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The nervous system is a highly specialized network whose principal components are cells called neurons. Neurons are interconnected to each other in complex arrangements, and have the property of conducting, using electrochemical signals, a great variety of stimuli both within the nervous tissue as well as from and towards most of the other tissues. Thus, neurons coordinate multiple functions in organisms.

Neuroscience Portal

The Human Nervous System. Red is CNS and blue is PNS.

Nervous systems are found in many multicellular animals but differ greatly in complexity between species.

Contents 1 Nervous system in humans 1.1 Central nervous system 1.2 Peripheral nervous system 2 Microanatomy 2.1 Neurons 2.2 Glial cells 3 Physiological division 4 Development 5 Non-humans 5.1 Vertebrates 5.2 Worms 5.3 Arthropoda 5.4 Development 6 External links

Nervous system in humans

The human nervous system can be observed both with gross anatomy, (which describes the parts that are large enough to be seen with the plain eye,) and microanatomy, (which describes the system at a cellular level.) At gross anatomy, the nervous system can be grouped in distinct organs, these being actually stations which the neural pathways cross through. Thus, with a didactical purpose, these organs, according to their ubication, can be divided in two parts: the central nervous system (CNS) and the peripheral nervous system (PNS).

Central nervous system

Main article: Central nervous system

See also: List of regions in the human brain

The central nervous system (CNS) represents the largest part of the nervous system, including the brain and the spinal cord. The CNS is contained within the dorsal cavity, with the brain within the cranial cavity, and the spinal cord in the spinal cavity. The CNS is covered by the meninges. The brain is also protected by the skull, and the spinal cord is also protected by the vertebrae. The nervous system can be connected into many systems that can function together. The two systems are central nervous system (CNS)and the peripheral nervous system (PNS). Italic text\'\'\'\'Bold text\'\'\'

Central nervous system	Brain	Prosencephalon	Telencephalon	Rhinencephalon, Amygdala, Hippocampus, Neocortex, Lateral ventricles
			Diencephalon	Epithalamus, Thalamus, Hypothalamus, Subthalamus, Pituitary gland, Pineal gland, Third ventricle
		Brain stem	Mesencephalon	Tectum, Cerebral peduncle, Pretectum, Mesencephalic duct
			Rhombencephalon	Metencephalon	Pons, Cerebellum,
				Myelencephalon	Medulla oblongata
	Spinal cord

Peripheral nervous system

Main article: Peripheral nervous system

The PNS consists of all the other nervous structures that do not lie within it. The large majority of what are commonly called nerves (which are actually axonal processes of nerve cells) are considered to be PNS.

Microanatomy

The nervous system is, on a small scale, primarily made up of neurons. However, glial cells also play a major role.

Neurons

Main article: Neuron

They are the core components of both the central nervous system & peripheral nervous system.

Glial cells

Main article: Glial cell

Glial cells are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system. In the human brain, glia are estimated to outnumber neurons by about 10 to 1.sfn.org Society for Neuroscience, 2000

Glial cells provide support and protection for neurons. They are thus known as the "glue" of the nervous system. The four main functions of glial cells are to surround neurons and hold them in place, to supply nutrients and oxygen to neurons, to insulate one neuron from another, and to destroy pathogens and remove dead neurons.

Physiological division

A less anatomical but much more functional division of the human nervous system is that classifying it according to the role that the different neural pathways play, regardless whether these cross through the CNS or the PNS:

The somatic nervous system is responsible for coordinating the body\'s movements, and also for receiving external stimuli. It is the system that regulates activities that are under conscious control.

The autonomic nervous system is then split into the sympathetic division, parasympathetic division, and enteric division. The sympathetic nervous system responds to impending danger or stress, and is responsible for the increase of one\'s heartbeat and blood pressure, among other physiological changes, along with the sense of excitement one feels due to the increase of adrenaline in the system. The parasympathetic nervous system, on the other hand, is evident when a person is resting and feels relaxed, and is responsible for such things as the constriction of the pupil, the slowing of the heart, the dilation of the blood vessels, and the stimulation of the digestive and genitourinary systems. The role of the enteric nervous system is to manage every aspect of digestion, from the esophagus to the stomach, small intestine and colon.

In turn, these pathways can be divided according to the direction in which they conduct stimuli:

• Afferent system by sensory neurons, which carry impulses from a receptor to the CNS
• Efferent system by motor neurons, which carry impulses from the CNS to an effector
• Relay system by relay neurons (also called interneurons), which transmit impulses between the sensory and motor neurones.

A useful mnemonic to remember the nature of Afferent vs Efferent is SAME DAVE: Sensory Afferent, Motor Efferent; Dorsal Afferent, Ventral Efferent

However, there are relay neurons in the CNS as well.

The junction between two neurones is called a synapse. There is a very narrow gap (about 20nm in width) between the neurons - the synaptic cleft, where an action potential is transmitted from one neuron to a neighboring one. They do this by relaying the message with the use of neurotransmitters which the next neuron then receives the electrical signal, known as a nerve impulse. The nerve impulse is determined by the neurotransmitter to then carry the message to its appropriate destination. These nerve impulses are a change in ion balance in the nerve cell, which the central nervous system can then interpret. The fact that the nervous system uses a mixture of electrical and chemical signals makes it incredibly fast, which is necessary to acknowledge the presence of danger. For example, a hand touching a hot stove. If the nervous system was only comprised of chemical signals, the body would not tell the arm to move fast enough to escape dangerous burns. So the speed of the nervous system is a necessity for life.

Physiological division	Somatic nervous system	Afferent system
		Efferent system
	Autonomic nervous system	Sympathetic	Afferent system
			Efferent system
		Parasympathetic	Afferent system
			Efferent system

Development

Main article: Neural development in humans

Some landmarks of embryonic neural development include the birth and differentiation of neurons from stem cell precursors, the migration of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of axons from neurons and guidance of the motile growth cone through the embryo towards postsynaptic partners, the generation of synapses between these axons and their postsynaptic partners, and finally the lifelong changes in synapses which are thought to underlie learning and memory.

Non-humans

Vertebrates

The nervous system of all vertebrate animals, is often divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord.

Worms

Planaria, a type of flatworm, have dual nerve cords running along the length of the body and merging at the tail and the mouth. These nerve cords are connected by transverse nerves like the rungs of a ladder. These transverse nerves help coordinate the two sides of the animal. Two large ganglia at the head end function similar to a simple brain. Photoreceptors on the animal\'s eyespots provide sensory information on light and dark.

The nervous system of the roundworm Caenorhabditis elegans has been mapped out to the cellular level. Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are known. In this species, the nervous system is sexually dimorphic; the nervous systems of the two sexes, males and hermaphrodites, have different numbers of neurons and groups of neurons that perform sex-specific functions. In C. elegans, males have exactly 383 neurons, while hermaphrodites have exactly 302 neurons [1]

Arthropoda

Arthropods, such as insects and crustaceans, have a nervous system made up of a series of ganglia, connected by a ventral nerve cord made up of two parallel connectives running along the length of the belly [2]. Typically, each body segment has one ganglion on each side, though some ganglia are fused to form the brain and other large ganglia [3].

The head segment contains the brain, also known as the supraesophageal ganglion. In the insect nervous system, the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum. Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia. It controls the mouthparts, the salivary glands and certain muscles.

Many arthropods have well-developed sensory organs, including compound eyes for vision and antennae for olfaction and pheromone sensation. The sensory information from these organs is processed by the brain.

Development

Main article: Neural development

Neural development in most species have many similarities neural development in humans.

External links

• Neuroscience for Kids
• The Human Brain Project Homepage

Reference:

• Kimball\'s Biology Pages, CNS
• Kimball\'s Biology Pages, PNS

v • d • e Human organ systems
Cardiovascular system • Digestive system • Endocrine system • Immune system • Integumentary system • Lymphatic system • Muscular system • Nervous system • Reproductive system • Respiratory system • Skeletal system • Urinary system

v • d • e Nervous system
Central nervous system (Brain, Spinal cord) • Peripheral nervous system • Somatic nervous system • Autonomic nervous system (Sympathetic, Parasympathetic) • Enteric nervous system • Sensory system

v • d • e Nervous system, receptors: somatosensory system
Medial lemniscus	Touch/mechanoreceptors: Pacinian corpuscles • Meissner\'s corpuscles • Merkel\'s discs • Ruffini endings • Free nerve endings • Hair cells • Baroreceptor Proprioception: Golgi organ • Muscle spindle (Intrafusal muscle fiber • Nuclear chain fiber • Nuclear bag fiber)
Spinothalamic tract	Pain: Nociceptors Temperature: Thermoreceptors

v • d • e Prenatal development/Mammalian development of nervous system
General neural development/ neurulation/neurula	Notochord - Neuroectoderm - Neural plate - Neural folds - Neural groove Neural crest - Neural tube (Neuromere/Rhombomere, Cephalic flexure) Alar plate - Basal plate
Eye development	Optic vesicles - Optic stalk - Optic cup - Lens placode
Auditory development	Auditory vesicle - Auditory pit

v • d • e Histology: nervous tissue
Neurons (gray matter)	soma, axon (axon hillock, axoplasm, axolemma, neurofibril/neurofilament), dendrite (Nissl body, dendritic spine, apical dendrite, basal dendrite) types (bipolar, pseudounipolar, multipolar, pyramidal, Purkinje, granule)
Afferent nerve/Sensory nerve/Sensory neuron	GSA, GVA, SSA, SVA, fibers (Ia, Ib or Golgi, II or Aβ, III or Aδ or fast pain, IV or C or slow pain)
Efferent nerve/Motor nerve/Motor neuron	GSE, GVE, SVE, Upper motor neuron, Lower motor neuron (α motorneuron, γ motorneuron)
Synapses	neuropil, synaptic vesicle, neuromuscular junction, electrical synapse - Interneuron (Renshaw)
Sensory receptors	Free nerve ending, Meissner\'s corpuscle, Merkel nerve ending, Muscle spindle, Pacinian corpuscle, Ruffini ending, Olfactory receptor neuron, Photoreceptor cell, Hair cell, Taste bud
Glial cells	astrocyte, oligodendrocyte, ependymal cells, microglia, radial glia
Myelination (white matter)	Schwann cell, oligodendrocyte, nodes of Ranvier, internode, Schmidt-Lanterman incisures, neurolemma
Related connective tissues	epineurium, perineurium, endoneurium, nerve fascicle, meninges

v • d • e Pathology of the nervous system, primarily CNS (G00-G47, 320-349)
Inflammatory diseases of the CNS	Meningitis (Arachnoiditis) - Encephalitis - Myelitis - Encephalomyelitis (Acute disseminated) - Tropical spastic paraparesis
Systemic atrophies primarily affecting the CNS	Huntington\'s disease - Spinocerebellar ataxia (Friedreich\'s ataxia, Ataxia telangiectasia, Hereditary spastic paraplegia) Spinal muscular atrophy: Werdnig-Hoffman disease - Kugelberg-Welander disease - Fazio Londe syndrome - MND (Amyotrophic lateral sclerosis (ALS), Progressive muscular atrophy (PMA), Progressive bulbar, Pseudobulbar, PLS)
Extrapyramidal and movement disorders	Parkinson\'s disease - Neuroleptic malignant syndrome - Postencephalitic parkinsonism - Pantothenate kinase-associated neurodegeneration - Progressive supranuclear palsy - Striatonigral degeneration - Dystonia (Spasmodic torticollis, Meige\'s syndrome, Blepharospasm) - Essential tremor - Myoclonus - Chorea (Choreoathetosis) - Restless legs syndrome - Stiff person syndrome
Other degenerative / demyelinating diseases	Alzheimer\'s disease - Pick\'s disease - Alpers\' disease - Dementia with Lewy bodies - Leigh\'s disease - Multiple sclerosis - Devic\'s disease - Central pontine myelinolysis - Transverse myelitis
Seizure/epilepsy	Focal (Simple partial, Complex partial) - Generalised (Tonic-clonic, Absence, Atonic, Benign familial neonatal) - Lennox-Gastaut - West - Epilepsia partialis continua - Status epilepticus (Complex partial status epilepticus)
Headache	Migraine (Familial hemiplegic) - Cluster - Vascular - Tension
Vascular	Transient ischemic attack (Amaurosis fugax, Transient global amnesia) - Cerebrovascular disease (MCA, ACA, PCA, Foville\'s syndrome, Millard-Gubler syndrome, Lateral medullary syndrome, Weber\'s syndrome, Lacunar stroke)
Sleep disorders	Insomnia - Hypersomnia - Sleep apnea (Ondine\'s curse) - Narcolepsy - Cataplexy - Kleine-Levin syndrome - Circadian rhythm sleep disorder - Delayed sleep phase syndrome - Advanced sleep phase syndrome
Other	Hydrocephalus (Normal pressure) - Idiopathic intracranial hypertension - Encephalopathy - Brain herniation - Cerebral edema - Reye\'s syndrome - Syringomyelia - Syringobulbia - Spinal cord compression

v • d • e Surgery, Nervous system: neurosurgical procedures (ICD-9-CM V3 01-05)
Skull, brain, and cerebral meninges	Craniotomy - Decompressive craniectomy - Lobotomy - Hemispherectomy - Ventriculostomy - Anterior temporal lobectomy
Spinal cord and spinal canal	Spinal cord and roots (Cordotomy - Rhizotomy) - Intervertebral discs (Discectomy - Intervertebral disc annuloplasty - Intervertebral disc arthroplasty) - Vertebral bones (Laminotomy - Laminectomy - Laminoplasty - Corpectomy - Facetectomy - Foraminotomy - Vertebral fusion - Vertebral fixation) - Lumbar puncture
Cranial and peripheral nerves	Ganglionectomy - Nerve block
Sympathetic nerves or ganglia	Endoscopic thoracic sympathectomy
Other	Axotomy - Hypophysectomy - Vagotomy

v • d • e Systems and systems science
Systems categories	Conceptual systems · Physical systems · Social systems · Systems · Systems theory · Systems science · Systems scientists
Systems	Biological system · Complex system · Complex adaptive system · Conceptual system · Cultural system · Dynamical system · Economic system · Ecosystem · Formal system · Global Positioning System · Human anatomy · Information systems · Legal systems of the world · Systems of measurement · Metric system · Nervous system · Nonlinearity · Operating system · Physical system · Political system · Sensory system · Social structure · Solar System
Theoretical fields	Chaos theory · Complex systems · Control theory · Cybernetics · Scientific holism · Sociotechnical systems theory · Systems biology · System dynamics · Systems ecology · Systems engineering · Systems science · Systems theory
Systems scientists	Russell L. Ackoff · William Ross Ashby · Gregory Bateson · Stafford Beer · Ludwig von Bertalanffy  · Kenneth E. Boulding · Peter Checkland · C. West Churchman · Heinz von Foerster · Charles François · Jay Wright Forrester · Ralph W. Gerard · Debora Hammond · George Klir · Niklas Luhmann · Humberto Maturana · Donella Meadows · Mihajlo D. Mesarovic · Howard T. Odum · Talcott Parsons · Ilya Prigogine · Anatol Rapoport · Francisco Varela · John N. Warfield · Norbert Wiener

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