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Lecture notes, cheat sheets
Histology. Nervous system
Directory / Lecture notes, cheat sheets Table of contents (expand) SECTION II. PRIVATE HISTOLOGY Topic 18. NERVOUS SYSTEM From an anatomical point of view, the nervous system is divided into central (brain and spinal cord) and peripheral (peripheral nerve nodes, trunks and endings). The morphological substrate of the reflex activity of the nervous system is reflex arcs, which are a chain of neurons of various functional significance, the bodies of which are located in different parts of the nervous system - both in the peripheral nodes and in the gray matter of the central nervous system. From a physiological point of view, the nervous system is divided into somatic (or cerebrospinal), which innervates the entire human body, except for internal organs, vessels and glands, and autonomous (or autonomic), which regulates the activity of these organs. Spinal nodes The first neuron of each reflex arc is the receptor nerve cell. Most of these cells are concentrated in the spinal nodes located along the posterior roots of the spinal cord. The spinal ganglion is surrounded by a connective tissue capsule. From the capsule, thin layers of connective tissue penetrate into the parenchyma of the node, which forms its skeleton, and blood vessels pass through it in the node. The dendrites of the nerve cell of the spinal ganglion go as part of the sensitive part of the mixed spinal nerves to the periphery and end there with receptors. Neurites together form the posterior roots of the spinal cord, carrying nerve impulses either to the gray matter of the spinal cord, or along its posterior funiculus to the medulla oblongata. The dendrites and neurites of the cells in the node and outside it are covered with membranes of lemmocytes. The nerve cells of the spinal nodes are surrounded by a layer of glial cells, which are here called mantle gliocytes. They can be recognized by the round nuclei surrounding the body of the neuron. Outside, the glial sheath of the body of the neuron is covered with a delicate, fine-fibred connective tissue sheath. The cells of this membrane are characterized by an oval-shaped nucleus. The structure of the peripheral nerves is described in the general histology section. Spinal cord It consists of two symmetrical halves, delimited from each other in front by a deep median fissure, and behind by a connective tissue septum. The inner part of the spinal cord is darker - this is its gray matter. On the periphery of it is a lighter white matter. The gray matter on the cross section of the brain is seen in the form of a butterfly. The protrusions of the gray matter are called horns. There are anterior, or ventral, posterior, or dorsal, and lateral, or lateral, horns. The gray matter of the spinal cord consists of multipolar neurons, non-myelinated and thin myelinated fibers, and neuroglia. The white matter of the spinal cord is formed by a set of longitudinally oriented predominantly myelinated fibers of nerve cells. The bundles of nerve fibers that communicate between different parts of the nervous system are called the pathways of the spinal cord. In the middle part of the posterior horn of the spinal cord is the own nucleus of the posterior horn. It consists of bundle cells, the axons of which, passing through the anterior white commissure to the opposite side of the spinal cord into the lateral funiculus of the white matter, form the ventral spinocerebellar and spinothalamic pathways and go to the cerebellum and thalamus. Interneurons are diffusely located in the posterior horns. These are small cells whose axons terminate within the gray matter of the spinal cord of the same (associative cells) or opposite (commissural cells) side. The dorsal nucleus, or Clark's nucleus, consists of large cells with branched dendrites. Their axons cross the gray matter, enter the lateral funiculus of the white matter of the same side, and ascend to the cerebellum as part of the dorsal spinocerebellar tract. The medial intermediate nucleus is located in the intermediate zone, the neurites of its cells join the ventral spinocerebellar tract of the same side, the lateral intermediate nucleus is located in the lateral horns and is a group of associative cells of the sympathetic reflex arc. The axons of these cells leave the spinal cord together with the somatic motor fibers as part of the anterior roots and separate from them in the form of white connecting branches of the sympathetic trunk. The largest neurons of the spinal cord are located in the anterior horns, they also form nuclei from the bodies of nerve cells, the roots of which form the bulk of the fibers of the anterior roots. As part of the mixed spinal nerves, they enter the periphery and end with motor endings in the skeletal muscles. The white matter of the spinal cord is composed of myelin fibers running longitudinally. The bundles of nerve fibers that communicate between different parts of the nervous system are called the pathways of the spinal cord. Brain In the brain, gray and white matter are also distinguished, but the distribution of these two components is more complicated here than in the spinal cord. The main part of the gray matter of the brain is located on the surface of the cerebrum and cerebellum, forming their cortex. The other (smaller) part forms numerous nuclei of the brain stem. Brain stem. All nuclei of the gray matter of the brainstem are composed of multipolar nerve cells. They have endings of neurite cells of the spinal ganglia. Also in the brain stem there are a large number of nuclei designed to switch nerve impulses from the spinal cord and brain stem to the cortex and from the cortex to the spinal cord's own apparatus. The medulla oblongata has a large number of nuclei of its own apparatus of cranial nerves, which are mainly located in the bottom of the IV ventricle. In addition to these nuclei, there are nuclei in the medulla oblongata that switch impulses entering it to other parts of the brain. These kernels include the lower olives. In the central region of the medulla oblongata is located the reticular substance, in which there are numerous nerve fibers running in different directions and together forming a network. This network contains small groups of multipolar neurons with long few dendrites. Their axons spread in ascending (to the cerebral cortex and cerebellum) and descending directions. The reticular substance is a complex reflex center associated with the spinal cord, cerebellum, cerebral cortex and hypothalamic region. The main bundles of myelinated nerve fibers of the white matter of the medulla oblongata are represented by cortico-spinal bundles - pyramids of the medulla oblongata, lying in its ventral part. The bridge of the brain consists of a large number of transversely running nerve fibers and nuclei lying between them. In the basal part of the bridge, the transverse fibers are separated by pyramidal paths into two groups - posterior and anterior. The midbrain consists of the gray matter of the quadrigemina and the cerebral peduncles, which are formed by a mass of myelinated nerve fibers coming from the cerebral cortex. The tegmentum contains a central gray matter composed of large multipolar and smaller spindle-shaped cells and fibers. The diencephalon is basically the optic tubercle. Ventral to it is a hypothalamic (hypothalamic) region rich in small nuclei. The visual hillock contains many nuclei delimited from each other by layers of white matter, they are interconnected by associative fibers. In the ventral nuclei of the thalamic region, ascending sensory pathways end, from which nerve impulses are transmitted to the cortex. Nerve impulses to the visual hillock from the brain go along the extrapyramidal motor pathway. In the caudal group of nuclei (in the pillow of the thalamus), the fibers of the optic pathway end. The hypothalamic region is a vegetative center of the brain that regulates the main metabolic processes: body temperature, blood pressure, water, fat metabolism, etc. Cerebellum The main function of the cerebellum is to ensure balance and coordination of movements. It has a connection with the brain stem through afferent and efferent pathways, which together form three pairs of cerebellar peduncles. On the surface of the cerebellum there are many convolutions and grooves. Gray matter forms the cerebellar cortex, a smaller part of it lies deep in the white matter in the form of central nuclei. In the center of each gyrus there is a thin layer of white matter, covered with a layer of gray matter - the bark. There are three layers in the cerebellar cortex: outer (molecular), middle (ganglionic) and inner (granular). Efferent neurons of the cerebellar cortex - pear-shaped cells (or Purkinje cells) make up the ganglion layer. Only their neurites, leaving the cerebellar cortex, form the initial link of its efferent inhibitory pathways. All other nerve cells of the cerebellar cortex are intercalary associative neurons that transmit nerve impulses to pear-shaped cells. In the ganglionic layer, the cells are arranged strictly in one row, their cords, branching abundantly, penetrate the entire thickness of the molecular layer. All branches of the dendrites are located only in one plane, perpendicular to the direction of the convolutions, therefore, with a transverse and longitudinal section of the convolutions, the dendrites of the pear-shaped cells look different. The molecular layer consists of two main types of nerve cells: basket and stellate. Basket cells are located in the lower third of the molecular layer. They have thin long dendrites, which branch mainly in a plane located transversely to the gyrus. The long neurites of the cells always run across the gyrus and parallel to the surface above the piriform cells. The stellate cells are located above the basket cells. There are two forms of stellate cells: small stellate cells, which are equipped with thin short dendrites and weakly branched neurites (they form synapses on the dendrites of pear-shaped cells), and large stellate cells, which have long and highly branched dendrites and neurites (their branches connect with the dendrites of pear-shaped cells). cells, but some of them reach the bodies of pear-shaped cells and are part of the so-called baskets). Together, the described cells of the molecular layer represent a single system. The granular layer is represented by special cellular forms in the form of grains. These cells are small in size, have 3 - 4 short dendrites, ending in the same layer with terminal branches in the form of a bird's foot. Entering into a synaptic connection with the endings of excitatory afferent (mossy) fibers entering the cerebellum, the dendrites of the granule cells form characteristic structures called cerebellar glomeruli. The processes of granule cells, reaching the molecular layer, form in it T-shaped divisions into two branches, oriented parallel to the surface of the cortex along the gyri of the cerebellum. These fibers, running in parallel, cross the branching of the dendrites of many pear-shaped cells and form synapses with them and the dendrites of basket cells and stellate cells. Thus, the neurites of the granule cells transmit the excitation they receive from mossy fibers over a considerable distance to many pear-shaped cells. The next type of cells are spindle-shaped horizontal cells. They are located mainly between the granular and ganglionic layers, from their elongated bodies long, horizontally extending dendrites extend in both directions, ending in the ganglionic and granular layers. Afferent fibers entering the cerebellar cortex are represented by two types: mossy and so-called climbing fibers. Mossy fibers are part of the olivocerebellar and cerebellopontine tracts and have a stimulating effect on the piriform cells. They end in the glomeruli of the granular layer of the cerebellum, where they come into contact with the dendrites of the granule cells. Climbing fibers enter the cerebellar cortex along the spinocerebellar and vestibulocerebellar pathways. They cross the granular layer, adjoin pear-shaped cells and spread along their dendrites, ending on their surface with synapses. These fibers transmit excitation to pear-shaped cells. When various pathological processes occur in pear-shaped cells, it leads to a disorder in the coordination of movement. cerebral cortex It is represented by a layer of gray matter about 3 mm thick. It is very well represented (developed) in the anterior central gyrus, where the thickness of the cortex reaches 5 mm. A large number of furrows and convolutions increases the area of the gray matter of the brain. The cortex contains about 10 - 14 billion nerve cells. Different parts of the cortex differ from each other in the location and structure of the cells. Cytoarchitectonics of the cerebral cortex. The neurons of the cortex are very diverse in form, they are multipolar cells. They are divided into pyramidal, stellate, fusiform, arachnid and horizontal neurons. Pyramidal neurons make up the bulk of the cerebral cortex. Their bodies have the shape of a triangle, the apex of which faces the surface of the cortex. From the top and side surfaces of the body depart dendrites, ending in different layers of gray matter. Neurites originate from the base of the pyramidal cells, in some cells they are short, forming branches within a given area of the cortex, in others they are long, entering the white matter. Pyramidal cells of different layers of the cortex are different. Small cells are intercalary neurons, the neurites of which connect separate parts of the cortex of one hemisphere (associative neurons) or two hemispheres (commissural neurons). Large pyramids and their processes form pyramidal pathways that project impulses to the corresponding centers of the trunk and spinal cord. In each layer of cells of the cerebral cortex there is a predominance of some types of cells. There are several layers: 1) molecular; 2) external granular; 3) pyramidal; 4) internal granular; 5) ganglionic; 6) a layer of polymorphic cells. The molecular layer of the cortex contains a small number of small spindle-shaped cells. Their processes run parallel to the surface of the brain as part of the tangential plexus of nerve fibers of the molecular layer. In this case, the bulk of the fibers of this plexus is represented by branching of the dendrites of the underlying layers. The outer granular layer is a cluster of small neurons that have a different shape (mostly rounded) and stellate cells. The dendrites of these cells rise into the molecular layer, and the axons go into the white matter or, forming arcs, go to the tangential plexus of fibers of the molecular layer. The pyramidal layer is the largest in thickness, very well developed in the precentral gyrus. The sizes of pyramidal cells are different (within 10 - 40 microns). From the top of the pyramidal cell, the main dendrite departs, which is located in the molecular layer. The dendrites coming from the lateral surfaces of the pyramid and its base are of insignificant length and form synapses with adjacent cells of this layer. In this case, you need to know that the axon of the pyramidal cell always departs from its base. The inner granular layer in some areas of the cortex is very strongly developed (for example, in the visual cortex), but in some areas of the cortex it may be absent (in the precentral gyrus). This layer is formed by small stellate cells, it also includes a large number of horizontal fibers. The ganglionic layer of the cortex consists of large pyramidal cells, and the region of the precentral gyrus contains giant pyramids, first described by the Kyiv anatomist V. Ya. Bets in 1874 (Bets cells). Giant pyramids are characterized by the presence of large lumps of basophilic substance. The neurites of the cells of this layer form the main part of the cortico-spinal tracts of the spinal cord and terminate in synapses on the cells of its motor nuclei. The layer of polymorphic cells is formed by spindle-shaped neurons. The neurons of the inner zone are smaller and lie at a great distance from each other, while the neurons of the outer zone are larger. The neurites of the cells of the polymorphic layer go into the white matter as part of the efferent pathways of the brain. Dendrites reach the molecular layer of the cortex. It must be borne in mind that in different parts of the cerebral cortex, its different layers are represented differently. So, in the motor centers of the cortex, for example, in the anterior central gyrus, layers 3, 5 and 6 are highly developed and layers 2 and 4 are underdeveloped. This is the so-called agranular type of cortex. Descending pathways of the central nervous system originate from these areas. In the sensitive cortical centers, where the afferent conductors coming from the organs of smell, hearing and vision end, the layers containing large and medium pyramids are poorly developed, while the granular layers (2nd and 4th) reach their maximum development. This type is called the granular type of the cortex. Myeloarchitectonics of the cortex. In the cerebral hemispheres, the following types of fibers can be distinguished: associative fibers (connect individual parts of the cortex of one hemisphere), commissural (connect the cortex of different hemispheres) and projection fibers, both afferent and efferent (connect the cortex with the nuclei of the lower parts of the central nervous system). The autonomic (or autonomic) nervous system, according to various properties, is divided into sympathetic and parasympathetic. In most cases, both of these species simultaneously take part in the innervation of organs and have an opposite effect on them. So, for example, if irritation of the sympathetic nerves delays intestinal motility, then irritation of the parasympathetic nerves excites it. The autonomic nervous system also consists of central sections, represented by the nuclei of the gray matter of the brain and spinal cord, and peripheral sections - nerve nodes and plexuses. The nuclei of the central division of the autonomic nervous system are located in the middle and medulla oblongata, as well as in the lateral horns of the thoracic, lumbar and sacral segments of the spinal cord. The nuclei of the craniobulbar and sacral divisions belong to the parasympathetic, and the nuclei of the thoracolumbar division belong to the sympathetic nervous system. The multipolar nerve cells of these nuclei are associative neurons of the reflex arcs of the autonomic nervous system. Their processes leave the central nervous system through the anterior roots or cranial nerves and end in synapses on the neurons of one of the peripheral ganglia. These are the preganglionic fibers of the autonomic nervous system. The preganglionic fibers of the sympathetic and parasympathetic autonomic nervous system are cholinergic. The axons of the nerve cells of the peripheral ganglions emerge from the ganglia in the form of postganglionic fibers and form terminal apparatuses in the tissues of the working organs. Thus, morphologically, the autonomic nervous system differs from the somatic one in that the efferent link of its reflex arcs is always binomial. It consists of central neurons with their axons in the form of preganglionic fibers and peripheral neurons located in peripheral nodes. Only the axons of the latter - postganglionic fibers - reach the tissues of the organs and enter into a synaptic connection with them. Preganglionic fibers in most cases are covered with a myelin sheath, which explains the white color of the connecting branches that carry sympathetic preganglionic fibers from the anterior roots to the ganglia of the sympathetic border column. Postganglionic fibers are thinner and in most cases do not have a myelin sheath: these are fibers of gray connecting branches that run from the nodes of the sympathetic border trunk to the peripheral spinal nerves. The peripheral nodes of the autonomic nervous system lie both outside the organs (sympathetic prevertebral and paravertebral ganglia, parasympathetic nodes of the head), and in the wall of organs as part of the intramural nerve plexuses that occur in the digestive tract, heart, uterus, bladder, etc. Sheaths of the brain and spinal cord The brain and spinal cord are covered with three types of membranes: soft (directly adjacent to the tissues of the brain), arachnoid and hard (bordering on the bone tissue of the skull and spine). The pia mater covers the brain tissue, it is delimited from it only by the marginal glial membrane. In this shell there are a large number of blood vessels that feed the brain, and numerous nerve fibers, terminal apparatus and single nerve cells. The arachnoid is a very delicate, loose layer of fibrous connective tissue. Between it and the pia mater lies the subarachnoid space, which communicates with the ventricles of the brain and contains cerebrospinal fluid. The dura mater is formed by dense fibrous connective tissue, it consists of a large number of elastic fibers. In the cranial cavity, it is tightly fused with the periosteum. In the spinal canal, the dura mater is delimited from the vertebral periosteum by an epidural space filled with a layer of loose fibrous unformed connective tissue, which provides it with some mobility. The subdural space contains a small amount of fluid. Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu. << Back: nervous tissue >> Forward: The cardiovascular system
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