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Module 1.2

Apply the Concept
▶ ▶Think back to a stressful moment when you felt your sympathetic nervous system kick in. What was your body preparing you
for? Were you able to sense your parasympathetic nervous system’s response when the challenge had passed?
Answers to the Examine the Concept questions can be found in Appendix C at the end of the book.

The Central Nervous System

From the process of neurons “talking” to other neurons arises the complexity of the central
nervous system’s brain and spinal cord.
Copyright © Bedford, Freeman & Worth Publishers.
It is the brain that enables our humanity — our thinking, feeling, and acting. Tens
of billions of neurons, each communicating with thousands of other neurons, yield an
ever-changing wiring web. By one estimate — projecting from neuron counts in small brain
samples — our brain contains some 128 billion neurons (Barrett, 2020).
Just as individual pixels combine to form a picture, the brain’s individual neu-
rons cluster into work groups called neural networks. To understand why, Stephen
Kosslyn and Olivier Koenig (1992, p. 12) have invited us to “think about why cities
exist; why don’t people distribute themselves more evenly across the countryside?”
Like people networking with people, neurons network with nearby neurons with
which they can have short, fast connections; each layer’s cells connect with various
cells in the neural network’s next layer. Learning — to play the violin, speak a for-
eign language, or solve a math problem — occurs as experience strengthens connec-
tions. To paraphrase one neuropsychologist, neurons that fire together, wire together
(Hebb, 1949). Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution. © Tom Swick/Cartoonstock.com
The other part of the CNS, the spinal cord, is a two-way information highway
connecting the peripheral nervous system and the brain. Ascending neural fibers
send up sensory information, and descending fibers send back motor-control infor-
mation. The neural pathways governing our reflexes, our automatic responses to
stimuli, illustrate the spinal cord’s work. A simple spinal reflex pathway — the reflex arc — is
composed of a single sensory neuron and a single motor neuron. These often communi-
cate through a spinal cord interneuron. The knee-jerk reflex, for example, involves one such
simple pathway (from the peripheral nervous system to the central nervous system’s spinal
cord, and back out through the peripheral nervous system). A headless warm body could
do it.
Another neural circuit enables the pain reflex (Figure 1.2-3). When your finger touches
a flame, neural activity (excited by the heat) travels via sensory neurons to interneurons in
your spinal cord. These interneurons respond by activating motor neurons leading to the
muscles in your arm. Because the simple pain-reflex pathway runs through the spinal cord
and right back out, your hand jerks away from the candle’s flame before your brain receives
and responds to the information that causes you to feel pain. That’s why it feels as if your
hand jerks away not by your choice, but on its own.
Information travels to and from the brain by way of the spinal cord. Were the top of
your spinal cord severed, you would not feel pain from your paralyzed body below. Nor
would you feel pleasure. With your brain literally out of touch with your body, you would
lose all sensation and voluntary movement in body regions with sensory and motor con- reflex a simple, automatic
nections to the spinal cord below its point of injury. You would exhibit the knee-jerk reflex response to a sensory stimulus,
without feeling the tap. To produce bodily pain or pleasure, the sensory information must such as the knee-jerk reflex.
reach the brain.

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