Reading 48


This passage is taken from a book titled Welcome to Your Brain by Sandra Aamodt and Sam Wang (© 2008 by Sandra Aamodt and Sam Wang).

  1. The idea that your brain can
  2. generate dreams, memory,
  3. breathing, and every mental
  4. process in your life may seem
  5. hard to believe—but it’s true.
  6. This is particularly impressive
  7. in view of the brain’s size.
  8. Considering its many functions,
  9. the brain is packed into a very
  10. small space. Billions of neurons
  11. and additional supporting cells
  12. communicate with one another
  13. using an astronomical number
  14. of synaptic connections—and
  15. the entire operation fits into
  16. an object weighting about
  17. three pounds, the size of a
  18. small cantaloupe.
  19. Like a cantaloupe—and the
  20. rest of your body—your brain
  21. is made of cells. Brain cells
  22. come in two types: neurons,
  23. which talk to one another and
  24. to the rest of the body, and
  25. glial cells, which provide
  26. essential support to keep the
  27. whole show going. Your brain
  28. is made up of about
  29. one-hundred billion neurons—
  30. which have a long, skinny,
  31. complicated shape—and many
  32. more glial cells.
  33. From a distance, the brains of
  34. different animals do not look
  35. alike. They all work according
  36. to the same principles,
  37. however. Signals within a
  38. neuron are carried by
  39. electricity.
  40. Each neuron has a net excess
  41. density of negative charge on
  42. the inside of the membrane
  43. that surrounds it relative to
  44. the outside, due to an uneven
  45. distribution of positive and
  46. negative ions like potassium
  47. and chloride. The unequal
  48. distribution of charge creates
  49. a voltage difference across
  50. the membrane, like a much
  51. smaller version of the voltage
  52. difference that allows a
  53. nine-volt battery to give a
  54. shock to your tongue.
  55. (Actively moving ions across
  56. the membrane to maintain
  57. this charge distribution
  58. requires more energy than
  59. anything else that the brain
  60. does.)
  61. To send electrical signals
  62. from one part of the neuron
  63. to another, the neuron opens
  64. channels that allow the ions
  65. to move across the membrane,
  66. creating a current that carries
  67. an electrical signal down the
  68. membrane. Neurons receive
  69. inputs through branched,
  70. treelike structures called
  71. dendrites, which put together
  72. information from a bunch of
  73. different sources. The neurons
  74. then sends an electrical signal
  75. down a long, wirelike structure,
  76. called an axon, which triggers
  77. chemical signal to another
  78. neuron, and so on.
  79. Axons can carry signals over
  80. long distances; your longest
  81. axons run from your spine to
  82. the tips of your toes. In
  83. contrast, the longest known
  84. axons in whales are sixty feet
  85. in length. The longest axons
  86. belonging to the shrew are a
  87. mere two inches. In all cases,
  88. electrical signals spread using
  89. similar molecules and according
  90. to the same biological principles.
  91. Neurons pass information down
  92. their axons by generating small
  93. electrical signals that last a
  94. thousandth of a second. These
  95. signals are called “spikes”
  96. because they represent sudden
  97. increases in the electrical
  98. currents in a neuron. Spikes—
  99. otherwise known as action
  100. potential—look the same
  101. whether they come from squid,
  102. rats, or Uncle Fred, making
  103. them a huge success story in the
  104. evolutionary history of animals.
  105. Racing down axons at speeds
  106. up to several hundred feet per
  107. second, spikes bring signals
  108. from your brain to your hand
  109. fast enough to escape the bite
  110. of a dog or the heat of a frying
  111. pan. They help all animals
  112. getaway from imminent
  113. danger—fast.