1.To describe the main components of the nervous tissue: neurocytes and glyocytes.

Transcription

1 Volgograd state medical university Department of histology, embryology, cytology Lecture: NERVOUS TISSUE for the 2 nd course English medium students Volgograd,, 2017

2 Objectives: 1.To describe the main components of the nervous tissue: neurocytes and glyocytes. 2.To demonstrate the neuron types, their structural and functional characteristics. 3.To classify gliocytes and provide their characteristics. 4.To explain the process of myelination of the nerve fibers. 5.To evaluate the types of the nerve endings. 6.To compare different types of synapses and enumerate their classifications.

3 Nervous Tissue The Nervous Tissue is composed of two types of cells: 1. EXCITABLE CELLS OR NEURONS 2. NON-EXCITABLE CELLS INCLUDE: (a) Neuroglia (b) Ependymal cells, lining the cavities of brain and spinal cord (c) Capsular cells, satellite cells or amphicytes of cerebrospinal ganglia (d) Schwann cells etc.

4 The nervous system is organized anatomically into the central nervous system (CNS)which comprises the brain and the spinal cord, and peripheral nervous system (PNS) located outside of the CNS. PNS includes cranial nerves, spinal nerves and their associated ganglia. Functionally the PNS is divided into a sensory (afferent) component, which receives and transmits impulses to the CNS for processing, and a motor (efferent) component, which originates in the CNS and transmits impulses to effector organs throughout the body. The motor component is further subdivided as follows: 1) In the somatic system impulses originating in the CNS are transmitted directly, via a single neuron, to skeletal muscles. 2) In the autonomous system impulses from CNS first are transmitted to a autonomous ganglionvia one neuron, a second neuron originating in the autonomic ganglion then transmits the impulses to smooth muscles, cardiac muscles, or glands. dendrite Nissl granules axon hillock in CNS in PNS Structure of Neuron perikaryon nucleus nucleolus axon (unmyelinated) axon myelin sheath axon Scwann cell node of Ranvier telodendron

5 Neurons range in diameter from 5 to 150 mcm, they are among both the smallest an the largest cells in the body. Нейроны это и самые крупные, и самые мелкие клетки в организме (от 5 до 150 мкм). Neurons are composed of a cell body (perikaryon, soma), dendrites, and an axon. The soma contains a nucleus with dispersed chromatin indicative of a rich synthetic activity, and a prominent nucleolus. The cell body contains a highly develped RER organized into aggregates of parallel cisternae. In the cytoplasm between cycternae are numerous ribosomes suggesting that these cells synthesize both structural proteins and proteins for transport. When stained by basic dyes RER and free ribosomes forming large conglomerates appera under light microscope as basophylic granular areas called Nissl bodies. The number of Nissl granules varies according to the neuron Parts of Neuron terminal boutons synapse nucleus perikaryon axon hillock dendrite axon

6 dendrite Structure of a Neuron Nissl granules synapse nucleolus Golgi apparatus mitochondria nucleus neurofilaments The Golgi apparatus is located only in the cell body around the periphery of the nucleus. Neurofilaments (intermediate filaments with a diameter 10 nm are abundant in the perikaryons and cell processes. The contain neurofilament protein of low (L) medium (M) and high (H) molecular weight, like NF200. axon axonal hillock microtubules

7 In the cytoplasm there may be granules of melanin (byproduct of neurotransmitter metabolism, like epinephrine and dopamine), lipofuscin (pigment of wear and tear) and lipid droplets (not only as a sign of degeneration but as an energy reservoir). When stained by silver impregnation, the cytoplasm of the neurons reveals neurofibrils which are the product of condensation of cytoskeleton elements (microtubules, actin filaments associated with the plasmalemma and neurofilaments). (A) (B) (C) (D) Neurons: (A) H & E staining; (B) Nissl method; (C) Gold method; (D) Gold/toluidine blue.

8 Neurons: Gold method Neurotubules of the cutoskeleton of the neurons contain microtuule-associated protein MAP-2 in the perikaryon and the dendrites) and MAP-3 in the axons only.

9 Classification of the Neurons: 1) Unipolar (neuroblasts during prenatal development) 2) Pseudounipolar (in the spinal and cranial ganglions) 3) Bipolar (olfactory neurons, nerve cells in the spiral ganglion of the Corti s organ and retina of the eye) 4) Multipolar (most neurons in the body). axon terminals bipolar neuron dendrites perikaryon axon collateral branches pseudounipolar neuron perikaryon dendrite axon multipolar neurons Neurons dendrites axon perikaryon

10 axon Axon is the 1 st process axon terminals which appears in the neuron duing its development. The neurons may have one or several dendrites which are principal signal reception and processing sites on the neuron but only one axon which propagates nerve impulse from the perykaryon. All axons originate from the short pyramid region, the axon hillock, which usually arises from the perikaryon. Macromolecules and organelles are synthesized in the cell body and are transported continuously by an anterograde flow along the axon to its terminals. Neuron Types nucleus cell soma bipolar neuron axon collateral branches unipolar neuron axon axon hillock multipolar neuron dendrite dendrite Nissl substance dendrite

11 Anterograde flow occurs at 3 distinct speeds. A slow stream (a few mm per day) transports proteins and microfilaments. A flow of intermediate speed transports mitochondria and a fast stream (100 times more rapid) transports the sunstances in vesicles that are needed at the axon terminal during neurotransmission. Simultaneously with anterograde flow, a retrograde flow in the opposite direction transports several molecules including material taken by endocytosis (including viruses and toxins) to the cell body. pyramidal neuron glomerular neuron amacrine neuron no axon Purkinje neuron Golgi neuron Neuron Types stellate neuron

12 Photomicrograph showing normal neuron somata (N) of ventral gray horn of spinal cord (Thionine stain)

13 Photomicrograph showing a chromatolysed neuron soma (Ch)

14 Ax NR M Teased, myelinated nerve fiber. l.s. Paraffin section. x 540. This longitudinal section of a single myelinated nerve fiber displays its axon (Ax) and the neurokeratin network, the remnants of the dissolved myelin (M). Note the node of Ranvier (NR), a region where two Schwann cells meet. It is here, where the axon is not covered by myelin, that saltatory conduction of impulses occur. Observe that Schmidt-Lanterman incisures (SL) are clearly evident. These are regions where the cytoplasm of Schwann cells is trapped in the myelin sheath

15 Myelinated nerve fibres: R= Node of Ranvier; S= Nucleus of Schwann cell Nerve fibers may be myelinated and unmyelinated. The axis cylinder of the nerve fiber is formed by the axon while the myelin sheath by the plasmalemma of the Schwann cell which is a variety of oligodendrogliocytes in the peripheral nervous system.

16 nucleus of Schwann cell mesaxon Schwann cell Schwann cell axis cylinder internal mesaxon external mesaxon Scwann cell axis cylinder Process of Myelination The mesaxon formed by the plasmalemma of the Scwann cell wraps around the axon forming its myelin sheat. The nucleus of the Schwann cell remain under its plasmalemma.

17 Nonmyelinated nerve fibers mesaxon cytoplasm and nucleus of Scwann cell axons А axon, С cytoplasm of Schwann cell In the unmyelinated nerve fibers the mesaxon is too short to wrap around the axon. The fiber of the cable type is shown on the picture.

18 Myelinated Nerve Fibers axon mesaxon cytoplasm of Schwann cell nucleus of Schwann cell myelin sheath А- axon М myelin sheath С cytoplasm of Schwann cell S nucleus of Schwann cell

19 Myelinated Nerve Fiber endoneurium cytoplasm of Schwann cell nucleus of Schwann cell Schmidt- Lanterman cleft А axon S schwann cell basement membrane of the Schwann cell node of Ranvier axon myelin sheath Schmidt- Lanterman cleft contains portions of the cytoplasm of the Schwann cell.

20 myelin sheath The synapse is responsible for the unidirectional transmission of the nerve impulse. All synapses of the human body are chemical, while in some other mammals (rats) chemical synapses co-exist with elecrical ones. Synapses may be axodendritic, axosomatic and axoaxonic. Synapse axon terminal bouton neurofilaments and microtubules synaptic vesicle presynaptic membrane synaptic cleft postsynaptic membrane postsynaptic web effector cell

21 motor axon nerve Motor end plate cytoskeleton myelin sheath Schwann cell nucleus synaptic vesicles mitochondria external lamina Schwann cytoplasm primary /synaptic secondary \cleft cell muscle cell nucleus myofibrils Neurotransmitters contained in the synaptic vesicles are released into the synaptic cleft by exocytosis and act on the receptors of the postsynaptic membrane.

22 MOTOR INNERVATION OF SKELETAL MUSCLE DIAGRAM OF THE MOTOR END PLATE Schwann cell axon axon terminal external lamina secretory granule skeletal muscle fiber mitochondria synaptic cleft junctional folds The sarcoplasmic membrane in the region the motor ens plate contains specialized receptors which, when activated by acetylcholine, permit muscle cell membrane depolarization.

23 Glial cells are 10 times more abundant in the mammalian brain than in the neurons. They surround both cell bodies and their processes that occupy the interneuronal spaces. Neuroglia ependimal cells tanycytes glial macrophages astrocytes pericapillary end foot pericapillary glial macrophage oligodendrocyte astrocyte subpial end foot pia mater

24 protoplasmic astrocyte NEUROGLIA microglia fibrous astrocyte oligodendrocyte Glial macrophages (microglia) belong to the system of the mononuclear phagocytes.

25 Fibrous astrocytes Protoplasmic astrocytes Internodular oligodendrocytes Perineuronal oligodendrocytes Ependymocytes MACROGLIA Pale oval or spheric nuclei, coarse chromatin, long thin unbranched processes Pale oval or spheric nuclei, coarse chromatin, short thik branched processes Small cells (6-8mcm), oval or spherical, with dark stained nuclei and less processes Same Simple cuboidal and cylindric lining Mainly in the white matter of the CNS Mainly in the gray matter of the CNS Surround axons of the white matter, form myelin the the CNS. In the gray substance are connected to the neuron bodies Line ventricles of the brain and the central canal of the spinal cord.

26 А Neuroglia О М Nuclei of the glial cells are prominent but the details of the cytoplasm merge into the neuropil.. Cells types may be identified by the character of their nuclei. Oligodendrocytes are most numerous with rounded nuclei surrounded by ill-defined clear perinuclear halo. Astrocytes are fewer in number with larger polygonal nuclei, which commonly contain central nucleoli.. The nuclei of microglial cells are rod-shaped or commashaped and stain more densely than those of the others.

27 At the surface of brain astrocytes line a BM and form external glia limitans which surrounds the whole CNS, while capillaries of the CNS are invested by well developed BM almost completely surrounded by end-feet of astrocytes forming perivascular glia limitans. GLIOCYTES pia mater perivascular process pericapillary macrophage capillary glial macrophage astrocyte subpial process ventricle brain surface astrocyte basement membrane myelin oligodendrocyte neuron glial macrophage ependyme

28 N Mi P n Microglia. Silver stain. x 540. This photomicrograph is of a section of the cerebral cortex, demonstrating the nuclei (N) of nerve cells as well as the presence of microglia (Mi). Note that microglia are very small and possess a dense nucleus (n) along with numerous cell processes (P).

29 FA BV Astrocytes. Silver stain. x 132. This photomicrograph of the white matter of the cerebrum presents a matted appearance due to the interweaving of various nerve cell and glial cell processes. Note also the presence of blood vessels (BV) passing horizontally across the field. The long processes of the fibrous astrocytes (FA) approach the blood vessels (arrows) and assist in the formation of the blood-brain barrier

30 adipocytes Structure of a peripheral nerve myelinated nerve fibers epineurium perineurium myelin endoneurium Schwann cell myelinated nerve fiber Endoneurium, perineurium and epineurium are composed of fibrous connective tissue.

31 perineurium main artery to nerve epineurium axons Structure of a peripheral nerve axons and endoneurium nerve trunk fascicle

32 Peripheral nerve. Silver stain. x.s. Silver-stained sections of myelinated nerve fibers display the large clear spaces (arrow) indicative of the dissolved myelin. The axons (Ax) stain well as dark, dense structures, and the delicate endoneurium (En) is also evident. Ax En

33 A MS S Ep P BV Peripheral nerve. x.132. This transverse section presents portions of two fascicles, each surrounded by perineurium (P). The intervening loose connective tissue of the epineurium (Ep) with its blood vessels (BV) is clearly evident. The perineurium forms a septum (S), which subdivides this fascicle into two compartments. Note that the axons (Ax) are in the center of the myelin sheath (MS) and that occasionally a crescentshaped nucleus of a Schwann cell (arrow) is evident. The denser, smaller nuclei (arrows) belong to endoneurial cells

34 Peripheral nerve. l.s. Monkey. x 132. The longitudinal section of the peripheral nerve fascicle presented in this photomicrograph is enveloped by its perineurium (P), composed of an outer connective tissue layer (CT) and an inner layer of flattened epithelioid cells (E). The perineurium conducts small blood vessels (BV), which are branches of larger vessels traveling in the surrounding epineurium, a structure composed of loose connective tissue with numerous fat cells. The peripheral nerve is composed of numerous nonmyelinated and myelinated nerve fibers. The dense nuclei (arrows) within the nerve fascicle belong to Schwann cells and endoneurial cells. CT P BV E

35 Peripheral nerve. l.s. x 270. A distinguishing characteristic of longitudinal sections of peripheral nerves is that they appear to follow a ziz-zag course, particularly evident in this photomicrograph. The sinuous course of these fibers is accentuated by the presence of nuclei of Schwann cells (ScC), fibroblasts (F), and endothelial cells of capillaries belonging to the endoneurium. Many of these nerve fibers are myelinated (M), as corroborated by the presence of the nodes of Ranvier (NR) and neurokeratin around the axons(ax). NR Ax F ScC M

36 Structure of Cervico-Spinal Ganglion perikaryon connective tissue capsule perikaryon capsular (satellite) cells nerve fibers Neurons of the spinal ganglions are pseudounipolar, they are surrounded ny the capsular (satellite) cells which are a variety of oligodendrocytes.

37 MN C S Ss Sympathetic ganglion. l.s. x 132. Sympathetic ganglia are structures that receive axons of presynaptic cells, whose soma are within the central nervous system. Located within the ganglion are soma of postsynaptic neurons upon which the presynaptic cell axons synapse. These ganglia are enveloped by a collagenous connective tissue capsule (C) that sends septa (S) containing blood vessels (BV) within the substance of the ganglion. The arrangement of the cell bodies of the multipolar neurons (MN) within the ganglion appears to be haphazard. This very vascular structure contains numerous nuclei that belong to endothelial cells (arrow), Nuclein of the supporting cells (SS) surrounding the nerve cell bodies are visible.

38 Li Ss n N BV Sympathetic ganglion. l.s. x 540. This photomicrograph presents a higher magnification of a region similar to the previous figure. Although neurons of the sympathetic ganglion are multipolar, their processes are not evident in this specimen stained with hematoxylin and eosin. The nucleus (N) with its prominent nucleolus (n) is clearly visible. The cytoplasm contains lipofuscin (Li), a yellowish pigment that is prevalent in neurons of older individuals. The clear space between the soma and the supporting cells (SS) is a shrinkage artifact.

39 C So Cc f BV Sensory ganglion. l.s. Human. x 132. The dorsal root ganglion provides a good representative example of a sensory ganglion. It possesses a vascular (BV) connective tissue capsule (C) that also envelops its sensory root. The neurons of the dorsal root ganglion are pseudounipolar in morphology; therefore, their somata (So) appear spherical in shape. The fiberts (f), many of which are myelinated, alternate with rows of cell bodies. Note that some somata are large while others are small. Each soma is surrounded by neuroectodermally capsule cells (Cc). derived

40 Cc F N Ax Nc Sensory ganglion. l.s. Human. Paraffin section. x 270. The spherical cell bodies display their centrally located nuclei (N) and nucleoli (n). Observe that both small and large somata are present in the field, and that the nuclei are not always in the plane of section. Hematoxylin and eosin stains the somata a more or less homogeneous pink, so that organelles such as Nissl substance are not visible. However, the nuclei and cytoplasm of capsule cells (Cc) are clearly evident. Moreover, the small, elongated, densely staining nuclei of fibroblasts (F) are also noted to surround somata, just peripheral to the capsule cells. Axons (Ax) of myelinated nerve fibers belong to the large pseudounipolar neurons.