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Module 1.3a

information, conducting it
toward the cell body (Poirazi & Figure 1.3-2
Papoutsi, 2020). From there, the Neurons communicating
cell’s single lengthy axon fiber Our billions of neurons exist in a
passes the message through vast and densely interconnected
web. As part of a fascinating
its terminal branches to other electrochemical communication
neurons or to muscles or glands process, one neuron’s terminal
(Figure 1.3-2). Dendrites listen; branches send messages to
neighboring dendrites.
axons speak.
Unlike the short dendrites,
axons may be very long, pro-
Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
jecting several feet through the David Scharf/Science Source
body. A human neuron carrying
orders from the brain to a leg
Copyright © Bedford, Freeman & Worth Publishers.
muscle, for example, has a cell
body and axon roughly on the scale of a basketball attached to a rope that’s 4 miles
(6.4 kilometers) long. Much as home electrical wire is insulated, some axons are encased
in a myelin sheath, a layer of fatty tissue that insulates them and speeds their impulses.
As myelin is laid down up to about age 25, neural efficiency, judgment, and self-control
grow (Nakamura et al., 2018; Van Munster et al., 2015). If the myelin sheath degener-
ates, this deterioration leads to diminished control and slower reaction time. In extreme
cases, it results in the disease known as multiple sclerosis. Communication to muscles
and brain regions slows. The result: diminished muscle control and sometimes impaired
cognition.
Supporting our billions of nerve cells are spidery glial cells (“glue cells”). Neurons are
like queen bees; on their own, they cannot feed or sheathe themselves. Glial cells are the axon the segmented neuron
worker bees. They provide nutrients and insulating myelin, guide neural connections, and extension that passes messages
clean up waste after neurons send messages to one another. Glia also play a role in learning, through its branches to other
neurons or to muscles or glands.
thinking, and memory. By “chatting” with neurons, they participate in information trans- myelin [MY-uh-lin] sheath a
mission and memory (Fields, 2013; Martín et al., 2015). fatty tissue layer segmentally
In more complex animal brains, the encasing the axons of some
proportion of glia to neurons increases. A neurons; it enables vastly greater
postmortem analysis of Albert Einstein’s transmission speed as neural
brain did not find more or larger-than- impulses hop from one node to
usual neurons. However, it did reveal a the next.
much greater concentration of glial cells glial cells (glia) cells in the
nervous system that support,
than found in the average Albert’s head nourish, and protect neurons;
(Fields, 2004). Einstein’s glial cells likely they may also play a role in
kept his brain abuzz with activity. learning, thinking, and memory.
action potential a neural
The Neural Impulse impulse; a brief electrical charge
that travels down an axon.
Neurons transmit messages when stim-
ulated by our senses or by neighboring
neurons. A neuron sends a message
by firing an impulse, called the action
potential — a brief electrical charge that Managing multiple sclerosis
travels down its axon. Actor Selma Blair’s multiple sclerosis
results from a loss of the myelin
Depending on the type of fiber, a sheath that insulates her motor axons
neural impulse travels at speeds ranging and speeds their neural impulses.
from a sluggish 2 miles (3 kilometers) John Sheerer/Getty Images She has discussed her challenges
per hour to more than 200 miles openly, including difficulty speaking
and walking. In 2018, Blair attended a
(320 kilometers) per hour. But even its Hollywood event using a cane.

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