electrical signals in animals

• Aquatic animal with internal solute concentration of 500 mOsm/L


  Placed in fluid with solute concentration of about 700 mOsm/L
• Adaptations of desert animals do not likely include

  • Shells, scales, and thick skin
  • A nocturnal activity pattern
  • Conservation of metabolic water
  • Nitrogenous waste excreted as ammonia
  • The production of concentrated urine
• Two-solute model explaining urine production in the nephron

  • NaCl moves out of the nephron and into the interstitial fluid in the descending loop of Henle
  • The fluid entering the distal convoluted tubule is more concentrated in NaCl than is the fluid entering the proximal convoluted tubule
  • The transport epithelium in the ascending loop of Henle is relatively impermeable to water
  • All urea movements along the nephron are from the interstitial fluid into tubule fluid
  • The ratio of NaCl to urea in interstitial fluid is about the same all along the length of the nephron
• Neurons


  Nerve cells that transfer information within the body
• Signals used by neurons to communicate

  • Electrical signals (long-distance)
  • Chemical signals (short-distance)
• Nervous system organization


  • Central nervous system (CNS) where integration takes place, including the brain and a nerve cord
  • Peripheral nervous system (PNS) which carries information into and out of the CNS, with neurons bundled together forming nerves
• Synapse


  Junction between an axon and another cell, where information is transmitted from a presynaptic cell (a neuron) to a postsynaptic cell (a neuron, muscle, or gland cell) using chemical messengers called neurotransmitters
• Resting potential


  The membrane potential of a neuron not sending signals, established by ion pumps and ion channels
• Formation of the resting potential

  1. Sodium-potassium pumps maintain K+ and Na+ gradients across the plasma membrane

  2. Chemical potential energy is converted to electrical potential as ion channels open

  3. Buildup of negative charge within the neuron is the major source of membrane potential
• Graded potentials


  Changes in polarization where the magnitude of the change varies with the strength of the stimulus, not the nerve signals that travel along axons
• Action potential


  A massive change in membrane voltage that transmits signals over long distances, arising from voltage-gated ion channels
• Generation of action potentials

  1. Voltage-gated Na+ channels open first and Na+ flows into the cell, crossing the threshold and increasing the membrane potential (rising phase)

  2. Voltage-gated Na+ channels become inactivated, voltage-gated K+ channels open and K+ flows out of the cell (falling phase)

  3. Membrane permeability to K+ is initially higher than at rest, then voltage-gated K+ channels close and resting potential is restored (undershoot)
• Refractory period


  After an action potential, a temporary inactivation of the Na+ channels prevents a second action potential from being initiated
• Threshold


  Minimum level of stimulation required to trigger an action potential
• Resting potential


  The membrane potential of a neuron when it is not actively transmitting an action potential
• Action potential


  1. Depolarization

  2. Rising phase

  3. Falling phase

  4. Undershoot
• Refractory period


  • After an action potential, a second action potential cannot be initiated due to temporary inactivation of Na+ channels
• Conduction of action potentials

  1. Depolarization of neighboring region of axon membrane

  2. Action potentials travel in one direction

  3. Inactivated Na+ channels behind the zone of depolarization prevent backward travel
• Axon structure


  • Speed of action potential increases with axon diameter
  • Myelin sheath insulates axons and increases speed (saltatory conduction)
• Electrical synapse


  Electrical current flows directly from one neuron to another
• Chemical synapse


  Chemical neurotransmitter carries information across the synaptic cleft
• Synaptic transmission


  1. Neurotransmitter synthesis and packaging

  2. Release of neurotransmitter

  3. Diffusion across synaptic cleft

  4. Binding to receptors on postsynaptic cell
• Excitatory postsynaptic potential (EPSP)

  Depolarization that brings the membrane potential toward threshold
• Inhibitory postsynaptic potential (IPSP)


  Hyperpolarization that moves the membrane potential farther from threshold
• Summation of postsynaptic potentials

  1. Temporal summation

  2. Spatial summation
• Neurotransmitter groups


  • Acetylcholine
  • Biogenic amines
  • Amino acids
  • Neuropeptides
  • Gases
• Neurotoxins can disrupt neurotransmitter signaling in various ways