Charles John Huffam Dickens was an English writer and social critic. He created some of the world's best-known fictional characters and is regarded by many as the greatest novelist of the Victorian era, his works enjoyed unprecedented popularity during his lifetime, by the 20th century critics and scholars had recognised him as a literary genius. His novels and short stories are still read today. Born in Portsmouth, Dickens left school to work in a factory when his father was incarcerated in a debtors' prison. Despite his lack of formal education, he edited a weekly journal for 20 years, wrote 15 novels, five novellas, hundreds of short stories and non-fiction articles and performed readings extensively, was an indefatigable letter writer, campaigned vigorously for children's rights and other social reforms. Dickens's literary success began with the 1836 serial publication of The Pickwick Papers. Within a few years he had become an international literary celebrity, famous for his humour and keen observation of character and society.
His novels, most published in monthly or weekly instalments, pioneered the serial publication of narrative fiction, which became the dominant Victorian mode for novel publication. Cliffhanger endings in his serial publications kept readers in suspense; the instalment format allowed Dickens to evaluate his audience's reaction, he modified his plot and character development based on such feedback. For example, when his wife's chiropodist expressed distress at the way Miss Mowcher in David Copperfield seemed to reflect her disabilities, Dickens improved the character with positive features, his plots were constructed, he wove elements from topical events into his narratives. Masses of the illiterate poor chipped in ha'pennies to have each new monthly episode read to them, opening up and inspiring a new class of readers. Dickens was regarded as the literary colossus of his age, his 1843 novella, A Christmas Carol, remains popular and continues to inspire adaptations in every artistic genre. Oliver Twist and Great Expectations are frequently adapted, like many of his novels, evoke images of early Victorian London.
His 1859 novel, A Tale of Two Cities, set in London and Paris, is his best-known work of historical fiction. Dickens has been praised by fellow writers—from Leo Tolstoy to George Orwell, G. K. Chesterton and Tom Wolfe—for his realism, prose style, unique characterisations, social criticism. On the other hand, Oscar Wilde, Henry James, Virginia Woolf complained of a lack of psychological depth, loose writing, a vein of sentimentalism; the term Dickensian is used to describe something, reminiscent of Dickens and his writings, such as poor social conditions or comically repulsive characters. Charles John Huffam Dickens was born on 7 February 1812, at 1 Mile End Terrace, Landport in Portsea Island, the second of eight children of Elizabeth Dickens and John Dickens, his father was temporarily stationed in the district. He asked Christopher Huffam, rigger to His Majesty's Navy and head of an established firm, to act as godfather to Charles. Huffam is thought to be the inspiration for Paul Dombey, the owner of a shipping company in Dickens's novel Dombey and Son.
In January 1815, John Dickens was called back to London, the family moved to Norfolk Street, Fitzrovia. When Charles was four, they relocated to Sheerness, thence to Chatham, where he spent his formative years until the age of 11, his early life seems to have been idyllic, though he thought himself a "very small and not-over-particularly-taken-care-of boy". Charles spent time outdoors, but read voraciously, including the picaresque novels of Tobias Smollett and Henry Fielding, as well as Robinson Crusoe and Gil Blas, he reread The Arabian Nights and the Collected Farces of Elizabeth Inchbald. He retained poignant memories of childhood, helped by an excellent memory of people and events, which he used in his writing, his father's brief work as a clerk in the Navy Pay Office afforded him a few years of private education, first at a dame school, at a school run by William Giles, a dissenter, in Chatham. This period came to an end in June 1822, when John Dickens was recalled to Navy Pay Office headquarters at Somerset House, the family moved to Camden Town in London.
The family had left Kent amidst mounting debts, living beyond his means, John Dickens was forced by his creditors into the Marshalsea debtors' prison in Southwark, London in 1824. His wife and youngest children joined him there. Charles 12 years old, boarded with Elizabeth Roylance, a family friend, at 112 College Place, Camden Town. Roylance was "a reduced old lady, long known to our family", whom Dickens immortalised, "with a few alterations and embellishments", as "Mrs Pipchin" in Dombey and Son, he lived in a back-attic in the house of an agent for the Insolvent Court, Archibald Russell, "a fat, good-natured, kind old gentleman... with a quiet old wife" and lame son, in Lant Street in Southwark. They provided the inspiration for the Garlands in The Old Curiosity Shop. On Sundays—with his sister Frances, free from her studies at the Royal Academy of Music—he spent the day at the Marshalsea. Dickens used the prison as a setting in Little Dorrit. To pay for his board and to help his family, Dickens was forced to leave school and work ten-hour days at Warren's Blacking Warehouse, on Hungerford Stairs, near the present Charing Cross railway station, where he earned six shillings
Bleak House is a novel by Charles Dickens, first published as a 20 episode serial between March 1852 and September 1853. The novel has many characters and several sub-plots, is told by the novel's heroine, Esther Summerson, by an omniscient narrator. At the centre of Bleak House is a long-running legal case in the Court of Chancery and Jarndyce, which came about because a testator wrote several conflicting wills. In a preface to the 1853 first edition, Dickens claimed there were many actual precedents for his fictional case. One such was the Thellusson v Woodford case in which a will read in 1797 was contested and not determined until 1859. Though the legal profession criticised Dickens's satire as exaggerated, this novel helped support a judicial reform movement which culminated in the enactment of legal reform in the 1870s. There is some debate among scholars as to; the English legal historian Sir William Holdsworth sets the action in 1827. Sir Leicester Dedlock and his wife Honoria live on his estate at Chesney Wold.
Unknown to Sir Leicester, before she married, Lady Dedlock had a lover, Captain Hawdon, had a daughter by him. Lady Dedlock believes; the daughter, Esther, is in Lady Dedlock's sister. Esther does not know. After Miss Barbary dies, John Jarndyce becomes Esther's guardian and assigns the Chancery lawyer "Conversation" Kenge to take charge of her future. After attending school for six years, Esther moves in with him at Bleak House. Jarndyce assumes custody of two other wards, Richard Carstone and Ada Clare, they are beneficiaries in one of the wills at issue in Jarndyce. Richard and Ada soon fall in love, but though Mr Jarndyce does not oppose the match, he stipulates that Richard must first choose a profession. Richard first tries a career in medicine, Esther meets Allan Woodcourt, a physician, at the house of Richard's tutor; when Richard mentions the prospect of gaining from the resolution of Jarndyce and Jarndyce, John Jarndyce beseeches him never to put faith in what he calls "the family curse".
Meanwhile, Lady Dedlock is a beneficiary under one of the wills. Early in the book, while listening to the reading of an affidavit by the family solicitor, Mr Tulkinghorn, she recognises the handwriting on the copy; the sight affects her so much she faints, which Tulkinghorn notices and investigates. He traces a pauper known only as "Nemo", in London. Nemo has died, the only person to identify him is a street-sweeper, a poor homeless boy named Jo, who lives in a grim and poverty-stricken part of the city known as Tom-All-Alone's. Lady Dedlock is investigating, disguised as her maid, Mademoiselle Hortense. Lady Dedlock pays Jo to take her to Nemo's grave. Meanwhile, Tulkinghorn is concerned Lady Dedlock's secret could threaten the interests of Sir Leicester and watches her even enlisting her maid to spy on her, he enlists Inspector Bucket to run Jo out of town, to eliminate any loose ends that might connect Nemo to the Dedlocks. Esther sees Lady Dedlock at church and talks with her at Chesney Wold – though neither woman recognises their connection.
Lady Dedlock does discover that Esther is her child. However, Esther has become sick after nursing the homeless boy Jo. Lady Dedlock waits. Though Esther and Lady Dedlock are happy to be reunited, Lady Dedlock tells Esther they must never acknowledge their connection again. Upon her recovery, Esther finds that Richard, having failed at several professions, has disobeyed his guardian and is trying to push Jarndyce and Jarndyce to conclusion in his and Ada's favour. In the process, Richard declines in health, he and Ada have secretly married, Ada is pregnant. Esther has her own romance when Mr Woodcourt returns to England, having survived a shipwreck, continues to seek her company despite her disfigurement. Esther has agreed to marry her guardian, John Jarndyce. Hortense and Tulkinghorn discover the truth about Lady Dedlock's past. After a confrontation with Tulkinghorn, Lady Dedlock flees her home, leaving a note apologising for her conduct. Tulkinghorn dismisses Hortense, no longer of any use to him.
Feeling abandoned and betrayed, Hortense kills Tulkinghorn and seeks to frame Lady Dedlock for his murder. Sir Leicester, discovering his lawyer's death and his wife's flight, suffers a catastrophic stroke, but he manages to communicate that he forgives his wife and wants her to return. Inspector Bucket, who has investigated several matters related to Jarndyce and Jarndyce, accepts Sir Leicester's commission to find Lady Dedlock. At first he suspects Lady Dedlock of the murder but is able to clear her of suspicion after discovering Hortense's guilt, he requests Esther's help to find her. Lady Dedlock has no way to know of her husband's forgiveness or that she has been cleared of suspicion, she wanders the country in cold weather before dying at the cemetery of her former lover, Captain Hawdon. Esther and Bucket find her there. Progress in Jarndyce and Jarndyce seems to take a turn for the better when a will is found, which revokes all previous wills and leaves the bulk of the estate to Richard and Ada.
Meanwhile, John Jarndyce cancels his engagement to Esther. The
Optical coherence tomography
Optical coherence tomography is an imaging technique that uses low-coherence light to capture micrometer-resolution, two- and three-dimensional images from within optical scattering media. It is used for industrial nondestructive testing. Optical coherence tomography is based on low-coherence interferometry employing near-infrared light; the use of long wavelength light allows it to penetrate into the scattering medium. Confocal microscopy, another optical technique penetrates less into the sample but with higher resolution. Depending on the properties of the light source, optical coherence tomography has achieved sub-micrometer resolution. Optical coherence tomography is one of a class of optical tomographic techniques. Commercially available optical coherence tomography systems are employed in diverse applications, including art conservation and diagnostic medicine, notably in ophthalmology and optometry where it can be used to obtain detailed images from within the retina, it has begun to be used in interventional cardiology to help diagnose coronary artery disease, in dermatology to improve diagnosis.
A recent implementation of optical coherence tomography, frequency-domain optical coherence tomography, provides advantages in the signal-to-noise ratio provided, thus permitting faster signal acquisition. Starting from white-light interferometry for in vivo ocular eye measurements imaging of biological tissue of the human eye, was investigated by multiple groups worldwide. A first two-dimensional in vivo depiction of a human eye fundus along a horizontal meridian based on white light interferometric depth scans was presented at the ICO-15 SAT conference in 1990. Further developed in 1990 by Naohiro Tanno a professor at Yamagata University, in particular since 1991 by Huang et al. in Prof. James Fujimoto laboratory at Massachusetts Institute of Technology, optical coherence tomography with micrometer resolution and cross-sectional imaging capabilities has become a prominent biomedical tissue-imaging technique. First in vivo OCT images – displaying retinal structures – were published in 1993 and first endoscopic images in 1997.
OCT has been used for various art conservation projects, where it is used to analyze different layers in a painting. OCT has interesting advantages over other medical imaging systems. Medical ultrasonography, magnetic resonance imaging, confocal microscopy, OCT are differently suited to morphological tissue imaging: while the first two have whole body but low resolution imaging capability, the third one can provide images with resolutions well below 1 micrometer, between 0 and 100 micrometers in depth, the fourth can probe as deep as 500 micrometers, but with a lower resolution. OCT is based on low-coherence interferometry. In conventional interferometry with long coherence length, interference of light occurs over a distance of meters. In OCT, this interference is shortened to a distance of micrometers, owing to the use of broad-bandwidth light sources. Light with broad bandwidths can be generated by using superluminescent diodes or lasers with short pulses. White light is an example of a broadband source with lower power.
Light in an OCT system is broken into two arms -- a reference arm. The combination of reflected light from the sample arm and reference light from the reference arm gives rise to an interference pattern, but only if light from both arms have traveled the "same" optical distance. By scanning the mirror in the reference arm, a reflectivity profile of the sample can be obtained. Areas of the sample that reflect back a lot of light will create greater interference than areas that don't. Any light, outside the short coherence length will not interfere; this reflectivity profile, called an A-scan, contains information about the spatial dimensions and location of structures within the item of interest. A cross-sectional tomograph may be achieved by laterally combining a series of these axial depth scans. A face imaging at an acquired depth is possible depending on the imaging engine used. Optical Coherence Tomography, or ‘OCT’, is a technique for obtaining sub-surface images of translucent or opaque materials at a resolution equivalent to a low-power microscope.
It is ‘optical ultrasound’, imaging reflections from within tissue to provide cross-sectional images. OCT has attracted interest among the medical community because it provides tissue morphology imagery at much higher resolution than other imaging modalities such as MRI or ultrasound; the key benefits of OCT are: Live sub-surface images at near-microscopic resolution Instant, direct imaging of tissue morphology No preparation of the sample or subject No ionizing radiationOCT delivers high resolution because it is based on light, rather than sound or radio frequency. An optical beam is directed at the tissue, a small portion of this light that reflects from sub-
The human eye is an organ which reacts to light and pressure. As a sense organ, the mammalian eye allows vision. Human eyes help to provide a three dimensional, moving image coloured in daylight. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth; the human eye can differentiate between about 10 million colors and is capable of detecting a single photon. Similar to the eyes of other mammals, the human eye's non-image-forming photosensitive ganglion cells in the retina receive light signals which affect adjustment of the size of the pupil and suppression of the hormone melatonin and entrainment of the body clock; the eye is not shaped like a perfect sphere, rather it is a fused two-piece unit, composed of the anterior segment and the posterior segment. The anterior segment is made up of the cornea and lens; the cornea is transparent and more curved, is linked to the larger posterior segment, composed of the vitreous, retina and the outer white shell called the sclera.
The cornea is about 11.5 mm in diameter, 1/2 mm in thickness near its center. The posterior chamber constitutes the remaining five-sixths; the cornea and sclera are connected by an area termed the limbus. The iris is the pigmented circular structure concentrically surrounding the center of the eye, the pupil, which appears to be black; the size of the pupil, which controls the amount of light entering the eye, is adjusted by the iris' dilator and sphincter muscles. Light energy enters the eye through the cornea, through the pupil and through the lens; the lens shape is controlled by the ciliary muscle. Photons of light falling on the light-sensitive cells of the retina are converted into electrical signals that are transmitted to the brain by the optic nerve and interpreted as sight and vision. Dimensions differ among adults by only one or two millimetres, remarkably consistent across different ethnicities; the vertical measure less than the horizontal, is about 24 mm. The transverse size of a human adult eye is 24.2 mm and the sagittal size is 23.7 mm with no significant difference between sexes and age groups.
Strong correlation has been found between the width of the orbit. The typical adult eye has an anterior to posterior diameter of 24 millimetres, a volume of six cubic centimetres, a mass of 7.5 grams.. The eyeball grows increasing from about 16–17 millimetres at birth to 22.5–23 mm by three years of age. By age 12, the eye attains its full size; the eye is made up of layers, enclosing various anatomical structures. The outermost layer, known as the fibrous tunic, is composed of the sclera; the middle layer, known as the vascular tunic or uvea, consists of the choroid, ciliary body, pigmented epithelium and iris. The innermost is the retina, which gets its oxygenation from the blood vessels of the choroid as well as the retinal vessels; the spaces of the eye are filled with the aqueous humour anteriorly, between the cornea and lens, the vitreous body, a jelly-like substance, behind the lens, filling the entire posterior cavity. The aqueous humour is a clear watery fluid, contained in two areas: the anterior chamber between the cornea and the iris, the posterior chamber between the iris and the lens.
The lens is suspended to the ciliary body by the suspensory ligament, made up of hundreds of fine transparent fibers which transmit muscular forces to change the shape of the lens for accommodation. The vitreous body is a clear substance composed of water and proteins, which give it a jelly-like and sticky composition; the approximate field of view of an individual human eye varies by facial anatomy, but is 30° superior, 45° nasal, 70° inferior, 100° temporal. For both eyes combined visual field is 200 ° horizontal, it is 13700 square degrees for binocular vision. When viewed at large angles from the side, the iris and pupil may still be visible by the viewer, indicating the person has peripheral vision possible at that angle. About 15° temporal and 1.5° below the horizontal is the blind spot created by the optic nerve nasally, 7.5° high and 5.5° wide. The retina has a static contrast ratio of around 100:1; as soon as the eye moves to acquire a target, it re-adjusts its exposure by adjusting the iris, which adjusts the size of the pupil.
Initial dark adaptation takes place in four seconds of profound, uninterrupted darkness. The process is nonlinear and multifaceted, so an interruption by light exposure requires restarting the dark adaptation process over again. Full adaptation is dependent on good blood flow; the human eye can detect a luminance range of 1014, or one hundred trillion, from 10−6 cd/m2, or one millionth of a candela per square meter to 108 cd/m2 or one hundred million candelas per square meter. This range does not include looking at the midday lightning discharge. At the low end o
Inflammation is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, is a protective response involving immune cells, blood vessels, molecular mediators. The function of inflammation is to eliminate the initial cause of cell injury, clear out necrotic cells and tissues damaged from the original insult and the inflammatory process, initiate tissue repair; the five classical signs of inflammation are heat, redness and loss of function. Inflammation is a generic response, therefore it is considered as a mechanism of innate immunity, as compared to adaptive immunity, specific for each pathogen. Too little inflammation could lead to progressive tissue destruction by the harmful stimulus and compromise the survival of the organism. In contrast, chronic inflammation may lead to a host of diseases, such as hay fever, atherosclerosis, rheumatoid arthritis, cancer. Inflammation is therefore closely regulated by the body. Inflammation can be classified as either chronic.
Acute inflammation is the initial response of the body to harmful stimuli and is achieved by the increased movement of plasma and leukocytes from the blood into the injured tissues. A series of biochemical events propagates and matures the inflammatory response, involving the local vascular system, the immune system, various cells within the injured tissue. Prolonged inflammation, known as chronic inflammation, leads to a progressive shift in the type of cells present at the site of inflammation, such as mononuclear cells, is characterized by simultaneous destruction and healing of the tissue from the inflammatory process. Inflammation is not a synonym for infection. Infection describes the interaction between the action of microbial invasion and the reaction of the body's inflammatory response—the two components are considered together when discussing an infection, the word is used to imply a microbial invasive cause for the observed inflammatory reaction. Inflammation on the other hand describes purely the body's immunovascular response, whatever the cause may be.
But because of how the two are correlated, words ending in the suffix -itis are sometimes informally described as referring to infection. For example, the word urethritis means only "urethral inflammation", but clinical health care providers discuss urethritis as a urethral infection because urethral microbial invasion is the most common cause of urethritis, it is useful to differentiate inflammation and infection because there are typical situations in pathology and medical diagnosis where inflammation is not driven by microbial invasion – for example, trauma and autoimmune diseases including type III hypersensitivity. Conversely, there is pathology where microbial invasion does not cause the classic inflammatory response – for example, parasitosis or eosinophilia. Acute inflammation is a short-term process appearing within a few minutes or hours and begins to cease upon the removal of the injurious stimulus, it involves a coordinated and systemic mobilization response locally of various immune and neurological mediators of acute inflammation.
In a normal healthy response, it becomes activated, clears the pathogen and begins a repair process and ceases. It is characterized by five cardinal signs:An acronym that may be used to remember the key symptoms is "PRISH", for pain, immobility and heat; the traditional names for signs of inflammation come from Latin: Dolor Calor Rubor Tumor Functio laesa The first four were described by Celsus, while loss of function was added by Galen. However, the addition of this fifth sign has been ascribed to Thomas Sydenham and Virchow. Redness and heat are due to increased blood flow at body core temperature to the inflamed site. Loss of function has multiple causes. Acute inflammation of the lung does not cause pain unless the inflammation involves the parietal pleura, which does have pain-sensitive nerve endings; the process of acute inflammation is initiated by resident immune cells present in the involved tissue resident macrophages, dendritic cells, Kupffer cells and mast cells. These cells possess surface receptors known as pattern recognition receptors, which recognize two subclasses of molecules: pathogen-associated molecular patterns and damage-associated molecular patterns.
PAMPs are compounds that are associated with various pathogens, but which are distinguishable from host molecules. DAMPs are compounds that are associated with host-related cell damage. At the onset of an infection, burn, or other injuries, these cells undergo activation and release inflammatory mediators responsible for the clinical signs of inflammation. Vasodilation and its resulting increased blood flow causes increased heat. Increased permeability of the blood vessels results in an exudation of plasma proteins and fluid into the tissue, which manifests itself as swelling; some of the released mediators such as bradykinin increase the sensitivity to pain. The mediator molecules alter the blood vessels to
Visual perception is the ability to interpret the surrounding environment using light in the visible spectrum reflected by the objects in the environment. This is different from visual acuity, which refers to how a person sees. A person can have problem with visual perceptual processing if he/she has 20/20 vision; the resulting perception is known as visual perception, sight, or vision. The various physiological components involved in vision are referred to collectively as the visual system, are the focus of much research in linguistics, cognitive science and molecular biology, collectively referred to as vision science; the visual system in animals allows individuals to assimilate information from their surroundings. The act of seeing starts when the cornea and the lens of the eye focuses light from its surroundings onto a light-sensitive membrane in the back of the eye, called the retina; the retina is part of the brain, isolated to serve as a transducer for the conversion of light into neuronal signals.
Based on feedback from the visual system, the lens of the eye adjusts its thickness to focus light on the photoreceptive cells of the retina known as the rods and cones, which detect the photons of light and respond by producing neural impulses. These signals are processed via complex feedforward and feedback processes by different parts of the brain, from the retina upstream to central ganglia in the brain. Note that up until now much of the above paragraph could apply to octopuses, worms and things more primitive. However, the following applies to mammals and birds: The retina in these more complex animals sends fibers to the lateral geniculate nucleus, to the primary and secondary visual cortex of the brain. Signals from the retina can travel directly from the retina to the superior colliculus; the perception of objects and the totality of the visual scene is accomplished by the visual association cortex. The visual association cortex combines all sensory information perceived by the striate cortex which contains thousands of modules that are part of modular neural networks.
The neurons in the striate cortex send axons to the extrastriate cortex, a region in the visual association cortex that surrounds the striate cortex. The human visual system is believed to perceive visible light in the range of wavelengths between 370 and 730 nanometers of the electromagnetic spectrum. However, some research suggests that humans can perceive light in wavelengths down to 340 nanometers the young; the major problem in visual perception is that what people see is not a translation of retinal stimuli. Thus people interested in perception have long struggled to explain what visual processing does to create what is seen. There were two major ancient Greek schools, providing a primitive explanation of how vision is carried out in the body; the first was the "emission theory" which maintained that vision occurs when rays emanate from the eyes and are intercepted by visual objects. If an object was seen directly it was by'means of rays' coming out of the eyes and again falling on the object.
A refracted image was, seen by'means of rays' as well, which came out of the eyes, traversed through the air, after refraction, fell on the visible object, sighted as the result of the movement of the rays from the eye. This theory was championed by scholars like their followers; the second school advocated the so-called'intro-mission' approach which sees vision as coming from something entering the eyes representative of the object. With its main propagators Aristotle and their followers, this theory seems to have some contact with modern theories of what vision is, but it remained only a speculation lacking any experimental foundation. Both schools of thought relied upon the principle that "like is only known by like", thus upon the notion that the eye was composed of some "internal fire" which interacted with the "external fire" of visible light and made vision possible. Plato makes this assertion in his dialogue Timaeus, in his De Sensu. Alhazen carried out many investigations and experiments on visual perception, extended the work of Ptolemy on binocular vision, commented on the anatomical works of Galen.
He was the first person to explain that vision occurs when light bounces on an object and is directed to one's eyes. Leonardo da Vinci is believed to be the first to recognize the special optical qualities of the eye, he wrote "The function of the human eye... was described by a large number of authors in a certain way. But I found it to be different." His main experimental finding was that there is only a distinct and clear vision at the line of sight—the optical line that ends at the fovea. Although he did not use these words he is the father of the modern distinction between foveal and peripheral vision. Issac Newton was the first to discover through experimentation, by isolating individual colors of the spectrum of light passing through a prism, that the visually perceived color of objects appeared due to the character
An axon, or nerve fiber, is a long, slender projection of a nerve cell, or neuron, in vertebrates, that conducts electrical impulses known as action potentials away from the nerve cell body. The function of the axon is to transmit information to different neurons and glands. In certain sensory neurons, such as those for touch and warmth, the axons are called afferent nerve fibers and the electrical impulse travels along these from the periphery to the cell body, from the cell body to the spinal cord along another branch of the same axon. Axon dysfunction has caused many inherited and acquired neurological disorders which can affect both the peripheral and central neurons. Nerve fibers are classed into three types – group A nerve fibers, group B nerve fibers, group C nerve fibers. Groups A and B are myelinated, group C are unmyelinated; these groups include both sensory fibers and motor fibers. Another classification groups only the sensory fibers as Type I, Type II, Type III, Type IV. An axon is one of two types of cytoplasmic protrusions from the cell body of a neuron.
Axons are distinguished from dendrites by several features, including shape and function. Some types of neurons have no transmit signals from their dendrites. In some species, axons can emanate from dendrites and these are known as axon-carrying dendrites. No neuron has more than one axon. Axons are covered by a membrane known as an axolemma. Most axons branch, in some cases profusely; the end branches of an axon are called telodendria. The swollen end of a telodendron is known as the axon terminal which joins the dendron or cell body of another neuron forming a synaptic connection. Axons make contact with other cells—usually other neurons but sometimes muscle or gland cells—at junctions called synapses. In some circumstances, the axon of one neuron may form a synapse with the dendrites of the same neuron, resulting in an autapse. At a synapse, the membrane of the axon adjoins the membrane of the target cell, special molecular structures serve to transmit electrical or electrochemical signals across the gap.
Some synaptic junctions appear along the length of an axon as it extends—these are called en passant synapses and can be in the hundreds or the thousands along one axon. Other synapses appear as terminals at the ends of axonal branches. A single axon, with all its branches taken together, can innervate multiple parts of the brain and generate thousands of synaptic terminals. A bundle of axons make a nerve tract in the central nervous system, a fascicle in the peripheral nervous system. In placental mammals the largest white matter tract in the brain is the corpus callosum, formed of some 20 million axons in the human brain. Axons are the primary transmission lines of the nervous system, as bundles they form nerves; some axons can extend up to more while others extend as little as one millimeter. The longest axons in the human body are those of the sciatic nerve, which run from the base of the spinal cord to the big toe of each foot; the diameter of axons is variable. Most individual axons are microscopic in diameter.
The largest mammalian axons can reach a diameter of up to 20 µm. The squid giant axon, specialized to conduct signals rapidly, is close to 1 millimetre in diameter, the size of a small pencil lead; the numbers of axonal telodendria can differ from one nerve fiber to the next. Axons in the central nervous system show multiple telodendria, with many synaptic end points. In comparison, the cerebellar granule cell axon is characterized by a single T-shaped branch node from which two parallel fibers extend. Elaborate branching allows for the simultaneous transmission of messages to a large number of target neurons within a single region of the brain. There are two types of axons in the nervous system: unmyelinated axons. Myelin is a layer of a fatty insulating substance, formed by two types of glial cells Schwann cells and oligodendrocytes. In the peripheral nervous system Schwann cells form the myelin sheath of a myelinated axon. In the central nervous system oligodendrocytes form the insulating myelin.
Along myelinated nerve fibers, gaps in the myelin sheath known as nodes of Ranvier occur at evenly spaced intervals. The myelination enables an rapid mode of electrical impulse propagation called saltatory conduction; the myelinated axons from the cortical neurons form the bulk of the neural tissue called white matter in the brain. The myelin gives the white appearance to the tissue in contrast to the grey matter of the cerebral cortex which contains the neuronal cell bodies. A similar arrangement is seen in the cerebellum. Bundles of myelinated axons make up the nerve tracts in the CNS. Where these tracts cross the midline of the brain to connect opposite regions they are called commissures; the largest of these is the corpus callosum that connects the two cerebral hemispheres, this has around 20 million axons. The structure of a neuron is seen to consist of two separate functional regions, or compartments – the cell body together with the dendrites as one region, the axonal region as the other.
The axonal region or compart