Drugs and the sympathetic nervous system Flashcards
What is the purpose of the autonomic nervous system?
Regulate organ function and homeostasis, not subject to voluntary control
Describe efferent autonomic nerves
Transmit impulses from CNS to periphery organs
Control heart, blood vessels, gut, bladder eyes, endocrine and exocrine glands
Describe afferent autonomic nerves
Periphery to CNS
Baroreceptors and chemoreceptors sensors in organs
Vagus, splanchnic and pelvic nerves
Contrast sympathetic preganglionic and postganglionic fibres
Pre= myelinated, cell bodies in lateral horns of the spinal segments T1-L2 (thoracic lumbar outflow), paravertebral ganglionic chains run from cervical to sacral region, synapse in ganglia Post= unmyelinated, longer, run all the way to effector organ
What are the exceptions to preganglionic fibres?
Do not synapse in sympathetic chains- terminate in separate cervical or abdominal ganglia/ travel in the greater splanchnic nerve and directly synapse with chromaffin cells in adrenal medulla
What is the adrenal medulla?
Adrenal glands located on superior aspect of each kidney, synthesises and stores catecholamines (similar to postganglionic nerve endings), additional enzyme converts noradrenaline into adrenaline, transforms neural impulses into hormone secretion
What is the role of the sympathetic nervous system?
Enables body to be prepared for flight or flight response
What is the affect of the SNS on the heart?
- Inc heart rate by actions on pacemaker cells in SAN and AVN, inc conduction rate of electrical impulses
- Inc contractility of ventricle myocardial muscle
- Inc stroke volume
- Inc cardiac output
What is the affect of the SNS on blood vessels?
- Vasoconstriction= smooth muscle contractions in most arterial circulations increases resistance to flow thereby reducing local blood flow, inc peripheral vascular resistance and bp
- Vasodilation= smooth muscle relaxation in large arteries and arterioles supplying skeletal muscle, inc muscle blood flow
- Venoconstriction= smooth muscle contraction in veins, mobilises blood from venal reservoir into circulation, inc venous return to heart
What is the affect of the SNS on the kidney?
- renal blood vessels= constricts renal arterioles reducing blood flow and urine production
- Renin secretion= stimulates release of enzyme from juxtaglomerular apparatus, catalyses the conversion of angiotensinogen into angiotensin 1 which is converted into angiotensin 2, potent vasoconstrictor and stimulates release of aldosterone (sodium retaining hormone)
What is the affect of the SNS on the lungs?
Bronchodilation= relaxation of smooth muscle surrounding bronchi and bronchioles, increases diameter, reduces resistance to airflow so improved ventilation, more effective transfer of oxygen and CO2
What is the affect of the SNS on the GI tract?
- Salivary glands= stimulates more viscous secretions
- Bowel wall= relaxes smooth muscle of gut to reduce peristalsis to conserve energy
- Sphincters= constricts smooth muscle so reduces onward transit of food and bowel contents
- Liver= stimulates glycogenolysis and gluconeogenesis
What is the affect of the SNS in adipose tissue?
Adipocytes= stimulates lipolysis, breaking down triglyceride stores to release free fatty acids as energy source for muscles
What is the affect of the SNS on the bladder?
- Bladder wall= relaxes smooth muscle to reduce possibility of voiding
- Sphincters= constricts smooth muscle to reduce possibility of voiding
- Male prostate= constricts smooth muscle so more difficult to urinate
What is the affect of the SNS on the genital tract?
- Male= ejaculation
- Female= contraction of myometrium of pregnant uterus during labour (alpha)/ relaxation of myometrium (Beta2)
What are the other sympathetic effects?
- Pupillary dilation in eyes
- Muscle tremor
- Platelet aggregation
- Skin sweating
What are the main endogenous mediators involved in regulation of SNS?
Pre= ACh, nicotinic receptor, adrenal medulla releases adrenaline and noradrenaline Post= noradrenaline, sweat glands ACh at muscarinic receptors
What are catecholamines?
Adrenaline and noradrenaline neurotransmitters
Synthesised from amino acid phenylalanine
Terminal branches of post fibres have swellings/ varicosities- form synaptic contact with effector organ, site of synthesis and storage of noradrenaline.
Released from granules
What is the action of noradrenaline terminated by?
- Diffusion from the site of action
- Reuptake back into the presynaptic nerve ending, inactivated by enzyme monoamine oxidase (MAO) in mitochondria
- Metabolism locally by enzyme catechol-O-methyl-transferase (COMT)
What are the subgroups of adrenoreceptors?
Alpha 1= vasoconstriction of blood vessels
Alpha 2= presynaptic membrane, negative feedback, feedback inhibition of NA on own release (self-regulation)
Beta 1= Inc rate and force of contraction in heart
Beta 2= vasodilation in blood vessels, bronchial relaxation in lung
What drugs are used at each receptor?
Alpha agonists= adrenaline (a1), noradrenaline (a1), phenylephrine (a1), clonidine (a2), ephedrine (a1)
Alpha antagonists= prazosin (a1)
Beta agonists= adrenaline (b1,2), isoprenaline (b1,2), salbutamol (b2), dobutamine (b1)
Beta antagonists= propranolol (b1,2), atenolol (b1)
What are the clinical uses of alpha agonists?
Rarely used Cardiovascular collapse (urgent intensive treatment of shocked patients- a1), nasal congestion (a1), hypertension (a2) Adverse effects- a1= hypertension, tachycardia, angina
What are the clinical uses of alpha antagonists?
Alpha-blockers, zosin suffix, prazosin, doxazosin, Tamsulosin
Hypertension, benign prostatic hypertrophy
Adverse; hypotension, dizziness, nasal congestion
What are the clinical uses of beta agonists?
Non-selective and selective (b1 dobutamine, b2 salbutamol)
Asthma (b2 salbutamol inhaled), premature labour (b2 salbutamol IV), severe heart failure (b1 dobutamine), anaphylaxis and cardiac arrest (non selective adrenaline)
Adverse; b1= tachycardia and palpitations
b2= tremor, hypokalemia
What are the clinical uses of beta antagonists?
Beta- blockers, olol, non-selective propranolol, selective b1 atenolol (cardio selective)
b1= hypertension, angina pectoris, heart failure
b2= tremor
Adverse= b1- bradycardia and heart failure
b2= bronchospasm, cold peripheries, lethargy
