The nervous system is considered to be the most complex morphological structure of the human body . It consists of over 100 billion neurons that provide integration and coordination of the body with the external and internal environment. It can be considered a real computer, which constantly receives information from external or internal environment, analyzes the information received and develop appropriate responses.

The nervous system is divided morphologically and functionally :

- Somatic nervous system receives information from musculo -articular or cutaneous receptors as well as sensory information ( visual, auditory, gustatory , olfactory or equilibrium) . It is controlled voluntarily and its main effector organ is the striated muscle ;- Autonomic nervous system receives information from osmoreceptors , baroreceptors and chemoreceptors located in the structure of different tissues or organs . It can not be controlled voluntarily , and effectors are represented by heart , endocrine or exocrine glandular cells and smooth muscles of internal organs.

Both the somatic nervous system and the autonomic have two distinct components: a central component (central nervous system ) and a peripheral component ( peripheral nervous system )

The nervous system is formed of of parenchyma and stroma .CNS parenchyma consists of all nerve cells. Stroma is composed of glial cells , blood vessels and fine connective elements .At all levels of the CNS there are two different structures, morphologically and functionally :

Gray matter White matter

The gray matter is arranged in columns ( bone marrow ) , islands (in the brainstem and diencephalon ) or forms the cortex (the area of the cerebral hemispheres and cerebellum).

Histologically it is formed of:Neuron bodiesDendritesThe initial unmyelinated portion of the axonGlial cellsCapillary network which allow a highly oxidative metabolism specific for neurons

Nerve cells have multiple extensions that connect to each other through various synapses forming a plexiform network called neuropil.

The white matter is composed of:Myelinated axons arranged parallel, grouped in bundles or cordsBetween the axons there are many glial cells , and in particular oligodendrocyte that have a role in the formation of the myelin sheath surrounding axons . Capillaries (less numerous in the white matter)

Macroscopically, the spinal cord presents: - Anterior median fissure; - Posterior median sulcus; - Two collateral grooves arranged symmetrically on the side of the median plane, places of emergence of the anterior roots of the spinal nerves; - Two collateral grooves , arranged a side of posterior median sulcus, representing the place of penetration of the posterior spinal nerve roots in the spinal cord.The gray matter is arranged in the center, surrounded by white matter. In longitudinal section it has the form of a continuous column for the entire spinal cord length. In cross-section is typically arranged, symmetrically in relation to the median plane, in the form of the letter "H" or two crescent joined together by a strip of gray matter called the gray commissure. Gray commissure is traversed throughout its length by the ependymal canal, which continues in the brain with the ventricular cavities.Each symmetrical half of the gray matter presents :- An anterior horn, shorter and with a greater transverse diameter consisting of a head and a base. It contains large motor neurons, whose axons form the anterior root of the spinal nerve .- A posterior horn narrower and longer , consisting of a head, cervix and base. It contains smaller oval sensory or association neurons - A lateral horn more obvious between C7 - L2 disposed between the base of the anterior horn and posterior horn base , consisting of small neurons , spherical , fusiform or oval with autonomic function . They are sympathetic preganglionic autonomic neurons ( cervico -dorsal region of the spinal cord ) or parasympathetic ( sacral region of the spinal cord ) .- The reticular substance ( Deiters ) consists of small islands of gray matter separated by fine myelinated nerve fibers, arranged in the angle between the posterior horn and lateral horn.Histologically, the gray matter contains neurons, glial cells, amyelinated nerve fibers representing dendrites and axons, blood vessels and a small amount of perivascular connective tissue .In anterior horns are many types of neurons. The most numerous are the alpha neurons . These neurons are called radicular neurons because their axons pass in white matter , receive the myelin sheath and enters the anterior spinal nerve root structure. They are stellate large, multipolar with a diameter between 60-120 m. Their dendrites make synapses with pyramidal and extrapyramidal tract axons or with the axons of sensory neurons in the posterior horns.These motor neurons innervate striated skeletal muscle fibers .Gamma neurons are multipolar neurons , small size , with various shapes that innervate muscle fibers within motor neuromuscular spindles.

The glial cells in the gray matter are the parenchymal glial cell( macroglia, protoplasmic macroglia, microglia, oligodendrocyte rare) and epithelial glial cell(ependymal) that guards the ependymal canal.The posterior horns contain sensory neurons , polymorphic , oval , fusiform , of varying sizes , from 6-70 micrometres arranged in several layers ( layer Rolando , the Clarke etc) incompletely defined. Their dendrites realize synapses in the posterior horn with axons of the pseudo-unipolar neurons in the spinal ganglia and their axons pass in white matter , entering the constitution of the ascending tracts.Other neurons in the posterior horns are association neurons , which connect sensory neurons and the motor of the anterior horns at the same level or at different levels of the neurax.

The lateral horns have autonomic neurons: sensitive in the posterior half and motor, preganglionic in the anterior half. They are oval or fusiform neurons with a diameter of 15 to 20 m. Their dendrites realize synapses with the axons of sensory neurons in the spinal ganglia. Their axon join the somato-motor fibers of the anterior roots, forming preganglionic fibers that stop in the paravertebral sympathetic ganglia or pelvic parasympathetic ganglia.

Regarding the length of the axon:Golgi I- long axon neurons. Their axon passes in the white matter of the spinal cord. radicular neurons- anterior horn and anterior half of lateral horn- forms the anterior root of the spinal nervecord neurons- posterior horn and posterior half of lateral horn- ascendant tractsGolgi II- short axon neurons. Their axon doesnt leave the gray matter. They have association role.Regarding their function:Sensory neurons (autonomic and somatic)Motor neurons(autonomic and somatic)Association neuronsNeuronal extensions (dendrites and axons) present in the gray matter are devoid of myelin sheath and participate in the neuropil.The white matter of the spinal cord is disposed outside the gray matter and is organized into three pairs of cords :- Anterior cords that communicate with each other through the white anterior commissure exchanging nerve extensions;- Lateral cords delimitated by the anterior and posterior roots of the spinal nerves ;- Posterior cords disposed between the dorsal roots of the spinal nerves and the posterior median sulcus.

The spinal cords white matter consists of myelinated nerve fibers grouped into bundles . These fibers are either endogenous or exogenous .

The endogenous fibers are represented by axons that of neurons located in the gray matter of the spinal cord at different levels. These fibers can be: short fibers and long fibers.

Endogenous short fibers are called spino - spinal fibers because arise in the spinal cord and realize synapses with other neurons in different levels of the spinal cord. This type of fibers form the fundamental fascicle .Endogenous long fibers have the perikaryon located in the dorsal horns of the spinal cord and myelinated axons pass into the lateral or anterior cord and reach the thalamus , cerebellum and brainstem . These fibers form the main ascendant sensitive fascicles:- Spino - thalamic lateral fascicle located in the lateral cord, leading thermal and pain sensitivity;- Spino - thalamic anterior fascicle- anterior cord- tactile sensitivity ;- Direct spino cerebellar fascicle (Flechsing) and cross spino cerebellar fascicle (Gowers) located in the lateral cords , leading unconscious proprioceptive sensitivity

Exogenous fibers are the myelinated axons with the neuronal body located outside the spinal cord. Regarding their path these fibers are classified into: fibers ascending and descending fibers .

I. exogenous ascending fibers have the perikaryon located in the spinal ganglion. Their axons enter the spinal cord through the posterior root directly into the posterior cord , have an ascendig path towards the medulla oblongata forming the spino - bulbar fascicles ( Goll and Burdach ) . These bundles are sensitive and lead fine tactile sensitivity and conscious proprioceptive sensitivity.

II . Exogenous descending fibers originate in the upper floors of the central nervous system ( cerebral cortex , midbrain , pons, medulla oblongata , cerebellum ) . The descending tracts are large pyramidal and extrapyramidal pathways. They are effector pathways controlling voluntary and involuntary motility.

The pyramid tracts called cortico -spinal pathways have the perikaryon located in the cerebral cortex , the motor neocortex . In their descending path they go through the cerebral peduncle, the pons and medulla oblongata , through the bulbar pyramids. Here, the medial fibers cross forming the decussation of pyramids and the lateral ones do not cross, thus forming in the spinal cord two fascicles with symetric arrangement regarding the median plane: Direct pyramid fascicle- anterior cords of the white matterCross pyramid fascicle- lateral cord of the white matter The extrapyramid tracts have the perikaryon located both in the cortex and in various nuclei located in the midbrain , pons, medulla , cerebellum . The main extrapyramid tracts are Tecto -spinal fascicle, Rubro-spinal , nigro -spinal , olivo -spinal , reticulo -spinal etc .

Descending fibers , both pyramid and extrapyramid make synapses with motor neurons in the anterior horns of the spinal cord.

Formed of: - medulla - pons - midbrainGray matter: Aggregates of neurons (known as nuclei or gray matter isles)- the parenchymaWhite matter: spino-talamic ascendant tracts and cortico-spinal descendent tracts- myelinated axons

The stroma is formed of glial cells and connective tissue septa with blood vessels originating in the meninges.

In the brainstem the reticular substance is better developed than in the spinal cord. It is made up of islands of nerve cells, different in shape, size and function, separated by many nerve fibers, more or less organized in fascicles. Regarding the histologic appearance of the nuclei of the reticular substance two areas were described: - The median area- rich in neurons: - The lateral area- poorer in nerve cells. The reticular substances nuclei receive afferences directly from the spinal cord, the medulla and pons nuclei, the cerebellum, hippothalamus and even from the cerebral cortex. They send efferent fibers to the spinal reticular substance, to autonomic neurons in the brainstem, hypothalamus, thalamus and cerebral cortex. In the nuclei of the reticular substance several groups of dopaminergic neurons, noradrenergic and serotonergic have been observed.

Called " intermediate brain " , diencephalon consists of several nerve formations : the thalamus , hypothalamus , metathalamus and epithalamus .

The thalamus is a bulky mass of gray matter located in the dorsal diencephalon . Its structure is uneven because of the division of neurons in several groups of nuclei. These nuclei contain neurons Golgi type I with long axons that project to the cerebral cortex and Golgi type II neurons that have short axons with association function.

The hypothalamus , located in the floor of the third ventricle , consists of several nuclei : anterior , middle and posterior . It receives afferences from the retina, the olfactory mucosa , the reticular substance and cerebral cortex and sends efferent fibers to the hippocampus, midbrain reticular substance and to the autonomic nuclei of brainstem and spinal cord. Also , the hypothalamus is connected structurally with the neurohypophisys through the anterior nuclei and controls the secretory activity of the adenohypophysis by middle nuclei . Hypothalamic nuclei also have autonomic sympathetic and parasympathetic function.

The metathalamusul is made up of four neuronal groups known as the geniculate bodies :- Medial geniculate corpus - relay station for auditory pathways ;- Lateral geniculate corpus - relay station for optical fibers .

The cerebellum is located in the lower floor of the posterior skull, behind the brainstem , under the occipital lobe of the cerebral hemispheres .It consists of :- Two cerebellar hemispheres ( neo-cerebelum ) ;- Vermis ( paleo-cerebellum ) an elongated midian part that joins the cerebellar hemispheres.It is formed of gray matter and white matter.The gray matter is disposed on the surface of the cerebellum and forms cerebellar cortex. It has a thickness of about 1 mm and covers the cerebellar hemisphere and vermis . Inside the cerebellar masses the gray matter forms islands called cerebellar nuclei (dentate, emboliform , globose and fastigii) .The white matter is located on the inside of the cerebellum consisting of afferent and efferent myelinated nerve fibers.

The gray matterThe cortex of the cerebellum has a specific cellular architecture. Thus, from the surface to the depth three different layers are visible in light microscopy:- Molecular layer; - Purkinje cell layer; - Granular layer.

The molecular layer is well developed. It consists of few stellate cells and Golgi type II cell with associative function. It also contains many dendrites of Purkinje cells from the underlying layer and axons originating in the granular layer. The fibers are oriented both perpendicular to the surface of the cerebellar cortex as well as parallel to it. This layer contains numerous synapses being an associative layer.

Purkinje cells layer consists of a single row of Purkinje cells . The dendrites of these cells climb in the molecular layer where they realize synapses with the " T " axon of the granular cells. Their axon descends through the granular layer in white matter where it realizes synapses with the neurons of the dentate nucleus. In the granular layer, the Purkinje cell axons emit collaterals to neighboring making axo - axonal synapses. The body Purkinje cell synapses with ' terminal panier " cells present in the granular layer and with the hanger" fibers originating in the pons nuclei.

The granular layer is made up of a large number of granular cells ( about 2 million/mm3 ) . Dendrites of these cells form a special synapse called "cerebellar glomerulus". Cerebellar glomerulus is a cluster of synapses made between granular cell dendrites and axons of "muscle" cells or Golgi cell axons, wrapped by glial capsule.Axons ascend to the molecular layer where they bifurcate in " T " and create synapses with Purkinje cell dendrites .The white matterContains myelinated afferent and efferent nerve fibers. These fibers are either endogenous or exogenous .Exogenous fibers are afferent fibers that have the perikaryon in the spinal cord , the medulla , the pons, or even brain cortex. They form spino - cerebellar tracts, bulbo - cerebellar , reticulo cerebellar or cortico- ponto cerebellar tracts .Some of these afferent fibers , called hanger fibers and muscle fibers, performs special synapses in the cerebellar cortex.

Hanger fibers originate in the medullas ovlivar nucleus, they pass through the cerebellar white substance as myelinated fibers and penetrate the cerebellum cortex where they lose their myelin sheath. After traversing the granular layer give numerous branches ending "clinging like vines" around Purkinje cell dendrites or body. They have direct excitatory action on Purkinje cells and Golgi cells.

Muscle fibers are thick , wavy , highlighted more easily . They come from many fascicles ( spino - , Ponto- , vestibulo - and reticulo - cerebellar ) . In the cerebellar cortex they lose their myelin sheath and give numerous branches forming the cerebellar glomerulus. On their length they have short and thin branches like muscle bushes. They have excitatory effect on granular cells.Endogenous fibers originate in the cerebellar cortex or cerebellar nuclei. Purkinje cell axons, cross the granular layer, they gain the myelin sheath and head towards the cerebellar nuclei where they realize synapses. The axons of these neurons form tracts that go to the vestibular nuclei in the medulla, to the reticular substance of the brainstem or to the red nucleus or towards the thalamus.

Glial cells of the cerebellum are: - Fibrous and protoplasmic astrocytes are more numerous in the cerebellar cortex; - The microglia present in both the gray matter and white matter; - The oligodendrocyte is present especially in the white matter; - Fananas glial cell in the molecular layer; - Bergman glial cell also known as radial epithelial cells present in the Purkinje cells layer.

Cerebral hemispheres are the most developed segment of the CNS , they occupy over 80 % of the skull. They are formed of: - The gray matter formes the cerebral cortex on the surface of hemispheres with a thickness of 3-5 mm and the striatum at the base of the cerebral hemispheres;- White substance disposed within the cerebral hemispheres , composed of myelinated nerve fibers that come and go from the cerebral cortex or linking different areas of the cerebral hemispheres.

The gray matterThe cortex represents the largest concentration of neurons and synapses. It is considered that it could contain between 14 and 26 billion neurons. The number of synapses is huge , some authors showing that there are about 1012 synapses/cm3 of cerebral cortex . The total area of the cerebral cortex was estimated at 1924 cm2.

In terms of structural , functional and phylogenetic in the cerebral cortex there are two areas :paleo-cortex or allo-cortex , the oldest representing 1/12 of the surface of the cortex . It has a cellullar arangement forming two layers of neurons and is located in the hippocampus ;neo-cortex or isocortex , the largest area ( dealing 11/12 of the surface of the cortex ) and the structurally complex . It is composed of 6 layers of neurons .

Histologically, the cerebral cortex is formed of:- Parenchyma , formed by all neurons and neuronal extensions;- Stroma composed of glial cells , blood vessels and fine perivascular connective tissue Cytoarchitectonics of the cerebral cortexThe distribution of neurons in the cerebral cortex is extremely varied, which made many authors identify 20 to over 200 different cortical areas morphologically different. With all this great variability in the isocortex six layers of overlapped nerve cells can be identified. From the surface to the depth of the cerebral hemispheres, there are :- Molecular or plexiform layer , just under the meninges , is poor in nerve cells, but is rich in dendrites and axons of neurons belonging to the underlying layers . Neurons are fusiform , pyriform cells Cajal and Golgi II cells with short axon . This layer has association function.- External granular layer is richer in neurons than the above layer . It consists of small granular cells, particularly neurons Golgi type II, Martinotti cells with ascending axon and small and medium pyramidal neurons. It has association function .- External pyramidal layer consists mainly of small and medium-sized pyramidal cells and rare granular neurons. Dendrites of pyramidal cells move towards the surface of the cerebral cortex and they branch especially in the molecular layer. The axon goes towards the white matter of the cerebral hemispheres. It has motor function.- Internal granular layer has a high density of granular neurons and few medium size pyramidal cells small . It has receptor function.- Internal pyramidal layer contains mainly pyramidal cells small , medium and large ones ( Betz cells ). Because of these large cells , some authors have called the internal pyramidal layer the ganglionar layer. It has the motor function.- Polymorphic layer consists of a mixture of the various cell sizes and shapes . It is rich in nerve fibers and has association funtion.

The uneven distribution of nerve cells in the isocortex led to its division: - Homeotypic isocortex (containing all 6 cell layers), subdivided into: - isocortex of frontal type- pyramidal cells; - isocortex of parietal type- granular cells; - Heterotypic isocortex (characterized by the almost complete absence of some layers): - Agranular isocortex type present in the motor area, characterized by the absence of the granular layer and the presence of a large number of pyramidal cells; - Granular isocortex type characterized by the absence of pyramidal cells and granular layers pronounced development.

The allocortexul or arhipalium is characterized by a reduced development and absence of stratification. Its structure has only layers 1 and 6.Myelo-architectonics of the cerebral cortexIs formed by the arrangement of neuronal extensionsNeuronal extensions (dendrites and axons) in the cerebral cortex are the amyelinic nerve fibers, afferent, efferent or association. These fibers are classified into:- Tangential fibers- horizontal, parallel to the surface, grouped in layers- Superficial stria or tangential plexus of Exner in the molecular layer- Lamina difibrosa located in the external granular layer (less developed); - Bechterew stria located in the superficial part of the external pyramidal layer; - External Baillarger's stria located in the internal granular layer; - Internal Baillarger's stria located in the internal pyramidal layer; - Infrastriated layer located in the polymorphic layer.- Radial fibers are formed mainly by the dendrites and axons of neurons that form afferent and efferent cortical fascicles

The white matterRepresents the most voluminous component of the cerebral hemispheres. Disposed under the cortex, the white matter is formed only of nerve fibers wrapped in myelin sheath and glial cells. In its structure different nerve fibers categories can be observed: - Afferent fibers that form the sensory-sensory pathways specific and nonspecific; - Efferent or motor fibers that emerge from the cortex; - Association fibers: Unilateral association fibers Bilateral association fibers passing from one hemisphere to the other, found in the structure of the corpus callosum and white commissure (anterior and posterior).Glial cells of the cerebral hemispheresParenchymal glial cell- astrocyte, microglia and oligodendrocyteEpithelial (Ependymal) glial cells- covers the cavities of the CNS, forming in the lateral ventricle the choroid plexus.

Choroid PlexusAre found in roof of 3rd & 4th ventricles and walls of lateral ventricles. Composed of fibrovascular core of loose connective tissue and blood vessels covered by specialized ependymal cells. These cells contain microvilli and cilia on their apical surface.Main function is to produce cerebrospinal fluid. CSF passes from ventricles to subarachnoid space and then enters the venous circulation via the arachnoid villi.

1. Dura mater (pachymeninges) dense irregular connective tissue. There is an epidural space above and a subdural space below.2. Arachnoid avascular connective tissue covered on both sides by a simple squamous epithelium. Sends down fibrous trabeculae into the subarachnoid space to connect the arachnoid with the pia mater. Subarachnoid space is filled with CSF and communicates with the ventricles of the brain; it also contains blood vessels (subarachnoid hemorrhage).Arachnoid villi (Pacchioni granules) perforate through the dura mater, allowing CSF to flow into the venous sinuses. In old persons they can calcify generating nodules.3. Pia mater loose connective tissue, very vascular. Partially follows blood vessels that enter the brain, resulting in a perivascular space around these vessels lined by pia mater. On its inner surface it is doubled by a glial membrane formed of glial cells extensions.

Blood-Brain Barrier functional barrier composed of endothelial cells with tight junctions, their basement membranes, and astrocyte vascular processes. It has selective permeability.

Is formed of nerves and nervous ganglia

Nerve Fibers -- an axon or collection of axons, plus any surrounding sheaths of ectodermal origin. The presence or absence of a sheath, and the nature of the sheath (cytoplasm vs. myelin) are used to further characterize nerve fibers.1. Unmyelinated Fibersa. Small axons in the PNS are embedded in clefts of the cytoplasm of Schwann cells. Each Schwann cell can sheathe (but not myelinate) a dozen axons. b. In the CNS, the majority of axons are unmyelinated. The axons are NOT sheathed by any type of cell. 2. Myelinated FibersAxons that become myelinated are generally larger than 1 M in diameter. Myelin is deposited just beyond the axon hillock and continues to near the region of termination of the axon. a. Myelin is laid down by Schwann cells. The membranes of the Schwann cell wrap around the axon several times and fuse to form a myelin sheath. The sheath is interrupted by gaps called nodes of Ranvier which represent the spaces between adjacent Schwann cells along the length of the axon. A Schwann cell can only myelinate a single axon; however, each axon is myelinated by several Schwann cells.b. In the CNS, myelin is laid down by oligodendrocytes. Each cell can myelinate several axons.

Nerves = groups or bundles of nerve fibers covered by connective tissue.1.Epineurium the outer layer of dense connective tissue. It surrounds several nerve bundles. Contains blood vessels and lymphatics.2.Perineurium connective tissue and flattened epithelial cells that surrounds each bundle (fascicle) of nerve fibers. Cells are joined by tight junctions to provide a barrier to passage of most macromolecules (blood-nerve barrier).3.Endoneurium thin layer of reticular fibers that surround individual nerve fibers.

On the trail of the posterior spinal nerve root in the spinal canal , there is one small , oval structure called the spinal ganglion. In longitudinal section the following histological structures are observed:- Fibrous connective capsule on the outside ;- A cortical area ;- Axial zone .The capsule is a connective, fibrous structure tissue, composed of collagen fibers arranged in lamellae and rare reticulin fibers and fibroblasts . Fine connective tissue septa and blood vessels penetrate inside the ganglion forming the organs stroma. Cortical area is disposed just under the capsula and it is formed of a cluster of neurons and glial cells. There are two types of neurons :- Pseudounipolar somatosensory neurons ;- Pseudounipolari autonomic-sensoty neurons ;The perikaryons are spherical shaped with sizes ranging from 15 to 70 and even 120 microns . They have a single amyelinic extension that forms a glomerulus surrounding the neuronal body and then divides in two branches: The axon penetrates the spinal cord through the posterior rootThe dendrite- towards the periphery, forming the sensory nervous endingBoth the neuronal body and its extension are covered by a continuous glial capsule formed of satellite peri-neuronal cells (isolation role)

They are formed of groups of neurons located on the trail of vegetative nerves. Thus , one side and the other of the spine there are two paravertebral sympathetic chain ganglia; around some blood vessels, or in the vicinity of some organs there are autonomic nerve plexus ( celiac plexus , heart , etc. ) and in the walls of internal organs there are intramural ganglia.

Autonomic ganglia, whether sympathetic or parasympathetic , presents a similar structural organization.

At the periphery the ganglion has a fibrous capsule which sends inner connective septa containing blood capillaries .The neuronal bodies are arranged in small islands between connective bundles and bundles of myelinated axonal fibers ( preganglionic ) and amyelinic (postganglionic ) . Perineuronal satellite cells form a capsule enclosing each perikaryon isolating it from the connective tissue.

Autonomic neurons are multipolar neurons with a variable diameter of from 15 to 60 microns. They have many dendrites of variable length and a single axon is distributed towards effector organ ( smooth muscle cells , glandular cells ).In autonomic ganglia we can find cells with endocrine function, secreting catecholamines (adrenaline and noradrenaline ).