Nervous System and Nervous Tissue

1. Nervous System and Nervous Tissue

2. Nervous System Master control and communication Functions (system level and cell level) • Sensory input – monitoring stimuli • Integration – interpretation of sensory input • Motor output – response to stimuli

3. Cellular v. System level PNS Dendrites: input Cell body: integration Axon: output CNS PNS

4. Nervous System Organization Central nervous system (CNS) Form: Brain and spinal cord Function: Integration and command center Peripheral nervous system (PNS) Form: Paired spinal and cranial nerves Function: Carries messages to and from the spinal cord and brain

5. Central Nervous System Peripheral Nervous System Sensory (afferent) Motor (efferent) Somatic (voluntary) Autonomic (involuntary) Sympathetic (Action! Go!) Parasympathetic (Stop! )

6. Peripheral Nervous System (PNS) INPUTS: Sensory (afferent) division • Sensory afferent fibers – from skin, skeletal muscles, and joints to the brain • Visceral afferent fibers – from visceral organs to the brain OUTPUTS: Motor (efferent) division • Transmits impulses from the CNS to effector organs

7. Motor Division Organization Somatic nervous system (SNS) • Conscious control of skeletal muscles Autonomic nervous system (ANS) • Regulates involuntary muscle (smooth and cardiac) and glands • Sympathetic (Stimulates = Go!) • Parasympathetic (Conserves = Stop!)

8. Nervous System Cell Types Neurons • Transmit electrical signals Neuroglia (“nerve glue”) • Supporting cells • Neuroglia in the CNS • Astrocytes • Microglia • Ependymal cells • Oligodendrocytes • Neuroglia in the PNS • Satellite cells • Schwann cells

9. Neurons • Structural units of the nervous system • Long-lived (100+ years) • Amitotic (no centrioles = can’t divide) • High metabolic rate (glucose gobblers!)

10. Neuron Classification (function) Sensory (afferent) • transmit impulses toward the CNS Motor (efferent) • transmit impulses away from the CNS Interneurons (association neurons) • shuttle signals through CNS pathways

11. Neuron (nerve cell) Neuron cell body Cell body Dendrites Dendritic spine (a) Impulse direction Node of Ranvier Nissl bodies Axon terminals (secretory component) Axon hillock Schwann cell (b)

12. Nerve Cell Body (Soma) • Contains nucleus and nucleolus • Major biosynthetic center • Focal point for the outgrowth of neuronal processes (dendrites and axons) • Axon hillock – where axons arise

13. Neuronal processes (fibers) Dendrites • Numerous • Short and tapering • Diffusely branched • Contain “spines” where synapses form Axons • One per cell • Long (up to 4 ft. in length) • Form synapses at terminals (release neurotransmitters) • Anterograde and retrograde transport (out and back!)

14. Supporting Cells: Neuroglia • Provide a supportive scaffolding for neurons • Segregate and insulate neurons • Guide young neurons to the proper connections • Promote health and growth • Help regulate neurotransmitter levels • Phagocytosis

15. Astrocytes • Most abundant and versatile • Cling to neurons and synaptic endings • Cover capillaries (blood-brain barrier) • Support and brace neurons • Guide migration of young neurons • Control the chemical environment

16. Microglia • Monitor health of neurons • Transform into macrophages to remove cellular debris, microbes and dead neurons NOTE: Normal immune system cells can’t enter CNS

17. EpendymalCells Shape: squamous to columnar (often ciliated) Location: Line the central cavities of the brain and spinal column Function: Circulate cerebrospinal fluid

18. Oligodendrocytes • Wrap CNS axons like a jelly roll • Form insulating myelin sheath

19. Schwann Cells and Satellite Cells Schwann cells • Surround axons of the PNS • Form insulating myelin sheath Satellite cells • Surround neuron cell bodies Nodes of Ranvier

20. Myelin Sheath and Neurilemma Myelin Sheath • White, fatty sheath protects long axons • Electrically insulates fibers • Increases the speed of nerve impulses Neurilemma • remaining nucleus and cytoplasm of a Schwann cell

21. Axons of the CNS • Both myelinated and unmyelinated fibers are present • Oligodendrocytes insulate up to 60 axons each White matter: dense collections of myelinated fibers Gray matter: mostly soma and unmyelinated fibers

22. Action Potentials (nerve impulse) • Electrical impulses carried along the length of axons • Always the same regardless of stimulus • Based on changes in ion concentrations across plasma membrane • This is HOW the nervous system functions

23. Electricity Definitions Voltage (V) • potential energy from separation of charges (+ and -) • For neurons, measured in millivolts Current (I) • the flow of electrical charge between two points Insulator • substance with high electrical resistance • Think myelin sheath!

24. Ion Channels Passive (leakage) channels: always open Voltage-gated channels: open and close in response to membrane potential Ligand-gated (chemically gated) channels: open when a specific neurotransmitter binds Mechanically gated channels: open and close in response to physical forces

25. Let’s review! The sodium-potassium pump

26. Voltage-Gated Channels

27. Chemically Gated Channels

28. Gated Channels When gated channels are open: • Ions move along electro-chemical gradients • Takes into account charge differences • Takes into account concentration differences • An electrical current is created • Voltage changes across the membrane

29. Measuring Membrane Potential

30. Resting Membrane Potential Resting membrane potential (–70 mV) • The inside of a cell membrane has more negative charges than outside the membrane • Major differences are in Na+ and K+

31. Changes in Membrane Potential Depolarization • the inside of the membrane becomes less negative Hyperpolarization • the inside of the membrane becomes more negative than the resting potential Repolarization • the membrane returns to its resting membrane potential

32. Action Potentials = nerve impulse • Principal means of neural communication • A brief reversal of membrane polarity • All or nothing event • Maintain their strength over distance • Generated only by muscle cells and neurons

33. Phases of an Action Potential • Resting state • Depolarization • Repolarization • Hyperpolarization • Return to resting potential

34. Action Potential: Resting State • Na+ and K+ GATED channels are closed • Each Na+ channel has two voltage-regulated gates • Activation gates • Inactivation gates

35. Action Potential: Depolarization • Na+ permeability increases; membrane potential reverses • Na+ gates are opened, but K+ gates are closed • Threshold: critical level of depolarization (-55 to -50 mV) • Once threshold is passed,action potential fires

36. Action Potential: Repolarization • Sodium inactivation gates close • Voltage-sensitive K+ gates open • K+ rushes out • Interior of the neuronis negative again

37. Action Potential: Hyperpolarization • Potassium gates remain open • Excess K+ leaves cell • Membrane becomes hyperpolarized • Neuron is insensitive to stimuli until resting potential is restored

38. Return to resting potential:Sodium-potassium pump Repolarization • ONLY restores the electrical differences across the membrane • DOES NOT restore the resting ionic conditions Sodium-potassium pump restores ionic conditions • More sodium outside • More potassium inside

39. Na+ Sodium channel Potassium channel Activation gates K+ Inactivation gate Na+ Na+ Resting state 1 K+ K+ Hyperpolarization Depolarization Na+ 4 2 K+ Repolarization 3

40. ACTION! • http://outreach.mcb.harvard.edu/animations/actionpotential.swf • http://www.youtube.com/watch?v=SCasruJT-DU • http://bcs.whfreeman.com/thelifewire/content/chp44/4402s.swf • http://www.blackwellpublishing.com/matthews/channel.html

41. Absolute and Relative Refractory Periods

42. Refractory Periods Absolute refractory period (NO WAY! NO HOW!) • Neuron CANNOT generate an action potential • Ensures that each action potential is separate event • Enforces one-way transmission of nerve impulses Relative refractory period (Well, maybe…) • Threshold is elevated • Only strong stimuli can generate action potentials

43. Propagation of an Action Potential Voltage at 2 ms +30 Membrane potential (mV)) Voltage at 0 ms Voltage at 4 ms –70 (a) Time = 0 ms (b) Time = 2 ms (c) Time = 4 ms Resting potential Peak of action potential Hyperpolarization

44. Action Potential Frequency Stronger stimuli generate more frequent action potentials

45. How fast does a signal travel? Velocity determined by • Axon diameter • the larger the diameter, the faster the impulse • Presence of a myelin sheath • Myleinated neurons have much faster impulses • Why? Node-jumping! (Saltatory conduction)

46. Saltatory Conduction

47. Multiple Sclerosis (MS) Cause: Autoimmune disease with symptoms appearing in young adults (women at highest risk) • UNKNOWN environmental and genetic factors Symptoms: visual disturbances, weakness, loss of muscular control, incontinence Physiology • Myelin sheaths in the CNS are destroyed, producing a hardened lesion (scleroses) • Shunting and short-circuiting of nerve impulses occurs • Alternating periods of relapse and remission

48. Multiple Sclerosis Treatment • Drugs that modify immune response Prognosis • Medications can prevent symptoms from worsening • Reduce complications • Reduce disability • HOWEVER, not all drugs work long-term in all patients

49. Synapses Junction for cell  cell communication • Neuron neuron • Neuron  effector cell Presynaptic neuron • Conducts impulses toward the synapse Postsynaptic neuron/cell • Receives signal • May/may not act on signal

50. Synapses