Lecture 10 Flashcards
What does the ANS (involuntary) regulate/exert control over?
-heart rate, BP, body temp (homeostatic functions)
-co-ordinating the body’s response to exercise and stress
Exerts control over:
-smooth muscle
-exocrine secretion
-rate and force of the contraction of the heart
What are the 2/3 divisions of the ANS + how are they divided into these categories?
Parasympathetic (cranial/sacral origin)
Sympathetic (thoracic/lumbar origin)
Based on anatomical grounds only rather than type of neurotransmitter
(some include the enteric nervous system-surround GI tract-parasympathetic + sympathetic fibres)
Where are the preganglionic cell bodies?
CNS
Difference in length of preganglionic/post ganglionic neurones in sympathetic/parasympathetic neurones:
Sympathetic
- pre:short (synapse in ganglion)
- post:long (innervate target tissue)
Parasympathetic
- pre:long
- post:short (close to/within target tissue)
Can a tissue be innervated by parasympathetic + sympathetic nerves?
Yes.
They often have opposite effects to work together to maintain balance.
(Parasympathetic NS is more dominant under basal conditions, whereas sympathetic activity is increased under stress)
Can sympathetic drive be independently regulated?
Yes. Sympathetic activity to heart can be increased without increasing the activity of the GI tract
(Fight/flight is more generalised activation of sympathetic nervous system)
What does the ANS do/not do in the CVS?
Controls:
- HR
- force of contraction of the heart
- peripheral resistance of blood vessels
Does not:
- initiate electrical activity in the heart
- denervated heart still beats, at a faster rate, as at rest it is normally under influence of vagus nerve (parasympathetic input)
What is the structure of parasympathetic nerves to the heart?
- preganglionic fibres from 10th cranial nerve: Vagus nerve
- synapse with postganglionic cells on epicardial surface/ within walls at AV/SA node (not much innervation of myocardium itself- targets nodes: meaning it can change HR but NOT force of contraction)
- postganglionic cells release ACh
- acts on M2 receptors (G-protein coupled)
- decreases HR, decreases AV node conduction velocity
What do the postganglionic sympathetic fibres innervate?
-from postganglionic fibres from sympathetic trunk
-innervate SA/AV node AND myocardium
Release noradrenaline
Acts on B1 adrenoreceptors (B2/3 adrenoceptors are present in the heart also)
-increasing force of contraction (positive inotropic effect-due to innervation of the myocardium)
-increasing heart rate
How does ANS effect pacemaker potential (slow depolarisation to threshold)?
- sympathetic activity will steepen pacemaker potential (quicker to reach threshold-AP fire faster): NA on beta 1 receptors- G protein, Gs, increase cAMP, HCN channels open allowing Na+ influx= reach TV
- parasympathetic activity will lengthen pacemaker potential (longer to reach threshold): ACh on M2 receptors- G protein, Gi, decreases cAMP + beta/gamma unit increases potassium conductance
How does NA increased force of contraction of heart?
-acts on B1 receptors in myocardium and nodes: (Gs coupled) causing an increase in cAMP-> activates PKA
-phosphorylation of Ca2+ channels (L type), increases Ca2+ entry during plateau phase
-increased uptake of Ca2+ in ER
= increased force of contraction
What is the more common innervation of vessels?
-sympathetic innervation (except specialised tissue e.g. erectile tissue which have parasympathetic)
What type of receptor is present in vascular smooth muscle?
Alpha 1 adrenoceptors
Coronary and skeletal muscle vasculature also have some B2 adrenoceptors
How you how control the diameter of vessels by sympathetic innervation?
-normal vasomotor tone (some sympathetic output (NA) acting on alpha 1 receptors)
(Basal tone allows constriction and dilation)
-relaxation: decrease sympathetic outflow, alllowing vasodilation
-constriction: increase sympathetic output: allowing vasoconstriction
Which blood vessels have B2 adrenoceptors as well as A1, and what is the effect of NA/adrenaline on these?
- skeletal muscle vessels
- myocardium vessels
- liver vessels
-noradrenaline from sympathetic nervous system acts on A1 adrenoceptors to constrict vessel (not all or nothing response- just to regulate BP)
-if you have an increase in circulating adrenaline (fight/flight response), theses act on B2 receptors, adrenaline can also activate A1 receptors at higher concentrations
-B2= dilation
(Predominant effect will be constriction if very high levels of adrenaline, at normal levels they act on B2 receptors for dilation)