Essential Cell Biology 5th edition

General Principles of Cell Signaling
535
the cell that secretes them. Thus, they act as local mediators on nearby
cells (Figure 16–3B). Many of the signal molecules that regulate inflammation
at the site of an infection or that control cell proliferation in a
healing wound function in this way. In some cases, cells can respond to
the local mediators that they themselves produce, a form of paracrine
communication called autocrine signaling; cancer cells sometimes promote
their own survival and proliferation in this way.
Neuronal signaling is a third form of cell communication. Like endocrine
cells, nerve cells (neurons) can deliver messages over long distances.
In the case of neuronal signaling, however, a message is not broadcast
widely but is instead delivered quickly and specifically to individual target
cells through private lines. As described in Chapter 12, the axon of
a neuron terminates at specialized junctions (synapses) on target cells
that can lie far from the neuronal cell body (Figure 16–3C). The axons
that extend from the spinal cord to the big toe in an adult human, for
example, can be more than a meter in length. When activated by signals
from the environment or from other nerve cells, a neuron sends electrical
impulses racing along its axon at speeds of up to 100 m/sec. On reaching
the axon terminal, these electrical signals are converted into a chemical
form: each electrical impulse stimulates the nerve terminal to release
a pulse of an extracellular signal molecule called a neurotransmitter.
The neurotransmitter then diffuses across the narrow (<100 nm) gap that
separates the membrane of the axon terminal from that of the target cell,
reaching its destination in less than 1 msec.
A fourth style of signal-mediated cell–cell communication—the most intimate
and short-range of all—does not require the release of a secreted
molecule. Instead, the cells make direct physical contact through signal
molecules lodged in the plasma membrane of the signaling cell
and receptor proteins embedded in the plasma membrane of the target
cell (Figure 16–3D). During embryonic development, for example, such
(A)
ENDOCRINE
endocrine cell
receptor
target cell
(B)
PARACRINE
signaling
cell
(C)
bloodstream
NEURONAL
neuron
cell
body
axon
hormone
target cell
nerve
terminal
neurotransmitter
synapse
target cell
(D)
local
mediator
CONTACT-DEPENDENT
signaling cell
membranebound
signal
molecule
target cell
target
cells
Figure 16–3 Animal cells use
extracellular signal molecules to
communicate with one another
in various ways. (A) Hormones
produced in endocrine glands are
secreted into the bloodstream and
are distributed widely throughout the
body. (B) Paracrine signals are released
by cells into the extracellular fluid in
their neighborhood and act locally.
(C) Neuronal signals are transmitted
electrically along a nerve cell axon.
When this electrical signal reaches the
nerve terminal, it causes the release
of neurotransmitters onto adjacent
target cells. (D) In contact-dependent
signaling, a cell-surface-bound signal
molecule binds to a receptor protein
on an adjacent cell. Many of the same
types of signal molecules are used for
endocrine, paracrine, and neuronal
signaling. The crucial differences lie in
the speed and selectivity with which the
signals are delivered to their targets.