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| ==Overview== | ==Overview== | ||
| Human encephalization is especially pronounced in the ]. The human brain not only is larger in proportion to the human body than brains of other animals of the same size; much more of the human brain is |
Human encephalization is especially pronounced in the ]. The human brain not only is larger in proportion to the human body than brains of other animals of the same size; much more of the human brain is neocortex than in other animals. Profound capacities for language, planning, extended memory, empathy and fabrication all can be related to structural features of enlarged ]s that form about a third of the neocortex. In humans, the ], ] and ] make up a smaller proportion of the brain than those of older species. | ||
| 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 in some ways to those of the most basic ]s. Human consciousness involves both the extended capacity of the modern neocortex in particular as well as profoundly developed protypical structures of the ]. | 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 in some ways to those of the most basic ]s. Human consciousness involves both the extended capacity of the modern neocortex in particular as well as profoundly developed protypical structures of the ]. | ||
Revision as of 06:00, 1 March 2004
In the anatomy of animals, the human brain is the most complex computational structure among all species on Earth. Humans' unique capacity for intelligent behavior results both from a larger brain size typical of a larger animal, and from encephalization, which is an increase of brain size in relation to body size.
Overview
Human encephalization is especially pronounced in the neocortex. The human brain not only is larger in proportion to the human body than brains of other animals of the same size; much more of the human brain is neocortex than in other animals. Profound capacities for language, planning, extended memory, empathy and fabrication all can be related to structural features of enlarged frontal lobes that form about a third of the neocortex. In humans, the medulla, metencephalon and diencephalon make up a smaller proportion of the brain than those of older species.
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 in some ways to those of the most basic vertebrates. Human consciousness involves both the extended capacity of the modern neocortex in particular as well as profoundly developed protypical structures of the paleopallium.
Anatomy
The adult human brain usually weighs about 1 - 1.5 kilograms in an average volume of 1,600 cubic centimetres.
A bulbuous cerebral cortex is formed of convoluted gray matter internally supported by deep brain white matter and separated by a prominant central fissure. A well-developed cerebellum is visible at the back of the brain. Brain stem structures are almost completely enveloped by the cerebellum and telencephalon, with the only the medulla oblongata visible as it merges with the spinal cord.
The blood supply to the brain involves several arteries that enter the brain and communicate in a circle called the circle of Willis. Blood is then drained from the brain through a network of sinuses that drain into the right and left internal jugular veins.
The brain is suspended in cerebrospinal fluid (CSF) which also fills spaces called ventricles inside it. The dense fluid protects the brain and spinal cord from shock; a brain that weights 1500gms in air weighs only 50gms when suspended in CSF. (Livingston, 1965). Fluid movement within the brain is limited by the blood-brain barrier, brain-cerebrospinal fluid barrier and the blood-cerebrospinal fluid barrier.
The brain is easily damaged by compression, so the fluid surrounding the central nervous system must be maintained at a constant volume. Humans are estimated to produce about 500 ml or more of cerebrospinal fluid each day, with only about 15 percent of the body's estimated 150 ml of CSF at any given time located in the ventricles of the brain. The remainder fills the subarachnoid space which separates the soft tissues of the brain and spinal cord from the hard surrounding bones (skull and vertebrae). Elevated levels of CSF are associated with traumatic brain injuries and a pediatric disease know as hydrocephalus. Increased fluid pressure can result in permanant brain injury and death.
Function
The human brain is the seat of cognitive processes related to perception, interpretation, imagination and memory, of which a person might or might not be aware. Beyond cognitive functions, the brain regulates autonomic processes related to essential body functions such as respiration and heartbeat.
Extended neocortical capacity allows humans extensive control over emotional behavior, but neural pathways between emotive centers of the brain stem and cerebral motor control areas are shorter than those connecting complex cognitive areas in the neocortex with incoming sensory information from the brain stem. Powerful emotional pathways can modulate spontaneous emotive expression regardless attempts at cerebral self-control. Emotive stability in humans is associated with planning, experience and an environment that is both stable and stimulating, especially during early developmental years.
The 19th Century discovery of a primary motor control area mapped to correspond with regions of the body led to popular belief that the brain was organized around a homunculus. A distorted figure drawn to represent the body's motor map in the pre-frontal cortex was popularly recognized as the brain's homonculus, but function of the human brain is far more complex.
The human brain appears to have no localized center of conscious control. Like the brains of other vertebrates, it derives consciousness from interaction among numerous systems within the brain. Executive decision-making functions rely on cerebral activities, especially those of the frontal lobes, but redundant and complementary processes within the brain result in a diffuse assignment of executive control that can be difficult to attribute to any single locale.
Mid-brain functions include routing, selecting, mapping and cataloguing information, including information perceived from the environment and information that is remembered and processed throughout the cerebral cortex. Endrocrine functions housed in the mid-brain play a leading role in modulating arousal of the cortex and of autonomic systems.
Voluntary muscular movements are transmitted through neural networks of the cerebrum to basal ganglia, as part of a complex feedback system that regulates movement. Complex muscle movements such as a series of notes played on a keyboard or a series of footsteps can be initiated as a packaged concept from the cognitive functions of the cerebral cortex to be executed as a series of steps with the help of the cerebellum.
Nerves from the brain stem complex where autonomic functions are modulated join nerves routing messages to and from the cerebrum in a bundle that passes through the spinal column to related parts of a body. Twelve cranial nerves, including some that inervate parts of the head, follow pathways from the medulla oblongata outside the spinal cord.
A definite description of the biological basis for consciousness so far eludes the best efforts of the current generation of researchers. But reasonable assumptions based on observable behaviors and on related internal responses have provided the basis for general classification of elements of consciousness and of likely neural regions associated with those elements. Researchers know people loose consciousness and regain it, they have identified partial losses of consciousness associated with particular neuropathologies and they know that certain conscious activities are impossible without particular neural structures.
Correlation of particular conscious activities with likely neural structures suggest three levels of consciousness in humans. A protoself represents the most basic level of consciousness shared with animals as primitive as amoeba. A core consciousness similar to that of other vertebrates lets humans see and hear their environment. An extended consciousness allows us to develop profound narratives describing our own lives and environment.