Brain Tumor Targeting Drug Delivery Systems: Advanced Nanoscience for Theranostics Applications E-Book
44,81 €
Mehr erfahren.
- Herausgeber: Bentham Science Publishers
- Kategorie: Fachliteratur
- Sprache: Englisch
- Veröffentlichungsjahr: 2000
Brain Tumor Targeting Drug Delivery Systems: Advanced Nanoscience for Theranostics Applications is a comprehensive reference focused on the latest advancements in nanotechnology for brain tumor therapy. With practical insights and cutting-edge research, this book equips readers with the knowledge to develop innovative drug delivery systems for effective brain tumor diagnosis and treatment.
Structured into insightful chapters, this book covers the anatomy, physiology, and pathophysiology of the brain, addressing barriers to targeted drug delivery strategies. Chapters explore theranostics-based delivery systems, including polymeric nanoparticles, liposomes, dendrimers, nanoemulsions, micelles, and inorganic nanoparticles, for precise brain tumor diagnosis and treatment.
This informative resource is designed for students and research scholars in pharmacology, pharmaceutical industry scientists, professors, and clinical medicine researchers. With comprehensive chapters and references for further reading, this book facilitates easy understanding of the intricate nanomedical technology, empowering researchers to make significant strides in the field of brain tumor therapy.
Key Features:
Structured chapters for easy understanding of nanotechnology concepts
In-depth coverage of theranostics-based delivery systems for brain tumor diagnosis and treatment
References for further reading and exploring new advances in drug delivery systems
Das E-Book können Sie in Legimi-Apps oder einer beliebigen App lesen, die das folgende Format unterstützen:
Seitenzahl: 427
Ähnliche
BENTHAM SCIENCE PUBLISHERS LTD.
End User License Agreement (for non-institutional, personal use)
This is an agreement between you and Bentham Science Publishers Ltd. Please read this License Agreement carefully before using the ebook/echapter/ejournal (“Work”). Your use of the Work constitutes your agreement to the terms and conditions set forth in this License Agreement. If you do not agree to these terms and conditions then you should not use the Work.
Bentham Science Publishers agrees to grant you a non-exclusive, non-transferable limited license to use the Work subject to and in accordance with the following terms and conditions. This License Agreement is for non-library, personal use only. For a library / institutional / multi user license in respect of the Work, please contact: [email protected].
Usage Rules:
All rights reserved: The Work is the subject of copyright and Bentham Science Publishers either owns the Work (and the copyright in it) or is licensed to distribute the Work. You shall not copy, reproduce, modify, remove, delete, augment, add to, publish, transmit, sell, resell, create derivative works from, or in any way exploit the Work or make the Work available for others to do any of the same, in any form or by any means, in whole or in part, in each case without the prior written permission of Bentham Science Publishers, unless stated otherwise in this License Agreement.You may download a copy of the Work on one occasion to one personal computer (including tablet, laptop, desktop, or other such devices). You may make one back-up copy of the Work to avoid losing it.The unauthorised use or distribution of copyrighted or other proprietary content is illegal and could subject you to liability for substantial money damages. You will be liable for any damage resulting from your misuse of the Work or any violation of this License Agreement, including any infringement by you of copyrights or proprietary rights.Disclaimer:
Bentham Science Publishers does not guarantee that the information in the Work is error-free, or warrant that it will meet your requirements or that access to the Work will be uninterrupted or error-free. The Work is provided "as is" without warranty of any kind, either express or implied or statutory, including, without limitation, implied warranties of merchantability and fitness for a particular purpose. The entire risk as to the results and performance of the Work is assumed by you. No responsibility is assumed by Bentham Science Publishers, its staff, editors and/or authors for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products instruction, advertisements or ideas contained in the Work.
Limitation of Liability:
In no event will Bentham Science Publishers, its staff, editors and/or authors, be liable for any damages, including, without limitation, special, incidental and/or consequential damages and/or damages for lost data and/or profits arising out of (whether directly or indirectly) the use or inability to use the Work. The entire liability of Bentham Science Publishers shall be limited to the amount actually paid by you for the Work.
General:
Any dispute or claim arising out of or in connection with this License Agreement or the Work (including non-contractual disputes or claims) will be governed by and construed in accordance with the laws of Singapore. Each party agrees that the courts of the state of Singapore shall have exclusive jurisdiction to settle any dispute or claim arising out of or in connection with this License Agreement or the Work (including non-contractual disputes or claims).Your rights under this License Agreement will automatically terminate without notice and without the need for a court order if at any point you breach any terms of this License Agreement. In no event will any delay or failure by Bentham Science Publishers in enforcing your compliance with this License Agreement constitute a waiver of any of its rights.You acknowledge that you have read this License Agreement, and agree to be bound by its terms and conditions. To the extent that any other terms and conditions presented on any website of Bentham Science Publishers conflict with, or are inconsistent with, the terms and conditions set out in this License Agreement, you acknowledge that the terms and conditions set out in this License Agreement shall prevail.Bentham Science Publishers Pte. Ltd. 80 Robinson Road #02-00 Singapore 068898 Singapore Email: [email protected]
PREFACE
There are still many unmet medical requirements, including brain tumours. Brain tumours can now be treated with considerably less toxicity and better pharmacokinetics and pharmacodynamics because of recent nano-drug delivery technologies. It is still difficult to treat brain tumours due to their rapid growth and poor prognoses, even with surgery, radiation, and chemotherapies. To combat the disease, therapeutic delivery methods that maximise drug accumulation in the tumour location and decrease toxicity in normal brain and peripheral tissue are a potential new strategy. The fact that brain tumours differ in many ways from tumours in other tissues means that drug delivery to brain tumours can take advantage of the constantly changing vascular characteristics and microenvironment. For brain tumour theranostics, nanocarrier-based delivery methods address brain architecture and tumours in addition to the advances and problems in delivering medicines across the blood brain barrier. Theranostics combines diagnostics and therapeutics. A growing number of people are becoming interested in individualised therapy and diagnostics approaches. As well as conserving money, this method also limits the negative effects of a specific goal.
This book contains several sections on nanotechnology, including the most recent developments in the field and practical advice on how to build more effective nanocarriers for medication and gene delivery. There is much helpful information in this book that will help readers to create innovative drug delivery systems for brain tumour therapy that will help to boost nanomedical technology. The key features highlighted in this book are various theranostic-based delivery systems for brain tumour diagnosis and treatment. This book will be of interest to many academicians, scientists, and researchers. It will enable them to understand the possible prospects of nanotechnology for delivering nanocarriers that can better diagnose and cure brain tumours in the future.
List of Contributors
Anatomy and Physiology of the Brain: Pathophysiology of Brain Tumor
Amitha Muraleedharan1,*,Nikhil Ponnoor Anto1Abstract
The brain is an efficient processor of information. It is the most complex and sensitive organ in the body and is responsible for all functions of the body, including serving as the coordinating center for all sensations, mobility, emotions, and intellect. The magnitude of its myriad function is often realized usually when there is a disruption of the nervous system due to injury, disease, or inherited predispositions. Neuroscience is the field of study that endeavors to make sense of such diverse questions; at the same time, it points the way toward the effective treatment of dysfunctions. The two-way channel of information: findings from the laboratory leading towards stricter criteria for diagnosing brain disorders and more effective methods for treating them and in turn, the clinician's increasingly acute skills of diagnosis and observation that supply the research scientist with more precise data for study in the lab diligently expands the field of neuroscience. Tumors of the brain produce neurological manifestations through several mechanisms. Stronger hypotheses about the mechanism of a disease can point the way toward more effective treatments and new possibilities for a cure. In highly complex disorders of the brain, in which many factors genetic, environmental, epidemiological, even social and psychological—play a part, broadly based hypotheses are exceedingly useful. With the advancements in technology and a better understanding of brain anatomy and physiology, the quest to discover an efficient cure for life-threatening tumors of the brain is underway.
INTRODUCTION
The nervous system is a very complex structure that can be divided into two major regions: the central nervous system (CNS) which consists of the brain and spinal cord and the peripheral nervous system (PNS) which is an extensive net-
work of nerves that consists of (i) Craniospinal nerves having 12 pairs of cranial nerves and 31 pairs of spinal nerves, (ii) Visceral nervous system comprising the sympathetic nervous system and parasympathetic nervous system connecting the CNS to the muscles and sensory structures [1, 2]. The spinal cord is a single structure, whereas the adult brain is divided into four major regions: (i) The cerebral hemispheres, comprised of the cerebral cortex, basal ganglia, white matter, hippocampi, and amygdalae; (ii) The diencephalon, with the thalamus and hypothalamus; (iii) The brain stem, consisting of the medulla, pons, and midbrain; and (iv) The cerebellum. The brain is the central control module of the body and coordinates activities like task-evoked responses, senses, movement, emotions, language, communication, thinking, and memory [3, 4]. In this book chapter, we discuss the anatomy of the brain, its functions, development, and pathology with a special focus on brain carcinogenesis.
THE ANATOMY OF THE HUMAN BRAIN
The brain is protected by the skull (cranium) which is in turn covered by the scalp. The scalp is composed of an outer layer of skin, which is loosely attached to the aponeurosis, a flat, broad tendon layer that anchors the superficial layers of the skin. The periosteum, below the aponeurosis, firmly encases the bones of the skull and provides protection, nutrition to the bone, and the capacity for bone repair. Below the skull are three layers of protective covering called the meninges that surround the brain and the spinal cord. The meningeal layer closest to the bones of the skull called the dura mater (meaning tough mother) is thick and tough and includes two layers; the periosteal layer lines the inner dome of the skull followed by the meningeal layer below. The space between the layers allows the passage of veins and arteries that supply blood to the brain. Below the dura mater lies the arachnoid mater (spider-like mother) which is comprised of a thin web-like connective tissue called arachnoid trabeculae and is devoid of nerves or blood vessels. The innermost meningeal layer is a delicate membrane called the pia mater (tender mother). The pia mater firmly adheres to the convoluted surface of the CNS, lining the inside of the sulci in the cerebral and cerebellar cortices, and is rich in veins and arteries. Between the arachnoid mater and pia mater is the subarachnoid space which is filled with cerebrospinal fluid (CSF), produced by the cells of the choroid plexus—areas in each ventricle of the brain (discussed further below). The CSF serves to deliver nutrients and removes waste from neural tissues and also provides a liquid cushion to the brain and spinal cord (Fig. 1) [5, 6].
Fig. (1)) The layers of the tissue surrounding the human brain including three meningeal membranes: the dura mater, the arachnoid mater, and pia mater.Cerebrum
The cerebrum, which appears to make up most of the brain mass, consists of two cerebral hemispheres demarked by a large separation called the longitudinal fissure. Each hemisphere has an inner core composed of white matter-the corpus callosum-and and an outer surface-the cerebral cortex- composed of gray matter. The corpus callosum, the largest of the five commissural nerve tracts, provides the major pathway for communication between the two hemispheres. According to the concept known as localization of function, different regions of the cerebral cortex can be associated with particular functions. In the early 1900s, an extensive study of the microscopic anatomy—the cytoarchitecture—of the cerebral cortex was undertaken by a German neuroscientist named Korbinian Brodmann who divided the cortex into 52 separate regions based on the histology of the cortex. The results from Brodmann’s work on the anatomy align very well with the functional differences within the cortex and resulted in a system of classification known as Brodmann’s areas, which is still used today to describe the anatomical distinctions within the cortex [7]. Each hemisphere is conventionally divided into four lobes namely the frontal lobe, temporal lobe, occipital lobe, and parietal lobe (Fig. 2).
Frontal lobe: positioned at the front of the brain, the lobe is associated with executive functions. Containing a majority of dopamine-sensitive neurons, the region is responsible for self-control, planning, reasoning, motivation, and abstract thinking. Broca’s area is responsible for the production of language, or controlling movements responsible for speech; in the vast majority of people, it is located only on the left side [4, 8, 9].Temporal lobe: It processes sensory information for the retention of memories, language, and emotions. The presence of olfactory nerves and auditory nerves makes it a major player in the sensations of smell and hearing [10].Occipital lobe: housing the visual cortex, the lobe is dedicated to vision [4].Parietal lobe: The main sensation associated with the parietal lobe is somatosensation. It integrates and interprets sensory information including vision, hearing, motor, sensory, spatial awareness, navigation, and memory function. Wernicke’s area is located here, which is responsible for understanding spoken and written language [9].Anterior to these regions is the prefrontal lobe, which serves cognitive functions that can be the basis of personality, short-term memory, and consciousness.Fig. (2))Lobes of the Cerebral Cortex: The cerebral cortex is divided into four lobes. Extensive folding increases the surface area available for cerebral functions.Subcortical Structures
Beneath the cerebral cortex lies a set of nuclei called the subcortical nuclei deep within the hemispheres that augment cortical processes (Fig. 3). The nuclei of the basal forebrain structures serve as the primary location for the production of acetylcholine which is then distributed throughout the cortex possibly leading to greater attention to sensory stimuli. Alzheimer’s disease is associated with the loss of neurons in these nuclei. The hippocampus and amygdala are medial-lobe structures that are involved in long-term memory formation and emotional responses. The basal ganglia or basal nuclei which include structures like the striatum, substantia nigra, and the subthalamic nucleus are involved in the control of voluntary motor movements, procedural learning, and influence the likelihood of movements taking place [7].
The Diencephalon
Diencephalon (through the brain) connects the cerebrum and the rest of the nervous system. The thalamus is a major processing region for sensory information and relays impulses between the cerebral cortex and the periphery, the spinal cord, or brain stem. Positioned in the center of the brain, the thalamus is involved in the regulation of consciousness, sleep, awareness, and alertness. The hypothalamus is largely involved in the regulation of homeostasis. It controls the autonomic nervous system and endocrine system by regulating the anterior pituitary secretions (e.g. LH) by releasing stimulating hormones (e.g. GnRH) into the hypophysial portal blood [11]. The secretions of the posterior pituitary (antidiuretic hormone, oxytocin) are also controlled by the hypothalamus. As part of the limbic system, certain parts of the hypothalamus are also involved in memory and emotion (Fig. 3) [7].
Brain Stem
The brain stem which includes the midbrain and hindbrain (pons and medulla) connects the cerebrum to the spinal cord (Fig. 3). The midbrain is a complex structure with a range of neuronal clusters that coordinate sensory representations of the visual, auditory, and somatosensory perceptual spaces. The pons is the main connection with the cerebellum. The pons and the medulla regulate several crucial functions, including the cardiovascular and respiratory systems and rates. The brain stem continues below the large opening in the occipital bone, called the foramen magnum, as the spinal cord and is protected by the vertebral column. A diffuse region of gray matter throughout the brain stem, known as the reticular formation, is related to sleep and wakefulness, such as general brain activity and attention [7, 12].
Cerebellum
The cerebellum (little brain) is largely responsible for comparing information from the cerebrum with the sensory feedback from the periphery through the spinal cord to fine-tune the precision and accuracy of motor activity. The cerebellum lies in the back of the cranial cavity, lying beneath the occipital lobes, separated by the cerebellar tentorium (Fig. 3). It is divided into three lobes; anterior lobe, posterior lobe, and floccondular lobe which function to coordinate voluntary muscle movements, maintain posture, balance, and equilibrium. The output of the cerebellum reaches the midbrain, which then sends a descending input to the spinal cord to correct the messages going to the skeletal muscles [7, 13].
Fig. (3)) Anatomy of the human brain (sagittal)Circulation and CSF
Blood is carried to the brain by two sets of arteries; the internal carotid arteries which enter the cranium through the carotid canal in the temporal bone and vertebral arteries which enter through the foramen magnum of the occipital bone. The internal carotid artery supplies most of the cerebrum, while the vertebral arteries supply the cerebellum, brainstem, and underside of the cerebrum. The two vertebral arteries then merge into the basilar artery, which gives rise to branches to the brain stem and cerebellum. The left and right internal carotid arteries and branches of the basilar artery all become the circle of Willis, a confluence of arteries that can maintain perfusion of the brain even if narrowing or a blockage limits flow through one part thus preventing brain damage. The external carotid arteries supply blood to the tissues on the surface of the cranium [14, 15].
The venous circulation of the brain is very different from the body. After passing through the CNS, blood returns to the circulation through a series of dural sinuses and veins. The superior sagittal sinus runs in the groove of the longitudinal fissure, where it absorbs CSF from the meninges. The superior sagittal sinus drains to the confluence of sinuses, along with the occipital sinuses and straight sinus, to then drain into the transverse sinuses. The transverse sinuses connect to the sigmoid sinuses, which then connect to the jugular veins. From there, the blood continues toward the heart to be pumped to the lungs for reoxygenation. The two jugular veins are the only drainage of the brain [15, 16].
The cerebrospinal fluid (CSF) is a modified transcellular fluid that circulates throughout and around the CNS. The ventricles are the open spaces within the brain where CSF is produced by filtering of the blood that is performed by a specialized membrane known as a choroid plexus which are networks of blood capillaries lined by ependymal cells in the walls of the ventricles. Specifically, CSF circulates through all of the ventricles to remove metabolic wastes from the interstitial fluids of nervous tissues to eventually emerge into the subarachnoid space where it is reabsorbed into the blood. The CSF also acts as a mechanical buffer; by remaining inside and outside the CNS, it equalizes mechanical pressure thus acting as a cushion between the soft and delicate tissues of the brain and the rigid cranium [6, 14].
There are four ventricles within the brain, all of which developed from the original hollow space within the neural tube, the central canal. The first two are named the lateral ventricles and are deep within the cerebrum. These ventricles are connected to the third ventricle by two openings called the interventricular foramina. The third ventricle is the space between the left and right sides of the diencephalon, which opens into the cerebral aqueduct that passes through the midbrain. The aqueduct opens into the fourth ventricle, which is the space between the cerebellum and the pons and upper medulla. Three separate openings, the middle and two lateral apertures, drain the cerebrospinal fluid from the fourth ventricle to the cisterna magna one of the major cisterns. From the fourth ventricle, the CSF flows into the subarachnoid space where it bathes and cushions the brain. The single median aperture and the pair of lateral apertures connect to the subarachnoid space so that CSF can flow through the ventricles and around the CNS [14, 17].
The tissues of the CNS have extra protection in that they are not exposed to blood or the immune system in the same way as other tissues. The blood vessels that supply the brain with nutrients and other chemical substances lie on top of the pia mater. The capillary endothelial cells joined by tight junctions control the transfer of blood components to the brain. In addition, cranial capillaries have far fewer fenestra (pore-like structures that are sealed by a membrane) and pinocytotic vesicles than other capillaries. As a result, materials in the circulatory system have a very limited ability to interact with the CNS directly. This phenomenon is referred to as the blood-brain barrier (BBB) [5, 14]. The barrier is less permeable to larger molecules but is still permeable to water, carbon dioxide, oxygen, and most fat-soluble substances (including anesthetics and alcohol). The blood-brain barrier is not present in the circumventricular organs—which are structures in the brain that may need to respond to changes in body fluids—such as the pineal gland, area postrema, and some areas of the hypothalamus. There is a similar blood-cerebrospinal fluid barrier, which serves the same purpose as the blood-brain barrier, but facilitates the transport of different substances into the brain due to the distinct structural characteristics between the two barrier systems. Morphological characteristics of the blood-brain barrier show; a) The high electron density of endothelial cytoplasm, b) Thicker basement membrane, c) Absence of perivascular connective tissue, d) Complete covering of the endothelial processes by astrocytic processes, and e) Small number or absence of cytoplasmic vesicles in endothelial cells. Lateral zonulae occludens of the capillary endothelium force solutes to pass through the cytoplasm of astrocytes which restrains the passage of molecule through its plasma membrane [18]. The blood-brain barrier maintains the constancy of the environment of the neurons in the CNS in such a way that the multiple homeostatic mechanisms of the ionic transfer during neuronal activity is maintained. While nutrient molecules, such as glucose or amino acids, can pass through the BBB, it is impermeable to larger molecules causing problems with drug delivery to the CNS. Pharmaceutical companies are thus challenged to design drugs that can cross the BBB as well as have an effect on the nervous system to treat conditions like neurodegenerative diseases and intracranial brain tumors [5]. Recently, a paravascular pathway, also known as the “glymphatic” pathway, has been described as a system for waste clearance in the brain. According to this model, cerebrospinal fluid (CSF) enters the paravascular spaces surrounding penetrating arteries of the brain, mixes with interstitial fluid (ISF) and solutes in the parenchyma, and exits along the paravascular spaces of draining veins [19].
MICROANATOMY
The human brain is primarily composed of neurons, glial cells, neural stem cells, and blood vessels. Neurons are the structural and functional unit of the nervous system. They are responsible for the computation and communication that the nervous system provides. They are electrically active and release chemical signals to target cells via specialized connections called synapses [20]. Glial cells, or glia, are known to play a supporting role in the nervous tissue. Ongoing research pursues an expanded role for glial cells in signalling, but neurons are still considered the basis of this function. Neurons are important, but without glial support, they would not be able to perform their function.
The first way to classify a neuron is by the number of processes attached to the cell body. Using the standard model of neurons, one of these processes is the axon, and the rest are dendrites. Because information flows through the neuron from dendrites or cell bodies toward the axon, these names are based on the neuron's polarity. Axon is the process of a nerve cell that carries impulse away from it. Nerve fibers that carry impulses to the CNS are termed afferent and those carrying impulses from the CNS to the periphery are known as efferent. It arises from the part of the cell called the axon hillock. It is generally long with few branches and contains no Nissl granules. The axon hillock also has the greatest density of voltage-dependent sodium channels which makes it the most easily excited part of the neuron and the spike initiation zone for the axon. In electrophysiological terms, it has the most negative threshold potential. Dendrites collect impulses from other neurons and carry them to the cell body. The short cellular extensions with specific branching patterns resemble a dendritic tree, hence the name [21] (Fig. 4).
Neurons are typically classified into three types based on their function; i.) Sensory neurons respond to stimuli such as touch, sound, or light that affect the cells of the sensory organs, and they send signals to the spinal cord or brain, ii.) Motor neurons receive signals from the brain and spinal cord to control everything from muscle contractions to glandular output, iii.) Interneurons connect neurons to other neurons within the same region of the brain or spinal cord. A group of connected neurons is called a neural circuit [22]. Neurons can also be classified according to the number of their processes: 1) Unipolar cells have only one process emerging from the cell. True unipolar cells are only found in invertebrate animals, so the unipolar cells in humans are more appropriately called “pseudo-unipolar” cells. All developing neuroblasts pass through a stage when they have only one process-the axon. Human unipolar cells have an axon that emerges from the cell body, but it splits so that the axon can extend over a very long distance. At one end of the axon are dendrites, and at the other end, the axon forms synaptic connections with a target. Unipolar cells are exclusively sensory neurons and have two unique characteristics.
Fig. (4))Parts of a Neuron: The major parts of the neuron are labelled on a multipolar neuron from the CNS.First, their dendrites are receiving sensory information, sometimes directly from the stimulus itself. Secondly, the cell bodies of unipolar neurons are always found in ganglia. Sensory reception is a peripheral function (those dendrites are in the periphery, perhaps in the skin) so the cell body is in the periphery, though closer to the CNS in a ganglion. The axon projects from the dendrite endings, past the cell body in a ganglion, and into the central nervous system; 2) Bipolar cells have two processes, which extend from each end of the cell body, opposite to each other. One is the axon and one the dendrite. Bipolar cells are not very common. They are found mainly in the olfactory neuro-epithelium (where smell stimuli are sensed), in the vestibular ganglion, in the spiral ganglion of the cochlea, and as part of the retina; 3) Multipolar neurons are all of the neurons that are not unipolar or bipolar. They have one axon and two or more dendrites (usually many more). Except for the unipolar sensory ganglion cells, and the specific bipolar cells mentioned above, all other neurons are multipolar. A few of the common types of multipolar neurons are the Purkinje cell of the cerebellar cortex, pyramidal cell of the motor cortex, small neuron from the spinal nucleus of the trigeminal nerve, and motor neuron from the ventral horn of the spinal cord [20, 22]. Some sources describe a fourth type of neuron, called an anaxonic neuron. Anaxonic neurons are very small, and if looked through a microscope at the standard resolution used in histology (approximately 400X to 1000X total magnification), one will not be able to distinguish process specifically as an axon or a dendrite. Any of those processes can function as an axon depending on the conditions at any given time and are therefore multipolar [20].
An alternate classification also exists where neurons are classified based on the length of their axon. (i) Golgi type I neuron has a very long axon that has an extensive course outside the gray matter of the CNS and passes the white matter. These cells form the bulk of the neurons which constitute the peripheral nerves and main fiber tracts of the brain and spinal cord. e.g., Pyramidal cell and Purkinje cell. (ii) Golgi type II neuron is stellate and has a short axon that does not leave the gray matter. These cells are found in the retina, the cerebellar, and the cerebral cortices. e.g., granule cell.
Glia (glial cells or neuroglia) are the non-neuronal cells in the nervous tissue and were first described by a German pathologist Rudolf Virchow in 1856. They maintain homeostasis, form myelin, and provide support and protection to neurons. In the central nervous system, glial cells include oligodendrocytes, astrocytes, ependymal cells, and microglia, and in the peripheral nervous system; glial cells include Schwann cells and satellite cells [23]. They have four main functions: (1) To surround neurons and hold them in place; (2) To supply nutrients and oxygen to neurons; (3) To insulate one neuron from another; (4) To destroy pathogens and remove dead neurons. Astrocytes are the most abundant type of glial cells in the CNS. In general, there are two types of astrocytes, protoplasmic and fibrous, similar in function but distinct in morphology and distribution. Protoplasmic astrocytes have short, thick, highly branched processes and are typically found in gray matter. Fibrous astrocytes have long, thin, less branched processes and are more commonly found in white matter. Some ways in which they support neurons in the central nervous system are by maintaining the concentration of chemicals in the extracellular space, removing excess signalling molecules, reacting to tissue damage, and contributing to the blood-brain barrier [20, 23-25]. Oligodendrocytes sometimes called just “oligo,” are the glial cells that coat the axon in the CNS. The few processes that extend from the cell body reach out and surround an axon to insulate it in myelin. One oligodendrocyte will provide the myelin for multiple axon segments, either for the same axon or for separate axons. The myelin sheath provides insulation to the axon that allows electrical signals to propagate more efficiently [23]. Ependymal cells are involved in the creation and secretion of cerebrospinal fluid (CSF) and beat their cilia on their apical surface to help circulate the CSF through the ventricular space and make up the blood-CSF barrier [23]. Microglia are specialized macrophages capable of phagocytosis. They are derived from the earliest wave of mononuclear cells that originate in the yolk sac and colonize the brain shortly after the neural precursors begin to differentiate. These cells are found in all regions of the brain and spinal cord. They are mobile within the brain and multiply when there is damage. In the healthy central nervous system, microglia processes constantly sample all aspects of their environment (neurons, other glial cells, and blood vessels). In a healthy brain, microglia direct the immune response to brain damage and play an important role in the inflammation that accompanies the damage [23, 26].
DEVELOPMENT
According to developmental neuroscientists, the essential stages in the development and shaping of the human brain are (i) Proliferation of a vast number of undifferentiated brain cells; (ii) Migration of the cells toward a predetermined location in the brain and the beginning of their differentiation into the specific type of cell appropriate to that location; (iii) Aggregation of similar types of cells into distinct regions; (iv) Formation of innumerable connections among neurons, both within and across regions; and (v) Competition among these connections, which results in the selective elimination of many and the stabilization of the 100 trillion or so that remain. These events do not occur in rigid sequence but overlap in time, from about 5 weeks after conception onward. After about 18 months of age, no more neurons are added, and the aggregation of cell types into distinct regions is roughly complete. But the pruning of excess connections—clearly a process of great importance for the shape of the mature brain—continues for years [27].
The formation of the nervous system begins with the process called neurulation which follows gastrulation and results in the formation of the neural tube. As the embryo develops, a portion of the ectoderm differentiates into a specialized region of neuroectoderm, which is the precursor for the tissue of the nervous system. Molecular signals induce cells in this region to differentiate into the neuroepithelium, forming a neural plate. Shortly after the neural plate has formed, its edges thicken and move upward to form the neural folds. A neural groove forms, visible as a line along the dorsal surface of the embryo. As the neural folds come together and converge, the underlying structure forms into a tube just beneath the ectoderm called the neural tube. This entire process leading up to the formation of the neural tube is called primary neurulation [28, 29]. Cells from the neural folds at the dorsal most portion of the neural tube then separate from the ectoderm to form a cluster of cells referred to as the neural crest, which runs lateral to the neural tube. The anterior end of the neural tube develops into the brain and the posterior portion forms the spinal cord. The neural crest migrates away from the nascent, or embryonic central nervous system that will form along the neural groove and develops into several parts of the peripheral nervous system, including the enteric nervous tissue. Many tissues that are not part of the nervous system also arise from the neural crest, such as craniofacial cartilage and bone, and melanocytes. The two open ends of the neural tube are called the anterior neuropore and the posterior neuropore. Different neural tube defects are caused when various parts of the neural tube fail to close. Failure to close the human posterior neural tube regions results in a condition called spina bifida, the severity of which depends on how much of the spinal cord remains exposed. Failure to close the anterior neural tube regions results in a lethal condition, anencephaly, where the forebrain remains in contact with the amniotic fluid and subsequently degenerates. The failure of the entire neural tube to close over the entire body axis is called craniorachischisis [29].
In secondary neurulation, the differentiation of the neural tube into various regions of the central nervous system takes place simultaneously. On the gross anatomical level, the neural tube and its lumen bulge and constrict to form the chambers of the brain and the spinal cord. At the tissue level, the cell populations within the wall of the neural tube rearrange themselves to form the different functional regions of the brain and the spinal cord. Finally, on the cellular level, the neuroepithelial cells themselves differentiate into the numerous types of nerve cells (neurons) and supportive cells (glia) present in the body. As the anterior end of the neural tube starts to develop into the brain, it undergoes a couple of enlargements; the result is the production of sac-like vesicles. Three primary vesicles form the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). The three vesicles further differentiate into five secondary vesicles. The prosencephalon enlarges into two new vesicles called the telencephalon and the diencephalon. The telencephalon will eventually form the cerebral hemispheres. The diencephalon gives rise to several adult structures like the thalamus and the hypothalamus. In the embryonic diencephalon, a structure known as the optic vesicles or eye cup develops, which will eventually become the nervous tissue of the eye called the retina. This is a rare example of nervous tissue developing as part of the CNS structures in the embryo, but becoming a peripheral structure in the fully formed nervous system. The mesencephalon does not differentiate into any finer divisions and its lumen eventually becomes the cerebral aqueduct. The midbrain is an established region of the brain at the primary vesicle stage of development and remains that way. The rest of the brain develops around it and constitutes a large percentage of the mass of the brain. The rhombencephalon develops into an anterior metencephalon and posterior myelencephalon. The metencephalon corresponds to the adult structure known as the pons and also gives rise to the cerebellum, the part of the brain responsible for coordinating movements, posture, and balance. The most significant connection between the cerebellum and the rest of the brain is at the pons because the pons and cerebellum develop out of the same vesicle. The myelencephalon corresponds to the adult structure known as the medulla oblongata whose neurons generate the nerves that regulate respiratory, gastrointestinal, and cardiovascular movements. The structures that come from the mesencephalon and rhombencephalon, except for the cerebellum, are collectively considered the brain stem, which specifically includes the midbrain, pons, and medulla. The rhombencephalon develops a segmental pattern that specifies the places where certain nerves originate. Periodic swellings called rhombomeres divide the rhombencephalon into smaller compartments and will form ganglia clusters of neuronal cell bodies whose axons form a cranial nerve [28, 29].
The neurons of the brain are divided into cortices (layers) and nuclei (clusters). New neurons are formed by mitosis in the neural tube. The neural precursors can migrate away from the neural tube and form a new layer. Neurons forming later have to migrate through the existing layers. This forms the cortical layers. The germinal zone at the lumen of the neural tube is called the ventricular zone. The new layer is called the mantle zone (gray matter). The cerebral cortex in humans has six layers, and the mantle zone is called the neocortex. Cell fates are often fixed as they undergo their last division. Neurons derived from the same stem cell may end up in different functional regions of the brain. Neural stem cells have been observed in the adult human brain. We now believe humans can continue making neurons throughout life, although at nowhere near the fetal rate [29].
PHYSIOLOGY
Neurons are electrically excitable. To understand how neurons communicate it is necessary to describe the role of an excitable membrane. In neurons and other excitable cells, regulation of the ionic environment is crucial for the development and maintenance of the specific signalling pathway for these cells, known as the electrical signalling system. This system is composed of two basic elements: i) A lipid bimolecular diffusion barrier, termed lipid bilayer, that separates cells from their environment, and ii) Two classes of macromolecule proteins, known as ion channels and ion carriers, that actively regulate the movement and distribution of ions across the lipid barrier in the plasma membrane, as well as in the endoplasmic reticulum (ER) and nuclear membranes; of special interest is the carrier protein referred to as the sodium/potassium pump that moves sodium ions (Na+) out of a cell and potassium ions (K+) into a cell, thus regulating ion concentration on both sides of the cell membrane [30, 31]. Neurons communicate with each other through two types of electrical signals: 1) Graded potential for short-distance communication and 2) Action potential for communication over long-distance. Graded potentials and action potentials occur because the membranes of neurons contain many different kinds of ion channels that allow specific charged particles to cross the membrane in response to an existing concentration gradient termed electrochemical gradient [32]. The ion channels can be classified based on the charge of the ions they transport; (1) Cation channels that most often allow sodium ions (Na+) to pass when opened, but sometimes allow potassium (K+) and/or calcium (Ca2+) ions as well, and (2) Anion channels that allow mainly chloride ions (Cl-) to pass but also minute quantities of other anions. Based on the mechanism of activation, ion channels are classified into four types: (1) A ligand-gated channel opens and closes in response to a specific chemical stimulus. A wide variety of chemical ligands including neurotransmitters, hormones, and particular ions can open or close ligand-gated channels. The neurotransmitter acetylcholine, for example, opens cation channels that allow Na+ and Ca2+ to diffuse inward and K+ to diffuse outward; (2) A mechanically-gated channel opens in response to a physical distortion of the cell membrane. Many channels associated with the sense of touch (somatosensation) are mechanically gated. Examples of mechanically gated channels are those found in auditory receptors in the ears, in receptors that monitor the stretching of internal organs, and in touch receptors and pressure receptors in the skin. (3) A voltage-gated channel is a channel that responds to changes in the electrical properties of the membrane in which it is embedded. Voltage-gated channels participate in the generation and conduction of action potentials. (4) A leakage channel is randomly gated. There is no actual event that opens the channel; instead, it has an intrinsic rate of switching between the open and closed states. Plasma membranes have many more K+ leakage channels than Na+ leakage channels, and the potassium ion leakage channels are leakier than the sodium ion leakage channels. Thus, the membrane’s permeability to K+ is much higher than its permeability to Na+. Leakage channels contribute to the resting transmembrane voltage of the excitable membrane. The resting membrane potential of a neuron is -70mV which describes the steady-state of a cell which is a dynamic process that is balanced by ion transport across the membrane [30, 33].
Action Potential
An action potential or impulse is a sequence of rapidly occurring events that lead to depolarization and subsequent repolarization of the neuronal membrane (Fig. 5). In the presence of a stimulus, the voltage-gated Na+ channels open and the ions rush into the cell driven by the concentration gradient. The potential difference rises from -70mV to 0mV to +30mV and is called the depolarization phase. This increase in voltage causes the opening of voltage-gated K+ channels and ions to accelerate out of the cell and the simultaneous closure of Na+. This causes the voltage to revert to -70mV and is called repolarization. Sometimes the K+ channels open for a prolonged period leading to the plunge of the voltage to -90mV called the after-hyperpolarization phase. The voltage returns to -70mV resting potential once the K+ channels close. An action potential occurs in the membrane of the axon of a neuron when depolarization reaches a certain level termed the threshold (about -55 mV) and the stimulus is termed threshold stimulus. While an action potential is in progress, another one cannot be initiated. That effect is referred to as the refractory period. There are two phases of the refractory period: the absolute refractory period and the relative refractory period. During the absolute phase, another action potential will not start. Once that channel is back to its resting conformation (less than -55 mV), a new action potential could be started, but only by a stronger stimulus than the one that initiated the current action potential. This is because of the flow of K+ out of the cell. Because that ion is rushing out, any Na+ that tries to enter will not depolarize the cell, but will only keep the cell from hyperpolarizing. This is termed as the relative refractory period
