Physiology of Parasympathetic NS Flashcards
Which known structure in the heart don’t receive parasympathetic innervation?
Parasympathetic nerves do not innervate ventricles and most blood vessels – so do not affect heart contractility (SV) or total peripheral resistance (BP)
What effect does stimulation of the vagus nerve have on the heart?
Vagus nerve releases ACh which acts at M2 receptors
- decrease frequency of pacemaker potential at SA node leading to reduction in heart rate. HR decreases so CO decreases.
- decrease electrical conduction through atria-ventricular node to balance reduction in heart rate to contraction
- decreases the force of contraction of atrial muscle.
How exactly does the vagus nerve reduce heart rate?
ACh inhibits a membrane conductance called the “pacemaker potential” at the S-A node. ACh also increases the potassium conductance and inhibits the inward calcium current. These actions decrease the slope of the pacemaker potential in the S-A node.
Parasympathetic nerves in the eye are the major influence in regulating what 3 things?
- pupil diameter
- intra-ocular pressure
- accommodation/focusing
How do parasympathetic nerves influence pupil diameter in the eye?
- Stimulation of M3 receptors lead to constriction of the circular smooth muscle of the iris (constrictor pupillae)
- Constriction of the pupil (miosis)
What is the parasympathetic supply of the eye?
Via occulomotor nerve and ciliary ganglion
How does change in pupil diameter via parasympathetic innervation affect intra-ocular pressure?
- Constriction of the pupil (M3) has a secondary action of opening the canal of Schlemm at the back of the pupil
- Drains aqueous humour from eye
- Reducing pressure within the eye
How does parasympathetic activity enable focusing on objects close to the eye?
- Ciliary muscle contracted
- Suspensory ligaments relaxed
- Bulged lens shape
- Reduced focal length
What effect does parasympathetic activity have on smooth muscle and how does it do this?
Causes contraction eg. bronchoconstriction, GI motility, bladder/rectum contraction
ACh acts on smooth muscle cells. ACh depolarizes smooth muscle cells to produce opening of voltage-dependent calcium channels. The resultant influx of Ca2+ ions produces contraction.
Discuss the parasympathetic control of micturition
- Micturition involves contraction of the detrusor muscle of the bladder (this is the smooth muscle in the body of the bladder) produced by the action of ACh on M3 receptors and is associated with (in man) voluntary relaxation of the external sphincter and a rise in intra-abdominal pressure.
Contraction of the internal sphincter involves impulses in sympathetic nerve fibres of the hypogastric nerve and parasympathetic fibres of the pelvic nerve.
Discuss parasympathetic innervation of the GI tract (motility)
- Stimulation of vagus nerve releases Ach which acts on M3 receptors
- Contraction of circular and longitudinal smooth muscle in GI tract
- Increased motility
- Vagus also contains afferent (sensory) fibres – peristaltic reflex control
Discuss the parasympathetic effects on secretions related to the GI tract
- Salivary glands -> facial + glossoph. stimulate acinar cells, increase amylase/mucins
- Gastric glands -> vagus stimulates parietal cells, increase gastric acid
- Pancreatic glands -> vagus stimulates acinar + islet cells, increase pancreatic secretions
- Pancreas -> vagus stimulates insulin secretion from beta-cells
Think of ‘rest and digest’
Which receptor is stimulated on bronchi smooth muscle to cause bronchoconstriction? Which drug is used as a bronchodilator here?
M3 receptors
Ipratropium, used in COPD - need to increase airway flow
What is an important clinical caution of using anti-muscaranic bronchodilators?
Caution to patients with bladder outflow problems and angle-closure glaucoma. Conditions also associated with elderly (COPD) - these are contraindications.
Most blood vessels do not receive a parasympathetic innervation. What is the main exception of this rule? (Hint: point + shoot)
Specialised sacral parasympathetic ‘vasodilator’ nerves innervate erectile tissue. Release NO (not ACh). NO is lipophilic/membrane-perm, causes relaxation of vascular smooth muscle cells composing the corpus cavernosum.
-> ERECTION
Viagra prevents breakdown of NO - increasing its vasodilator effects
