Autonomic nervous system Flashcards
What does Autonomic Nervous System regulate?
Physiological functions
(e.g. Heart rate, temperature, BP)
Give some examples of the physiological functions that the ANS controls
– Heart rate, BP, body temperature… etc (homeostasis)
– Co-ordinating the body’s response to exercise and stress
In ANS regulation largely voluntary or involuntary?
- Involuntary
What types of tissues does ANS exert control over? (3)
- Smooth muscle (Vascular and visceral)
- Exocrine secretion
- Rate and force of contraction in heart (HR)
2 divisions of ANS and what are these based on?
Sympathetic and Parasympathetic
Based on Anatomical grounds
Some text books include a 3rd division of the ANS, describe this
- Enterinc nervous system
- Network of neurones surrounding the GI Tract
- Is normally controlled via sympathetic and parasympathetic fibres (mix of both)
What is the parasympathetic NS responsible for?
‘rest and digest’ / basal conditions
What is the sympathetic nervous system responsible for?
‘fight or flight’ / stressful conditions (can be coordinated or independent in diff. tissues)
(s for stress and sympathetic)
Sympathetic ganglionic arrangement
Short preganglionic neurone
Long post ganglionic neurone to target tissue
Parasympathetic ganglion organisation
Long preganglionic neurone
Short postganglionic neurone (within target tissue walls)
What happens under stress?
Sympathetic system activity is increased
What autonomic system is more dominant under normal body conditions (basal)?
PNS - dominates under normal circumstances
What is the anatomical origin of the SNS?
- Thoracolumbar origin
What is the anatomical origin of the PNS?
- Craniosacral origin
What do preganglionic neurones of both PNS and SNS divisions release?
- Acetylcholine (ACh)
What does the acetylcholine released from the preganglionic neurones act on?
- Nicotinic acetylcholine receptors on the postganglionic cell (neuromuscular junction)
What do nicotinic MACh receptors have?
- An Ion channel
Describe how action potentials are fired to act on the effector cells
- Pre ganglionic neurone fires action potentials
- Causes release of acetylcholine from the terminal
- They act on the nicotinic acetylcholine receptors at the neuromuscular junction
- These receptors have an integral ion channel
- The acetylcholine binds to the receptor
- This opens the ion channel
- The ion channel is permeable to sodium and potassium ions but at resting membrane potentials, overwhelming you get an inward movement of sodium ions
- This depolarises the post ganglionic neurone to threshold
- And it can then fire action potentials
- The postganglionic neurone then releases a neurotransmitter (noradrenaline) onto the effector cell
Where are preganglionic cell bodies found in?
- CNS
What is the primary neurotransmitter in the parasympathetic neurotransmitter?
Acetylcholine
Where are preganglionic synapses found?
- Periphery
What receptor does sympathetic system usually act on and how?
Adrenaline/noradrenaline released and acts on Adrenergic receptors (eg a1/b1 or b2)
What receptor does parasympathetic system usually act on and how?
Ach binds to muscarinic receptors usually (eg M3/M2)
What happens if you denervate a heart?
Still beats but faster
Vagal influence from parasympathetic usually slows down
Parasympathetic supply to heart is via…
Preganglionic fibres of Vagus nerve (10th cranial nerve)
Where do preganglionic fibres synapse to postganglionic fibres in parasympathetic system?
On epicardial surface/within heart (SA/AV NODE!)
What do post ganglionic fibres release in parasympathetic heart stimulation and what does this bind to?
Ach
Binds to M2 receptors
Effects of Ach binding to M2 receptor
Decrease HR (-ve chronotropic effect) Decrease AV node conduction velocity
Sympathetic input to heart
Postganglionic fibres travel from sympathetic chain
What do the post ganglionic fibres of sympathetic system innervate?
SA NODE, AV NODE, MYOCARDIUM
different to parasympathetic as myocardium is included here
How does the sympathetic input occur in heart?
Release of Noradrenaline
NA binds to B1 adrenoreceptors
What effect does NA binding to B1 adrenoreceptors have on the heart?
Increase HR (+ve chronotropic effect) Increase force of contraction (+ve inotropic effect)
What sets the rhythm of the heart?
AP firing in the SA node (steadily depolarise towards threshold)
Sympathetic effects on AP firing in SA node
Noradrenaline binds to B1 receptors (Gs receptors)
Increase cAMP
Speeds up pacemaker potential (If - funny current)
Parasympathetic effects on AP firing SA node
Ach binds to M2 receptors (Gi receptors)
Decrease cAMP
Increase K+ conductance
decreases pacemaker potential slope
How does noradrenaline increase force of contraction?
NA acts on B1 receptors
Increase cAMP
Activates PKA
= phosphorylation of Ca2+ channels
What does phosphorylation of Ca2+ channels lead to?
Increase Ca2+ entry during plateau
Increase uptake of Ca2+ in SR (more available to release)
what innervation type do most vessels receive?
Sympathetic
What receptors are present in most vessels?
a1 adrenoreceptors
What vessels have B2 receptors?
skeletal muscle
myocardium
liver
What do you need to ensure you can have vasoconstriction and vasodilation?
Vasomotor tone
Already some constriction to allow vasodilation if needed
How does sympathetic output effect vessels?
Increased output = vasoconstriction
decreased output = vasodilation
How does circulating adrenaline affect vessels?
High affinity for b2 receptors than for a1
At high levels adrenaline can affect a1
What binds to what to allow for vasoconstriction/vasodilation?
More Noradrenaline will bind to a1
adrenaline binds to b2 but a1 in high levels
What effect does activating b2 adrenoreceptors have?
Vasodilation
What effect does activating a1 adrenoreceptors have?
Vasoconstriction
How does vasodilation occur from activating b2 receptor?
Increase cAMP PKA Open K+ channels inhibits MLCK (cannot phosphorylate MLC) Relaxation of smooth muscle
How does vasoconstriction occur from activation of a1 receptor?
IP3 production
Increase in Ca2+ from SR stores and influx
= Contraction
(DAG inhibits MLC phosphatase so stays phosphorylated)
What has the largest impact on ensuring adequate perfusion?
Metabolite concentration (eg adenosine, H+) STRONG vasodilator effect
How are changes in system communicated to brain?
Afferent nerves (towards brain) eg baroreceptors or atrial receptors
this then alters efferent pathway
What are baroreceptors?
Stretch receptors (increased arterial pressure = stretch)
Where are baroreceptors found?
Carotid sinus and aortic arch
What happens if baroreceptors detect stretch of blood vessels?
Stretch = high BP
Fire more action potentials towards medulla
Inhibit SNS, Activate PNS
=Bradycardia and Vasodilation
What is baroreceptor for?
Maintaining blood pressure over SHORT time
moment to moment
What happens to baroreceptors if there is continuous hypertension?
Reset to ‘new normal’
reset to higher levels
drugs acting on ANS
Sympathomimetics (a and b receptor agonists)
Adrenoreceptor antagonists
Cholinergics (muscarinic antagonists/agonists)
Sympathomimetics eg
Adrenaline
Dobutamine
salbutamol
Adrenaline function
Restore function and support circulation in cardiac arrest
Anaphylactic shock
(you get widespread vasodilation when this occurs, HIGH LEVELS of adrenaline stimulates a1 receptor)
Dobutamine
B1 agonist cardiogenic shock (pump failure)
Salbutamol
b2 agonist treats asthma (relaxes bronchioles0
a adrenoreceptor antagonists
Anti-hypertensive
eg a1 antagonist PRAZOSIN
Inhibits NA action on a1 receptors = vasodilation
B adrenoreceptor antagonists example
Propranolol = non selective
slows HR
reduce force of contraction
but BRONCHOCONSTRICTION
problem with propranolol
non selective
NOT GOOD for Asthmatic patient (causes bronchoconstriction from b2)
(use selective b1 like atenolol)
