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30.2: Stems - Biology

30.2: Stems - Biology


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Skills to Develop

  • Describe the main function and basic structure of stems
  • Compare and contrast the roles of dermal tissue, vascular tissue, and ground tissue
  • Distinguish between primary growth and secondary growth in stems
  • Summarize the origin of annual rings
  • List and describe examples of modified stems

Stems are a part of the shoot system of a plant. They may range in length from a few millimeters to hundreds of meters, and also vary in diameter, depending on the plant type. Stems are usually above ground, although the stems of some plants, such as the potato, also grow underground. Stems may be herbaceous (soft) or woody in nature. Their main function is to provide support to the plant, holding leaves, flowers and buds; in some cases, stems also store food for the plant. A stem may be unbranched, like that of a palm tree, or it may be highly branched, like that of a magnolia tree. The stem of the plant connects the roots to the leaves, helping to transport absorbed water and minerals to different parts of the plant. It also helps to transport the products of photosynthesis, namely sugars, from the leaves to the rest of the plant.

Plant stems, whether above or below ground, are characterized by the presence of nodes and internodes (Figure (PageIndex{1})). The stem region between two nodes is called an internode. The stalk that extends from the stem to the base of the leaf is the petiole. An axillary bud is usually found in the axil—the area between the base of a leaf and the stem—where it can give rise to a branch or a flower. The apex (tip) of the shoot contains the apical meristem within the apical bud.

Stem Anatomy

The stem and other plant organs arise from the ground tissue, and are primarily made up of simple tissues formed from three types of cells: parenchyma, collenchyma, and sclerenchyma cells.

Parenchyma cells are the most common plant cells (Figure (PageIndex{2})). They are found in the stem, the root, the inside of the leaf, and the pulp of the fruit. Parenchyma cells are responsible for metabolic functions, such as photosynthesis, and they help repair and heal wounds. Some parenchyma cells also store starch.

Collenchyma cells are elongated cells with unevenly thickened walls (Figure (PageIndex{3})). They provide structural support, mainly to the stem and leaves. These cells are alive at maturity and are usually found below the epidermis. The “strings” of a celery stalk are an example of collenchyma cells.

Sclerenchyma cells also provide support to the plant, but unlike collenchyma cells, many of them are dead at maturity. There are two types of sclerenchyma cells: fibers and sclereids. Both types have secondary cell walls that are thickened with deposits of lignin, an organic compound that is a key component of wood. Fibers are long, slender cells; sclereids are smaller-sized. Sclereids give pears their gritty texture. Humans use sclerenchyma fibers to make linen and rope (Figure (PageIndex{4})).

Art Connection

Which layers of the stem are made of parenchyma cells?

  1. cortex and pith
  2. phloem
  3. sclerenchyma
  4. xylem

Like the rest of the plant, the stem has three tissue systems: dermal, vascular, and ground tissue. Each is distinguished by characteristic cell types that perform specific tasks necessary for the plant’s growth and survival.

Dermal Tissue

The dermal tissue of the stem consists primarily of epidermis, a single layer of cells covering and protecting the underlying tissue. Woody plants have a tough, waterproof outer layer of cork cells commonly known as bark, which further protects the plant from damage. Epidermal cells are the most numerous and least differentiated of the cells in the epidermis. The epidermis of a leaf also contains openings known as stomata, through which the exchange of gases takes place (Figure (PageIndex{5})). Two cells, known as guard cells, surround each leaf stoma, controlling its opening and closing and thus regulating the uptake of carbon dioxide and the release of oxygen and water vapor. Trichomes are hair-like structures on the epidermal surface. They help to reduce transpiration (the loss of water by aboveground plant parts), increase solar reflectance, and store compounds that defend the leaves against predation by herbivores.

Vascular Tissue

The xylem and phloem that make up the vascular tissue of the stem are arranged in distinct strands called vascular bundles, which run up and down the length of the stem. When the stem is viewed in cross section, the vascular bundles of dicot stems are arranged in a ring. In plants with stems that live for more than one year, the individual bundles grow together and produce the characteristic growth rings. In monocot stems, the vascular bundles are randomly scattered throughout the ground tissue (Figure (PageIndex{6})).

Xylem tissue has three types of cells: xylem parenchyma, tracheids, and vessel elements. The latter two types conduct water and are dead at maturity. Tracheids are xylem cells with thick secondary cell walls that are lignified. Water moves from one tracheid to another through regions on the side walls known as pits, where secondary walls are absent. Vessel elements are xylem cells with thinner walls; they are shorter than tracheids. Each vessel element is connected to the next by means of a perforation plate at the end walls of the element. Water moves through the perforation plates to travel up the plant.

Phloem tissue is composed of sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers. A series of sieve-tube cells (also called sieve-tube elements) are arranged end to end to make up a long sieve tube, which transports organic substances such as sugars and amino acids. The sugars flow from one sieve-tube cell to the next through perforated sieve plates, which are found at the end junctions between two cells. Although still alive at maturity, the nucleus and other cell components of the sieve-tube cells have disintegrated. Companion cells are found alongside the sieve-tube cells, providing them with metabolic support. The companion cells contain more ribosomes and mitochondria than the sieve-tube cells, which lack some cellular organelles.

Ground Tissue

Ground tissue is mostly made up of parenchyma cells, but may also contain collenchyma and sclerenchyma cells that help support the stem. The ground tissue towards the interior of the vascular tissue in a stem or root is known as pith, while the layer of tissue between the vascular tissue and the epidermis is known as the cortex.

Growth in Stems

Growth in plants occurs as the stems and roots lengthen. Some plants, especially those that are woody, also increase in thickness during their life span. The increase in length of the shoot and the root is referred to as primary growth, and is the result of cell division in the shoot apical meristem. Secondary growth is characterized by an increase in thickness or girth of the plant, and is caused by cell division in the lateral meristem. Figure (PageIndex{7}) shows the areas of primary and secondary growth in a plant. Herbaceous plants mostly undergo primary growth, with hardly any secondary growth or increase in thickness. Secondary growth or “wood” is noticeable in woody plants; it occurs in some dicots, but occurs very rarely in monocots.

Some plant parts, such as stems and roots, continue to grow throughout a plant’s life: a phenomenon called indeterminate growth. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases when a plant part reaches a particular size.

Primary Growth

Most primary growth occurs at the apices, or tips, of stems and roots. Primary growth is a result of rapidly dividing cells in the apical meristems at the shoot tip and root tip. Subsequent cell elongation also contributes to primary growth. The growth of shoots and roots during primary growth enables plants to continuously seek water (roots) or sunlight (shoots).

The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.

Link to Learning

Watch this BBC Nature video showing how time-lapse photography captures plant growth at high speed.

Secondary Growth

The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems, which are lacking in herbaceous plants. Lateral meristems include the vascular cambium and, in woody plants, the cork cambium (see Figure 30.2.8). The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem (tracheids and vessel elements) to the inside, and secondary phloem (sieve elements and companion cells) to the outside. The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength.

In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells (bark) containing a waxy substance known as suberin that can repel water. The bark protects the plant against physical damage and helps reduce water loss. The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm. The periderm substitutes for the epidermis in mature plants. In some plants, the periderm has many openings, known as lenticels, which allow the interior cells to exchange gases with the outside atmosphere (Figure (PageIndex{8})). This supplies oxygen to the living and metabolically active cells of the cortex, xylem and phloem.

Annual Rings

The activity of the vascular cambium gives rise to annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter and their primary cell walls are not extensively thickened. This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids. It results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem (Figure (PageIndex{9})). An examination of the number of annual rings and their nature (such as their size and cell wall thickness) can reveal the age of the tree and the prevailing climatic conditions during each season.

Stem Modifications

Some plant species have modified stems that are especially suited to a particular habitat and environment (Figure (PageIndex{10})). A rhizome is a modified stem that grows horizontally underground and has nodes and internodes. Vertical shoots may arise from the buds on the rhizome of some plants, such as ginger and ferns. Corms are similar to rhizomes, except they are more rounded and fleshy (such as in gladiolus). Corms contain stored food that enables some plants to survive the winter. Stolons are stems that run almost parallel to the ground, or just below the surface, and can give rise to new plants at the nodes. Runners are a type of stolon that runs above the ground and produces new clone plants at nodes at varying intervals: strawberries are an example. Tubers are modified stems that may store starch, as seen in the potato (Solanum sp.). Tubers arise as swollen ends of stolons, and contain many adventitious or unusual buds (familiar to us as the “eyes” on potatoes). A bulb, which functions as an underground storage unit, is a modification of a stem that has the appearance of enlarged fleshy leaves emerging from the stem or surrounding the base of the stem, as seen in the iris.

Link to Learning

Watch botanist Wendy Hodgson, of Desert Botanical Garden in Phoenix, Arizona, explain how agave plants were cultivated for food hundreds of years ago in the Arizona desert in this video: Finding the Roots of an Ancient Crop.

Some aerial modifications of stems are tendrils and thorns (Figure (PageIndex{11})). Tendrils are slender, twining strands that enable a plant (like a vine or pumpkin) to seek support by climbing on other surfaces. Thorns are modified branches appearing as sharp outgrowths that protect the plant; common examples include roses, Osage orange and devil’s walking stick.

Summary

The stem of a plant bears the leaves, flowers, and fruits. Stems are characterized by the presence of nodes (the points of attachment for leaves or branches) and internodes (regions between nodes).

Plant organs are made up of simple and complex tissues. The stem has three tissue systems: dermal, vascular, and ground tissue. Dermal tissue is the outer covering of the plant. It contains epidermal cells, stomata, guard cells, and trichomes. Vascular tissue is made up of xylem and phloem tissues and conducts water, minerals, and photosynthetic products. Ground tissue is responsible for photosynthesis and support and is composed of parenchyma, collenchyma, and sclerenchyma cells.

Primary growth occurs at the tips of roots and shoots, causing an increase in length. Woody plants may also exhibit secondary growth, or increase in thickness. In woody plants, especially trees, annual rings may form as growth slows at the end of each season. Some plant species have modified stems that help to store food, propagate new plants, or discourage predators. Rhizomes, corms, stolons, runners, tubers, bulbs, tendrils, and thorns are examples of modified stems.

Art Connections

[link] Which layers of the stem are made of parenchyma cells?

  1. cortex and pith
  2. epidermis
  3. sclerenchyma
  4. epidermis and cortex.

[link] A and B. The cortex, pith, and epidermis are made of parenchyma cells.

Glossary

apical bud
bud formed at the tip of the shoot
axillary bud
bud located in the axil: the stem area where the petiole connects to the stem
bark
tough, waterproof, outer epidermal layer of cork cells
bulb
modified underground stem that consists of a large bud surrounded by numerous leaf scales
collenchyma cell
elongated plant cell with unevenly thickened walls; provides structural support to the stem and leaves
companion cell
phloem cell that is connected to sieve-tube cells; has large amounts of ribosomes and mitochondrion
corm
rounded, fleshy underground stem that contains stored food
cortex
ground tissue found between the vascular tissue and the epidermis in a stem or root
epidermis
single layer of cells found in plant dermal tissue; covers and protects underlying tissue
guard cells
paired cells on either side of a stoma that control stomatal opening and thereby regulate the movement of gases and water vapor
internode
region between nodes on the stem
lenticel
opening on the surface of mature woody stems that facilitates gas exchange
node
point along the stem at which leaves, flowers, or aerial roots originate
parenchyma cell
most common type of plant cell; found in the stem, root, leaf, and in fruit pulp; site of photosynthesis and starch storage
periderm
outermost covering of woody stems; consists of the cork cambium, cork cells, and the phelloderm
pith
ground tissue found towards the interior of the vascular tissue in a stem or root
primary growth
growth resulting in an increase in length of the stem and the root; caused by cell division in the shoot or root apical meristem
rhizome
modified underground stem that grows horizontally to the soil surface and has nodes and internodes
runner
stolon that runs above the ground and produces new clone plants at nodes
sclerenchyma cell
plant cell that has thick secondary walls and provides structural support; usually dead at maturity
secondary growth
growth resulting in an increase in thickness or girth; caused by the lateral meristem and cork cambium
sieve-tube cell
phloem cell arranged end to end to form a sieve tube that transports organic substances such as sugars and amino acids
stolon
modified stem that runs parallel to the ground and can give rise to new plants at the nodes
tendril
modified stem consisting of slender, twining strands used for support or climbing
thorn
modified stem branch appearing as a sharp outgrowth that protects the plant
tracheid
xylem cell with thick secondary walls that helps transport water
trichome
hair-like structure on the epidermal surface
tuber
modified underground stem adapted for starch storage; has many adventitious buds
vessel element
xylem cell that is shorter than a tracheid and has thinner walls

Lipid metabolic reprogramming in cancer cells

Many human diseases, including metabolic, immune and central nervous system disorders, as well as cancer, are the consequence of an alteration in lipid metabolic enzymes and their pathways. This illustrates the fundamental role played by lipids in maintaining membrane homeostasis and normal function in healthy cells. We reviewed the major lipid dysfunctions occurring during tumor development, as determined using systems biology approaches. In it, we provide detailed insight into the essential roles exerted by specific lipids in mediating intracellular oncogenic signaling, endoplasmic reticulum stress and bidirectional crosstalk between cells of the tumor microenvironment and cancer cells. Finally, we summarize the advances in ongoing research aimed at exploiting the dependency of cancer cells on lipids to abolish tumor progression.


Dolan DNA Learning Center

Since its founding in 1988, Cold Spring Harbor Laboratory’s DNA Learning Center (DNALC) has provided an environment where students and the public can learn about science by asking questions and doing experiments. The DNALC has four laboratory classrooms, one computer classroom, and a museum exhibit.

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Pluripotent Stem Cells

Michael J. Shamblott , . John D. Gearhart , in Handbook of Stem Cells (Second Edition) , 2013

Embryoid Body Formation and Analysis

Embryoid bodies (Ebs) form spontaneously in human EG cultures. Although this represents a loss of pluripotent EG cells from the culture, EBs provide evidence for the pluripotent status of the culture, and provide cellular material for subsequent culture and experimentation (see the section “EBD cells”). Initially, EBs provided the only direct evidence that human EG cultures were pluripotent, as all attempts to form teratomas in mice from human EG cells failed. To this day, there is no evidence of teratoma formation from human EG cells or their derivatives.


Yun-Bo Shi, Ph.D.

Dr. Yun-Bo Shi is a senior investigator and the head of Section on Molecular Morphogenesis, Laboratory of Gene Regulation and Development, Program on Cell Regulation and Metabolism, at NICHD. Dr. Shi received his B. S. degree from the Department of Chemistry, Wuhan University, China, in 1982 and his Ph. D. degree from the Department of Chemistry, University of California, Berkeley, in 1988. After postdoctoral training at the Carnegie Institution, Baltimore, MD, Dr. Shi established his own research group within the intramural research program of NICHD in 1992.

His laboratory has been studying the molecular basis of thyroid hormone regulation of vertebrate development by using Xenopus metamorphosis as a model system. Dr. Shi has published over 230 research papers and reviews/book chapters, edited three books, and written a monograph on amphibian metamorphosis. For his accomplishments, For his accomplishments, Dr. Shi has received many awards and recognitions, including the 2009 NIH APAO (Asian & Pacific Islander American Organization) Award for outstanding accomplishments in biomedical research and the 2008 Van Meter Award by the American Thyroid Association, which honors an investigator who has made outstanding contributions to research on the thyroid gland. In 2012, Dr. Shi was elected as a Fellow of the American Association for the Advancement of Science. Dr. Shi was an editor of Cell Research from 1997 to 2010 and the Editor-in-Chief of Cell and Bioscience from 2010-2020. Dr. Shi is currently an editor of Development, Growth, and Differentiation and has also served as a board member of many journals including Thyroid, Endocrinology, and Journal of Biological Chemistry.


Adult Stem Cells: Biology and Methods of Analysis Paperback – 26 September 2014

"The three parts that makes up this volume, the first dealing with the basic biology, the second with the characterization of the adult stem cells phenotype and the third with the regulation of the life span of these cells, are all targeted to the critical evaluation of the techniques we use to distinguish adult stem cell-renewal from cell survival. . The volume is well written and well illustrated . . Well done!" (Carlo Alberto Redi, European Journal of Histochemistry, Vol. 55, 2011)

From the Back Cover

This is comprehensive overview of a vital area of scientific enquiry, which covers a broad spectrum of issues. With contributions from some of the key researchers in the field, Adult Stem Cells: Biology and Methods of Analysis offers readers a historical perspective as well as unique insights into cutting-edge thoughts. The volume contextualizes the recent discovery of stem/progenitor cell populations resident in many adult tissues and organs. It confronts the complexities scientists face in trying to validate these cells, while it also describes and critically evaluates the methods currently used to assess stem cell self-renewal. The chapters also seek to distinguish this process from other aspects of cell survival, such as the regulation of life span, senescence, and immortalization at a molecular level.

The monograph begins with a section that examine the basic biology of adult stem cells, including chapters on the emerging role of microRNAs in regulating their fate and the molecular mechanisms that govern their self-renewal, the book moves on to analyze the varying methodologies employed in characterizing these elusive elements of our genetic make-up. The second section details in-vivo lineage tracing of tissue-specific stem cells, explores the neural stem cell paradigm, and considers the function of ABC transporters and aldehyde dehydrogenase in adult stem-cell biology. The final section shifts the focus to the life-span regulation and immortalization and features a chapter on the cancer stem cell paradigm.

This is an authoritative volume on one of the frontiers of genetic research, and will serve as a valuable resource, not just for established scientists but also for those now entering the field of stem cell biology.


Contents

The earliest study of the nervous system dates to ancient Egypt. Trepanation, the surgical practice of either drilling or scraping a hole into the skull for the purpose of curing head injuries or mental disorders, or relieving cranial pressure, was first recorded during the Neolithic period. Manuscripts dating to 1700 BC indicate that the Egyptians had some knowledge about symptoms of brain damage. [8]

Early views on the function of the brain regarded it to be a "cranial stuffing" of sorts. In Egypt, from the late Middle Kingdom onwards, the brain was regularly removed in preparation for mummification. It was believed at the time that the heart was the seat of intelligence. According to Herodotus, the first step of mummification was to "take a crooked piece of iron, and with it draw out the brain through the nostrils, thus getting rid of a portion, while the skull is cleared of the rest by rinsing with drugs." [9]

The view that the heart was the source of consciousness was not challenged until the time of the Greek physician Hippocrates. He believed that the brain was not only involved with sensation—since most specialized organs (e.g., eyes, ears, tongue) are located in the head near the brain—but was also the seat of intelligence. [10] Plato also speculated that the brain was the seat of the rational part of the soul. [11] Aristotle, however, believed the heart was the center of intelligence and that the brain regulated the amount of heat from the heart. [12] This view was generally accepted until the Roman physician Galen, a follower of Hippocrates and physician to Roman gladiators, observed that his patients lost their mental faculties when they had sustained damage to their brains. [13]

Abulcasis, Averroes, Avicenna, Avenzoar, and Maimonides, active in the Medieval Muslim world, described a number of medical problems related to the brain. In Renaissance Europe, Vesalius (1514–1564), René Descartes (1596–1650), Thomas Willis (1621–1675) and Jan Swammerdam (1637–1680) also made several contributions to neuroscience.

Luigi Galvani's pioneering work in the late 1700s set the stage for studying the electrical excitability of muscles and neurons. In the first half of the 19th century, Jean Pierre Flourens pioneered the experimental method of carrying out localized lesions of the brain in living animals describing their effects on motricity, sensibility and behavior. In 1843 Emil du Bois-Reymond demonstrated the electrical nature of the nerve signal, [14] whose speed Hermann von Helmholtz proceeded to measure, [15] and in 1875 Richard Caton found electrical phenomena in the cerebral hemispheres of rabbits and monkeys. [16] Adolf Beck published in 1890 similar observations of spontaneous electrical activity of the brain of rabbits and dogs. [17] Studies of the brain became more sophisticated after the invention of the microscope and the development of a staining procedure by Camillo Golgi during the late 1890s. The procedure used a silver chromate salt to reveal the intricate structures of individual neurons. His technique was used by Santiago Ramón y Cajal and led to the formation of the neuron doctrine, the hypothesis that the functional unit of the brain is the neuron. [18] Golgi and Ramón y Cajal shared the Nobel Prize in Physiology or Medicine in 1906 for their extensive observations, descriptions, and categorizations of neurons throughout the brain.

In parallel with this research, work with brain-damaged patients by Paul Broca suggested that certain regions of the brain were responsible for certain functions. At the time, Broca's findings were seen as a confirmation of Franz Joseph Gall's theory that language was localized and that certain psychological functions were localized in specific areas of the cerebral cortex. [19] [20] The localization of function hypothesis was supported by observations of epileptic patients conducted by John Hughlings Jackson, who correctly inferred the organization of the motor cortex by watching the progression of seizures through the body. Carl Wernicke further developed the theory of the specialization of specific brain structures in language comprehension and production. Modern research through neuroimaging techniques, still uses the Brodmann cerebral cytoarchitectonic map (referring to study of cell structure) anatomical definitions from this era in continuing to show that distinct areas of the cortex are activated in the execution of specific tasks. [21]

During the 20th century, neuroscience began to be recognized as a distinct academic discipline in its own right, rather than as studies of the nervous system within other disciplines. Eric Kandel and collaborators have cited David Rioch, Francis O. Schmitt, and Stephen Kuffler as having played critical roles in establishing the field. [22] Rioch originated the integration of basic anatomical and physiological research with clinical psychiatry at the Walter Reed Army Institute of Research, starting in the 1950s. During the same period, Schmitt established a neuroscience research program within the Biology Department at the Massachusetts Institute of Technology, bringing together biology, chemistry, physics, and mathematics. The first freestanding neuroscience department (then called Psychobiology) was founded in 1964 at the University of California, Irvine by James L. McGaugh. [23] This was followed by the Department of Neurobiology at Harvard Medical School, which was founded in 1966 by Stephen Kuffler. [24]

The understanding of neurons and of nervous system function became increasingly precise and molecular during the 20th century. For example, in 1952, Alan Lloyd Hodgkin and Andrew Huxley presented a mathematical model for transmission of electrical signals in neurons of the giant axon of a squid, which they called "action potentials", and how they are initiated and propagated, known as the Hodgkin–Huxley model. In 1961–1962, Richard FitzHugh and J. Nagumo simplified Hodgkin–Huxley, in what is called the FitzHugh–Nagumo model. In 1962, Bernard Katz modeled neurotransmission across the space between neurons known as synapses. Beginning in 1966, Eric Kandel and collaborators examined biochemical changes in neurons associated with learning and memory storage in Aplysia. In 1981 Catherine Morris and Harold Lecar combined these models in the Morris–Lecar model. Such increasingly quantitative work gave rise to numerous biological neuron models and models of neural computation.

As a result of the increasing interest about the nervous system, several prominent neuroscience organizations have been formed to provide a forum to all neuroscientists during the 20th century. For example, the International Brain Research Organization was founded in 1961, [25] the International Society for Neurochemistry in 1963, [26] the European Brain and Behaviour Society in 1968, [27] and the Society for Neuroscience in 1969. [28] Recently, the application of neuroscience research results has also given rise to applied disciplines as neuroeconomics, [29] neuroeducation, [30] neuroethics, [31] and neurolaw. [32]

Over time, brain research has gone through philosophical, experimental, and theoretical phases, with work on brain simulation predicted to be important in the future. [33]

The scientific study of the nervous system increased significantly during the second half of the twentieth century, principally due to advances in molecular biology, electrophysiology, and computational neuroscience. This has allowed neuroscientists to study the nervous system in all its aspects: how it is structured, how it works, how it develops, how it malfunctions, and how it can be changed.

For example, it has become possible to understand, in much detail, the complex processes occurring within a single neuron. Neurons are cells specialized for communication. They are able to communicate with neurons and other cell types through specialized junctions called synapses, at which electrical or electrochemical signals can be transmitted from one cell to another. Many neurons extrude a long thin filament of axoplasm called an axon, which may extend to distant parts of the body and are capable of rapidly carrying electrical signals, influencing the activity of other neurons, muscles, or glands at their termination points. A nervous system emerges from the assemblage of neurons that are connected to each other.

The vertebrate nervous system can be split into two parts: the central nervous system (defined as the brain and spinal cord), and the peripheral nervous system. In many species — including all vertebrates — the nervous system is the most complex organ system in the body, with most of the complexity residing in the brain. The human brain alone contains around one hundred billion neurons and one hundred trillion synapses it consists of thousands of distinguishable substructures, connected to each other in synaptic networks whose intricacies have only begun to be unraveled. At least one out of three of the approximately 20,000 genes belonging to the human genome is expressed mainly in the brain. [34]

Due to the high degree of plasticity of the human brain, the structure of its synapses and their resulting functions change throughout life. [35]

Making sense of the nervous system's dynamic complexity is a formidable research challenge. Ultimately, neuroscientists would like to understand every aspect of the nervous system, including how it works, how it develops, how it malfunctions, and how it can be altered or repaired. Analysis of the nervous system is therefore performed at multiple levels, ranging from the molecular and cellular levels to the systems and cognitive levels. The specific topics that form the main foci of research change over time, driven by an ever-expanding base of knowledge and the availability of increasingly sophisticated technical methods. Improvements in technology have been the primary drivers of progress. Developments in electron microscopy, computer science, electronics, functional neuroimaging, and genetics and genomics have all been major drivers of progress.

Molecular and cellular neuroscience Edit

Basic questions addressed in molecular neuroscience include the mechanisms by which neurons express and respond to molecular signals and how axons form complex connectivity patterns. At this level, tools from molecular biology and genetics are used to understand how neurons develop and how genetic changes affect biological functions. The morphology, molecular identity, and physiological characteristics of neurons and how they relate to different types of behavior are also of considerable interest.

Questions addressed in cellular neuroscience include the mechanisms of how neurons process signals physiologically and electrochemically. These questions include how signals are processed by neurites and somas and how neurotransmitters and electrical signals are used to process information in a neuron. Neurites are thin extensions from a neuronal cell body, consisting of dendrites (specialized to receive synaptic inputs from other neurons) and axons (specialized to conduct nerve impulses called action potentials). Somas are the cell bodies of the neurons and contain the nucleus.

Another major area of cellular neuroscience is the investigation of the development of the nervous system. Questions include the patterning and regionalization of the nervous system, neural stem cells, differentiation of neurons and glia (neurogenesis and gliogenesis), neuronal migration, axonal and dendritic development, trophic interactions, and synapse formation.

Computational neurogenetic modeling is concerned with the development of dynamic neuronal models for modeling brain functions with respect to genes and dynamic interactions between genes.

Neural circuits and systems Edit

Questions in systems neuroscience include how neural circuits are formed and used anatomically and physiologically to produce functions such as reflexes, multisensory integration, motor coordination, circadian rhythms, emotional responses, learning, and memory. In other words, they address how these neural circuits function in large-scale brain networks, and the mechanisms through which behaviors are generated. For example, systems level analysis addresses questions concerning specific sensory and motor modalities: how does vision work? How do songbirds learn new songs and bats localize with ultrasound? How does the somatosensory system process tactile information? The related fields of neuroethology and neuropsychology address the question of how neural substrates underlie specific animal and human behaviors. Neuroendocrinology and psychoneuroimmunology examine interactions between the nervous system and the endocrine and immune systems, respectively. Despite many advancements, the way that networks of neurons perform complex cognitive processes and behaviors is still poorly understood.

Cognitive and behavioral neuroscience Edit

Cognitive neuroscience addresses the questions of how psychological functions are produced by neural circuitry. The emergence of powerful new measurement techniques such as neuroimaging (e.g., fMRI, PET, SPECT), EEG, MEG, electrophysiology, optogenetics and human genetic analysis combined with sophisticated experimental techniques from cognitive psychology allows neuroscientists and psychologists to address abstract questions such as how cognition and emotion are mapped to specific neural substrates. Although many studies still hold a reductionist stance looking for the neurobiological basis of cognitive phenomena, recent research shows that there is an interesting interplay between neuroscientific findings and conceptual research, soliciting and integrating both perspectives. For example, neuroscience research on empathy solicited an interesting interdisciplinary debate involving philosophy, psychology and psychopathology. [36] Moreover, the neuroscientific identification of multiple memory systems related to different brain areas has challenged the idea of memory as a literal reproduction of the past, supporting a view of memory as a generative, constructive and dynamic process. [37]

Neuroscience is also allied with the social and behavioral sciences, as well as with nascent interdisciplinary fields. Examples of such alliances include neuroeconomics, decision theory, social neuroscience, and neuromarketing to address complex questions about interactions of the brain with its environment. A study into consumer responses for example uses EEG to investigate neural correlates associated with narrative transportation into stories about energy efficiency. [38]

Computational neuroscience Edit

Questions in computational neuroscience can span a wide range of levels of traditional analysis, such as development, structure, and cognitive functions of the brain. Research in this field utilizes mathematical models, theoretical analysis, and computer simulation to describe and verify biologically plausible neurons and nervous systems. For example, biological neuron models are mathematical descriptions of spiking neurons which can be used to describe both the behavior of single neurons as well as the dynamics of neural networks. Computational neuroscience is often referred to as theoretical neuroscience.

Nanoparticles in medicine are versatile in treating neurological disorders showing promising results in mediating drug transport across the blood brain barrier. [39] Implementing nanoparticles in antiepileptic drugs enhances their medical efficacy by increasing bioavailability in the bloodstream, as well as offering a measure of control in release time concentration. [39] Although nanoparticles can assist therapeutic drugs by adjusting physical properties to achieve desirable effects, inadvertent increases in toxicity often occur in preliminary drug trials. [40] Furthermore, production of nanomedicine for drug trials is economically consuming, hindering progress in their implementation. Computational models in nanoneuroscience provide alternatives to study the efficacy of nanotechnology-based medicines in neurological disorders while mitigating potential side effects and development costs. [39]

Nanomaterials often operate at length scales between classical and quantum regimes. [41] Due to the associated uncertainties at the length scales that nanomaterials operate, it is difficult to predict their behavior prior to in vivo studies. [39] Classically, the physical processes which occur throughout neurons are analogous to electrical circuits. Designers focus on such analogies and model brain activity as a neural circuit. [42] Success in computational modeling of neurons have led to the development of stereochemical models that accurately predict acetylcholine receptor-based synapses operating at microsecond time scales. [42]

Ultrafine nanoneedles for cellular manipulations are thinner than the smallest single walled carbon nanotubes. Computational quantum chemistry [43] is used to design ultrafine nanomaterials with highly symmetrical structures to optimize geometry, reactivity and stability. [41]

Behavior of nanomaterials are dominated by long ranged non-bonding interactions. [44] Electrochemical processes that occur throughout the brain generate an electric field which can inadvertently affect the behavior of some nanomaterials. [41] Molecular dynamics simulations can mitigate the development phase of nanomaterials as well as prevent neural toxicity of nanomaterials following in vivo clinical trials. [40] Testing nanomaterials using molecular dynamics optimizes nano characteristics for therapeutic purposes by testing different environment conditions, nanomaterial shape fabrications, nanomaterial surface properties, etc. without the need for in vivo experimentation. [45] Flexibility in molecular dynamic simulations allows medical practitioners to personalize treatment. Nanoparticle related data from translational nanoinformatics links neurological patient specific data to predict treatment response. [44]

Neuroscience and medicine Edit

Neurology, psychiatry, neurosurgery, psychosurgery, anesthesiology and pain medicine, neuropathology, neuroradiology, ophthalmology, otolaryngology, clinical neurophysiology, addiction medicine, and sleep medicine are some medical specialties that specifically address the diseases of the nervous system. These terms also refer to clinical disciplines involving diagnosis and treatment of these diseases.

Neurology works with diseases of the central and peripheral nervous systems, such as amyotrophic lateral sclerosis (ALS) and stroke, and their medical treatment. Psychiatry focuses on affective, behavioral, cognitive, and perceptual disorders. Anesthesiology focuses on perception of pain, and pharmacologic alteration of consciousness. Neuropathology focuses upon the classification and underlying pathogenic mechanisms of central and peripheral nervous system and muscle diseases, with an emphasis on morphologic, microscopic, and chemically observable alterations. Neurosurgery and psychosurgery work primarily with surgical treatment of diseases of the central and peripheral nervous systems.

Translational research Edit

Recently, the boundaries between various specialties have blurred, as they are all influenced by basic research in neuroscience. For example, brain imaging enables objective biological insight into mental illnesses, which can lead to faster diagnosis, more accurate prognosis, and improved monitoring of patient progress over time. [46]

Integrative neuroscience describes the effort to combine models and information from multiple levels of research to develop a coherent model of the nervous system. For example, brain imaging coupled with physiological numerical models and theories of fundamental mechanisms may shed light on psychiatric disorders. [47]

Modern neuroscience education and research activities can be very roughly categorized into the following major branches, based on the subject and scale of the system in examination as well as distinct experimental or curricular approaches. Individual neuroscientists, however, often work on questions that span several distinct subfields.

List of the major branches of neuroscience
Branch Description
Affective neuroscience Affective neuroscience is the study of the neural mechanisms involved in emotion, typically through experimentation on animal models. [48]
Behavioral neuroscience Behavioral neuroscience (also known as biological psychology, physiological psychology, biopsychology, or psychobiology) is the application of the principles of biology to the study of genetic, physiological, and developmental mechanisms of behavior in humans and non-human animals.
Cellular neuroscience Cellular neuroscience is the study of neurons at a cellular level including morphology and physiological properties.
Clinical neuroscience The scientific study of the biological mechanisms that underlie the disorders and diseases of the nervous system.
Cognitive neuroscience Cognitive neuroscience is the study of the biological mechanisms underlying cognition.
Computational neuroscience Computational neuroscience is the theoretical study of the nervous system.
Cultural neuroscience Cultural neuroscience is the study of how cultural values, practices and beliefs shape and are shaped by the mind, brain and genes across multiple timescales. [49]
Developmental neuroscience Developmental neuroscience studies the processes that generate, shape, and reshape the nervous system and seeks to describe the cellular basis of neural development to address underlying mechanisms.
Evolutionary neuroscience Evolutionary neuroscience studies the evolution of nervous systems.
Molecular neuroscience Molecular neuroscience studies the nervous system with molecular biology, molecular genetics, protein chemistry, and related methodologies.
Nanoneuroscience An interdisciplinary field that integrates nanotechnology and neuroscience.
Neural engineering Neural engineering uses engineering techniques to interact with, understand, repair, replace, or enhance neural systems.
Neuroanatomy Neuroanatomy is the study of the anatomy of nervous systems.
Neurochemistry Neurochemistry is the study of how neurochemicals interact and influence the function of neurons.
Neuroethology Neuroethology is the study of the neural basis of non-human animals behavior.
Neurogastronomy Neurogastronomy is the study of flavor and how it affects sensation, cognition, and memory. [50]
Neurogenetics Neurogenetics is the study of the genetical basis of the development and function of the nervous system.
Neuroimaging Neuroimaging includes the use of various techniques to either directly or indirectly image the structure and function of the brain.
Neuroimmunology Neuroimmunology is concerned with the interactions between the nervous and the immune system.
Neuroinformatics Neuroinformatics is a discipline within bioinformatics that conducts the organization of neuroscience data and application of computational models and analytical tools.
Neurolinguistics Neurolinguistics is the study of the neural mechanisms in the human brain that control the comprehension, production, and acquisition of language.
Neurophysics Neurophysicsis the branch of biophysics dealing with the development and use of physical methods to gain information about the nervous system.
Neurophysiology Neurophysiology is the study of the functioning of the nervous system, generally using physiological techniques that include measurement and stimulation with electrodes or optically with ion- or voltage-sensitive dyes or light-sensitive channels.
Neuropsychology Neuropsychology is a discipline that resides under the umbrellas of both psychology and neuroscience, and is involved in activities in the arenas of both basic science and applied science. In psychology, it is most closely associated with biopsychology, clinical psychology, cognitive psychology, and developmental psychology. In neuroscience, it is most closely associated with the cognitive, behavioral, social, and affective neuroscience areas. In the applied and medical domain, it is related to neurology and psychiatry.
Paleoneurobiology Paleoneurobiology is a field which combines techniques used in paleontology and archeology to study brain evolution, especially that of the human brain.
Social neuroscience Social neuroscience is an interdisciplinary field devoted to understanding how biological systems implement social processes and behavior, and to using biological concepts and methods to inform and refine theories of social processes and behavior.
Systems neuroscience Systems neuroscience is the study of the function of neural circuits and systems.

The largest professional neuroscience organization is the Society for Neuroscience (SFN), which is based in the United States but includes many members from other countries. Since its founding in 1969 the SFN has grown steadily: as of 2010 it recorded 40,290 members from 83 different countries. [51] Annual meetings, held each year in a different American city, draw attendance from researchers, postdoctoral fellows, graduate students, and undergraduates, as well as educational institutions, funding agencies, publishers, and hundreds of businesses that supply products used in research.

Other major organizations devoted to neuroscience include the International Brain Research Organization (IBRO), which holds its meetings in a country from a different part of the world each year, and the Federation of European Neuroscience Societies (FENS), which holds a meeting in a different European city every two years. FENS comprises a set of 32 national-level organizations, including the British Neuroscience Association, the German Neuroscience Society (Neurowissenschaftliche Gesellschaft), and the French Société des Neurosciences. The first National Honor Society in Neuroscience, Nu Rho Psi, was founded in 2006. Numerous youth neuroscience societies which support undergraduates, graduates and early career researchers also exist, like Project Encephalon. [52]

In 2013, the BRAIN Initiative was announced in the US. An International Brain Initiative was created in 2017, [53] currently integrated by more than seven national-level brain research initiatives (US, Europe, Allen Institute, Japan, China, Australia, Canada, Korea, Israel) [54] spanning four continents.

Public education and outreach Edit

In addition to conducting traditional research in laboratory settings, neuroscientists have also been involved in the promotion of awareness and knowledge about the nervous system among the general public and government officials. Such promotions have been done by both individual neuroscientists and large organizations. For example, individual neuroscientists have promoted neuroscience education among young students by organizing the International Brain Bee, which is an academic competition for high school or secondary school students worldwide. [55] In the United States, large organizations such as the Society for Neuroscience have promoted neuroscience education by developing a primer called Brain Facts, [56] collaborating with public school teachers to develop Neuroscience Core Concepts for K-12 teachers and students, [57] and cosponsoring a campaign with the Dana Foundation called Brain Awareness Week to increase public awareness about the progress and benefits of brain research. [58] In Canada, the CIHR Canadian National Brain Bee is held annually at McMaster University. [59]

Neuroscience educators formed Faculty for Undergraduate Neuroscience (FUN) in 1992 to share best practices and provide travel awards for undergraduates presenting at Society for Neuroscience meetings. [60]

Finally, neuroscientists have also collaborated with other education experts to study and refine educational techniques to optimize learning among students, an emerging field called educational neuroscience. [61] Federal agencies in the United States, such as the National Institute of Health (NIH) [62] and National Science Foundation (NSF), [63] have also funded research that pertains to best practices in teaching and learning of neuroscience concepts.


Engaging in science practices in classrooms predicts increases in undergraduates' STEM motivation, identity, and achievement: A short-term longitudinal study

Campbell Leaper, Department of Psychology, University of California, Santa Cruz, CA 95064.

Department of Psychology, University of California, Santa Cruz, California

Institute for Scientist & Engineer Educators, University of California, Santa Cruz, California

Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, California

Division of Physical and Biological Sciences, University of California, Santa Cruz, California

Institute for Scientist & Engineer Educators, University of California, Santa Cruz, California

Department of Psychology, University of California, Santa Cruz, California

Campbell Leaper, Department of Psychology, University of California, Santa Cruz, CA 95064.

Abstract

Our short-term longitudinal study explored undergraduate students' experiences with performing authentic science practices in the classroom in relation to their science achievement and course grades. In addition, classroom experiences (felt recognition as a scientist and perceived classroom climate) and changes over a 10-week academic term in STEM (science, technology, engineering, and mathematics) identity and motivation were tested as mediators. The sample comprised 1,079 undergraduate students from introductory biology classrooms (65.4% women, 37.6% Asian, 30.2% White, 25.1% Latinx). Using structural equation modeling (SEM), our hypothesized model was confirmed while controlling for class size and GPA. Performing science practices (e.g., hypothesizing or explaining results) positively predicted students' felt recognition as a scientist and felt recognition positively predicted perceived classroom climate. In turn, felt recognition and classroom climate predicted increases over time in students' STEM motivation (expectancy-value beliefs), STEM identity, and STEM career aspirations. Finally, these factors predicted students' course grade. Both recognition as a scientist and positive classroom climate were more strongly related to outcomes among underrepresented minority (URM) students. Findings have implications for why large-format courses that emphasize opportunities for students to learn science practices are related to positive STEM outcomes, as well as why they may prove especially helpful for URM students. Practical implications include the importance of recognition as a scientist from professors, teaching assistants, and classmates in addition to curriculum that engages students in the authentic practices of science.

Appendix S1. Supporting Information.

Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.


Stem modifications

Some plant species have modified stems that are especially suited to a particular habitat and environment ( [link] ). A rhizome is a modified stem that grows horizontally underground and has nodes and internodes. Vertical shoots may arise from the buds on the rhizome of some plants, such as ginger and ferns. Corms are similar to rhizomes, except they are more rounded and fleshy (such as in gladiolus). Corms contain stored food that enables some plants to survive the winter. Stolons are stems that run almost parallel to the ground, or just below the surface, and can give rise to new plants at the nodes. Runners are a type of stolon that runs above the ground and produces new clone plants at nodes at varying intervals: strawberries are an example. Tubers are modified stems that may store starch, as seen in the potato ( Solanum sp.). Tubers arise as swollen ends of stolons, and contain many adventitious or unusual buds (familiar to us as the &ldquoeyes&rdquo on potatoes). A bulb , which functions as an underground storage unit, is a modification of a stem that has the appearance of enlarged fleshy leaves emerging from the stem or surrounding the base of the stem, as seen in the iris.

Stem modifications enable plants to thrive in a variety of environments. Shown are (a) ginger ( Zingiber officinale ) rhizomes, (b) a carrion flower ( Amorphophallus titanum ) corm (c) Rhodes grass ( Chloris gayana ) stolons, (d) strawberry ( Fragaria ananassa ) runners, (e) potato ( Solanum tuberosum ) tubers, and (f) red onion ( Allium ) bulbs. (credit a: modification of work by Maja Dumat credit c: modification of work by Harry Rose credit d: modification of work by Rebecca Siegel credit e: modification of work by Scott Bauer, USDA ARS credit f: modification of work by Stephen Ausmus, USDA ARS)


Struggles with math and science classes can have long-term consequences. Students could change their majors, change their careers to something outside a STEM field, or drop out of college. As a result, they could alter their lifetime earning potential.

According to the Mathematical Association of America, struggling with math is “the most significant barrier” to earning a college degree.

Student success often depends largely on the professor. For example, at the University of California Los Angeles (UCLA), the same calculus course was named as one of the best courses and one of the worst courses, depending on who the professor was.

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Watch the video: The Life Hydrologic: Crash Course Kids # (May 2022).