encephalon n : that part of the central nervous system that includes all the higher nervous centers; enclosed within the skull; continuous with the spinal cord [syn: brain] [also: encephala (pl)]

English

Noun

1. Part of brain, area of central nervous system that includes all higher nervous centers, enclosed within the skull and continuous with the spinal cord.

In animals, the brain is the control center of the central nervous system, responsible for behavior. In mammals, the brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing, equilibrioception (balance), sense of taste, and olfaction (smell).

While all vertebrates have a brain, most invertebrates have either a centralized brain or collections of individual ganglia. Some animals such as cnidarians and echinoderms do not have a centralized brain, and instead have a decentralized nervous system, while animals such as sponges lack both a brain and nervous system entirely.

Brains can be extremely complex. For example, the human brain contains roughly 100 billion neurons, each linked to as many as 10,000 other neurons.

History

Early views on the function of the brain regarded it as little more than cranial stuffing. In Ancient Egypt, from the late Middle Kingdom onwards, in preparation for mummification, the brain was regularly removed, for it was the heart that was assumed to be the seat of intelligence. According to Herodotus, during the first step of mummification, "The most perfect practice is to extract as much of the brain as possible with an iron hook, and what the hook cannot reach is mixed with drugs." Over the next five-thousand years, this view came to be reversed; the brain is now known to be seat of intelligence, although idiomatic variations of the former remain, as in memorizing something "by heart".

The first thoughts on the field of psychology came from ancient philosophers, such as Aristotle. As thinkers became more in tune with biomedical research over time, as was the case with medieval psychologists such as Alhazen and Avicenna for example, the concepts of experimental psychology and clinical psychology began emerging. From that point, different branches of psychology emerged with different individuals creating new ideas, with modern psychologists such as Freud and Jung contributing to the field.

Mind and brain

portalpar Mind and Brain The distinction between the mind and the brain is fundamental in philosophy of mind. The mind-body problem is one of the central problems in the history of philosophy. The brain is the physical and biological matter contained within the skull, responsible for electrochemical neuronal processes. The mind, in contrast, consists in mental attributes, such as beliefs, desires, perceptions, and so on. There are scientifically demonstrable correlations between mental events and neuronal events; the philosophical question is whether these phenomena are identical, at least partially distinct, or related in some other way.

Philosophical positions on the mind-body problem fall into two main categories. The first category is dualism, according to which the mind exists independently of the brain. Dualist theories are further divided into substance dualism and property dualism. René Descartes is perhaps the most prominent substance dualist, while property dualism is more popular among contemporary dualists like David Chalmers. The second category is materialism, according to which mental phenomena are identical to neuronal phenomena. A third category of view, idealism, claims that only mental substances and phenomena exist. This view, most prominently held by 18th century Irish philosopher Bishop George Berkeley, has few contemporary adherents.

Both dualism and materialism face serious philosophical challenges. Dualism requires admitting non-physical substances or properties into ontology, which is in apparent conflict with the scientific world view. Materialism, on the other hand, must provide an explanation of how two seemingly different kinds of phenomena – the mental and the physical – could be identical. This is particularly challenging in that mental phenomena have certain characteristics – particularly intentionality and phenomenal character – that cannot (at least currently) be explained satisfactorily by a purely physical analysis of the brain.

Comparative anatomy

Three groups of animals have notably complex brains: the arthropods (insects, crustaceans, arachnids, and others), the cephalopods (octopuses, squids, and similar mollusks), and the craniates (vertebrates and hagfish). The brain of arthropods and cephalopods arises from twin parallel nerve cords that extend through the body of the animal. Arthropods have a central brain with three divisions and large optical lobes behind each eye for visual processing. In craniates, the brain is protected by the bones of the skull.

Mammals have a six-layered neocortex (or homotypic cortex, neopallium), in addition to having some parts of the brain that are allocortex. The cerebrum has two cerebral hemispheres. The cerebellum also has hemispheres. The telencephalic hemispheres are connected by the corpus callosum, another large white matter tract. 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 bilaterally symmetrical encephalization. Encephalization refers to the tendency for more complex organisms to gain larger brains through evolutionary time. Larger vertebrates develop a complex, layered and interconnected neuronal circuitry. In modern species most closely related to the first vertebrates, brains are covered with gray matter that has a three-layer structure (allocortex). Their brains also contain deep brain nuclei and fiber tracts forming the white matter. Most regions of the human cerebral cortex have six layers of neurons (neocortex). These highly specialized circuits make up systems which are the basis of perception, different types of action, and higher cognitive function.

Structure

Neurons are the cells that convey information to other cells; these constitute the essential class of brain cells.

In addition to neurons, the brain contains glial cells in a roughly 10:1 proportion to neurons. Glial cells ("glia" is Greek for “glue”) form a support system for neurons. They create the insulating myelin, provide structure to the neuronal network, manage waste, and clean up neurotransmitters. Most types of glia in the brain are present in the entire nervous system. Exceptions include the oligodendrocytes which myelinate neural axons (a role performed by Schwann cells in the peripheral nervous system). The myelin in the oligodendrocytes insulates the axons of some neurons. White matter in the brain is myelinated neurons, while grey matter contains mostly cell soma, dendrites, and unmyelinated portions of axons and glia. The space between neurons is filled with dendrites as well as unmyelinated segments of axons; this area is referred to as the neuropil.

In mammals, the brain is surrounded by connective tissues called the meninges, a system of membranes that separate the skull from the brain. This three-layered covering is composed of (from the outside in) the dura mater, arachnoid mater, and pia mater. The arachnoid and pia are physically connected and thus often considered as a single layer, the pia-arachnoid. Below the arachnoid is the subarachnoid space which contains cerebrospinal fluid, a substance that protects the nervous system. Blood vessels enter the central nervous system through the perivascular space above the pia mater. The cells in the blood vessel walls are joined tightly, forming the blood-brain barrier which protects the brain from toxins that might enter through the blood.

The brain is bathed in cerebrospinal fluid (CSF), which circulates between layers of the meninges and through cavities in the brain called ventricles. It is important both chemically for metabolism and mechanically for shock-prevention. For example, the human brain weighs about 1-1.5 kg or about 2-3 lb. The mass and density of the brain are such that it will begin to collapse under its own weight if unsupported by the CSF. The CSF allows the brain to float, easing the physical stress caused by the brain’s mass.

Function

Vertebrate brains receive signals through nerves arriving from the sensors of the organism. These signals are then processed throughout the central nervous system; reactions are formulated based upon reflex and learned experiences. A similarly extensive nerve network delivers signals from a brain to control important muscles throughout the body. Anatomically, the majority of afferent and efferent nerves (with the exception of the cranial nerves) are connected to the spinal cord, which then transfers the signals to and from 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 the motor cortex, cerebellum, and the basal ganglia. The system eventually projects to the spinal cord and then out to the muscle effectors. Nuclei in the brain stem control many involuntary muscle functions such as heart rate and breathing. In addition, many automatic acts (simple reflexes, locomotion) can be controlled by the spinal cord alone.

Brains also produce a portion of the body's hormones that can influence organs and glands elsewhere in a body—conversely, brains also react to hormones produced elsewhere in the body. In mammals, the hormones that regulate hormone production throughout the body are produced in the brain by the structure called the pituitary gland.

Evidence strongly suggests that developed brains derive consciousness from the complex interactions between the numerous systems within the brain. Cognitive processing in mammals occurs in the cerebral cortex but relies on midbrain and limbic functions as well. 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. This focusing of cognition is known as attention. Cognitive priorities are constantly shifted by a variety of factors such as hunger, fatigue, belief, unfamiliar information, or threat. The simplest dichotomy related to the processing of threats is the fight-or-flight response mediated by the amygdala and other limbic structures.

Neurotransmitter systems

Neurons expressing certain types of neurotransmitters sometimes form distinct systems, where activation of the system causes effects in large volumes of the brain, called volume transmission.

The major neurotransmitter systems are the noradrenaline (norepinephrine) system, the dopamine system, the serotonin system and the cholinergic system.

Drugs targeting the neurotransmitter of such systems affects the whole system, which explains the mode of action of many drugs;

• Cocaine, for example, blocks the reuptake of dopamine, leaving these neurotransmitters in the synaptic gap longer.
• Prozac is a selective serotonin reuptake inhibitor (SSRI), hence potentiating the effect of naturally released serotonin.
• AMPT prevents the conversion of tyrosine to L-DOPA, the precursor to dopamine; reserpine prevents dopamine storage within vesicles; and deprenyl inhibits monoamine oxidase (MAO)-B and thus increases dopamine levels.

Diseases may affect specific neurotransmitter systems. For example, Parkinson's disease is at least in part related to failure of dopaminergic cells in deep-brain nuclei, for example the substantia nigra. Treatments potentiating the effect of dopamine precursors have been proposed and effected, with moderate success.

A brief comparison of the major neurotransmitter systems follows: