Fundamentals of the Nervous System and Nervous Tissue

1. PART 3 Fundamentals of theNervous System and Nervous Tissue

2. 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

3. Myelin Sheaths in the PNS • Formed by Schwann cells (neurolemmacytes) • 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

4. Myelin Sheaths in the PNS Figure 12.14a, b

5. Myelin Sheaths in the PNS Figure 12.14c, d

6. Myelin Sheaths in the PNS Figure 12.15a

7. Myelin Sheaths in the PNS • Nodes of Ranvier – gaps along axon • Thick axons are myelinated • Thin axons are unmyelinated • Conduct impulses more slowly

8. Myelin Sheaths in the CNS • Oligodendrocytes form the myelin sheaths in the CNS • Have multiple processes • Coil around several different axons Figure 12.15b

9. 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

10. 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

11. Nerves • Nerves – cablelike organs in the PNS • Consists of numerous axons wrapped in connective tissue • Axon is surrounded by Schwann cells • You see many nerves in lab • Nerves of Brachial Plexus • Radial, axillary, median, musculocutaneous, ulnar • Nerves of lumbosacral plexus

12. Nerves • 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

13. Structure of a Nerve Figure 12.16a

14. 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

15. 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

16. 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

17. 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

18. Five Essential Components to the Reflex Arc • Motor neuron – conducts efferent impulses from integration center to an effector • Effector – muscle or gland cell • Responds to efferent impulses • Contracting or secreting

19. Five Essential Components to the Reflex Arc Figure 12.17

20. Types of Reflexes • Monosynaptic reflex • Simplest of all reflexes • Just one synapse • The fastest of all reflexes • Knee-jerk reflex

21. Types of Reflexes • Polysynaptic reflex • More common type of reflex • Most have a single interneuron between the sensory and motor neuron • Withdrawal reflexes

22. Types of Reflexes Figure 12.18a, b

23. 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

24. 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

25. 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

26. Simplified Design of the Nervous System Figure 12.19

27. 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

28. Neuronal Circuits Figure 12.20

29. Input Processing – not in notes • 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

30. Neural Processing Figure 12.21

31. Disorders of the Nervous System • Multiple sclerosis • Common cause of neural disability • An autoimmune disease • Immune system attacks the myelin around axons in the CNS • Varies widely in intensity among those affected • More women than men are affected • When men are affected disease develops quicker and is more devastating • Cause is incompletely understood

32. Nervous Tissue Throughout Life • Nervous system develops from the dorsal ectoderm • Invaginates to form the neural tube and neural crest • Neural tube walls begin as neuroepithelial cells • These cells divide and become neuroblasts

33. Nervous Tissue Throughout Life Figure 12.22

34. Neuronal Regeneration • Neural injuries may cause permanent dysfunction • If axons alone are destroyed, cells bodies often survive and the axons may regenerate • PNS – macrophages invade and destroy axon distal to the injury • Axon filaments grow peripherally from injured site • Partial recovery is sometimes possible

35. Neuronal Regeneration • CNS – neuroglia never form bands to guide re-growing axons and may hinder axon growth with growth-inhibiting chemicals • No effective regeneration after injury to the spinal cord and brain

36. Regeneration of the Peripheral Nerve Fiber Figure 12.23