Actions of Autonomic Neurotransmitters

As in the case of stimulation at neuromuscular junctions (see chapter 9, p. 302) and synapses (see chapter 10, p. 363), the actions of autonomic neurotransmitters result from their binding to protein receptors in the membranes of effector cells. Receptor binding alters the membrane. For example, the membrane's permeability to certain ions may increase, and in smooth muscle cells, an action potential followed by muscular contraction may result. Similarly, a gland cell may respond to a change in its membrane by secreting a product.

Acetylcholine can combine with two types of cholinergic receptors, called muscarinic receptors and nicotinic receptors. These receptor names come from muscarine, a toxin from a fungus that can activate mus-carinic receptors, and nicotine, the toxin of tobacco that can activate nicotinic receptors. The muscarinic receptors are located in the membranes of effector cells at the ends of all postganglionic parasympathetic nerve fibers and at the ends of the cholinergic sympathetic fibers. Responses from these receptors are excitatory and occur

Parasympathetic Nerve Trachea

Fibers to skin, blood vessels, and adipose tissue

Spinal

Sympathetic cord chain ganglia

Lacrimal gland

■ Parotid gland

Submandibular and sublingual salivary glands

Trachea

Preganglionic

Ganglion Spinal Cord Tissue

Trachea

Kidney

Postganglionic

Figure 11.39

The preganglionic fibers of the sympathetic division of the autonomic nervous system arise from the thoracic and lumbar regions of the spinal cord. Note that the adrenal medulla is innervated directly by a preganglionic fiber.

Adrenal gland

Preganglionic

Postganglionic

Kidney

Urinary bladder

Figure 11.39

The preganglionic fibers of the sympathetic division of the autonomic nervous system arise from the thoracic and lumbar regions of the spinal cord. Note that the adrenal medulla is innervated directly by a preganglionic fiber.

Medulla Nervous System

Figure

The preganglionic fibers of the parasympathetic division of the autonomic nervous system arise from the cranial nerves and the sacral region of the spinal cord.

Brain

ACh = acetylcholine (cholinergic) NE = norepinephrine (adrenergic)

Cranial parasympathetic neurons

Preganglionic fiber (axon)

Sympathetic neurons

Sacral parasympathetic neurons

Brain

Preganglionic fiber (axon)

Cranial parasympathetic neurons

Sympathetic neurons

Sacral parasympathetic neurons

Images Bladder Neurotransmitters

ACh = acetylcholine (cholinergic) NE = norepinephrine (adrenergic)

Figure 11.41

Most sympathetic fibers are adrenergic and secrete norepinephrine at the ends of the postganglionic fibers; parasympathetic fibers are cholinergic and secrete acetylcholine at the ends of the postganglionic fibers. The two arrangements of parasympathetic postganglionic fibers are seen in both cranial and sacral portions. Similarly, sympathetic paravertebral and collateral ganglia are seen in both the thoracic and lumbar portions of the nervous system.

Paravertebral Ganglion ganglion

Postganglionic fiber (axon) j" "¡visceral

'ganglion

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