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).
- The idea that your brain can
- generate dreams, memory,
- breathing, and every mental
- process in your life may seem
- hard to believe—but it’s true.
- This is particularly impressive
- in view of the brain’s size.
- Considering its many functions,
- the brain is packed into a very
- small space. Billions of neurons
- and additional supporting cells
- communicate with one another
- using an astronomical number
- of synaptic connections—and
- the entire operation fits into
- an object weighting about
- three pounds, the size of a
- small cantaloupe.
- Like a cantaloupe—and the
- rest of your body—your brain
- is made of cells. Brain cells
- come in two types: neurons,
- which talk to one another and
- to the rest of the body, and
- glial cells, which provide
- essential support to keep the
- whole show going. Your brain
- is made up of about
- one-hundred billion neurons—
- which have a long, skinny,
- complicated shape—and many
- more glial cells.
- From a distance, the brains of
- different animals do not look
- alike. They all work according
- to the same principles,
- however. Signals within a
- neuron are carried by
- electricity.
- Each neuron has a net excess
- density of negative charge on
- the inside of the membrane
- that surrounds it relative to
- the outside, due to an uneven
- distribution of positive and
- negative ions like potassium
- and chloride. The unequal
- distribution of charge creates
- a voltage difference across
- the membrane, like a much
- smaller version of the voltage
- difference that allows a
- nine-volt battery to give a
- shock to your tongue.
- (Actively moving ions across
- the membrane to maintain
- this charge distribution
- requires more energy than
- anything else that the brain
- does.)
- To send electrical signals
- from one part of the neuron
- to another, the neuron opens
- channels that allow the ions
- to move across the membrane,
- creating a current that carries
- an electrical signal down the
- membrane. Neurons receive
- inputs through branched,
- treelike structures called
- dendrites, which put together
- information from a bunch of
- different sources. The neurons
- then sends an electrical signal
- down a long, wirelike structure,
- called an axon, which triggers
- a chemical signal to another
- neuron, and so on.
- Axons can carry signals over
- long distances; your longest
- axons run from your spine to
- the tips of your toes. In
- contrast, the longest known
- axons in whales are sixty feet
- in length. The longest axons
- belonging to the shrew are a
- mere two inches. In all cases,
- electrical signals spread using
- similar molecules and according
- to the same biological principles.
- Neurons pass information down
- their axons by generating small
- electrical signals that last a
- thousandth of a second. These
- signals are called “spikes”
- because they represent sudden
- increases in the electrical
- currents in a neuron. Spikes—
- otherwise known as action
- potential—look the same
- whether they come from squid,
- rats, or Uncle Fred, making
- them a huge success story in the
- evolutionary history of animals.
- Racing down axons at speeds
- up to several hundred feet per
- second, spikes bring signals
- from your brain to your hand
- fast enough to escape the bite
- of a dog or the heat of a frying
- pan. They help all animals
- getaway from imminent
- danger—fast.