The Spinal Cord

The Central Nervous System: Making Sense of the World
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IS THAT YOUR FINAL ANSWER?
The geographically largest U.S. state
is (a) Montana, (b) Texas, (c) Alaska,
(d) Wyoming. Like contestants on shows
such as Who Wants to Be a Millionaire,
you must rely on your central nervous
system, and especially on the
information-processing power of your
brain, to understand the question,
recognize the correct option, and direct
movements of your vocal muscles to
make your answer heard. (The correct
choice in this case is c.)
plural of nucleus). The sidewalks and hallways of the CNS are axons that travel together
in bundles called fiber tracts, or pathways. The axon (hallway) from any given cell
(office) may merge with and leave many fiber tracts (sidewalks) and send branches out
to other tracts. Let’s consider a practical example of nervous system functioning to begin
learning our way around the “campus” of the brain.
It is 6 A.M. and your alarm clock goes off, creating the simple case of information
processing illustrated in Figure 2.1. Your ears receive sensory input in the form of sound
from the alarm. The sound is converted into neural signals and sent to the brain. Your
brain compares these signals with previous experiences stored in memory and correctly
associates the sound with “alarm clock.” Your muscle-guiding output is not yet at peak
performance, though, because your brain activity has not yet reached the waking state.
So you fumble to turn off the alarm, shuffle to the kitchen, and accidentally touch the
coffeemaker’s heating element. Things get more lively now. Heat energy activates sensory neurons in your fingers, generating action potentials that speed along fiber tracts
going into the spinal cord. Your motor neurons are reflexively activated by the CNS,
causing muscles in your arm to contract and quickly withdraw your hand.
The Spinal Cord
fiber tracts Bundles of axons that
travel together.
spinal cord The part of the central
nervous system that receives
information from the senses, passes
these signals to the brain, and sends
messages from the brain to the body.
reflexes Simple, involuntary,
unlearned behaviors directed by the
spinal cord without instructions from
the brain.
The spinal cord receives signals such as pain and touch from the senses and passes those
signals to the brain. Neuron fibers within the cord also carry signals downward from the
brain to the muscles. Some cells of the spinal cord can direct simple behaviors without
instructions from the brain. These behaviors are called reflexes, because the response
to the incoming signal is directly “reflected” back out, as shown in Figure 2.6. Spinal
reflexes, such as the one that pulled your hand away from the heat, are very fast because
they include few time-consuming synaptic links. Reflexes are called involuntary because
they occur without instructions from the brain. As reflexes occur, though, action potentials are also sent along fiber tracts to the brain. So you officially “know” you have been
burned a fraction of a second after your reflex got you out of trouble.
The spinal cord is an example of a feedback system. When touching something hot
sets off a simple reflex, one set of arm muscles contracts, and an opposing set of muscles relaxes. If this did not happen, the arm would go rigid. The muscles also have receptors that send information to the spinal cord to let it know how extended they are so
that adjustments can be made for a smooth contracting motion. Information about the