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Epithelial tissue (epithelium)
General characteristics of epithelium
Is avascular tissue (without blood supply – cells receive nourishment by diffusion from a highly vascular area of loose connective tissue just below the basement membrane called the lamina propria ) is highly cellular tissue – cells are arranged to form cohesive sheet or groups with no or little extracellular matrix displays a free surface – usualy luminal surface (turned to the lumen) opposite (basal) surface adheres to extracellular basement membrane or lamina basalis epithelial cells display polarity – apical (luminal), lateral and basal surfaces with structural specialization epithelial cells are specialised for absorption, secretion or to act as barrier lateral surfaces display junctional complexes for intercellular cohesion and communication
One type of epithelium may change into another type – metaplasia ( examples: pseudostratified ep. of respiratory passages transforms into stratified squamous ep. on the surface of epiglottis and soft palate )
Membrane specializations of epithelia Lateral surface Specialised structures are present in epithelia which link individual cells together. Two main adhesion types are distinguished:
- Cell membrane proteins acting as specialised cell adhesion molecules (CAMs)
- Specialised areas of the cell membrane incorporated into cell junctions.
Three types are recognized: occluding junctions, anchoring or adherence junctions and communicating junctions.
o Occluding junctions bind cell together to form an impermeable barrier Zonula occludens or tight junction o Anchoring junctions link the cytoskeleton of cells to each other and two underlying tissues Zonula adherens provides mechanical strength Macula adherens or desmosomes provides mechanical strength in tissues where there are tensile or shearing stresses, eg skin o Communications junctions allow direct cell-cell communication Gap junction or nexus allow rapid communication for coordinated action
Luminal (free, apical) surface
Microvilli – short finger-like projection of the cell membrane to increased surface area (regularly arranged microvilli in intestines – striated border , in kidney tubules – brush border ) Cilia – hair-like surface projections of cells involved in transport Glycocalyx – thin extracellular layer consisting of protein glycoprotein and sugar residues; stains PAS positive; can act as enzyme, CAM or for cell recognition
Basal surface
Basal invaginations or folds – greatly enhance surface area; folded membrane with ions pumps + mitochondria form basal labyrinth in kidney tubules.
Epithelial tissues are physically separated from underlying connective tissues by a basement membrane or basal lamina. The portion of an epithelial cell attached to the basement membrane is called its basal surface. The opposite side - facing the external environment, or lumen of a body cavity, is its apical surface. Basement membranes are composed of a special type of collagen and a substance called laminin (see below). The basement membrane helps epithelial cells orient themselves in relation to other tissues. After epithelial injury (e.g., an abrasion), the basement membrane serves as a scaffolding upon which new cells attach themselves during healing.
Cassification of epithelia
I. surface epithelium – is 1 or more layers of cells arranged into sheet;
According to number of layers
According to shape of cells in the outermost layer SURFACE EPITHELIUM simple layerd – squamous
- cuboid
- columnar
- pseudostratified columnar stratified – squamous non-cornified (non-keratinized)
- squamous cornified (keratinized)
- columnar
- transitional
STRATIFIED EPITHELIUM – consists of basal layer on basement membrane, several layers of polyhedral cells and surface layer. According to the shape of cells in this layer the epithelium is named (squamous, columnar, transitional)
Stratified squamous – resiting to mechanical influences (press)
- non-keratinised (mouth cavity, vocal cords, vagina, anus)
- keratinised (epidermis of the skin) – the cells are released continously from the surface
Is not described in human organism
Stratified columnar – 2 or more layers of cells, columnar cells form the upper layer (two- layered in ductus epididymis and ductus deferens, more-layered male urethra, conjunctive)
Transitional - stratified, top layer dome or umbrella shaped. (only in some urinary passages – renal pelvis, ureter and urinary bladder). Epithelium change the shape of cells and number of layers according to wall conditions of urinary passages – distansion or contraction.
Epithelia with special functions: resorptive, sensory, respiratory, myoepithelial cells
II. glandular epithelium – multicellular epithelial structures that specialize in synthesizing and secreting complex molecules.
Glandular cell
rough endoplasmic reticulum Golgi apparatus secretory granules nucleus with nucleoli
CLASSIFICATION OF GLANDS
GLANDULAR
EPITHELIUM
unicellular Single cells in coverinrg epithelium – (Paneth cells, goblet cells, enteroendocrine cells, Leydig cells) multicellular Accordin of mechanism of secretion endocrine exocrine – merocrine, apokrine,holocrine According to loclalization intraepithelial extraepithelial According to arrangement of ducts simple branched compound According to type of secretory portions tubular alveolar (acinar) tuboalveolar According to product properties mucous serous mixed
According of mechanism of secretion
endocrine – glands withou ducts, product is released into the blood through the wall of capilareis exocrine – secretory cells of exocrine glands release their products into ducts in three different ways:
merocrine apocrine holocrine
Functions of epithelia:
Barrier: Epithelial tissue commonly functions as a covering or lining for organs and other tissues (e.g., skin, mucous membranes, pleural cavity, etc.). Epithelial cells serve as selective barriers between the environment and the internal structures of the body. They protect underlying tissues from drying, and from mechanical and chemical injury. Tight junctions between individual cells play an important role in the barrier function of epithelium. Some barrier epithelial cells have motile cilia that propel fluid or particulate matter over tissue surfaces (e.g., cells lining the bronchi). Absorption: Epithelial cells are found in those organs (e.g., intestine) which are involved in absorption of substances important for life. These cells often microvilli which increase cell surface area in order to facilitate absorption. Secretion: The secretory cells of endocrine and exocrine glands are epithelia. Sensory: Many of the more complex sensory receptors of the nervous system are derived from specialized epithelia called neuroepithelia (e.g., the rods and cones of the retina, olfactory receptors of the nose, taste receptors on the tongue, etc.). Sensory receptors function by converting mechanical, chemical, or electromagnetic signals from the environment into nerve impulses which can be processed by the nervous system. Contractility: Some very specialized epithelial cells (myoepithelia) contain the contractile proteins myosin and actin similar to muscle. Myoepithelia are associated with the ducts of sweat, salivary, lacrimal, amd mammary glands and assist in the secretory process.
Origin: Epithelial tissues are derived from all three primary germ cell layers.
Ectoderm: The epithelial cells of the skin and oral cavity (epidermis) are derived from ectoderm. Epithelial cells covering the cornea and lens, as well as sensory receptors of the eyes, ears, and nose, are also ectodermal in origin. Mesoderm: The epithelial lining of blood vessels (endothelium) is derived from mesoderm. The epithelial lining of the pleural and peritoneal cavities (mesothelium) also originate from mesodermal cells. Endoderm: The epithelial lining of the respiratory system and digestive tracts - as well as the functional cells (parenchyma) of the liver, pancreas, gallbladder, thyroid, and parathyroid, are derived from endoderm.
Connective Tissue (CT)
forms an extensive compartment in the body and can be considered as the "glue" that holds the body and organs together.
- the most diverse of the four tissue types with a wide variety of functions,
- 3 types of CT: connective tissue proper / cartilage / bone
- consistency is soft gel-like (areolar CT) to hard (bone),
- originate frome embryonic CT = mesenchyme (derivate of mesoderm)
- function of CT: supporting, nutritive (diffusion of nutritives from blood vessels)
All types of CT consist of cells and intercellular matrix secreted by some of those cells. The
intercellular matrix consists of fibres and ground substance (interfibrilar substance).
Types of CT Classification Cells intercellular matrix
Fibres Ground
substance
CT proper Mesenchyme
Mucose CT Loose CT Dense CT
- regular
- irregular Reticular CT Adipose CT
- white fat
- brown fat
Fixed: fibroblasts and fibrocytes, reticular cells, pigment cells, adipose cells, histiocytes, mast cells, plasma cells, Free: macrophages, leukocytes
collagen reticular elastic
Cartilage Hyaline
Elastic Fibrocartilage
chondroblasts and chondrocytes
collagen elastic
Bone Fibrilar
Lamellar
osteoblasts, osteocytes
collagen mineralized
Ground substance : occupies the space between the cells and fibres of CT,
- it is colorless, transparent, homogenous substance of mucose consistency,
- it has a high water content,
- in specially prepared sections, has an amorphous appearance. In routine preparations, it is lost during the fixation and dehydration process, and only cells and fibres can be seen.
Ground substance consists of:
- glycosaminoglycans – ( earlier called mucopolysaccharides ) – are polymers of hexosamines (glucosamine or galactosamine) and uronic acids. The main of them are hyaluronic acid , chondroitinsulfate, dermatansulfate, keratansulfate, heparansulfate. Sulfate and carboxyl groups makes glycosaminoglycans highly negatively charged. The high density of negative charges attracts water, forming a hydrated gel. This gel permits the rapid diffusion of water- soluble molecules but inhibits the movement of large molecules and bacteria. Hyaluronic acid is extremely long and rigid, consisting of a chain of several thousand sugars.
- proteoglycans – ( aggrecan, syndecan, fibroglycan) are very large macro-molecules, consisting of a core protein to which many glycosaminoglycan molecules are covalently attached.
- glycoproteins – (fibronectin, chondronectin, laminin, osteocalcin, osteonectin, osteopontin) differ from proteoglycans: protein core is the main part of their moleculeto which oligosaccharide chains are attached.
Fibres : There are three types of fibres secreted by connective tissue cells: collagen fibres , reticular fibres , and elastic fibres. The abundance and preponderance of different types of fibres varies in different CTs. Each type of fibre is formed by proteins made of long peptide chains.
- they produce precursors of all types of fibres (collagen, reticular, elastin) and ground substance. Reticular cell:
- stellate cells
- production of collagen III of reticular fibres
Pigment cells: (melanocytes)
- production of pigment melanin
- cells originate from neuroectoderm Adipose (fat) cells:
- also called adipocytes, these cells are specialized to store neutral fat.
- white fat – univacuolar cells (one large lipid droplet)
- brown fat – multivacuolar cells ( several small lipid droplets)
Mobile cells: Histiocytes:
- are phagocytic cells (fixed macrophages), after activation transform into migrating form – free macrophages Mast cells:
- with granules containing histamine, heparin and anaphylactic factors. When released in response to an antigen, they cause hypersensitivity reactions, allergy and anaphylaxis. Plasma cell:
- with voluminous cytoplasm and typical appearance of nucleus („clock“). Plasma cells are derived from B-lymphocytes and produce antibodies against a specific antigen. They have a limited migratory ability and a short life. Undifferentiated mesenchyme cells: These are cells that retain the multiple potentials of embryonic mesenchyme cells. They are found in the tunica adventitia (the outer layer of CT) of venules.
Wandering cells : Neutrophils: Neutrophils are white blood cells that act as phagocytes in the early stages of acute inflammation. Eosinophils: Eosinophils are white blood cells that are found in the lamina propria of the GI tract, and at sites of allergic reaction and parasitic infection. Basophils: Basophils are white blood cells that are similar to mast cells in having vasoactive agents released in response to an allergen. Lymphocytes: These are cells responsible for immune responses that circulate in the blood. Normally, only small numbers are found in the CTs throughout the body. The number increases dramatically at certain sites of tissue inflammation. They are also very numerous in the lamina propria of the respiratory and gastrointestinal tracts, where they are involved in immunosurveillance. The lamina propria is a layer of loose CT lying immediately beneath the epithelium. Monocytes: Monocytes are white blood cells that will give rise to all the phagocytes of the mononuclear phagocytic system.
Classification of connective tissues: CTs are classified on the basis of types and relative abundance of cells, fibres and ground substance, and on the organization of fibres.
Mesenchyme: Mesenchyme contains fairly uniform appearing, small spindle-shaped cells whose processes extend and contact those of other cells to form a three dimensional cellular network. A semi-fluid ground substance fills the intercellular spaces. Fibres are present, but are very fine and sparse.
Mucose CT or Wharton's jelly , is present in the umbilical cord (Its adult counterpart is found in the iris of the eye and in dental pulp of deciduous teeth.) In mucous CT, the ground substance is more viscous or jelly-like than in mesenchyme. Fibroblasts are the predominant cell type, and the number of fibres increases with age. Loose (areolar) CT:
- cellular type of CT – all types of cells
- abundant ground substance and thin and relatively sparse fibres (mainly collagenous)
- viscous gel-like consistency
- is important for the diffusion of oxygen and nutrients, phagocytosis; connective function examples of occurrence: beneath epithelia as lamina propria, submucosa – distinct layer of the wall of hollow tubular organs Dense irregular CT:
- collagenous fibres form a bundles running in various directions (hence irregular)
- fibroblasts are scarce and usually the only cell type present
- little ground substance is present. examples of occurrence: is found on the outside of many organs as fibrous capsule, in the dermis of the skin and as a sclera. Dense regular CT:
- collagenous fibres are packed in dense regular arrays, between which lie rows of cells. examples of occurrence: in tendons, ligaments, (some also contain large amounts of elastic fibres and are called elastic ligaments), and aponeuroses. Elastic CT:
- bundles of elastic fibres (elastin causes yellow colour of CT)
- cells and ground substance are scarce
- mechanical functions examples of occurrence: ligamenta flava (spinal column), vocal ligaments; elastic fibres are present as elastic membranes in the wall of blood vessels Reticular CT:
- consists of reticular cells and reticular fibres
- serves as supporting network for free cells (lymfocytes or hematopoietic cells) examples of occurrence: some lymph organs, bone marrow. Adipose CT: Adipocytes, which are specialized to store fat, are found throughout loose connective tissue. When adipocytes are the predominant cell present, the tissue is called adipose tissue. In white or unilocular fat, adipocytes contain a single, large lipid droplet surrounded by a thin layer of cytoplasm. The lipid mass compresses the nucleus to an eccentric position, producing a "signet ring" appearance. A different kind of adipose tissue is known as brown or multilocular fat. Brown fat contains fat droplets of varying sizes. The cells are smaller than those of white fat, with an eccentric round nucleus. Brown fat has a very limited distribution in adult humans, but is found in many animals. In hibernating animals, the oxidation of brown fat warms the blood flowing through it during arousal from hibernation. Human newborns, whose large surface to volume ratio can result in heat loss, also have a lot of brown fat. Most of it disappears during the first decade of life.
Cartilage
- type of connective tissue whose cells, called chondrocytes , secrete extracellular substances of ground mater and proteins of collagen or elsatic fibres
FIBERS: Elastins (proteins of elastic fibers) Chondrocytes (in lacunae) are dispersed difusely, are not arranged in isogenic groups. MICROSCOPIC APPEARANCE: Under low light, when you focus in and out, you can see refraction of the elastic fibers under the microscope. Elastic fibers can be visualized by staining with orcein or resorcin fuchsin. DISTRIBUTION: Auditory tube + auricle of ear Epiglottis Small laryngeal cartilages c) FIBROCARTILAGE: Found in places where high stress occurs. DISTRIBUTION: It never occurs alone. It is closely associated with either dense connective tissue or with hyaline cartilage. Intervertebral Disks Articular Disks Pubic Symphysis STRUCTURE: It has no perichondrium GROWTH: It grows more like connective tissue (i.e. interstitial growth) due to the absence of a perichondrium. FIBERS: Collagen Type I Chondrocytes are not numerous, they are flattened and arranged in rows due to press of thich bundles of collagen fibers.
Bone
Bone is a connective tissue that is characterized by a mineralized extracellular matrix. The matrix is secreted by cells called osteocytes.
UNIQUE QUALITIES OF BONE
HYDROXYAPATITE: Calcium Phosphate crystals. It prevents diffusion of metabolites. It prevents interstitial growth -- all bone growth occurs from the periosteum. A CANALICULAR SYSTEM: Tiny canals connect one haversian system to the other. VASCULARITY: All bone cells are in close proximity to vessels! APPOSITIONAL GROWTH: All growth occurs by appositional growth. BONE RECONSTRUCTION: Bone is dynamic tissue, constantly changing shape. LONG-BONE GROSS STRUCTURE :
- DIAPHYSIS: The shaft, with a medullary cavity on inside.
- EPIPHYSIS: The ends.
- METAPHYSIS: The site of ossification, between the diaphysis and epiphysis.
- ARTICULAR CARTILAGE: Hyaline cartilage covering compact bone at the ends of long bones. It lacks perichondrium.
- PERIOSTEUM: Osteogenic potential around the outside.
- ENDOSTEUM: Lines the marrow cavity and also has osteogenic potential. In Skull, the endosteum is the dura mater , and it has limited osteogenic potential which is important in fracture healing.
GENERAL PROPERTIES OF BONE
BONE: Consists of cells in lacunae, in an extracellular matrix
BONE CELL TYPES
A) OSTEOPROGENITOR CELLS: The Stem-Cells of bone. DISTRIBUTION: Found on the inner lining of the periosteum and endosteum. Found lining vascular canals. B) OSTEOBLASTS: They are secretory cells. SECRETE: They secrete the bone matrix. ALKALINE PHOSPHATASE which calcifies the matrix. They have polarity and resemble other secretory cells. C) OSTEOCYTES: They are osteoblasts that have become trapped in their own matrix. They are found in lacunae, between layers of lamellae , in the matrix of cortical bone. CANALICULI: Fine cytoplasmic extensions of the osteocytes running perpendicular to the haversian canals. D) OSTEOCLASTS: Large, multinucleate cells derived from monocytes. They have acid hydrolases. Osteoclasts have many lysosomes and are eosinophilic. HOWSHIP'S LACUNAE: The spaces for bone resorption, between the osteoclast and the bone resorption surface.
BONE-MATRIX
COLLAGEN: Type I Collagen = 85% - 90% of total bone protein. NON-COLLAGEN PROTEINS: Small percentage but very important. Cell Attachment Proteins: Fibronectin, Osteopontin Proteoglycans OSTEOCALCIN: i mportant in bone turnover. HYDROXYAPATITE: Bone salts (Calcium Phosphate) composes the non-protein inorganic part.
TYPES OF BONE
WOVEN BONE - collagen fibers are not arrenged into lamellae and running in different directions, they form a network in gound substance. Osteocytes in lacunae are situated in this network. COMPACT BONE - is recognized as:
- Lamellar bone : Bone arranged in concentric layers called lamellae , with Haversian Canals containing blood vessels in the center. Often found around the outside of large bones.
Compact lamellar bone : 1.Haversian Lamellae: Lamellae around central Haversian Canals , which contain blood vessels and nerves. Osteocytes are within the lamellae, with canaliculi radiating toward the central haversian canal.
Primary ossification centers close around the time of birth. Thereafter, long- bone growth occurs from the secondary ossification centers. SECONDARY OSSIFICATION CENTER: Forms at the epiphyseal plate. The orderly columns of chondrocytes are not seen here. Growth occurs from the epiphysis downward, toward the epiphyseal plate. EPIPHYSEAL CLOSURE: The end of longitudinal growth in long bone, when the primary ossification center overtakes (i.e. calcifies) the secondary ossification center, and hence long-bone growth ceases.
OSSIFICATION ZONES
RESERVE ZONE: This is the zone of normal hyaline cartilage. Little cellular activity or cell division is occurring.
PROLIFERATIVE ZONE: Chondrocytes are multiplying and arrange themselves in long parallel isogenous columns: This is the main zone responsible for growth of the long axis of the bone.
ZONE of HYPERTROPHY: Chondrocytes are hypertrophying and secreting alkaline phosphatase.
CALCIFICATION ZONE: Matrix around the hypertrophied cells calcified, trapping the chondrocytes in the matrix. Chondrocytes are dying here (due to no nutrients.).
„LINE of errosion“
OSSIFORM ZONE: calcified cartilage matrix = spicules are covered with osteoblasts are recruited from the blind end of vascular bud. OSTEOBLASTS produce bone matrix = osteoid, which is later calcified trapping osteoblasts in mineralized matrix - ossein
FORMATION OF THE MARROW CAVITY: Develops from the early cavities in trabecular bone, formed by erosions in the trabeculae. FORMATION OF HAVERSIAN SYSTEMS: Trabeculae of the long bone appear as stalactites, hanging down from the epiphyses. These trabeculae enclose tunnels where chondrocytes once resided. The vascular bud enters through those tunnels, and haversian canals form around those tunnels. Bone builds up on either side of the vascular tunnel. The two ridges then fuse, enclosing the vascular tunnel. What was periosteum forming on both sides has now become the endosteum of the tunnel. Ossification proceeds inward. Endosteal cells deposit bone until the vessel is completely enclosed.
THE ARF CYCLE
: The process of BONE-REMODELING, which occurs during growth and in mature bone. It explains the interdependence between osteoclastic and osteoblastic activity in bone- remodeling, which explains why Osteoporosis is difficult to treat.
Activation: Osteoclasts are activated and begin secreting acids to resorb bone. Resorption: Osteoclastic resorption occurs. Reversal: Resorption stops and osteoblasts take over. Formation: Osteoblasts form bone on the opposing surface to complete the bone reforming process.
PARATHYROID HORMONE: Enhances the rate of bone-turnover. Ultimately it takes Ca+2^ from the bone and puts it into the blood.
PTH indirectly stimulates Osteoclasts to resorb bone. PTH stimulation of Osteoclasts is mediated by Osteoblasts. Again this shows interdependence of the two cell types. PTH stimulates the release of soluble factors from osteoblasts. Those factors stimulate the osteoclastic activity. Osteoclasts have no PTH-Receptors. PTH also stimulates the differentiation of monocytes into osteoclasts, and increases the amount of osteoclastic ruffled borders. PTH also directly stimulates Bone-Lining Cells to transfer Ca+2. We need Parathyroid Hormone to survive and maintain Calcium homeostasis. We can live without Calcitonin.
VITAMIN-D: 1,25-Dihydroxy-Vit-D stim. osteoblasts to synthesis the bone matrix & alkaline phosphatase.
CALCITONIN: Takes Ca+2^ from the blood and deposits it into the bone.
Calcitonin inhibits osteoclastic activity by binding directly to osteoclasts. Osteoclasts do have Calcitonin receptors. Calcitonin also directly stimulates osteoblastic activity. Although Calcitonin opposes PTH, it does not completely counteract PTH. The effect of PTH is more important.
TRANSFORMING GROWTH FACTOR (TGF-beta): One connection between osteoblasts and osteoclasts.
TGF-beta is found in bone in its inactive form. Osteoclastic activity stimulates TGF-beta : increased acid production --> activation of TGF-beta in bone. TGF-beta, in turn, inhibits further osteoclastic activity (negative feedback) and promotes osteoblastic activity. TGF-beta, as well as other cytokines, ( IL-1 , Tumor Necrosis Factor ), appear to be involved in differentiation of monocytes into osteoclasts.
MECHANISMS OF CALCIFICATION
MATRIX VESICLES: Matrix vesicles are inside the osteoblasts. They accumulate the mineral and control the rate at which calcification proceeds. Intravesicular Calcium Accumulation : Ca+2^ binds to vesicle and combines with phosphates from the phospholipids to form the calcium salt. Extravesicular Calcium Accumulation: The hydroxyapatite crystals get exposed to the extracellular space. Vesicular Alkaline Phosphatase is required for external calcification to occur.
NEURON
Nerve cells are very variable in appearance, shape and size, but all neurons have a cell body, also called soma or perikarion , and processes extending from the nerve cell to communicate with other cells. There are two types of processes: dendrites that receive impulses and axons (neurits) that transmit impulses. All nerve cells have one axon, which is usually the longest process that extends from the cell and one or more (hundreds) dendrites, these are generally shorter and thicker than the axon. The junction where a nerve cell communicates with another nerve cell or an effector cell (eg. muscle fibre) is called a synapse, which can be chemical or electric. The terminal part of the axon with chemical synapses releases substances called a neurotransmitter which acts on the membrane of the other cell.
Main parts of neuron
Dendrites Cell body Axon (neurit) Axon terminal
Cell body – PERIKARION: contains nucleus and most cytoplasm with organelles:
- nucleus – round or oval, very light, with prominent nucleolus
- rough ER (called Nissl´ substance) – involved in synthesis of proteins (neurotransmitters)
- other usual organelles (mitochondria, Golgi apparatus, lysosomes)
- neurofibrils – neurofilaments and neurotubules
- pigment lipofuscin
DENDRITES – input structure – receive signals; number of dendrites: one – several hundreds
short, branched processes with structure similar to perikarion (cytoplasm + organelles + neurofibrils) incoming signals summate to initiate action potential highly branched tree structure
Classification of neurons according to number of processes (dendrites):
- Multipolar neuron – several dendrites extend from body found in brain & spinal cord
- Bipolar neuron – one dendrite and one axon (in retina of eye)
- Unipolar neuron – one process only, link to axon (sensory neurons)
- Pseudounipolar neuron – one short process divides later into dendrite and axon (spinal ganglia)
AXON – only one
no protein synthesis here Trigger zone - where nerve impulses arise Axon hillock – the cone-shaped base of the axon, its cytoplasm is free of rER (Nissl substance) Axons terminal - end with fine branching with „terminal boutons“ – mitochondria and synaptic vesicles containing neurotransmitters Axon hillock and terminal are not covered with oligodendrocytes (in CNS) or Schwann cells (in PNS) Serves for impulses transmission and for axonal transport of neurotransmitters and nutrients
Classification of neurons according to length of axon:
- Golgi type I – long axon (up to 1 m) – somatic motor neurons
- Golgi type II - short axon (in μm)
Classification of neurons according to function:
- sensitive neurons – (afferent) conduct informations from receptors to CNS
- motor neurons – (efferent) conduct infirmations from CNS to effector cells: somatomotor to skeletal muscle and visceromotor to smooth muscle cells, cardiomyocytes or glandular cells
- interneurons (97 %)
Sheaths of axons:
Schwann sheath (neurilemma) – Schwann cells surround the axon (gray fibers)
Myelin sheath – lipoprotein product of Schwann cells in PNS and oligodendrocytes in CNS
- electrically insulates axon - inreases speed of nerve impulse
- wraps around one axon many times and has a lamellar appearance
Many axons are wrapped in a lipid-rich covering called myelin. This myelin sheath insulates the axon from the surrounding extracellular component and increases the rate of electrical conduction. The myelin sheath is discontinuous at intervals called the nodes of Ranvier. The area covered with myelin is called internodal area (internodium). In myelinated axons, the voltage reversal (that is, the impulse propagation) can occur only at the nodes, and the impulse "jumps" from node to node. This is called saltatory conduction. In unmyelinated axons, the impulse is conducted more slowly, moving as a wave of voltage reversal along the axon.
Synapses
- NEURON – NEURON
Presynaptic neuron - conducts signal to a synapse // synaptic vesicles with neurotransmiter Synaptic cleft (20-30 nm thick) Postsynaptic neuron - conducts signal from a synapse // receptors on cell membrane
Effector cells – muscle cells (in smooth muscle = neuromuscular spindle , in skeletal muscle = motor-end-plate ), cardiomyocytes, glandular cells
Chemical Synapses
Presynaptic cell releases neurotransmitters from synaptic vesicles Act on the postsynaptic cell (help initiate AP) Neurotransmitters can excite or inhibit Neurotransmitters (acetylcholine, serotonin, norepinepherine and epinephrin, dopamine, GABA, …)
Neurotransmiter must be removed to prevent continual firing of neurons
