Neurophysiology Part 2

1
Neurophysiology Part 2

• Communication Along Between Neurons A Closer
  Look
• Highlights of Transmission within between
  Neurons (Chapter 6)

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Summary to date

• Understanding the connection between behavior,
  consciousness creative thought and their
  association to neuronal activity goes beyond
  understanding AP and the stimulation of muscle to
  contract or glands to secrete
• All neurons carry information by electrical
  signals based on the movement of specific ions
  across the plasma membrane
• Behavior depends not on the firing of a single AP
  but on activity of many neurons working together
• Signals move along membrane via 2 methods

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General Review of Transmission

• Electronically conducted potentials (like along a
  wire) Eg sensory neuron detects pressure
  pressure stimulus is translated such that it
  changes membrane potential (Vm) change is
  graded depending on strength of stimulus at
  sensory neuron this is called receptor
  potential this signal travels like a signal
  through a wire because sensory membranes that
  receive stimuli lack voltage-gated ion channels
  these neurons cannot generate APs thus signal
  cannot be propagated regeneratively receptor
  potentials progressively decay cannot travel
  widely passive electrotonic or decremental
  transmission

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Review of Transmission cont

• 2. Generation of an AP - for long distance
  travel, sensory signals must be transformed into
  APs occurs if spike-initiating zone of a
  sensory neuron contains high density of
  voltage-gated ion channels - regenerating
  without decrement
• At axon terminals of sensory neurons, signal must
  cross synapse typically transfer via
  neurotransmittors for success,
  neurotransmittors must change Vm of the
  post-synaptic neuron - more APs higher
  frequency of APs cause more neurotransmitter
  molecules to be released post-synaptic
  potential (psp) providing psp is sufficiently
  large, brings spike-initiating zone of post-syn
  neuron to threshold triggers AP

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Propagation of Action Potentials

• Non-spiking neurons so small that electrotonic
  conduction is sufficient no APs generated but
  still large enough to trigger release of
  neurotransmittors at synapse
• However, majority of communication depends on
  propagation of APs over over again as AP
  travels, each activated patch of membrane excites
  neighboring patches opening closing of Na
  voltage-gated K voltage-gated channel
  sequentially along the axon propagates the AP
  (backwards travel prevented as membrane patch
  just behind the AP is in refractory phase)

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Saltatory Conduction

• Special glial cells (Schwann cells in PNS
  oligodendrocytes in CNS) wrap around segments of
  axons producing layers of fatty membranes
  collectively called myelin producing 2 effects
• Increase effective transmembrane resistance (as
  layers increase, so does resistance)
• Decrease effective transmembrane capacitance
  (capacitance deceases because myelin is very
  thick) less capacitance current required to
  change Vm more charge can flow down axon to
  depolarize the next segment

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Saltatory Conduction cont

• NB myelin would not improve conduction if it
  completely covered axon because electrotonically
  conducted current would eventually decrease to 0
  as a function of distance thus, myelin sheath is
  segmented separated by short, unmyelinated gaps
  nodes of Ranvier regions of axon under myelin
  wrapping internodes
• Internodal axonal membrane has no voltage-gated
  Na or K channel AP initiated at one node
  electrotonically depolarizes mem at next node
  i.e.jumping from node to node APs produced
  only in small area of membrane exposed at NofR

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Saltatory Conduction defined

• Defn discontinuous regenerative
  depolarizations that take place at NofR
• NS of all vertebrates includes both myelinated
  unmyelinated neurons velocity of signal will
  vary depending on types of fibers

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Transmission of Information between Neurons the
Synapse

• 2 types electrical chemical
• gt 50 NTs identified with more types of activities
  initiated than first thought (typical being at
  the neuromuscular junction) - once thought that
  NT only caused a post syn. cell to depolarize or
  hyperpolarize, now know that they
• can increase or decrease of ion channels
  inserted into the membrane of post syn cell
• alter the excitability of post syn cell by
  changing rate at which ion channels open close
  or
• modify the sensitivity of channels to activating
  signals

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Electrical Synapses

• Transfer of information between cells by direct
  ionic coupling
• Plasma membranes of pre- postsyn cells are in
  close apposition communication between cells
  occurs via protein channels called gap junctions
• Ions flow directly from one cell into the other
  this is rapid is similar to propagation of AP
  along an axon because both depend on passive
  spread of local current ahead of the AP to
  depolarize excite a neighboring region (but
  little flexibility in synaptic transmission that
  chemical synapses offer)

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Chemical Synapses Structure Function

• Summary of events
• AP to axon terminal of pre-syn. neuron
• NTs stored in synaptic vesicles
• NTs released by exocytosis to synaptic cleft
• Bind to specific receptor of post-syn neuron
  opening ligand-gated ion channels
• Brief ionic current flows through mem of post syn
  neuron see fig. 6-10 p. 169

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Chemical Synapses cont

• 2 types fast slow those at neuromuscular
  junctions fast slow affect post syn cell by
  activating receptors that alter levels of signal
  molecules with the post syn cell eventually
  modify ion channels, rather than by directly
  changing the conductance through ligand-gated ion
  channel multiple steps makes it slower
• Fast small molecules syn packaged within axon
  terminals slow larger molecules synthesized
  from 1 or more aas in the soma biogenic animes
  if 1, neuropeptide if gt 1 onset slower but
  effects may be longer

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Chemical Synapses cont

• Whether fast or slow, controlled the same
• AP arrives at axon terminal
• Activates Ca2 channels allowing Ca to enter
• Increase Ca2 initiates exocytosis NT dumped
  into syn cleft
• (in fast syn vesicles fuse with plasma membrane
  release contents at specialized sites called
  active zones slow release at many sites)

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Fast Chemical Synapses

• Neuromuscular junction as prime example
• Structural specializations of pre syn terminal,
  post-syn membrane associated Schwann cells at
  the motor endplate (aka neuronmuscular junc)
• Axon of motor neuron forms small branches which
  lie in longitudinal depression along surface of
  muscle fiber (transverse folds junctional
  folds)
• Above each fold active zone filled with
  vesicles (typically containing 10 to the 5th
  vesicles) cleft is filled mucopolysaccharide
  glues together pre post mem (membrane is
  recycled) Acetylcholine (Ach) is transmitter
  released at neuromuscular junctions

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ACh

• ACh binds to ACh-specific receptors in post-syn
  membrane causing ion channels selective for Na
  K to open briefly
• Simultaneously, Acetylcholinesterase (AChE)
  hydrolyses ACh limits the time ACH is active

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Synaptic Currents

• Post-synaptic current (psc) the change in the
  rate of ion flow across post-synaptic membrane
  (PSM) generally, ion-specific channels at PSM
  open (or close) when NT binds to receptor sites
  changing amt of ionic current crossing mem
  direction intensity of psc (controlled by the
  size of conductance thro open channels,
  electrochemical force charge on perm ions)
  determines polarity amplitude of post syn
  potential release of transmitter by pre syn
  term followed by psc of ions moving down
  electrochem gradients thro open channels in post
  syn mem

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Synaptic Currents cont

• psc cont depolarizing psc at NMJ consists of
  influx of Na (which can be partly cancelled by
  influx of K) both Na K pass thro same post
  syn Ach-activated channels, indicating these
  channels have broader ion selectivity than to
  highly selective voltage gated Na K
• psc shorted lived than post syn potentials
  (Ach-activated channels open momentarily because
  ACH is radidly removed by enz degradation
  channels close psc ceases to flow

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PostSynaptic Excitation Inhibition

• Any change in Vm at post syn mem that increases
  the prob than an AP will be initiated in post syn
  cell excitatory postsynaptic potential (epsp)
  vs.
• Any change in Vm that reduces the prob of an AP
  in post syn cell inhibitory post syn potential
  (ipsp)
• Reversal potential mem potential where no
  current flows thro mem ion channels, even though
  channels are open equilibrium.steady-state
  potential for ion (s) that are conducted thro
  open channels

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Post-Synaptic Excitation Inhibition

• Any change in Vm at post syn mem that increases
  the prob that an AP will be initiated in post syn
  cell excitatory postsynaptic potential (epsp)
  vs.
• Any change in Vm that reduces the prob of an AP
  in post syn cell inhibitory postsyn potential
  (ipsp)
• e.g. ACh is excitatory at NMJ where it opens
  channels to allow Na K to cross post-syn mem
  but also is inhibitory at terminals of
  parasympathetic neurons innervating vertebrate
  heart where it causes K selective channels to
  remain open longer, thus reducing the freq of
  spontaneous depolarizations that drive the heart
  beat

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More terminology

• Neuromodulation (modulation of synatic
  transmission) results in transient changes in
  how effectively a presyn neuron can control
  events in neurons post syn to it
• Synaptic plasticity capacity for long-lasting
  or permanent changes in synaptic efficacy as a
  result of experience

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Integration

• As opposed to triggering of a single AP from one
  neuron to another as studied thus far, must note
  that even the simplest behavior typically
  requires that many neurons (1000s) must act in a
  coordinated fashion each integrating
  inhibitory/excitatory/long-acting/short-acting
  signals either producing more APs or not
  focus of much contemporary neuroscience research
  discussed in detail in the balance of chapter 6