NERVE TISSUE NERVE TISSUE GENERAL CONCEPTS •Nerve tissue is highly specialized to employ modifications in membrane electrical potentials to relay signals throughout the body. Neurons form intricate circuits that: •Relay sensory information from the internal and external environments. •Integrate information among millions of neurons •Transmit effector signals to muscles and glands. •Anatomical subdivisions of nervous tissue • •Central nervous system (CNS) •Brain •Spinal cord • •Peripheral nervous system (PNS) •Nerves •Ganglia (singular, ganglion) • •Motor and sensory components NERVE TISSUE NERVE TISSUE CELLS OF NERVE TISSUE •Neurons •Functional units of the nervous system; receive, process, store, and transmit information to and from other neurons, muscle cells, or glands •Composed of a cell body, dendrites, axon and synapses •Form complex and highly integrated circuits •Supportive (glial) cells •Provide metabolic and structural support for neurons, insulate neurons via a myelin sheath, maintain homeostasis, and perform phagocytic functions • •Comprised of: •Astrocytes •Oligodendrocytes •Microglia •ependymal cells in the CNS •Schwann cells in the PNS NERVE TISSUE STRUCTURE OF A "TYPICAL" NEURON Cell body (soma, perikaryon) •Nucleus •Large, spherical, usually centrally located in the soma •Highly euchromatic with a large, prominent nucleolus • •Cytoplasm •Well-developed cytoskeleton •Intermediate filaments (neurofilaments) •Microtubules •Abundant rough endoplasmic reticulum and polysomes (Nissl substance) •Well-developed Golgi apparatus •Numerous mitochondria F09_06.jpg NERVE TISSUE STRUCTURE OF A "TYPICAL" NEURON Dendrite(s) •Usually multiple and highly branched at acute angles • •May possess spines small membranous elevations which form excitatory synapses • •Collectively, form the majority of the receptive field of a neuron; conduct impulses toward the cell body • •Cytoplasmic components •Microtubules and neurofilaments •Rough endoplasmic reticulum and polysomes •Smooth endoplasmic reticulum •Mitochondria NERVE TISSUE STRUCTURE OF A "TYPICAL" NEURON Axon •Usually only one per neuron •Generally, of smaller caliber and longer than dendrites •Branches at right angles, fewer branches than dendrites •Cytoplasmic components •Microtubules and neurofilaments •Lacks rough endoplasmic reticulum and polysomes •Smooth endoplasmic reticulum •Mitochondria •Axon hillock. Region of the cell body where axon originates •Devoid of rough endoplasmic reticulum and so stains pale •Continuous with initial segment of the axon that is a highly electrically excitable zone for initiation of nervous impulse •Usually, ensheathed by supporting cells •Transmits impulses away from the cell body to •Neurons •Effector structures such as muscle and glands •Branches extensively near its target, each branch ends in a swelling, the terminal bouton, which is the presynaptic element of a synapse NERVE TISSUE TYPES OF NEURONS BY SHAPE AND FUNCTION •Multipolar neuron. Most numerous and structurally diverse type •Efferent. Motor, carrying impulses out of the CNS or innervating smooth muscle from autonomic ganglia •Integrative function, axons remain in the CNS •Found throughout the CNS and in autonomic ganglia in the PNS •(Pseudo)unipolar neuron •Afferent. Sensory function, carrying impulses from peripheral receptors into the CNS •Found in selected areas of the CNS and in sensory ganglia of cranial nerves and spinal nerves (dorsal root ganglia) • •Bipolar neuron •Afferent. Sensory function •Found associated with organs of special sense (retina of the eye, olfactory epithelium, vestibular and cochlear ganglia of the inner ear) •Developmental stage for all neurons NERVE TISSUE ARRANGEMENT OF NEURONAL CELL BODIES AND THEIR PROCESSES In both CNS and PNS, cell bodies are found in clusters or layers and axons travel in bundles. These groupings are based on common functions and/or common connections. NERVE TISSUE THE REFLEX ARC The reflex arc is the simplest neuronal circuit. These circuits provide rapid, stereotyped reactions to help maintain homeostasis. To begin the reflex, a pseudounipolar, sensory neuron is activated by a receptor. The axon carries an afferent signal from the skin into the spinal cord where it synapses on a multipolar association neuron or interneuron. The interneuron signals a multipolar, motor neuron whose axon then carries an efferent signal to skeletal muscle to initiate contraction. NERVE TISSUE SYNAPSE •The function of the synapse is to alter the membrane potential of the postsynaptic target cell to either facilitate or inhibit the likelihood of the stimulus to be propagated by the postsynaptic cell. Most neurons receive thousands of synaptic contacts, both stimulatory and inhibitory, and the algebraic sum of these inputs determines whether the postsynaptic cell will depolarize. •Classified according to postsynaptic target: •Axodendritic. Most common •Axosomatic •Axoaxonic. Occur mostly at presynaptic terminals •Neuromuscular junction NERVE TISSUE SYNAPSE •Structure of the synapse •Presynaptic component •Distal end of the axon branches, each branch terminating in a swelling or button called the terminal bouton. •Boutons with neurotransmitter-containing synaptic vesicles and numerous mitochondria. •Synaptic gap/cleft. Separation (20-30 nm) between pre- and postsynaptic cells. •Postsynaptic component •Formed by the membrane of the postsynaptic neuron or muscle cell and contains receptors for neurotransmitters •Membrane shows a postsynaptic density or thickening on its cyto-plasmic side. •"Bouton-like" swellings along the length of an axon, allow a single axon to contact many distant cells. Common in smooth muscle innervation. 17 F09_07.jpg 18 Kromě chemických synapsí ještě ELEKTRICKÉ – signály přenášeny ionty, které pronikají spojovacím komplexem podobným nexu Excitační synapse – viz výše Nebo hyperpolarizace bez přenosu nervového impulzu – inhibiční synapse NERVE TISSUE SUPPORTIVE (GLIAL) CELLS •10x more abundant than neurons •Support and protect neurons •Supporting cells of the CNS • •Astrocytes •Oligodendrocytes •Ependymal cells •Microglial cells • •Supporting cells of the PNS • •Schwann cells •Satelite cells • White matter – axons, glial cells Grey matter – glial cells, neuronal cell bodies, neuropil Neuropil – dense network of astrocyte processes, axons and dendrites in gray matter. NERVE TISSUE SUPPORTIVE CELLS •Supporting cells of the CNS • •Astrocytes • •Stellate morphology • •Types: •Fibrous astrocytes in white matter •Protoplasmic astrocytes in gray matter • •Functions: •Physical support •Transport nutrients •Maintain ionic homeostasis •Take up neurotransmitters • •Form glial scars (gliosis) e. NERVE TISSUE SUPPORTIVE CELLS •Supporting cells of the CNS • •Oligodendrocytes • •Present in white and gray matter •Oligodendrocytes located in the white matter of the CNS produce the myelin sheath. •They can myelinate multiple axons 3. NERVE TISSUE SUPPORTIVE CELLS •Supporting cells of the CNS • •Ependymal cells •Line central canal of spinal cord and ventricles of brain •Cuboidal to columnar, with cilia and microvilli • C. NERVE TISSUE SUPPORTIVE CELLS •Supporting cells of the CNS • •Microglia • •Small cells with short irregular precesses, elongated nuclei •Originate from monocytes and have similar phagocytic function •Provide immune surveillance and produce immunomodulatory compounds NERVE TISSUE Main components of the peripheral nervous system •Nerves. •Bundle of nerve fibres (axons) surrounded by glial cells (Schwann cells) and connective tissue • •Ganglia. •Collections of neuron cell bodies • •Specialized nerve endings • NERVE TISSUE SUPPORTIVE CELLS •Supporting cells of the PNS • •Satellite cells • •surround cell bodies in ganglia • NERVE TISSUE SUPPORTIVE CELLS •Supporting cells of the PNS • •Schwann cells • •Surround unmyelinated axons. Numerous axons indent the Schwann cell cytoplasm and, therefore, are surrounded by a single plasma membrane. • •Produce the myelin sheath around axons NERVE TISSUE Unmyelinated axons NERVE TISSUE Myelinated axons NERVE TISSUE MYELIN SHEATH •The myelin sheath is formed by the plasma membrane of supporting cells wrapping around the axon. The sheath consists of multilamellar, lipid-rich segments produced by Schwann cells in the PNS and oligodendrocytes in the CNS. •Functions: •Increases speed of conduction (saltatory conduction) •Insulates the axon •Similar structure in CNS and PNS with some differences in protein composition •Organization: •Internode. Single myelin segment •Paranode. Ends of each internode where they attach to the axon •Node of Ranvier. Specialized region of the axon between myelin internodes where depolarization occurs • •In the PNS, each Schwann cell associates with only one axon and forms a single internode of myelin. •In the CNS, each oligodendrocyte associates with many axons (i.e., each oligodendrocyte forms multiple internodes on different axons). NERVE TISSUE CONNECTIVE TISSUE INVESTMENTS OF NERVOUS TISSUE •Central nervous system (meninges) • •Pia mater •Thin membrane lying directly on the surface of the brain and spinal cord •Accompanies larger blood vessels into the brain and spinal cord •Arachnoid membrane •Separated from pia mater by connective tissue trabeculae •Encloses the subarachnoid space, which contains blood vessels and the cerebrospinal fluid (CSF) produced by the cells of the choroid plexus •Together with pia mater, constitute the leptomeninges; inflam¬mation of these membranes produces meningitis •Dura mater •Outermost of the meninges •Dense connective tissue that includes the periosteum of the skull NERVE TISSUE CONNECTIVE TISSUE INVESTMENTS OF NERVOUS TISSUE •Peripheral nervous system • •Epineurium. Dense connective tissue surrounding fascicles and the entire nerve •Perineurium. Dense tissue surrounding groups of axons and their surrounding Schwann cells, forming fascicles; forms the blood-nerve barrier •Endoneurium. Delicate connective tissue surrounding Schwann cells; includes the basal lamina secreted by Schwann cells as well as reticular fibers • 44 Příčný řez nervem EM – epineurium (hustá vazivová síť bohatá na Col – kolagenní vlákna a F – fibroblasty), perineurium (několik vrstev plochých buněk pevně k sobě vázaných, hráz průniku makromolekul do nervu), endoneurium (hlavně retikulární vlákna – RF syntetizovaná Schwannovými buňkami – SC) Periferní nerv obsahující myelinizovaná M i nemyelinizovaná vlákna U; RF na příčném řezu – z endoneuria, S – jádro S.buňky; P, šipky - buňky perineuria – bariéra Výřez –část axonu s četnými neurofilamenty a mikrotubuly Histo dva malé nervy a tenká krycí vrstva; arrowheads – jádra S.buňky, šipka – axon, C - kapilára 46 Příčný řez nervem EM – epineurium (hustá vazivová síť bohatá na Col – kolagenní vlákna a F – fibroblasty), perineurium (několik vrstev plochých buněk pevně k sobě vázaných, hráz průniku makromolekul do nervu), endoneurium (hlavně retikulární vlákna – RF syntetizovaná Schwannovými buňkami – SC) Periferní nerv obsahující myelinizovaná M i nemyelinizovaná vlákna U; RF na příčném řezu – z endoneuria, S – jádro S.buňky; P, šipky - buňky perineuria – bariéra Výřez –část axonu s četnými neurofilamenty a mikrotubuly Histo dva malé nervy a tenká krycí vrstva; arrowheads – jádra S.buňky, šipka – axon, C - kapilára