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Chapter 12: Nervous Tissue Objectives Describe the anatomical and functional divisions of the nervous system. Sketch and label the structure of a typical neuron, describe the functions of each component, and classify neurons on the basis of their structure and function. Describe the locations and functions of the various types of neuroglia. Explain how the resting potential is created and maintained. Describe the events involved in the generation and propagation of an action potential. Discuss the factors that affect the speed with which action potentials are propagated. Describe the structure of a synapse, and explain the mechanism involved in synaptic activity. Describe the major types of neurotransmitters and neuromodulators, and discuss their effects on postsynaptic membranes. Discuss the interactions that enable information processing to occur in neural tissue. An Introduction to the Nervous System Neural Tissue o Contains two kinds of cells N_____________________: cells that send and receive signals N_____________________ (glial cells): cells that support and protect neurons An Introduction to the Nervous System Organs of the Nervous System o Brain and spinal cord o Sensory receptors of sense organs (eyes, ears, etc.) o Nerves connect nervous system with other systems Divisions of the Nervous System Anatomical Divisions of the Nervous System o _____________________nervous system (CNS) o Peripheral nervous system (PNS) Chapter 12: Neural Tissue Page 1 Divisions of the Nervous System The Central Nervous System (CNS) o Consists of the _____________________ and _____________________ o Contains neural tissue, connective tissues, and blood vessels o Functions of the CNS Are to process and coordinate: sensory data: from inside and outside body motor commands: control activities of peripheral organs (e.g., skeletal muscles) higher functions of brain: intelligence, memory, learning, emotion Divisions of the Nervous System The Peripheral Nervous System (PNS) o Includes all neural tissue _____________________the CNS o Functions of the PNS Deliver sensory information to the CNS Carry motor commands to peripheral tissues and systems The Peripheral Nervous System (PNS) o Nerves (also called peripheral nerves) Bundles of axons with connective tissues and blood vessels Carry sensory information and motor commands in PNS: ___________________nerves— connect to brain spinal nerves—attach to spinal cord Divisions of the Nervous System Functional Divisions of the PNS o _____________________division Carries sensory information From PNS sensory receptors to CNS o _____________________division Carries motor commands From CNS to PNS muscles and glands Divisions of the Nervous System Functional Divisions of the PNS o The efferent division _____________________nervous system (SNS): controls skeletal muscle contractions Chapter 12: Neural Tissue Page 2 Autonomic nervous system (ANS): Controls contractions of smooth muscle, cardiac muscle and glandular secretions o _____________________division o parasympathetic division Neurons Neurons o The basic functional units of the nervous system Neurons The Structure of Neurons o cell body (_____________________) o short, branched dendrites o long, single axon Major Organelles of the Cell Body o Nucleus o Perikaryon (cytoplasm) o Mitochondria o RER and ribosomes (produce neurotransmitters) o Cytoskeleton _____________________bodies o Dense areas of RER and ribosomes o Make neural tissue appear gray (gray matter) D_____________________ o Highly branched Chapter 12: Neural Tissue Page 3 The _____________________ o Carries electrical signal (action potential) to target Neurons Structures of the Axon o Axon hillock “_____________________” o Telodendria Fine extensions of distal axon o Synaptic terminals Tips of telodendria Neurons The Synapse o The synaptic knob Contains synaptic vesicles of _____________________ Neurotransmitters: are chemical messengers affect receptors of postsynaptic membrane Neurons The Structure of Neurons o The synapse Presynaptic cell: neuron that sends message _____________________cell: cell that receives message The synaptic cleft: the small gap that separates the presynaptic membrane and the postsynaptic membrane Neurons Types of Synapses o _____________________junction Synapse between neuron and muscle o Neuroglandular junction Synapse between neuron and gland Neurons Chapter 12: Neural Tissue Page 4 Recycling Neurotransmitters o Axoplasmic transport Neurotubules within the axon Transport raw materials Between cell body and synaptic knob Powered by mitochondria, kinesin, and dynein Neurons Four Structural Classifications of Neurons o Anaxonic neurons Found in brain and sense organs o Bipolar neurons Found in special sensory organs (sight, smell, hearing) o Unipolar neurons Found in sensory neurons of PNS o Multipolar neurons Common in the CNS Include all skeletal muscle motor neurons Neurons _____________________Neurons o Small o All cell processes look alike Bipolar Neurons o Are small o One dendrite, one axon _____________________Neurons o Have very long axons o Fused dendrites and axon o Cell body to one side Multipolar Neurons o Have very long axons o Multiple dendrites, one axon Neurons Three Functional Classifications of Neurons o Sensory neurons Afferent neurons of PNS o _____________________neurons Efferent neurons of PNS o I_____________________ Association neurons Chapter 12: Neural Tissue Page 5 Neurons Three Types of Sensory Receptors o Interoceptors Monitor internal systems Internal senses o Exteroceptors External senses Distance senses o P_____________________ Monitor position and movement Neurons Motor Neurons Carry instructions from CNS to peripheral effectors Via efferent fibers (axons) Two major efferent systems o Somatic nervous system (SNS) o Autonomic (visceral) nervous system (ANS) Neurons Interneurons o Most are located in brain, spinal cord, and autonomic ganglia o Between sensory and motor neurons Neuroglia Two types of cells of nervous tissue o Neurons perform all communication, information processing, and control functions of the nervous system o _____________________preserve physical and biochemical structure of neural tissue and are essential to survival and function of neurons Chapter 12: Neural Tissue Page 6 Neuroglia Four Types of Neuroglia in the CNS o Ependymal cells o Astrocytes o O_____________________ o Microglia Neuroglia Four Types of Neuroglia in the CNS o Ependymal cells Line central canal of spinal cord and ventricles of brain: secrete cerebrospinal fluid (CSF) circulate CSF o A_____________________ Maintain blood–brain barrier (isolates CNS) Control interstitial environment o Oligodendrocytes Processes contact other neuron cell bodies Wrap around axons to form myelin sheaths o Microglia Migrate through neural tissue Clean up cellular debris, waste products, and pathogens Neuroglia Ganglia o Masses of neuron cell bodies o Surrounded by neuroglia o Found in the PNS Neuroglia Neuroglia of the Peripheral Nervous System o Satellite cells Surround ganglia Regulate environment around neuron o S_____________________ cells Also called neurilemmocytes Form myelin sheath (neurilemma) many Schwann cells sheath entire axon Chapter 12: Neural Tissue Page 7 Neuroglia Oligodendrocytes and Schwann Cells o Myelination increases speed of action potentials myelin insulates myelinated axons makes nerves appear white o Nodes and internodes internodes: myelinated segments of axon _____________________ (also called nodes of Ranvier) gaps between internodes where axons may branch Neuroglia Neural Responses to Injuries o Wallerian degeneration Axon distal to injury degenerates o Schwann cells Form path for _____________________growth Wrap new axon in myelin Neuroglia Nerve Regeneration in CNS o Limited by chemicals released by _____________________that Block growth Produce scar tissue Disorders of Nervous Tissue Multiple _____________________ o Disorder of oligodendrocytes o Weakness, incoordination, visual impairment and speech disturbances o Most common among women 20-40 o Autoimmune disorder or viral o No cure Tumors o G_____________________ o Multiple neurofibromatosis Transmembrane Potential Ion Movements and Electrical Signals o All plasma (cell) membranes produce electrical signals by ion movements Chapter 12: Neural Tissue Page 8 o Transmembrane potential is particularly important to neurons Transmembrane Potential Five Main Membrane Processes in Neural Activities Resting potential o The transmembrane potential of resting cell Graded potential o Temporary, localized change in resting potential _____________________potential o Is an electrical impulse o Produced by graded potential o Propagates along surface of axon to synapse Synaptic activity o Releases neurotransmitters at presynaptic membrane o Produces graded potentials in postsynaptic membrane Information processing o Response (integration of stimuli) of postsynaptic cell Transmembrane Potential Differing ionic concentrations between cytosol and extracellular fluid Selectively permeable membrane Transport mechanisms Passive Forces - Gradients _____________________gradients Electrical gradients o Potassium ions leave cytosol (ICF) more rapidly than sodium enters o Makes interior more negative Electrochemical gradients o Sum of chemical and electrical forces acting across the membrane Active Forces Sodium Potassium Exchange Pump Resting Potential Undisturbed cell High permeability of potassium Low permeability of Sodium Chapter 12: Neural Tissue Page 9 Sodium-potassium pump stabilization Balanced at -70 mV Transmembrane Potential Three Classes of Gated Channels _____________________gated channels o Open in presence of specific chemicals o Found on neuron cell body and dendrites _____________________-gated channels o Respond to changes in transmembrane potential o Found in neural axons, skeletal muscle sarcolemma, cardiac muscle _____________________gated channels o Respond to membrane distortion o Found in sensory receptors (touch, pressure, vibration) Initiating a Change in the Transmembrane Potential Chemically regulated channels o Acetylcholine Mechanically regulated channels o Pressure (membrane distortion) The Transmembrane Potential of an Excitable Membrane Voltage regulated channels o Excitable membrane Transmembrane Potential _____________________Potentials o Also called local potentials o Changes in transmembrane potential That cannot spread far from site of stimulation o Any stimulus that opens a gated channel Produces a graded potential Chapter 12: Neural Tissue Page 10 Transmembrane Potential Graded Potentials R_____________________ o When the stimulus is removed, transmembrane potential returns to normal Hyperpolarization o Increasing the negativity of the resting potential o Result of opening a potassium channel o Opposite effect of opening a sodium channel o Positive ions move out, not into cell Graded Potential Transmembrane potential is most affected at point of stimulus and then diminishes with distance Spreads passively May involve either depolarization or hyperpolarization The stronger the stimulus the greater the affect Transmembrane Potential Graded Potentials o Effects of graded potentials At cell dendrites or cell bodies Action Potential Four Steps in the Generation of Action Potentials o Step 1: Depolarization to _____________________ o Step 2: Activation of Na+ channels Rapid depolarization Na+ ions rush into cytoplasm Inner membrane changes from negative to positive o Step 3: Inactivation of Na+ channels, activation of K+ channels At +30 mV Inactivation gates close (Na+ channel inactivation) K+ channels open Chapter 12: Neural Tissue Page 11 Repolarization begins o Step 4: Return to normal permeability K+ channels begin to close: when membrane reaches normal resting potential (–70 mV) + K channels finish closing: membrane is hyperpolarized to -90 mV transmembrane potential returns to resting level: action potential is over Action Potential The Refractory Period Absolute _____________________period o Sodium channels open or inactivated o No action potential possible Relative refractory period o Membrane potential almost normal o Very large stimulus can initiate action potential Action Potential Propagation of Action Potentials o Two methods of propagating action potentials Continuous propagation: unmyelinated axons Saltatory propagation: myelinated axons Axon Diameter and Speed Three Groups of Axons o Type A fibers o Type B fibers o Type C fibers These groups are classified by o Diameter o Myelination o Speed of action potentials Axon Diameter and Speed Type A Fibers o M_____________________ o Large diameter o High speed (140 m/sec) o Carry rapid information to/from CNS o For example, position, balance, touch, and motor impulses Chapter 12: Neural Tissue Page 12 Type B Fibers o Myelinated o _____________________diameter o Medium speed (18 m/sec) o Carry intermediate signals o For example, sensory information, peripheral effectors Type C Fibers o Unmyelinated o _____________________diameter o Slow speed (1 m/sec) o Carry slower information o For example, involuntary muscle, gland controls Synapses Synaptic Activity o Action potentials (nerve impulses) Are transmitted from presynaptic neuron To postsynaptic neuron (or other postsynaptic cell) Across a synapse Synapses Two Classes of Neurotransmitters o Excitatory neurotransmitters Cause depolarization of postsynaptic membranes Promote action potentials o _____________________neurotransmitters Cause hyperpolarization of postsynaptic membranes Suppress action potentials Synapses The Effect of a Neurotransmitter o On a postsynaptic membrane Depends on the receptor Not on the neurotransmitter o For example, _____________________ (ACh) Usually promotes action potentials But inhibits cardiac neuromuscular junctions Synapses Cholinergic Synapses o Any synapse that releases ACh All neuromuscular junctions with skeletal muscle fibers Chapter 12: Neural Tissue Page 13 Many synapses in CNS All neuron-to-neuron synapses in PNS All neuromuscular and neuroglandular junctions of ANS parasympathetic division Synapses Events at a _____________________Synapse o Action potential arrives, depolarizes synaptic knob o Calcium ions enter synaptic knob, trigger exocytosis of Ach o ACh binds to receptors, depolarizes postsynaptic membrane o AChE breaks ACh into acetate and choline Synapses Synaptic Delay o Fewer synapses mean faster response o Reflexes may involve only one synapse Synapses Synaptic _____________________ o Occurs when neurotransmitter cannot recycle fast enough to meet demands of intense stimuli o Synapse inactive until ACh is replenished Synapses Mechanism of Drug Actions Chemical Synapses The effect of a neurotransmitter on the postsynaptic membrane depends on the properties of the receptor, not on the nature of the neurotransmitter. Neurotransmitters and Neuromodulators Other Neurotransmitters o At least 50 neurotransmitters other than ACh, including Some amino acids Peptides Prostaglandins ATP Some dissolved gases Chapter 12: Neural Tissue Page 14 Neurotransmitters and Neuromodulators Important Neurotransmitters o Other than acetylcholine Norepinephrine (NE) D_____________________ Serotonin Gamma aminobutyric acid (GABA) Neurotransmitters and Neuromodulators Norepinephrine (NE) o Released by adrenergic synapses o Excitatory and depolarizing effect o Widely distributed in brain and portions of ANS Dopamine o A CNS neurotransmitter o May be excitatory or inhibitory o Involved in Parkinson disease, cocaine use S_____________________ o A CNS neurotransmitter o Affects attention and emotional states Gamma Aminobutyric Acid (GABA) o Inhibitory effect o Functions in CNS o Not well understood Information Processing Postsynaptic Potentials o Graded potentials developed in a postsynaptic cell In response to neurotransmitters Two Types of Postsynaptic Potentials o Excitatory postsynaptic potential (EPSP) Graded depolarization of postsynaptic membrane o Inhibitory postsynaptic potential (IPSP) Graded hyperpolarization of postsynaptic membrane Information Processing Inhibition o A neuron that receives many IPSPs Is _____________________from producing an action potential Because the stimulation needed to reach threshold is increased Chapter 12: Neural Tissue Page 15 Summation o To trigger an action potential o One EPSP is not enough o EPSPs (and IPSPs) combine through summation: temporal summation spatial summation Information Processing _____________________Summation o Multiple times o Rapid, repeated stimuli at one synapse Spatial Summation o Multiple locations o Many stimuli, arrive at multiple synapses Chapter 12: Neural Tissue Page 16