The Siamese cat is one of the first distinctly recognized breeds of Asian cat. Derived from the Wichianmat landrace, one of several varieties of cat native to Thailand, the Siamese became one of the most popular breeds in Europe and North America in the 19th century; the refined modern Siamese is characterized by blue almond-shaped eyes. The International Cat Association describes the modern Siamese as affectionate, social and playful into adulthood enjoying a game of fetch. Siamese tend to seek human interaction and like companionship from other cats; the Siamese is among the foundation stock of several other breeds developed by crossbreeding with other cats. The Siamese cat comes in two distinct variations: traditional, with an apple-shaped head and a chubby body. A description and depiction of the Wichienmaat first appears in a collection of ancient manuscripts called the Tamra Maew thought to originate from the Ayutthaya Kingdom. Over a dozen are now kept in the National Library of Thailand, while others have resurfaced outside of Thailand and are now in the British Library and National Library of Australia.
In addition to the old Siamese cat, the Tamra Maew describes other heritage cats of Thailand including the Korat cat which are still bred for preservation in Thailand today and have become popular in other countries, Konja cat, Suphalak. When the capital city Ayutthaya was sacked on 7 April 1767 at the end of the Burmese-Siamese war the Burmese army burned everything in sight and returned to Burma taking Siamese noblemen and royal family members with them as captives. Buddha images were hacked apart for their gold, all the royal treasures were stolen. Thai legend has it that the Burmese King Hsinbyushin found and read the poem for the Thai cats in the Tamra Maew; the poem describes the all Thai cats as being as rare as gold, anyone that owns this cat will become wealthy. He told his army to round up all the Suphalak cats and bring them back to Burma along with the other treasures. Today in Thailand this legend is told as a humorous explanation as to why the all Thai cats are so rare; the pointed cat known in the West as "Siamese", recognized for its distinctive markings, is one of several breeds of cats from Siam described and illustrated in manuscripts called "Tamra Maew", estimated to have been written from the 14th to the 18th century.
In 1878, U. S. President Rutherford B. Hayes received the first documented Siamese to reach the United States, a cat named "Siam" sent by the American Consul in Bangkok. In 1884, the British Consul-General in Bangkok, Edward Blencowe Gould, brought a breeding pair of the cats and Mia, back to Britain as a gift for his sister, Lilian Jane Gould. In 1885, Gould's UK cats Pho and Mia produced three Siamese kittens—Duen Ngai and Khromata—who were shown with their parents that same year at London's Crystal Palace Show, their unique appearance and distinct behaviour attracted attention but all three of the kittens died soon after the show, their cause of death not documented. By 1886, another pair was imported to the UK by her sister, Ada. Compared to the British Shorthair and Persian cats that were familiar to most Britons, these Siamese imports were longer and less "cobby" in body types, had heads that were less rounded with wedge-shaped muzzles and had larger ears; these differences and the pointed coat pattern, which had not been seen before in cats by Westerners, produced a strong impression—one early viewer described them as "an unnatural nightmare of a cat."
Over the next several years, fanciers imported a small number of cats, which together formed the base breeding pool for the entire breed in Britain. It is believed that most Siamese in Britain today are descended from about eleven of these original imports. In their early days in Britain, they were called the "Royal Cat of Siam", reflecting reports that they had been kept only by Siamese royalty. Research has not shown evidence of any organised royal breeding programme in Siam; the original Siamese imports were medium-sized, rather long-bodied, graceful cats with moderately wedge-shaped heads and ears that were comparatively large but in proportion to the size of the head. The cats were not extreme in either way. In the 1950s–1960s, as the Siamese was increasing in popularity, many breeders and cat show judges began to favor the more slender look; as a result of generations of selective breeding, they created long, fine-boned, narrow-headed cats. By the mid
The tiger is the largest species among the Felidae and classified in the genus Panthera. It is most recognizable for its dark vertical stripes on reddish-orange fur with a lighter underside, it is an apex predator preying on ungulates such as deer and bovids. It is territorial and a solitary but social predator, requiring large contiguous areas of habitat, which support its requirements for prey and rearing of its offspring. Tiger cubs stay with their mother for about two years, before they become independent and leave their mother's home range to establish their own; the tiger once ranged from Eastern Anatolia Region in the west to the Amur River basin, in the south from the foothills of the Himalayas to Bali in the Sunda islands. Since the early 20th century, tiger populations have lost at least 93% of their historic range and have been extirpated in Western and Central Asia, from the islands of Java and Bali, in large areas of Southeast and South Asia and China. Today's tiger range is fragmented, stretching from Siberian temperate forests to subtropical and tropical forests on the Indian subcontinent and Sumatra.
The tiger is listed as Endangered on the IUCN Red List since 1986. As of 2015, the global wild tiger population was estimated to number between 3,062 and 3,948 mature individuals, down from around 100,000 at the start of the 20th century, with most remaining populations occurring in small pockets isolated from each other. Major reasons for population decline include habitat destruction, habitat fragmentation and poaching. This, coupled with the fact that it lives in some of the more densely populated places on Earth, has caused significant conflicts with humans; the tiger is among the most popular of the world's charismatic megafauna. It featured prominently in ancient mythology and folklore and continues to be depicted in modern films and literature, appearing on many flags, coats of arms and as mascots for sporting teams; the tiger is the national animal of India, Bangladesh and South Korea. The Middle English tigre and Old English tigras derive from Latin tigris; this was a borrowing of Classical Greek τίγρις'tigris', a foreign borrowing of unknown origin meaning'tiger' as well as the river Tigris.
The origin may have been the Persian word tigra meaning'pointed or sharp', the Avestan word tigrhi'arrow' referring to the speed of the tiger's leap, although these words are not known to have any meanings associated with tigers. The generic name Panthera is traceable to the Old French word'pantère', the Latin word panthera, the Ancient Greek word πάνθηρ'panther'; the Sanskrit word पाण्डर pând-ara means'pale yellow, white'. In 1758, Carl Linnaeus described the tiger in his work Systema Naturae and gave it the scientific name Felis tigris. In 1929, the British taxonomist Reginald Innes Pocock subordinated the species under the genus Panthera using the scientific name Panthera tigris; the tiger's closest living relatives were thought to be the Panthera species lion and jaguar. Results of genetic analysis indicate that about 2.88 million years ago, the tiger and the snow leopard lineages diverged from the other Panthera species, that both may be more related to each other than to the lion and jaguar.
P. T. palaeosinensis from the Early Pleistocene of northern China is the most primitive known tiger to date. Fossil remains of Panthera zdanskyi were excavated in Gansu province of northwestern China; this species lived at the beginning of the Pleistocene about two million years ago, is considered to be a sister taxon of the modern tiger. It was about the size of a jaguar and had a different coat pattern. Despite being considered more "primitive", it was functionally and also ecologically similar to the modern tiger. Northwestern China is thought to be the origin of the tiger lineage. Tigers grew in size in response to adaptive radiations of prey species like deer and bovids, which may have occurred in Southeast Asia during the early Pleistocene. Panthera tigris trinilensis lived about 1.2 million years ago and is known from fossils excavated near Trinil in Java. The Wanhsien, Ngandong and Japanese tigers became extinct in prehistoric times. Tigers reached India and northern Asia in the late Pleistocene, reaching eastern Beringia and Sakhalin.
Some fossil skulls are morphologically distinct from lion skulls, which could indicate tiger presence in Alaska during the last glacial period, about 100,000 years ago. Tiger fossils found in the island of Palawan were smaller than mainland tiger fossils due to insular dwarfism. Fossil remains of tigers were excavated in Sri Lanka, Japan, Sarawak dating to the late Pliocene and Early Holocene; the Bornean tiger was present in Borneo between the Late Pleistocene and the Holocene, but may have gone extinct in prehistoric times. The potential tiger range during the Late Pleistocene and Holocene was predicted applying ecological niche modelling based on more than 500 tiger locality records combined with bioclimatic data; the resulting model shows a contiguous tiger range from southern India to Siberia at the Last Glacial Maximum, indicating an unobstructed gene flow between tiger populations in mainland Asia throughout the Late Pleistocene and Holocene. The tiger populations on the Sunda Islands and mainland Asia were separated during interglacial periods.
Results of a phylogeographic study indicate that all living tigers had a common ancestor 72,000–108,000 years ago. The tiger's full genome sequence was published in 2013, it was found to have similar repeat composition to other cat genomes and an appreciably conserved synteny. Following Linnaeus's first descriptions of t
Decussation is used in biological contexts to describe a crossing. The anatomical term chiasma is named after the Greek uppercase'Χ', chi). Examples include: In the brain, where nerve fibers obliquely cross from one lateral part to the other, to say they cross at a level other than their origin. See for examples Decussation of pyramids and sensory decussation. Decussation describes the point where the nerves cross from one side of the brain to the other, the nerves from the left side of the body decussate to the right side of the brain and the nerves from the right side of the body decussate to the left brain, however depending on the function of the nerves the level of decussation is variable. In neuroanatomy the term chiasma is reserved for the crossing of nerves outside the brain, such as the optic chiasm. In botanical leaf taxology, the word decussate describes an opposite pattern of leaves which has successive pairs at right angles to each other. In effect, successive pairs of leaves cross each other.
Basil is a classic example of a decussate leaf pattern. In tooth enamel, where bundles of rods cross each other as they travel from the enamel-dentine junction to the outer enamel surface, or near to it. In taxonomic description where decussate markings or structures occur, names such as decussatus or decussata or otherwise in part containing "decuss..." are common in the specific epithet. The origin of the contralateral organization, the optic chiasm and the major decussations on the nervous system of vertebrates has been a long standing puzzle to scientists. For long the visual map theory of Ramón y Cajal has been the most popular theory. More scientists have realized that this theory has some severe flaws. According to the current theory, the decussations are caused by an axial twist which makes it so that the anterior head, along with the forebrain, is turned by 180° with respect to the rest of the body. Commissure Why does the nervous system decussate?: Stanford Neuroblog Media related to Decussation at Wikimedia Commons
Stereopsis is a term, most used to refer to the perception of depth and 3-dimensional structure obtained on the basis of visual information deriving from two eyes by individuals with developed binocular vision. Because the eyes of humans, many animals, are located at different lateral positions on the head, binocular vision results in two different images projected to the retinas of the eyes; the differences are in the relative horizontal position of objects in the two images. These positional differences are referred to as horizontal disparities or, more binocular disparities. Disparities are processed in the visual cortex of the brain to yield depth perception. While binocular disparities are present when viewing a real 3-dimensional scene with two eyes, they can be simulated by artificially presenting two different images separately to each eye using a method called stereoscopy; the perception of depth in such cases is referred to as "stereoscopic depth". The perception of depth and 3-dimensional structure is, possible with information visible from one eye alone, such as differences in object size and motion parallax, though the impression of depth in these cases is not as vivid as that obtained from binocular disparities.
Therefore, the term stereopsis can refer to the unique impression of depth associated with binocular vision. It has been suggested that the impression of "real" separation in depth is linked to the precision with which depth is derived, that a conscious awareness of this precision – perceived as an impression of interactability and realness – may help guide the planning of motor action. There are two distinct aspects to stereopsis: coarse stereopsis and fine stereopsis, provide depth information of different degree of spatial and temporal precision. Coarse stereopsis appears to be used to judge stereoscopic motion in the periphery, it provides the sense of being immersed in one's surroundings and is therefore sometimes referred to as qualitative stereopsis. Coarse stereopsis is important for orientation in space while moving, for example when descending a flight of stairs. Fine stereopsis is based on static differences, it allows the individual to determine the depth of objects in the central visual area and is therefore called quantitative stereopsis.
It is measured in random-dot tests. Fine stereopsis is important for fine-motor tasks such as threading a needle; the stereopsis which an individual can achieve is limited by the level of visual acuity of the poorer eye. In particular, patients who have comparatively lower visual acuity tend to need larger spatial frequencies to be present in the input images, else they cannot achieve stereopsis. Fine stereopsis requires both eyes to have a good visual acuity in order to detect small spatial differences, is disrupted by early visual deprivation. There are indications that in the course of the development of the visual system in infants, coarse stereopsis may develop before fine stereopsis and that coarse stereopsis guides the vergence movements which are needed in order for fine stereopsis to develop in a subsequent stage. Furthermore, there are indications that coarse stereopsis is the mechanism that keeps the two eyes aligned after strabismus surgery, it has been suggested to distinguish between two different types of stereoscopic depth perception: static depth perception and motion-in-depth perception.
Some individuals who have strabismus and show no depth perception using static stereotests do perceive motion in depth when tested using dynamic random dot stereograms. One study found the combination of motion stereopsis and no static stereopsis to be present only in exotropes, not in esotropes. There are strong indications that the stereoscopic mechanism consists of at least two perceptual mechanisms three. Coarse and fine stereopsis are processed by two different physiological subsystems, with a coarse stereopsis being derived from diplopic stimuli and yielding only a vague impression of depth magnitude. Coarse stereopsis appears to be associated with the magno pathway which processes low spatial frequency disparities and motion, fine stereopsis with the parvo pathway which processes high spatial frequency disparities; the coarse stereoscopic system seems to be able to provide residual binocular depth information in some individuals who lack fine stereopsis. Individuals have been found to integrate the various stimuli, for example stereoscopic cues and motion occlusion, in different ways.
How the brain combines the different cues – including stereo, vergence angle and monocular cues – for sensing motion in depth and 3D object position is an area of active research in vision science and neighboring disciplines. Not everyone has the same ability to see using stereopsis. One study shows that 97.3% are able to distinguish depth at horizontal disparities of 2.3 minutes of arc or smaller, at least 80% could distinguish depth at horizontal differences of 30 seconds of arc. Stereopsis has a positive impact on exercising practical tasks such
The visual system is the part of the central nervous system which gives organisms the ability to process visual detail as sight, as well as enabling the formation of several non-image photo response functions. It detects and interprets information from visible light to build a representation of the surrounding environment; the visual system carries out a number of complex tasks, including the reception of light and the formation of monocular representations. The psychological process of visual information is known as visual perception, a lack of, called blindness. Non-image forming visual functions, independent of visual perception, include the pupillary light reflex and circadian photoentrainment; this article describes the visual system of mammals, humans in particular, although other "higher" animals have similar visual systems. Together the cornea and lens shine it on the retina; the retina transduces this image into electrical pulses using cones. The optic nerve carries these pulses through the optic canal.
Upon reaching the optic chiasm the nerve fibers decussate. The fibers branch and terminate in three places. Most of the optic nerve fibers end in the lateral geniculate nucleus. Before the LGN forwards the pulses to V1 of the visual cortex it gauges the range of objects and tags every major object with a velocity tag; these tags predict object movement. The LGN sends some fibers to V2 and V3. V1 performs edge-detection to understand spatial organization. V2 receives them. Pulvinar is responsible for visual attention. V2 serves much the same function as V1, however, it handles illusory contours, determining depth by comparing left and right pulses, foreground distinguishment. V2 connects to V1 - V5. V3 helps process ‘global motion’ of objects. V3 connects to V1, V2, the inferior temporal cortex. V4 recognizes simple shapes, gets input from V1, V2, V3, LGN, pulvinar. V5’s outputs include V4 and its surrounding area, eye-movement motor cortices. V5’s functionality is similar to that of the other V’s, however, it integrates local object motion into global motion on a complex level.
V6 works in conjunction with V5 on motion analysis. V5 analyzes self-motion. V6’s primary input is V1, with V5 additions. V6 houses the topographical map for vision. V6 outputs to the region directly around it. V6A has direct connections including the premotor cortex; the inferior temporal gyrus recognizes complex shapes and faces or, in conjunction with the hippocampus, creates new memories. The pretectal area is seven unique nuclei. Anterior and medial pretectal nuclei inhibit pain, aid in REM, aid the accommodation reflex, respectively; the Edinger-Westphal nucleus moderates pupil dilation and aids in convergence of the eyes and lens adjustment. Nuclei of the optic tract are involved in smooth pursuit eye movement and the accommodation reflex, as well as REM; the suprachiasmatic nucleus is the region of the hypothalamus that halts production of melatonin at first light. The eye the retina The optic nerve The optic chiasma The optic tract The lateral geniculate body The optic radiation The visual cortex The visual association cortex.
These are divided into posterior pathways. The anterior visual pathway refers to structures involved in vision before the lateral geniculate nucleus; the posterior visual pathway refers to structures after this point. Light entering the eye is refracted, it passes through the pupil and is further refracted by the lens. The cornea and lens act together as a compound lens to project an inverted image onto the retina; the retina consists of a large number of photoreceptor cells which contain particular protein molecules called opsins. In humans, two types of opsins are involved in conscious vision: cone opsins. An opsin absorbs a photon and transmits a signal to the cell through a signal transduction pathway, resulting in hyper-polarization of the photoreceptor. Rods and cones differ in function. Rods are found in the periphery of the retina and are used to see at low levels of light. Cones are found in the center of the retina. There are three types of cones that differ in the wavelengths of light they absorb.
Cones are used to distinguish color and other features of the visual world at normal levels of light. In the retina, the photoreceptors synapse directly onto bipolar cells, which in turn synapse onto ganglion cells of the outermost layer, which will conduct action potentials to the br
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
Semaphorins are a class of secreted and membrane proteins that were identified as axonal growth cone guidance molecules. They act as short-range inhibitory signals and signal through multimeric receptor complexes. Semaphorins are cues to deflect axons from inappropriate regions important in neural system development; the major class of proteins that act as their receptors are called plexins, with neuropilins as their co-receptors in many cases. The main receptors for semaphorins are plexins, which have established roles in regulating Rho-family GTPases. Recent work shows that plexins can influence R-Ras, which, in turn, can regulate integrins; such regulation is a common feature of semaphorin signalling and contributes to our understanding of semaphorin biology. Every semaphorin is characterised by the expression of a specific region of about 500 amino acids called the sema domain. Semaphorins were named after the English word Semaphore, which originated from Greek, meaning sign-bearer; the Semaphorins are grouped into eight major classes based on structure and phylogenetic tree analyses.
The first seven are ordered by number, from class 1 to class 7. The eighth group is class V. Classes 1 and 2 are found in invertebrates only, whilst classes 3, 4, 6, 7 are found in vertebrates only. Class 5 is found in both vertebrates and invertebrates, class V is specific to viruses. Classes 1 and 6 are considered to be homologues of each other; the same applies to classes 2 and 3. Each class of Semaphorin has many subgroups of different molecules that share similar characteristics. For example, Class 3 Semaphorins range from SEMA3A to SEMA3G. In humans, the genes are: SEMA3A, SEMA3B, SEMA3C, SEMA3D, SEMA3E, SEMA3F, SEMA3G SEMA4A, SEMA4B, SEMA4C, SEMA4D, SEMA4F, SEMA4G SEMA5A, SEMA5B SEMA6A, SEMA6B, SEMA6C, SEMA6D SEMA7A Different semaphorins use different types of receptors: Most Semaphorins use receptors in the group of proteins known as plexins. Class 3 semaphorins signal through heterocomplexes of neuropilins, Class A Plexins, cell adhesion molecules, the makeup of these complexes provides specificity for binding and transducing signals from different Class 3 Semaphorins.
Class 7 Semaphorin are thought to use integrins as their receptors. Semaphorins are versatile, their discovery was in regards to axon guidance in the limb buds of grasshoppers in 1992, but since it has been discovered that semaphorins have a role in many processes. They not only guide axons in development, but have major roles in immune function and the development of bones. Class 3 semaphorins are one of the most versatile semaphorin classes, in which Sema3a is the most studied. During development and their receptors may be involved in the sorting of pools of motor neurons and the modulation of pathfinding for afferent and efferent axons from and to these pools. For instance, Sema3a repels axons from the dorsal root ganglia, facial nerves, vagal nerves, olfactory-sensory, cortical nerves, hippocampal nerves and cerebellar nerves. Class 3 semaphorins have an important function after traumatic central nervous system injuries, such as spinal cord injury, they regulate neuronal and non-neuronal cells associated with the traumatic injury due to their presence in the scar tissue.
Class 3 semaphorins modulate axonal regrowth, re-vascularisation, re-myelination and the immune response after central nervous system trauma. Semaphorins at the US National Library of Medicine Medical Subject Headings