Neuroscience and Human Development Across the Life Span

Introduction: Human Development and the Brain

One of the most noticeable aspects of human beings involves the changes in shape, size, form, and function of the individual from a newly formed fetus to a fully grown adult. As the single most successful organism on Earth, human beings have developed, through millions of years of evolutionary adaptation, integrated yet malleable systems involving biological, physiological, emotional, and intellectual components. This paper reviews some of the most prominent theories of human development, discusses the nexus of human development and the neurological processes involved in the human body, and analyzes the developmental and life-progression processes human beings experience from birth through death.

Much of the success of human beings is attributable to the very design of the human body, including a large bi-pedal frame, a brain that is disproportionately large relative to body size, and an extended period of childhood during which significant formative transitions occur (Ulijaszek et al., 2000). With an unusually large brain compared to other organisms on Earth, humans have certain distinct advantages in the struggle for survival and dominance over other animals. For example, our ability to reason, analyze, and interpret information quickly has provided a tactical and strategic advantage over would-be predators. Our innate abilities, limited as they may be at birth, are honed as we develop new skill sets during infancy, early childhood, and adolescence. By adulthood, with some luck, humans have had time to develop behaviors through learning and experience that provide an opportunity to become successful, social, problem-solving beings.

Human growth and development encompasses a wide spectrum of attributes: structural, behavioral, physiological, humanistic, psychological, and cognitive skills are but a few of the developments humans undergo during the life span. While many life-span and human development theories attempt to provide a descriptive analysis—a theoretical framework for understanding the myriad changes humans experience from fertilization to death—such theories do not always account for the variety among human beings, the differences among each individual, and the unique qualities that make each of us distinct. With that in mind, it is best to understand theories of human development as guidelines, as a foundation for better understanding humans in a general sense. This is not to suggest, however, that such theories are not beneficial or utilitarian. From a practical perspective, human development theories can provide people with an increased awareness of the self across the life span.

With an increased awareness and a desire to reflect, knowledge and intellect provide the human being with an important ability that further serves to separate us from other animals: our ability to think about our own thinking, to metacognate and contemplate the meaning of life. In turn, our capacity to become fully sentient is realized. With daily advances in technology, only now are we able to fully recognize and appreciate the intricacies of our own bodies and minds in ways that allow us to help those in need. For example, with current medical knowledge, we are able to care for the vulnerable and provide meaningful support for those who are nearing end of life. While human development and life-span theories are as numerous as they are varied, several prominent theorists stand apart in our attempt to better understand the changes that occur during life.

In an effort to develop viable and reliable models of human development through the entire life course, some theorists have attempted to extend the range of human development theories beyond the formative stages of early childhood. Kastenbaum (1993) observes that disengagement theory was the first substantive and innovative theory to consider the middle and later adult years; consequently, the term "mid-life crisis" emerged as an influential concept a few years later.

Life-span theories and human development models form the foundation for understanding adult development as well as the aging process. Sigelman and Rider (2006, p. 2) define development as the entire set of "systematic changes and continuities" that occur in the individual from birth to death. These systematic changes and continuities occur in three broad domains: physical development, cognitive development, and psychosocial development (Sigelman & Rider, 2006). Physical development includes normative physical attributes during the growth and decline of the human body, including the proper functioning of all combined physiological systems, physical manifestations of aging, sensory-motor responses, and the collective physical accommodations that humans develop as a result of the aging process (Sigelman & Rider, 2006). Cognitive development includes the set of changes and adaptations that occur in perception, language, learning, memory, problem solving, and the full range of mental functioning. Psychosocial development, Sigelman and Rider (2006, p. 3) note, includes "interpersonal aspects of development, such as motives, emotions, personality traits, interpersonal skills and relationships, and roles played in the family and in the larger society."

With this working definition of human development in mind, it is important to note that life-span theorists do not all agree on either the ways in which people grow and develop or exactly why people develop the way they do. All developmental theories involve some element of progression from one stage to another. This progression, however, does not necessarily mean "change" in the sense of improvement. Life-stage development theorists differ on the nuances of each life stage but generally agree that incremental progressions throughout the life span provide for unique and identifiable segments in human development.

The Nervous System

Life-span perspectives suggest that an individual's adult experiences should be contextualized; childhood and adolescence are integral components involving a myriad of experiences, thoughts, and feelings that must be considered in order to understand the adult. Dividing human development into two distinctly separate phases, the life-span perspective involves both an early phase (childhood and adolescence) and a later phase (young adulthood, middle age, and old age). "The early phase is characterized by rapid age-related increases in people's size and abilities. The later phase is defined by slow changes in size while abilities continue to develop in response to environmental adaptation" (Cavanaugh, 2005, p. 3).

Adult development is a complex, multi-faceted phenomenon; understanding how an adult develops requires a variety of perspectives, which may include behavioral, physiological, and cognitive approaches (Cavanaugh & Fields, 2006). Within the domain of intellectual functioning, cognition refers to the processes through which knowledge is acquired and problems are solved. Cognitive development refers not just to the structural development of the brain but also to the development of one's knowledge. Piaget indicated that the highest cognitive stage of development for adults is formal operations, while suggesting that some adults progress beyond formal operations to more advanced forms of thought (Sigelman & Rider, 2009). With this in mind, the following sections focus on the cognitive aspects of growth and development in adults, providing an in-depth discussion of the brain, neuroscience, and their relation to adult development and learning.

All organisms receive information in the form of external stimuli, process that information, and produce appropriate responses. While this process may take only a fraction of a second, for most living organisms these functions are performed by two interconnected systems working in tandem: the nervous system and the endocrine system.

The nervous system is composed of large networks of nerve cells that perform three interconnecting functions. First, it allows organisms to receive information from a variety of sensory modalities; sight, smell, touch, taste, and sound all provide people with a range of stimuli every day. How the human brain processes such information, and how the nervous system responds to brain signals, dictates how people react and feel in a given situation. Harris (2010) notes that the nervous system provides responses to stimuli quickly because information transmission is achieved by electrical and chemical impulses within and between nerve cells. The nervous system allows an individual to respond appropriately to perceived stimuli primarily by controlling muscles and glands. These three functions can be accomplished within a few milliseconds (Harris, 2010).

Neurons and Their Electrical Activity

The nervous system is divided into two separate systems: the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and the spinal cord, while the peripheral nervous system exists outside the central nervous system and is comprised of nerves and ganglia.

The peripheral nervous system consists of two separately functioning components: the sensory division and the motor division. The sensory division conveys signals from sensory receptors to the central nervous system. Sensory neurons transmit reactive responses from the periphery to the central nervous system, while the motor division conducts action potentials from effector organs such as muscles and glands. Motor neurons, in contrast, transmit action potentials from the central nervous system toward the periphery (Seeley et al., 2005).

The nervous system is composed of millions of nerve cells called neurons. Neurons are the parenchyma of the nervous system, performing every function from simple sensory processing to complex thinking and analysis. Upon receipt of stimuli, neurons transmit responsive signals to other neurons or to effector organs. Clark (2005) observes that the anatomy of a neuron is composed of four main parts: the cell body, the dendrites, the axon, and the nerve fibers.

Varying in diameter and containing a single nucleus, the cell body is the primary component of the neuron. The nucleus of the neuron provides information for protein synthesis and contains most of the cell's organelles. Seeley et al. (2005) write that the cell body contains large numbers of mitochondria owing to its high metabolic demands, as well as abundant rough endoplasmic reticulum referred to as Nissl bodies.

The dendrites of a neuron are cytoplasmic extensions that reach out from the cell body and contain a full array of cellular organelles, such as mitochondria, chromatophilic substance, and ribosomes. The most important feature of a dendrite is its electrical activity. Dendrites receive information from other neurons and transmit it toward the cell body, producing electrical impulses referred to as graded potentials. Graded potentials can have varying degrees of depolarization or hyperpolarization. These graded potentials arise in the dendrites or the cell body as a result of various stimuli and are important in initiating action potentials in neurons. As a graded potential passes through the cell body, it may initiate an action potential at the base of the axon (Clark, 2005).

An axon is a long cell process extending from the neuron cell body. Each neuron contains only one axon. The axon has a plasma membrane called the axolemma and a cytoplasm called the axoplasm. Unlike dendrites, there are no chromatophilic substances found in axons. Axons may branch distally into axon terminals called telodendria, which end in sacs called synaptic end bulbs. Synaptic end bulbs are parts of synapses or neuroeffector junctions. Axons carry action potentials away from the cell body toward the synaptic end bulbs; these action potentials require the axolemma to have many voltage-gated ion channels. The release of neurotransmitters from synaptic vesicles into the synaptic cleft is caused by these action potentials. A mechanism of active movement in the axon called axonal transport expends energy to move substances in both directions through the axoplasm at approximately 300 mm per day. This mechanism involves the cytoskeleton and is used to deliver organelles and remove wastes back to the cell body (Clark, 2005).

Nerve fibers are collections of axons or dendrites and may be surrounded by myelin, which provides additional layers of insulation. Axons are surrounded by cell processes of oligodendrocytes in the central nervous system and by Schwann cells in the peripheral nervous system. Myelin sheaths are repeatedly wrapped around axon segments to form tightly wrapped cell membranes that prevent almost all electrical current flow through the cell membrane. Gaps exist between the myelin sheaths known as the nodes of Ranvier, occurring approximately every millimeter between oligodendrocyte segments or between individual Schwann cells. Current flows easily between the extracellular fluid and the axon at the nodes of Ranvier, allowing action potentials to develop (Seeley et al., 2005).

Conclusion

Having reviewed the scholarly literature concerning the effects of neuroscience on human development and life-span theories, it appears that science is increasingly attempting to account for the myriad vagaries and nuances in human behavior as a matter of brain-based chemical reactions and the collection of assorted impulses associated with environmental stimuli. While human beings are certainly the most successful species on Earth, we are not yet fully aware of our own potential, and we have yet to discover and account for the full range of differences among all people. With this in mind, it is important for researchers to continue investigating how neuroscience can explain human behaviors, emotions, and actions without eclipsing the broader picture of human pursuits and humanity as a whole.