Nervous system I

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Chapter 10

• Nervous system I

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Function of neurons

• Excitability (irritability)-ability to respond to
  a stimulus
• conductivity-electrical signals quickly reach
  other cells at distant locations
• Secretion-electrical signal reaches end of neuron
  and stimulates the release of neurotransmitter
  that stimulates the next cell

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Types of neurons

• Sensory (afferent) neurons-detect changes in
  environment (stimuli) and transmit this
  information INTO the central nervous system
• Cells that respond to stimuli are called
  receptors
• Interneurons (association) neurons-found in the
  CNS and connect the sensory and motor (90 of
  all neurons are this type)
• Motor (efferent) neurons-send signals to muscle
  and glands which carry out the response to
  stimuli (cells or organs that carry out the
  response are called effectors)

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Structural Classification of Neurons

• http//www.utsa.edu/tsi/2000tsi/people/Jordan/Anat
  omy20Pages/Classification20of20neurons.html
• Unipolar-a single process leading away from the
  cells body (sensory neurons from skin)
• Bipolar-one axon and one dendrite (olfactory
  cells of nasal cavity)
• Multipolar-one axon and many dendrites, most
  common
• http//www.getbodysmart.com

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Subdivision of the Nervous System

• Central Nervous System-Brain and Spinal Cord
• Contains gray matter and white matter
• Peripheral Nervous System-everything not in the
  CNS
• Contains nerves and ganglia
• Nerves are bundles of nerve fibers wrapped in
  fibrous connective tissue (more in a minute!)
• Ganglia are swellings where the nerve cell bodies
  are located

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Subdivisions of the PNS

• Functionally divided into sensory and motor
  subdivisions
• Each of these is divided into somatic and
  visceral subdivisions
• Put it all together you have
• Somatic sensory-signals from skin, muscle, bone
• Visceral sensory-signals from viscera
• Somatic motor-signals to skeletal muscle
• Visceral motor (autonomic)-signals to glands,
  cardiac and smooth muscle
• Have sympathetic and parasympathetic divisions

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More on nerves

• A nerve is a bundle of thousands of axons, plus
  blood vessels, and connective tissue that lie
  outside the brain and spinal cord
• Nerves coming from the brain are called cranial
  nerves
• Nerves coming from the spinal cord are called
  spinal nerves
• Each of these is protected by 3 layers
  endoneurium, perineurium, epineurium

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Cells of Nervous system

• http//psych.hanover.edu/Krantz/neural/struct3.htm
  l for a quiz over structure
• 3 main parts dendrites, cell body, axon

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Structure of cells of the nervous system

• Neurons-control center is cell body and contains
  the nucleus
• The soma or cell body has mitochondria,
  cytoplasm, lysosomes, golgi bodies, ER, and
  cytoskeleton
• The rough ER is called Nissl bodies and are
  unique to neurons
• No mitosis after adolescence, but unspecialized
  stem cells can divide in CNS and become neurons
• Lipofuscin pigment collects with age and pushes
  nucleus to side (no apparent problems associated
  with this)

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Dendrites

• -branches that receive signals from other neurons
  and send them into the cell body
• Neurons can have one or many (receive more
  information with many)

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Axons

• Axon hillock is a mound coming off the cell body
  where the axon arises
• Neurons have a single axon that is specialized
  for receiving signals from the cell body
• May branch (axon collateral)
• Cytoplasm of axon is axoplasm, cell membrane is
  axolemma
• Synaptic knob is found at the terminal end of the
  axon contain synaptic vesicles with
  neurotransmitter
• forms a synapse with the muscle, gland, or other
  nerve

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Axonal transport

• All the proteins needed by neuron are made in
  soma and travel by axonal transport
• Travel down the axon is by anterograde transport
• Return used synaptic vesicles, etc and
  information up axon by retrograde transport
• Also have axoplasmic flow which is only
  antergrade and slow, and governs regeneration
  speed of damaged nerve fibers

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Neurogliahttp//www.cliffsnotes.com/WileyCDA/Clif
fsReviewTopic/Neuroglia.topicArticleId-22032,artic
leId-21933.html

• Outnumber neurons 50 to 1
• Bind neurons together and support them
• 6 types
• Oligodendrocytes-forms myelin sheath around nerve
  fiber and insulates it, speeds up the impulse
  conduction
• Astrocytes-star shaped, most abundant type in
  CNS, help form Blood Brain Barrier, contact blood
  vessels

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Neuroglia continued

• Ependymal cells -produce CSF in CNS
• Microglia-small macrophages that phagocytize dead
  cells in CNS
• Schwann cells -envelop nerve fibers in PNS and
  produce myelin sheath, assist in regeneration of
  damaged nerve fibers
• Satellite cells -surround neuron cell bodies in
  ganglia of PNS, little known of function
• http//www.jsmarcussen.com/gbs/uk/damage.htm for
  article on Guillain-Barr Syndrome (GBS)

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Myelin

• Myelin sheath is insulating layer around a nerve
  fiber, segmented by Nodes of Ranvier
• Formed by oligodendrocytes in CNS and Schwann
  cells in PNS
• Little myelin in brain at birth, develops rapidly
  in infancy and completed in adolescence (high
  lipid diet important)

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Oligodendrocytes

• Arm like processes of oligodendrocytes reach out
  to nerve fibers and spiral around them
• Almost no cytoplasm between the membranes
• Takes many oligodendrocytes to cover a nerve fiber

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Schwann Cells

• Spiral around a single nerve fiber
• Puts down around 100 layers of membrane
• Surrounded by neurilemma which is the outermost
  coil
• Neurilemma contains the nucleus and most of the
  cytoplasm (not in CNS)
• Surrounding this is a basement membrane and
  endoneurium that is not found in CNS

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Speed of impulses

• Depends on myelin and diameter of nerve fiber
• Large myelinated fibers are fastest-120 m/sec due
  to saltatory conduction
• Small myelinated fibers-3-15 m/sec
• Small unmyelinated fibers-0.5-2.0 m/sec

http//www.brainviews.com/abFiles/AniSalt.htm Anim
ation
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Regeneration of nerve fibers

• Damaged Peripheral nerve fiber can regenerate if
  soma is intact and some neurilemma remains
• First damaged myelin sheath and axon degenerate
  and are removed
• Next a regeneration tube forms by neurilemma and
  endoneurium
• Axon stump puts out sprouts and one will find
  its way into the tube
• Grows 3-5 mm per day, other sprouts are
  reabsorbed
• Must make sure connection is the same!

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Animations for nerve impulse

• http//www.biology4all.com/resources_library/sourc
  e/63.swf
• http//www.biologymad.com/NervousSystem/nerveimpul
  ses.htm

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Resting Membrane Potential

• Living cells are polarized (has potential like a
  charge in a battery)
• The charge difference across the plasma membrane
  is the resting membrane potential (RMP)
• This is typically about -70 mV (millivolts)
• The negative value means there are more
  negatively charged particles on the inside of the
  membrane than on the outside

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How does this happen?

• Electrical currents in the body are created by
  the flow of ions such as Na and K though
  openings or channels in the membrane
• Gated channels can be opened and closed by
  various stimuli, enabling cells to turn currents
  off and on
• The RMP is due to the fact that electrolytes are
  unevenly distributed between ECF outside the
  plasma membrane and ICF inside

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RMP continued

• Depends on 3 things
• diffusion of ions
• Selective permeability of membrane allowing some
  ions to move more easily
• Electrical attraction of anions and cations to
  each other
• Potassium has the greatest influence because the
  plasma membrane is more permeable to these ions

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Electrolytes and RMP

• Sodium ions are 12 times as concentrated in the
  ECF as the ICF (more outside than inside)
• Potassium is more concentrated in the ICF than
  the ECF (about 40 times at equilibrium)-more
  inside than outside
• Both follow laws of diffusion and travel down
  concentration gradients (greater to less
  concentration)
• Sodium leaks in and potassium leaks out, so a
  sodium/potassium pump moves them back at a ratio
  of 3 Na out for every 2 K in (this requires 1
  ATP)

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Local Potentials (LP)

• When neuron is stimulated the response usually
  begins at the dendrite, then the soma, and into
  the axon and ends at synaptic bulb
• A signal (pain, chemical, whatever) triggers
  sodium channels to open and sodium will rush
  inside the cell
• This changes the charge on the inside so that it
  becomes less negative
• Once this begins it is depolarization
• These sodium ions diffuse for a short distance
  and produce a current that travels from pt. of
  stimulation toward trigger zone (short range
  change is local potential)

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Four difference between local and action
potentials

• 1. local potentials are graded they vary in
  strength depending on strength of stimulus
• 2. they are decremental they get weaker as they
  spread because K leaks out
• 3. they are reversible when stimulation
  ceases, K diffuses out quickly and cell returns
  to resting potential
• 4. local potentials can be inhibitory or
  excitatory
• http//faculty.washington.edu/chudler/ap.html
  scan down page to find a puzzle

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Action Potentials

• This zone is a more dramatic change
• Occur only where there are lots of voltage
  regulated gates
• Most of the soma only has 50-75 gates per square
  micrometer, no action potential possible
• The trigger zone has 350-500 gates per square
  micrometer
• If the excitatory local potential reaches the
  trigger zone with enough strength, you get an
  action potential (rapid up/down shift in voltage)

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Events in Action Potential

• When sodium arrives at axon hillock, they
  depolarize the membrane there (LP)
• If LP rises to threshold level (-55 mV) voltage
  regulated gates open
• Neuron fires and produces an AP and Na and K
  gates open
• Na is fast, K is slower
• Acts like positive feedback so more and more Na
  rushes in and voltage rises fast (depolarization)

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Continued

• Once voltage measures 0 mV the Na gates
  inactivate and close, but it takes time for all
  to close
• Ending voltage is 35 mV (approximate)
• Membrane is now positive inside, negative outside
• By this time the K gates are fully open and K
  rush out (repolarization)
• Potassium gates stay open longer so amount of K
  that leaves is greater than what was there
  resulting in hyperpolarization (1 or 2 mV
  difference)
• Ion diffusion will eventually restore the balance
  found in RMP

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AP vs LP

• Action potentials follow All OR None Law meaning
  if the threshold is reached it responds
  completely
• APs are non-decremental and do not get weaker
  with distance from trigger point
• APs are irreversible, if threshold is reached it
  cant be stopped (like firing a gun)

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Refractory Period

• Period of resistance to restimulation
• 2 phases
• Absolute refractory -no stimulus of any strength
  can trigger a new action potential
• Corresponds to opening of sodium channels (gates)
• Relative refractory- a strong stimulus can
  trigger a new AP
• Lasts until K gates close and hyperpolarization
  is complete

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Myelinated vs unmyelinated

• In unmyelinated fibers Na gates line the entire
  length
• AP in trigger zone causes excitation and
  depolarization immediately distal to spot
• This repeats down the entire axon (like a wave in
  a football stadium)
• With myelinated fibers the only way a nerve
  signal can travel is by jumping from Nodes of
  Ranvier to Nodes of Ranvier (saltatory conduction)

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Synapse

• Impulses pass from neuron to neuron or other
  cells at the synapse
• The presynaptic neuron is before the synapse
• The postsynaptic neuron is after the synapse
• http//www.mind.ilstu.edu/flash/synapse_1.swf
  play this clip!

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Synapses continued

• Presynaptic neuron can synapse with dendrite,
  soma, or axon of another nerve
• This is axodendritic, axosomatic, or axoaxonic
  synapse
• Axons can have a huge number of synapses (in
  cerebellum of brain one neuron can have 100,000
  synapses)

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Neurotransmitter

• Acetylcholine was first discovered
• There are over 100 neurotransmitters
• 3 main categories according to chemical
  composition
• 1. Acetylcholine
• 2. Amino acids-glycine, glutamine, aspartate,
  GABA
• 3. Monoamines or biogenic amines made from
  amino acids
• -include catecholamines like epinephrine and
• NE and indolamines like serotonin and
• histamine

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Neurotransmitters

• http//thebrain.mcgill.ca/flash/i/i_01/i_01_m/i_01
  _m_ana/i_01_m_ana.html

• A fourth category is neuropeptides, differ
  because they are stored in secretory granules
• Substance P mediates pain transmission
• Beta endorphin-produces runners high
• Enkephalins-act as pain relievers by inhibiting
  substance P
• See tables 10.4-10.6

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Neural integration

• The ability of neurons to process ALL
  information, store and recall, and make decisions
• Post-synaptic potentials
• EPSP-excitatory postsynaptic potential-any change
  that makes a neuron more likely to fire (usually
  Na entering) glutamate and aspartate
• IPSP-inhibitory postsynaptic potential-hyperpolari
  zation will make the postsynaptic cell less
  likely to fire (extra K leaving) glycine and
  GABA)

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Summation, Facilitation, Inhibition

• Neuron may receive input from 1000s of
  Pre-synaptic neurons at the same time
• Some are EPSP and some are IPSP, net effect
  determines what will happen
• Summation is the process of adding the
  post-synaptic potentials and responding to net
  effects
• Facilitation is when one neurons enhances the
  effect of another
• Presynaptic inhibition is opposite of
  facilitation and is used to reduce unwanted
  synaptic transmissions

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Neuronal pools and circuits

• Thousands to miIlions of interneurons that
  control body function
• Follow a neuronal pathway
• 1. diverging circuit-one nerve fiber branches
  and synapses with several postsynaptic cells
• 2. converging circuit-input from many nerve
  fibers is funneled into one
• 3. Reverberating circuit-linear sequence where C
  sends collateral back to A. Every time C fires
  it stimulates A and C
• 4. Parallel after-discharge circuit-single input
  diverges to stimulate several chains and
  eventually re-converge to output neuron