McKenna's Pharmacology for Nursing, 2e

451

C H A P T E R 2 9 Introduction to the autonomic nervous system

■■ TABLE 29.3 Comparing the effects of autonomic stimulation Effector site Sympathetic reaction

Parasympathetic reaction

Eye structures Iris radial muscle

Contraction (pupil dilates)

—

Iris sphincter muscle

— —

Contraction (pupil constricts)

Ciliary muscle

Contraction (lens accommodates for near vision)

↑ Secretions

Lacrimal glands

—

↑ Rate, contractility

↓ Rate

Heart

↑ Atrioventricular conduction

↓ Atrioventricular conduction

Blood vessels Skin, mucous membranes

Constriction

— —

Skeletal muscle Bronchial muscle

Dilation

Relaxation (dilation)

Constriction

Gastrointestinal system Muscle motility and tone

↓ Activity Contraction ↓ Secretions

↑ Activity Relaxation ↑ Activity

Sphincters Secretions

Salivary glands

Thick secretions

Copious, watery secretions

Gallbladder

Relaxation

Contraction

Liver

Glyconeogenesis

—

Urinary bladder Detrusor muscle

Relaxation Contraction

Contraction Relaxation

Trigone muscle and sphincter

Sex organs Male

Emission

Erection (vascular dilation)

Female

Uterine relaxation

—

Skin structures Sweat glands

↑ Sweating

— —

Piloerector muscles

Contracted (goosebumps)

—, no reaction or response.

Cholinergic receptors Cholinergic receptors or ACh receptors are found on organs and muscles. They have been classified as muscarinic receptors and nicotinic receptors . This clas- sification is based on very early research of the ANS that used muscarine (a plant alkaloid from mushrooms) and nicotine (a plant alkaloid found in tobacco plants) to study the actions of the parasympathetic system. Muscarinic receptors As the name implies, muscarinic receptors are receptors that can be stimulated by muscarine. They are found in visceral effector organs, such as the GI tract, bladder and heart, in sweat glands and in some vascular smooth muscle. Stimulation of muscarinic receptors causes pupil constriction, increased GI motility and secretions (including saliva), increased urinary bladder contraction and a slowing of the heart rate. Nicotinic receptors Nicotinic receptors are located in the CNS, the adrenal medulla, the autonomic ganglia and the neuromuscu- lar junction. Stimulation of nicotinic receptors causes muscle contractions, autonomic responses such as signs and symptoms of a stress reaction and release of noradrenaline and adrenaline from the adrenal medulla.

2. Postganglionic nerves of the parasympathetic system and a few SNS nerves, such as those that re-enter the spinal cord and cause general body reactions such as sweating

3. Motor nerves on skeletal muscles 4. Cholinergic nerves within the CNS Acetylcholine synthesis and storage

ACh is an ester of acetic acid and an organic alcohol called choline. Cholinergic nerves use choline, obtained in the diet, to produce ACh. The last step in the pro- duction of the neurotransmitter involves choline acetyltransferase, an enzyme that is also produced within cholinergic nerves. Just like noradrenaline, the ACh is produced in the nerve and travels to the end of the axons, where it is packaged into vesicles. To be a cholinergic nerve, the nerve must contain all of the enzymes and building blocks necessary to produce ACh. Acetylcholine release The vesicles full of ACh move to the nerve membrane; when an action potential reaches the nerve terminal, calcium entering the cell causes the membrane to contract and secrete the neurotransmitter into the synaptic cleft. The ACh travels across the synaptic cleft and reacts with very specific cholinergic receptor sites on the effector cell (see Figure 29.5).