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12 PART 1 Fundamentals of the Nervous System and Nervous Tissue Pages 349-367 PowerPoint® Lecture Presentations prepared by Leslie Hendon University of Alabama, Birmingham © 2014 Pearson Education, Inc. Nervous System • Master control and communication system • Has three overlapping functions • Sensory receptors monitor changes inside and outside the body • Change—a stimulus • Gathered information—sensory input • Processes and interprets sensory input • Makes decisions—integration • Dictates a response by activating effector organs • Response—motor output © 2014 Pearson Education, Inc. Nervous System Sensory input Integration Motor output © 2014 Pearson Education, Inc. Figure 12.1 Basic Divisions of the Nervous System • Central nervous system (CNS) • Brain and spinal cord • Integrating and command center • Peripheral nervous system (PNS) • Outside the CNS • Consists of nerves extending from brain and spinal cord • Cranial nerves • Spinal nerves • Peripheral nerves link all regions of the body to the CNS • Ganglia are clusters of neuronal cell bodies © 2014 Pearson Education, Inc. Basic Divisions of the Nervous System Brain CNS Spinal cord Nerves PNS Ganglia © 2014 Pearson Education, Inc. Figure 12.2 Sensory Input and Motor Output • Sensory (afferent) signals picked up by sensor receptors • Carried by nerve fibers of PNS to the CNS • Motor (efferent) signals are carried away from the CNS • Innervate muscles and glands © 2014 Pearson Education, Inc. Sensory Input and Motor Output • Divided according to region they serve • Somatic body region • Visceral body region • Results in four main subdivisions • Somatic sensory • Visceral sensory • Somatic motor • Visceral motor (autonomic nervous system) © 2014 Pearson Education, Inc. Types of Sensory and Motor Information Central nervous system (CNS) Peripheral nervous system (PNS) Brain and spinal cord Integrative and control centers Cranial nerves and spinal nerves Communication lines between the CNS and the rest of the body Sensory (afferent) division Motor (efferent) division Somatic and visceral sensory nerve fibers Conducts impulses from receptors to the CNS Somatic sensory fiber Visceral sensory fiber Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands) Somatic nervous system Skin Somatic motor (voluntary) Conducts impulses from the CNS to skeletal muscles Stomach Autonomic nervous system (ANS) Visceral motor (involuntary) Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Mobilizes body systems during activity Sympathetic motor fiber of ANS Parasympathetic division Conserves energy Promotes housekeeping functions during rest Heart Structure Function Sensory (afferent) division of PNS Parasympathetic motor fiber of ANS Bladder Motor (efferent) division of PNS © 2014 Pearson Education, Inc. Figure 12.3 Basic Divisions of the Nervous System • Somatic sensory • General somatic senses—receptors are widely spread • Touch • Pain • Vibration • Pressure • Temperature © 2014 Pearson Education, Inc. Basic Divisions of the Nervous System • Somatic sensory (continued) • Proprioceptive senses—detect stretch in tendons and muscle • Body sense—position and movement of body in space • Special somatic senses • Hearing • Balance • Vision © 2014 Pearson Education, Inc. Basic Divisions of the Nervous System • Visceral sensory • General visceral senses—stretch, pain, temperature, nausea, and hunger • Widely felt in digestive and urinary tracts, and reproductive organs • Special visceral senses • Taste & smell • Somatic motor • General somatic motor—signals contraction of skeletal muscles • Under our voluntary control • Often called “voluntary nervous system” © 2014 Pearson Education, Inc. Basic Divisions of the Nervous System • Visceral motor • Regulates the contraction of smooth and cardiac muscle • Makes up autonomic nervous system • Controls function of visceral organs • Often called “involuntary nervous system” • Autonomic nervous system © 2014 Pearson Education, Inc. Nervous Tissue • Cells are densely packed and intertwined • Two main cell types • Neurons—transmit electrical signals • Support cells (neuroglial cells in CNS) • Nonexcitable • Surround and wrap neurons © 2014 Pearson Education, Inc. The Neuron • The human body contains billions of neurons • Basic structural unit of the nervous system • Specialized cells conduct electrical impulses along the plasma membrane • Nerve impulse (action potential) • Other special characteristics • Longevity • Do not divide • High metabolic rate • Neurons die after 5 minutes without oxygen © 2014 Pearson Education, Inc. The Cell Body • Cell body (soma) • Perikaryon—around the nucleus • Size of cell body varies from 5–140µm • Contains usual organelles plus other structures • Chromatophilic bodies (Nissl bodies) • Clusters of rough ER and free ribosomes • Stain darkly and renew membranes of the cell • Neurofibrils—bundles of intermediate filaments • Form a network between chromatophilic bodies © 2014 Pearson Education, Inc. The Cell Body • Most neuronal cell bodies are • Located within the CNS • Protected by bones of the skull and vertebral column • Ganglia—clusters of cell bodies • Lie along nerves in the PNS © 2014 Pearson Education, Inc. Structure of a Typical Large Neuron Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Dendrites Neuron cell body Nucleus with nucleolus Neurofibril Nucleus Chromatophilic (Nissl) bodies (b) Axon (impulse generating and conducting region) Nucleolus Nissl bodies Axon hillock (a) © 2014 Pearson Education, Inc. Neurilemma Impulse direction Schwann cell (one internode) Nuclei of neuroglial cells Node of Ranvier Axon terminals (secretory region) Terminal branches Figure 12.4 Neuron Processes • Dendrites • Extensively branching from the cell body • Transmit electrical signals toward the cell body • Chromatophilic bodies—only extend into the basal part of dendrites and to the base of the axon hillock • Function as receptive sites for receiving signals from other neurons © 2014 Pearson Education, Inc. Neuron Processes • Axons • Neuron has only one • Impulse generator and conductor • Transmits impulses away from the cell body • Chromatophilic bodies are absent • No protein synthesis in axon © 2014 Pearson Education, Inc. Neuron Processes • Axons (continued) • Neurofilaments, actin microfilaments, and microtubules • Provide strength along length of axon • Aid in the transport of substances to and from the cell body • Axonal transport © 2014 Pearson Education, Inc. Neuron Processes • Axons (continued) • Branches along length are infrequent • Axon collaterals • Multiple branches at end of axon • Terminal branches (telodendria) • End in knobs called axon terminals (also called end bulbs or boutons) © 2014 Pearson Education, Inc. Neuron Processes • Nerve impulse • Generated at the initial segment of the axon • Conducted along the axon • Releases neurotransmitters at axon terminals • Neurotransmitters—excite or inhibit neurons • Neuron receives and sends signals © 2014 Pearson Education, Inc. Synapses • Site at which neurons communicate • Signals pass across synapse in one direction • Presynaptic neuron • Conducts signal toward a synapse • Postsynaptic neuron • Transmits electrical activity away from a synapse © 2014 Pearson Education, Inc. Two Neurons Communicating at a Synapse Presynaptic neuron Axon Axon terminal at synapse Postsynaptic neuron Synapse Dendrite (a) Two neurons connected by synapses © 2014 Pearson Education, Inc. Figure 12.5a Types of Synapses • Axodendritic • Between axon terminals of one neuron and dendrites of another • Most common type of synapse • Axosomatic • Between axons and neuronal cell bodies © 2014 Pearson Education, Inc. Some Important Types of Synapses Presynaptic axon Nerve impulses Microtubule Neurofilament Axon terminal Vesicle releasing neurotransmitter Mitochondrion Synaptic vesicles Synaptic cleft Postsynaptic dendrite (b) Enlarged view of the synapse © 2014 Pearson Education, Inc. Figure 12.5b Synapses • Elaborate cell junctions • Axodendritic synapses—representative type • Synaptic vesicles on presynaptic side • Membrane-bound sacs containing neurotransmitters • Mitochondria abundant in axon terminals • Synaptic cleft • Separates the plasma membrane of the two neurons © 2014 Pearson Education, Inc. Classification of Neurons • Structural classification • Multipolar—possess more than two processes • Numerous dendrites and one axon • Bipolar—possess two processes • Rare neurons • Found in some special sensory organs • Unipolar (pseudounipolar)—possess one short, single process • Start as bipolar neurons during development © 2014 Pearson Education, Inc. Table 12.2 Comparison of Structural Classes of Neurons (1 of 3) © 2014 Pearson Education, Inc. Table 12.2 Comparison of Structural Classes of Neurons (2 of 3) © 2014 Pearson Education, Inc. Neurons Classified by Structure © 2014 Pearson Education, Inc. Functional Classification of Neurons • Functional classification is • According to the direction the nerve impulse travels relative to the CNS • Types of neurons • Sensory neurons • Motor Neurons • Interneurons © 2014 Pearson Education, Inc. Functional Classification of Neurons • Sensory neurons • Transmit impulses toward the CNS • Virtually all are unipolar neurons • Cell bodies in ganglia outside the CNS • Short, single process divides into • The central process—runs centrally into the CNS • The peripheral process— extends peripherally to the receptors © 2014 Pearson Education, Inc. Functional Classification of Neurons • Motor (efferent) neurons • Carry impulses away from the CNS to effector organs • Most motor neurons are multipolar • Cell bodies are within the CNS • Form junctions with effector cells • Interneurons (association neurons)— most are multipolar • Lie between motor and sensory neurons © 2014 Pearson Education, Inc. Neurons Classified by Function © 2014 Pearson Education, Inc. Supporting Cells • Six types of supporting cells • Four in the CNS • Two in the PNS • Provide supportive functions for neurons • Cover nonsynaptic regions of the neurons © 2014 Pearson Education, Inc. Neuroglial in the CNS • Neuroglia • Glial cells have branching processes and a central cell body • Outnumber neurons 10 to 1 • Make up half the mass of the brain • Can divide throughout life © 2014 Pearson Education, Inc. Neuroglia in the CNS • Astrocytes are the most abundant glial cell type • Sense when neurons release glutamate • Extract blood sugar from capillaries for energy • Take up and release ions to control environment around neurons • Involved in synapse formation in developing neural tissue • Produce molecules necessary for neuronal growth (BDTF) • Propagate calcium signals involved with memory © 2014 Pearson Education, Inc. Neuroglia in the CNS Capillary Neuron Astrocyte (a) Astrocytes are the most abundant CNS neuroglia. © 2014 Pearson Education, Inc. Figure 12.6a Neuroglia in the CNS • Microglia—smallest and least abundant glial cell • Phagocytes—the macrophages of the CNS • Engulf invading microorganisms and dead neurons • Derive from blood cells called monocytes © 2014 Pearson Education, Inc. Neuroglia in the CNS Neuron Microglial cell (b) Microglial cells are defensive cells in the CNS. © 2014 Pearson Education, Inc. Figure 12.6b Neuroglia in the CNS • Ependymal cells • Line the central cavity of the spinal cord and brain • Bear cilia—help circulate the cerebrospinal fluid • Oligodendrocytes—have few branches • Wrap their cell processes around axons in CNS • Produce myelin sheaths © 2014 Pearson Education, Inc. Neuroglia in the CNS Fluid-filled cavity Ependymal cells Brain or spinal cord tissue (c) Ependymal cells line cerebrospinal fluid–filled cavities. Myelin sheath Process of oligodendrocyte Nerve fibers (d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers. © 2014 Pearson Education, Inc. Figure 12.6c, d Neuroglia in the PNS • Satellite cells—surround neuron cell bodies within ganglia • Schwann cells (neurolemmocytes)—surround axons in the PNS • Form myelin sheath around axons of the PNS Satellite cells Cell body of neuron Schwann cells (forming myelin sheath) Nerve fiber (e) Satellite cells and Schwann cells (which form myelin) surround neurons in the PNS. © 2014 Pearson Education, Inc. Figure 12.6e Myelin Sheaths • Segmented structures composed of the lipoprotein myelin • Surround thicker axons • Form an insulating layer • Prevent leakage of electrical current • Increase the speed of impulse conduction © 2014 Pearson Education, Inc. Myelin Sheaths in the PNS • Formed by Schwann cells • Develop during fetal period and in the first year of postnatal life • Schwann cells wrap in concentric layers around the axon • Cover the axon in a tightly packed coil of membranes • Neurilemma • Material external to myelin layers © 2014 Pearson Education, Inc. Myelin Sheaths in the PNS (a) Myelinated axon in PNS An axon wrapped with a fatty insulating sheath formed from Schwann cells Schwann cell plasma membrane Schwann cell cytoplasm Axon 1 A Schwann cell envelops an axon. Schwann cell nucleus Myelin sheath Schwann cell cytoplasm Neurilemma 2 The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers. Neurilemma Myelin sheath © 2014 Pearson Education, Inc. Axon Cross section of a myelinated axon (TEM 30,000) 3 The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath. Figure 12.7a Unmyelinated Axons in the PNS (b) Unmyelinated axons in PNS Axons that are not covered with an insulating sheath Schwann cell Schwann cell Axons Schwann cell nucleus 1 A Schwann Neurilemma cell surrounds multiple axons. Axons Cross section of unmyelinated axons (TEM 11,000) 2 Each axon is encircled by the Schwann cell plasma membrane. © 2014 Pearson Education, Inc. Figure 12.7b Myelin Sheaths in the PNS • Nodes of Ranvier—gaps along axon • Thick axons are myelinated • Thin axons are unmyelinated • Conduct impulses more slowly Myelin Sheaths in the CNS • Oligodendrocytes form the myelin sheaths in the CNS • Have multiple processes • Coil around several different axons © 2014 Pearson Education, Inc. Nerves • Nerves—cablelike organs in the PNS • Consists of numerous axons wrapped in connective tissue • Axon is surrounded by Schwann cells • Endoneurium—layer of delicate connective tissue surrounding the axon • Perineurium—connective tissue wrapping surrounding a nerve fascicle • Nerve fascicles—groups of axons bound into bundles • Epineurium—whole nerve is surrounded by tough fibrous sheath © 2014 Pearson Education, Inc. Figure 12.9 Structure of a nerve. Axon Myelin sheath Endoneurium Perineurium Blood vessels Fascicle Epineurium Myelinated axons Myelin sheath Fascicle Epineurium Schwann cell nucleus Axon Myelin Myelin sheath gap © 2014 Pearson Education, Inc. Gray and White Matter in the CNS • Gray matter • Is gray-colored and surrounds hollow central cavities of the CNS • Forms H-shaped region in the spinal cord • Dorsal half contains cell bodies of interneurons • Ventral half contains cell bodies of motor neurons • Primarily composed of neuronal cell bodies, dendrites, unmyelinated axons • Surrounds white matter of CNS in cerebral cortex and cerebellum © 2014 Pearson Education, Inc. Figure 12.8 Cross section through the spinal cord. PNS CNS Spinal nerve Gray matter • Short nonmyelinated interneurons • Cell bodies of interneurons and motor neurons • Neuroglia Motor (efferent) fiber White matter • Fiber tracts of myelinated and nonmyelinated axons • Neuroglia Sensory (afferent) fiber Central cavity © 2014 Pearson Education, Inc. Gray and White Matter in the CNS • White matter • Lies external to the gray matter of the CNS • Composed of myelinated axons • Consists of axons passing between specific regions of the CNS • Tracts are bundles of axons traveling to similar destinations © 2014 Pearson Education, Inc. Integration Between the PNS and CNS • The CNS and PNS are functionally interrelated • Nerves of the PNS • Information pathways to and from body periphery • Afferent PNS fibers respond to sensory stimuli • Efferent PNS fibers transmit motor stimuli from CNS to muscles and glands © 2014 Pearson Education, Inc. Integration Between the PNS and CNS • Nerves of the CNS • Composed on interneurons that • Process and receive sensory information • Direct information to specific CNS regions • Initiate appropriate motor responses • Transport information from one area of the CNS to another © 2014 Pearson Education, Inc. Reflex Arcs • Reflex arcs—simple chains of neurons • Explain reflex behaviors • Determine structural plan of the nervous system • Responsible for reflexes • Rapid, autonomic motor responses • Can be visceral or somatic © 2014 Pearson Education, Inc. Five Essential Components to the Reflex Arc • Receptor—site where stimulus acts • Sensory neuron—transmits afferent impulses to the CNS • Integration center—consists of one or more synapses in the CNS • Motor neuron—conducts efferent impulses from integration center to an effector • Effector—muscle or gland cell • Responds to efferent impulses • Contracting or secreting © 2014 Pearson Education, Inc. Figure 12.10 Components of a reflex arc. Stimulus Skin 1 Receptor Cell body Synapse Axon Interneuron 2 Sensory neuron 3 Integration center 4 Motor neuron 5 Effector Spinal cord (in cross section) © 2014 Pearson Education, Inc. Figure 12.11a Types of reflex arcs. 1 Sensory (stretch) receptor 2 Sensory (afferent) neuron 3 4 Motor (efferent) neuron 5 Effector organ Monosynaptic stretch reflex © 2014 Pearson Education, Inc. Figure 12.11b Types of reflex arcs. 1 Sensory receptor 2 Sensory (afferent) neuron 3 Interneuron 4 Motor (efferent) neuron 5 Effector organ Polysynaptic withdrawal reflex © 2014 Pearson Education, Inc. Neuronal Circuits • Diverging circuit—one presynaptic neuron synapses with several other neurons (divergence) • Converging circuit—many neurons synapse on a single postsynaptic neuron (convergence) • Reverberating circuit—circuit that receives feedback via a collateral axon from a neuron in the circuit © 2014 Pearson Education, Inc. Figure 12.12 Types of neuronal circuits. Input Input 1 Input 2 Many outputs Diverging circuit to multiple pathways Input 3 Output Converging circuit Input Output Reverberating circuit © 2014 Pearson Education, Inc. Input Processing • Serial processing • Neurons pass a signal to a specific destination along a single pathway from one to another • Parallel processing • Input is delivered along many pathways; a single sensory stimulus results in multiple perceptions © 2014 Pearson Education, Inc. Simplified Design of the Nervous System • Three-neuron reflex arcs • Basis of the structural plan of the nervous system • Similar reflexes are associated with the brain • Sensory neurons—located dorsally • Cell bodies outside the CNS in sensory ganglia • Central processes enter dorsal aspect of the spinal cord • Motor neurons—located ventrally • Axons exit the ventral aspect of the spinal cord © 2014 Pearson Education, Inc. Simplified Design of the Nervous System • Interneurons—located centrally • Synapse with sensory neurons • Interneurons are neurons confined to CNS • Long chains of interneurons between sensory and motor neurons © 2014 Pearson Education, Inc. Figure 12.13 A sensory stimulus elicits signals through several neuronal pathways. (1 of 2) © 2014 Pearson Education, Inc. Figure 12.13 A sensory stimulus elicits signals through several neuronal pathways. (2 of 2) © 2014 Pearson Education, Inc.