The Nervous System: Overview
The Nervous System: Overview
Yahia BELLOUCHE
Keywords
Central Nervous System, Peripheral Nervous System, Autonomic Nervous system, neuron, dendrite, axon.
Cellular level of the Nervous System organization
Basing on the cell concept, the neuron (nervous cell) is recognized as the nervous system’s most fundamental unit, the origin of the majority of our brain functions. The neurons are polymorphic highly differentiated cells characterized -besides the standard subcellular components- by an extensive branching out of the cellular soma, while these branches (dendrites, axons) assure a transport function for signals from and out of the soma, this last, is the site of activity regulation, m
essage integration and mediators synthesis, these functions originate from an arsenal of genetically coded proteins and one of the most developed cytoskeletal systems in existence, which highlights the remarkable level of neurons specialization and explains the loss of regeneration capacity for most of neural cellular types known for some as the “Noble tissue”. In addition, the neurons are excitable cells, which implicates the existence of an ephemeral ionic conduction change leading to a set of ionic movements through a lipid bilayer membrane and a membrane potential values change defining a local depolarization in the centripetal compartment (dendrites and soma), and a self-maintained.
Adepolarization state called action potential in the centrifugal compartment (axons); while local potentials are mainly passive and of an electrotonic nature and later ones are only possible because of a group of specific ionic channels (Voltage dependent sodium and potassium channels mainly). Induced by a stimulus ( which can be electrical, mechanic, chemical or physical), this excitation is the platform of information coding and neural interactions, reinforced by some focal intercellular connections called synapses where the signal “jumps” from a cell to another through liberation of chemical substances in the synaptic cleft for most of them (some electrical synapses use direct cytosol connection), these linking points are the source of neural circuits diversity, in the other hand, the nervous system contains many other variable cellular types such as astrocytes, oligodendrocytes and microglia whose main role is to maintain the parameters of the microenvironment within the limits compatible with the neural functioning requirements and participate in neural interaction optimization by increasing conduction velocity and synaptogenesis dynamics within the CNS.
Histological /Anatomic levels
In a macroscopic view of the nervous system, we can distinguish two morphological entities within its different parts, a first, grey (called grey matter), hosts mostly the somas, which qualifies it to play a role of processing centers, while the other is white (White matter) where we can find the axons of the somas covered by a myelin sheath (enhancing the membrane’s capacitance) who plays mostly the role of linking cables.
Central Nervous System
Anatomically speaking, the nervous system is divided into two parts: The Central Nervous System (CNS) resulting from the organization of nervous tissue around a fluid core, ventricles (Brain), and continues with the medullary central cavity (Spinal cord). At the top, around the lateral ventricles, the cerebral hemispheres, with a grey multilobar cortex covering a white matter mass that include some grey nuclei called basal ganglia. Occupying the summit of neural functioning diagram, the cortex is the treating center for almost every neural impulse and the source of the output to peripheral effectors later on with the linking done by the white mass under it, with relative specialization of some areas, as of the basal ganglia, it has been established that they are (with the cerebellum) the filters of neural output towards motor effector especially. The diencephalon (regroups the thalamus, hypothalamus and the epithalamus) around the third ventricle, are mainly projection and cortical excitability regulators, except the hypothalamus, hormonal and metabolic regulation center through its inferior branch, the pituitary gland which interacts and commands almost every gland in thebody, under it, the brain stem, one of the most conserved structures in the animal world, with the midbrain, the pons and the medulla oblongata, the emergence locus for almost all cranial nerves with their respective nuclei and the regulating center for some basic functions such as cardiac and respiratory functions, finally, all nervous tracts from and to the forebrain pass by the brain stem. Behind, the cerebellum with a cerebrum-like but a smaller structure, delimiting the fourth ventricle; while the coordination and control of motor responses, equilibrium and unconscious proprioception are known functions of the cerebellum, its role in cognition and other emotional aspects are still beyond our understanding.
Continuing the medulla oblongata from the bottom, the spinal cord, the last part of the CNS, is a cylindrical bundle that extends for about forty-seven centimeters, organized in segments (each segment corresponds to the vertebrae of the spinal nerve exit), the spinal cord is the main direct center of command for limbs skeletal muscles by means of the motoneurons (contained in the anterior column) that receive the superior nervous impulses along with the peripheral input from the muscles and the sensory organs, and the relay of the majority of the peripheral sensory input through the different tracts heading towards the brain.
Giving the fragility of the nervous tissue, this last represents one of the most well-protected organs in the body, with some solid bony structures (skull and the vertebral column), a three layered membrane assures more flexible reinforcement of the bones (dura mater, pia mater and arachnoid mater), finally, the CNS is also provided with a shock absorbing apparatus (the fluid core) filled with the cerebrospinal fluid, which has for a role, nutrition and immunity of the nervous system also.
Peripheral Nervous System / Autonomic Nervous System
Outside the CNS, some nervous structures exist, englobing the nerves (a pack of neural fibers -axons- enclosed in a connective sheath along with some blood vessels, assuring a bidirectional impulse transport from and to the CNS) cranial (originating from the brain stem to innervate the cranial side of the body) or spinal (originating from the fusion of the anterior and posterior spinal roots) they can be , the ganglia (housing neural somas and synapses) which are assimilated to peripheral regional treating centers and the peripheral sensory organs(scattered in different organs and tissues) along with the effectors (glands and muscles mainly). All these structure are regrouped under the name of the Peripheral Nervous System.
This third division is, in reality, more of a functional division, the ANS combines structure from both the CNS (Limbic system, hypothalamus and the lateral column of the spinal cord) and the PNS (ganglia and nerves), with its two branches, sympathetic and parasympathetic , it regulates unconsciously the heart and respiratory rates, digestion rhythm and the general state of our bodies (overall excitation, pupillary diameter adjustments) basing on the visceral feed-back (mechanic, osmotic, barometric and chemo-receptors in smooth muscles, heart and the different organs) and nature of our surroundings, determining the classic reactions of “Fight and Flight” or “Feed and breed”. The ANS is implicated on many other functions, unclear for the most, but the latest results suggest a role in systemic nervous coordination and even, cognition.
As stated before, the nervous system accomplishes a wide range of tasks, and for that, a single cell isolated model isn’t likely functional, as a matter of fact, the overall functioning of the CNS is based on the interaction between many populations of neurons, with a network where each neuron receives (by the dendrites) and emits (with the axon) millions of electrochemical impulses every day, the nature of this connections (linear, loops, divergent or convergent), their number and the nature of the synapses (inhibitory or excitatory) and the specialization of the composing elements determine the nature of these circuits, taking for instance the simple well studied reflex arc, an external dangerous stimulus causes the stimulation of a peripheral sensory organ in the tegument, the impulse is transmitted to the corresponding segment of the spinal cord through the sensory nerve, which (supposing a monosynaptic reflex) -after entering the spinal cord- will transmit an excitatory message to the motoneuron that will cause muscle contraction after receiving the output through the motor nerve, the circuits of other functions are more complicated, including polysynaptic, divergent and many nervous centers interactions generating tons of intrinsic electrical discharge patterns. From this analysis, we can distinguish three functional sectors of the nervous system that work in a linear way: Sensory, integrative and motor sectors.
Sensory Nervous System
It is the body’s main tool to understand what happens in the surroundings (environment) and inside of it (interior milieu), the sensory nervous system is a set of receptors that code the data they receive about the corresponding stimuli (and their parameters) into an electrical impulse in a process called transduction. The receptors are specialized structures with the ability to receive certain perturbations (pressure, vibration, light, sounds, heat…..), to recognize them, and to define their parameters (intensity, modality and topography) to eventually, transform this data into a wave of action potentials to be transmitted later on, to the corresponding treating areas in the CNS, while the function model of some modalities is relatively simple (like somatic sensory receptors), nothing is simple about the functioning of others (the eye for instance, contains more than seven neural populations and connects with almost every structure in the superior CNS).
Integrative Nervous System
Here again, the activity of neural circuits is on its best, after the arrival of sensory input to the concerned areas, a data treatment sequence is initiated, implicating the interaction of many neural populations, pre-determined program sequences comparison and other sensory modalities feedback, the CNS comes to the adequate responses and determines the modification and the adjustments compatible with the current state, this complex process is called integration and unlike the relatively clear lateralization and somatotopic arrangement in the sensory and the motor sector of the nervous system, the integrative function is very heterogeneous and has little respect to principles of area specialization in the CNS, to this special complex sector, attributed most of the high mental processes of memory, emotions (limbic system), language and abstract thinking, faculties englobed by the name of cognition.
Motor Nervous System After determining the output, the CNS sends back these adjustments via the motor pathways to the effectors (Muscles or glands mainly) in order to keep the integrity of the body and maintain the different biological parameters of the internal milieu within the limits of physiological conditions. Worth mentioning at last, that nothing is absolute about all of these divisions, as of the appearance of new proofs every day, especially in Neuroscience research fields, about new different functioning patterns, which can only support the complexity of this system and the extend of its adaptation capacities, and new model is emerging more today, one that suggests a combination of area specialization and overall discharge patterns, a model, that only days can confirm or refute.
References
- Arthur F. Dalley, Keith L. Moore, Anne M.R. Agur (2010). Clinically oriented anatomy (6th ed., [International ed.]. ed.). Philadelphia [etc.]: Lippincott Williams & Wilkins, Wolters Kluwer. ISBN 0-652-60547-1-978. - Bear, Mark F.; Barry W. Connors; Michael A. Paradiso (2007). Neuroscience: Exploring the Brain: Third Edition. Philadelphia, PA, USA: Lippincott Williams & Wilkins. ISBN 4-6003-7817-0-978. - Campbell, Neil A.; Jane B. Reece; Lisa A. Urry; Michael L. Cain; Steven A. Wasserman; Peter V. Minorsky; Robert B. Jackson (2008). Biology: Eighth Edition. San Francisco, CA, USA: Pearson / Benjamin Cummings. ISBN 4-6844-8053-0-978. - Kandel ER, Schwartz JH (2012). Principles of neural science (5. ed.). Appleton & Lange: McGraw Hill. ISBN 8-139011-07-0-978. - Kent, George C.; Robert K. Carr (2001). Comparative Anatomy of the Vertebrates: Ninth Edition. New York, NY, USA: McGraw-Hill Higher Education. ISBN 5-303869-07-0. - Maton, Anthea; Jean Hopkins; Charles William McLaughlin; Susan Johnson; Maryanna Quon Warner; David LaHart; Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 1-981176-13-0. - Romer, A.S. (1949): The Vertebrate Body. W.B. Saunders, Philadelphia. (2nd ed. 3 ;1955rd ed. 4 ;1962th ed. 1970).
Adepolarization state called action potential in the centrifugal compartment (axons); while local potentials are mainly passive and of an electrotonic nature and later ones are only possible because of a group of specific ionic channels (Voltage dependent sodium and potassium channels mainly). Induced by a stimulus ( which can be electrical, mechanic, chemical or physical), this excitation is the platform of information coding and neural interactions, reinforced by some focal intercellular connections called synapses where the signal “jumps” from a cell to another through liberation of chemical substances in the synaptic cleft for most of them (some electrical synapses use direct cytosol connection), these linking points are the source of neural circuits diversity, in the other hand, the nervous system contains many other variable cellular types such as astrocytes, oligodendrocytes and microglia whose main role is to maintain the parameters of the microenvironment within the limits compatible with the neural functioning requirements and participate in neural interaction optimization by increasing conduction velocity and synaptogenesis dynamics within the CNS.
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| Schéma de Hafida KRIDI |
Histological /Anatomic levels
In a macroscopic view of the nervous system, we can distinguish two morphological entities within its different parts, a first, grey (called grey matter), hosts mostly the somas, which qualifies it to play a role of processing centers, while the other is white (White matter) where we can find the axons of the somas covered by a myelin sheath (enhancing the membrane’s capacitance) who plays mostly the role of linking cables.
Central Nervous System
Anatomically speaking, the nervous system is divided into two parts: The Central Nervous System (CNS) resulting from the organization of nervous tissue around a fluid core, ventricles (Brain), and continues with the medullary central cavity (Spinal cord). At the top, around the lateral ventricles, the cerebral hemispheres, with a grey multilobar cortex covering a white matter mass that include some grey nuclei called basal ganglia. Occupying the summit of neural functioning diagram, the cortex is the treating center for almost every neural impulse and the source of the output to peripheral effectors later on with the linking done by the white mass under it, with relative specialization of some areas, as of the basal ganglia, it has been established that they are (with the cerebellum) the filters of neural output towards motor effector especially. The diencephalon (regroups the thalamus, hypothalamus and the epithalamus) around the third ventricle, are mainly projection and cortical excitability regulators, except the hypothalamus, hormonal and metabolic regulation center through its inferior branch, the pituitary gland which interacts and commands almost every gland in thebody, under it, the brain stem, one of the most conserved structures in the animal world, with the midbrain, the pons and the medulla oblongata, the emergence locus for almost all cranial nerves with their respective nuclei and the regulating center for some basic functions such as cardiac and respiratory functions, finally, all nervous tracts from and to the forebrain pass by the brain stem. Behind, the cerebellum with a cerebrum-like but a smaller structure, delimiting the fourth ventricle; while the coordination and control of motor responses, equilibrium and unconscious proprioception are known functions of the cerebellum, its role in cognition and other emotional aspects are still beyond our understanding.
Continuing the medulla oblongata from the bottom, the spinal cord, the last part of the CNS, is a cylindrical bundle that extends for about forty-seven centimeters, organized in segments (each segment corresponds to the vertebrae of the spinal nerve exit), the spinal cord is the main direct center of command for limbs skeletal muscles by means of the motoneurons (contained in the anterior column) that receive the superior nervous impulses along with the peripheral input from the muscles and the sensory organs, and the relay of the majority of the peripheral sensory input through the different tracts heading towards the brain.
Giving the fragility of the nervous tissue, this last represents one of the most well-protected organs in the body, with some solid bony structures (skull and the vertebral column), a three layered membrane assures more flexible reinforcement of the bones (dura mater, pia mater and arachnoid mater), finally, the CNS is also provided with a shock absorbing apparatus (the fluid core) filled with the cerebrospinal fluid, which has for a role, nutrition and immunity of the nervous system also.
Peripheral Nervous System / Autonomic Nervous System
Outside the CNS, some nervous structures exist, englobing the nerves (a pack of neural fibers -axons- enclosed in a connective sheath along with some blood vessels, assuring a bidirectional impulse transport from and to the CNS) cranial (originating from the brain stem to innervate the cranial side of the body) or spinal (originating from the fusion of the anterior and posterior spinal roots) they can be , the ganglia (housing neural somas and synapses) which are assimilated to peripheral regional treating centers and the peripheral sensory organs(scattered in different organs and tissues) along with the effectors (glands and muscles mainly). All these structure are regrouped under the name of the Peripheral Nervous System.
This third division is, in reality, more of a functional division, the ANS combines structure from both the CNS (Limbic system, hypothalamus and the lateral column of the spinal cord) and the PNS (ganglia and nerves), with its two branches, sympathetic and parasympathetic , it regulates unconsciously the heart and respiratory rates, digestion rhythm and the general state of our bodies (overall excitation, pupillary diameter adjustments) basing on the visceral feed-back (mechanic, osmotic, barometric and chemo-receptors in smooth muscles, heart and the different organs) and nature of our surroundings, determining the classic reactions of “Fight and Flight” or “Feed and breed”. The ANS is implicated on many other functions, unclear for the most, but the latest results suggest a role in systemic nervous coordination and even, cognition.
As stated before, the nervous system accomplishes a wide range of tasks, and for that, a single cell isolated model isn’t likely functional, as a matter of fact, the overall functioning of the CNS is based on the interaction between many populations of neurons, with a network where each neuron receives (by the dendrites) and emits (with the axon) millions of electrochemical impulses every day, the nature of this connections (linear, loops, divergent or convergent), their number and the nature of the synapses (inhibitory or excitatory) and the specialization of the composing elements determine the nature of these circuits, taking for instance the simple well studied reflex arc, an external dangerous stimulus causes the stimulation of a peripheral sensory organ in the tegument, the impulse is transmitted to the corresponding segment of the spinal cord through the sensory nerve, which (supposing a monosynaptic reflex) -after entering the spinal cord- will transmit an excitatory message to the motoneuron that will cause muscle contraction after receiving the output through the motor nerve, the circuits of other functions are more complicated, including polysynaptic, divergent and many nervous centers interactions generating tons of intrinsic electrical discharge patterns. From this analysis, we can distinguish three functional sectors of the nervous system that work in a linear way: Sensory, integrative and motor sectors.
Sensory Nervous System
It is the body’s main tool to understand what happens in the surroundings (environment) and inside of it (interior milieu), the sensory nervous system is a set of receptors that code the data they receive about the corresponding stimuli (and their parameters) into an electrical impulse in a process called transduction. The receptors are specialized structures with the ability to receive certain perturbations (pressure, vibration, light, sounds, heat…..), to recognize them, and to define their parameters (intensity, modality and topography) to eventually, transform this data into a wave of action potentials to be transmitted later on, to the corresponding treating areas in the CNS, while the function model of some modalities is relatively simple (like somatic sensory receptors), nothing is simple about the functioning of others (the eye for instance, contains more than seven neural populations and connects with almost every structure in the superior CNS).
Integrative Nervous System
Here again, the activity of neural circuits is on its best, after the arrival of sensory input to the concerned areas, a data treatment sequence is initiated, implicating the interaction of many neural populations, pre-determined program sequences comparison and other sensory modalities feedback, the CNS comes to the adequate responses and determines the modification and the adjustments compatible with the current state, this complex process is called integration and unlike the relatively clear lateralization and somatotopic arrangement in the sensory and the motor sector of the nervous system, the integrative function is very heterogeneous and has little respect to principles of area specialization in the CNS, to this special complex sector, attributed most of the high mental processes of memory, emotions (limbic system), language and abstract thinking, faculties englobed by the name of cognition.
Motor Nervous System After determining the output, the CNS sends back these adjustments via the motor pathways to the effectors (Muscles or glands mainly) in order to keep the integrity of the body and maintain the different biological parameters of the internal milieu within the limits of physiological conditions. Worth mentioning at last, that nothing is absolute about all of these divisions, as of the appearance of new proofs every day, especially in Neuroscience research fields, about new different functioning patterns, which can only support the complexity of this system and the extend of its adaptation capacities, and new model is emerging more today, one that suggests a combination of area specialization and overall discharge patterns, a model, that only days can confirm or refute.
References
- Arthur F. Dalley, Keith L. Moore, Anne M.R. Agur (2010). Clinically oriented anatomy (6th ed., [International ed.]. ed.). Philadelphia [etc.]: Lippincott Williams & Wilkins, Wolters Kluwer. ISBN 0-652-60547-1-978. - Bear, Mark F.; Barry W. Connors; Michael A. Paradiso (2007). Neuroscience: Exploring the Brain: Third Edition. Philadelphia, PA, USA: Lippincott Williams & Wilkins. ISBN 4-6003-7817-0-978. - Campbell, Neil A.; Jane B. Reece; Lisa A. Urry; Michael L. Cain; Steven A. Wasserman; Peter V. Minorsky; Robert B. Jackson (2008). Biology: Eighth Edition. San Francisco, CA, USA: Pearson / Benjamin Cummings. ISBN 4-6844-8053-0-978. - Kandel ER, Schwartz JH (2012). Principles of neural science (5. ed.). Appleton & Lange: McGraw Hill. ISBN 8-139011-07-0-978. - Kent, George C.; Robert K. Carr (2001). Comparative Anatomy of the Vertebrates: Ninth Edition. New York, NY, USA: McGraw-Hill Higher Education. ISBN 5-303869-07-0. - Maton, Anthea; Jean Hopkins; Charles William McLaughlin; Susan Johnson; Maryanna Quon Warner; David LaHart; Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 1-981176-13-0. - Romer, A.S. (1949): The Vertebrate Body. W.B. Saunders, Philadelphia. (2nd ed. 3 ;1955rd ed. 4 ;1962th ed. 1970).
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