Receptors for autonomic neurotransmitters

As discussed in the previous section, all the effects of the ANS in tissues and organs throughout the body, including smooth muscle contraction or relaxation; alteration of myocardial activity; and increased or decreased glandular secretion, are carried out by only three substances: acetylcholine, norepi-nephrine, and epinephrine. Furthermore, each of these substances may stimulate activity in some tissues and inhibit activity in others. How can this

PARASYMPATHETIC SYSTEM

Brainstem

Spinal cord (S2-S4)

Terminal ganglion

Terminal ganglion

Cardiac muscle Smooth muscle Glands

Cardiac muscle Smooth muscle Glands

SYMPATHETIC SYSTEM

Spinal cord (T1-L2) Sympathetic ganglion chain or

Collateral ganglia

Innervated tissue

Innervated tissue o—<0

SPina| cord (T-L2) Sympathetic ganglion chain

Sweat glands

Sweat glands a

Spinal cord

Adrenal Medulla

Adrenal Medulla

Epinephrine (80%) Norepinephrine(20%)

Epinephrine (80%) Norepinephrine(20%)

Circulation

Circulation

Figure 9.2 Autonomic nerve pathways. All preganglionic neurons release acetylcholine (Ach), which binds to nicotinic receptors (N) on the postganglionic neurons. All postganglionic neurons in the parasympathetic system and some sympathetic postganglionic neurons innervating sweat glands release Ach that binds to muscarinic (M) receptors on the cells of the effector tissue. The remaining postganglionic neurons of the sympathetic system release norepinephrine (NE), which binds to alpha (a) or beta (b) receptors on cells of the effector tissue. The cells of the adrenal medulla, which are modified postganglionic neurons in the sympathetic system, release epinephrine (EPI) and NE into the circulation.

or wide variety of effects on many different tissues be carried out by so few neurotransmitters or hormones? The effect caused by any of these substances is determined by receptor distribution in a particular tissue and biochemical properties of the cells in that tissue — specifically, the second messenger and enzyme systems present in the cell.

The neurotransmitters of the ANS and the circulating catecholamines bind to specific receptors on the cell membranes of effector tissue. Each receptor is coupled to a G protein also embedded within the plasma membrane. Receptor stimulation causes activation of the G protein and formation of an intracellular chemical, the second messenger. (The neurotransmitter molecule, which cannot enter the cell, is the first messenger.) The function of intracellular second messenger molecules is to elicit tissue-specific biochemical events within the cell that alter the cell's activity. In this way, a given neurotransmitter may stimulate the same type of receptor on two different types of tissue and cause two different responses due to the presence of different biochemical pathways within each tissue.

Acetylcholine binds to two types of cholinergic receptors:

• Nicotinic receptors

• Muscarinic receptors

Nicotinic receptors are found on the cell bodies of all sympathetic and parasympathetic postganglionic neurons in the ganglia of the ANS. Ace-tylcholine released from the preganglionic neurons binds to these nicotinic receptors and causes a rapid increase in the cellular permeability to Na+ and Ca++ ions. The resulting influx of these two cations causes depolarization and excitation of postganglionic neurons in the ANS pathways. Mus-carinic receptors are found on cell membranes of effector tissues and are linked to G proteins and second messenger systems that carry out the intracellular effects. Acetylcholine released from all parasympathetic post-ganglionic neurons, and some sympathetic postganglionic neurons traveling to sweat glands, binds to these receptors. Muscarinic receptors may be inhibitory or excitatory, depending on the tissue upon which they are found. For example, muscarinic receptor stimulation in the myocardium is inhibitory and decreases heart rate, while stimulation of these receptors in the lungs is excitatory and causes contraction of airway smooth muscle and bronchoconstriction.

The two classes of adrenergic receptors for norepinephrine and epineph-rine are:

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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