Chapter 5b Nerve Cells

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Chapter 5b Nerve Cells

• Chris Rorden
• University of South Carolina
• Norman J. Arnold School of Public Health
• Department of Communication Sciences and
  Disorders
• University of South Carolina

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Hierarchy of Organism Structures

• Organism
• Organ Systems
• Organs
• Tissues
• Cells
• Organelles
• Organic Molecules

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Cell components

• Channels
• Structural Proteins
• Sodium-Potasium Pump (Na-K)
• Extracellular fluid
• Intracellular fluid
• Membranes lipids attached to proteins.
• Lipids (fats) do not dissolve in water
• Separates extra and intra-cellular fluids.

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Cell membranes

• Lipoproteins line up in double layer with protein
  (head) to outside and lipid tail to inside of
  membrane

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Resting Potentials

• All Cells have General Characteristic of
  Irritability.
• Need Irritability to Respond to Outside
  Influences.
• Well Developed in Neurons.
• Intracellular Fluid is -70 mvolts as Compared to
  Extracellular Fluid.

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Why?

• Uneven distribution of
• Positively charged sodium
• Positively charged potassium
• Negatively charged chloride ions
• Other negatively charged proteins.
• Channels Open to Selectively Allow Movement of
  Ions.
• Na-K Pump Helps to Keep Resting Potential.

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Intra vs Extracellular fluid
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Graded local potentials

• Mechanical or Chemical Event Affects Neuronal
  Membrane
• Neuron Becomes Perturbed (Perturbation)
• Channels Open Causing Negative Ions to Flow Out
  or Positive Ions to Flow in

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Changes in resting potential

• Resting Potential Becomes Less than -70 mvolts
  Depolarization
• Resting Potential Becomes More than -70 mvolts
  Hyperpolarization
• If voltage exceeds threshold (-55mV) the neuron
  fires.

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Movement of Graded Potentials

• Potential changes can occur in soma, along
  dendrite or initial portions of axon
• Spreads along membrane, effect becomes smaller.
• If depolatrization is at least 10mv at axon
  hillock, action potential is triggered
• Smaller changes in potential will not influence
  neuron.

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Action potential

• During an action potential
• Membrane is Depolarized, then Sodium (Positive
  Charge) Flows into Cell Causing Interior
  Potential to Become Positive.
• Impulse Occurs travels down axon to terminals
• Absolute Refractory Period
• After Impulse Fires, Over Reaction Drives
  Interior Charge to -80 or -90 mV
• Any Additional Charge Would be Hard to Activate
  Until Cell Returned to Normal Resting State of
  -70mV

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Impulse conduction

• Neighboring Areas of the Cell Undergo Positive
  Charge Changes
• The Impulse is Carried Through Continuous Short
  Distance Action Potentials
• Myelin Speeds up the Impulse Through Saltatory
  Conduction

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An action potential
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Impulses Between Cells

• Synapse
• When a neuron fires, it pours neurotransmitters
  into the synaptic clefts of its terminals.
• These neurotransmitters influence the
  post-synaptic membrane, either polarizing
  (inhibiting) or depolarizing (excting) the target
  neuron.

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Conduction Velocities

• Dependent on Size of Axon and Whether it is
  Myelinated or Not
• Myelinated Fibers Conduct at 6m/sec Times Size of
  Fiber
• ( 3um x 6m/sec18m/sec)
• Unmyelinated Fiber Diameter of 1 um Conducts
  Impulse at lt1m/sec

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Neuronal Response to Injury

• Two Types
• Axonal (Retrograde) Reaction Occurs When
  Sectioning of Axon Interrupts Information that
  returns to Cell Body and Interferes with Support
  Reprogramming
• Wallerian Degeneration Occurs When Axon
  Degenerates in Region Detached from cell Body

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Axonal Reaction

• Series of internal changes
• Chromatolysis degenerative process
• Begins between axon hillock and cell nucleus
• Nissl bodies
• Displacement of nucleus from center of soma
• If RNA Production and Protein Synthesis Increase,
  Cell May Survive and Return to Normal Size

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Wallerian Degeneration

• Axon Dependent on Cytoplasm from Cell Body
• Without Nourishment, Distal Portion of Axon
  Becomes Swollen and Begins Degenerating in 12-20
  Hours
• After 7 Days, Macrophagic Process (Cleanup)
  Begins and Takes 3-6 Months

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Neuroglial Responses

• Glial cells multiply in Number Hyperplasia
• Increase in Size Hypertrophy
• Neurophils (Scavenger White Blood Cells) Arrive
  at Injury
• Astrocytes Form a Glial Scar
• Microglia Cells Ingest Debris
• Cells May Return to Function

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Axonal Regeneration

• PNS
• Ends of Axon are Cleaned
• Sheath of Schwan Cell Guides Axon to Reconnect
• Grows 4 mm/day
• May Have Mismatch of Axons
• CNS
• Minimal restoration after injury
• Growth occurs, but not significant enough to be
  functional

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Neuro-transmitters

• Two Types
• Small molecules transient effects
• Acetylcholine, Norepinephrine, Dopamine,
  Serotonin, Glutamate, Y-aminobutyric acid (GABA)
• Large Molecules - Longer Effects
• Peptides Table 5.4

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Neurotransmitter Acetylcholine

• Major Player in the PNS
• Released in Synapses Where it is Released to
  Facilitate Stimulation of Synapse
• Needed for Continuous Nerve Impulses
• Most Studied Neurotransmitter
• After Use, Picked Up By Acetylcholinesterase
• Regulates Forebrain and Inhibits Basal Ganglia
• Example Scopolamine used for motion sickness.
  Blocks acetylcholine receptors

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Related Diseases

• Myasthenia Gravis
• Affects Acetylcholine receptors
• Behavioral Example Fatigue in Speaking
• Alzheimer's Disease
• Implication of Deficient Projections in Cortex,
  Hippocampus, and Orbito-frontal Cortex

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Dopamine

• Cells are Located in Upper Midbrain and Project
  Ipsilaterally
• Mesostriatal - Midbrain and Striatum
• Substantia Nigra to Basal Ganglia
• Results in Parkinsons Disease
• Mesocortical - Midbrain and Cortex
• Can Result in Problems of Cognition and
  Motivation
• Can be Affected by Drug Abuse to Gain Pleasurable
  Feelings

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Dopamine

• Parkinson's disease loss of dopamine in the
  neostriatum
• Treatment increase dopamine
• Schizophrenia Too much dopamine
• Treatment Block some (D2) dopamine receptors.
• Problem Overdose or prolonged dose leads to
  Parkinson's disease-like tremors (tardive
  dyskinesia)

Not enough DA Parkinsons
Normal
Too much DA Schizophrenia
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Norepinephrine

• Pons and Medulla
• Reticular Formation and Locus Ceruleus
• Project to Diencephalon, Limbic Structures and
  Cerebral Cortex, Brainstem, Cerebellar Cortex and
  Spinal Cord
• Maintain Attention and Vigilance
• May be Related to Handedness Due to Asymmetry in
  Thalamus

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Serotonin

• Found Primarily in Blood Platelets and GI Tract
• Terminals at Most Levels of Brainstem and in
  Cerebrum
• Involved in General Activity of CNS and in Sleep
  Patterning
• Increased Concentration of Serotonin, Decreases
  Depression and Pain (Prozac)

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Y-Aminobutyric Acid (GABA)

• Major Player in the CNS
• Pyramidal (Motor Cortex) Cells Rich in GABA
• Present in Hippocampus, Cortex of Cerebrum and
  Cerebellum
• Suppress Firing of Projection Neurons
• Implicated in Huntingtons Disease
• Reduced GABA Causes High Amount of Dopamine and
  Acetylcholine and Uncontrolled Movements

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Peptides

• Important in Pain Management
• Examples
• Enkephalin
• Endorphins
• Substance P

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Drug Treatments

• Blocking Enzymatic Breakdown of Neurotransmitter
• Allows for Increased Neurotransmitter to Continue
  Function
• e.g. Myasthenia Gravis
• Regulating Activity of Postsynaptic Membrane
• Blocking Effects of Released Neurotransmitter
  Causing Problem