Human Physiology, 14th edition (2016)

252 Chapter 9

parasympathetic axons often innervate the same target cells,

where they release different neurotransmitters that promote

different (and usually antagonistic) effects.

Responses to Adrenergic Stimulation

Adrenergic stimulation—by epinephrine in the blood and by

norepinephrine released from sympathetic nerve endings—

has both excitatory and inhibitory effects. The heart, dilatory

muscles of the iris, and the smooth muscles of many blood

vessels are stimulated to contract. The smooth muscles of the

bronchioles and of some blood vessels, however, are inhibited

from contracting; adrenergic chemicals, therefore, cause these

structures to dilate.

Since the cells of the adrenal medulla are embryologically
related to postganglionic sympathetic neurons, it is not sur-
prising that their hormones are epinephrine (about 85%) and
norepinephrine (about 15%). Epinephrine differs from nor-
epinephrine only in that the former has an additional methyl
(CH 3 ) group, as shown in figure  9.8. Epinephrine, norepi-
nephrine, and dopamine (a transmitter within the CNS) are
all derived from the amino acid tyrosine and are collectively
termed catecholamines ( fig. 9.8 ).
Where the axons of postganglionic autonomic neurons
enter into their target organs, they have numerous swellings,
called varicosities, that contain the neurotransmitter mole-
cules. Neurotransmitters can thereby be released along a length
of axon, rather than just at the axon terminal. Thus, autonomic
neurons are said to form synapses en passant (“synapses in
passing”) with their target cells ( fig.  9.9 ). Sympathetic and

Figure 9.7 Neurotransmitters of the autonomic motor system. ACh  5  acetylcholine; NE  5  norepinephrine;
E  5  epinephrine. Those nerves that release ACh are called cholinergic; those nerves that release NE are called adrenergic. The adrenal
medulla secretes both epinephrine (85%) and norepinephrine (15%) as hormones into the blood.

Adrenal

medulla

E, NE (hormones)

ACh

ACh

ACh

ACh

NE

NE

ACh

ACh ACh

Cranial

parasympathetic

nerves

Sympathetic

(thoracolumbar)

nerves

Sacral

parasympathetic

nerves

Terminal

ganglion

Paravertebral

ganglion

Circulation

Visceral

effectors

Visceral

effectors

Visceral

effectors

Collateral

ganglion

Visceral

effector

organs