Brain‎ > ‎

Brain High Level Architecture

Human brain is probably the most complex order (refer: Origin of Orders) emerged (refer: Emergence) in our known universe that wonders about itself. Still there are many unknowns about our brain, but nothing mystic!

A beautiful introduction from Carl Sagan:

Key Functions

Despite its complexity, it has some key functions (click the picture to enlarge it).

  • Senses both internal and external signals
  • Processes this incoming signals
  • Stores elements of this information (memory and learning)
  • Acts on the incoming input
Evolved Stages

Brain has evolved inside out (also refer:
Evolutionary view of Emotional and Thinking Brain, where only very high level functional division is discussed) in four major stages.

Brain stem: It is the oldest part. It conducts basic biological functions, the rhythms of life, such as heart beat and respiration.

Protoreptilian or R-complex: It is the seat of aggression, rituals, territoriality, and social hierarchy. It evolved 100s of millions of years ago in our reptilian ancestors.

Paleomammalian: Limbic system or mammal brain is evolved 10s of millions of years ago in mammals, but not yet primates like monkeys or apes. It controls moods, emotions, concerns, care for the young, etc.

Neomammalian: Cerebral cortex: evolved millions of years ago in primates (monkeys, apes, human). Here 2/3 of brain mass transformed into consciousness It's a realm of intuition and critical analysis; ideas and inspiration; read and write; mathematics and music; arts and science.
Hierarchy of Brain Function

The human brain is organized from the most simple (e.g., fewest cells: brainstem) to most complex (e.g., most cells and most synapses: frontal cortex). The various functions of the brain, from most simple and reflexive (e.g., regulation of body temperature) to most complex (e.g., abstract thought) are mediated in parallel with these various areas. These areas organize during development and change in the mature brain in a ‘use-dependent’ fashion. The more a certain neural system is activated, the more it will ‘build in’ this neural state -- creating an internal representation of the experience corresponding to this neural activation. This use-dependent capacity to make internal representations of the external or internal world is the basis for learning and memory.

Sequential Processing: All incoming sensory information first enters the Brain at the level of the spinal cord or brainstem. This means that the first place where patterns of activation are matched against previously stored templates is in these primitive areas. Indeed, the spinal cord and brainstem may process and act on incoming information before the integrated and interpreted signals even get up to the cortex (e.g., reflex withdrawal of a finger from fire).

Cortical Modulation

As the brain develops in this sequential and hierarchical fashion, and the more complex limbic, subcortical and cortical areas organize, they begin to modulate, moderate and ‘control’ the more primitive and ‘reactive’ lower portions of the brain. These various brain areas develop, organize and become fully functional at different times during childhood. At birth, for example, the brainstem areas responsible for regulating cardiovascular and respiratory function must be intact while the cortical areas responsible for abstract cognition have years before they will be fullyfunctional.


The human brain is very plastic - meaning that it is capable of changing in response to patterned, repetitive activation (e.g., reading or hearing a new language, learning a new motor skill such as typing). Recalling, however, that the brain is not just “one” large mass of equivalent tissue - recalling that the brain has a hierarchical and complex organization and that different systems mediate different functions, it stands to reason that not all parts of the brain -once developed - are as easy to modify or change with experience. Simply stated, not all parts of the brain are equally plastic. The plasticity of the cortex is much greater than the plasticity of the brainstem.