Nervous Tissue and The Nervous System PDF

Nervous Tissue and The Nervous System The Nervous System the most complex system in the body formed by a network of many billion nerve cells (neurons), assisted by many more supporting cells called glial cells two Anatomically, the two major divisions: Central nervous system (CNS)...

Nervous Tissue and The Nervous System The Nervous System the most complex system in the body formed by a network of many billion nerve cells (neurons), assisted by many more supporting cells called glial cells two Anatomically, the two major divisions: Central nervous system (CNS) 1. brain 2. spinal cord Peripheral nervous system (PNS) conducting impulses to and from the CNS (sensory and motor nerves, respectively) and ganglia that are small aggregates of nerve cells outside the CNS. 1. cranial nerves 2. spinal nerves 3. peripheral nerves Nervous Tissue The Nervous System Functionally, the two major divisions: 1. Sensory division (afferent) Somatic – sensory input perceived consciously (eg, from eyes ears, skin, musculoskeletal structures) Visceral – sensory input not perceived consciously (eg, from internal organs and cardiovascular structures) 2. Motor division (efferent) Somatic – motor output controlled consciously or voluntarily (eg, by skeletal muscle effectors) Autonomic – motor output not controlled consciously (eg, by heart or gland effectors) Nervous Tissue  The Nervous System Autonomic – motor output not controlled consciously (eg, by heart or gland effectors) ANS have pathways involving two neurons: 1. a preganglionic neuron with the cell body in the CNS 2. postganglionic neuron with the cell body in a ganglion. ANS has two divisions: (1) The parasympathetic division with its ganglia within or near the effector organs maintains normal body homeostasis. (2) The sympathetic division has its ganglia close to the CNS controls the body’s responses during emergencies and excitement. ANS components located in the wall of the digestive tract are sometimes referred to as the enteric nervous system. Nervous Tissue  Development of the Neural Tissue : NEURULATION The nervous system develops from the outermost of the three early embryonic layers, the ectoderm begins in the third week of development The notochord signals the ectoderm on the mid- dorsal side of the embryo to thicken to form the epithelial neural plate. The sides of this plate fold upward and grow toward each other medially, and within a few days fuse to form the neural tube. the neural crest separates from the neuroepithelium and becomes mesenchymal. Neural crest cells migrate extensively and differentiate as all the cells of the PNS, as well as a number of other non-neuronal cell types. Nervous Tissue The Nervous System Kinds of cells in the Nervous System 1. NEURONS The functional unit of the nervous system Usually has long processes Neurons respond to environmental changes (stimuli) by altering the ionic gradient that exists across their plasma membranes. All cells maintain electrical potential, but cells that can rapidly change this potential in response to stimuli (eg, neurons, muscle cells, some gland cells) are said to be excitable or irritable. Neurons react promptly to stimuli with a reversal of the ionic gradient (membrane depolarization) Nervous Tissue  The Nervous System Kinds of NEURONS in the Nervous System SENSORY NEURONS Afferent stimuli from receptors throughout the body. MOTOR NEURONS are efferent sending impulses to effector organs such as muscle fibers and glands. Somatic motor nerves Under voluntary control and typically innervate skeletal muscle Autonomic motor nerves control the involuntary unconscious activities of glands, cardiac muscle, and most smooth muscle. Interneurons establish relationships among other neurons, forming complex functional networks or Circuits in the CNS. Interneurons are either multipolar or anaxonic and comprise 99% of all neurons in adults. In the CNS most neuronal perikarya occur in the gray matter, with their axons concentrated in the white matter. In the PNS cell bodies are found in ganglia and in some sensory regions, such as the olfactory mucosa,Nervous and axons Tissueare bundled in ner  The Nervous System cells in the Nervous System In the CNS most neuronal perikarya occur in the gray matter, with their axons concentrated in the white matter. In the PNS cell bodies are found in ganglia and in some sensory regions, such as the olfactory mucosa, and axons are bundled in nerves. Nervous Tissue The Nervous System PARTS OF A NEURON 1. cell body (also called the perikaryon or soma) which contains the nucleus and most of the cell’s organelles and serves as the synthetic or trophic center for the entire neuron. 2. Dendrites the numerous elongated processes extending from the perikaryon receive stimuli from other neurons at unique sites called synapses. 3. axon (Gr. axon, axis) a single long process ending at synapses specialized to generate and conduct nerve impulses to other cells (nerve, muscle, and gland cells may also receive information from other neurons that mainly modifies the transmission of action potentials to those neurons Nervous Tissue  The Nervous System Neurons can be classified according to the number of processes extending from the cell body 1. Multipolar neurons with one axon and two or more dendrites the most common. 2. Bipolar neurons with one dendrite and one axon comprise the sensory neurons of the retina, the olfactory epithelium, and the inner ear. 3. Unipolar or pseudounipolar neurons Include all other sensory neurons, each have a single process that bifurcates close to the perikaryon, with the longer branch extending to a peripheral ending and the other toward the CNS. 4. Anaxonic neurons with many dendrites but no true axon, do not produce action potentials, but regulate electrical changes of adjacent CNS neurons Nervous Tissue Cell Body (Perikaryon or Soma) contains the nucleus and cytoplasm, exclusive of the cell processes acts as a trophic center, producing most cytoplasm for the processes. Most cell bodies are in contact with a great number of nerve endings conveying excitatory or inhibitory stimuli generated in other neurons. neuron has an unusually large, euchromatic nucleus with a prominent nucleolus, indicating intense synthetic activity. Cytoplasm contains free polyribosomes and highly developed RER Nissl substance, Nissl bodies Basophilic chromatophilic substance that are regions with concentrated RER and other polysomes abundant in large nerve cells such as motor neurons The Golgi apparatus is located only in the cell body, but mitochondria can be found throughout the cell and are usually abundant in the axon terminals. Nervous Tissue DENDRITES are typically short, small processes emerging and branching off the soma Usually covered with many synapses, dendrites are the principal signal reception and processing sites on neurons. The large number and extensive arborization of dendrites allow a single neuron to receive and integrate signals from many other nerve cells. Nervous Tissue AXONS Most neurons have only one axon, typically longer than its dendrites. Axons of the motor neurons that innervate the foot muscles have lengths of nearly a meter The plasma mem brane of the axon is called the axolemma The contents are known as axoplasm. Axons originate from a pyramid-shaped region of the perikaryon called the axon hillock branch less profusely than dendrites, but do undergo terminal arborization small axonal branch ends with a dilation , Terminal bouton that contacts another neuron or non-nerve cell at a synapse to initiate an impulse in that cell Nervous Tissue NERVE IMPULSES A nerve impulse, or action potential, travels along an axon like a spark moves along an explosive’s fuse. an electro chemical process initiated at the axon hillock when other impulses received at the cell body or dendrites meet a certain threshold. The action potential is propagated along the axon as a wave of membrane depolarization produced by voltage-gated Na+ and K+ channels in the axolemma that allow diffusion of these ions into and out of the axoplasm. The xtracellular compartment around all regions of the neuron is a very thin zone immediately outside the cell that is formed by enclosing glial cells which also regulate its ionic contents. In unstimulated neurons ATP-dependent Na-K pumps and other membrane proteins maintain an axoplasmic Na+ concentration only one-tenth of that outside the cell and a K+ level many times greater than the extracellular concentration. This produces a potential electrical difference across the axolemma of about –65 mV, with the inside negative to the outside. This difference is the axon’s resting potential. Nervous Tissue NERVE IMPULSES When the threshold for triggering an impulse is met, channels at the axon’s initial segment open and allow a very rapid influx of extracellular Na+ that makes the axoplasm positive in relation to the extracellular environment Shifts (depolarizes) the resting potential from negative to positive, to +30 mV. after the membrane depolarization, the voltage-gated Na+ channels close and those for K+ open, which rapidly returns the membrane to its resting potential. occurs in less than 1 millisecond. Nervous Tissue SYNAPTIC COMMUNICATION Synapses are sites where nerve impulses are transmitted from one neuron to another, or from neurons and other effector cells. ensures that transmission is unidirectional Synapses convert an electrical signal (nerve impulse) from the presynaptic cell into a chemical signal that affects the postsynaptic cell. Most synapses act by releasing neurotransmitters to either open or close ion channels or initiate second messenger cascades. Nervous Tissue SYNAPTIC COMMUNICATION A synapse has the following components: 1. presynaptic axon terminal (terminal bouton) contains mitochondria and numerous synaptic vesicles from which neurotransmitter is released by exocytosis contain a large number of synaptic vesicles containing eurotransmitters, numerous mitochondria, and smooth ER as asource of new membrane. Upon arrival of a nerve impulse, voltage-regulated Ca2+ channels permit Ca2+ entry that triggers neurotransmitter release into the synaptic cleft. 2. The postsynaptic membrane contains the neurotransmitter receptors and mechanisms to initiate an impulse at the postsynaptic neuron. postsynaptic cell membrane contains receptors for the neurotransmitter, and ion channels or other mechanisms to initiate a new impulse. At the presynaptic region the nerve impulse briefly opens calcium channels, promoting a Ca2+ influx that triggers neurotransmitter release by exocytosis Immediately the released neurotransmitter molecules diffuse across the synaptic cleft and bind receptors at the post synaptic region. Nervous Tissue SYNAPTIC COMMUNICATION Immediately the released neurotransmitter molecules diffuse across the synaptic cleft and bind receptors at the post synaptic region. This produces either an excitatory or an inhibitory effect at the postsynaptic membrane: Neurotransmitters from excitatory synapses cause postsynaptic Na+ channels to open, and the resulting Na+ influx initiates a depolarization wave in the postsynaptic neuron or effector cell At inhibitory synapses neurotransmitters open Cl- or other anion channels, causing influx of anions and hyperpolarization of the postsynaptic cell, making its membrane potential more negative and more resistant to depolarization. Nervous Tissue  The Nervous System GLIAL CELLS Glial cells support neuronal survival and activities ten times more abundant than neurons in the mammalian brain. Like neurons most glial cells develop from progenitor cells of the embryonic neural plate. Nervous Tissue  The Nervous System GLIAL CELLS 1. OLIGODENCROCYTES Nervous Tissue  The Nervous System GLIAL CELLS 1. OLIGODENDROCYTES Oligodendrocytes extend many processes, each of which becomes sheet-like and wraps repeatedly around a portion of a nearby CNS axon most cytoplasm gradually moves out of the growing extension, leaving multiple compacted layers of cell membrane collectively termed myelin. An axon’s full length is covered by the action of many oligodendrocytes. The resulting myelin sheath electrically insulates the axon and facilitates rapid transmission of nerve impulses. Found only in the CNS are the predominant glial cells in white matter, which is white because of the lipid concentrated in the wrapped membrane sheaths. Nervous Tissue  The Nervous System GLIAL CELLS 2. ASTROCYTES Also unique to the CNS have a large number of long radiating, branching processes Astrocytes originate from progenitor cells in the embryonic neural tube most numerous glial cells of the brain, as well as the most diverse structurally and functionally. Fibrous astrocytes are abundant in white matter predominate in the gray matter. Nervous Tissue  The Nervous System GLIAL CELLS 3. EPENDYMAL CELLS are columnar or cuboidal cells that line the fluid- filled ventricles of the brain and the central canal of the spinal cord In some CNS locations, the apical ends of ependymal cells have cilia, which facilitate the movement of cerebrospinal fluid (CSF), and long microvilli, which are likely involved in absorption. Nervous Tissue  The Nervous System GLIAL CELLS 4. MICROGLIA Less numerous than oligodendrocytes or astrocytes microglia are small cells with actively mobile processes evenly distributed throughout gray and white matter Microglial cells also constitute the major mechanism of immune defense in the CNS, removing any microbial invaders and secreting a number of immunoregulatory cytokines. from circulating blood monocytes, Nervous Tissue  The Nervous System GLIAL CELLS 5. SCHWANN CELLS sometimes called neurolemmocytes are found only in the PNS differentiate from precursors in the neural crest. Schwann cells are the counterparts to oligodendrocytes of the CNS Have trophic interactions with axons and most importantly forming their myelin sheathes. However unlike an oligodendrocyte A Schwann cell forms myelin around a portion of only one axon. Nervous Tissue  The Nervous System GLIAL CELLS 6. SATELLITE CELLS OF THE GANGLIA Also derived from the embryonic neural cres small satellite cells form a thin, intimate glial layer around each large neuronal cell body in the ganglia of the PNS exert a trophic or supportive effect on these neurons, insulating, nourishing, and regulating their microenvironments. Nervous Tissue  The Central Nervous System There are organized areas of white matter and gray matter Differences caused by the differential distribution of lipid-rich myelin. main components of white matter are myelinated axons often grouped together as tracts, and the myelin-producing oligodendrocytes. Astrocytes and microglia are also present, but very few neuronal cell bodies. Gray matter contains abundant neuronal cell bodies, dendrites, astrocytes, and microglial cells, and is where most synapses occur. Gray matter makes up the thick cortex or surface layer of both the cerebrum the cerebellum most white matter is found in deeper regions. Deep within the brain are localized, variously shaped darker areas called the cerebral nuclei, each containing large numbers of aggregated neuronal cell bodies. Nervous Tissue  The Central Nervous System 1. cerebral cortex six layers of neurons with different sizes and shapes. most conspicuous of these cells are the efferent pyramidal neurons Neurons of the cerebral cortex function in the integration of sensory information and the initiation of voluntary motor responses. 2. cerebellar cortex coordinates muscular activity throughout the body and is organized with three layers a. A thick outer molecular layer has much neuropil and scattered neuronal cell bodies. b. thin middle layer consists only of very large neurons called Purkinje cells c. thick inner granular layer contains various very small, densely packed neurons and little neuropil. Nervous Tissue  The Central Nervous System 3. Spinal Cord the white matter is peripheral and the gray matter forms a deeper, H-shaped mass The two anterior projections of this gray matter, the anterior horns, contain cell bodies of very large motor neurons whose axons make up the ventral roots of spinal nerves. The two posterior horns contain interneurons which receive sensory fibers from neurons in the spinal (dorsal root) ganglia. Near the middle of the cord the gray matter surrounds a small central canal, which develops from the lumen of the neural tube, is continuous with the ventricles of the brain, is lined by ependymal cells, and contains CSF. Nervous Tissue  The Peripheral Nervous System MENINGES 1. DURA MATER thick external dura mater consists of dense irregular connective tissue organized as an outer periosteal layer continuous with the periosteum of the skull, and an inner meningeal layer Around the spinal cord the dura mater is separated from the periosteum of the vertebrae by the epidural space, which contains a plexus of thin-walled veins and loose connective tissue The dura mater may be separated from the arachnoid by formation of a thin subdural space. Nervous Tissue  The Peripheral Nervous System MENINGES 2. ARACHNOID has two components: (1) a sheet of connective tissue in contact with the dura mater and (2) system of loosely arranged trabeculae composed of collagen and fibroblasts, continuous with the underlying pia mater layer. Surrounding trabeculae is a large, sponge-like cavity, the subarachnoid space, filled with CSF. The subarachnoid space communicates with the ventricles of the brain where the CSF is produced. The connective tissue of the arachnoid is avascular but larger blood vessels run through it Nervous Tissue  The Peripheral Nervous System The main components of the peripheral nervous system (PNS) 1. Nerves 2. Ganglia 3. nerve endings. Nerves are bundles of nerve fibers (axons) surrounded by Schwann cells and layers of connective tissue. Nervous Tissue  The Peripheral Nervous System NERVE FIBERS are analogous to tracts in the CNS containing axons enclosed within sheaths of glial cells specialized to facilitate axonal function axons are sheathed by Schwann cells, or neurolemmocytes The sheath may or may not form myelin around the axons, depending on their diameter. Nervous Tissue  The Peripheral Nervous System MYELINATED NERVE FIBERS As axons are engulfed along their length by a series of differentiating neurolemmocytes and become myelinated nerve fibers. The plasma membrane of each covering Schwann cell fuses with itself at an area termed the mesaxon and a wide, flattened process of the cell continues to extend itself, moving circumferentially around the axon many times The multiple layers of Schwann cell membrane unite as a thick myelin sheath. Composed mainly of lipid bilayers and membrane proteins, myelin is a large lipoprotein complex Nervous Tissue  The Peripheral Nervous System MYELINATED NERVE FIBERS the myelin sheath serves to insulate axons and maintain a constant ionic microenvironment most suitable for action potentials. NODES OF RANVIER Nodal gaps adjacent Schwann cells on an axon the myelin sheath the axon is only partially covered by interdigitating Schwann cell processes. The axolemma is exposed to ions in the interstitial fluid and has a much higher concentration of voltage-gated Na+ channels, which renew the action potential and produce saltatory conduction of nerve impulses, Allows rapid movement from node to node. Nervous Tissue  The Peripheral Nervous System UNMYELINATED NERVE FIBERS the smallest-diameter axons of peripheral nerves are still enveloped within simple folds of Schwann cells the glial cell does not form the multiple wrapping of a myelin sheath In unmyelinated fibers, each Schwann cell can enclose portions of many axons with small diameters. nodes of Ranvier are not seen along unmyelinated nerve fibers. their impulse conduction is not saltatory and is much slower than that of myelinated axons. Nervous Tissue  The Peripheral Nervous System GANGLIA ovoid structures containing neuronal cell bodies and their surrounding glial satellite cells supported by delicate connective tissue and surrounded by a denser capsule. serve as relay stations to transmit nerve impulses The direction of the nerve impulse determines whether the ganglion will be a sensory or an autonomic ganglion. Nervous Tissue  The Peripheral Nervous System SENSORY GANGLIA Sensory ganglia receive afferent impulses that go to the CNS. Sensory ganglia are associated with both cranial nerves (cranial ganglia) and the dorsal roots of the spinal nerves (spinal ganglia). The large neuronal cell bodies of ganglia are associated with thin, sheet-like extensions of small glial satellite cells relay information from the ganglion’s nerve endings to the gray matter of the spinal cord via synapses with local neurons. Nervous Tissue  The Peripheral Nervous System AUTONOMIC GANGLIA Autonomic ganglia are small bulbous dilations in autonomic nerves, usually with multipolar neurons. Some are located within certain organs, especially in the walls of the digestive tract, where they constitute the intramural ganglia. The capsules of these ganglia may be poorly defined among the sacral portion of the spinal cord. 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