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Chapter 12

Chapter 12. Nervous Tissue Lecture Outline. INTRODUCTION. The nervous system , along with the endocrine system, helps to keep controlled conditions within limits that maintain health and helps to maintain homeostasis.

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Chapter 12

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  1. Chapter 12 Nervous Tissue Lecture Outline

  2. INTRODUCTION • The nervous system, along with the endocrine system, helps to keep controlled conditions within limits that maintain health and helps to maintain homeostasis. • The nervous system is responsible for all our behaviors, memories, and movements. • The branch of medical science that deals with the normal functioning and disorders of the nervous system is called neurology. Principles of Human Anatomy and Physiology, 11e

  3. Chapter 12Nervous Tissue • Controls and integrates all body activities within limits that maintain life • Three basic functions • sensing changes with sensory receptors • fullness of stomach or sun on your face • interpreting and remembering those changes • reacting to those changes with effectors • muscular contractions • glandular secretions Principles of Human Anatomy and Physiology, 11e

  4. Major Structures of the Nervous System • Brain, cranial nerves, spinal cord, spinal nerves, ganglia, enteric plexuses and sensory receptors Principles of Human Anatomy and Physiology, 11e

  5. Structures of the Nervous System - Overview • Twelve pairs of cranial nerves emerge from the base of the brain through foramina of the skull. • A nerve is a bundle of hundreds or thousands of axons, each of which courses along a defined path and serves a specific region of the body. • The spinal cord connects to the brain through the foramen magnum of the skull and is encircled by the bones of the vertebral column. • Thirty-one pairs of spinal nerves emerge from the spinal cord, each serving a specific region of the body. • Ganglia, located outside the brain and spinal cord, are small masses of nervous tissue, containing primarily cell bodies of neurons. • Enteric plexuses help regulate the digestive system. • Sensory receptors are either parts of neurons or specialized cells that monitor changes in the internal or external environment. Principles of Human Anatomy and Physiology, 11e

  6. Functions of the Nervous Systems • The sensory function of the nervous system is to sense changes in the internal and external environment through sensory receptors. • Sensory (afferent) neurons serve this function. • The integrative function is to analyze the sensory information, store some aspects, and make decisions regarding appropriate behaviors. • Association or interneurons serve this function. • The motor function is to respond to stimuli by initiating action. • Motor(efferent) neurons serve this function. Principles of Human Anatomy and Physiology, 11e

  7. Nervous System Divisions • Central nervous system (CNS) • consists of the brain and spinal cord • Peripheral nervous system (PNS) • consists of cranial and spinal nerves that contain both sensory and motor fibers • connects CNS to muscles, glands & all sensory receptors Principles of Human Anatomy and Physiology, 11e

  8. Subdivisions of the PNS • Somatic (voluntary) nervous system (SNS) • neurons from cutaneous and special sensory receptors to the CNS • motor neurons to skeletal muscle tissue • Autonomic (involuntary) nervous systems • sensory neurons from visceral organs to CNS • motor neurons to smooth & cardiacmuscle and glands • sympathetic division (speeds up heart rate) • parasympathetic division (slow down heart rate) • Enteric nervous system (ENS) • involuntary sensory & motor neurons control GI tract • neurons function independently of ANS & CNS Principles of Human Anatomy and Physiology, 11e

  9. Organization of the Nervous System • CNS is brain and spinal cord • PNS is everything else Principles of Human Anatomy and Physiology, 11e

  10. Enteric NS • The enteric nervous system (ENS) consists of neurons in enteric plexuses that extend the length of the GI tract. • Many neurons of the enteric plexuses function independently of the ANS and CNS. • Sensory neurons of the ENS monitor chemical changes within the GI tract and stretching of its walls, whereas enteric motor neurons govern contraction of GI tract organs, and activity of the GI tract endocrine cells. Principles of Human Anatomy and Physiology, 11e

  11. HISTOLOGY OF THE NERVOUS SYSTEM Principles of Human Anatomy and Physiology, 11e

  12. Neuronal Structure & Function Principles of Human Anatomy and Physiology, 11e

  13. Neurons • Functional unit of nervous system • Have capacity to produce action potentials • electrical excitability • Cell body • single nucleus with prominent nucleolus • Nissl bodies (chromatophilic substance) • rough ER & free ribosomes for protein synthesis • neurofilaments give cell shape and support • microtubules move material inside cell • lipofuscin pigment clumps (harmless aging) • Cell processes = dendrites & axons Principles of Human Anatomy and Physiology, 11e

  14. Parts of a Neuron Neuroglial cells Nucleus with Nucleolus Axons or Dendrites Cell body Principles of Human Anatomy and Physiology, 11e

  15. Cell membrane • The dendrites are the receiving or input portions of a neuron. • The axon conducts nerve impulses from the neuron to the dendrites or cell body of another neuron or to an effector organ of the body (muscle or gland). Principles of Human Anatomy and Physiology, 11e

  16. Dendrites • Conducts impulses towards the cell body • Typically short, highly branched & unmyelinated • Surfaces specialized for contact with other neurons • Contains neurofibrils & Nissl bodies Principles of Human Anatomy and Physiology, 11e

  17. Axons • Conduct impulses away from cell body • Long, thin cylindrical process of cell • Arises at axon hillock • Impulses arise from initial segment (trigger zone) • Side branches (collaterals) end in fine processes called axon terminals • Swollen tips called synaptic end bulbs contain vesicles filled with neurotransmitters Synaptic boutons Principles of Human Anatomy and Physiology, 11e

  18. Axonal Transport • Cell body is location for most protein synthesis • neurotransmitters & repair proteins • Axonal transport system moves substances • slow axonal flow • movement in one direction only -- away from cell body • movement at 1-5 mm per day • fast axonal flow • moves organelles & materials along surface of microtubules • at 200-400 mm per day • transports in either direction • for use or for recycling in cell body Principles of Human Anatomy and Physiology, 11e

  19. Axonal Transport & Disease • Fast axonal transport route by which toxins or pathogens reach neuron cell bodies • tetanus (Clostridium tetani bacteria) • disrupts motor neurons causing painful muscle spasms • Bacteria enter the body through a laceration or puncture injury • more serious if wound is in head or neck because of shorter transit time Principles of Human Anatomy and Physiology, 11e

  20. Diversity in Neurons • Both structural and functional features are used to classify the various neurons in the body. • On the basis of the number of processes extending from the cell body (structure), neurons are classified as multipolar, biopolar, and unipolar (Figure 12.4). • Most neurons in the body are interneurons and are often named for the histologist who first described them or for an aspect of their shape or appearance. Examples are Purkinje cells (Figure 12.5a) or Renshaw cells (Figure 12.5b). Principles of Human Anatomy and Physiology, 11e

  21. Structural Classification of Neurons • Based on number of processes found on cell body • multipolar = several dendrites & one axon • most common cell type • bipolar neurons = one main dendrite & one axon • found in retina, inner ear & olfactory • unipolar neurons = one process only(develops from a bipolar) • are always sensory neurons Principles of Human Anatomy and Physiology, 11e

  22. Functional Classification of Neurons • Sensory (afferent) neurons • transport sensory information from skin, muscles, joints, sense organs & viscera to CNS • Motor (efferent) neurons • send motor nerve impulses to muscles & glands • Interneurons (association) neurons • connect sensory to motor neurons • 90% of neurons in the body Principles of Human Anatomy and Physiology, 11e

  23. Association or Interneurons Principles of Human Anatomy and Physiology, 11e

  24. Neuroglial Cells • Half of the volume of the CNS • Smaller cells than neurons • 50X more numerous • Cells can divide • rapid mitosis in tumor formation (gliomas) • 4 cell types in CNS • astrocytes, oligodendrocytes, microglia & ependymal • 2 cell types in PNS • schwann and satellite cells Principles of Human Anatomy and Physiology, 11e

  25. Astrocytes • Star-shaped cells • Form blood-brain barrier by covering blood capillaries • Metabolize neurotransmitters • Regulate K+ balance • Provide structural support Principles of Human Anatomy and Physiology, 11e

  26. Microglia • Small cells found near blood vessels • Phagocytic role -- clear away dead cells • Derived from cells that also gave rise to macrophages & monocytes Principles of Human Anatomy and Physiology, 11e

  27. Ependymal cells • Form epithelial membrane lining cerebral cavities & central canal • Produce cerebrospinal fluid (CSF) Principles of Human Anatomy and Physiology, 11e

  28. Satellite Cells • Flat cells surrounding neuronal cell bodies in peripheral ganglia • Support neurons in the PNS ganglia Principles of Human Anatomy and Physiology, 11e

  29. Oligodendrocytes • Most common glial cell type • Each forms myelin sheath around more than one axons in CNS • Analogous to Schwann cells of PNS Principles of Human Anatomy and Physiology, 11e

  30. Myelination • A multilayered lipid and protein covering called the myelin sheath and produced by Schwann cells and oligodendrocytes surrounds the axons of most neurons (Figure 12.8a). • The sheath electrically insulates the axon and increases the speed of nerve impulse conduction. Principles of Human Anatomy and Physiology, 11e

  31. Schwann Cell • Cells encircling PNS axons • Each cell produces part of the myelin sheath surrounding an axon in the PNS Principles of Human Anatomy and Physiology, 11e

  32. Axon Coverings in PNS • All axons surrounded by a lipid & protein covering (myelin sheath) produced by Schwann cells • Neurilemma is cytoplasm & nucleusof Schwann cell • gaps called nodes of Ranvier • Myelinated fibers appear white • jelly-roll like wrappings made of lipoprotein = myelin • acts as electrical insulator • speeds conduction of nerve impulses • Unmyelinated fibers • slow, small diameter fibers • only surrounded by neurilemma but no myelin sheath wrapping Principles of Human Anatomy and Physiology, 11e

  33. Myelination in PNS • Schwann cells myelinate (wrap around) axons in the PNS during fetal development • Schwann cell cytoplasm & nucleus forms outermost layer of neurolemma with inner portion being the myelin sheath • Tube guides growing axons that are repairing themselves Principles of Human Anatomy and Physiology, 11e

  34. Myelination in the CNS • Oligodendrocytes myelinate axons in the CNS • Broad, flat cell processes wrap about CNS axons, but the cell bodies do not surround the axons • No neurilemma is formed • Little regrowth after injury is possible due to the lack of a distinct tube or neurilemma Principles of Human Anatomy and Physiology, 11e

  35. Gray and White Matter • White matter = myelinated processes (white in color) • Gray matter = nerve cell bodies, dendrites, axon terminals, bundles of unmyelinated axons and neuroglia (gray color) • In the spinal cord = gray matter forms an H-shaped inner core surrounded by white matter • In the brain = a thin outer shell of gray matter covers the surface & is found in clusters called nuclei inside the CNS • A nucleus is a mass of nerve cell bodies and dendrites inside the CNS. Principles of Human Anatomy and Physiology, 11e

  36. Electrical Signals in Neurons • Neurons are electrically excitable due to the voltage difference across their membrane • Communicate with 2 types of electric signals • action potentials that can travel long distances • graded potentials that are local membrane changes only • In living cells, a flow of ions occurs through ion channels in the cell membrane Principles of Human Anatomy and Physiology, 11e

  37. Two Types of Ion Channels • Leakage (nongated) channels are always open • nerve cells have more K+ than Na+ leakage channels • as a result, membrane permeability to K+ is higher • explains resting membrane potential of -70mV in nerve tissue • Gated channels open and close in response to a stimulus • results in neuron excitability Principles of Human Anatomy and Physiology, 11e

  38. Ion Channels • Gated ion channels respond to voltage changes, ligands (chemicals), and mechanical pressure. • Voltage-gated channels respond to a direct change in the membrane potential (Figure 12.10a). • Ligand-gated channels respond to a specific chemical stimulus (Figure 12.10b). • Mechanically gated ion channels respond to mechanical vibration or pressure. Principles of Human Anatomy and Physiology, 11e

  39. Gated Ion Channels Principles of Human Anatomy and Physiology, 11e

  40. Resting Membrane Potential • Negative ions along inside of cell membrane & positive ions along outside • potential energy difference at rest is -70 mV • cell is “polarized” • Resting potential exists because • concentration of ions different inside & outside • extracellular fluid rich in Na+ and Cl • cytosol full of K+, organic phosphate & amino acids • membrane permeability differs for Na+ and K+ • 50-100 greater permeability for K+ • inward flow of Na+ can’t keep up with outward flow of K+ • Na+/K+ pump removes Na+ as fast as it leaks in Principles of Human Anatomy and Physiology, 11e

  41. Principles of Human Anatomy and Physiology, 11e

  42. Graded Potentials • Small deviations from resting potential of -70mV • hyperpolarization = membrane has become more negative • depolarization = membrane has become more positive • The signals are graded, meaning they vary in amplitude (size), depending on the strength of the stimulus and localized. • Graded potentials occur most often in the dendrites and cell body of a neuron. Principles of Human Anatomy and Physiology, 11e

  43. How do Graded Potentials Arise? • Source of stimuli • mechanical stimulation of membranes with mechanical gated ion channels (pressure) • chemical stimulation of membranes with ligand gated ion channels (neurotransmitter) • Graded/postsynaptic/receptor or generator potential • ions flow through ion channels and change membrane potential locally • amount of change varies with strength of stimuli • Flow of current (ions) is local change only Principles of Human Anatomy and Physiology, 11e

  44. Generation of an Action Potential • An action potential (AP) or impulse is a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential (depolarization) and then restore it to the resting state (repolarization). • During an action potential, voltage-gated Na+ and K+ channels open in sequence (Figure 12.13). • According to the all-or-none principle, if a stimulus reaches threshold, the action potential is always the same. • A stronger stimulus will not cause a larger impulse. Principles of Human Anatomy and Physiology, 11e

  45. Action Potential • Series of rapidly occurring events that change and then restore the membrane potential of a cell to its resting state • Ion channels open, Na+ rushes in (depolarization), K+ rushes out (repolarization) • All-or-none principal = with stimulation, either happens one specific way or not at all (lasts 1/1000 of a second) • Travels (spreads) over surface of cell without dying out Principles of Human Anatomy and Physiology, 11e

  46. Depolarizing Phase of Action Potential • Chemical or mechanical stimuluscaused a graded potential to reachat least (-55mV or threshold) • Voltage-gated Na+ channels open& Na+ rushes into cell • in resting membrane, inactivation gate of sodium channel is open & activation gate is closed (Na+ can not get in) • when threshold (-55mV) is reached, both open & Na+ enters • inactivation gate closes again in few ten-thousandths of second • only a total of 20,000 Na+ actually enter the cell, but they change the membrane potential considerably(up to +30mV) • Positive feedback process Principles of Human Anatomy and Physiology, 11e

  47. Repolarizing Phase of Action Potential • When threshold potential of-55mV is reached, voltage-gated K+ channels open • K+ channel opening is muchslower than Na+ channelopening which caused depolarization • When K+ channels finally do open, the Na+ channels have already closed (Na+ inflow stops) • K+ outflow returns membrane potential to -70mV • If enough K+ leaves the cell, it will reach a -90mV membrane potential and enter the after-hyperpolarizing phase • K+ channels close and the membrane potential returns to the resting potential of -70mV Principles of Human Anatomy and Physiology, 11e

  48. Refractory Period of Action Potential • Period of time during whichneuron can not generateanother action potential • Absolute refractory period • even very strong stimulus willnot begin another AP • inactivated Na+ channels must return to the resting state before they can be reopened • large fibers have absolute refractory period of 0.4 msec and up to 1000 impulses per second are possible • Relative refractory period • a suprathreshold stimulus will be able to start an AP • K+ channels are still open, but Na+ channels have closed Principles of Human Anatomy and Physiology, 11e

  49. The Action Potential: Summarized • Resting membrane potential is -70mV • Depolarization is the change from -70mV to +30 mV • Repolarization is the reversal from +30 mV back to -70 mV) Principles of Human Anatomy and Physiology, 11e

  50. Local Anesthetics • Local anesthetics and certain neurotoxins • Prevent opening of voltage-gated Na+ channels • Nerve impulses cannot pass the anesthetized region Examples: • Novocaine and lidocaine Principles of Human Anatomy and Physiology, 11e

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