Brain: Encyclopedia II - Brain - The importance of the brain

Brain - The importance of the brain

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) ^[1]. The brain of arthropods and cephalopods arises from twin parallel nerve cords that extend through the body of the animal. In arthropods, the brain consists of a central brain with three divisions and large optical lobes behind each eye for visual processing^[2].

The brain of craniates develops from the anterior section of a single dorsal nerve cord, which later becomes the spinal cord^[3]. 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) ^[4]. 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 ^[5]. 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 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 million billion synaptic connections, making it one of the most densely connected network systems in the known universe; however, more complex structures may exist.

Brain - 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 illnesses, 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.

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.

Brain - 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.

Adapted from the Wikipedia article "The importance of the brain", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki