NERVOUS SYSTEM

• Nervous system


  Controls all physiological and psychological reactions, even the endocrine system
• There is no "you" or "me" without the nervous system
• The nervous system is the most distinctive feature of the human species
• Principal functions of the nervous system

  • Sensory input
  • Integration
  • Motor output
• Sensory input


  Information detected by sensory receptors
• Integration


  Processing of sensory input and deciding on a response
• Motor output


  Response activated by the nervous system
• Main parts of the nervous system


  • Central nervous system (brain and spinal cord)
  • Peripheral nervous system (nerves branching off from brain and spine)
• Divisions of the peripheral nervous system


  • Sensory (afferent) division
  • Motor (efferent) division
• Subdivisions of the motor division

  • Somatic (voluntary) nervous system
  • Autonomic (involuntary) nervous system
• Divisions of the autonomic nervous system

  • Sympathetic division
  • Parasympathetic division
• Nervous tissue is densely packed with cells, with less than 20% extracellular space
• Main types of cells in nervous tissue


  • Neurons (nerve cells)
  • Neuroglia (glial cells)
• Neurons


  • Respond to stimuli and transmit signals
  • Account for a small part of nervous tissue
• Glial cells


  • Outnumber neurons 10 to 1
  • Serve many important functions
• Types of glial cells in the central nervous system

  • Astrocytes
  • Microglia
  • Ependymal cells
  • Oligodendrocytes
• Types of glial cells in the peripheral nervous system

  • Satellite cells
  • Schwann cells
• Neurons


  • Some of the longest-lived cells in the body
  • Irreplaceable - cannot divide once they take on their roles
  • Have a high metabolic rate and require abundant glucose and oxygen
• Basic neuron structure


  • Soma (cell body)
  • Dendrites
  • Axon
• Types of neurons based on number of processes

  • Multipolar neurons
  • Bipolar neurons
  • Unipolar neurons
• Types of neurons based on function

  • Sensory (afferent) neurons
  • Motor (efferent) neurons
  • Interneurons (association neurons)
• How the nervous system responds to a spider on the knee

  1. Sensory neurons in the skin detect the spider

  2. Signal travels up the sensory neuron to the spinal cord

  3. Interneurons in the spinal cord process the signal

  4. Some interneurons trigger a reflex response to kick the leg

  5. Other interneurons send the signal to the brain to be further processed and elicit a conscious response (scream or calmly remove the spider)
• The next lesson will cover how nerve cells use chemistry and electricity to communicate with each other
• Action potential


  The electrical impulse that zips down a neuron's axon to its neighboring neurons
• Action potential


  • It only transmits at one uniform strength and speed
  • The frequency or number of pulses can vary
• Neuron


  A cell that can transmit electrical impulses
• Voltage


  The measure of potential energy generated by separated charges, measured in millivolts
• Membrane potential


  The difference in charge between the inside and outside of a cell
• Resting membrane potential


  The membrane potential of a neuron at rest, around -70 millivolts
• Establishing resting membrane potential

  1. More positive sodium ions outside the cell

  2. More negative potassium ions and proteins inside the cell

  3. Sodium-potassium pump maintains the gradient
• Ion channels


  Proteins that allow the passage of ions across the cell membrane
• Voltage-gated channels


  Ion channels that open and close in response to changes in membrane potential
• Ligand-gated channels


  Ion channels that open when a specific neurotransmitter or hormone binds to them
• Mechanically-gated channels


  Ion channels that open in response to physical stretching of the membrane
• Graded potential


  A small, localized change in membrane potential
• Action potential


  A large, all-or-nothing change in membrane potential that triggers the opening of voltage-gated channels
• Generation of action potential


  1. Stimulus depolarizes membrane to threshold (-55 mV)

  2. Voltage-gated sodium channels open

  3. Massive influx of sodium ions

  4. Membrane potential briefly goes positive

  5. Voltage-gated potassium channels open

  6. Potassium ions flow out, repolarizing the membrane
• Refractory period


  The period after an action potential when the neuron cannot respond to another stimulus
• Action potentials


  • Always have the same initial threshold and peak amplitude
  • Frequency of action potentials varies with stimulus strength
• Conduction velocity


  The speed at which action potentials travel along an axon