Chapter 12 Nervous Tissue

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Chapter 12Nervous Tissue

• Overview of the nervous system
• Cells of the nervous system
• Electrophysiology of neurons
• Synapses
• Neural integration

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Electrophysiology of Neurons

• Communication between neurons is based on
  producing electrical potentials and currents
• Electrical potential the difference in
  concentration of charged particles between two
  points
• Electrical current - flow of charged particles
  from one point to another
• Living cells are polarized
• unequal electrolytes distribution between ECF/ICF
• diffusion of ions down their concentration
  gradients
• selective permeability of plasma membrane
• electrical attraction of cations and anions

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Resting Membrane Potential

• Resting membrane potential is -70 mV with a
  relatively negative charge on the inside of cell
  membranes
• Explanation for -70 mV resting potential
• membrane very permeable to K
• cytoplasmic anions can not escape due to size or
  charge (PO42-, SO42-, organic acids, proteins)
• membrane much less permeable to Na
• Na/K pumps out 3 Na for every 2 K it brings
  in
• works continuously requires great deal of ATP
• necessitates glucose oxygen be supplied to
  nerve tissue
• resting does not mean inactive

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Resting Membrane Potential

• Na concentrated outside of cell (ECF)
• K concentrated inside cell (ICF)

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Chemical Stimulus - Excitation
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Local Potentials

• Local disturbances in membrane potential
• stimulus binds to receptors on neuron
• opens ligand-gated Na channels
• Na rushes in down concentration and electrical
  gradients
• Depolarization- when membrane voltage shifts to a
  less negative value
• Na diffuses for short distance inside membrane
  producing a change in voltage called a local
  potential
• Produces signal from point of stimulus to trigger
  zone
• Characteristics of local potentials
• are graded (more stimuli open more Na
  gates?increase voltage change)
• are decremental (get weaker the farther they
  spread)
• are reversible (as K diffuses out of cell and
  reverses effect of Na inflow)
• can be either excitatory or inhibitory (depends
  on neurotransmitter)

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Action Potentials

• More dramatic change in membrane potential
  produced at the trigger zone (has high density of
  voltage-gated channels)
• If local potential spreads to trigger zone and
  threshold potential (-55mV) is reached,
  voltage-gated Na channels open (Na enters
  causing depolarization)
• Na channels start to close at 0mV
• (peaks at 35)
• K gates fully open, K exits
• until repolarization occurs
• Hyperpolarization occurs due to
• slow K channels still open prior
• to returning to resting potential

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Action Potentials

• Called a spike
• Characteristics of AP
• Follows an all-or-none law
• If threshold is reached, neuron will fire (AP
  will occur)
• Exceeding the threshold will not produce a
  stronger AP
• Nondecremental
• do not get weaker with distance
• Irreversible
• once started goes to completion and can not be
  stopped

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The Refractory Period

• The interval after the nerve fires when the
  neuron cannot generate another action potential
  Period of resistance to stimulation
• Absolute refractory period
• immediately after AP
• during repolarization
• no stimulus will trigger 2nd AP
• Relative refractory period
• during hyperpolarization
• only especially strong stimulus will trigger new
  AP
• Refractory period is occurring only to a small
  patch of membrane at one time (quickly recovers)

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Impulse Conduction in Unmyelinated Fibers

• Threshold voltage in trigger zone begins impulse
• Nerve signal (impulse)
• -a chain reaction of
• sequential opening of
• voltage-gated Nachannels
• down entire length of axon
• nondecremental
• travels at 2m/sec

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Impulse Conduction in Myelinated FibersSaltatory
Conduction

• Voltage-gated channels needed for APs
• fewer than 25 per ?m2 in myelin-covered regions
• up to 12,000 per ?m2 in nodes of Ranvier
• Fast Na diffusion occurs between nodes (in
  sheathed areas)

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Saltatory Conduction of Myelinated Fiber

• Action potentials jump from node of Ranvier to
  node of Ranvier.

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Nerve Synapse

• Between Two Neurons
• 1st is presynaptic neuron releases
  neurotransmitter
• 2nd is postsynaptic neuron responds to
  neurotransmitter
• Synapse may be
• Axodendritic (with dendrite of postsynaptic
  neuron)
• axosomatic (with soma of postsynaptic neuron)
• Axoaxonic (with axon of postsynaptic neuron)
• Number of synapses on postsynaptic neuron vary
• 8000 on spinal motor neuron
• 100,000 on neuron in cerebellum

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Chemical Synapse Structure

• Presynaptic neurons have synaptic vesicles with
  neurotransmitter and postsynaptic have receptors

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Neurotransmitters

• Chemical substance by which each neuron
  communicates with others to process information
  and send messages
• Four major categories (100 neurotransmitter
  types)
• Characterisitics
• Synthesized by presynaptic neuron
• Released when signal reaches synapse
• Bind to receptors on postsynaptic neuron
• Alter physiology of the postsynaptic neuron
  receptors

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Types of Neurotransmitters

• Acetylcholine
• Formed from acetic acid and choline
• Amino acid neurotransmitters
• GABA (Gamma Amino Butyric Acid)
• Glycine
• Aspartic acid

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Types of Neurotransmitters

• Monoamines (Biogenic amines)
• Catecholamines
• Epinephrine
• Norepinephrine
• Dopamine
• Indolamines
• Serotonin
• Histamine

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Types of Neurotransmitters

• Neuropeptides
• Chains of 2 to 40 amino acids
• Enkephalin
• Substance P
• Cholecystokinin
• Beta- endorphin

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