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Nervous System • Master control and communication system Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Nervous System: Functions • Three overlapping functions • Sensory receptors monitor changes inside and outside the body • Change – a stimulus • Gathered information – sensory input • CNS Processes and interprets sensory input • Makes decisions – integration • Dictates a response by activating effector organs • Response – motor output Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Basic Divisions of the Nervous System: CNS • Central nervous system (CNS) • Brain and spinal cord • Integrating and command center Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Basic Divisions of the Nervous System: PNS • Peripheral nervous system (PNS) • Outside the CNS • Nerves extending from brain and spinal cord • Cranial nerves • Spinal nerves • Link all regions of the body to the CNS Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Sensory Input and Motor Output • Sensory signals picked up by sensory receptors • Carried by afferent nerve fibers of PNS to the CNS • Motor signals are carried away from the CNS • Carried by efferent nerve fibers of PNS to effectors • Innervate muscles and glands Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Somatic Sensory • Somatic sensory • General somatic senses – receptors are widely spread • Touch, pain, vibration, pressure, and temperature • Proprioceptive senses – detect stretch in tendons and • muscle Body sense – position and movement of body in space • Special somatic senses • Hearing, balance, vision, and smell Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Visceral Sensory • Visceral sensory • General visceral senses – stretch, pain, temperature, nausea, and hunger • Widely felt in digestive and urinary tracts, reproductive organs • Special visceral senses – taste Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Somatic Motor • Somatic motor • General somatic motor – signals contraction of skeletal muscles • Under voluntary control • Often called “voluntary nervous system” Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Visceral Motor • Visceral motor • Regulates the contraction of smooth and cardiac • • • muscle and gland secretion Makes up autonomic nervous system Controls function of visceral organs Often called “involuntary nervous system” Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Peripheral Nervous System Summary Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.3 Types of Sensory and Motor Information Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.3 Types of Sensory and Motor Information Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.3 Nervous Tissue • Cells are densely packed and intertwined • Two main cell types • Neurons – transmit electrical signals • Support cells (neuroglial cells) – nonexcitable • Surround and wrap neurons Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings 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 • Graded potentials • Action potentials Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings The Neuron: Special Characteristics • Longevity – can live and function for a lifetime • Do not divide – fetal neurons lose their ability to • undergo mitosis; neural stem cells are an exception High metabolic rate – require abundant oxygen and glucose Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Neuron Structure Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings The Cell Body or Soma (also called Perikaryon) • Size varies from 5–140µm • Contains nucleus, 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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Nissl Body Staining Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings The Cell Body • Most neuronal cell bodies • Located within the CNS (clustered in nuclei) • Protected by bones of the skull and vertebral column • Ganglia – clusters of cell bodies in PNS Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Cell Body Structure Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.4 Neuron Processes: Dendrites • Dendrites • Extensively branching from • • • the cell body Transmit electrical signals (graded potentials) toward the cell body Chromatophilic bodies – only extend into the basal part of dendrites Function as receptive sites Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Dendritic Spines Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Neuron Processes: Axons • Axons (nerve fibers) • Neuron has only one, but it can • • • • branch Impulse generator and conductor Transmits action potentials away from the cell body Chromatophilic bodies absent No protein synthesis in axon Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Neuron Processes: Axons • Axons • 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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Neuron Processes • Axons • 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) Neuron Structure Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Neuron Processes: Action Potentials • Nerve impulse (action potential) • 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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Two Neurons Communicating at a Synapse Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.6 Structure of a Synapses PLAY Synapse Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.8a, b Signals Carried by Neurons: Resting Membrane Potential • • • Plasma membranes of neurons conduct electrical signals Resting neuron – membrane is polarized Inner, cytoplasmic side is negatively charged Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Changes in Membrane Potential • Signals occur as changes in membrane potential Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Directional Signals • • Stimulation of the neuron depolarization Inhibition of the neuron hyperpolarization Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Action Potentials Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.9a, b Action Potentials on Axons • Strong depolarizing stimulus applied to the axon hillock triggers • Action potential • • • Membrane becomes positive internally Action potential travels the length of the axon Membrane repolarizes itself Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Action Potentials on Axons Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.9c–e Graded Potentials on Dendrites and the Cell Body • • Natural stimuli applied to dendrites and the cell body • Receptive zone of the neuron Membrane stimulation causes local depolarization • A graded potential – inner surface becomes less • negative Depolarization spreads from receptive zone to the axon hillock • Acts as the trigger that initiates an action potential in the axon Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Synaptic Potentials • Excitatory synapses • Neurotransmitters alter the permeability of the • postsynaptic membrane Leads to an inflow of positive ions • Depolarizes the postsynaptic membrane • Drives the postsynaptic neuron toward impulse generation Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Synaptic Potentials • Inhibitory synapses • The external surface of the postsynaptic membrane becomes more positive • Reduces the ability of the postsynaptic neuron to generate an action potential Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Classification of Neurons • • Structural Classification Functional Classification Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Structural Classification of Neurons Classification based on number of processes • Multipolar • Bipolar • Unipolar (pseudounipolar) Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Multipolar Neurons Possess more than two processes Numerous dendrites and one axon Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.10a–c Bipolar Neurons Possess two processes Rare neurons – found in some special sensory organs Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.10a–c Unipolar (Pseudounipolar) Neurons Possess one single process Start as bipolar neurons during development Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.10a–c Afferent neurons • Afferent (sensory) neurons – transmit impulses toward the CNS • Virtually all are pseudounipolar neurons (some true • bipolar) 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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Afferent Neurons Periphery Sensory receptors Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings CNS Axon terminals Efferent Neurons • Efferent (motor) neurons • Carry impulses away from the CNS to effector • • • organs Most efferent neurons are multipolar Cell bodies are within the CNS Form junctions with effector cells Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Interneurons • Interneurons (association neurons) – most are multipolar • Lie between afferent and efferent neurons • Confined to the CNS Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Neurons Classified by Function Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.11 Variety of Interneurons • Purkinje cell, stellate cell, granule cell, and basket cell • Located in the cerebellum • Pyramidal cell – located in the cerebral cortex Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Variety of Interneurons Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Glial Cells (Supporting Cells) • Six types of glial cells • Four in the CNS • Two in the PNS • • Provide supportive functions for neurons Cover nonsynaptic regions of the neurons Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Supporting Cells (Neuroglial Cells) in the CNS • Neuroglia – usually only refers to supporting cells in the CNS, but can be used for PNS • 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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Types of Glial Cells in the CNS • • • • Astrocytes Microglia Ependymal Cells Oligodendrocytes Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Astrocytes • Astrocytes – most abundant glial cell type • Take up and release ions to control the environment • • • • around neurons Recapture and recycle neurotransmitters Involved with synapse formation in developing neural tissue Produce molecules necessary for neural growth (BDTF) Propagate calcium signals that may be involved in memory Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Astrocytes Necessary for development and maintenance of theblood brain barrier Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.12a Microglia • Microglia – smallest and least abundant • Phagocytes – • • the macrophages of the CNS Engulf invading microorganisms and dead neurons Derived from blood cells called monocytes Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.12b Ependymal Cells • Ependymal cells • Line the central cavity of the spinal cord and brain • Bear cilia – help circulate the cerebrospinal fluid Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Oligodendrocytes • Oligodendrocytes – have few branches • Wrap their cell processes around axons in CNS • Produce myelin sheaths Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Supporting Cells 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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.13 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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Myelin Sheaths in the PNS • • Nodes of Ranvier – gaps along axon Allow current exchange across axon membrane Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Myelin Sheaths in the PNS • Thick axons are myelinated • Fast conduction velocity • Thin axons are unmyelinated • Slow conduction velocity Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Myelin Sheaths in the PNS Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.14a Myelin Sheaths in the PNS – myelinated axon Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.15b Myelin Sheaths in the PNS – unmyelinated axons Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.15b Myelin Sheaths in the CNS • Oligodendrocytes form the myelin sheaths in the CNS • Have multiple processes • Coil around several different axons Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Oligodendrocytes Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Nerves • • • Nerves – cordlike organs in the PNS Consists of numerous axons wrapped in connective tissue Axon is surrounded by Schwann cells Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Nerves • • • • Endoneurium – layer of delicate connective tissue surrounding the axon Nerve fascicles – groups of axons bound into bundles Perineurium – connective tissue wrapping surrounding a nerve fascicle Epineurium – whole nerve is surrounded by tough fibrous sheath Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Simplified Design of the Nervous System • 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 • Interneurons – located centrally • Provide communication between sensory and motor neurons and between levels of the CNS Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Example of Neuronal Organization: Reflexes • Reflex arcs – simple neural pathways • Responsible for reflexes • Rapid, autonomic motor responses • Can be visceral or somatic Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Five Essential Components to the Reflex Arc • • • • • Receptor – detects the stimulus Afferent (sensory neuron) – transmits impulses to the CNS Integration center – consists of one or more synapses in the CNS Efferent (motor neuron) – conducts impulses from integration center to an effector Effector – muscle or gland cell • Responds to efferent impulses • Contraction or secretion Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Example of the Five Components to the Reflex Arc Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.17 Reflex Classification • • • • Monosynaptic or polysynaptic Spinal or cranial Somatic or autonomic Innate or learned Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Types of Reflexes: Number of Classes • Monosynaptic reflex – simplest of all reflexes • Just one synapse • The fastest of all reflexes • Example – knee-jerk reflex • Polysynaptic reflex – more common type of reflex • Most have a single interneuron between the sensory • and motor neuron Example – withdrawal reflexes Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Monosynaptic Reflex Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.18a, b Polysynaptic Reflex Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.18a, b Spinal vs Cranial Reflexes • Spinal = spinal cord integration center • Ex. Knee-jerk reflex • Cranial = brain as integration center • Ex. Pupillary light reflex Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Somatic vs Autonomic Reflexes • Somatic = motor neurons to skeletal muscles • Ex. Knee-jerk reflex • Autonomic = autonomic neurons to smooth muscle and glands • Ex. Pupillary light reflex Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Innate vs Learned Reflexes • Innate = born-with • Knee-jerk reflex, pupillary reflex • Learned = develops based on experiences • Pavlov’s dogs salivation in response to bell Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Gray versus White Matter in the Central Nervous System • Gray matter • Cell bodies • Dendrites • Synapses •White matter •Axons (myelin) Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Gray Matter in the Spinal Cord • Gray matter in the spinal cord • H-shaped (butterfly) region – surrounds central cavity • Dorsal half contains cell bodies of interneurons • Ventral half contains cell bodies of motor neurons • Cell bodies are clustered in the gray matter Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings White Matter in the Spinal Cord • White matter in the spinal cord • Located externally to the gray matter • Contains no neuronal cell bodies, but millions of axons • Myelin sheath – white color • Consists of axons running between different parts of the CNS • Tracts – bundles of axons traveling to similar destinations Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Gray Matter in Brain • Cortex and nuclei Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings White Matter in Brain • Pathways, tracts and commissures Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Disorders of the Nervous System • Multiple sclerosis – common cause of neural disability • Varies widely in intensity among those affected • Cause is incompletely understood • An autoimmune disease • Immune system attacks the myelin around axons in the CNS Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings