ANS applied physiology

Question 1

Where do the preganglionic SNS neurons arise from?

Answer 1

Lateral horn of the spinal cord

Question 2

Describe the course of these pre-ganglionic SNS fibres

Answer 2

Descend 1 - 2 sebments within the spinal cord before emerge along with the posterior segmental roots. They then synapse in the ganglia of the paravertebral sympathetic chain to give rise to LONG post-ganglionic neurons

Question 3

As the SNS is ‘thoracolumbar’ how is the head and neck supplied with

Answer 3

The cervical (stellate) ganglia:
Superior
Middle
Inferior

Formed from preganglionic fibers emerging from the first 3 thoracic segments

Question 4

What anatomical course do sympathetic fibres usually follow

Answer 4

the arterial blood supply to these organs

Question 5

Describe the structure and function of the Adrenal medulla

Answer 5

It is a specialized sympathetic ganglion in which post-ganglionic cells are modified into secretory cells rather than nerve fibres –> consequently the output of this gland is neuronal rather than neural:

Noradrenalin ± 70 %
Adrenalin ± 29 %
Dopamine ± 1 %

Question 6

From which cranial nerves does parasympathetic cranial (craniosacral) outflow occur

Answer 6

3, 7, 9, 10

Question 7

Where are PSNS ganglia found and how does the length of preganglionic PSNS fibers compare to postganglionic PSNS fibres

Answer 7

Ganglia are near target organs and as such pre-ganglionic neurons are significantly longer than PSNS postganglionic neurons

Question 8

Where is ACh the neurotransmitter in the ANS and PNS

Answer 8

1. All ANS ganglia (including adrenal medulla)
2. All PSNS post ganglionic nerve endings
3. At 2 places in the SNS
   - –> Apocrine sweat glands
   - –> Vasodilation in blood vessels of skeletal muscle

PNS –> Dominant neurotransmitter in the motor system including the neuromuscular junction

Question 9

Summarize basic function, distribution of the adrenergic receptors

Answer 9

alpha 1 - arteriolar smooth muscle + –> VC
alpha 2 - Pre-synaptic + –> - SNS
beta 1 - heart + inotropy and chronotropy
beta 2 - lungs + bronchodilation and reduce secretions

Question 10

Describe the redistribution of blood flow consequent to the ‘fight or flight response’

Answer 10

Redistribution of blood flow to “fight or flight” organs with sustained perfusion of the brain.

Blood diverted away from skin/liver/GIT/kidney via alpha adrenergic vasoconstriction

Blood diverted toward heart, muscle, lungs, brain via Beta beta adrenergic and vasodilation (muscarinic in muscle)

Question 11

Describe the overall organization of the ANS

Answer 11

PSNS (Craniosacral)

• CN 3,7,9,10 innervate 4 PSNS ganlgia in the head and neck. Ciliary (Eye), Sphenopalatine (lacrimal), Submandibular (Submaxillary and sublingual glands), Otic (parotid glands)
• The Vagus nerve then supplies the vocal chords to the colon
• The pelvic nerve supplies the colon/rectum/bladder/sex organs

SNS
The sympathetic chain runs paravertebrally
T1 - T4
Supply head and neck:
3 cervical ganglion (Supplied from T1 - T#3): Superior/middle and inferior supply SNS to head and neck. These nerves follow the course of the carotid artery and its branches.

Also supply heart/lungs

T5 - T12 via the Greater and small splanchnic nerves –> supplies stomach to sex organs via three ganglia: Celiac, Superior mesenteric, inferior mesenteric ganglion

Question 12

Summarize the organization of the ANS

Answer 12

PSNS

• 4 ganglia in the head
• CN 3,7,9,
• Vagus: Subglottic to colon
• Pelvic: Colon to sex organs

SNS

• 3 ganglia in the neck
• 3 ganglia in the abdomen: greater/small splanchnic nerves also here
• No ganglion for thoracic structures
• T1 - T4: Head to Lungs
• T4 - T12: Stomach to sex organs

Question 13

SNS effects on pulmonary vasculature

Answer 13

pulmonary vessel constriction

Question 14

Does the PSNS effect force of contraction in the heart

Answer 14

Yes

Question 15

What is the heart rate of transplant patients and why?

Answer 15

± 110
Vagal nerve transected

Explanation
Adults: PSNS dominates –> Resting heart rate 70 - 80 bpm
Kids: SNS dominates –> HR ± 100

Question 16

What pharmacological strategies are available to stimulate parasympathetic activity - name the agents and their clinical use

Answer 16

Muscarinic agonists - Ophthalmology: Constrict pupil, opening flow to aqueous humor and decreasing IOP.

• Pilocarpine
• Carbacol
• Methacholine

Anticholinesterase - Anaesthetics and myasthenia gravis

• Increase Ach available at M receptors and at NMJ
• Neostigmine
• Pyridostigmine (myasthenia)
• Organophosphates

Question 17

What pharmacological strategies are available to inhibit parasympathetic activity. Name the drugs and differentiate between them

Answer 17

Muscarinic antagonists - from the plant Atropa belladonna (Deadly nightshade)

• Atropine - crosses BBB with central effects (confusion in elderly)
• Glycopyrrolate - does not cross BBB
• Hyoscine - used for GI and HPB colic
• Homatropine

Question 18

Describe the effects of antimuscarinic drugs on the CNS, CVS, RSP and GIT

Answer 18

CNS - atropine is a mild stimulant and can cause confusion in the elderly

CVS - Tachycardia - vagus blocked allowing unopposed SNS. IF brady due to hypoxia - no tachycardia

RSP - Bronchodilatation and reduces secretions

GIT - Anti-sialogogue. Reduced gastrin secretion. Reduced secretions. GI motility is reduced not abolished.

Question 19

Describe the mechanism of action of adrenalin and noradrenalin at different doses

Answer 19

Adrenalin
Low doses: beta effects (initial decrease SVR)
High doses: alpha effects increase with increasing dose

Noradrenalin
Predominant alpha 1 effects with some beta effects all doses

Question 20

Describe the mechanism of action of dopamine and dobutamine at different doses

Answer 20

Dopamine and dobutamine

Low doses: Stimulate dopamine receptors
High doses: Stimulate adrenergic receptors

Dopamine (similar to adrenalin)
Low dose: Beta 2
High dose: Alpha 1

Dobutamine
Beta 1 (minimal beta 2 and alpha 1 with slight decrease SVR)
–> shift of perfusion from GIT to heart and skeletal muscle but with overall improved splanchnic perfusion

Question 21

What is the mechanism of action, dose, onset and duration of Ephedrine?

Answer 21

Dual mechanism of action

1. Release NA of endogenous noradrenaline from sympathetic terminals
2. Direct effect on alpha and beta receptors

Dose: 2.5 - 10 mg titrated to BP
Onset: 1 minute
Duration: 1 hour

Question 22

What are the receptor specific adrenegic receptor agonists? What is their mechanism dose, onset, duration of action

Answer 22

Phenylephrine
Alpha 1 agonist –> increase SVR
Dose: 50 - 100ug per dose titrated to BP

Question 23

Explain how to use a paediatric infusion set to estimate infusion rate

Answer 23

Dimensional analysis

60 drops/ml
_________

1 drop/second

= 60 s/ml

= 60s/ml x 1 min/60s

= 1 min/ml

AND

60 drops/ml
_________

2 drop/s

= 30s/ml

= 30s/ml x 1min/60s

= 1min/2ml

= 2ml/min

SO NUMBER OF DROPS PER SECOND = NUMBER of MLS PER MINUTE

Question 24

What is the mechanism of action of clonidine and dexmedetomidine?

Answer 24

Clonidine

• alpha 2 predominant
• also alpha 1

Dexmedetomidine (more specific for alpha 2)
- alpha 2 (8 x more selective than clonidine)

Stimulation of alpha 2 receptors

• -> Sedation (central alpha 2 effects)
• -> Anxiolytic
• -> Analgaesia without respiratory depression
• -> Decrease SVR

Question 25

What is the Richmond Agitation Sedation Score

Answer 25

+4 Violent +3 Aggressive (pulls tubes/catheters) +2 Agitated (frequent movements/fights vent) +1 Restless (anxious but movement not vigorous) 0 Alert and calm - 1 Drowsy: eyes open with contact to voice >10s - 2 Light sedation: eye open with contact to voice < 10s - 3 Mod sedation: eye open to voice - 4 Deep sedation: eye open to physical stimulation -5 Unarousable: No response to voice or physical

Question 26

Give examples of selective beta 2 receptor agonist and their use

Answer 26

Asthma Salbutamol Terbutaline Fenoterol

Question 27

Give examples of the selective alpha 1 blockers and their use

Answer 27

Phenoxybenzamine - Great for phaeochromocytoma as the drugs binds covalently to alpha receptor which destroys the receptor - Non-competitive alpha 1 antagonist Prazosin - alpha 1 antagonist - t1/2 short --> requires QID dosing Doxazosin - Longer half life - Used as antihypertensive and preop in phaeo.

Question 28

What is the only available parenteral beta blocker readily available in RSA

Answer 28

Labetalol

Question 29

What is the mechanism of action of labetalol

Answer 29

10 X greater affinity for beta receptors than alpha receptors but does block both types of receptor

Question 30

How does the dosing of beta blockers differ depending on oral vs intravenous administration and why is this the case

Answer 30

Beta blockers have high first pass metabolism | IV doses are therefore significantly lower than oral doses

Question 31

What is the major disadvantage of non-selective Beta antagonists

Answer 31

Beta antagonism in the lungs --> increasing airway resistance in asthmatics

Question 32

Describe the metabolic effects of beta bolckers

Answer 32

1. Inhibit lipolysis | 2. Inhibit gluconeogenesis (risk of hypoglycaemia)