The human brain is the anatomical and physiological part of an animal’s body-as-a-whole that enables and governs virtually all life functions. It accomplishes this through its unique ability to continually change its internal states in order to adapt the organism’s behavior in response to the moment-by-moment demands of its physical and social environment. In short, the Organism’s brain mediates received Sensations and expressed Responses. (The brain’s nature is thus symbolized: S-O-R.) The brain accomplishes this herculean task through a dynamic network of billions of neurons, trillions of neural-synaptic connections, and millions of molecular-genetic reactions.
The brain stores the fluid history of its prior internal states (based on learning and memories) which enables it to respond to future stimuli with increasingly effective adaptation and efficiency. Thus, the brain depends on logging and time-stamping its chronology of experiences, learning from the past and modifying itself accordingly. This enables brain states to anticipate and adapt to the future with as few surprises as possible. The brain builds a repertoire of patterned responses necessary for automatic biological processes (e.g., digestion, circulation, and breathing) and behavioral habits (e.g., walking, talking, and routine activities of daily living). It monitors whether, in any situation, it is appropriate to rely on those automated responses versus engaging novel cognitive control requiring heightened attention, reason, and judgment in order to negotiate unique or unfamiliar stimuli and responses.
The centralized brain—by extension through its peripheral nervous system—infiltrates all bodily tissues and unites the entire organism. The nervous system is the first tissue to differentiate and emerge in the embryo, and as it develops it permeates virtually all organ systems. Functions occurring anywhere within the body are communicated to the brain and are, in turn, accounted for by the brain in formulating responses anywhere in the body. The brain is the mediator of life’s constant flow of stimuli and responses, ever morphing for each person into a unique coalescence of form and function: an intellect and personality bridging genetic foundation and the life-time flow of idiosyncratic experience.
Brain (and the greater nervous system) is the most dynamic organ of the body as it constantly changes its biological (physical and chemical) structure in order to accommodate and assimilate the moment-to-moment experiences of life. These biological adaptations are reflected in all behaviors: from simple motor responses to the most complex thought processes and emotions. Brain is the great unifier that aims to integrate the continuous stream of external sensation and internal thought, and produces the seamless flow of motor and mental responses so that an organism’s life occurs as an organized, on-going event, unique for each person. The brain is, at any split millisecond in time, a coded snapshot of our past behaviors, predicted future behaviors, perceptual propensities, personality characteristics, beliefs, moral values, and more.
Brain embodies each one-of-a-kind personality—the expression of thousands of genes, experiences, and response histories, the accumulation of which produces a unique, whole person. With its network of hundreds of billions of neurons representing its memories and providing the basis for its sensation-response repertoire—not in fixed, linear form but in probabilistic, nonlinear form—each person yields a mix of high and low predictable behaviors, depending in part on the familiarity of any given situation. There are so many parallel processes at work in the brain that its coded state at any point in time represents multiple neural trajectories that coalesce into an experienced singularity. Just what thoughts and behaviors that singularity yields is a probabilistic function of its neural network history.
[This concise definition of brain raises the issue whether some animal species (bacteria, for instance, or even higher phyla) actually have a brain. Rather than exploring this question here, the present description is confined to brain exhibited by Homo sapiens.]
The human brain, in its usual (normal) form, exhibits the capability to use an extensive and generative language and to create and use elaborate tools in fulfilling its capacity to respond to environment and social experience. It accomplishes this through a matrix of more than one hundred billion neural cells that continually connect and disconnect with each other, forming and changing at any given point into a network with more than one hundred trillion synaptic connections. The dizzying array of connections at any moment reflects the developmental state of the organism in the process of accomplishing motivations and goals, and earning rewards (e.g., to continue living, reproducing, and experiencing pleasure, satisfaction, and contentment).
Adaptation— the primary function of brain (within the organism-as-a-whole)—refers to virtually any response the organism makes following exposure to some stimulation. An adaptation may range from an instantaneous and simple reflex to a lengthy and complex process of reasoning, judgment, and decision-making. In the case of humans (and other higher phyla), stimulation may come from outside or from within the embodied brain. External stimulation affects receptor cells sensitive to light (photons), hearing (air pressure waves), touch (physical pressure), and taste and smell (molecular forms). Internal stimulation is the product of brain cells continually influencing one another in complex and interactive ways producing what is normally observed as the flow of “thought.” Internal stimulation may also be received from the functions of other bodily organ systems (proprioception from muscle movements, for example). Each instance of an adaptive action depends on one’s history of prior acts of adaptation. Thus, the diversity, probability, and flexibility of adaptive responses, over time, characterize individuals in ways that reveal the robustness of the brain in contributing to evolutionary success. (A robust brain accomplishes a higher probability of successful adaptations in order to contribute to sustaining the species and moving it towards a gradually stronger position among species.)
The brain, as with all organ systems, is composed of cells (neurons and glia, in the case of the nervous system). Unlike other organ system, however, nervous system cells are interconnected in a vast and dynamical array (i.e., the synaptic network) that, through molecular processes, facilitate as much communication as necessary in order to achieve integrative (life-coordinating and adaptive) purposes. Unlike skin, bone, and many internal organs whose cells function collectively as structural components like bricks of a wall, neural cells function as communication lines like a telephone switching network in a city, creating a potential connection of any telephone with any other telephone in the city. The brain is constructed with about 100 billion neurons, the peripheral nervous system with many times more than that. The set of brain neurons contains hundreds of trillions of points of potential interconnection (synapses) that constantly change in functional strength and number, representing the moment-by-moment interactions of the body with its environment. These interactions include moving the body, thinking, expressing thought through language, and experiencing emotions. There are also billions of non-neural cells that function within the nervous system, such as glia, providing a host of physiological contributions to mental processes on which neurons depend.
The cells of the brain and peripheral nervous system emerge during embryogenesis and remain, essentially, until the body dies. With a few exceptions, neurons do not regularly die then regenerate. Most other body cells go through a cycle of apoptosis (cell death) and regeneration. It is understandable why neurons could not possibly exhibit this biological property. Given the intricate communication networks neurons create, and the adaptive genetic-physiological intracellular states, the loss of a few neurons could disrupt thousands of points of potential communication, diminishing the smoothly acquired role of each neuron in regulating some sensory, mental, emotional, or motor function. Although a few isolated neurons lost here and there from time-to-time will not make much of a noticeable difference (because of connectivity redundancy the system builds into itself), the brain depends on minimal loss of neurons and their maximum interconnectivity potentials (and on-going re-connectivity as the product of experience) in order to remain optimally functional vis-à-vis changing interactions with the environment.
The brain—along with its entourage of peripheral neurons, glia, blood vessels, supportive cells, and molecules—commands every function, thought, and emotion that flows through us, moment-to-moment defining each unique life experience. Neural “firing” (neuron activation proceeding to activation of more “downstream” neurons) occurs at breakneck speed and in dizzying volleys in order to accomplish these feats. During any given fraction of a second, thousands upon thousands of neurons will be in one or another phase of “firing,” each neuron taking only one two-hundredth of a second to accomplish its individual task. Through orchestrated and rhythmic volleys along pathways of dynamically connected neurons, the brain accomplishes each and every task that, collectively, defines life—from breathing, circulating the blood, experiencing emotions, thinking, lifting a fork to take a bite of food, talking, planning an activity, having a flash of insight, writing, abiding by moral conduct, and so forth. Taking in a bombarding stream of sensation, attending to what is important, and producing a barrage of motor and mental responses is a monumental feat—that usually works out well, but not always.
From the moment of birth (actually, even before birth), the brain is like a teaming stew that takes on epicurean flavors and textures as it simmers. As the brain encounters environments and experiences, the neural stew begins to make (dynamically, both strong and weak) connections that memorialize those places and events. Learning—the psychological term referring, biologically, to specifically organized and retained neural connections and gene/molecular states—takes place in order for the brain to represent its on-going flow of experiences, emotions, motivations, accomplishments, and setbacks (from which it learns anew). Because of our awareness that the brain tends to “think” in terms of big chunks, we are not typically aware of all the tiny bits and pieces of experience that write themselves in the brain’s “wiring.” But, that it does. And the “wiring” is plastic in that it can change—sometimes quickly and sometimes taking days, weeks, or years—as each new experience weaves itself within and among our prior experiences. Although dynamic, a stable underlying pattern of one’s unique neural “wiring” at any given moment is most typically referred to collectively as one’s personality (including intelligence, temperament, memory, beliefs, traits, and so forth). One’s brain is at any moment a framework onto which new experiences are interwoven.
The brain is an animal’s key to life—in all its aspects. It writes and tweaks its situation, lying between the body and the environment, so that each new experience melds into the history each prior experience has added to the weave. The brain holds an almost unfathomable record of one’s life: the pre-wiring that genes provide and the post-wiring that experiences provide. Virtually every influence—from genetics to nutrition, to one’s experience of being loved and raised, to every thought and dream—crafts the big picture and the very essence of who we are. The moment-to-moment travails of life spilling into the big weave, tweaking the big picture ever so slightly—and in some cases not so slightly—coalesce to form our incredible brain. While usually giving it little thought, it is the three-pound organ that defines us in every way.
Paradoxically simple and complex, the brain is arguably the most intriguing system known to man. We persist in working to understand it better.