Reading 48

Neurons

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. a 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.