9.4-6

• What do receptors detect and link to effectors?

  Stimuli
• How do nerves function in the nervous system?

  Nerves carry information from receptors to regulator areas and effectors
• What is a typical nerve cell called?

  Neuron
• What are the main components of a neuron?

  A neuron consists of a cell body (soma), nucleus, dendrites, and axons
• How is information carried from cell to cell in neurons?

  By electrical impulses called action potentials generated by changes in ions across the neuron membrane
• Where is the nucleus of a neuron located?

  In the soma
• What are the two main types of fibers in neurons?

  • Axons: long and rarely branched, except at the ends
  • Dendrites: shorter and have many branches
• What is the function of dendrites in a neuron?

  Dendrites receive input from other neurons
• What is the primary function of axons?

  Axons are the main pathway for the conduction of impulses
• What covers axons in neurons?

  Schwann cells
• What is the myelin sheath?

  The myelin sheath is a layer of cells wrapping the axon that is rich in lipids
• What is the node of Ranvier?

  A gap between Schwann cells where the axon is exposed
• What types of neurons are associated with stimuli and responses?

  • Sensory (afferent) neurons: associated with stimuli
  • Motor (efferent) neurons: associated with response
  • Interneurons: link sensory and motor neurons
• What accompanies the conduction of a nerve impulse along a neuron?

  Chemical and electrical changes along the neuron membrane
• What does it mean for cells to be polarized?

  There is a difference in charge between the two sides of the membrane
• What is the charge inside the cell membrane compared to the outside?

  The inside of the cell membrane is more negatively charged than the outside
• What ions are unevenly distributed across the neuron membrane?

  Sodium (Na+) and potassium (K+) ions
• What is the sodium pump?

  A mechanism that pushes sodium ions out through special membrane channels against a concentration gradient
• How do potassium ions balance the outward movement of sodium ions?

  Potassium ions flow from the extracellular fluid into the cell
• How many potassium ions move into the cytoplasm for every three sodium ions pumped out?

  Two potassium ions
• What does the polarization of the neuron create?

  An electrical potential difference between the two sides of the membrane
• Where does the impulse start in a neuron?

  At the dendrite endings as a chemical signal
• What happens if the stimulus is strong enough?

  It brings about a change in the potential of the axon membrane that passes along the membrane to the synapse
• What occurs at the synapse?

  Chemicals are released to stimulate the dendrites of the next neuron
• What are the stages of signal transduction in neurons?

  1. Initiation at the dendrites

  2. Transmission through the cytoplasm to the axon

  3. Release of chemicals at the synapse
• How can the conduction of a nerve impulse be likened?

  To a wave of electrochemical changes
• What is the resting potential of a neuron?

  –70 mV
• What happens during depolarization of a neuron?

  Sodium channels open and sodium ions move into the cytoplasm
• What is the result of depolarization?

  The inside of the neuron becomes more positive relative to the outside
• What is action potential?


  A burst of electrical energy that passes along the axon
• What happens after the action potential?

  Sodium channels close and potassium channels open
• What is the relative refractory period?

  The time taken for the membrane to restore resting potential
• What occurs during the relative refractory period?

  No further action potentials can be generated in the section of membrane
• What are the steps of sodium pump restoration?

  1. Sodium ions actively pumped out of cytoplasm

  2. Potassium ions re-enter

  3. Resting potential achieved
• What is the threshold potential for an action potential to occur?

  A change from –70 mV to –55 mV
• What happens if the stimulus exceeds the threshold level?

  It will not increase the size of the action potential but can increase the frequency of action potentials
• What is the all-or-nothing response in action potentials?

  The magnitude of the action potential is the same regardless of the strength of the stimulus above threshold
• How does the action potential move along the neuron membrane?


  Once generated, it moves along the neuron membrane
• What initiates the depolarization of the next part of the neuron membrane?

  Depolarization of one segment of the neuron membrane
• How does conduction velocity vary in neurons?

  • Varies according to the type of neuron
  • Larger diameter of the axon increases conduction speed
  • Myelin sheath increases conduction speed by generating action potentials only at the nodes of Ranvier