development proceeds, a flap called the indentation nerve (neural groove) appears along the center line. These folds into the inside and then closed at the top. At this point SSP were coming, looks like the structure of cylindrical called the neural tube, SST will be the place looks like two lines of tissue called the neural crest (neural crest), which is on top of the neural tube. The series of stages from the disk nerves to neural tube and neural crest known as neurulasi.
In the early 20th century, a series of famous experiments by Hans Spemann and Hilde Mangold showed that the formation of neural networks "induced" by a signal from a group of mesodermal called "regulatory region" (organizer region). However, for decades, the nature of the induction process can not be known, until finally it was solved by genetic approach in the 1990s. Induction of neural networks require inhibition of a gene called bone morphogenetic proteins (bone morphogenetic protein, abbreviated as BMP). In particular, BMP4 protein appears to be involved. Two protein called Noggin and Chordin secreted by mesoderm seems capable of inhibiting BMP4 and thus induces the ectoderm to turn into nerve tissue. It seems a similar molecular mechanisms involved in a variety of different types of animals, including arthropods and vertebrates. However, in some animals, a kind of other molecules called fibroblast growth factors (Fibroblast Growth Factor, abbreviated as FGF) may play a role in the induction.
Induction of neural tissue causes the formation of neural precursor cells called neuroblasts. In Drosophila, neuroblasts split asymmetrically, so that the product is "ganglion stem cell" (mother ganglion cells, abbreviated as GMC), and the other is a neuroblasts. A GMC is divided once and produces a pair of neurons or glial cell mate. Overall, neuroblasts are able to produce the number of neurons or glia infinite.
As indicated in the study in 2008, a factor which is common to all bilateral organisms (including humans) are a group of molecules that secrete signaling molecules called neurotrophin that regulate the growth and survival of neurons. Zhu et al. identify DNT1, neurotrophin first discovered in flies. DNT1 structure similar to all neurotrophin known and is an important factor in determining the fate of neurons in Drosophila. Because neurotrophin now been identified in vertebrates and invertebrates, this evidence suggests that there is a natural neurotrophin common ancestor bilateral organisms and may represent a general mechanism for the formation of the nervous system

Pathology.

Central nervous system (CNS) is protected by a barrier (barrier), physical and chemical. Physically, the brain and spinal cord are surrounded by strong meningeal membranes, and wrapped by the skull and spinal vertebrae, which forms a strong physical protection. Chemically, the brain and spinal cord is isolated by the so-called blood-brain barrier, which prevents most types of chemicals move from the bloodstream into the inside of the SSP. This protection makes the SSP is less vulnerable than the SST; but, on the other hand, damage to the central nervous system tend to be more serious impact.

Although nerves tend to be under the skin except in a few places, such as the ulnar nerve near the elbow joint linkage, nerves tend to be exposed to physical damage, which can cause pain, loss of sensation, or loss of muscle control. Nerve damage can also be caused by swelling or bruising at the wrought nerve passes between a tight spinal canal, as happened in the hallway carpal syndrome. If a completely severed nerve, the nerve will regenerate, but for a long nerve, this process will probably take months to complete. In addition to the physical damage to peripheral neuropathy can be caused by other medical problems, including genetic conditions, metabolic conditions such as diabetes, inflammatory conditions such as Guillain-Barré syndrome, vitamin deficiency, infectious diseases such as leprosy or herpes zoster, or poisoning by toxins such as heavy metals. Many cases have no identifiable cause, and called idiopathic. Nerves can also lose its function for a time, resulting in a lack of sense - common causes include mechanical pressure, drop in temperature, or chemical interactions with drugs such as lidocaine.

Physical damage to the spinal cord may result in loss of sensation or movement. If an accident on the backbone to produce something that is not severe of swelling, symptoms only temporary, but if the nerve fibers in the spinal cord is destroyed, usually permanent loss of function. Experiments have shown that the spinal nerve fibers usually try to grow back in the same way as the nerve fibers, teapi in the bone marrow, the tissue damage normally produce scar tissue that can not be penetrated by nerves to grow back.

Thank you for reading this article. Written and posted by Bambang Sunarno. sunarnobambang86@gmail.com

author:

https://plus.google.com/105319704331231770941.

name: Bambang Sunarno.

http://primadonablog.blogspot.com/2015/09/nervous-system.html

DatePublished: September 9, 2015 at 18:09

Tag : Nervous system.

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