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Pharmacology-1 PHL 313

Parasympathetic Nervous System

Fourth LectureBy

Abdelkader Ashour, Ph.D.Phone: 4677212 Email: aeashour@ksu.edu.sa

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The anterior chamber of the eye, showing the pathway for

secretion and drainage of the aqueous humor

Muscarinic Effects on the EyeThe parasympathetic (muscarinic) nerves to the eye supply:

the ciliary muscle, which adjusts the curvature of the lens Contraction of the ciliarymuscle pulling the ciliary body forwards and inwards, thus relaxing the tension on the

suspensory ligament of the lens, allowing the lens to bulge more, reducing its focal lengt This parasympathetic reflex is necessary to accommodate the eye for near vision

the constrictor pupillae muscle, which is important not only for adjusting the pupil inresponse to changes in light intensity but also in regulating the intraocular pressure

The main use of muscarinic agonists is in treating glaucoma, by local instillation in the form ofeye drops. Pilocarpine is the most effective as, being a tertiary amine, it can cross the

conjunctival membrane

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Muscarinic Effects on the EyeAqueous humour is secreted slowly and continuously by the cells of the epithelium covering the

ciliary body, and it drains into the canal of Schlemm

The intraocular pressure is normally 10-15 mmHg above atmospheric, which keeps the eye

slightly distendedAbnormally raised intraocular pressure (associated with glaucoma) damages the eye and is one

of the commonest preventable causes of blindness

In acute glaucoma, drainage of aqueous humour becomes impeded when the pupil is dilated

because folding of the iris tissue occludes the drainage angle, causing the intraocular pressure to

rise. Activation of the constrictor pupillae muscle by muscarinic agonists in these circumstances

lowers the intraocular pressure

The anterior chamber of the eye, showing the pathway for

secretion and draina e of the a ueous humor

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Adverse Effects: (most of them are related to its non-selective action as a

muscarinic receptor agonist)

1. Excessive sweating

2. Excessive salivation

3. Bronchospasm and increased bronchial mucus secretion

4. Bradycardia, hypotension

5. Nausea and diarrhea

6. It may result in miosis when used chronically as an eye drop

Pilocarpine (Salagen)

Indications: It is more commonly used thanbethanechol to induce salivation, and also for

various purposes in ophthalmology

1. Treatment of primary open-angle glaucoma

and also to lower intraocular pressure prior to

surgery for acute angle-closure glaucoma

2. Treatment of symptoms of dry mouth from salivarygland hypofunction caused by radiotherapy for

cancer of the head and neck

Muscarinic Agonists (Cholinomimetics, Parasympathomimetics)

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Nonselective Muscarinic Antagonists

The classical muscarinic antagonists are derived from plants and are

nonselective competitive antagonists. Atropa belladonna contains atropine.

Hyoscyamus niger contains primarily scopolamine and hyoscine

Clinically, atropine is used for raising heart rate during situations where vagal

activity is pronounced (for example, vasovagal syncope). It is also used for

dilating the pupils. Its most widespread current use is in pre-anesthetic

preparation of patients; in this situation, atropine reduces respiratory tractsecretions and thus facilitates intubation

Muscarinic Antagonists

Ipratropium is used by inhalation as a bronchodilator

Cyclopentolate and tropicamide are developed for ophthalmic use and

administered as eye drops (for dilating the pupil to aid examination of the eye)

Oxybutinin and tolterodine are new drugs developed forurinary incontinence

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Side effects of muscarinic antagonists include:

constipation

urinary retention

xerostomia (dry mouth)

hypohidrosis (decreased sweating)

mydriasis (dilated pupils)

precipitation of glaucoma

decreased lacrimation

tachycardia

decreased respiratory secretions

Selective Muscarinic Antagonists

Pirenzepine shows selectivity for the M1 muscarinic receptor

Because of the importance of this receptor in mediating gastric acid release,

M1 antagonists such as pirenzepine help patients with ulcer disease or

gastric acid hyper-secretion

Pirenzepine reduces gastric acid secretion with fewer adverseeffects than

atropine and other less selective agents. Pirenzepine passes the BBB, but

only to a small extent, therefore it has no central effects

Muscarinic Antagonists