bio 14

• Nervous System

  • Master control and communication system of the body
  • Uses electrical impulses to communicate with body cells
  • Rapid
  • Specific

  
• Nervous System vs Endocrine System

  • Nervous system: Fast acting, messages travel via nerve impulses and neurotransmitters, acts on a limited area, effects usually short-lived
  • Endocrine system: Slow acting, messages travel via hormones, can act on tissues throughout the body, effects can be long lasting
• Central Nervous System


  Component of nervous system that processes information from sensory to generate motor output: Brain and spinal cord

  
• Peripheral Nervous System


  Component of nervous system that transmits signals to and from the central nervous system

  
• Sensory Neurons


  Transmit information such as touch, light, taste, sound, blood pressure and muscle tension

  
• Motor Neurons


  Transmit nervous impulses from CNS to effector cells (muscle and glands)

  
• Nerve


  • Bundle of axons
  • Axons can range from <1 millimeter to as long as 1 meter (base of spinal cord to large toe)
• Neurons


  • Interneurons: Sum up info received from sensory neurons directly or indirectly and send impulses to motor neurons
  • Sensory neurons: Take sensory (external/internal) info from receptor to CNS
  • Motor neurons: Transmit nervous impulses from CNS to effector cells

  
• Neuron Structure: Cell Body

  Region containing nucleus cytoplasm, organelles
• Neuron Structure: Dendrites


  Branched extensions that receive signals from other neurons

  
• Neuron Structure: Axon


  Single long extension that transmits signals to other neurons

  
• Neuron Structure: Axon Hillock


  Region joining the cell body with the axon

  
• Neuron Structure: Synapse


  Connection between axon and dendrite of two neurons

  
• Neuron Structure: Axon Terminal


  Axon Terminal is the distal end of an axon at the synapse
• Neuron Structure Summary


  • Dendrites: Branched structures that receive input
  • Cell body: Location of many organelles, also receives input
  • Axon hillock: Cone shaped region important in action potential generation
  • Axon: Action potentials run along the axon towards another neuron or effector
  • Axon terminals: Structures at end of axon that communicate with another neuron or effector

  
• Neuroglia


  Protect, support, nourish neuronal cells
• Neuroglia in the CNS

  • Astrocytes: Support and brace and anchor neurons to blood capillaries, determine permeability and exchange between capillaries and neurons, control chemical environment around neurons, guide neuron and synapse development
  • Microglial Cells: Monitor health of neurons, can transform into macrophages in presence of debris/invaders
  • Ependymal Cells: Line cavities of the brain and spinal cord, cilia assist with circulation of cerebrospinal fluid
  • Oligodendrocytes: Wrap around nerve fibers in the central nervous system, produce myelin sheaths

  
• Myelin Sheath


  • White, fatty material covering axons
  • Protects and insulates fibers
  • Speeds nerve impulse transmission
• Schwann Cells


  • Form myelin sheath by wrapping around neuron
  • Gaps between Schwann cells called nodes of Ranvier
• Satellite Cells


  Protect and cushion neuron body
• Neuron Function


  • Excitability: Ability to respond to stimuli
  • Conductivity: Ability to transmit impulses

  
• Luigi Galvani discovered (by accident) that if you apply an electrical signal to a dead frog's leg nerve, the dead frog would kick!
• Neurons carry information as electrical signals

  
• Nerve Impulses


  • Means by which neurons communicate with one another
  • Caused by the movement of ions across the cell membrane
  • Changes voltage across the cell membrane

  
• Plasma Membrane


  • Phospholipid bilayer with embedded proteins
  • Phospholipid bilayer acts as insulator that separates charges
  • Resists flow, but thin enough that charges can interact
  • Difficult for charged molecules to cross hydrophobic region unaided
• Cell Membrane Potential


  • Difference in the amount of electrical charge inside vs. outside of neuron
  • Caused by imbalance of ions inside vs outside cell
  • Creates a source of potential energy
• Sodium Potassium Pump


  • Requires ATP
  • Pumps 2 K+ into cell
  • Pumps 3 Na+ out of cell
• Leaky Potassium Channels

  • Allow potassium to flow out of cell according to chemical gradient (high to low concentration)
  • Outward flow of potassium contributes to electric potential across membrane
• Resting Membrane Potential

  • Action of sodium potassium pump and leaky potassium channel generates a negative membrane potential (negative inside cell vs outside)
  • Potential energy (chemical/electric) can be translated into real energy when voltage gated ion channels are open
• Neuron at Resting Potential: concentration of K+ is highest inside the cell, concentration of Na+ is highest outside the cell
• Action Potential


  • Threshold: When membrane potential drops beyond a certain limit, voltage gated Na+ channels all open simultaneously
  • Depolarization: "All or nothing" response
  • Repolarization: Sodium channels inactivated temporarily preventing another action potential (refractory period), as membrane potential becomes positive voltage gated potassium channels open
  • Hyperpolarization (undershoot): Voltage gated potassium channels are slow to close, membrane potential briefly undershoots the resting membrane potential briefly
• Gated Channels


  • Chemically gated channels: Respond to molecules that bind to or alter the channel
  • Voltage gated channels: Respond to changes in membrane potential
• When channels are opened: Ion permeability increases, Ions flow quickly across the membrane, Ions travel down their concentration gradients, Movement of ions creates flow of current across membrane, Membrane potential (voltage across the membrane) changes
• As membrane depolarizes action potential spreads along the length of the axon, Refractory period ensures that the action potential occurs in one direction only
• Lesson Review


  • Ions that govern electrical and chemical potential energy in a neuron are sodium and potassium. They move through the membrane via pumps and leaky channels.

  1. Axon: Single long extension that transmits signals to other neurons. Dendrite: Branched extensions that receive signals from other neurons. Synapse: Connection between axon and dendrite of two neurons.
  2. Ion pump: Sodium Potassium Pump. Leaky channel: Leaky potassium channels.
  3. Membrane potential is established by the sodium potassium pump and leaky potassium channels.
  4. Stages of action potential: Threshold, Depolarization, Repolarization, Hyperpolarization (undershoot).