human nervous system
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The central nervous system receives sensory information from the peripheral nervous system and …
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Prenatal and postnatal development of the human nervous system
Almost all nerve cells, or neurons, are generated during prenatal life, and in most cases they are not replaced by new neurons thereafter. Morphologically, the nervous system first appears about 18 days after conception, with the genesis of a neural plate. Functionally, it appears with the first sign of a reflex activity during the second prenatal month, when stimulation by touch of the upper lip evokes a withdrawal response of the head. Many reflexes of the head, trunk, and extremities can be elicited in the third month.
During its development the nervous system undergoes remarkable changes to attain its complex organization. In order to produce the estimated 1 trillion neurons present in the mature brain, an average of 2.5 million neurons must be generated per minute during the entire prenatal life. This includes the formation of neuronal circuits comprising 100 trillion synapses, as each potential neuron is ultimately connected with either a selected set of other neurons or specific targets such as sensory endings. Moreover, synaptic connections with other neurons are made at precise locations on the cell membranes of target neurons. The totality of these events is not thought to be the exclusive product of the genetic code, for there are simply not enough genes to account for such complexity. Rather, the differentiation and subsequent development of embryonic cells into mature neurons and glial cells are achieved by two sets of influences: (1) specific subsets of genes and (2) environmental stimuli from within and outside the embryo. Genetic influences are critical to the development of the nervous system in ordered and temporally timed sequences. Cell differentiation, for example, depends on a series of signals that regulate transcription, the process in which deoxyribonucleic acid (DNA) molecules give rise to ribonucleic acid (RNA) molecules, which in turn express the genetic messages that control cellular activity. Environmental influences derived from the embryo itself include cellular signals that consist of diffusible molecular factors (see below Neuronal development). External environmental factors include nutrition, sensory experience, social interaction, and even learning. All of these are essential for the proper differentiation of individual neurons and for fine-tuning the details of synaptic connections. Thus, the nervous system requires continuous stimulation over an entire lifetime in order to sustain functional activity.
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"nervous system, human." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 4 Mar. 2008 <http://www.britannica.com/eb/article-9110704>. - APA style:
nervous system, human. ( 2008). In Encyclopædia Britannica. Retrieved March 4, 2008, from Encyclopædia Britannica Online: http://www.britannica.com/eb/article-9110704
human nervous system
Neuronal development
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Development of the human embryo. Embryo of 18 days at disk or shield stage, (Ja) …
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In the second week of prenatal life, the rapidly growing blastocyst (the bundle of cells into which a fertilized ovum divides) flattens into what is called the embryonic disk. The embryonic disk soon acquires three layers: the ectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer). Within the mesoderm grows the notochord, an axial rod that serves as a temporary backbone. Both the mesoderm and notochord release a chemical that instructs and induces adjacent undifferentiated ectoderm cells to thicken along what will become the dorsal midline of the body, forming the neural plate. The neural plate is composed of neural precursor cells, known as neuroepithelial cells, which develop into the neural tube (see below Morphological development). Neuroepithelial cells then commence to divide, diversify, and give rise to immature neurons and neuroglia, which in turn migrate from the neural tube to their final location. Each neuron forms dendrites and an axon; axons elongate and form branches, the terminals of which form synaptic connections with a select set of target neurons or muscle fibres.
The remarkable events of this early development involve an orderly migration of billions of neurons, the growth of their axons (many of which extend widely throughout the brain), and the formation of thousands of synapses between individual axons and their target neurons. The migration and growth of neurons are dependent, at least in part, on chemical and physical influences. The growing tips of axons (called growth cones) apparently recognize and respond to various molecular signals, which guide axons and nerve branches to their appropriate targets and eliminate those that try to synapse with inappropriate targets. Once a synaptic connection has been established, a target cell releases a trophic factor (e.g., nerve growth factor) that is essential for the survival of the neuron synapsing with it. Physical guidance cues are involved in contact guidance, or the migration of immature neurons along a scaffold of glial fibres.
In some regions of the developing nervous system, synaptic contacts are not initially precise or stable and are followed later by an ordered reorganization, including the elimination of many cells and synapses. The instability of some synaptic connections persists until a so-called critical period is reached, prior to which environmental influences have a significant role in the proper differentiation of neurons and in fine-tuning many synaptic connections. Following the critical period, synaptic connections become stable and are unlikely to be altered by environmental influences. This suggests that certain skills and sensory activities can be influenced during development (including postnatal life), and for some intellectual skills this adaptability presumably persists into adulthood and late life.
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human nervous system
Morphological development
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Prenatal and postnatal development of the human nervous system > Morphological development
Development of the human embryo. Embryo of 23 days showing (K) growth of the amnion, (L) …
Encyclopædia Britannica, Inc.
By 18 days after fertilization, the ectoderm of the embryonic disk thickens along what will become the dorsal midline of the body, forming the neural plate and, slightly later, the primordial eye, ear, and nose. The neural plate elongates, and its lateral edges rise and unite in the midline to form the neural tube, which will develop into the central nervous system. The neural tube detaches from the skin ectoderm and sinks beneath the surface. At this stage, groupings of ectodermal cells, called neural crests, develop as a column on each side of the neural tube. The cephalic (head) portion of the neural tube differentiates into the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain), and the caudal portion becomes the spinal cord. The neural crests develop into most of the elements (e.g., ganglia and nerves) of the peripheral nervous system. This stage is reached at the end of the first embryonic month.
The cells of the central nervous system originate from the ventricular zone of the neural tube—that is, the layer of neuroepithelial cells lining the central cavity of the tube. These cells differentiate and proliferate into neuroblasts, which are the precursors of neurons, and glioblasts, from which neuroglia develop. With a few exceptions, the neuroblasts, glioblasts, and their derived cells do not divide and multiply once they have migrated from the ventricular zone into the gray and white matter of the nervous system. Most neurons are generated before birth, although not all are fully differentiated. (One exception is the neurons of the olfactory nerve, which are generated continuously throughout life.) This effectively implies that an individual is born with a full complement of nerve cells.
By mid-fetal life the slender primordial brain of the neural-tube stage differentiates into a globular-shaped brain. Although fully mature size and shape are not obtained until puberty, the main outlines of the brain are recognizable by the end of the third fetal month. This early development is the product of several factors: the formation of three flexures (cephalic, pontine, and cervical); the differential enlargement of various regions, especially the cerebrum and the cerebellum; the massive growth of the cerebral hemispheres over the sides of the midbrain and of the cerebellum at the hindbrain; and the formations of convolutions (sulci and gyri) in the cerebral cortex and folia of the cerebellar cortex. The central and calcarine sulci are discernible by the fifth fetal month, and all major gyri and sulci are normally present by the seventh month. Many minor sulci and gyri appear after birth.
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human nervous system
Postnatal changes
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Prenatal and postnatal development of the human nervous system > Postnatal changes
The postnatal growth of the human brain is rapid and massive, especially during the first two years. By two years after birth, the size of the brain and the proportion of its parts are basically those of an adult. The typical brain of a full-term infant weighs 350 grams (12 ounces) at birth, 1,000 grams at the end of the first year, about 1,300 grams at puberty, and about 1,500 grams at adulthood. This increase is attributable mainly to the growth of preexisting neurons, new glial cells, and the myelination of axons. The trebling of weight during the first year (a growth rate unique to humans) may be an adaptation that is essential to the survival of humans as a species with a large brain. Birth occurs at a developmental stage when the infant is not so helpless as to be unable to survive, yet is small enough to be delivered out of the maternal pelvis. If the brain was much larger (enough, say, to support intelligent behaviour), normal delivery would not be possible.
Between the ages of 20 and 75, it is estimated that an average of 50,000 neurons atrophy or die each day. In a healthy person, this loss is roughly equal to 10 percent of the original neuronal complement. By the age of 75, the weight of the brain is reduced from its maximum at maturity by about one-tenth, the flow of blood through the brain by almost one-fifth, and the number of functional taste buds by about two-thirds. A loss of neurons does not necessarily imply a comparable loss of function; however, some loss may be compensated for by the formation from viable neurons of new branches of nerve fibres and by the formation of new synapses.
Charles R. Noback
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human nervous system
The central nervous system
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Anatomy of the human nervous system > The central nervous system
Lateral view of the right cerebral hemisphere of the human brain, shown in situ within the skull. A …
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The central nervous system consists of the brain and spinal cord, both derived from the embryonic neural tube. Both are surrounded by protective membranes called the meninges, and both float in a crystal-clear cerebrospinal fluid. The brain is encased in a bony vault, the neurocranium, while the cylindrical and elongated spinal cord lies in the vertebral canal, which is formed by successive vertebrae connected by dense ligaments.
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human nervous system
The brain
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Anatomy of the human nervous system > The central nervous system > The brain
Medial view of the left hemisphere of the human brain.
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The brain weighs about 1,500 grams (3 pounds) and constitutes about 2 percent of total body weight. It consists of three major divisions: (1) the massive paired hemispheres of the cerebrum, (2) the brainstem, consisting of the thalamus, hypothalamus, epithalamus, subthalamus, midbrain, pons, and medulla oblongata, and (3) the cerebellum.
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"nervous system, human." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 4 Mar. 2008 <http://www.britannica.com/eb/article-75529>. - APA style:
nervous system, human. ( 2008). In Encyclopædia Britannica. Retrieved March 4, 2008, from Encyclopædia Britannica Online: http://www.britannica.com/eb/article-75529
human nervous system
Cerebrum
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Anatomy of the human nervous system > The central nervous system > The brain > Cerebrum
Dissection of the left hemisphere of the brain, showing the internal capsule and middle cerebellar …
Original preparation by J. Klingler, Anatomical Museum, Basel, Switz.
The cerebrum, derived from the telencephalon, is the largest, uppermost portion of the brain. It is involved with sensory integration, control of voluntary movement, and higher intellectual functions, such as speech and abstract thought. The outer layer of the duplicate cerebral hemispheres is composed of a convoluted (wrinkled) outer layer of gray matter, called the cerebral cortex. Beneath the cerebral cortex is an inner core of white matter, which is composed of myelinated commissural nerve fibres connecting the cerebral hemispheres via the corpus callosum, and association fibres connecting different regions of a single hemisphere. Myelinated fibres projecting to and from the cerebral cortex form a concentrated fan-shaped band, known as the internal capsule. The internal capsule consists of an anterior limb and a larger posterior limb and is abruptly curved, with the apex directed toward the centre of the brain; the junction is called the genu. The cerebrum also contains groups of subcortical neuronal masses known as basal ganglia.
Upper surface of the brain, showing division of the cerebrum into two hemispheres by the …
From N. Gluhbegovic and T.H. Williams, The Human Brain: A Photographic Guide (1980), J.B. Lippincott Co./Harper & Row
The cerebral hemispheres are partially separated from each other by a deep groove called the longitudinal fissure. At the base of the longitudinal fissure lies a thick band of white matter called the corpus callosum. The corpus callosum provides a communication link between corresponding regions of the cerebral hemispheres.
Left lateral surface of the brain, showing various lobes of the hemisphere.
From N. Gluhbegovic and T.H. Williams, The Human Brain: A Photographic Guide (1980), J.B. Lippincott Co./Harper & Row
Each cerebral hemisphere supplies motor function to the opposite, or contralateral, side of the body from which it receives sensory input. In other words, the left hemisphere controls the right half of the body, and vice versa. Each hemisphere also receives impulses conveying the senses of touch and vision, largely from the contralateral half of the body, while auditory input comes from both sides. Pathways conveying the senses of smell and taste to the cerebral cortex are ipsilateral (that is, they do not cross to the opposite hemisphere).
In spite of this arrangement, the cerebral hemispheres are not functionally equal. In each individual, one hemisphere is dominant. The dominant hemisphere controls language, mathematical and analytical functions, and handedness. The nondominant hemisphere controls simple spatial concepts, recognition of faces, some auditory aspects, and emotion. (For further discussion of cerebral dominance, see below Functions of the human nervous system: Higher cerebral functions.)
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human nervous system
Lobes of the cerebral cortex
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Anatomy of the human nervous system > The central nervous system > The brain > Cerebrum > Lobes of the cerebral cortex
Lateral view of the right cerebral hemisphere of the human brain, shown in situ within the skull. A …
Encyclopædia Britannica, Inc.
The cerebral cortex is highly convoluted; the crest of a single convolution is known as a gyrus, and the fissure between two gyri is known as a sulcus. Sulci and gyri form a more or less constant pattern, on the basis of which the surface of each cerebral hemisphere is commonly divided into four lobes: (1) frontal, (2) parietal, (3) temporal, and (4) occipital. Two major sulci located on the lateral, or side, surface of each hemisphere distinguish these lobes. The central sulcus, or fissure of Rolando, separates the frontal and parietal lobes, and the deeper lateral sulcus, or fissure of Sylvius, forms the boundary between the temporal lobe and the frontal and parietal lobes.
Functional areas of the human brain.
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The frontal lobe, the largest of the cerebral lobes, lies rostral to the central sulcus (that is, toward the nose from the sulcus). One important structure in the frontal lobe is the precentral gyrus, which constitutes the primary motor region of the brain. When parts of the gyrus are electrically stimulated in concious patients (under local anesthesia), they produce localized movements on the opposite side of the body that are interpreted by the patients as voluntary. Injury to parts of the precentral gyrus results in paralysis on the contralateral half of the body. Parts of the inferior frontal lobe (close to the lateral sulcus) constitute the Broca area, a region involved with speech (see below Functions of the human nervous system: Language).
The parietal lobe, posterior to the central sulcus, is divided into three parts: (1) the postcentral gyrus, (2) the superior parietal lobule, and (3) the inferior parietal lobule. The postcentral gyrus receives sensory input from the contralateral half of the body. The sequential representation is the same as in the primary motor area, with sensations from the head being represented in inferior parts of the gyrus and impulses from the lower extremities being represented in superior portions. The superior parietal lobule, located caudal to (that is, below and behind) the postcentral gyrus, lies above the intraparietal sulcus. This lobule is regarded as an association cortex, an area that is not involved in either sensory or motor processing, although part of the superior parietal lobule may be concerned with motor function. The inferior parietal lobule (composed of the angular and supramarginal gyri) is a cortical region involved with the integration of multiple sensory signals.
In both the parietal and frontal lobes, each primary sensory or motor area is close to, or surrounded by, a smaller secondary area. The primary sensory area receives input only from the thalamus, while the secondary sensory area receives input from the thalamus, the primary sensory area, or both. The motor areas receive input from the thalamus as well as the sensory areas of the cerebral cortex.
The temporal lobe, inferior to the lateral sulcus, fills the middle fossa, or hollow area, of the skull. The outer surface of the temporal lobe is an association area made up of the superior, middle, and inferior temporal gyri. Near the margin of the lateral sulcus, two transverse temporal gyri constitute the primary auditory area of the brain. The sensation of hearing is represented here in a tonotopic fashion—that is, with different frequencies represented on different parts of the area. The transverse gyri are surrounded by a less finely tuned secondary auditory area. A medial, or inner, protrusion near the ventral surface of the temporal lobe, known as the uncus, constitutes a large part of the primary olfactory area.
The occipital lobe lies caudal to the parieto-occipital sulcus, which joins the calcarine sulcus in a Y-shaped formation. Cortex on both banks of the calcarine sulcus constitutes the primary visual area, which receives input from the contralateral visual field via the optic radiation. The visual field is represented near the calcarine sulcus in a retinotopic fashion—that is, with upper quadrants of the visual field laid out along the inferior bank of the sulcus and lower quadrants of the visual field represented on the upper bank. Central vision is represented mostly caudally and peripheral vision rostrally.
Not visible from the surface of the cerebrum is the insular, or central, lobe, an invaginated triangular area on the medial surface of the lateral sulcus; it can be seen in the intact brain only by separating the frontal and parietal lobes from the temporal lobe. The insular lobe is thought to be involved in sensory and motor visceral functions as well as taste perception.
The limbic lobe is a synthetic lobe located on the medial margin (or limbus) of the hemisphere. Composed of adjacent portions of the frontal, parietal, and temporal lobes that surround the corpus callosum, the limbic lobe is involved with autonomic and related somatic behavioral activities. The limbic lobe receives input from thalamic nuclei that are connected with parts of the hypothalamus and with the hippocampal formation, a primitive cortical structure within the inferior horn of the lateral ventricle.
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nervous system, human. ( 2008). In Encyclopædia Britannica. Retrieved March 4, 2008, from Encyclopædia Britannica Online: http://www.britannica.com/eb/article-75531
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