Chapter 12 – Introduction to the Nervous System

1. Chapter 12 – Introduction to the Nervous System Organization Cell Types

2. Review What 3 parts make up the nervous system? • Brain • Spinal cord • Nerves

3. http://www.nlm.nih.gov/medlineplus/ency/images/ency/fullsize/19588.jpghttp://www.nlm.nih.gov/medlineplus/ency/images/ency/fullsize/19588.jpg

4. Functions of the Nervous System • Detect changes (stimuli) in the internal or external environment • Evaluate the information • Initiate a change in muscles or glands Goal – maintain homeostasis What does this remind you of??

5. Organization of the Nervous System • Central nervous system (CNS) • Brain and spinal cord • Peripheral nervous system (PNS) • Nervous tissue in the outer regions of the nervous system • Cranial nerves: originates in the brain • Spinal nerves : originates from the spinal cord • Central fibers: extend from cell body towards the CNS • Peripheral fibers: extend from cell body away from CNS

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7. Afferent vs Efferent Nervous pathways are organized into division based on the direction they carry information • Afferent division: incoming information (sensory) • Efferent division: outgoing information (motor) (Efferent = Exit)

8. Somatic & Autonomic Nervous Systems Nervous pathways are also organized according to the type of effectors (organs) they regulate • Somatic nervous system (SNS) • Somatic sensory division (afferent) • Somatic motor division (efferent)

9. Somatic & Autonomic Nervous Systems cont… • Autonomic nervous system (ANS): Carry information to the autonomic or visceral effectors (smooth & cardiac muscles and glands) • Visceral sensory division (afferent) • Efferent pathways • Sympathetic division – “fight or flight” • Parasympathic division – “rest and repair”

10. Figure 12-2

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12. Review What are the two main cell types in the nervous system? (Hint: we talked about this when we covered tissue types) Answer: neurons and glia

13. Cells of the Nervous System Neurons: excitable cells that conduct information Glia (also neuroglia or glial cells): support cells, do not conduct information • Most numerous • Glia = glue

14. Types of Glia Five major types: • Astrocytes • Microglia • Ependymal cells • Oligodendrocytes • Schwann cells

15. Astrocytes (12-3A) • Star-shaped, largest, most numerous • Cell extension connect neurons and capillaries • Transfer nutrients from blood to neuron • Help form blood-brain barrier (BBB) http://astrocyte.info/astrocytes1.jpg

16. Blood-Brain Barrier • Helps maintain stable environment for normal brain function • “feet” of astrocytes wrap around capillaries in brain • Regulates passage of ions • Water, oxygen, CO2, glucose and alcohol pass freely • Important for drug research • Parkinson’s Disease

17. Microglia (12-3B) • Engulf and destroy cellular debris (phagocytosis) • Enlarge during times of inflammation and degeneration

18. Ependymal cells (12-3C) • Similar to epithelial cells • Forms thin sheets that line the fluid-filled cavities of the brain and spinal cord • Some cells help produce the fluid that fills these cavities (cerebral spinal fluid - CSF) • Cilia may be present to help circulate fluid http://www.lab.anhb.uwa.edu.au/mb140/corepages/nervous/Images/epen100he.jpg

19. Oligodendrocytes (12-3D) • Hold nerve fibers together • Produce myelin sheaths in CNS http://4.bp.blogspot.com/_XzEk6ORFLFg/SUQ4IitreiI/AAAAAAAAAD4/XrmtzSv1eGU/s400/article_ms_01.gif http://blustein.tripod.com/Oligodendrocytes/08-zoom.jpg

20. Multiple Sclerosis (MS) • Most common myelin disorder • Characterized by: • myelin loss and destruction  injury and death  plaque like lesions • Impaired nerve conduction  weakness, loss of coordination, vision and speech problems • Remissions & relapses • Autoimmune or viral infection • Women 20-40 yrs • No known cure

21. Multiple Sclerosis (MS) http://www.riversideonline.com/source/images/image_popup/ww5r308_big.jpg

22. Schwann cells (12-3E) • Only in PNS • Support nerve fibers & form myelin sheaths • Satellite cells (12-3G) • Types of schwann cell that covers a neuron’s cell body

23. http://legacy.owensboro.kctcs.edu/gcaplan/anat/images/Image425.gifhttp://legacy.owensboro.kctcs.edu/gcaplan/anat/images/Image425.gif

24. Neurons All neurons have 3 parts: • Cell body (soma) • Axon • One or more dendrites

25. Neuron Anatomy • Soma resembles other cells • Nissl bodies – part of rough ER; contain proteins necessary for nerve signal transmission & nerve regeneration • Dendrites – branch out from soma; receptors; conduct impulse towards soma • Axon – process that extends from the soma at a tapered portion called the axon hillock • Axon collaterals: side branches • Telodendria: distal branches of axon • Synaptic knob: ends of telodendria

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27. Neuron Anatomy • Myelin sheaths: areas of insulation produced by Schwann cells; increases speed of nerve impulse • Myelinated = white matter • Unmyelinated = gray matter • Nodes of Ranvier: breaks in myelin sheath btwn Schwann cells • Synapse: junction btwn two neurons or btwn a neuron and an effector

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29. Structural Classification of Neurons • Multipolar • One axon, several dendrites • Most numerous • Bipolar • One axon, one dendrite • Least numerous • Retina, inner ear, olfactory pathway • Unipolar • Axon is a single process that branches into a central process (towards CNS) and a peripheral process (towards PNS) • Dendrites at distal end of peripheral process • Always sensory neurons

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31. Functional Classification of Neurons • Afferent • Sensory • Towards CNS • Efferent • Motor • Towards muscles & glands • Interneurons • Connect afferent & efferent neurons • Lie within CNS

32. Reflex Arc

33. Examples of Reflex Arcs • Ipsilateral • Contralateral • intersegmental

34. Nerves vs Tracts • Nerves – bundles of parallel neurons held together by fibrous CT in the PNS • Tracts – bundles of parallel neurons in the CNS

35. Nerve Fibers • Remember the difference between nerves and tracts? • Endoneurium: surrounds each nerve fiber • Perineurium: surrounds fascicles (bundles of nerve fibers • Epineurium: surrounds a complete nerve (PNS) or tract (CNS)

37. Review: Gray vs White Matter • White matter – myelinated nerve fibers • Myelin sheaths help increase the speed of an action potential • Gray matter – unmyelinated nerve fibers & cell bodies • Ganglia: regions of gray matter in PNS

38. Nerve Fiber Repair • Nervous tissue has a limited repair capacity b/c mature neurons are incapable of cell division • Repair can take place if soma and neurilemma remain intact

39. Steps of Nerve Fiber Repair • Injury • Distal axon and myelin sheaths degenerates • Remaining neurilemma & endoneurium forms a “tunnel” from the injury to the effector • Proteins produced in the nissl bodies help extend a new axon down the tunnel to the effector

41. Nerve Impulses • Neurons are specialized to initiate and conduct signals  nerve impulses • Exhibit excitability & conductivity • Nerve impulse wave of electrical fluctuation that travels along the plasma membrane

42. Membrane Potentials • Difference in charges across the plasma membrane • Inside slightly negative • Outside slightly positive • Result in a difference in electrical charges  membrane potential • Stored potential energy • Analogy = water behind a dam

43. Membrane Potentials • Membrane potential creates a polarized membrane • Membrane has – pole & + pole • Potential difference of a polarized membrane is measured in millivolts (mV) • The sign indicates the charge of the inside of a polarized membrane

44. Resting Membrane Potential (RMP) • When not conducting electrical signals, a membrane is “resting” • -70mV • RMP maintained by ionic imbalance across membrane • Sodium-Potassium Pump • Pumps 3 Na+ out for every 2 K+ pumps in • Creates an electrical gradient (more positive on outside)

45. Resting Membrane Potential (RMP)

46. Local Potential • Local potential - The slight shift away from the RMP • Isolated to a particular region of the plasma membrane • Stimulus-gated Na+ channels open  Na+ enters  membrane potential to moves closer to zero (depolarization) • Stimulus-gated K+ channels open  K+ exits  membrane potential away from zero (hyperpolarization) • **Local potentials do not spread to the end of the axon**

47. Local Potentials

48. Action Potentials Definitions: • Membrane potential of an active neuron (one that is conducting an impulse • Action potential = nerve impulse • An electrical fluctuation that travels along the plasma membrane

49. Steps of Producing an Action Potential (table 12-1) • A stimulus triggers stimulus-gated Na+ channels to open  Na+ diffuses inside the cell  depolarization • Threshold potential is reached (-59mV)  voltage-gated Na+ channels open  depolarization continues • Action potential peaks at +30mV, voltage-gated Na+ channels close • Voltage-gated K+ channels open  K+ diffuses outward  repolarization • Brief period of hyperpolarization (below -70mV)  RMP is restored by Na+/K+ pump