Brain

:For other articles about other subjects named brain see brain (disambiguation). For information about the human brain in specific, please see its article.

Inside brains

Despite the variance of the of the species in which the brain can be found there are many common features in its cellular make-up, structure and function. On a cellular level the brain is composed of two classes of cell, neurons and glia both of which contain various groups of cell. Neurons connect to form neural circuits which may be viewed in a similar (but not the same) light to synthetic electrical circuits. Of course, these do not form simple electrical one-to-one circuits, neurons typically connect to at least a thousand other neurons{{ref|1}}. These highly specific circuits are the link between perception, action and higher cognitive functions; they make up systems. The brain is divided into sections which are both spatially, compositionally and mostly functionally. In mammals these are often the cerebrum (a large, dorsal section with convoluted surface), cerebellum (smaller, ventral structure with a rippled surface), and the brain stem. These sections may then further divided into hemispheres, lobes, cortexes, areas, and so forth.

Related Topics:
Species - Neuron - Glia - Neural circuits - Electrical circuit - Higher cognitive function - Cerebrum - Brain stem - Hemisphere - Lobe

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Histology

The chief class of neuron in the brain is the interneuron (defined as being any which does not leave the central nervous system), however afferent and efferent neurons are also present and are introduced via the brain stem. The brain contains about 10 glia 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'). Much of the 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). In fact, oligosaccharides are the defining factor between white matter and grey matter in the brain—white matter is composed of myelinated (insulated) axons, where grey matter contains cell soma, dendrites and unmyelinated portions of axons and glia.

Related Topics:
Interneuron - Central nervous system - Afferent - Efferent neuron - Greek - Nervous system - Oligodendrocyte - Axon - Schwann cell - White matter - Grey matter - Myelin - Soma - Dendrite

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In mammals, apart from the above nerve tissue, the brain also contains a certain amount of [[connective tissue called the meninges. It 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 (brain)|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.

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The brain is suspended in cerebrospinal fluid, which circulates between layers of the meninges and through cavities in the brain called ventricles. It functions as both chemically (in metabolism) and physically (in shock-prevention) important.

Related Topics:
Cerebrospinal fluid - Ventricle - Metabolism

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Anatomy

Although the histology of the brain is common to all those who have one, the higher, 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.

Related Topics:
Vertebrate - Invertebrate

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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.

Related Topics:
Insect - Optical lobe - Protocerebrum - Deutocerebrum - Tritocerebrum - Eye - Mushroom bodies - Smell - Central body complex - Antennal lobes - Olfactory bulb - Mechanosensory - Neuropil - Touch receptor - Head - Antennae - Flies - Moth

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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.

Related Topics:
Cephalopod - Supraesophageal mass - Subesophageal mass - Esophagus - Basal lobe - Dorsal magnocellular lobe

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Vertebrates

Function

Vertebrate brains receive signals through nerves arriving from most portions of a body, interpret those signals and formulate reactions based on prior experiences and on physical needs. A similarly extensive nerve network delivers signals from a brain to control muscles throughout a body.

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Sensory input is processed by the brain to recognize danger, find food, and identify potential mates, among other goals. Visual, touch, and auditory sensory pathways of vertebrates are routed to 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.

Related Topics:
Thalamus - Sensory system - Visual system - Auditory system - Somatosensory system - Olfactory bulb - Olfactory system - Taste - Gustatory system

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To control movement, the brain has several parallel systems of muscle control. The motor system controls voluntary muscle movement, aided by feedback loops in the cerebellum and the basal ganglia. Nuclei in the brainstem control many involuntary muscle functions such as heartrate and breathing.

Related Topics:
Movement - Muscle - Motor system - Feedback loop - Cerebellum - Basal ganglia

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Brains also produce hormones that can influence organs elsewhere in a body and brains 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.

Related Topics:
Hormones - Circulatory system - Pituitary gland

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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 vertebrates, sensory processing involves progressively rostral regions of the brain among newer species.

Related Topics:
Consciousness - Cerebral cortex - Limbic - Thalamus - Hippocampus

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Hormones, sensory information, autonomic processes, and cognitive processes alike can exert strong influence on the regulation of brain activities. Stimulus from any source can trigger a general arousal process that decreases reliance on cortical processes, or that enhances and focuses cortical processes. Such diverse causes as hunger, fatigue, beliefs, unfamiliar information or actual threats can trigger an alert response that can exert control over cognitive priorities.

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