Human Physiology, 14th edition (2016)

154 Chapter 6

a functional connection, or synapse, between the axon end-

ing and the target cell. There is a small synaptic gap, or cleft,

between the two cells, and chemical regulators called neu-

rotransmitters are released by the axon endings ( fig. 6.29 b ).

In endocrine signaling, the cells of endocrine glands

secrete chemical regulators called hormones into the extracel-

lular fluid. The hormones enter the blood and are carried by

the blood to all the cells in the body. Only the target cells for

a particular hormone, however, can respond to the hormone

( fig. 6.29 c ).

In order for a target cell to respond to a hormone, neuro-

transmitter, or paracrine regulator, it must have specific

receptor proteins for these molecules. A typical cell can

have a few million receptor proteins. Of these, about 10,000

to 100,000 receptors can be of a given type in certain cells.

Taking into account the total number of receptor genes, the

alternative splicing of exons that can be produced from these

genes, and the possible posttranslational modifications of

proteins (chapter 3), scientists have estimated that the 200 dif-

ferent cell types found in the human body may have as many

as 30,000 different types of receptor proteins for different

from one cell to the next because their plasma membranes are
very close together, and their cytoplasm is continuous through
tiny gap junctions that couple the cells together (see chapter 7,
fig. 7.21). In these cases, ions and regulatory molecules can
travel by diffusion through the cytoplasm of adjoining cells.
In most cases, however, cells signal each other by releasing
chemicals into the extracellular environment. In these cases,
cell signaling can be divided into three general categories:
(1) paracrine signaling; (2) synaptic signaling; and (3) endo-
crine signaling.
In paracrine signaling ( fig.  6.29 a ), cells within an organ
secrete regulatory molecules that diffuse through the extracellu-
lar matrix to nearby target cells (those that respond to the regu-
latory molecule). Paracrine regulation is considered to be local,
because it involves the cells of a particular organ. Numerous
paracrine regulators have been discovered that regulate organ
growth and coordinate the activities of the different cells and
tissues within an organ.
Synaptic signaling refers to the means by which neurons
regulate their target cells. The axon of a neuron (see chap-
ter 1, fig. 1.11) is said to innervate its target organ through

Figure 6.29 Chemical signaling between cells.
( a ) In paracrine signaling, regulatory molecules are released by
the cells of an organ and target other cells in the same organ.
( b ) In synaptic signaling, the axon of a neuron releases a
chemical neurotransmitter, which regulates a target cell. ( c ) In
endocrine signaling, an endocrine gland secretes hormones into
the blood, which carries the hormones to the target organs.

(a)

(b)

(c)

Paracrine

regulator

Neuron Axon

Neurotransmitter

Hormone

Endocrine

gland

Target

organ

Figure 6.30 How regulatory molecules influence

their target cells. Regulatory molecules that are polar bond

to receptor proteins on the plasma membrane of a target cell,

and the activated receptors send second messengers into the

cytoplasm that mediate the actions of the hormone. Nonpolar

regulatory molecules pass through the plasma membrane and

bind to receptors within the cell. The activated receptors act in

the nucleus to influence genetic expression.

Regulatory molecule

(neurotransmitter, hormone,

or paracrine regulator)

Second

messengers

If regulator is

polar (water-soluble)

Nucleus

Receptor

Nucleus

mRNA

If regulator is

nonpolar (lipid-soluble)