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Neurons are a major class of Cells in the Nervous System . Neurons are sometimes called nerve cells, though this term is technically imprecise since many neurons do not form nerves. In Vertebrate s, they are found in the Brain , the Spinal Cord and in the Nerve s and Ganglia of the Peripheral Nervous System , and their primary role is to process and transmit neural information. One important characteristic of neurons is that they have Excitable Membranes which allow them to generate and propagate electrical signals. The concept of a neuron as the primary computational unit of the nervous system was devised by Spanish anatomist Santiago Ramón Y Cajal . Cajal proposed that neurons were discrete cells which communicated with each other via specialized junctions. This became known as the Neuron Doctrine , one of the central tenets of modern neuroscience. However, it is important to note that Cajal would not have been able to observe the structure of individulal neurons and their processes, and in turn devise the Neuron Doctrine, if his rival, Camillo Golgi , (for whom the Golgi Apparatus is named after) had not developed his highly specific " Golgi Staining Method ." When the Golgi Stain is applied to neurons, it binds the cell's Microtubules and gives stained cells a black outline when light is shone through them. ANATOMY AND HISTOLOGY Many highly-specialized types of neurons exist, and these differ widely in appearance. Neurons have cellular extensions known as ''processes'' which they use to send and receive information. Neurons are highly asymmetric in shape, and consist of:
Although the canonical view of the neuron is to assign strictly defined and dedicated functions to its various anatomical components, the fact that dendrites and axons very often act contrary to their so-called main function is but one small glimpse into the complex integrative capacity of every nerve cell. Nervous systems bear little resemblance to simple Feed-forward Input/Output Circuits , and this understanding begins by appreciating the global signaling capacity of individual neurons. Axons and dendrites in the central nervous system are typically only about a Micrometer thick, while some of those in the peripheral nervous system are much thicker. The soma is usually about 10–25 micrometers in diameter and not much larger than the Cell Nucleus it contains. The longest axon of a human Motoneuron can be over a meter long, reaching from the base of the spine to the toes, while Giraffe s have single axons running along the whole length of their necks, several meters in length. Much of what we currently know about axonal function comes from studying the Squid Giant Axon , an ideal experimental preparation for research due to its relatively immense size (0.5–1 millimeters thick, several centimeters long). CLASSES Functional classification There are three functional classes of neurons: afferent neurons, efferent neurons, and interneurons.
''Afferent'' and ''efferent'' can also refer to neurons which convey information from one region of the brain to another. Structural classification Most neurons can be anatomically characterized into one of three categories:
CONNECTIVITY ''Main article: Synapse '' Neurons communicate with one another and to other cells through Synapse s, where the axon terminal of one cell impinges upon a dendrite or soma of another, or less commonly to an axon. Neurons such as the Purkinje Cell s in the Cerebellum , can have over 1000 dendrites each, enabling connections with tens of thousands of other cells. Synapses can either be Excitatory or Inhibitory and will either respectively increase or decrease activity in the target neuron. Neurons can also communicate via Electrical Synapse s, which are direct, electrically-conductive Junctions between cells. In a chemical synapse, the process of synaptic transmission is as follows: When an Action Potential reaches the axon terminal, the wave of changing charges opens Voltage-gated Calcium Channels , thus allowing Calcium Ions to enter the presynaptic terminal. Calcium causes synaptic vesicles filled with Neurotransmitter molecules to fuse with the membrane and then release their contents into the synaptic cleft. The neurotransmitters diffuse across the synaptic cleft and activate Receptors on the postsynaptic neuron. The Human Brain has a gigantic number of synapses. Each of 100 billion neurons has on average 7,000 synaptic connections to other neurons. Most authorities estimate total number of synapses at 1,000 trillion for a three-year-old child. This number declines with age, stabilizing by adulthood. Estimates vary for an adult, ranging from 100 to 500 trillion synapses. {Link without Title} ADAPTATIONS TO CARRYING ACTION POTENTIALS The cell membrane in the axon and soma contain Voltage-gated Ion Channel s which allow the neuron to generate and propagate an electrical impulse known as an Action Potential . Substantial early knowledge of neuron electrical activity came from experiments with into the giant squid axons, accurate measurements could be made of the Membrane Potential . Electrical activity can be produced in neurons by a number of stimuli. Pressure , stretch, Chemical Transmitters , and electrical current passing across the nerve membrane as a result of a potential difference in voltage all can initiate nerve activity {Link without Title} . The narrow cross-section of axons lessens the metabolic expense of carrying s in the central nervous system and Schwann Cell s in the peripheral nervous system. The sheath enables the action potentials to travel Faster than in unmyelinated axons of the same diameter whilst simultaneously spending less energy to "recharge" the action potential after. The myelin sheath in peripheral nerves normally runs along the axon in sections about 1 mm long, punctuated by unsheathed Nodes Of Ranvier which contain a high density of voltage-gated ion channels. Multiple Sclerosis is a neurological disorder which results from abnormal demyelination of peripheral nerves. Neurons with demyelinated axons do not conduct electrical signals properly. HISTOLOGY AND INTERNAL STRUCTURE Hippocampus showing various classes of cells.]] Nerve cell bodies stained with basophilic dyes will show numerous microscopic clumps of Nissl substance (named after German psychiatrist and neuropathologist Franz Nissl , 1860–1919), which consists of rough Endoplasmic Reticulum and associated Ribosomes . The prominence of the Nissl substance can be explained by the fact that nerve cells are metabolically very active, and hence are involved in large amounts of Protein Synthesis . The cell body of a neuron is supported by a complex meshwork of structural proteins called Neurofilament s, which are assembled into larger '''neurofibrils'''. Some neurons also contain pigment granules, such as '''neuromelanin''' (a brownish-black pigment, byproduct of synthesis of Catecholamine s) and ''' Lipofuscin ''' (yellowish-brown pigment that accumulates with age). CHALLENGES TO THE NEURON DOCTRINE While the neuron doctrine has remained a central tenet of modern neuroscience, recent studies challenging this view have suggested that the narrow confines of this doctrine need to be expanded. Among the most serious challenges to the neuron doctrine is the fact that Electrical Synapse s are more common in the central nervous system than previously thought. This means that rather than functioning as individual units, in some parts of the brain large ensembles of neurons may be active together in order to process neural information. A second challenge comes from the fact that Dendrites , like Axons , also have Voltage-gated Ion Channel s and can generate Electrical Potentials which convey information to and from the soma. This challenges the view that dendrites are simply passive recipients of information and axons the sole transmitters. It also suggests that the neuron is not simply active as a single element, but that complex computations can occur within a single neuron. Third, the role of Glia in processing neural information has begun to be appreciated. Neurons and glia make up the two chief cell types of the Central Nervous System . There are far more glial cells than neurons: It has been estimated that glial cells outnumber neurons by as many as 10:1. Recent experimental results have suggested that glial cells play a vital role in information processing among neurons, indicating that neurons may not be the sole information processing cells in the nervous system. Finally, the traditional view among neuroscientists that the primate brain is different from other organs in that it is not capable of repairing itself or growing new cells has been challenged by more recent research showing that neurogenesis, or the generation of new neurons, can be demonstrated in adult primate brains and that neurogenesis is environment-dependent rather than age-related, being halted by survival-type stress factors. [http://www.princeton.edu/pr/news/99/q4/1014-brain.htm NEURONS IN THE BRAIN The number of neurons contained within the brain varies dramatically across Species . For example the human brain has about 100 billion () neurons and 100 trillion () connections ( Synapses ) between them. In contrast, the nematode worm ('' Caenorhabditis Elegans '') has 302 neurons. Scientists have mapped all of the nematode's neurons. As a result, such worms are ideal candidates for neurobiological experiments and tests. Many properties of neurons, ranging from the type of Neurotransmitter used to Ion Channel composition are maintained across species, allowing scientists to study processes occurring in more complex organisms in much simpler experimental systems. Recent research has challenged the view that neurogenesis, or the generation of new neurons, does not occur in adult primate brains. This research has shown that neurogenesis can be environment-dependent in addition to being age-related and is halted by survival-type stress factors. [http://www.princeton.edu/pr/news/99/q4/1014-brain.htm SEE ALSO SOURCES
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