Reality → Life → Human → Brain
The anatomy of the human brain closely resembles the animalistic brain of all mammals. Its overall organization reflects evolutionary and embryonal development: closest to the spinal cord, the hindbrain fine-tunes motor functions; the midbrain plays an important role for sensory functions; and the forebrain processes and controls all motor, sensory and higher functions [1] . The brain exerts centralized control over the body's physiological processes and allows rapid reactions as well as gradual adaptations to changes in the environment: the cerebellum, a key component of the hindbrain, integrates inputs from the forebrain and the spinal cord for fast and coordinated motor signals; and the hypothalamus, a small region at the base of the forebrain, links with the endocrine system and plays a major role in regulating the slow release of hormones from various glands to adjust body functions to changes in the environment.
Some 100 billion neurons of the human brain, supported by several times more
glia cells, work together in miraculous, little understood ways. A neuron's dendritic branches
receive signals from other neurons. Dendrites can have from 1 to maybe 200,000 branches (Purkinje cells
of the cerebellum have the highest number). The received signals are processed in the neuron's body which contains a
nucleus, ER, and
Golgi, while mitochondria are accumulated at the cell connections. The output is conducted through
a single axon with multiple terminals that connect with dendrites of other neurons. On average, a neuron has a
few thousand connections. The number of possible connections in a network of 100 billion neurons is staggering. The electrical firing of neurons
is a fascinating process that has been studied in detail [2] .
Astrocyte-type glia cells are believed to play a major role in neural signaling, while
oligodendrocytes are responsible for myelination,
a process that increases transmission speed. How the brain's molecular structures with myriads of chemical and electrical interactions team up to achieve the
body's survival-oriented actions and reactions remains a mystery. Moreover, the ultimate puzzle is how the human brain can generate a mind that recognizes
itself, thinks, and can create masterpieces of art, science, and technology. Some scientists hypothesize that a hierarchical organization of neurons in the thin
layer of cortical gray matter may hold a key for explaining the brain's higher functions [3] .
The human brain (see Sheet) is distinguished by its greatly enlarged forebrain. It can be divided vertically into 3 distinct zones: a 2-3 mm thin outer layer of gray matter made up mainly of neural bodies; a thick mass of white matter consisting mainly of myelinated axons; and, at the base, a zone of dense cell agglomerations (basal ganglia).
In neurons, action potentials (sharp electric pulses, or spikes, of about 1 ms duration) are generated when the membrane potential exceeds a threshold. The process is controlled by selective voltage-gated ion channels and Na+/K+ pumps and works in a 'all-or-none'-manner, where spikes of constant amplitude are fired and stronger excitatory signals translate into more spikes. The transmission speed of the action potential is only in the order of 1 m/s for an unprotected axon, but can increase 10 to 100-fold for myelinated axons. At the axon's terminals, an arriving spike causes signal transfer to the dendrites of other neurons through chemical (most common) and electrical (rarer) synapses. In chemical synapses, the action potential triggers an influx of Ca++ ions and a cascade of actions by complex structures in the active zone, resulting in the release of neurotransmitters (mainly the amino acid glutamate and its derivative GABA, though some 100 other compounds may also be involved), which then pass through a cleft, where they can be influenced by extracellular molecules, before attaching briefly to specific receptors in the membrane of the receiving neuron. A new action potential is generated (or inhibited), and secondary messenger molecules may be released, while the neurotransmitters are recycled or destroyed. The whole process takes only milliseconds. In electrical synapses, signal transmission is still much faster, enabled by gap junctions with direct passage of smaller molecules from cell to cell. It is believed that 'hard-wired' synaptic connections are formed during early childhood and adolescence, a process thought to be critical for memory formation and learning and to be closely connected with myelination (white matter formation) of axons.
About 90% of the cortex (outer layer) of the human forebrain is distinguished by a 6-layered structure, called neocortex, that only occurs in mammals (i.e., has not evolved until 'only' 60 million years ago). The neocortex is responsible for all higher functions, including deliberate attention to sensory signals and control of movements, thinking, and cognition. The distinctly horizontal layer-structure conceals a more subtle organization into vertical cortical columns and minicolumns (there may be some 2 million columns, each containing some 100 minicolumns made up of some 100 neurons each). This structure has evoked theories about how the brain might work. One of the more plausible concepts is a hierarchical temporal memory that would support the human brain's extraordinary capacity for invariant object recognition and lightning-fast predictions as demonstrated by the ability to immediately recognize persons or things under greatly altered conditions, even as abstractions like caricatures, symbols, or CAPTCHAs (see also On Intelligence, by Jeff Hawkins).