Adrenergic and Cholinergic Synaptic Transmission

Acetylcholine (ACh) is the neurotransmitter of all preganglionic fibers (both sympathetic and parasympathetic). Acetylcholine is also the transmitter released by most parasympathetic post-

ganglionic fibers at their synapses with effector cells (fig. 9.7). Transmission at these synapses is thus said to be cholinergic.

The neurotransmitter released by most postganglionic sympathetic nerve fibers is norepinephrine (noradrenaline). Transmission at these synapses is thus said to be adrenergic. There are a few exceptions, however. Some sympathetic fibers that innervate blood vessels in skeletal muscles, as well as sympathetic fibers to sweat glands, release ACh (are cholinergic).

In view of the fact that the cells of the adrenal medulla are embryologically related to postganglionic sympathetic neurons, it is not surprising that the hormones they secrete should consist of epinephrine (about 85%) and norepinephrine (about 15%). Epinephrine differs from norepinephrine only in that the former has an additional methyl (CH3) group, as shown in figure 9.8. Epinephrine, norepinephrine, and dopamine (a transmitter within the CNS) are all derived from

Figure 9.7 Neurotransmitters of the autonomic motor system. ACh = acetylcholine; NE = norepinephrine; E = 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.

Figure 9.7 Neurotransmitters of the autonomic motor system. ACh = acetylcholine; NE = norepinephrine; E = 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.

The Autonomic Nervous System 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 molecules. 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 parasympathetic axons often innervate the same target cells, where they release different neurotransmitters that promote different (and usually antagonistic effects).

Varicosity

Tyrosine

(an amino acid)

DOPA

(dihydroxyphenylalanine)

Dopamine

(a neurotransmitter)

Norepinephrine

(a neurotransmitter and hormone)

Epinephrine

(major hormone of adrenal medulla)

H cOOH

OH H

OH H

H cOOH

OH H

OH H

■ 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 (CH3) group compared to norepinephrine.

Smooth muscle cell

Varicosity

Smooth muscle cell

Synapses en passant

Parasympathetic neuron neuron

-Sympathetic

Synapses en passant

Parasympathetic neuron

Axon of Sympathetic Neuron

Synaptic vesicle with norepinephrine (NE)

Axon of Sympathetic Neuron

Synaptic vesicle with norepinephrine (NE)

Axon of Parasympathetic Neuron

Synaptic vesicle with acetylcholine (ACh)

Axon of Parasympathetic Neuron

Synaptic vesicle with acetylcholine (ACh)

■ 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.

neuron

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