In biology, homology is the existence of shared ancestry between a pair of structures, or genes, in different taxa. A common example of homologous structures is the forelimbs of vertebrates, where the wings of bats, the arms of primates, the front flippers of whales and the forelegs of dogs and horses are all derived from the same ancestral tetrapod structure. Evolutionary biology explains homologous structures adapted to different purposes as the result of descent with modification from a common ancestor; the term was first applied to biology in a non-evolutionary context by the anatomist Richard Owen in 1843. Homology was explained by Charles Darwin's theory of evolution in 1859, but had been observed before this, from Aristotle onwards, it was explicitly analysed by Pierre Belon in 1555. In developmental biology, organs that developed in the embryo in the same manner and from similar origins, such as from matching primordia in successive segments of the same animal, are serially homologous.
Examples include the legs of a centipede, the maxillary palp and labial palp of an insect, the spinous processes of successive vertebrae in a vertebral column. Male and female reproductive organs are homologous if they develop from the same embryonic tissue, as do the ovaries and testicles of mammals including humans. Sequence homology between protein or DNA sequences is defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either a speciation event or a duplication event. Homology among proteins or DNA is inferred from their sequence similarity. Significant similarity is strong evidence that two sequences are related by divergent evolution from a common ancestor. Alignments of multiple sequences are used to discover the homologous regions. Homology remains controversial in animal behaviour, but there is suggestive evidence that, for example, dominance hierarchies are homologous across the primates. Homology was noticed by Aristotle, was explicitly analysed by Pierre Belon in his 1555 Book of Birds, where he systematically compared the skeletons of birds and humans.
The pattern of similarity was interpreted as part of the static great chain of being through the mediaeval and early modern periods: it was not seen as implying evolutionary change. In the German Naturphilosophie tradition, homology was of special interest as demonstrating unity in nature. In 1790, Goethe stated his foliar theory in his essay "Metamorphosis of Plants", showing that flower part are derived from leaves; the serial homology of limbs was described late in the 18th century. The French zoologist Etienne Geoffroy Saint-Hilaire showed in 1818 in his theorie d'analogue that structures were shared between fishes, reptiles and mammals; when Geoffroy went further and sought homologies between Georges Cuvier's embranchements, such as vertebrates and molluscs, his claims triggered the 1830 Cuvier-Geoffroy debate. Geoffroy stated the principle of connections, namely that what is important is the relative position of different structures and their connections to each other; the Estonian embryologist Karl Ernst von Baer stated what are now called von Baer's laws in 1828, noting that related animals begin their development as similar embryos and diverge: thus, animals in the same family are more related and diverge than animals which are only in the same order and have fewer homologies.
Von Baer's theory recognises that each taxon has distinctive shared features, that embryonic development parallels the taxonomic hierarchy: not the same as recapitulation theory. The term "homology" was first used in biology by the anatomist Richard Owen in 1843 when studying the similarities of vertebrate fins and limbs, defining it as the "same organ in different animals under every variety of form and function", contrasting it with the matching term "analogy" which he used to describe different structures with the same function. Owen codified 3 main criteria for determining if features were homologous: position and composition. In 1859, Charles Darwin explained homologous structures as meaning that the organisms concerned shared a body plan from a common ancestor, that taxa were branches of a single tree of life; the word homology, coined in about 1656, is derived from the Greek ὁμόλογος homologos from ὁμός homos "same" and λόγος logos "relation". Biological structures or sequences in different taxa are homologous if they are derived from a common ancestor.
Homology thus implies divergent evolution. For example, many insects possess two pairs of flying wings. In beetles, the first pair of wings has evolved into a pair of hard wing covers, while in Dipteran flies the second pair of wings has evolved into small halteres used for balance; the forelimbs of ancestral vertebrates have evolved into the front flippers of whales, the wings of birds, the running forelegs of dogs and horses, the short forelegs of frogs and lizards, the grasping hands of primates including humans. The same major forearm bones are found in fossils of lobe-finned fish such as Eusthenopteron; the opposite of homologous organs are analogous organs which do similar jobs in two taxa that were not present in their most recent common ancestor but rather evolved separately. For example, the wings of insects and birds evolved independently in separated groups, converged functionally to support powered flight, so they are analogous; the wings of a sycamore maple seed and the wings of a bird are analogous but not homologous, as they develop from quite different structures.
A structure can be only analogous at another. Pterosaur and bat wings are analogous as wings
Alertness is the state of active attention by high sensory awareness such as being watchful and prompt to meet danger or emergency, or being quick to perceive and act. It is related to psychology as well as to physiology. A lack of alertness is a symptom of a number of conditions, including narcolepsy, attention deficit disorder, chronic fatigue syndrome, Addison's disease, or sleep deprivation. Pronounced lack of alertness can be graded as an altered level of consciousness; the word is formed from "alert", which comes from the Italian "all'erta" People who have to be alert during their jobs, such as air traffic controllers or pilots face challenges maintaining their alertness. Research shows that for people "...engaged in attention-intensive and monotonous tasks, retaining a constant level of alertness is rare if not impossible." If people employed in safety-related or transportation jobs have lapses in alertness, this "may lead to severe consequences in occupations ranging from air traffic control to monitoring of nuclear power plants."
During the Second World War, US soldiers and aviators were given benzedrine, an amphetamine drug, to increase their alertness during long periods on duty. While air force pilots are able to use the drug to remain awake during combat flights, the use of amphetamines by commercial airline pilots is forbidden. British troops used 72 million amphetamine tablets in the second world war and the RAF used so many that "Methedrine won the Battle of Britain" according to one report. American bomber pilots use amphetamines to stay awake during long missions; the Tarnak Farm incident, in which an American F-16 pilot killed several friendly Canadian soldiers on the ground, was blamed by the pilot on his use of amphetamine. A nonjudicial hearing rejected the pilot's claim. Amphetamines are used by high-school students as a study and test-taking aid. Amphetamine increases energy levels and motivation, allowing students to study for an extended period of time; these drugs are acquired through ADHD prescriptions to students and peers, rather than illicitly produced drugs.
Cocaine is used to increase alertness. Eugeroics including Modafinil have gained popularity with the US Military. Vigilance is an important trait for animals. A reduction in alertness is observed for animals that live in larger groups. Studies on vigilance have been conducted on various animals including the scaly-breasted munia
Amphibians are ectothermic, tetrapod vertebrates of the class Amphibia. Modern amphibians are all Lissamphibia, they inhabit a wide variety of habitats, with most species living within terrestrial, arboreal or freshwater aquatic ecosystems. Thus amphibians start out as larvae living in water, but some species have developed behavioural adaptations to bypass this; the young undergo metamorphosis from larva with gills to an adult air-breathing form with lungs. Amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely on their skin, they are superficially similar to lizards but, along with mammals and birds, reptiles are amniotes and do not require water bodies in which to breed. With their complex reproductive needs and permeable skins, amphibians are ecological indicators; the earliest amphibians evolved in the Devonian period from sarcopterygian fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land.
They diversified and became dominant during the Carboniferous and Permian periods, but were displaced by reptiles and other vertebrates. Over time, amphibians shrank in size and decreased in diversity, leaving only the modern subclass Lissamphibia; the three modern orders of amphibians are Anura and Apoda. The number of known amphibian species is 8,000, of which nearly 90% are frogs; the smallest amphibian in the world is a frog from New Guinea with a length of just 7.7 mm. The largest living amphibian is the 1.8 m Chinese giant salamander, but this is dwarfed by the extinct 9 m Prionosuchus from the middle Permian of Brazil. The study of amphibians is called batrachology, while the study of both reptiles and amphibians is called herpetology; the word "amphibian" is derived from the Ancient Greek term ἀμφίβιος, which means "both kinds of life", ἀμφί meaning "of both kinds" and βιος meaning "life". The term was used as a general adjective for animals that could live on land or in water, including seals and otters.
Traditionally, the class Amphibia includes all tetrapod vertebrates. Amphibia in its widest sense was divided into three subclasses, two of which are extinct: Subclass Lepospondyli† Subclass Temnospondyli† Subclass Lissamphibia Salientia: Jurassic to present—6,200 current species in 53 families Caudata: Jurassic to present—652 current species in 9 families Gymnophiona: Jurassic to present—192 current species in 10 families The actual number of species in each group depends on the taxonomic classification followed; the two most common systems are the classification adopted by the website AmphibiaWeb, University of California and the classification by herpetologist Darrel Frost and the American Museum of Natural History, available as the online reference database "Amphibian Species of the World". The numbers of species cited above follows Frost and the total number of known amphibian species as of March 31, 2019 is 8,000, of which nearly 90% are frogs. With the phylogenetic classification, the taxon Labyrinthodontia has been discarded as it is a polyparaphyletic group without unique defining features apart from shared primitive characteristics.
Classification varies according to the preferred phylogeny of the author and whether they use a stem-based or a node-based classification. Traditionally, amphibians as a class are defined as all tetrapods with a larval stage, while the group that includes the common ancestors of all living amphibians and all their descendants is called Lissamphibia; the phylogeny of Paleozoic amphibians is uncertain, Lissamphibia may fall within extinct groups, like the Temnospondyli or the Lepospondyli, in some analyses in the amniotes. This means that advocates of phylogenetic nomenclature have removed a large number of basal Devonian and Carboniferous amphibian-type tetrapod groups that were placed in Amphibia in Linnaean taxonomy, included them elsewhere under cladistic taxonomy. If the common ancestor of amphibians and amniotes is included in Amphibia, it becomes a paraphyletic group. All modern amphibians are included in the subclass Lissamphibia, considered a clade, a group of species that have evolved from a common ancestor.
The three modern orders are Anura and Gymnophiona. It has been suggested that salamanders arose separately from a Temnospondyl-like ancestor, that caecilians are the sister group of the advanced reptiliomorph amphibians, thus of amniotes. Although the fossils of several older proto-frogs with primitive characteristics are known, the oldest "true frog" is Prosalirus bitis, from the Early Jurassic Kayenta Formation of Arizona, it is anatomically similar to modern frogs. The oldest known caecilian is another Early Jurassic species, Eocaecilia micropodia from Arizona; the earliest salamander is Beiyanerpeton jianpingensis from the Late Jurassic of northeastern China. Authorities disagree as to whether Salientia is a superorder that includes the order Anura, or whether
Lateral geniculate nucleus
The lateral geniculate nucleus is a relay center in the thalamus for the visual pathway. It receives a major sensory input from the retina; the LGN is the main central connection for the optic nerve to the occipital lobe the primary visual cortex. In humans, each LGN has six layers of neurons alternating with optic fibers; the LGN is a small, ventral projection at the termination of the optic tract on each side of the brain. The LGN and the medial geniculate nucleus which deals with auditory information are both thalamic nuclei and so are present in both hemispheres; the LGN receives information directly from the ascending retinal ganglion cells via the optic tract and from the reticular activating system. Neurons of the LGN send their axons through the optic radiation, a direct pathway to the primary visual cortex. In addition, the LGN receives many strong feedback connections from the primary visual cortex. In humans as well as other mammals, the two strongest pathways linking the eye to the brain are those projecting to the dorsal part of the LGN in the thalamus, to the superior colliculus.
Both the left and right hemisphere of the brain have a lateral geniculate nucleus, named after its resemblance to a bent knee. In humans as well as in many other primates, the LGN has layers of magnocellular cells and parvocellular cells that are interleaved with layers of koniocellular cells. In humans the LGN is described as having six distinctive layers; the inner two layers, are magnocellular layers, while the outer four layers, are parvocellular layers. An additional set of neurons, known as the koniocellular layers, are found ventral to each of the magnocellular and parvocellular layers; this layering is variable between primate species, extra leafleting is variable within species. Size relates to cell body, dendritic tree and receptive fieldThe magnocellular and koniocellular layers of the LGN correspond with the named types of retinal ganglion cells. Retinal P ganglion cells send axons to a parvocellular layer, M ganglion cells send axons to a magnocellular layer, K ganglion cells send axons to a koniocellular layer.
Koniocellular cells are functionally and neurochemically distinct from M and P cells and provide a third channel to the visual cortex. They project their axons between the layers of the lateral geniculate nucleus where M and P cells project, their role in visual perception is presently unclear. The parvo- and magnocellular fibers were thought to dominate the Ungerleider–Mishkin ventral stream and dorsal stream, respectively. However, new evidence has accumulated showing that the two streams appear to feed on a more mixture of different types of nerve fibers; the other major retino–cortical visual pathway is the tectopulvinar pathway, routing through the superior colliculus and thalamic pulvinar nucleus onto posterior parietal cortex and visual area MT. Layer 1, 2 Large cells, called magnocellular pathways Input from Y-ganglion cells Very rapid conduction Colour blind systemLayer 3–6 Parvocellular Input from X- ganglion cells Colour vision Moderate velocity. Both the LGN in the right hemisphere and the LGN in the left hemisphere receive input from each eye.
However, each LGN only receives information from one half of the visual field. This occurs due to axons of the ganglion cells from the inner halves of the retina decussating through the optic chiasma; the axons of the ganglion cells from the outer half of the retina remain on the same side of the brain. Therefore, the right hemisphere receives visual information from the left visual field, the left hemisphere receives visual information from the right visual field. Within one LGN, the visual information is divided among the various layers as follows: the eye on the same side sends information to layers 2, 3 and 5 the eye on the opposite side sends information to layers 1, 4 and 6; this description applies to the LGN of many primates, but not all. The sequence of layers receiving information from the ipsilateral and contralateral eyes is different in the tarsier; some neuroscientists suggested that "this apparent difference distinguishes tarsiers from all other primates, reinforcing the view that they arose in an early, independent line of primate evolution".
In visual perception, the right eye gets information from the right side of the world, as well as the left side of the world. You can confirm this by covering your left eye: the right eye still sees to your left and right, although on the left side your field of view may be blocked by your nose; the LGN receives input from the retina. In some species, such as rodents, the principle neurons in the LGN receive strong inputs from the retina. However, the retina only accounts for a small percentage of LGN input in these cases; as much as 95% of input in the LGN comes from the visual cortex, superior colliculus, thalamic reticular nuclei, local LGN interneurons. Regions in the brainstem that are not involved in visual perception project to the LGN, such as the mesencephalic reticular formation, dorsal raphe nucleus, periaqueuctal grey matter, the locus coeruleus; the LGN receives some inputs from the optic tectum. These non-reti
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
The cerebellum is a major feature of the hindbrain of all vertebrates. Although smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or larger. In humans, the cerebellum plays an important role in motor control, it may be involved in some cognitive functions such as attention and language as well as in regulating fear and pleasure responses, but its movement-related functions are the most solidly established. The human cerebellum does not initiate movement, but contributes to coordination and accurate timing: it receives input from sensory systems of the spinal cord and from other parts of the brain, integrates these inputs to fine-tune motor activity. Cerebellar damage produces disorders in fine movement, equilibrium and motor learning in humans. Anatomically, the human cerebellum has the appearance of a separate structure attached to the bottom of the brain, tucked underneath the cerebral hemispheres, its cortical surface is covered with finely spaced parallel grooves, in striking contrast to the broad irregular convolutions of the cerebral cortex.
These parallel grooves conceal the fact that the cerebellar cortex is a continuous thin layer of tissue folded in the style of an accordion. Within this thin layer are several types of neurons with a regular arrangement, the most important being Purkinje cells and granule cells; this complex neural organization gives rise to a massive signal-processing capability, but all of the output from the cerebellar cortex passes through a set of small deep nuclei lying in the white matter interior of the cerebellum. In addition to its direct role in motor control, the cerebellum is necessary for several types of motor learning, most notably learning to adjust to changes in sensorimotor relationships. Several theoretical models have been developed to explain sensorimotor calibration in terms of synaptic plasticity within the cerebellum; these models derive from those formulated by David Marr and James Albus, based on the observation that each cerebellar Purkinje cell receives two different types of input: one comprises thousands of weak inputs from the parallel fibers of the granule cells.
The basic concept of the Marr–Albus theory is that the climbing fiber serves as a "teaching signal", which induces a long-lasting change in the strength of parallel fiber inputs. Observations of long-term depression in parallel fiber inputs have provided support for theories of this type, but their validity remains controversial. At the level of gross anatomy, the cerebellum consists of a folded layer of cortex, with white matter underneath and a fluid-filled ventricle at the base. Four deep cerebellar nuclei are embedded in the white matter; each part of the cortex consists of the same small set of neuronal elements, laid out in a stereotyped geometry. At an intermediate level, the cerebellum and its auxiliary structures can be separated into several hundred or thousand independently functioning modules called "microzones" or "microcompartments"; the cerebellum is located in the posterior cranial fossa. The fourth ventricle and medulla are in front of the cerebellum, it is separated from the overlying cerebrum by a layer of leathery dura mater, the tentorium cerebelli.
Anatomists classify the cerebellum as part of the metencephalon, which includes the pons. Like the cerebral cortex, the cerebellum is divided into two hemispheres. A set of large folds is, by convention, used to divide the overall structure into 10 smaller "lobules"; because of its large number of tiny granule cells, the cerebellum contains more neurons than the total from the rest of the brain, but takes up only 10% of the total brain volume. The number of neurons in the cerebellum is related to the number of neurons in the neocortex. There are about 3.6 times as many neurons in the cerebellum as in the neocortex, a ratio, conserved across many different mammalian species. The unusual surface appearance of the cerebellum conceals the fact that most of its volume is made up of a tightly folded layer of gray matter: the cerebellar cortex; each ridge or gyrus in this layer is called a folium. It is estimated that, if the human cerebellar cortex were unfolded, it would give rise to a layer of neural tissue about 1 meter long and averaging 5 centimeters wide—a total surface area of about 500 square cm, packed within a volume of dimensions 6 cm × 5 cm × 10 cm.
Underneath the gray matter of the cortex lies white matter, made up of myelinated nerve fibers running to and from the cortex. Embedded within the white matter—which is sometimes called the arbor vitae because of its branched, tree-like appearance in cross-section—are four deep cerebellar nuclei, composed of gray matter. Connecting the cerebellum to different parts of the nervous system are three paired cerebellar peduncles; these are the superior cerebellar peduncle, the middle cerebellar peduncle and the inferior cerebellar peduncle, named by their position relative to the vermis. The superior cerebellar peduncle is an output to the cerebral cortex, carrying efferent fibers via thalamic nuclei to upper motor neurons in the cerebral cortex; the fibers arise from the deep cerebellar nuclei. The middle cerebellar peduncle is connected to the pons and receives all of its input from the pons from the pontine nuclei; the input to the pons is from the cerebral cortex and is relayed from the pontine nuclei via transverse pontine fibers to the cerebellum
Cerebrospinal fluid is a clear, colorless body fluid found in the brain and spinal cord. It is produced by the specialised ependymal cells in the choroid plexuses of the ventricles of the brain, absorbed in the arachnoid granulations. There is about 125mL of CSF at any one time, about 500 mL is generated every day. CSF acts as a cushion or buffer for the brain, providing basic mechanical and immunological protection to the brain inside the skull. CSF serves a vital function in cerebral autoregulation of cerebral blood flow. CSF occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord, it fills the ventricles of the brain and sulci, as well as the central canal of the spinal cord. There is a connection from the subarachnoid space to the bony labyrinth of the inner ear via the perilymphatic duct where the perilymph is continuous with the cerebrospinal fluid. A sample of CSF can be taken via lumbar puncture; this can reveal the intracranial pressure, as well as indicate diseases including infections of the brain or its surrounding meninges.
Although noted by Hippocrates, it was only in the 18th century that Emanuel Swedenborg is credited with its rediscovery, as late as 1914 that Harvey W. Cushing demonstrated CSF was secreted by the choroid plexus. There is about 125–150 mL of CSF at any one time; this CSF circulates within the ventricular system of the brain. The ventricles are a series of cavities filled with CSF; the majority of CSF is produced from within the two lateral ventricles. From here, CSF passes through the interventricular foramina to the third ventricle the cerebral aqueduct to the fourth ventricle. From the fourth ventricle, the fluid passes into the subarachnoid space through four openings – the central canal of the spinal cord, the median aperture, the two lateral apertures. CSF is present within the subarachnoid space, which covers the brain, spinal cord, stretches below the end of the spinal cord to the sacrum. There is a connection from the subarachnoid space to the bony labyrinth of the inner ear making the cerebrospinal fluid continuous with the perilymph in 93% of people.
CSF moves in a single outward direction from the ventricles, but multidirectionally in the subarachnoid space. Fluid movement is pulsatile, matching the pressure waves generated in blood vessels by the beating of the heart; some authors dispute this, posing that there is no unidirectional CSF circulation, but cardiac cycle-dependent bi-directional systolic-diastolic to-and-fro cranio-spinal CSF movements. CSF is derived from blood plasma and is similar to it, except that CSF is nearly protein-free compared with plasma and has some different electrolyte levels. Due to the way it is produced, CSF has a higher chloride level than plasma, an equivalent sodium level. CSF contains 0.3% plasma proteins, or 15 to 40 mg/dL, depending on sampling site. In general, globular proteins and albumin are in lower concentration in ventricular CSF compared to lumbar or cisternal fluid; this continuous flow into the venous system dilutes the concentration of larger, lipid-insoluble molecules penetrating the brain and CSF.
CSF is free of red blood cells, at most contains only a few white blood cells. Any white blood cell count higher. At around the third week of development, the embryo is a three-layered disc, covered with ectoderm and endoderm. A tube-like formation develops in the midline, called the notochord; the notochord releases extracellular molecules that affect the transformation of the overlying ectoderm into nervous tissue. The neural tube, forming from the ectoderm, contains CSF prior to the development of the choroid plexuses; the open neuropores of the neural tube close after the first month of development, CSF pressure increases. As the brain develops, by the fourth week of embryological development three swellings have formed within the embryo around the canal, near where the head will develop; these swellings represent different components of the central nervous system: the prosencephalon and rhombencephalon. Subarachnoid spaces are first evident around the 32nd day of development near the rhombencephalon.
At this time, the first choroid plexus can be seen, found in the fourth ventricle, although the time at which they first secrete CSF is not yet known. The developing forebrain surrounds the neural cord; as the forebrain develops, the neural cord within it becomes a ventricle forming the lateral ventricles. Along the inner surface of both ventricles, the ventricular wall remains thin, a choroid plexus develops and releasing CSF. CSF fills the neural canal. Arachnoid villi are formed around the 35th week of development, with aracnhoid granulations noted around the 39th, continuing developing until 18 months of age; the subcommissural organ secretes SCO-spondin, which forms Reissner's fiber within CSF assisting movement through the cerebral aqueduct. It disappears during early development. CSF serves several purposes: Buoyancy: The actual mass of the human brain is about 1400–1500 grams; the brain therefore exists in neutral buoyancy, which allows the brain to maintain its density without being impaired by its own weight, which would cut off blood supply and kill neurons in the lower sections without CSF.
Protection: CSF protects the brain tissue from injury when jolted or hit, by providing a fluid buffer that acts as a shock absorber from some forms of mechanical injury. Prevention of brain ischemia: The prevention of brai