Neuroscience and Adult Development

Neuroscience and Human Development

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 adaptations, integrated yet malleable systems involving biological, physiological, emotional and intellectual components. This paper will review some of the most prominent theories of human development, discuss the nexus of human development and the neurological processes involved in the human body, and analyze the development 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 body, a brain that is disproportionately large relative to that of body size, as well as an extended period of childhood, during which significant formative transitions occur (Ulijaszek et al., 2000). With an unusually large brain than 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, to analyze, and interpret information quickly has provided a tactile and strategic advantage over would be predators. Our innate abilities, limited as they may be at birth, are honed as we develop new skills sets during infancy, early childhood and adolescence. By adulthood, with some luck, humans have had time to develop behaviors through learning and activities that provide us an opportunity to be successful, social, problem-solving beings.

Human growth and development encompasses a wide spectrum of attributes for the human being; structural, behavioral, physiological, humanistic, psychological, and cognitive skills are but a few of the developments that 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 seemingly account for the varieties of humans, the differences among each human being and the unique qualities that make each of us an individual. 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. However, this isn't to suggest 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 during the life span. With an increased awareness and a desire to reflect, knowledge and intellect provide the human being with, arguably, 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 ability to fully become sentient is realized. With an increased awareness and daily advances in technology, only now are we able to fully recognize and appreciate the intricacies of our own bodies, our own minds so that we can help those in need. For example, with current medical knowledge, we are able to mend the weak and provide for meaningful care for those who are nearing end of life. While human development theories and life span theories are as numbered 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, rather than simply focus on the formative stages of development that occur in early childhood. Kastenbaum (1993) observes that so called 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 alternative a few years later.

Life span theories and human development theoretical models form the foundation for understanding adult development as well as the aging process.

Sigelman and Rider (2006, pg. 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 and Rider, 2006). Physical development, of course, 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, as well as the collective physical accommodations that humans develop as a result of the aging process (Sigelman and Rider, 2006). Cognitive development includes the set of changes and adaptations that occur in perception, language, learning, memory, problem solving and the gamut of mental functioning. Psychosocial development, Sigelman and Rider (2006, pg. 3) note, include "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, 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 they way that they do.

All developmental theories involve some element of progression from one stage to another. This progression, however, does not necessarily mean "change." Life stage development theorists differ on the nuisances of each life stage, but seem to agree that incremental progressions throughout the lifespan provide for unique and identifiable segments in human development. Again, this is not to suggest that "progression" imparts a sense of "better" or "improved."

Life span perspectives suggest that an individual's adult experiences should be contextualized; that childhood and adolescence are integral components, involving a myriad of experiences, thoughts, and feelings that must be considered 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 the environment adaptation" (Cavanaugh, 2005, pg. 3).

Adult development is a complex, multi-faceted phenomenon; understanding how an adult develops requires a variety of perspectives. Such perspectives may include behavioral, physiological, and cognitive approaches (Cavanaugh & Fields, 2006). Within the gambit 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 as well. Piaget indicated that the highest cognitive stage of development for adult people is formal operations, suggesting that some adults progress beyond formal operations to more advanced forms of thought (Sigelman & Rider, 2009). With this in mind, this study focuses on the cognitive aspect of growth and development of the adult. Specifically, this research aims to provide an in-depth discussion about the brain and neuroscience and its relation to adult development and learning.

The Human Brain and Neuroscience

All organisms receive information in the form of some external stimuli; to process information received, and to produce appropriate responses. While this process may take only a fraction of a second, it is interesting to note that, for most living organisms, these functions are performed by two interconnected systems working in tandem to provide the human with a viable set of reactionary responses; the nervous system and the endocrine system.

The Nervous System

The nervous system is composed of large networks of nerve cells that perform three interconnecting functions. First, the nervous system allows organisms to receive information from a variety of sensory modalities in the well-functioning human being; sight, smell, touch, taste and sound all provide people with a range of stimuli every day. How the human brain processes such information, as well as 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 given that the speed of the information transmission is achieved by electrical and chemical impulses within and between nerve cells. The nervous system allows an individual to respond, act appropriately in response to the perceived stimuli primarily by controlling muscles and glands. The three functions can be accomplished within a few milliseconds (Harris, 2010).

The nervous system is bifurcated 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 subsists outside the central nervous system 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 provides appropriate responses 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. In contrast, motor neurons transmit action potentials from the central nervous system toward the periphery (Seeley et al., 2005).

Neurons and their Electrical Activity

The nervous system is composed of millions of nerve cells called neurons. Neurons are the parenchyma of the nervous system which performs every function of the nervous system from simple sensory functions to complex thinking and analysis. Neurons, upon receipt of stimuli, 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. Given the importance of each of the neuron components, it is important to discuss how each work separately and in tandem to achieve efficient and appropriate responses in the human body.

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 organelles of the neuron. Seeley et al., (2005) write that the cell body contains large numbers of mitochondria because of its high metabolic function and also abundant rough endoplasmic reticulum's which referred to as Nissl bodies.

The dendrites of a neuron are cytoplasmic extensions that reach out from the cell body like arms 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 them toward the cell body, then produce 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 in the cell body as a result of various stimuli and are important in initiating action potentials in neurons. As the graded potential passes through a cell body, it may initiate an action potential at the base of another cytoplasmic projection which is 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 which is called the axolemma, and a cytoplasm which is called the axoplasm. Unlike dendrites, there are no chromatophilic substances found in axons. Axons may branch distally into axon terminals called telodendria. These end in sacs called synaptic end bulbs. Synaptic end bulbs are parts of synapses or neuroeffector junctions. Axons also play an important role in the electrical impulse activities of neurons. They carry action potentials away from the perikaryon toward the synaptic end bulbs, and these action potentials require the axolemma to have many volt-gaged ion channels. The releases of neurotransmitters from synaptic vesicles into the synaptic cleft are caused by these action potentials. A mechanism of active movement in the axon is called axonal transport. It expends energy to move substances in both directions in the axoplasm approximately 300 mm per day. This mechanism involves the cytoskeleton, and is used to deliver organelles and wastes back to the cell body (Clark, 2005).

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

The Central Nervous System

The central nervous system consists of the brain and the spinal cord. The brain is contained within the cranial cavity while the spinal cord is inside of the vertebral column. The peripheral part of the brain is comprised of grey matter while the inside of the brain, the medulla, is comprised of white matter. Both the brain and the spinal cord are completely surrounded by three meninges or membranes which lay between the skull and the brain. Meninges are connective tissue membranes that serve to protect the brain and the spinal cord from injuries. The function of the meninges is to cushion the tissues of the brain and the spinal cord should some physical trauma occur. Bhise and Yadav (2008) note that the three protective meninges are: the dura mater, arachnoid mater, and the pia mater.

As the thickest and most superficial of the three meninges, the dura mater folds extend into the longitudinal fissure between the two cerebral hemispheres as well as between the cerebrum and cerebellum. The dura mater contains spaces called dural venous sinuses within the folds in the dura mater. These sinuses collect blood from the small veins of the brain. The dural venous sinuses empty their collected blood into the internal jugular veins, which then exits the skull. The dura mater is tightly attached to the periosteum of the skull. The dura mater of the spinal cord contains a space between the vertebrae referred to as the epidural space, which is used for the administration of anesthetics during surgery (Seeley et al., 2005). The second meningeal membrane is the arachnoid mater which is composed of thin and wispy connective tissues that cover the brain and the spinal cord. The space between the dura mater and the arachnoid mater is called the subarachnoid space, which generally provides a space containing a very small amount of serous fluid. It is a delicate serous membrane that contains cerebrospinal fluid (Bhise & Yadav, 2008).

The last meningeal membrane is the pia mater. Tightly bound to the surface of the brain and the spinal cord the pia mater is adjacent to the arachnoid mater. The space between the arachnoid mater and the pia mater is called the subarachnoid space, which contains blood vessels and is filled with cerebrospinal fluid. Seely et al. (2005) writes that the function of the pia is to protect the nervous tissue as well as to supply blood and nourishment to the central nervous tissue (Seeley, et al., 2005).

The central nervous system contains fluid-filled cavities called ventricles. These are irregularly shaped cavities that contain cerebrospinal fluid. There are four ventricles in the central nervous system; the right and left lateral ventricles, as well as the third and fourth ventricles. Collectively, these ventricles produce cerebrospinal fluid that serve to nourish and provide protection to the nervous tissues (Seeley et al., 2005). Within the cerebral hemispheres, the lateral ventricles lay on either side of the median plane just below the corpus calosum, separated by a thin membrane called septum lucidum. Blood capillaries are present in the lateral ventricles. It is also lined internally by means of ciliated epithelium called choroid plexus where cerebrospinal fluid is derived (Bhise & Yadav, 2008). As a smaller midline cavity, the third ventricle, located in the center of the diencephalon between the two halves of the thalamus, is filled with cerebrospinal fluid and it is connected by holes to the lateral ventricles known as interventricular foramina (Bhise & Yadav, 2008).

The fourth ventricle, connected to the third ventricle by the cerebral aqueduct, a narrow canal, is located at the base of the cerebellum. This fourth ventricle is present below and behind the third ventricle and between the cerebellum and pons varolii. The fourth ventricle is Connected continuously with the central canal of the spinal cord, the fourth ventricle opens into the subarachnoid space through foramina in its walls and roof (Seeley, et al., 2005).

With an abundant supply of cerebrospinal fluid, the central nervous system contains a cerebrospinal fluid produced by the choroid plexuses. These are specialized structures made of ependymal cells which are located in the ventricles. Cerebrospinal fluid fills the brain ventricles, the central canal of the spinal cord, as well as the subarachnoid space. Flowing from the lateral ventricles into the third ventricle and then through the cerebral aqueduct in the fourth ventricle, the Cerebrospinal fluid incrementally enters the central canal of the spinal cord. Cerebrospinal fluid exits from the fourth ventricle through small openings and enters the subarachnoid space. Arachnoid granulations are masses of arachnoid tissue that penetrate into the superior sagittal sinus, while cerebrospinal fluid passes from the subarachnoid space into the blood through these granulations (Seeley et al., 2005).

One of the main functions of the cerebrospinal fluid is to protect and support the delicate structures of the brain and the spinal cord. Also, the cerebrospinal fluid maintains uniform pressure around the brain structure. The cerebrospinal fluid provides a cushion for the brain and the spinal cord and serves to protect the brain and spinal cord in the event of injury or severe trauma. Lastly, the cerebrospinal fluid keeps the brain and the spinal cord moist as there may be an interchange of substances between the fluid and nerve cells (Bhise & Yadav, 2008).

The Human Brain

The major regions of the human brain are the brainstem, the diencephalon, the cerebrum, and the cerebellum. The brainstem connects the spinal cord to the brain. It is composed of the medulla oblongata, pons, and midbrain and contains several nuclei involved in vital body functions such as the regulation of heart rate, blood pressure, and breathing. Bear et al., (2007) observe that disruption of these vital functions from trauma to the brainstem typically results in death.

As the most inferior portion of the brainstem the medulla oblongata is connected continuously with the spinal cord. The medulla oblongata extends from the level of the foramen magnum to the pons and contains ascending and descending nerve tracts as well as discrete nuclei which serve to help regulate heart rate,, blood vessel diameter, breathing, swallowing, vomiting, coughing, sneezing, balance, and coordination. There are two "pyramids" which are prominent enlargements on the anterior surface of the medulla oblongata containing descending nerve tracts, which transmit action potentials from the brain to motor neurons of the spinal cord. These pyramids are also involved in the conscious control of skeletal muscles (Bear et al., 2007).

The pons, immediately superior to the medulla oblongata, contains several nuclei, and ascending and descending nerve tracts. Some of the nuclei in the pons are responsible in relaying information between the cerebrum and the cerebellum. Several nuclei of the medulla oblongata extend into the lower part of the pons which further provides a regulator for breathing, swallowing, and balance. Other nuclei in the pons control activities such as chewing and salivation (Seeley et al., 2005).

As the smallest region of the brainstem, the midbrain is found just superior to the pons. The dorsal part of the midbrain is composed of four colliculi; two inferior, two superior. The two inferior colliculi are major relay centers for the auditory nerve pathways in the central nervous system. The two superior colliculi are involved in controlling visual reflexes. Also, the midbrain contains nuclei involved in the coordination of eye movements, as well as in the control of pupil diameter and lens shape. The midbrain has a substantia nigra, a black nuclear mass that is also part of the basal nuclei, which is involved in the regulation of body movements. The rest of the midbrain is composed of large ascending tracts from the spinal cord to the cerebrum and descending tracts from the cerebrum to the spinal cord or cerebellum (Seeley et al., 2005).

Scattered throughout the brainstem are a group of nuclei called the reticular formation that provide crucial regulatory brain functions involving cyclical motor functions such as respiration, walking, and chewing. The reticular activating system is composed mainly of reticular formations that serve to arouse from slumber, as well as maintaining consciousness and regulating the sleep-wake cycle. In fact, these reticular formations are so important that, when damage occurs to these regulatory mechanisms, coma may result (Bear et al., 2007).

The cerebellum literally means little brain. It is attached to the brainstem by cerebellar peduncles. These large connections provide means of communication between the cerebellum and other parts of the central nervous system. The cerebellar cortex is composed of gray matter and it also has gyri and sulci. It consists of gray nuclei and white nerve tracts on the inside. The cerebellum is involved in balance, maintenance of muscle tone, and coordination of fine motor movement. The cerebellum also compares information about the intended movement from the motor cortex with sensory information from the moving structures as action potentials from proprioceptive neurons reach the cerebellum. Another function of the cerebellum involves learning motor skills such as playing the piano or driving a car (Bear et al., 2007).

Located between the brainstem and the cerebrum, the diencephalon consists of the thalamus, epithalamus, and the hypothalamus. The thalamus is the largest part of the diencephalon. Shaped like a yo-yo, with two large lateral parts connected in the center by a small interthalamic adhesion, the thalamus consists of a cluster of nuclei responsible for most sensory input that ascends through the spinal cord. The thalamus also influences mood and registers an un-localized, uncomfortable perception of pain (Seeley et al., 2005).

The epithalamus is a small area located superior and posterior to the thalamus which is involved in the emotional and visceral response to odors because of few small nuclei in the epithalamus. The epithalamus also contains a pineal body which is an endocrine gland that may influence the onset of puberty, as well as play a role in controlling some long-term cycles that are influenced by the light-dark cycle (Bear et al., 2007).

The hypothalamus is very important in maintaining homeostasis. It is the most inferior part of the diencephalon and it contains several small nuclei. The hypothalamus plays a crucial role in the control of body temperature, hunger, and thirst. It is responsible for sensations such as sexual pleasure, feeling relaxed and rested after a meal, rage, and fear. Nervous perspiration in response to stress or feeling hungry as a result of depression and other emotional responses which seem to be inappropriate to the circumstances also involve the hypothalamus. A funnel-shaped stalk in the hypothalamus, the infundibulum, extends to the pituitary gland and provides the hypothalamus a major role in controlling the secretion of hormones from the pituitary gland. Bear et al. (2007) note that there are also mamillary bodies on the posterior portion of the hypothalamus that are involved in emotional responses to odors and in memory. As the largest part of the brain, the cerebrum is divided into two hemispheres by a longitudinal fissure: the left and the right hemispheres. Each of these hemispheres contain folds called gyri which increases the area of the cerebral cortex. It also has intervening grooves called sulci. Each hemisphere is divided into four lobes named for the skull bones overlying them. While the frontal lobe is responsible in the control of voluntary motor functions, motivation, aggression, mood, and smell reception, the parietal lobe is the main center for the reception and conscious perception of most sensory information such as touch, pain, temperature, balance, and taste. The occipital lobe functions in the reception and perception of visual stimuli. The last lobe, the temporal lobe, is involved in smell and hearing sensations and plays an important role in memory (Seeley et al., 2005).

Neuroscience and its Relationship to Adult Development and Learning

The Neuroscience of Learning and Memory

This paper will now focus on the study of neuroscience as it pertains to adult development, specifically adult learning. The field of cognitive neuroscience attempts to relate cognition to neuroscience in order to understand how thought is implemented in the brain.

Cognitive neuroscience has revealed that there are a many relatively independent memory systems in the human brain. Long-term memory depends on different neural substances than does working memory, and working memory depends on different neural substances than sensory memory. Moreover, the executive system that controls these memory systems also depends on different neural substrates than do the core memory systems themselves.

Donald Hebb proposed one of the first neural theories of learning. Hebb proposed that some form of physiological change invariably occurred when two connected neurons are frequently active at the same time, thereby providing a physiological basis for memory as well as an increase in the likelihood in their future connectivity; learning (Guadagnoli et al., 2008). Evidence for synaptic strengthening was discovered in neural circuits of the mollusk Aplysia and in hippocampal neurons of the rabbit empirically supported Hebb's principle of learning. The principle of Hebbian learning provides an explicit account of how patterns of activities in a network of neurons can be stored in a pattern of synaptic connections, thereby serving as a neural substrate of memory.

Hebbian learning is a powerful mechanism, however, operating in conjunction with recurrent connectivity without other constraints would be problematic for the formation of memory. Because neurons are highly interconnected, excitatory activity in a few neurons tends to spread to neighboring neurons. Guadagnoli et al. (2008) suggest that this conflict is further exacerbated by the presence of recurrent connectivity, allowing activity to reverberate in the network creating mutually reinforcing activity. As activity progresses to a network of neurons, the more active neurons tend to increasingly excite each other and at the same time increasingly inhibit less active neurons. In this way, neurons become specialized; they specifically respond to increasingly predictable patterns of input.

In recent years, a number of theories and frameworks have emerged in an effort to address both the potentials and limitations of effective cognitive and social functioning during the adult years. Such frameworks have served to facilitate characteristics of adult development by integrating observations that would otherwise have been a collection of meaningless or poorly understood phenomena. While certain accommodations are made by adults in response to the decline in other abilities, cognitive functioning during the adult years suggest that adults do not necessarily follow predictable patterns of behaviors solely based on increased age (Demick & Andreoletti, 2003).

The processes and outcomes of learning influence the nature and course of adult development, and reciprocally, developmental variables influence the processes and products of learning. The concepts of learning and development can be distinguished along two dimensions. First, in terms of the inclusiveness or scope of the behavior and of the antecedents of change, learning refers to the effects of practice or experience on behavior whereas development refers to a wider variety of influences that are associated with time-related change. It is generally determined that developmental change is multi-determined and multidirectional (Demick & Andreoletti, 2003).