Human Physiology, 14th edition (2016)

The Autonomic Nervous System 253

Because excitatory and inhibitory effects can be produced

in different tissues by the same neurotransmitter, the responses

must depend on the characteristics of the effector cells. To

some degree, this is due to the presence of different membrane

receptor proteins for the catecholamine neurotransmitters.

(The interaction of neurotransmitters and receptor proteins in

the postsynaptic membrane was described in chapter 7.) The

two major classes of these receptor proteins are designated

alpha- ( a ) and beta- ( b ) adrenergic receptors.

Experiments have revealed that each class of adrener-

gic receptor has two major subtypes. These are designated

by subscripts: a 1 and a 2 ;  b 1 and b 2. (There is also a more

Figure 9.8 The catecholamine family of
molecules. Catecholamines are derived from the amino
acid tyrosine, and include both neurotransmitters (dopamine
and norepinephrine) and a hormone (epinephrine). Notice that
epinephrine has an additional methyl (CH 3 ) group compared to
norepinephrine.

HO

OH

H

C

OH

H

C

H

H

CH 3

N

HO

HO

H

C

OH

H

C

H

NH 2

HO

HO

H

C

H

H

C

H

NH 2

HO

HO

H

C

H

H

C NH 2

COOH

HO

H

C

H

H

C NH 2

COOH

Tyrosine

(an amino acid)

DOPA

(dihydroxyphenylalanine)

Dopamine

(a neurotransmitter)

Norepinephrine

(a neurotransmitter

and hormone)

Epinephrine

(major hormone of

adrenal medulla)

Figure 9.9 Sympathetic and parasympathetic

axons release different neurotransmitters. ( a ) The axons

of autonomic neurons have varicosities that form synapses en

passant with the target cells. ( b ) In general, sympathetic axons

release norepinephrine, which binds to its adrenergic receptors,

while parasympathetic neurons release acetylcholine, which

binds to its cholinergic receptors (discussed in chapter 7). In

most cases, these two neurotransmitters elicit antagonistic

responses from smooth muscles.

Varicosity

Sympathetic

neuron

Synapses

Smooth muscle cell en passant

Parasympathetic

neuron

(a)

Synaptic vesicle

with norepinephrine (NE)

Synaptic vesicle

with acetylcholine (ACh)

Axon of Sympathetic

Neuron

Axon of Parasympathetic

Neuron

Adrenergic

receptors

Smooth

muscle cell

Cholinergic

receptors

ACh

Antagonistic effects

(b)

NE