Chapter 5 The Nervous System • MHR 1475.2 How the Neuron Works
Aneuron is formed by the same process of cell
division as occurs in other cells. A neuron has a
single nucleus, numerous mitochondria, ribosomes,
lysosomes, and other organelles. However, neurons
also have very specialized structures that make
them different from other cells and that enable
them to perform their unique functions.
As mentioned previously, a neuron is composed
of dendrites, a cell body, and an axon. The axon
sends a wave of depolarization along its length,
which is part of the high-speed network that sends
impulses from one part of the body to another. The
wave of depolarization is primarily the movement
of two positive ions (Na+and K+) from one side of
the axon’s cell membrane to the other.
The Neuron at Rest
When a neuron is at rest, the outside of the
membrane of the neuron is positively charged
compared to the inside. This is the result of the
uneven distribution of positively charged ions
(cations) and negatively charged ions (anions).
Outside the cell there are high concentrations of
sodium ions (Na+) and lower concentrations of
potassium ions (K+). Chloride (Cl−) is the dominant
anion exterior to the cell. Inside the cell there is a
high concentration of K+, a lower concentration of
Na+, and the dominant anions are negatively
charged proteins, amino acids, phosphates, and
sulfates. The membrane has specialized channels
or gates for the movement of Na+, K+, and Cl−, but
the larger anions (such as proteins and amino
acids) are trapped within the cell. As shown in
Figure 5.11 on the following page, the movement of
Na+and K+is critical to the wave of depolarization.
At rest, the membrane is 50 times more permeable
to K+than to Na+. That is, while Na+is moving into
the cell, there is more K+diffusing out of the cell.
As this happens, the inside of the cell becomes
increasingly negatively charged because the larger
anions are trapped inside. Although the increasing
negative charge within the cell attracts both the
Na+and K+, this force is offset by the Na+/K+pump,
which is found in the cell membrane. The Na+/K+
pump uses active transport to pull three Na+cations
from the inside of the cell to the outside. In
exchange, two K+cations are pulled from outside
to inside the cell, thereby increasing the difference
in charge. The final result is a relatively negative
charge inside the cell compared to the outside. This
charge (due to the unequal distribution of cations
and anions) can be measured using tiny micro-
electrodes placed inside and outside the membrane.
At rest, the difference in charge is approximately
–70 mV. This difference is referred to as the resting
potential.The All-or-none Principle
Sensory neurons can be stimulated by chemicals,
light, heat, or the mechanical distortion of their
membrane. Motor neurons and the neurons of the
central nervous system are usually stimulated by
neurotransmitters, which are chemicals secreted
by other neurons. Neurons can also be stimulated
experimentally using an electrical current. If the
neuron is given a mild electrical stimulus, there is
a brief and small change in the charge of the cell
membrane near the point of stimulation. The axon
itself does not send a wave of depolarization along
its length. However, if the electrical stimulus is
strong enough (that is, if it reaches the threshold of
stimulus), a wave of depolarization will sweep
along the surface of the axon.
An axon is governed by the all-or-none principle.
If an axon is stimulated sufficiently (above the
threshold), the axon will trigger an impulse down
the length of the axon. The strength of the response
is uniform along the entire length of the axon.
Also the strength of response in a single neuron is
independent of the strength of the stimulus. An
axon cannot send a mild or strong response; it can
only respond or not respond. The threshold ofEXPECTATIONS
Describe the function of neurons.
Explain the role of neurotransmitters in the central nervous system.
Describe how neurons respond to a stimulus.