Neuronal Physiology

• EXCITABLE CELLS SUCH AS NEURONS AND MUSCLES EVOLVED FOR RAPID SIGNALING, COORDINATION, AND MOVEMENT
• Patch clamp


  Technique to measure the electrical activity of individual cells

  
• Techniques involved: microelectrodes; patch clamping
• Membrane potential


  Electrical potential difference across the cell membrane

  
• Terminology


  • Resting Potential: -70 in a typical neuron
  • Depolarization: change in potential that makes the membrane less polarized than at resting potential
  • Repolarization: membrane returns to resting potential after having been depolarized
  • Hyperpolarization: A change in potential that makes the membrane more polarized than at resting potential

  
• Electrical signals are produced by changes in ion movement through ion channels across the plasma membrane

  
• Types of gated channels

  • Voltage-gated ion channels
  • Chemically-gated channels
  • Mechanically-gated channels
  • Thermally-gated channels
• Graded potential


  Local changes in membrane potential that occur in varying grades or degrees of magnitude or strength

  
• The stronger a triggering event, the larger the resultant graded potential
• The longer the duration of the triggering event, the longer the duration of the graded potential
• Active area


  Temporarily depolarized region

  
• Inactive areas


  Still at resting membrane potential

  
• Current


  Flow of electrical charges
• Resistance


  Hindrance to electrical charge movement
• Graded potentials spread by passive current flow and are impeded by resistance

  
• Graded potentials die out over short distances
• Action potential


  Brief, rapid, large (100 mV) changes in membrane potential during which the potential actually reverses so that the inside of the excitable cell transiently becomes more positive than the outside

  
• Action potentials are propagated in a nondecremental fashion
• Resting membrane potential: -70 mV

  
• Depolarization proceeds slowly
• Threshold potential: -50 to -55 mV or 10 to 15 mV above resting membrane potential

  
• Explosive depolarization; sharp upward deflection as the potential rapidly reverses itself so that the inside of the cell becomes positive compared to the outside
• Overshoot: potential is reversed (between 0 mV and 30 mV)
• Peak potential: usually 30 mV to 40 mV
• Repolarization: dropping back to resting potential
• Threshold is a critical all-or-none event
• Na+ channel


  Conformations of channels
• K+ channel


  Permeability changes and ion fluxes during an action potential

  
• Contiguous conduction


  Spread of the action potential along every patch of membrane

  
• Saltatory conduction


  Clinical correlation
• Chemical synapse


  • Disadvantage: slower
  • Advantage: operate in one direction only; allow for various kinds of signaling events
• Serotonin


  Plays an important role in the control of sleep, pain, aggression, sexual behavior, and food intake
• Effects of SSRIs in dogs

  • Reduction in the intensity of obsessive–compulsive disorders such as excessive licking behavior, tail mutilation, separation anxieties, dominance-related aggression, and so forth
• Indication of SSRI in cats

  • Treatment of psychogenic alopecia (unnatural loss of hair), offensive aggression, and urine spraying
• Prozac


  • Treatment of cribbing (force-swallowing gulps of air) in horses and bulimia (episodes of binge eating that continue until terminated by abdominal pain or vomiting) in pigs
• Neurotransmitter


  Carries the signal across a fast synapse and opens a chemically gated channel
• Slow synapses


  Synapses that lead to responses mediated by second messengers
• Fast synapses


  NTs usually function by changing the conformation of chemically gated channels
• Acetylcholine


  A fast excitatory neurotransmitter that links electrical signals in motor neurons with electrical signals in skeletal muscle cells
• Muscle fiber


  Single muscle cell