12 PART 1

1. 12 PART 1 Fundamentals of the Nervous System andNervous Tissue Pages 349-367

2. 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

3. Sensory input Integration Motor output Nervous System Figure 12.1

4. 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

5. Brain CNS Spinal cord Nerves PNS Ganglia Basic Divisions of the Nervous System Figure 12.2

6. 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

7. 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)

8. Central nervous system (CNS) Peripheral nervous system (PNS) Brain and spinal cord Cranial nerves and spinal nerves Integrative and control centers Communication lines between the CNS and the rest of the body Sensory (afferent) division Motor (efferent) division Somatic and visceral sensory nerve fibers Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands) Conducts impulses from receptors to the CNS Somatic sensory fiber Somatic nervous system Autonomic nervous system (ANS) Skin Somatic motor (voluntary) Visceral motor (involuntary) Conducts impulses from the CNS to skeletal muscles Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Visceral sensory fiber Stomach Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Parasympathetic division Mobilizes body systems during activity Conserves energy Promotes house- keeping functions during rest Sympathetic motor fiber of ANS Heart Structure Function Sensory (afferent) division of PNS Bladder Parasympathetic motor fiber of ANS Motor (efferent) division of PNS Types of Sensory and Motor Information Figure 12.3

9. Basic Divisions of the Nervous System • Somatic sensory • General somatic senses—receptors are widely spread • Touch • Pain • Vibration • Pressure • Temperature

10. 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

11. 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”

12. 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

13. 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

14. 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

15. 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

16. 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

17. Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Dendrites Neuron cell body Nucleus with nucleolus Neurofibril Nucleus Chromatophilic (Nissl) bodies (b) Nuclei of neuroglial cells Axon (impulse generating and conducting region) Impulse direction Nucleolus Nissl bodies Node of Ranvier Axon terminals (secretory region) Axon hillock Schwann cell (one inter- node) Neurilemma (a) Terminal branches Structure of a Typical Large Neuron Figure 12.4

18. 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

19. 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

20. 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

21. 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)

22. 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

23. 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

24. Two Neurons Communicating at a Synapse Presynaptic neuron Axon Axon terminal at synapse Postsynaptic neuron Synapse Dendrite (a) Two neurons connected by synapses Figure 12.5a

25. 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

26. Nerve impulses Presynaptic axon Microtubule Neurofilament Axon terminal Mitochondrion Vesicle releasing neurotransmitter Synaptic vesicles Synaptic cleft Postsynaptic dendrite (b) Enlarged view of the synapse Some Important Types of Synapses Figure 12.5b

27. 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

28. 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

29. Table 12.2 Comparison of Structural Classes of Neurons (1 of 3)

30. Table 12.2 Comparison of Structural Classes of Neurons (2 of 3)

31. Neurons Classified by Structure

32. 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

33. 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

34. 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 • Confined to the CNS

35. Neurons Classified by Function

36. 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

37. 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

38. 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

39. Capillary Neuron Astrocyte (a) Astrocytes are the most abundant CNS neuroglia. Neuroglia in the CNS Figure 12.6a

40. 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

41. Neuron Microglial cell (b) Microglial cells are defensive cells in the CNS. Neuroglia in the CNS Figure 12.6b

42. 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

43. 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. Neuroglia in the CNS Figure 12.6c, d

44. 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. 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 Figure 12.6e

45. 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

46. 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

47. (a) Myelinated axon in PNS An axon wrapped with a fatty insulating sheath formed from Schwann cells Myelin sheath Schwann cell plasma membrane 1 A Schwann cell envelops an axon. Schwann cell cytoplasm Schwann cell cytoplasm Axon Axon Schwann cell nucleus Neurilemma 2 The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers. 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. Neurilemma Myelin sheath Myelin Sheaths in the PNS Figure 12.7a

48. (b) Unmyelinated axons in PNS Axons that are not covered with an insulating sheath Schwann cell Schwann cell Axons Neurilemma A Schwann cell surrounds multiple axons. 1 Schwann cell nucleus Axons Cross section of unmyelinated axons (TEM 11,000) 2 Each axon is encircled by the Schwann cell plasma membrane. Unmyelinated Axons in the PNS Figure 12.7b

49. Myelin Sheaths in the PNS • Nodes of Ranvier—gaps along axon • Thick axons are myelinated • Thin axons are unmyelinated • Conduct impulses more slowly • Oligodendrocytes form the myelin sheaths in the CNS • Have multiple processes • Coil around several different axons Myelin Sheaths in the CNS

50. 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