Drawing by Santiago Ramón y Cajal of neurons in the pigeon cerebellum. Santiago Ramón y Cajal ( May 1 1852 &ndash October 17 1934) was a Spanish histologist, Physician, and (A) Denotes Purkinje cells, an example of a bipolar neuron. For the cells of the Electrical conduction system of the heart, see Purkinje fibers Purkinje cells (or Purkinje neurons) are a class of (B) Denotes granule cells which are multipolar. Granular cell also refers to Juxtaglomerular cell in the kidney In Neuroscience, granule cells refer to tiny neurons (a type of

Neurons (IPA: /njˈɒɹɒns/, also known as neurones and nerve cells) are electrically excitable cells in the nervous system that process and transmit information. The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called The nervous system is a Network of specialized cells that communicate information about an animal's surroundings and itself Neurons are the core components of the brain, and spinal cord in vertebrates and ventral nerve cord in invertebrates, and peripheral nerves. The brain is the center of the Nervous system in animals All Vertebrates and the majority of Invertebrates have a brain The spinal cord is a long thin tubular bundle of Nerves that is an extension of the Central nervous system from the brain and is enclosed in and protected The ventral nerve cords make up the Nervous system of some phyla of the Invertebrates particularly within the Nematodes Annelids and

Overview

Structure of a typical neuron

Neuron
Dendrite Soma Axon Nucleus Node of Ranvier Axon Terminal Schwann cell Myelin sheath

Fully developed neurons are permanently amitotic. Dendrites (from Greek δένδρον déndron, “tree” are the branched projections of a Neuron that act to conduct the electrochemical The soma, or cyton or perikaryon, is the bulbous end of a Neuron, containing the Cell nucleus. An axon or nerve fiber is a long slender projectionof a nerve cell or Neuron, that conducts electrical impulses away from the neuron's Cell In Cell biology, the nucleus (pl nuclei; from Latin la ''nucleus'' or la ''nuculeus'' "little nut" or kernel is a membrane-enclosed Nodes of Ranvier are known as the gaps (about 1 micrometer in diameter formed between myelin sheath cells along axons or nerve fibers Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands Named after the German physiologist Theodor Schwann, Schwann cells (also referred to as neurolemmocytes) are a variety of Glial cell that mainly Myelin is an electrically-insulating Dielectric Phospholipid layer that surrounds only the Axons of many Neurons It is an outgrowth Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei ^[1] (they do not divide); however, new recent research shows that additional neurons throughout the brain can originate from neural stem cells found in the subventricular zone and subgranular zone through the process of neurogenesis. Stem cells are cells found in most if not all multi-cellular Organisms. Subventricular zone ( SVZ) is a paired brain structure situated throughout the lateral walls of the Lateral ventricles. Subgranular zone ( SGZ) is a brain region in the Dentate gyrus where adult Neurogenesis occurs Neurogenesis ( birth of Neurons ' is the process by which neurons are created ^{[2][3][4][5][6][7]}

Neurons are typically composed of a soma, or cell body, a dendritic tree and an axon. The soma, or cyton or perikaryon, is the bulbous end of a Neuron, containing the Cell nucleus. Dendrites (from Greek δένδρον déndron, “tree” are the branched projections of a Neuron that act to conduct the electrochemical An axon or nerve fiber is a long slender projectionof a nerve cell or Neuron, that conducts electrical impulses away from the neuron's Cell 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 synapses, in a process known as synaptic transmission. Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands An electrical synapse is a mechanical and electrically conductive link between two abutting Neuron cells that is formed at a narrow gap between the pre- Neurotransmission (latin transmissio = passage crossing from transmitto = send let through also called synaptic transmission, is an electrical movement 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. In Neurophysiology, the action potential is a self-regenerating Wave of Electrochemical activity that allows Nerve cells to carry a signal Membrane potential (or transmembrane potential) is the Voltage difference (or Electrical potential difference between the interior and exterior of a This is also known as a wave of depolarization.

History

The neuron's place as the primary functional unit of the nervous system was first recognized in the early 20th century through the work of the Spanish anatomist Santiago Ramón y Cajal. Santiago Ramón y Cajal ( May 1 1852 &ndash October 17 1934) was a Spanish histologist, Physician, and ^[8] Cajal proposed that neurons were discrete cells that communicated with each other via specialized junctions, or spaces, between cells. ^[8] This became known as the neuron doctrine, one of the central tenets of modern neuroscience. The neuron doctrine is the now fundamental idea that Neurons are the basic structural and functional units of the Nervous system. ^[8] To observe the structure of individual neurons, Cajal used a silver staining method developed by his rival, Camillo Golgi. Golgi's method is a Nervous tissue Staining technique discovered by Italian Physician and Scientist Camillo Golgi (1843-1926 Camillo Golgi ( July 7, 1843 &ndash January 21, 1926) was an Italian Physician and Scientist. ^[8] The Golgi stain is an extremely useful method for neuroanatomical investigations because, for reasons unknown, it stains a very small percentage of cells in a tissue, so one is able to see the complete microstructure of individual neurons without much overlap from other cells in the densely packed brain. ^[9]

Anatomy and histology

Diagram of a typical myelinated vertebrate neuron. Myelin is an electrically-insulating Dielectric Phospholipid layer that surrounds only the Axons of many Neurons It is an outgrowth Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes

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. ^[10]

• The soma is the central part of the neuron. The soma, or cyton or perikaryon, is the bulbous end of a Neuron, containing the Cell nucleus. It contains the nucleus of the cell, and therefore is where most protein synthesis occurs. In Cell biology, the nucleus (pl nuclei; from Latin la ''nucleus'' or la ''nuculeus'' "little nut" or kernel is a membrane-enclosed Protein biosynthesis (synthesis is the process in which cells build Proteins The term is sometimes used to refer only to protein translation but more The nucleus ranges from 3 to 18 micrometers in diameter. ^[11]

• The dendrites of a neuron are cellular extensions with many branches, and metaphorically this overall shape and structure is referred to as a dendritic tree. Dendrites (from Greek δένδρον déndron, “tree” are the branched projections of a Neuron that act to conduct the electrochemical This is where the majority of input to the neuron occurs. Information outflow (i. e. from dendrites to other neurons) can also occur, but not across chemical synapses; there, the backflow of a nerve impulse is inhibited by the fact that an axon does not possess chemoreceptors and dendrites cannot secrete neurotransmitter chemicals. This unidirectionality of a chemical synapse explains why nerve impulses are conducted only in one direction.

• The axon is a finer, cable-like projection which can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. An axon or nerve fiber is a long slender projectionof a nerve cell or Neuron, that conducts electrical impulses away from the neuron's Cell The axon carries nerve signals away from the soma (and also carry some types of information back to it). Many neurons have only one axon, but this axon may - and usually will - undergo extensive branching, enabling communication with many target cells. The part of the axon where it emerges from the soma is called the axon hillock. "Hillock" redirects here A hillock is also a small Hill. Besides being an anatomical structure, the axon hillock is also the part of the neuron that has the greatest density of voltage-dependent sodium channels. This makes it the most easily-excited part of the neuron and the spike initiation zone for the axon: in neurological terms it has the most negative hyperpolarized action potential threshold. Hyperpolarization any change in a cell's Membrane potential that makes it more polarized In Neurophysiology, the action potential is a self-regenerating Wave of Electrochemical activity that allows Nerve cells to carry a signal While the axon and axon hillock are generally involved in information outflow, this region can also receive input from other neurons.

• The axon terminal contains synapses, specialized structures where neurotransmitter chemicals are released in order to communicate with target neurons. Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands See Chemical synapse for an introduction to concepts and terminology used in this article

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. A micrometre ( American spelling: micrometer; symbol µm) is one millionth of a Metre, or equivalently one thousandth of a Millimetre 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. In Vertebrates the term motor neuron (or motoneuron) classically applies to Neurons located in the Central nervous system (or CNS that project Sensory neurons have axons that run from the toes to the dorsal columns, over 1. The posterior column refers to the area of white matter in the dorsomedial side of the Spinal cord. 5 meters in adults. Giraffes have single axons several meters in length running along the entire length of their necks. The giraffe ( Giraffa camelopardalis) is an African Even-toed ungulate Mammal, the tallest of all land-living Animal Species 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. The squid giant axon is the very large (up to 1 mm in diameter typically around 0 5–1 millimeters thick, several centimeters long).

Classes

Image of pyramidal neurons in mouse cerebral cortex expressing green fluorescent protein. The cerebral cortex is a structure within the Brain that plays a key role in Memory, Attention, perceptual Awareness, Thought, The green fluorescent protein ( GFP) is composed of 238 Amino acids (26 The red staining indicates GABAergic interneurons. Gamma-aminobutyric acid (GABA is the chief inhibitory Neurotransmitter in the Mammalian Central nervous system. Source PLoS Biology [1]

SMI32-stained pyramidal neurons in cerebral cortex. The cerebral cortex is a structure within the Brain that plays a key role in Memory, Attention, perceptual Awareness, Thought,

Structural classification

Polarity

Most neurons can be anatomically characterized as:

• Unipolar or pseudounipolar: dendrite and axon emerging from same process. A pseudounipolar neuron ( pseudo - false uni - one is a sensory Neuron in the Peripheral nervous system.
• Bipolar: axon and single dendrite on opposite ends of the soma. A bipolar cell is a type of Neuron which has two extensions Bipolar cells are specialized sensory neurons for the transmission of special senses
• Multipolar: more than two dendrites:
  • Golgi I: neurons with long-projecting axonal processes; examples are pyramidal cells, Purkinje cells, and anterior horn cells. A multipolar neuron is a type of Neuron that possesses a single (usually long Axon and many Dendrites allowing for the integration of a great deal of information A pyramidal cell (or pyramidal neuron, or projection neuron) is a multipolar Neuron located in the Hippocampus and Cerebral For the cells of the Electrical conduction system of the heart, see Purkinje fibers Purkinje cells (or Purkinje neurons) are a class of The anterior horn of the Spinal cord (or anterior cornu, or anterior column, or ventral horn) is the ventral (front Grey matter
  • Golgi II: neurons whose axonal process projects locally; the best example is the granule cell. Granular cell also refers to Juxtaglomerular cell in the kidney In Neuroscience, granule cells refer to tiny neurons (a type of

Other

Furthermore, some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are:

• Basket cells, neurons with dilated and knotty dendrites in the cerebellum. Basket cells are inhibitory GABAergic Interneurons found in several brain regions the molecular layer of the Cerebellum, the Hippocampus The cerebellum ( Latin: "little brain" is a region of the Brain that plays an important role in the integration of sensory perception
• Betz cells, large motor neurons. Betz cells are Pyramidal cell Neurons located within the fifth layer of the Grey matter in the Primary motor cortex.
• Medium spiny neurons, most neurons in the corpus striatum. The medium spiny neurons are a special type of inhibitory cells representing approximately 90% of the Neurons within the Corpus striatum of the Basal ganglia The corpus striatum (striated body is a compound structure consisting of the Striatum and the Globus pallidus.
• Purkinje cells, huge neurons in the cerebellum, a type of Golgi I multipolar neuron. For the cells of the Electrical conduction system of the heart, see Purkinje fibers Purkinje cells (or Purkinje neurons) are a class of The cerebellum ( Latin: "little brain" is a region of the Brain that plays an important role in the integration of sensory perception
• Pyramidal cells, neurons with triangular soma, a type of Golgi I. A pyramidal cell (or pyramidal neuron, or projection neuron) is a multipolar Neuron located in the Hippocampus and Cerebral
• Renshaw cells, neurons with both ends linked to alpha motor neurons. Renshaw cells are inhibitory Interneurons found in the Gray matter of the Spinal cord, and are associated in two ways with an Alpha motor neuron Alpha motor neurons ( α-MNs) are large Lower motor neurons of the Brainstem and Spinal cord.
• Granule cells, a type of as Golgi II neuron. Granular cell also refers to Juxtaglomerular cell in the kidney In Neuroscience, granule cells refer to tiny neurons (a type of
• anterior horn cells, motoneurons located in the spinal cord. The anterior horn of the Spinal cord (or anterior cornu, or anterior column, or ventral horn) is the ventral (front Grey matter In Vertebrates the term motor neuron (or motoneuron) classically applies to Neurons located in the Central nervous system (or CNS that project

Functional classification

Direction

• Afferent neurons convey information from tissues and organs into the central nervous system and are sometimes also called sensory neurons. In the Nervous system, afferent neurons --otherwise known as sensory or receptor Neurons -carry nerve impulses from receptors or sense
• Efferent neurons transmit signals from the central nervous system to the effector cells and are sometimes called motor neurons. In the Nervous system, efferent nerves – otherwise known as motor or effector Neurons – carry nerve impulses away from the Effector cells are a type of Lymphocyte that are actively engaged in secreting antibodies.
• Interneurons connect neurons within specific regions of the central nervous system. An interneuron (also called association neuron, local circuit neuron or relay neuron) is a neuron which connects Afferent neurons and Efferent

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

• Excitatory neurons excite their target neurons. In Neuroscience, an excitatory postsynaptic potential ( EPSP) is a temporary depolarization of postsynaptic Membrane potential caused by the flow of positively Excitatory neurons in the central nervous system, including the brain, are often glutamatergic. Glutamic acid (abbreviated as Glu or E) is one of the 20 Alpha Amino acids It is not among the human Essential amino acids Its Neurons of the peripheral nervous system, such as spinal motoneurons that synapse onto muscle cells, often use acetylcholine as their excitatory neurotransmitter. The spinal cord is a long thin tubular bundle of Nerves that is an extension of the Central nervous system from the brain and is enclosed in and protected In Vertebrates the term motor neuron (or motoneuron) classically applies to Neurons located in the Central nervous system (or CNS that project The Chemical compound acetylcholine (often abbreviated ACh) is a Neurotransmitter in both the Peripheral nervous system (PNS and Central However, this is just a general tendency that may not always be true. It is not the neurotransmitter that decides excitatory or inhibitory action, but rather it is the postsynaptic receptor that is responsible for the action of the neurotransmitter.
• Inhibitory neurons inhibit their target neurons. An Inhibitory Postsynaptic Potential (commonly abbreviated as IPSP) is the change in membrane voltage of a postsynaptic Neuron which results from synaptic Inhibitory neurons are often interneurons. The output of some brain structures (neostriatum, globus pallidus, cerebellum) are inhibitory. The primary inhibitory neurotransmitters are GABA and glycine. Gamma-aminobutyric acid (GABA is the chief inhibitory Neurotransmitter in the Mammalian Central nervous system. Glycine (abbreviated as Gly or G) is the Organic compound with the formula NH2CH2COOH
• Modulatory neurons evoke more complex effects termed neuromodulation. In neuroscience neuromodulation is the process in which several classes of Neurotransmitters in the nervous system regulate diverse populations of Neurons (one These neurons use such neurotransmitters as dopamine, acetylcholine, serotonin and others. Dopamine is a Hormone and Neurotransmitter occurring in a wide variety of animals including both vertebrates and invertebrates The Chemical compound acetylcholine (often abbreviated ACh) is a Neurotransmitter in both the Peripheral nervous system (PNS and Central Serotonin (ˌsɛrəˈtoʊnən ( 5-hydroxytryptamine, or 5-HT) is a Monoamine Neurotransmitter synthesized in serotonergic Neurons

Discharge patterns

Neurons can be classified according to their electrophysiological characteristics:

• Tonic or regular spiking. Electrophysiology (from Greek grc ἥλεκτρον ēlektron, "amber" the [[Electron#Etymology|etymology of "electron"]] grc φύσις Some neurons are typically constantly (or tonically) active. Example: interneurons in neurostriatum.
• Phasic or bursting. Neurons that fire in bursts are called phasic.
• Fast spiking. Some neurons are notable for their fast firing rates, for example some types of cortical inhibitory interneurons, cells in globus pallidus.
• Thin-spike. Action potentials of some neurons are more narrow compared to the others. For example, interneurons in prefrontal cortex are thin-spike neurons.

Neurotransmitter released

Some examples are

• cholinergic neurons
• GABAergic neurons
• glutamatergic neurons
• dopaminergic neurons
• 5-hydroxytryptamine neurons (5-HT; serotonin)

Connectivity

Main article: Synapse

Neurons communicate with one another via synapses, where the axon terminal of one cell impinges upon another neuron's dendrite, soma or, less commonly, axon. Serotonin (ˌsɛrəˈtoʊnən ( 5-hydroxytryptamine, or 5-HT) is a Monoamine Neurotransmitter synthesized in serotonergic Neurons Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands Neurons such as Purkinje cells 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. For the cells of the Electrical conduction system of the heart, see Purkinje fibers Purkinje cells (or Purkinje neurons) are a class of The cerebellum ( Latin: "little brain" is a region of the Brain that plays an important role in the integration of sensory perception The supraoptic nucleus ( SON) is a nucleus of Magnocellular neurosecretory cells in the Hypothalamus of the mammalian brain Synapses can be excitatory or inhibitory and will either increase or decrease activity in the target neuron. In Neuroscience, an excitatory postsynaptic potential ( EPSP) is a temporary depolarization of postsynaptic Membrane potential caused by the flow of positively An Inhibitory Postsynaptic Potential (commonly abbreviated as IPSP) is the change in membrane voltage of a postsynaptic Neuron which results from synaptic Some neurons also communicate via electrical synapses, which are direct, electrically-conductive junctions between cells. An electrical synapse is a mechanical and electrically conductive link between two abutting Neuron cells that is formed at a narrow gap between the pre- A gap junction or nexus is a specialized Intercellular connection between certain animal cell -types

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. Voltage-dependent calcium channels (VDCC are a group of voltage-gated Ion channels found in excitable cells ( e Calcium (Ca2+ plays a vital role in the Anatomy, Physiology and Biochemistry of Organisms and of the cell, particularly 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. In Biochemistry, a receptor is a Protein molecule embedded in either the Plasma membrane or Cytoplasm of a cell to which a mobile signaling

The human brain has a huge number of synapses. The human brain controls the Central nervous system (CNS by way of the Cranial nerves and Spinal cord, the Peripheral nervous system (PNS Each of the 10¹¹ (one hundred billion) neurons 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 10¹⁵ synapses (1 quadrillion). This number declines with age, stabilizing by adulthood. Estimates vary for an adult, ranging from 10¹⁴ to 5 x 10¹⁴ synapses (0. 1 to 0. 5 quadrillion). ^[12]

Mechanisms for propagating action potentials

A signal propagating down an axon to the cell body and dendrites of the next cell.

The cell membrane in the axon and soma contain voltage-gated ion channels which allow the neuron to generate and propagate an electrical impulse (an action potential). Voltage-gated ion channels are a class of transmembrane Ion channels that are activated by changes in electrical Potential difference near the channel these In Neurophysiology, the action potential is a self-regenerating Wave of Electrochemical activity that allows Nerve cells to carry a signal Substantial early knowledge of neuron electrical activity came from experiments with squid giant axons. The squid giant axon is the very large (up to 1 mm in diameter typically around 0 In 1937, John Zachary Young suggested that the giant squid axon can be used to study neuronal electrical properties. John Zachary Young ( 18 March, 1907 &ndash 4 July, 1997) generally known as 'JZ' was an English Zoologist and Neurophysiologist ^[13] As they are much larger than human neurons, but similar in nature, it was easier to study them with the technology of that time. By inserting electrodes into the giant squid axons, accurate measurements could be made of the membrane potential. Electrophysiology (from Greek grc ἥλεκτρον ēlektron, "amber" the [[Electron#Etymology|etymology of "electron"]] grc φύσις Membrane potential (or transmembrane potential) is the Voltage difference (or Electrical potential difference between the interior and exterior of a

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. A mechanoreceptor is a Sensory receptor that responds to mechanical pressure or distortion ^[14]

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 myelin around their axons. Myelin is an electrically-insulating Dielectric Phospholipid layer that surrounds only the Axons of many Neurons It is an outgrowth The sheaths are formed by glial cells: oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. Glial cells, commonly called neuroglia or simply glia (Greek for "glue" are non- Neuronal cells that provide support and nutrition Origin Oligodendroglia arise during development from an Oligodendrocyte precursor cell, which can be identified by its expression of a number of Antigens, including Named after the German physiologist Theodor Schwann, Schwann cells (also referred to as neurolemmocytes) are a variety of Glial cell that mainly The sheath enables action potentials to travel faster than in unmyelinated axons of the same diameter, whilst using less energy. For Saltation definition and other use disambiguation see Saltation Saltatory conduction (from the Latin saltare, to hop or leap is 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. Nodes of Ranvier are known as the gaps (about 1 micrometer in diameter formed between myelin sheath cells along axons or nerve fibers Multiple sclerosis is a neurological disorder that results from demyelination of axons in the central nervous system. Multiple sclerosis (abbreviated MS also known as disseminated sclerosis or encephalomyelitis disseminata) is an autoimmune condition in which the In Vertebrates the central nervous system ( CNS) is the part of the Nervous system which is enclosed in the Meninges.

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.

All-or-none principle

The conduction of nerve impulses is an example of an all-or-none response. All or none (AON is a Stock market term usually mentioned when trading securities In other words, if a neuron responds at all, then it must respond completely. It is important to note that a greater intensity of stimulation produces more impulses per second, not a stronger impulse; in this way it is similar to a boolean function in computer programming. In Mathematics, a (finitary Boolean function is a function of the form f: B k &rarr B, where B  = {0 1}

Histology and internal structure

Golgi-stained neurons in human hippocampal tissue. The hippocampus is a part of the Forebrain, located in the medial Temporal lobe.

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 smooth endoplasmic reticulum and associated ribosomal RNA. Franz Nissl ( September 9, 1860 &ndash August 11, 1919) was a German Neuropathologist. The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle Ribosomal RNA ( rRNA) is the central component of the Ribosome, the protein manufacturing machinery of all living cells. 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 neurofilaments, which are assembled into larger neurofibrils. Neurofilaments are the 10 nanometer (10nm or Intermediate filaments found specifically in neurons Some neurons also contain pigment granules, such as neuromelanin (a brownish-black pigment, byproduct of synthesis of catecholamines) and lipofuscin (yellowish-brown pigment that accumulates with age). Lipofuscin is the name given to finely granular yellow brown Pigment granules composed of Lipid -containing residues of Lysosomal digestion

There are different internal structural characteristics between axons and dendrites. Axons typically almost never contain ribosomes, except some in the initial segment. Ribosomes ( from ribo nucleic acid and "Greek soma ( meaning body") are complexes of RNA and Protein that Dendrites contain granular endoplasmic reticulum or ribosomes, with diminishing amounts with distance from the cell body. The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle Ribosomes ( from ribo nucleic acid and "Greek soma ( meaning body") are complexes of RNA and Protein that

The neuron doctrine

The neuron doctrine is the now fundamental idea that neurons are the basic structural and functional units of the nervous system. The neuron doctrine is the now fundamental idea that Neurons are the basic structural and functional units of the Nervous system. The nervous system is a Network of specialized cells that communicate information about an animal's surroundings and itself The theory was put forward by Santiago Ramón y Cajal in the late 19th century. Santiago Ramón y Cajal ( May 1 1852 &ndash October 17 1934) was a Spanish histologist, Physician, and It held that neurons are discrete cells (not connected in a meshwork), acting as metabolically distinct units. Cajal further extended this to the Law of Dynamic Polarization, which states that neural transmission goes only in one direction, from dendrites toward axons. ^[15] As with all doctrines, there are some exceptions. For example glial cells may also play a role in information processing. Glial cells, commonly called neuroglia or simply glia (Greek for "glue" are non- Neuronal cells that provide support and nutrition ^[16] Also, electrical synapses are more common than previously thought,^[17] meaning that there are direct-cytoplasmic connections between neurons. An electrical synapse is a mechanical and electrically conductive link between two abutting Neuron cells that is formed at a narrow gap between the pre- 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 neurons^[18] and axons can receive synaptic inputs.

Neurons in the brain

The number of neurons in the brain varies dramatically from species to species. ^[19] One estimate puts the human brain at about 100 billion (10¹¹) neurons and 100 trillion (10¹⁴) synapses. ^[19] 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. Caenorhabditis elegans (ˌsiːnoʊræbˈdaɪtɪs ˈɛlɪgænz is a free-living Nematode (roundworm about 1  mm in length which By contrast, the fruit fly Drosophila melanogaster has around 300,000 neurons (which do spike) and exhibits many complex behaviors. Drosophila melanogaster (from the Greek for black-bellied dew-lover) is a two-winged insect that belongs to the Diptera, the order 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. Ion channels are pore-forming Proteins that help establish and control the small Voltage Gradient across the Plasma membrane of all living

Neurologic diseases

Alzheimer's disease: Alzheimer's disease (AD), also known simply as Alzheimer's, is a neurodegenerative disease characterized by progressive cognitive deterioration together with declining activities of daily living and neuropsychiatric symptoms or behavioral changes. Alzheimer's disease ( AD) also called Alzheimer disease or simply Alzheimer's, is the most common form of Dementia. The most striking early symptom is loss of short-term memory (amnesia), which usually manifests as minor forgetfulness that becomes steadily more pronounced with illness progression, with relative preservation of older memories. As the disorder progresses, cognitive (intellectual) impairment extends to the domains of language (aphasia), skilled movements (apraxia), recognition (agnosia), and functions such as decision-making and planning get impaired.

Parkinson's disease: Parkinson's disease (also known as Parkinson disease or PD) is a degenerative disorder of the central nervous system that often impairs the sufferer's motor skills and speech. Parkinson's disease (also known as Parkinson disease or PD) is a degenerative disorder of the Central nervous system that often impairs the sufferer's Parkinson's disease belongs to a group of conditions called movement disorders. It is characterized by muscle rigidity, tremor, a slowing of physical movement (bradykinesia), and in extreme cases, a loss of physical movement (akinesia). The primary symptoms are the results of decreased stimulation of the motor cortex by the basal ganglia, normally caused by the insufficient formation and action of dopamine, which is produced in the dopaminergic neurons of the brain. Secondary symptoms may include high level cognitive dysfunction and subtle language problems. PD is both chronic and progressive.

Myasthenia Gravis: Myasthenia gravis is a neuromuscular disease leading to fluctuating muscle weakness and fatigability. Myasthenia gravis (literally "serious muscle-weakness" from Greek μύς "muscle" "weakness" and Latin gravis Weakness is typically caused by circulating antibodies that block acetylcholine receptors at the post-synaptic neuromuscular junction, inhibiting the stimulative effect of the neurotransmitter acetylcholine. Myasthenia is treated with immunosuppressants, cholinesterase inhibitors and, in selected cases, thymectomy.

Demyelination

Demyelination is the act of demyelinating, or the loss of the myelin sheath insulating the nerves. When myelin degrades, conduction of signals along the nerve can be impaired or lost, and the nerve eventually withers. This leads to certain neurodegenerative disorders like multiple sclerosis, chronic inflammatory demyelinating polyneuropathy.

Axonal degeneration

Although most injury responses include a calcium influx signaling to promote resealing of severed parts, axonal injuries initially lead to acute axonal degeneration (AAD), which is rapid separation of the proximal and distal ends within 30 minutes of injury. Degeneration follows with swelling of the axolemma, and eventually leads to bead like formation. Granular disintegration of the axonal cytoskeleton and inner organelles occurs after axolemma degradation. Early changes include accumulation of mitochondria in the paranodal regions at the site of injury. Endoplasmic reticulum degrades and mitochondria swell up and eventually disintegrate. The disintegration is dependent on Ubiquitin and Calpain proteases (caused by influx of calcium ion), suggesting that axonal degeneration is an active process. Thus the axon undergoes complete fragmentation. The process takes about roughly 24 hrs in the PNS, and longer in the CNS. The signaling pathways leading to axolemma degeneration are currently unknown.

References

Sources

• Kandel E. R. , Schwartz, J. H. , Jessell, T. M. 2000. Principles of Neural Science, 4th ed. , McGraw-Hill, New York.
• Bullock, T. H. , Bennett, M. V. L. , Johnston, D. , Josephson, R. , Marder, E. , Fields R. D. 2005. The Neuron Doctrine, Redux, Science, V. 310, p. 791-793.
• Ramón y Cajal, S. 1933 Histology, 10th ed. , Wood, Baltimore.
• Roberts A. , Bush B. M. H. 1981. Neurones Without Impulses. Cambridge University Press, Cambridge.
• Peters, A. , Palay, S. L. , Webster, H, D. , 1991 The Fine Structure of the Nervous System, 3rd ed. , Oxford, New York.

External links

• NeuronBankan online neuromics tool for cataloging neuronal types and synaptic connectivity.
• High Resolution Neuroanatomical Images of Primate and Non-Primate Brains.