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Cerebrospinal Fluid

Cerebrospinal fluid

Cerebrospinal fluid (CSF) is a clear bodily fluid that occupies the subarachnoid space in the brain (the space between the skull and the cerebral cortex—more specifically, between the arachnoid and pia layers of the meninges). It is basically a saline solution and acts as a "cushion" or buffer for the cortex.

Physiology

Cerebrospinal fluid also occupies the ventricular system of the brain and the spinal cord. It is mainly produced by the choroid plexus, but also by the ependymal lining of the brain's ventricles. The CSF is formed by the choroid plexus of the ventricles circulates through the interventricular foramina into the third ventricle and then via the mesencephalic duct (cerebral aqueduct) into the fourth ventricle. From there, the fluid passes to the subarachnoid space through two lateral apertures and one median aperature and is then absorbed by the venous system to the blood circulation. The total amount of cerebrospinal fluid is about 150 ml, and about 500 ml is produced every day, which indicates its very active circulation.

Pathology

The cerebrospinal fluid has many putative roles including mechanical protection of the brain, distribution of neuroendocrine factors, and facilitation of pulsatile cerebral blood flow. Understanding cardiovascular dynamics is valuable as the flow pattern of arterial blood must be tightly regulated within the brain in order to assure consistent brain oxygenation. CSF movement allows arterial expansion and contraction by acting like a spring, which prevents wide changes in intracranial blood flow. When disorders of CSF flow occur, they may therefore impact not only CSF movement, but may also impact intracranial blood flow and subsequent neuronal and glial vulnerabilities. The venous system is also important in this equation. Infants and patients shunted as small children may have particularly unexpected relationships between pressure and ventricular size, possibly due in part to venous pressure dynamics. This may have significant treatment implications but the underlying pathophysiology needs to be further explored. CSF connections with the lymphatic system have been demonstrated in several mammalian systems. Preliminary data suggest that these CSF-lymph connections form around the time that the CSF secretory capacity of the choroid plexus is developing (in utero). There may be some relationship between CSF disorders, including hydrocephalus and impaired CSF lymphatic transport.

Diagnosis and therapy

Cerebrospinal fluid can be tested for the diagnosis of a variety of neurological diseases. Usually, it is obtained by a procedure called lumbar puncture in an attempt to count the cells in the fluid and to detect the levels of protein and glucose. These parameters alone may be extremely beneficial in the diagnosis of central nervous system infections (especially meningitis and subarachnoid hemorrhage). Moreover, a cerebrospinal fluid culture examination may yield the microorganism that has caused the infection. By using more sophisticated methods, such as the detection of the oligoclonal bands, an ongoing inflammatory condition (for example, multiple sclerosis) can be recognized. Lumbar puncture can also be performed to measure the intracranial pressure, which might be increased in certain types of hydrocephalus. Category:Central nervous system Category:Neurology ja:脳脊髄液

Subarachnoid space

: meninges

Brain

In the anatomy of animals, the brain, or encephalon (Greek for "in the head"), is the higher, supervisory center of the nervous system. The term 'brain' is typically used in connection with vertebrate nervous systems, and less often with regard to the nervous system of invertebrates. In the latter, neural control is performed by collections of ganglia. The brain is an extremely complex organ: the human brain is a collection of 100 billion neurons, each linked with up to 25,000 others. This huge number of interconnecting neurons, often referred to as a neural ensemble, is what makes the brain intelligent—enabling humans to analyze sensory signals, control the body, and think. In most animals, the brain is located in the head, close to the primary sensory apparatus and the mouth. Hippocrates considered the brain to be the seat of thought, while Aristotle believed it to be a cooling system for the blood. Today the study of the mind and brain consists of Neuroscience, the field of biology that studies the brain at its various levels of organization (from single neurons to functional systems such as visual system, auditory system, motor system and others); and psychology, the study of the cognition that arises from the neural function of the brain. Attempts have also been made to directly "read" the brain, which has been accomplished in a rudimentary manner through a brain-computer interface. In recent years, several institutions and bodies have undertaken research on recreating the neural structure of the brain with aim to produce human-like cognition and intelligence in computers. The brain controls and coordinates most movement, behavior and homeostatic body functions (such as heartbeat, blood pressure, fluid balance and body temperature). The brain is responsible for cognition, emotion, memory, motor learning and other kinds of learning. However, many behaviors, such as simple reflexes and basic locomotion, can be executed under spinal cord control alone.

The importance of the brain

The brain in animals

Three groups of animals, with some exceptions, have notably complex brains: the arthropods (insects and crustaceans), the cephalopods (octopuses, squid, and similar mollusks), and the craniates (vertebrates and their cousins). The brain of arthropods and cephalopods arises from twin parallel nerve cords that extend through the body of the animal. In arthropod, the brain consists of a central brain with three divisions and large optical lobes behind each eye for visual processing. eye The brain of craniates develops from the anterior section of a single dorsal nerve cord, which later becomes the spinal cord. In craniates, the brain is protected by the bones of the skull. In vertebrates, increasing complexity in the cerebral cortex correlates with height on the phylogenetic and evolutionary tree. Primitive vertebrates, like fish, reptiles, and amphibians have cortices with fewer than six layers of neurons, a structure known as allocortex (also named heterotypic cortex) (Martin, 1996). More complex vertebrates such as mammals have developed a six-layered neocortex (other terms: homotypic cortex, neocortex, neopallium), in addition to having some parts of the brain that are allocortex (Martin, 1996). In mammals, increasing convolutions of the brain, called gyri, are characteristic of animals with more advanced brains. These convolutions evolved to provide a larger surface area for a greater number of neurons, while keeping the volume of the brain compact enough to fit inside the skull.

The human brain

The structure of the human brain is different from that of other animals in several significant ways. These differences have allowed for many abilities over and above those of other animals, such as advanced cognitive skills. Human encephalization is especially pronounced in the neocortex, the most complex part of the cerebral cortex. The proportion of the human brain that is devoted to the neocortex—and the most advanced part within it, the prefrontal cortex—is larger than in all other animals. Humans enjoy unique neural capacities, but much of the human neuroarchitecture is shared with ancient species. Basic systems that alert the nervous system to stimulus, that sense events in the environment, and that monitor the condition of the body are similar to those of the most basic vertebrates. The neural circuitry underlying human consciousness includes both the advanced neocortex and protypical structures of the brain stem. The human brain also has a a million billion synaptic connections, making it one of the most densely connected network systems in the known universe; however, more complex structures may exist.

Pathology of the brain

The loss of function in the brain fulfills some definitions of death. Injuries to the brain tend to affect large areas of the organ, sometimes causing major deficits in intelligence, memory and control of the body. Head trauma, caused, for example, by vehicle and industrial accidents, is a leading cause of death in youth and middle age. In these cases, more damage is typically caused by resultant swelling (edema) than by the impact itself. Stroke, caused by the blockage of blood vessels in the brain, is another major cause of death from brain damage. Other problems in the brain can be more accurately classified as diseases rather than injuries. Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, motor neurone disease, and Huntington's disease, are caused by the gradual death of individual neurons, leading to decrements in movement control, memory, and cognition. Currently, only the symptoms of these diseases can be treated, but stem cell research may offer a cure. Mental illness, such as clinical depression, schizophrenia, bipolar disorder, and post-traumatic stress disorder, are brain diseases that impact on the personality and typically on other aspects of mental and somatic function. These disorders may be treated by psychiatric therapy, by pharmaceutical intervention, or by a combination of treatments; therapeutic effectiveness varies significantly among individuals. pharmaceutical Some infectious diseases affecting the brain are caused by viral and bacterial infection(s). Infection of the meninges, the membrane that covers the brain, can lead to meningitis. Bovine spongiform encephalopathy (also known as mad cow disease), is deadly in cattle and is linked to prions. Kuru is a similar prion-borne degenerative brain disease affecting humans. Both are linked to the ingestion of neural tissue, and may be an evolutionary defense against cannibalism. Viral or bacterial causes have been substantiated in multiple sclerosis, Parkinson's disease, Lyme disease, encephalopathy and encephalomyelitis. Some brain disorders are congenital. Tay-Sachs disease, Fragile X syndrome, Down syndrome, and Tourette syndrome are all linked to genetic and chromosomal errors. Malfunctions in the embryonic development of the brain can be caused by genetic factors, by drug use, and disease during a mother's pregnancy.

Other matters

Some philosophers consider that "brain" is synonymous with "mind", while others (such as strong AI theorists) believe that the mind is analogous to software and the brain to hardware. This issue—related to the mind-body problem—and many other issues, are the subjects of the area of the philosophy of mind. Questions asked in this field typically relate to the nature of consciousness and whether non-human animals are conscious beings. Computer scientists have produced computer systems called neural networks, loosely based on the structure of neuron connections in the brain. Artificial intelligence seeks to replicate brain function—although not necessarily brain mechanisms—but as yet is an immature science. Creating algorithms to mimic a biological brain is extremely difficult because the brain is not a static arrangement of circuits, but a network of vastly interconnected neurons that are constantly changing their connectivity and sensitivity. More recent work in both neuroscience and artificial intelligence models the brain using the mathematical tools of chaos theory and dynamical systems. Brain activity can be detected by electrodes, raising the possibility of "brain-computer interface". The reverse path has been demonstrated: brain implants have been used to generate artificial hearing and (crude and experimental) artificial vision for deaf and blind people; brain pacemakers are now commonly used to regulate brain activity in conditions such as Parkinson's disease. Both of these avenues of research are confronted with potentially serious ethical implications. For example, by placing electrodes in the brain and using a remote control, researchers have been able to remotely control the movements of a rat, combining commands of what to do with the stimulation of the brain pleasure centers. This raises the possibility of creating an electronically controlled biological "ratbot" that could be used in dangerous circumstances.

The biology of the brain

Despite the variance of the species in which the brain is found there are many common features in its cellular make-up, its structure and its function. On a cellular level, the brain is composed of two classes of cell, neurons and glia, both of which contain several different cell types which perform different functions. Interconnected neurons form neural networks (or neural ensembles). These networks are similar to man-made electrical circuits in that they contain circuit elements (neurons) connected by biological wires (nerve fibers). Of course, these do not form simple one-to-one electrical circuits (as is the case in many man-made circuits), neurons typically connect to at least a thousand other neurons. These highly specialized circuits make up systems which are the basis of perception, action and higher cognitive function. The brain contains anatomical and functional divides. In mammals, the most obvious partitioning of the brain is into the cerebrum (Latin for "brain", a large, anterior part that consists of two convoluted hemispheres and deep nuclei), cerebellum (Latin for "small brain", a smaller, structure behind the cerebrum with two rippled hemispheres and deep cerebellar nuclei), and brain stem (an elongated structure connecting the brain to the spinal cord). These parts are further divided into hemispheres, lobes, gyri, cortices, cytoarchitectonic and functional areas, nuclei, layers, fiber tracks and so forth. In summary, the chemical and electrical impulses continually passing through the cells of the brain produce all control, action and cognitive function in the body.

Histology

lobe Neurons, the cells that generate action potentials and convey them to other cells, constitute the chief class of brain cells. In each particular brain area, input (or afferent) neurons, output (or efferent) neurons and interneurons are typically found. Input neurons are recipients of projections from other brain areas. Output neurons project to the other areas. Interneurons are the neurons which do not leave the area. In addition to neurons, the brain contains glial cells in the proportion roughly 10 glial cells to every neuron; these are traditionally seen to perform supportive roles to neurons and fill out the space between them (hence its name, Greek for 'glue'). Most types of glia in the brain (and the rest of the central nervous system) are present in the entire nervous system, exceptions include oligodendrocytes which insulate neural axons (a role performed by Schwann cells in the peripheral nervous system). Oligosaccharides are the defining factor between white matter and grey matter in the brain—white matter is composed of myelinated (insulated) axons, whereas grey matter contains mostly cell soma, dendrites and unmyelinated portions of axons and glia and a smaller proportion of myelinated axons. In mammals, the brain also contains a certain amount of connective tissue called the meninges which is a system of membranes that separate the skull from the brain. The three-layered covering is made of, from the outside in, dura mater, arachnoid and pia mater (the latter two are connected and thus often considered as a single layer, the pia-arachnoid). Below the arachnoid is the subarachnoid space which contains cerebrospinal fluid which protects the nervous system. Blood vessels enter the central nervous system through the perivascular space above the pia mater. A blood-brain barrier protects the brain from unwanted substances that might enter it through the blood. The brain is suspended in cerebrospinal fluid, which circulates between layers of the meninges and through cavities in the brain called ventricles. It is important both chemically (metabolism) and mechanically (shock-prevention).

Anatomy

Although the histology of the brain is common to all those who have one, the structural anatomy is not. Apart from the general nature of the brain to order into lobes and suchforth, the lobes into which it has evolved are not common across the vertebrate/invertebrate divide. There are further dissimilarities within invertebrates, though vertebrates tend to share certain commonalities.

Invertebrates

In insects, the brain can be divided into four parts, the optical lobes, the protocerebrum, the deutocerebrum, and the tritocerebrum. The optical lobes are positioned behind each eye and process visual stimuli (Butler, 2000). The protocerebrum contains the mushroom bodies, which respond to smell, and the central body complex. The deutocerebrum includes the antennal lobes, which are similar to the mammalian olfactory bulb, and the mechanosensory neuropils which receive information from touch receptors on the head and antennae. The antennal lobes of flies and moths are quite complex. In cephalopods, the brain is divided into two regions: the supraesophageal mass and the subesophageal mass. These parts are divided by the animal's esophagus. The supra- and subesophageal masses are connected to each other on either side of the esophagus by the basal lobes and the dorsal magnocellular lobes. The large optic lobes are sometimes not considered to be part of the brain proper since the optic lobes anatomically separate from the brain and are joined to the brain by the optic stalks. However, the optic lobes perform much of the visual processing and can be functionally considered to be a part of the brain.

Vertebrates

In vertebrates, a gross division into three major parts is used: hindbrain (medulla oblongata and metencephalon), midbrain (mesencephalon) and forebrain (diencephalon and telencephalon). Varied taxonomies have been used by assorted schools at various times in history for the study of diverse species. An anterior part of the telencephalon called the cerebrum makes up the largest section of the mammalian brain and in humans, its surface has many deep fissures (sulci) and convolutions (gyri), giving a wrinkled appearance to the brain. In most vertebrates the metencephalon is the highest integration center in the brain, whereas in mammals this role has been adopted by the cerebrum. Behind (or in humans, below) the cerebrum is the cerebellum, a convoluted structure whose neural circuitry is often compared with crystal structure. Cerebellum participates in the control of movement. The cerebellum attaches to the hindbrain in a structure called the pons. The cerebrum and the cerebellum consist each of two halves (hemispheres). The corpus callosum connects the two hemispheres of the cerebrum. An outgrowth of the telencephalon called the olfactory bulb is a major structure in many animals, but in humans and other primates, it is relatively small. Vertebrate nervous systems are distinguished by encephalization and bilateral symmetry. Encephalization refers to the tendency for more complex organisms to gain a larger-size brains through evolutionary time. Larger vertebrates develop a complex of layered, networked and convoluted grey matter and white matter. Grey matter refers to tissue mostly comprised of neurons and can be found on the surface of cerebral cortex, as well as in clusters called nuclei deep within the brain. White matter refers to axons and their surrounding myelin insulation, which gives this tissue its white color. White matter is found in bundles of fibers known as tracts which connect the different parts of the brain. In modern species most closely related to the first vertebrates, brains are covered with gray matter that has a three-layer structure. Their brains also contain deep brain nucleus and fiber tracks forming the white matter. Most regions of the human cerebral cortex have six layers of neurons, a structure known as neocortex.

Brain Regions in Vertebrates

According to the hierarchy based on embryonic and evolutionary development, chordate brains are composed of the following regions:
- RHOMBENCEPHALON (Greek for "rhomboid brain")
  - Myelencephalon (Greek for "brain marrow", also called medulla oblongata which means "long marrow" in Latin)
  - Metencephalon (Greek for "after the brain"; also called hindbrain)
    - pons
    - cerebellum
- MESENCEPHALON (Greek for "middle brain", also called midbrain)
  - tectum
  - midbrain tegmentum
  - substantia nigra
  - crus cerebri (also called cerebral peduncles and pedunculus cerebri)
- PROSENCEPHALON
  - Diencephalon (Greek for "brain in between")
    - thalamus
    - hypothalamus (Greek for "under the thalamus")
    - pituitary gland
    - epithalamus
    - pineal gland
  - Telencephalon (Greek for "end brain", i.e. the most rostral part of the brain; also called forebrain)
    - TELENCEPHALON NUCLEI
      - putamen
      - caudate nucleus
      - putamen
      - globus pallidus
      - amygdala
    - CEREBRAL CORTEX
    - Archipallium (Greek for "first cloak", i.e. cortex that developed first; also called archeocortex)
      - hippocampus
    - Paleopallium (Greek for "ancient cloak"; also called "paleocortex")
      - priform(olfactory) cortex
      - parahippocampal gyrus
    - Neopallium (Greek for "new cloak"; also called "paleocortex"; also called neocortex and isocortex)
      - frontal lobe
      - temporal lobe
      - parietal lobe
      - occipital lobe
      - insula
      - cingulate cortex In addition, the brain is often subdivided into the following major parts:
- BRAINSTEM
  - Medulla
  - Pons
  - Midbrain
- CEREBELLUM
  - Cerebellar cortex
  - Cerebellar nuclei
- BASAL GANGLIA (some midbrain nuclei, such as substantia nigra are usually considered as basal ganglia)
  - Striatum (caudate nucleus and putamen)
  - Globus pallidus
- HIPPOCAMPUS
- AMYGDALA
- THALAMUS
- HYPOTHALAMUS
- CEREBRAL CORTEX Yet alternative classifications arrange brain areas into functional systems:
- Limbic system
- Sensory systems
  - Visual system
  - Olfactory system
  - Gustatory system
  - Auditory system
  - Somatosensory system
- Motor system
- Associative areas

Function

Associative areas Vertebrate brains receive signals through nerves arriving from the sensors of the organism, interpret those signals and formulate reactions based on built-in programs and learned experiences. A similarly extensive nerve network delivers signals from a brain to control muscles throughout a body. Anatomically, the majority of afferent and efferent nerves (with the exception of cranial nerves) are connected to the spinal cord, which then transfers the signals to the brain. Sensory input is processed by the brain to recognize danger, find food, identify potential mates and perform more sophisticated functions. Visual, touch, and auditory sensory pathways of vertebrates are routed to specific nuclei of the thalamus and then to regions of the cerebral cortex that are specific to each sensory system: the visual system, the auditory system and the somatosensory system. Olfactory pathways are routed to the olfactory bulb, then to various parts of the olfactory system. Taste is routed through the brainstem and then to other portions of the gustatory system. To control movement, the brain has several parallel systems of muscle control. The motor system controls voluntary muscle movement, aided by motor areas of the cerebral cortex, the cerebellum and the basal ganglia — the system that eventually projects to the spinal cord. Nuclei in the brainstem control many involuntary muscle functions such as heartrate and breathing. In addition, many automatic acts (simple reflexes, locomotion) can be controlled by the spinal cord alone. Brains also produce hormones that can influence organs and glands elsewhere in a body - conversely, brains also react to hormones produced elsewhere in the body. In mammals, most of these hormones are released into the circulatory system by a structure called the pituitary gland. It is hypothesized that developed brains derive consciousness from interaction among numerous systems within the brain. Cognitive processing in mammals occurs in the cerebral cortex but relies on mid-brain and limbic functions as well, especially those of the thalamus and hippocampus. Among "younger" (in an evolutionary sense) vertebrates, advanced processing involves progressively rostral (forward) regions of the brain. Hormones, incoming sensory information, and cognitive processing performed by the brain determine the brain state. Stimulus from any source can trigger a general arousal process that focuses cortical operations to processing of the new information. Cognitive priorities are constantly shifted by a variety of factors, such as hunger, fatigue, beliefs, unfamiliar information or threats. The simplest dichotomy related to processing of threats is the fight-or-flight response mediated by the amygdala, among other structures.

The study of the brain

Fields of study

Several areas of science specifically study the brain. Neuroscience seeks to understand the nervous system, including the brain, from a biological perspective. Psychology seeks to understand behavior and the brain. The terms neurology and psychiatry usually refer to medical applications of neuroscience and psychology, respectively. Cognitive science seeks to unify neuroscience and psychology with other fields that concern themselves with the brain, such as computer science (in Artificial intelligence and similar fields) and philosophy.

Methods of observation

Each method for observing activity in the brain has its advantages and drawbacks. Electrophysiology, in which wire electrodes are implanted in the brain, allows scientists to record the electrical activity of individual neurons or fields of neurons, but since it requires invasive surgery, this is a technique usually reserved for lab animals. By placing electrodes on the scalp, electroencephalography (EEG) measures brain waves, which are the mass changes in electrical current from the cerebral cortex, but can only detect changes over large areas of the brain and very little sub-cortical activity. Functional magnetic resonance imaging (fMRI) measures changes in blood flow in the brain, but the activity of neurons is not directly measured, nor can it be distinguished whether this activity is inhibitory or excitatory. Similarly, a PET (Positron Emission Tomography) Scan, is able to monitor glucose intake in different areas within the brain which is correlated the level of activity in that region. Behavioral tests can measure symptoms of disease and mental performance, but only provide indirect measurements of brain function and may not be practical in all animals. Finally, post-mortem analysis of the brain allows for the study of anatomy and protein expression patterns, but is only possible after the human or animal is dead.

History

Ancient Greeks had differing views on the function of the brain. Hippocrates believed the brain to be the seat of intelligence, but Aristotle held that the brain was a cooling mechanism for the blood, while the heart was the seat of intelligence. He reasoned that humans are more rational than the beasts because they have a proportionally larger brain to cool their hot-bloodedness (Bear, 2001). During the Roman Empire, the anatomist Galen dissected the brains of sheep. He concluded that since the cerebellum was hard on touch, it must control the muscles, while since the cerebrum was soft, it must be where the senses were processed. Galen further theorized that the brain functioned by movement of fluids through the ventricles (Bear, 2001). In the Age of Reason, René Descartes espoused a fluid mechanical view of the brain similar to Galen's theories. However, Descartes thought that although this explanation was adequate to explain the brain functions of animals, the higher mental functions of humans were accomplished by the soul. This theoretical separation of the mind and brain became known as the mind-body problem (Bear, 2001). In the mid-1600s, however, great progress in describing the anatomy of the brain was achieved with the works of English anatomist Thomas Willis and Flemish anatomist Vesalius. They dispelled many of the notions of Galen and Descartes and discovered many facts about the macro structure of the brain of animals and humans. In the 1700s, Luigi Galvani showed that electrically stimulating the sciatic nerve of a dissected frog caused movement of the attached muscle. His experiments led scientists away from the fluid mechanical theory of the brain and toward an electrical theory. In the 19th century, Galvani's work led to the development of research in bioelectricity and to the discovery of the membrane potential and action potential by researchers such as Emil du Bois-Reymond. The scientists of the 1800s debated whether areas of the brain corresponded to specific functions or if the brain functioned as a whole (the "aggregate field theory"). Jean Pierre Flourens championed the aggregate field theory in opposition to the pseudoscience of phrenology, founded by Franz Joseph Gall. However, the work of Paul Pierre Broca, Karl Wernicke, and Korbinian Brodmann eventually helped to show that areas of the brain had specific functions, though some functions were repeated, an idea known as parallel distributed processing (Kandel, 2001). As the 20th century approached, the anatomical works of Santiago Ramon y Cajal and Camillo Golgi laid the foundation for the study of individual neurons in the brain. Charles Scott Sherrington and Edgar Douglas Adrian furthered the study of neurons with the new techniques of electrodes and the electroencephalogram (EEG). Neurotransmitters were discovered and investigated by a number of scientists, including Otto Loewi, Henry Hallett Dale, Arvid Carlsson and many others. Modern Neuroscience experiences rapid development. The scientists use a variety of approaches to study the brain at different levels — from the molecules to systems. Extensive knowledge has been accumulated about the electrophysiological properties of different types of neurons and their responsiveness to neurotransmitters. Recordings from the brain of awake, behaving animals pioneered by Edward Evarts help to decode neuronal firing during different behaviors and cognitive processes. Miguel Nicolelis introduce multielectrode recording techniques which led to creation of brain-computer interfaces. Rapidly developing brain imaging allows scientists to study the brain in living humans and animals in ways that their predecessors could not.

The brain as a food

Like most other internal organs, the brain can serve as nourishment. For example, in the Southern United States canned pork brain in gravy can be purchased for consumption as food. This form of brain is often fried with scrambled egg to produce the famous "Eggs n' Brains". The brain of animals also features in the cuisine of France such as in the dish tête de veau, or head of calf. Although it might consist only of the outer meat of the skull and jaw, the full meal includes the brain, tongue and glands (the latter form being the favorite food of president Jacques Chirac). Similar delicacies from around the world include Mexican tacos de sesos (tacos made with cattle brain) and squirrel brain in the US South. The Anyang tribe of Cameroon practiced a tradition in which a new chief would consume the brain of a hunted gorilla while another senior member of the tribe would eat the heart. Consuming the brain and other nerve tissue of animals is not without its risks. The first problem is that the brain is made up of 60% fat due to the myelin (which by itself is 70% fat) insulating the axons of neurons and glia. As an example, a 5 oz. (0.14 kg) can of "Pork Brains in Milk Gravy", a single serving, contains 3500 milligrams of cholesterol, 1170% of our recommended daily intake. More importantly, humans can contract fatal transmissible spongiform encephalopathies such as Creutzfeldt-Jakob disease and other (prion diseases), as well as Bovine Spongiform Encephalopathy (colloquially known as "mad cow" disease) through the consumption of the infected nerve tissue of cattle and other animals - However, "there is no evidence that people can get mad cow disease from eating muscle meat". Another prion disease called kuru has been traced to a mourning ritual among the Fore people of Papua New Guinea in which those close to the dead would eat their brain to create a sense of immortality. Some archaeological evidence suggests that the mourning rituals of European neanderthals also involved the consumption of the brain. The practice of eating another human's brain has been depicted by Hollywood in Hannibal (film) and countless zombie movies. It is not only humans who eat the brains of other animals. The two species of chimpanzee, though generally vegetarian, are known to eat the brains of monkeys to obtain fat in their diet.

External links


- [http://www.stanford.edu/group/hopes/basics/braintut/ab0.html Brain Tutorial]
- [http://brainmuseum.org/ Comparative Mammalian Brain Collection]
- [http://www.rmcybernetics.com/science/cybernetics/ai_vision_perception_brain.htm RMCybernetics - The Brain and Artificial Intelligence]
- [http://braininfo.rprc.washington.edu BrainInfo for Neuroanatomy]
- [http://faculty.washington.edu/chudler/neurok.html Neuroscience for kids]
- [http://3dscience.com/advancedsearch.asp?stS=brain&cboMatch=Any&selectcategory=0&txtMinPrice=&txtMaxPrice= Free Brain Medical Clip Art].
- [http://purl.net/net/neurowiki neuroscience wiki]
- [http://www.brainmaps.org/ BrainMaps.org], interactive high-resolution digital brain atlas based on scanned images of serial sections of both primate and non-primate brains

Related topics


- A/S ratio
- Avian pallium
- Brain damage
- Brain-computer interface
- Coma
- Human brain
- Persistent vegetative state
- Regions in the human brain
- The Memory-Prediction Framework
- Metastability in the brain
- Neuroendocrinology
- Traumatic brain injury

References


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Notes

The following are the sources for individual facts, statistics and information included in the article:
- Statistic from page 161 of Basic Histology: Text and Atlas, 10th ed. by L.C. Junqueira, and J. Carneiro.
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- Category:Central nervous system Category:Cerebrum ja:脳 ko:뇌 simple:Brain th:สมอง

Cerebral cortex

The cerebral cortex is a brain structure in vertebrates, including humans. It is the outermost layer of the cerebrum and has a grey color. (Hence the name "grey matter". Grey matter is formed by neurons and their fibers, and white matter below the grey matter of the cortex is formed predominantly by nerve fibers interconnecting cortical areas with each other and with subcortical structures.) The human cerebral cortex is 2-4 mm (0.08-0.16 inches) thick and is folded. In the "higher" animals (especially the higher mammals), the surface of the cerebral cortes becomes folded. This creates grooves on the surface of the brain called "sulci" (singular = "sulcus"). The bumps or ridges on the surface of the brain are called "gyri" (singular = "gyrus"). The folding of the cortex increases the cortical surface area. The cerebral cortex, made up of four lobes, is involved in many complex brain functions including memory, attention, perceptual awareness, "thinking", language and consciousness. The cerebral cortex receives sensory information from many different sensory organs eg: eyes, ears, etc. and processes the information. Areas that receive that particular information are called sensory areas. The two hemispheres receive the information from the opposite sides of the body. Sensory information is relayed to the cortex by the thalamus. Parts of the cortex that receive this information are called primary sensory areas. Other areas receive impulses from the primary sensory areas and integrate the information coming in from different types of receptors (i.e., modalities). These are often called association areas and make up a great deal of the cortex in all primates, humans included. Thus, the cortex is commonly described as comprised of the primary sensory areas, the motor areas and the association areas. Association areas can be grouped the following way: # in the parietal, temporal and occipital lobes. It is involved in producing our perceptions resulting from what our eyes see, ears hear and other sensory organs tell us about the position of different parts of our body and relate them to the position of other objects in the environment # in the frontal lobe. Called prefrontal association complex and involved in planning actions and movement, as well as abstract thought # in the limbic association area. Involved in emotion and memory In humans, the association areas of the left hemisphere, especially the parietal-temporal-occipital complex are responsible for our understanding and use of language. The motor areas are located in both hemispheres of the cortex. They are shaped like a pair of headphones stretching from ear to ear. The motor areas are very closely related to the control of voluntary movements, especially fine fragmented movements performed by the hand. The right half of the motor area controls the left side of your body and vice versa. Two areas of the cortex are commonly referred to as motor:
- Primary motor cortex: Executing voluntary movements
- Secondary motor areas in premotor cortex: Selecting voluntary movements In addition, motor functions have been described for
- Posterior Parietal Cortex: Guiding voluntary movements in space
- Dorsolateral Prefrontal Cortex: Deciding which voluntary movements to make according to higher-order instructions, rules and self-generated thoughts

Development

The cerebral cortex develops from the neural plate, a specialised part of the embryonic ectoderm. The neural plate folds and closes to form the neural tube. From the cavity inside the neural tube develops the ventricular system, and from the epithelial cells of its walls, the neurones and glial cells. The most frontal part of the neural tube, the telencephalon gives rise to the cerebral hemispheres and the neocortex. Most cortical neurones are generated within the ventricular zone close to the ventricles. Initially, progenitor cells in the ventricular zone divide symmetrically, producing two progenitor cells by mitotic cycle. Then, some progenitor cells begin to divide asymmetrically, producing one postmitotic cell that migrates and leaves the ventricular zone, and a daughter cell that continues to divide or that eventually dies. The postmitotic cells will become neurones.

Laminar pattern

The standard areas of cortex (isocortex) is characterized as having six distinct layers. From outside inward: # Molecular layer # External granular layer # External pyramidal layer # Internal granular layer # Internal pyramidal layer # Multiform layer After migration (interestingly, the inner layers are formed first during development), neurons form efferents and receive afferent connections characteristic of its layer.
- The molecular layer I contains few scattered neurons and consists mainly of extensions of apical dendrites and horizontally oriented axons, and some Cajal-Retzius and spiny stellate neurons can be found.
- The external granular layer II contains small pyramidal neurons and numerous stellate neurons.
- The external pyramidal layer III contains predominantly small and medium sized pyramidal neurons, as well as non-pyramidal neurons with vertically oriented intracortical axons. Layers I--III are the main target of interhemispheric corticocortical afferents, and layer III is the principal source of corticocortical efferents.
- The internal granular layer IV contains different types of stellate and pyramidal neurons, and is the main target of thalamocortical afferents as well as intra-hemispheric corticocortical afferents.
- The internal pyramidal layer V contains large pyramidal neurons (as the Betz cells in the primary motor cortex) as well as interneurons, and it is the principal source of efferent for all the motor-related subcortical structures.
- The multiform layer VI contains few large pyramidal and many small spindle-like pyramidal and multiform neurons. The layer VI sends efferent fibres to the thalamus establishing a very precise reciprocal interconnection between the cortex and the thalamus (Creutzfeldt, 1995). The cortical layers are not simply stacked one over the other, they develop characteristic connections between different layers, which define the basic structure of the cortical columns in the mature cortex (Mountcastle, 1997). There are no actual borders between the layers, and neurons cross layer boundaries with their dendrites and axons trees all over. The pyramidal cells (the majority of the neurons) span at least three layers, and in many cases all the layers. Thus it is not obvious that the layers have any functional significance. Theorists such as Jeff Hawkins have posited that these layers, particularly in the neocortex, form part of a laminar memory system of classification and lateral association which underpins human cognitive function. Although new, it brings an intriguing perspective on the unusual structural consistency of the most physically large cortex of the brain.

Classification

Based on the differences in lamination the cerebral cortex can be classified into two major groups:
- Isocortex (homotypical cortex), the part of the cortex with six layers.
- Allocortex (heterotypical cortex) with variable number of layers, e.g., olfactory cortex and hippocampus. Auxiliary classes are:
- Mesocortex, classification between isocortex and allocortex where layers 2, 3 and 4 are merged.
- Proisocortex, Brodmann areas 24, 25, 32.
- Periallocortex is cortical areas adjacent to allocortex. Based on supposed developmental differences the following classification also appears:
- Neocortex or Neopallium that corresponds to the isocortex.
- Archicortex
- Paleocortex In addition, cortex may be classified on the basis of gross topographical conventions into the following:
- Temporal Cortex
- Parietal Cortex
- Frontal Cortex
- Occipital Cortex
- Limbic Cortex
- Insular Cortex

See also


- Cortical column
- Frontal lobe
- Limbic lobe
- List of regions in the human brain
- Microgyrus
- Occipital lobe
- Parietal lobe
- Temporal lobe
- Cerebral hemisphere
- Brain-computer interface

References


- Angevine, J. and Sidman, R. 1961. Autoradiographic study of cell migration during histogenesis of cerebral cortex in the mouse. Nature, 192:766-768
- Creuzfeldt, O. 1995. Cortex Cerebri. Springer-Verlag.
- Marin-Padilla, M. 2001. Evolución de la estructura de la neocorteza del mamífero: Nueva teoría citoarquitectónica. Rev. Neurol, 33(9):843-853
- Mountcastle, V. 1997. The columnar organization of the neocortex. Brain, 120:701-722
- Ogawa, M. et al. 1995. The reeler gene-associated antigen on Cajal-Retzius neurons is a crucial molecule for laminar organization of cortical neurones. Neuron, 14:899-912
- Rakic, P. 1988. Specification of cerebral cortical areas. Science, 241:170-241

External links


- [http://braininfo.rprc.washington.edu/Scripts/hiercentraldirectory.aspx?ID=20&toback=0 See more images and get more information from BrainInfo]
- [http://brainmaps.org Brain Atlas, Brain Maps, Neuroinformatics]
- [http://webvision.med.utah.edu/VisualCortex.html Webvision - The primary visual cortex] Comprehensive article about the structure and function of the primary visual cortex.
- [http://webvision.med.utah.edu/imageswv/BasicCells.jpg Webvision - Basic cell types] Image of the basic cell types of the monkey cerebral cortex.
- [http://info.med.yale.edu/chldstdy/plomdevelop/development/Arch1.html Development of the Cerebral Cortex] Different topics on cortical development in the form of columns written by leading scientists. Category:Cerebrum ja:大脳皮質

Pia mater

The pia mater (Latin: "tender mother", itself a translation from Arabic) is the delicate innermost layer of the meninges - the membranes surrounding the brain and spinal cord. The thin, mesh-like pia mater closely envelops the entire surface of the brain, running down into the fissures of the cortex. It joins with the ependyma which lines the ventricles to form choroid plexuses that produce cerebrospinal fluid. Category:Central nervous system Category:Neuroscience

Saline (medicine)

In medicine saline is a solution of sodium chloride in sterile water, used commonly for intravenous infusion, cleaning contact lenses, and nasal irrigation or jala neti. Sodium chloride (NaCl) is ordinary salt. Saline solutions are available in various concentrations for different purposes. Normal saline is the solution of 0.9% w/v of NaCl. It contains 154 meq/l of Na and Cl and has about the same degree of osmolality as blood (referred to as isotonic), about 300 mosm/l. Normal saline (NS) is therefore used frequently in intravenous drips (IVs) for patients who cannot take fluids orally and have developed severe dehydration. Normal saline is typically the first fluid used when dehydration is severe enough to threaten the adequacy of blood circulation and is the safest fluid to give quickly in large volumes. Other concentrations of saline are frequently used for other purposes, such as supplying extra water to a dehydrated patient or supplying the daily water and salt needs ("maintenance" needs) of a patient who is unable to take them by mouth. Because infusing a solution of low osmolality can cause problems, intravenous solutions with reduced saline concentrations typically have dextrose (glucose) added to maintain a safe osmolality while providing less sodium chloride. As the molecular weight of dextrose is greater, this has the same osmolality as normal saline but contributes less sodium to the circulation. Because dextrose monohydrate (MW 198 in contrast to MW 180 for glucose) is the commercial form of dextrose used in these preparations, 5% dextrose actually contains only 4.5 g/dl of glucose. Concentrations commonly used include #Half-normal saline (0.45% NaCl), often with "D5" (5% dextrose), contains 77 meq/l of Na and Cl and 4.5 g/l glucose. #Quarter-normal saline (0.22% NaCl) has 39 meq/l of Na and Cl and always contains 5% dextrose for osmolality reasons. #Dextrose (glucose) 4% in 0.18% saline is used sometimes for maintenance replacement. The amount of normal saline infused depends largely on the needs of the patient (e.g. ongoing diarrhoea or heart failure) but is typically between 1.5 and 3 litres a day for an adult.

See also


- Intravenous therapy
- Lactated Ringer's solution Category:Medical treatments

Ventricular system

The ventricular system is structure continuous with the central canal of the spinal cord and filled with cerebrospinal fluid (CSF) and is embryologically derived from the centre of the neural tube. The ventricular system serves to bathe and cushion the brain and spinal cord within their bony confines.

The Path of the Cerebrospinal Fluid

CSF is produced by the glial cells of the choroid plexus within the lateral ventricles, the 2 largest which are found within the cerebral hemispheres. From there it flows via the foramina of Monro into the 3rd ventricle, to the 4th ventricle via the cerebral aqueduct in the brainstem where it can pass into the central canal of the spinal cord or into the cisterns, subarachnoid spaces around the brain, via 3 small foramina - the central foramen of Magendie and the 2 larger foramina of Luschka. The fluid then flows around the superior sagittal sinus to be reabsorbed via the arachnoid villi into the venous system. CSF within the spinal cord can flow all the way down to the lumbar cistern at the end of the cord around the cauda equina where lumbar punctures are performed. The aqueduct between the third and fourth ventricles is very small, as are the foramina, which means that they can be blocked, causing high pressure in the lateral ventricles. This is hydrocephalus, otherwise known as water on the brain and is extremely serious both due to the damage caused by the pressure and nature of whatever caused the block, possibly a tumour or inflammatory swelling.

The 4 Ventricles

The 2 lateral ventricles are relatively large and C-shaped, roughly wrapping around the dorsal aspects of the basal ganglia. The choroid plexus is found here and it is here within the embryo that the successive generation of neurons gives rise to the 6-layered structure of the neocortex, constructed from the inside out during development. They extend into the temporal and occipital lobes via the temporal and occipital horns, respectively. The lateral ventricles both communicate to the third ventricle, found centrally within the mid-brain. This then communicates the CSF to the fourth ventricle, found within the hind-brain. The foramina to the subarachnoid space are found here, allowing the CSF to surround the spinal cord, brainstem, cerebellum and cerebral cortex. Diseases of the ventricular system include abnormal enlargement (hydrocephalus) and inflammation of the CSF-spaces (meningitis, ventriculitis) caused by infection or introduction of blood following trauma or haemorrhage. Interestingly, scientific study of CAT scans of the ventricles in the late 1970s revolutionized the study of mental illness. Researchers found that patients with schizophrenia had enlarged ventricles compared to healthy subjects. This became the first "evidence" that mental illness was biological in origin and led to a reinvigoration of the study of such conditions via modern scientific techniques.

Protection of the Brain

The brain and spinal cord are covered by a series of tough membranes called meninges, which protect these organs from rubbing against the bones of the skull and spine. The cerebrospinal fluid within the skull and spine is found between the pia mater and the arachnoid meninges and this provides further cushioning.
- Purves D, Augustine GJ, Fitzpatrick D, Hall WC, Lamantia AS, McNamara JO, Williams SM, Neuroscience (third edition). Sinauer Associates Inc, July 2004. ISBN 0878937250 Some information derived from Edgley S et al, Neuroanatomy from the Department of Anatomy, University of Cambridge. Category:Central nervous system ja:側脳室

Spinal cord

The spinal cord is a part of the vertebrate nervous system that is enclosed in and protected by the vertebral column (it passes through the spinal canal). It consists of nerve cells. The cord conveys the 31 spinal nerve pairs of the peripheral nervous system, as well as central nervous system pathways that innervate skeletal muscles. The vertebral column consists of vertebrae described as belonging to 5 groups (called segments). These segments are (in order from top to bottom): the cervical, thoracic, and lumbar vertebrae, and the sacrum and coccyx.

Embryology

In the human fetus, the spinal cord extends all the way down to the sacral vertebrae. As a person matures, the spinal cord shortens relative to the rest of the body, so at adulthood, the spinal cord only reaches down to around the level of L1 (the first, i.e. highest, lumbar vertebra), where it terminates and the cauda equina begin - this is why lumbar punctures are usually carried out on an adult at the (lower) level of L3/L4.

Anatomy

The spinal cord originates inside the brain at the inferior end of the medulla oblongata, exiting the skull via the foramen magnum. It is wrapped in three layers of membranes, called meninges. The spinal cord carries sensory signals and motor innervation to most of the skeletal muscles in the body. Just about every voluntary muscle in the body below the head depends on the spinal cord for control. Similarly, most cutaneous sensation below the neck is transmitted via the spinal cord. Most of the sympathetic pathways and the lower (i.e. non-vagal) parasympathetic pathways also go through the spinal cord. A cross-section through the spinal cord reveals that there is a central canal that carries cerebrospinal fluid (CSF) surrounded by grey matter on the inside, and this is surrounded by white matter. (This is the opposite to the brain's cerebral cortex.) A section of the cord can be divided into neat symmeterical halves by the dorsal median sulcus and ventral median fissure. The dorsal (towards the back) side of the spinal cord carries sensory information. The neurons that bring somatosensory information to the spinal cord reside in the dorsal root ganglion. Sensation from the lower body travels up the gracile tract, while sensation from the upper body and arms travels up the cuneate tract, which lies lateral to the gracile tract. There is no cuneate tract in the lumbar part of the spinal cord as sensory information from the arms does not travel through this area. Motor information (signals coming from the brain to move the muscles) travels down the ventral (toward the front) half of the spinal cord. Motor neurons are located in the anterior (this means close to the front, in humans it means the same as ventral) horn of the grey matter. There are two main columns of neurons in the anterior horn, the medial and lateral motor columns. The spinal cord proper ends at the level of L1. It terminates at a conical point known as the conus medullaris, from which a strand of connective tissue, the filum terminale extends caudally and attaches to the dorsal surface of the first cocygeal vertebra. After the termination of the cord, the spinal nerves continue as dangling nerves called the cauda equinae (literally "horse's tail"). The actual cord is approxiamately cylindrical in shape, but the diameter varies at different vetebral levels. There are two enlargements, cervical and lumbar. The cervical enlargement is due to the cord segments from C5 to T1 which innervates the upper limb via the brachial plexus. The lumbar enlargement arise from segments L1 to S3 (only the region around L1/L2 is part of the spinal cord proper) and innervates the lower limbs via the lumbar and sacral plexuses. There is a higher proportion of white matter in the cervical (neck) part of the spinal cord. This is because information to and from the whole body (such as the feet) must pass through here. In contrast, the lumbar and sacral areas do not carry information from anywhere above them, so have less white matter.

Pathology


- Damage in the spinal cord, called myelopathy, can result in paraplegia or quadriplegia, depending on the level within the spinal cord of the damage.
- spinal tumor
- syringomyelia
- Brown-Sequard syndrome Category:Central nervous system
-

External links


- [http://www.spineuniverse.com/displayarticle.php/article1275.html Nerve Structures of the Spine] ja:脊髄

Choroid plexus

The choroid plexus is the area on the ventricles of the brain where cerebrospinal fluid (CSF) is produced. Choroid plexus is present in the superior part of the inferior horn of the lateral ventricles. It follows up along this boundary, continuous with the inferior of the body of the lateral ventricles. It passes into the interventricular foramen, and is present at the top of the third ventricle. There is also choroid plexus on the fourth ventricle, on the section closest to the bottom half of the cerebellum.

Structure of the choroid plexus

The choroid plexus consists of many capillaries, separated from the subarachnoid space by pia mater and choroid ependymal cells. Liquid filters through these cells from blood to become CSF. There is also much active transport of substances into, and out of, of the CSF as it's made. Category:Central nervous system

Ependyma

Ependyma is the thin epithelial membrane lining the ventricular system of the brain and the spinal cord canal. Ependyma is one of four types of neuroglia, and is itself lined with epithelial cilia. It is involved in the production of the CSF. Jonas Frisen and his colleagues at the Karolinska Institute in Stockholm believe that ependyma is the prime candidate for the location of neural stem cells. Category:Physiology

Interventricular foramina

The interventricular foramen (aka the foramen of Monro) joins the lateral ventricles of the brain with the anterior third ventricle. Cerebrospinal fluid then passes through the cerebral aqueduct (aka the aqueduct of Sylvius) into the fourth ventricle, and then through the median aperture (aka the foramen of Magendie) and the paired lateral apertures (aka the foramena of Lushka) into the thecal sac surrounding the spinal cord and the basilar cisterns of the skull.

Third ventricle

The third ventricle is one of the four connected fluid-filled cavities within the human brain. It is a median cleft between the two thalami, and is filled with cerebrospinal fluid (CSF). It is in the midline, between the left and right lateral ventricles. It communicates with the lateral ventricles anteriorly (in front), and with the mesencephalic duct (aqueduct of Sylvius) posteriorly (in back). Developmentally it represents the cavity of the diencephalon, though they are in front of the interventricular foramen which is derived from the median part of the telencephalon. The third ventricle is bounded by the thalamus and hypothalamus on both the left and right sides. The lamina terminalis forms the anterior wall of the third ventricle. There are two protrusions on the front of the third ventricle, the supra-optic recess (above the optic chiasma), and the infundibular recess (above the pituitary stalk). In casts of the ventricular system, a small 'hole' may be seen in the body of the third ventricle. This is formed where the two thalami are joined together at the interthalamic adhesion (not seen in all people). The four fluid-filled cavities in the brain, collectively the ventricular system, are the left and right lateral ventricles, the third ventricle, and the fourth ventricle. Category:Cerebrum ja:第三脳室

Mesencephalic duct

The mesencephalic duct, also known as the aqueduct of Silvius or the cerebral aqueduct, contains cerebrospinal fluid (CSF), is within the mesencephalon (or midbrain) and connects the third ventricle in the thalamus (or diencephalon) to the fourth ventricle, which is between the pons and cerebellum. A blockage in this duct is a cause of hydrocephalus. Category:Cerebrum

Oxygenation

Oxygenation refers to the amount of oxygen in a medium. In blood it may be taken to be synonymous with saturation, which describes the degree to which the oxygen-carrying capacity of haemoglobin is utilised (normally 98-100%). Oxygenation also refers to the process of adding oxygen to a medium such as water or body tissue. Claims have been made that oxygenation of human tissue prevent diseases, including cancer, however some regard these claims as unverifiable. Oxygenation of various fluorocarbon liquids has been used successfully in fluid breathing systems, allowing air-breathing animals (including humans) to breathe via liquids for short periods of time. Category:Blood

Mammal



- Subclass Multituberculata (extinct)
  - Plagiaulacida
  - Cimolodonta
- Subclass Palaeoryctoides (extinct)
- Subclass Triconodonta (extinct)
- Subclass Eutheria (includes extinct ancestors)/Placentalia (excludes extinct ancestors)
  - Afrosoricida
  - Artiodactyla
  - Carnivora
  - Cetacea
  - Chiroptera
  - Cimolesta (extinct)
  - Creodonta (extinct)
  - Condylarthra (extinct)
  - Dermoptera
  - Desmostylia (extinct)
  - Embrithopoda (extinct)
  - Hyracoidea
  - Insectivora
  - Lagomorpha
  - Litopterna (extinct)
  - Macroscelidea
  - Mesonychia (extinct)
  - Notoungulata (extinct)
  - Perissodactyla
  - Pholidota
  - Plesiadapiformes (extinct)
  - Primates
  - Proboscidea
  - Rodentia
  - Scandentia
  - Sirenia
  - Taeniodonta (extinct)
  - Tillodontia (extinct)
  - Tubulidentata
  - Xenarthra
- Subclass Marsupialia
  - Dasyuromorphia
  - Didelphimorphia
  - Diprotodontia
  - Microbiotheria
  - Notoryctemorphia
  - Paucituberculata
  - Peramelemorphia
- Subclass Monotremata
  - Monotremata The mammals are the class of vertebrate animals characterized by the presence of mammary glands, which in females produce milk for the nourishment of young; the presence of hair or fur; and which have endothermic or "warm-blooded" bodies. The brain regulates endothermic and circulatory systems, including a four-chambered heart. Mammals encompass some 5500 species, distributed in about 1200 genera, 152 families and up to 46 orders, though this varies depending on the classification scheme adopted. Phylogenetically, Mammalia is defined as all of the descendants of the last common ancestor of monotremes (e.g., echidnas) and therian mammals (placentals and marsupials).

Characteristics

While most mammals give birth to live young, there are a few mammals (the monotremes) that lay eggs. Live birth also occurs in a variety of non-mammalian species, such as guppies and hammerhead sharks; thus it is not a distinguishing characteristic of mammals. Although all mammals are endothermic, so are birds and so this is also not a main defining feature. While monotremes do not have nipples, they do have mammary glands, meaning that they meet all conditions for inclusion in the class Mammalia. It should be noted that the current trend in taxonomy is to emphasize common ancestry; the diagnostic characteristics are useful for identifying this ancestry, but if, for example, a cetacean were found that had no hair at all, it would still be classified as a mammal. Mammals have three bones in each ear and one (the dentary) on each side of the lower jaw; all other vertebrates with ears have one bone (the stapes) in the ear and at least three on each side of the jaw. A group of therapsids called cynodonts had three bones in the jaw, but the main jaw joint was the dentary and the other bones conducted sound. The extra jaw bones of other vertebrates are thought to be homologous with the malleus and incus of the mammal ear. All mammalian brains possess a neocortex. This brain region is unique to mammals. Mammals have integumentary systems made up of three layers: the outermost epidermis, the dermis, and the hypodermis. The epidermis is typically ten to thirty cells thick, its main function being to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is fifteen to forty times thicker than the epidermis. The dermis is made up of many components such as bony structures and blood vessels. The hypodermis is made up of adipose tissue. Its job is to store lipids, and to provide cushioning and insulation. The thickness of this layer varies widely from species to species. Most mammals are terrestrial, but a number are aquatic, including sirenia (manatees and dugongs) and the cetaceans (dolphins and whales). Whales are the largest of all animals. There are semi-aquatic species such as seals which come to land to breed but spend the majority of the time in water. True flight has evolved only once in mammals, the bats; mammals such as flying squirrels and flying lemurs are actually gliding animals. No mammals have hair naturally blue or green in colour. Some cetaceans, along with the mandrills appear to have shades of blue skin. Many mammals are indicated as having blue hair or fur, but in all cases, it will be found to be a shade of grey. The two-toed sloth can seem to have green fur, however, this colour is caused by algae growths.

Origins

Mammals belong among the amniotes, and in particular to a group called the synapsids, distinguished by the shape of their skulls, in particular the presence of a single hole where jaw muscles attach, called temporal fenestra. In comparison, dinosaurs, birds, and most reptiles are diapsids, with two temporal fenestrae; and turtles, with no temporal fenestra, are anapsids. From synapsids came the first mammal precursors, therapsids, and more specifically the eucynodonts, 220 million years ago (mya) during the Triassic period. Pre-mammalian ears began evolving in the late Permian to early Triassic to their current state, as three tiny bones (incus, malleus, and stapes) inside the skull; accompanied by the transformation of the lower jaw into a single bone. Other animals, including reptiles and pre-mammalian synapsids and therapsids, have several bones in the lower jaw, some of which are used for hearing; and a single ear-bone in the skull, the stapes. This transition is evidence of mammalian evolution from reptilian beginnings: from a single ear bone, and several lower jaw bones (for example the sailback pelycosaur, Dimetrodon) to progressively smaller "hearing jaw bones" (for example the cynodont, Probainognathus), and finally (possibly with Morganucodon, but definitely with Hadrocodium), true mammals with three ear bones in the skull and a single lower jaw bone. Hence pelycosaurs and cynodonts are sometimes called "mammal-like reptiles", though this is strictly incorrect since in modern parlance these two are not reptiles, but rather synapsids. During the Mesozoic Period mammals diversified into four main groups: multituberculates, monotremes, marsupials, and placentals. Multituberculates went extinct during the Oligocene, about 30 million years ago, but the three other mammal groups are all represented today. Most early mammals remained small and shrew-like throughout the Mesozoic, but rapidly developed into larger more diverse forms following the Cretaceous-Tertiary extinction event 65 mya. The names "Prototheria", "Metatheria" and "Eutheria" expressed the theory that Placentalia were descendants of Marsupialia, which were in turn descendants of Monotremata, but this theory has been refuted. However, Eutheria and Metatheria are often used in paleontology, especially with regards to mammals of the Mesozoic. Mammal evolutionary progression is below:
- Jawless fish: Cambrian period to mid Ordovician periods
- Bony fish: mid-Ordovician period to late Devonian period
- Amphibians: late Devonian period to early Carboniferous period
- Reptiles: late Carboniferous period
- Pelycosaurs (synapsids, or "mammal-like reptiles"): late Carboniferous period to very early Triassic period
- Cynodonts: Permian-Triassic
- Mammals: mid-Triassic period to today

In the Mesozoic

Most early mammals were small shrew-like animals that fed on insects. However, in January 2005, the discovery was reported of two fossils of Repenomamus around 130 million years old, one more than a meter in length, the other having remains of a baby dinosaur in its stomach (Nature, Jan. 15, 2005 [http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v433/n7022/full/433116b_fs.html].) The earliest mammals include:
- Eozostrodon: Triassic and Jurassic
- Deltatheridium: Cretaceous
- Jeholodens: mid-Cretaceous
- Megazostrodon: late Triassic and early Jurassic
- Triconodont: Triassic to Cretaceous
- Zalambdalestes: late Cretaceous Although mammals existed alongside the dinosaurs, mammals only began to dominate after the mass extinction of the dinosaurs 65 mya, in the Cenozoic.

In the Paleocene

During the next 8 million years, the Paleocene period (64–58 mya), mammals exploded into the ecological niches left by the extinction of the dinosaurs. Small rodent-like mammals still dominated, but medium and larger-sized mammals evolved.
- Ptilodus: multituberculate
- Pucadelphys andinus: an opposum-like marsupial
- Purgatorius: a primate-like mammal, placental
- Ectoconus: an early hoofed mammal, placental

Classification

Main article: Mammal classification George Gaylord Simpson's classic "Principles of Classification and a Classification of Mammals" (AMNH Bulletin v. 85, 1945) was the original source for the taxonomy listed here. Simpson laid out a systematics of mammal origins and relationships that was universally taught until the end of the 20th century. Since Simpson's 1945 classification, the paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics. Though field work gradually made Simpson's classification outdated, it remained the closest thing to an official classification of mammals.

Standardized textbook classification

A somewhat standardized classification system has been adopted by most current mammalogy classroom textbooks. The following taxonomy of extant and recently extinct mammals is taken from Vaughan et al. (2000). Class Mammalia
- Subclass Prototheria - monotremes: platypus and echidnas
- Subclass Theria - live-bearing mammals
  - Infraclass Metatheria - marsupials
  - Infraclass Eutheria - placentals

McKenna/Bell classification

In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and Susan K. Bell, which has resulted in the "McKenna/Bell classification". McKenna and Bell, Classification of Mammals: Above the species level, (1997) is the most comprehensive work to date on the systematics, relationships, and occurrences of all mammal taxa, living and extinct, down through the rank of genus. The new McKenna/Bell classification was quickly accepted by paleontologists. The authors work together as paleontologists at the American Museum of Natural History, New York. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia. The McKenna/Bell hierarchical listing of all of the terms used for mammal groups above the species includes extinct mammals as well as modern groups, and introduces some fine distinctions such as legions and sublegions (ranks which fall between classes and orders) that are likely to be glossed over by the layman. The published re-classification forms both a comprehensive and authoritative record of approved names and classifications and a list of invalid names. Click on the highlighted link for a [http://nasa.utep.edu/chih/chklist/mammals/keys/mammtab.htm table comparing the traditional and the new McKenna/Bell classifications of mammals] Extinct groups are represented by †. Class Mammalia
- Subclass Prototheria: monotremes: platypuses and echidnas
- Subclass Theriiformes: live-bearing mammals and their prehistoric relatives
  - Infraclass †Allotheria: multituberculates
  - Infraclass †Triconodonta: triconodonts
  - Infraclass Holotheria: modern live-bearing mammals and their prehistoric relatives
    - Supercohort Theria: live-bearing mammals
      - Cohort Marsupialia: marsupials
      -
- Magnorder Australidelphia: Australian marsupials and the monito-del-monte
      -
- Magnorder Ameridelphia: New World marsupials
      - Cohort Placentalia: placentals
      -
- Magnorder Xenarthra: xenarthrans
      -
- Magnorder Epitheria: epitheres
      -
  - Grandorder Anagalida: lagomorphs, rodents, and elephant shrews
      -
  - Grandorder Ferae: carnivorans, pangolins, creodonts, and relatives
      -
  - Grandorder Lipotyphla: insectivorans
      -
  - Grandorder Archonta: bats, primates, colugos, and tree shrews
      -
  - Grandorder Ungulata: ungulates
      -
    - Order Tubulidentata incertae sedis: aardvark
      -
    - Mirorder Eparctocyona: condylarths, whales, and artiodactyls
      -
    - Mirorder †Meridiungulata: South American ungulates
      -
    - Mirorder Altungulata: perissodactyls, elephants, manatees, and hyraxes

Molecular classification of mammals

Molecular studies based on DNA analysis have suggested new relationships among mammal families over the last few years. The most recent classification systems based on molecular studies have proposed four groups or lineages of placental mammals. Molecular clocks suggest that these clades diverged from early common ancestors in the Cretaceous, but fossils have not been found to corroborate this hypothesis. These molecular findings are consistent with mammal zoogeography: The first divergence was that of the Afrotheria 110–100 mya. The Afrotheria proceeded to evolve and diversify in the isolation of the African-Arabian continent. The Xenarthra, isolated in South America, diverged from the Boreoeutheria approximately 100–95 mya. The Boreoeutheria split into the Laurasiatheria and Euarchontoglires between 95 and 85 mya; both of these groups evolved on the northern continent of Laurasia. After tens of millions of years of relative isolation, Africa-Arabia collided with Eurasia, exchanging Afrotheria and Boreoeutheria. The formation of the Isthmus of Panama linked South America and North America, which facilitated the exchange of mammal species in the Great American Interchange. The traditional view that no placental mammals reached Australasia until about 5 million years ago when bats and murine rodents arrived has been challenged by recent evidence and may need to be reassessed. It should however be noted that these molecular results are still controversial because they are not reflected by morphological data and thus not accepted by many systematists.
- Group I: Afrotheria
  - Order Macroscelidea: elephant shrews (Africa).
  - Order Afrosoricida
  - Order Tubulidentata: aardvark (Africa south of the Sahara).
  - Clade