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of neurons in the pigeon cerebellum. (A) Denotes Purkinje Cell s, an example of a bipolar neuron. (B) Denotes Granule Cells which are multipolar.]] Neurons (also known as '''neurones''' and '''nerve cells''') are electrically excitable Cells in the Nervous System that process and transmit information. In Vertebrate animals, neurons are the core components of the Brain , Spinal Cord and peripheral nerves. Neurons are typically composed of a Soma , or cell body, a Dendritic Tree and an Axon . The majority of vertebrate neurons receive input on the cell body and dendritic tree, and transmit output via the axon. However, there is great heterogeneity throughout the nervous system and the animal kingdom, in the size, shape and function of neurons. Neurons communicate via Chemical and Electrical Synapse s, in a process known as Synaptic Transmission . The fundamental process that triggers synaptic transmission is the Action Potential , a propagating electrical signal that is generated by exploiting the Electrically Excitable Membrane of the neuron. This is also known as a wave of depolarization. HISTORY ANATOMY AND HISTOLOGY Neurons are highly specialized for the processing and transmission of cellular signals. Given the diversity of functions performed by neurons in different parts of the nervous system, there is, as expected, a wide variety in the shape, size, and electrochemical properties of neurons. For instance, the soma of a neuron can vary from 4 to 100 micrometers in diameter. The Neuron: Size Comparison
Although the canonical view of the neuron attributes dedicated functions to its various anatomical components, dendrites and axons often act in ways contrary to their so-called main function. Axons and dendrites in the central nervous system are typically only about one Micrometer thick, while some in the peripheral nervous system are much thicker. The soma is usually about 10–25 micrometers in diameter and often is 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. Sensory neurons have axons that run from the toes to the Dorsal Columns , over 1.5 meters in adults. Giraffe s have single axons several meters in length running along the entire length of their necks. Much of what is known about axonal function comes from studying the Squid Giant Axon , an ideal experimental preparation because of its relatively immense size (0.5–1 millimeters thick, several centimeters long). CLASSES expressing .]] Structural classification Polarity Most neurons can be anatomically characterized as:
Other Furthermore, some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are:
Functional classification Direction
''Afferent'' and ''efferent'' can also refer generally to neurons which, respectively, bring information to or send information from the brain region. Action on other neurons
Discharge patterns Neurons can be classified according to their Electrophysiological characteristics:
Neurotransmitter released Some examples are cholinergic, GABAergic, glutamatergic and dopaminergic neurons. CONNECTIVITY See Also: Synapse Neurons communicate with one another via 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 Purkinje Cell s in the Cerebellum can have over 1000 dendritic branches, making connections with tens of thousands of other cells; other neurons, such as the magnocellular neurons of the Supraoptic Nucleus , have only one or two dendrites, each of which receives thousands of synapses. Synapses can be Excitatory or Inhibitory and will either increase or decrease activity in the target neuron. Some neurons 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, it opens Voltage-gated Calcium Channels , allowing Calcium Ions to enter the terminal. Calcium causes synaptic vesicles filled with neurotransmitter molecules to fuse with the membrane, releasing 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 huge number of synapses. Each of the 1012 (one trillion) neurons (including Glia l cells) has on average 7,000 synaptic connections to other neurons. It has been estimated that the brain of a three-year-old child has about 1016 synapses (10 quadrillion). This number declines with age, stabilizing by adulthood. Estimates vary for an adult, ranging from 1015 to 5 x 1015 synapses (1 to 5 quadrillion).2 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 (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 difference in voltage can all initiate nerve activity. Electrical activity of nerves The narrow cross-section of axons lessens the metabolic expense of carrying action potentials, but thicker axons convey impulses more rapidly. To minimize metabolic expense while maintaining rapid conduction, many neurons have insulating sheaths of s in the central nervous system and Schwann Cell s in the peripheral nervous system. The sheath enables action potentials to travel Faster than in unmyelinated axons of the same diameter, whilst using less energy. 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 that results from demyelination of axons in the Central Nervous System . Some neurons do not generate action potentials, but instead generate a graded electrical signal, which in turn causes graded neurotransmitter release. Such nonspiking neurons tend to be sensory neurons or interneurons, because they cannot carry signals long distances. HISTOLOGY AND INTERNAL STRUCTURE tissue.]] Nerve cell bodies stained with basophilic dyes 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). There are different internal structural characteristics between axons and dendrites. Axons typically almost never contain Ribosomes , except some in the initial segment. Dendrites contain granular Endoplasmic Reticulum or Ribosomes , with diminishing amounts with distance from the cell body. THE NEURON DOCTRINE The ''', 2007 .. As with all doctrines, there are some exceptions. For example Glial Cell s may also play a role in information processing3. Also, Electrical Synapse s are more common than previously thought, 4 meaning that there are direct-cytoplasmic connections between neurons. In fact, there are examples of neurons forming even tighter coupling; the squid giant axon arises from the fusion of multiple neurons that retain individual cell bodies and the crayfish giant axon consists of a series of neurons with high conductance septate junctions. The Law of Dynamic Polarization also has important exceptions; dendrites can serve as synaptic output sites of neurons5 and axons can receive synaptic inputs. NEURONS IN THE BRAIN The number of neurons in the brain varies dramatically from species to species.Williams, R and Herrup, K (2001). "The Control of Neuron Number." Originally published in ''The Annual Review of Neuroscience'' 11:423–453 (1988). Last revised Sept 28, 2001. Retrieved from http://www.nervenet.org/papers/NUMBER_REV_1988.html on May 12, 2007. One estimate puts the human brain at about 100 billion () neurons and 100 trillion () synapses. By contrast, the nematode worm ('' Caenorhabditis Elegans '') has just 302 neurons making it an ideal experimental subject as scientists have been able to map all of the organism's neurons. By contrast, ''Drosophila melanogaster'' (the fruit fly) has around 300,000 neurons (which do spike) and exhibits many complex behaviors. Many properties of neurons, from the type of neurotransmitters used to Ion Channel composition, are maintained across species, allowing scientists to study processes occurring in more complex organisms in much simpler experimental systems. REFERENCES SOURCES
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