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Nervous System

Nervous System. Chapter 9 Pages 211-257. Chapter 9 Wordbytes. af - = toward 11. - ferrent = carried arachn - = spider 12. gangli - = swelling astro - = star 13. - glia = glue auto- = self 14. mening - =membrane dendro - = tree 15. micro- = small

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Nervous System

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  1. Nervous System Chapter 9 Pages 211-257

  2. Chapter 9 Wordbytes • af- = toward 11.-ferrent = carried • arachn- = spider 12.gangli- = swelling • astro- = star 13. -glia = glue • auto- = self 14.mening- =membrane • dendro- = tree 15. micro- = small • di- = 2, through16.neuro- = nerve • ef- = away from 17. –oid = similar to • encephalo- = brain18.oligo- = few • enter- = intestines 19. peri- = around • epen- = above 20.somat- = body

  3. Nervous System Overview • Master controller and communicator for the body • Responsible for all behavior • 3 functions: • Sensory input monitors changes inside/outside of body • Integration processes and interprets, then decides what should be done • Motor output causes a response in effector organs

  4. Organization—2 main parts: • Central Nervous System (CNS) = brain and spinal cord • Interprets incoming sensory info. and dictates motor responses • Peripheral Nervous System (PNS) = nerves from brain & in spinal cord • INPUT-Afferent or Sensory division • OUTPUT- Efferent or Motor division • Subdivided: Somatic (SNS—from CNS to skeletal muscles=voluntary) & Autonomic (ANS—regulate smooth & cardiac muscle, glands=involuntary)

  5. Major structures:

  6. Histology • Highly cellular—densely packed & tightly intertwined • 2 types of cells: • Neuron= nerve cell • Specialized for signal carrying & information processing • Neuroglia cells support, nourish & protect neurons • Neuroglia critical for homeostasis of interstitial fluid around neurons

  7. Supporting cells (Neuroglia) • ~ half the volume of CNS • Cells smaller than neurons • Can multiply and divide and fill in brain areas • Do not conduct nerve impulses

  8. Supporting Cells in CNS • Astrocytes most abundant and most versatile; blood-brain barrier • Oligodendrocytes have fewer branches; produce insulating myelin sheath in CNS • Microglia ovoid cells with thorny processes; provide defense (because immunity cells not allowed in CNS) • Ependymal cells squamous/columnar cells with cilia; produce cerebrospinal fluid (CSF)

  9. Supporting Cells in PNS • Schwann cells PNS cell support; produce & maintain myelin sheath, regenerate PNS axons • Satellite cells in PNS ganglia; support neurons in ganglia, regulate exchange of materials between neurons and interstitial fluid

  10. Neuron Characteristics • They conduct nerve impulses from one part of the body to another • They have extreme longevity live/function for a lifetime • They are amitotic lose their ability to divide • They have a high metabolic rate = need O2 and glucose

  11. Neuronal Structure • Cell body nucleus, cytoplasm with typical organelles; most within CNS = protected by cranial bones & vertebrae • Dendrites short, highly branched input structures emerging from cell body = high surface area to receive signals • Axon may be short or long, only one per neuron; conducts away from cell body toward another neuron or effector • Emerges at cone-shaped axon hillock • Axon terminals at end of axon with synaptic bulbs

  12. Figure 9.3 (Neurilemma) = impulse direction Pg. 391

  13. Myelination • Axons covered with a myelin sheath • Many layered lipid & protein creating insulations • Increases speed of nerve conduction. • Formed by: • Schwann cells in PNS (pg. 393 fig. 11.5) • Oligodendrocytes in CNS • Nodes of Ranvier= gaps in the myelin • Nodes are important for signal conduction • Some diseases destroy myelin multiple sclerosis & Tay-Sachs

  14. Gray and White Matter • White matter- primarily myelinated axons • Gray matter- nerve cell bodies, dendrites, unmyelinated axons, axon terminals & neuroglia • Spinal cord gray matter is centrally located

  15. Classification of Neurons • Structural according to # of processes: • Multipolar 3 or more; most common, especially in CNS • Bipolar 2 processes (an axon and a dendrite) that extend from opposite sides; found in special sense organs • Unipolar 1 process that divides like a T; found in ganglia in PNS

  16. Functional according to the direction impulse travels (Table 11.1) • Sensory (afferent) neurons transmit impulses from sensory receptors toward or into the CNS; mostly unipolar, with cell bodies in ganglia outside CNS • Motor (efferent) neurons carry impulses away from CNS to muscles and glands; multipolar, usually with cell bodies in CNS • Interneurons (association neurons) between motor & sensory neurons; most in CNS; 99% of neurons in body; mostly multipolar

  17. Neurophysiology • Neurons are highly irritable = responsive to stimuli • When stimulated, an electrical impulse (action potential) is conducted along its axon • Action potential underlies all functional activities of the nervous system

  18. Action Potentials • Action potentials = nerve impulses • Require a membrane potential • electrical charge difference across cell membrane – like a battery • Ion Channels allow ions to move by diffusion = current • If no action potential then resting cell has resting membrane potential

  19. Ion Channels • Allow specific ions to diffuse across membrane • Move from high concentration to low or toward area of opposite charge • Leakage channels • Gated channels- require trigger to open • Voltage- Gated channels respond to a change in membrane potential

  20. Resting Membrane Potential • Leakage channels • Cytosol high in K+ & interstitial fluid high in Na+(sodium –potassium pumps) • Leakage lets K+ through easily and Na+ poorly • inside is negative relative to outside • actual value depends on the relative leakage channel numbers

  21. Figure 9.4

  22. Graded Potentials • Short-lived, local changes to membrane potential • Cause current flows that decrease with distance • Magnitude varies with strength of stimulus

  23. Action Potential (AP) • Generated by neurons and muscle cells • Series of active events • Channels actively open & close • Some initial event is required to reach a voltage threshold (~ = - 55 mv) • Stimulus = any event bringing membrane to threshold

  24. Action Potential • Resting state • voltage-gated channels closed • Depolarizing phase- • membrane potential rises and becomes positive • Repolarizing phase- • potential restored to resting value ( PNa,  PK) • Undershoot • Potassium permeability continues

  25. Figure 9.5

  26. Active Events • Stimulus to reach threshold • Na+ channel opens=> • Na+ ions enter=> • positive potential=> • Causes K+ channel opening => • repolarization

  27. All- or –None Phenomenon • This sequence is always the same • If threshold then the same size of changes occur no larger or smaller APs • Stimulus must reach threshold to start • After one AP there is a short period before next can be triggered= absoluterefractory period each AP is a separate, all-or-none event; enforces one-way transmission of AP

  28. Conduction of Nerve Impulses • Each section triggers next locally • Refractory period keeps it going the right direction • unmyelinated fiber- continuous conduction • With myelin- saltatory conduction • Can only be triggered at nodes of Ranvier • Myelinated fibers faster & larger neurons faster

  29. Figure 9.6a

  30. Figure 9.6b

  31. The Syanpse • Synapse (to clasp or join)- junction that mediates information transfer from 1 neuron to another or from a neuron to an effector cell • Axodendritic or axosomaticsynapses – most synapses occur between the axonal ending of a neuron and the dendrites or cell body of other neurons

  32. Synaptic Transmission – Electrical synapse • Sequence of events at synapse • Triggered by voltage change of the Action Potential • Sending neuron = presynaptic • Receiving neuron = postsynaptic • Space between = synaptic cleft • Neurotransmitter carries signal across cleft

  33. Events at Synapse – Chemical synapse • AP arrives at presynaptic end bulb=> • Opens voltage gated Ca2+ channels=> • Ca2+ flows into cell • increased Ca2+ concentration => • exocytosis of synaptic vesicles=> • Neurotransmitter released into cleft • Diffuse across and bind to receptors in postsynaptic cell membrane

  34. Synaptic Transmission • Binding at receptors • Chemical trigger of ion channels • May depolarize or hyperpolarize postsynaptic cell membrane • If threshold reached at axon hillock then postsynaptic cell action potential results

  35. Synaptic Transmission • Finally the neurotransmitter must be removed from the cleft- • Diffusion away • Destroyed by enzymes in cleft • Transport back into presynaptic cell • Neuroglia destruction

  36. Figure 9.7

  37. Neurotransmitters • AcetylCholine (Ach)- common in PNS • Biogenic amines - Norepinephrine (NE), Dopamine (DA), serotonin, Histamine • Amino Acids- • Glutamate, Aspartate, gamma aminobutyric acid (GABA), glycine • Neuropeptides – endorphins • Novel Messengers - ATP/ Nitric oxide (NO)/ Carbon monoxide (CO)

  38. Development of Neurons • P. 422-424 • Neuroblasts • Growth cone • Programmed cell death

  39. Web sites: • http://www.sciencecases.org/chin/chin.asp • http://www.pbs.org/wgbh/nova/sciencenow/3204/01.html • http://www.getbodysmart.com/ap/nervoussystem/menu/menu.html • http://www.bbc.co.uk/science/humanbody/body/interactives/3djigsaw_02/index.swf?startPosition=nervous • http://learn.genetics.utah.edu/units/addiction/reward/madneuron.cfm • http://www.gpc.edu/~bbrown/peril/neurons/level1.htm

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