Adrenergic Receptors

Adrenergic Receptors

Since most sympathetic postganglionic neurons secrete NE (noradrenaline), they are called adrenergic fibers. The two major types of adrenergic receptors are alpha (α) and beta (β). Subdivisions of these types include α₁, α₂, β₁, β₂, and β₃. One or more of these subdivisions­ are present on organs that respond to either norepinephrine or epinephrine, and either excitatory or inhibitory effects can occur. The deter-mining factor is which the subdivision of the receptor is the denominating subdivision on the target organ. When NE binds to the β₁ receptors of cardiac muscle, for example, heart activity increases. When epineph-rine binds to the β₂ receptors of bronchiole smooth muscle, it relaxes, causing the bronchiole to dilate.

Drug Effects

Specific drugs can be prescribed using knowledge of how they will affect their target organs, based on cholin-ergic and adrenergic receptor subdivisions. Atropine is a drug that is administered before surgery to dry respira-tory secretions and prevent salivation. It is an anticho-linergic drug that blocks the muscarinic ACh receptors. Another use for atropine is to dilate the pupils of the eyes before examination. Neostigmine, another anticho-linergic, inhibits AChE, allowing ACh to accumulate in the synapses. It is used to treat myasthenia gravis, which impairs the activity of skeletal muscles because of inad-equate ACh stimulation. Depression may be relieved by using drugs that prolong NE’s activity upon postsynap-tic membranes of the brain.

Drugs that mimic the activity of one of the nor-mal autonomic neurotransmitters are called mimetics. Sympathomimetic drugs mimic the effects of sympa-thetic activity and include many over-the-counter cold and allergy medications. They stimulate a-adrenergic receptors to constrict nasal mucosa blood vessels, which inhibits secretions. Ideally, drugs that affect just a single subdivision of receptor will be used with more frequency in the future. Fairly recently, drugs that pri-marily activate β₂ receptors were discovered. They can be used for asthma to dilate lung bronchioles with-out activating the β₁ receptors (which would increase heart rate, an unneeded effect). TABLE 14-3 lists various drug classes that influence the ANS.

Autonomic Division Interactions

Because most visceral organs receive dual innervation from the parasympathetic and sympathetic divisions, both ANS divisions are partially activated. From both subsystems, action potentials fire regularly down the axons to cause antagonism that carefully controls visceral activity. However, one subsystem does dominate, and in rare cases, there is actual cooperation between them.

Antagonism is common in the gastrointestinal sys-tem, heart, and the respiratory system. The fight-or-flight syndrome involves sympathetic increases in heart rate, inhibition of digestion and elimination, and dilation of airways. Then, the parasympathetic division reverses these processes when the initial stressors have ceased. It is important to understand that the sympathetic division is still the primary controller of blood pressure, even when at rest. The vascular system is almost entirely innervated by sympathetic fibers. They maintain partial constriction of blood vessels continually, which is known as sympa-thetic (vasomotor) tone. Sympathetic fibers fire more quickly when blood pressure is insufficient to maintain blood flow. The blood vessels then constrict, and blood pressure increases. If it becomes too high, sympathetic fibers fire less quickly and vessels dilate. For the occa-sional treatment of hypertension, alpha blockers are used, which block the responses in these vasomotor fibers.

Inadequate blood flow to body tissues is also known as circulatory shock. When this occurs or when additional blood is required for activity in the skeletal muscles, the blood vessels that serve the abdominal viscera and skin strongly constrict. This shunts blood into the overall circulation to support the skeletal muscles and vital organs.

The parasympathetic division primarily controls the heart and smooth muscle of urinary and digestive organs. Parasympathetic tone is maintained, which can slow down the heart and maintain normal levels of ­urinary and digestive activity. This can be interrupted by the sympathetic division when certain stress-ors occur. Medications that block parasympathetic responses speed up heart activity and cause urine and feces to be retained. Most glands are activated by para-sympathetic fibers, except for sweat glands in the skin and the adrenal­ glands.

The external genitalia provide the primary example of cooperative ANS effects. During sexual activity, erec-tion of the penis or clitoris is produced by parasympa-thetic stimulation, which dilates the blood vessels. It is understandable why anxiety can impair sexual perfor-mance, because the sympathetic division takes control. Sympathetic stimulation causes reflex contractions of the vagina and ejaculation of semen by the penis.

1. Define the terms “preganglionic fiber” and “postganglionic fiber.”

2. Which neurotransmitters are used by the ANS?

3. Classify cholinergic and adrenergic receptors