Lect 17

• The Central Nervous System (CNS) consists of the brain and the spinal cord.
• The Peripheral Nervous System (PNS) consists of spinal and cranial nerves, autonomic ganglia, and contains both afferent and efferent axons.
• Afferent axons carry action potentials toward the CNS.
• Efferent axons carry action potentials away from the CNS.
• The Peripheral Nervous System (PNS) can be divided into the Somatic Nervous System (SNS) which innervates skeletal muscle, and the Autonomic Nervous System (ANS) which innervates cardiac muscle, smooth muscle, and exocrine glands.
• The Autonomic Nervous System (ANS) can be further divided into the Sympathetic Nervous System (SNS) and the Parasympathetic Nervous System (PSNS).
• The Autonomic Nervous System (ANS) has a higher number of neurons than the Somatic Nervous System (SNS).
• The Autonomic Nervous System (ANS) has a higher amount of myelin along peripheral axons than the Somatic Nervous System (SNS).
• Postganglionic neurons in the Autonomic Nervous System (ANS) usually have slowly conducted unmyelinated axons.
• The addition of EPI to local anesthetics to delay removal of the anesthetic from site of injection is a common practice.
• Phenylephrine causes splenic muscle contraction, discharges RBC into the circulation and reduces the size of the spleen.
• Drugs that are selective α-adrenergic agonists or have potent α-adrenergic activity and minimal β-adrenergic effects can cause significant vasoconstriction.
• NE is a potent constrictor of renal vascular beds.
• Phenylephrine administration reduces splenic area and thickness making room for the colon to dislodge from the nephrosplenic space.
• α1-adrenergic receptors are expressed on the smooth muscle of the splenic capsule.
• Nephrosplenic Entrapment in Horses occurs when the left ventral and dorsal colon migrate between the spleen and the body wall and become entrapped over the nephrosplenic ligament, resulting in an obstruction of the large colon.
• Organ/effector tissue Sympathetic stimulation Receptor Parasympathetic stimulation Receptor Eye Radial muscle, iris Pupillary dilation α1- - Sphincter muscle, iris - - Pupillary constriction M3, M2 Ciliary muscle Slight relaxation β2- Contraction M3, M2 Gland Lacrimal ↑ Secretion α1 ↑↑ Secretion M3, M2 Ophthalmic Pharmacology Modified from: Veterinary Pharmacology and Therapeutics
• The SNS activation to this target produces smooth muscle contraction.
• Tenth edition
• Phenylephrine administration (selective α1 adrenergic agonist) produces peripheral vasoconstriction and is used in circumstances where cardiac output is adequate.
• α1-adrenergic receptors are expressed on the radial dilator muscle of the iris (iris dilator muscle) and on conjunctival blood vessels.
• Adrenergic receptors are expressed at numerous sites in the urinary tract including ureters, detrusor muscle of the bladder body, bladder base, and internal urethral sphincter.
• β2-adrenergic receptor agonist, used in cattle.
• A single daily dose of phenylpropanolamine results in an increase in urethral pressure values and improved urinary incontinence.
• Urethral sphincter incompetence is the most common cause of acquired urinary incontinence in female dogs and cats.
• Following topical application, phenylephrine produces contraction of the smooth muscle of the conjunctival blood vessels and contraction of the iris dilator muscle.
• Horner’s syndrome is an ocular sympathetic palsy that causes miosis (constriction of the affected pupil), drooping of the upper eyelid (ptosis), an inward sinking of the eyeball (enophthalmos), and partial protrusion of the third eyelid (nictitating membrane).
• Extra-label drug use is strictly prohibited.
• These drugs are fed at the end of the animals feeding period only.
• In unilateral postganglionic Horner’s syndrome, the affected pupil should dilate within 20 minutes and other clinical signs will improve or completely resolve.
• β1-adrenergic receptor agonist with some β2-receptor activity, used in cattle, swine, and turkeys.
• Phenylephrine is a selective α1-adrenergic receptor agonist used in pupillary dilation (mydriasis) protocols for dogs prior to cataract surgery.
• In the United States, it is illegal to feed a βAA or any other drug in a manner that is inconsistent with its labeling as approved by the FDA.
• Not all βAA are approved for use in food animals.
• Stimulation of sympathetic nerves innervating these sites or activation of these receptors produces smooth muscle relaxation of the bladder body via β2 receptors, smooth muscle contraction at the bladder base via α1 receptors, and internal urethral sphincter contraction by activation of α1 receptors.
• β-Adrenergic agonists have been used as growth promotion agents for livestock in the United States since 1999.
• Adrenergic receptor selectivity varies depending on the tissue and the type of agonist.
• Adrenergic agonists can be classified as modified from: Veterinary Pharmacology and Therapeutics
• α2 adrenergic receptors are predominant in platelets, causing aggregation.
• α2 adrenergic receptors are also present in selected vascular smooth muscle, causing contraction.