4.1 Introduction to the autonomic nervous system Flashcards
The autonomic nervous system is divided into sympathetic and parasympathetic systems:
• Sympathetic system primarily takes care of the fight-or-flight response while the parasympathetic system takes care of the rest-and-digest response
• Many tissues receive innervation from both systems → tend to be of opposite effects
• Some organs (e.g. ___________________________) predominantly receive sympathetic input → may dilate or constrict blood vessels
o Differences in effect is mediated by the receptors (not the nerves)
• Salivary glands receive both sympathetic and parasympathetic inputs of similar effects (__________________), but the parasympathetic input is more important in terms of digestive function while the sympathetic input is for immune function
• Gut possesses its own nervous system (enteric nervous system) → local nervous control over GI function (independent from CNS influence)
o Responds to environmental changes within the gut (e.g. motility, blood flow, lumen environment)
liver, adipose, kidneys, blood vessels;
increased saliva production
effect of sympathetic on eye
dilation (pupil)
effect of parasympathetic on eye
constriction (pupil), contraction (ciliary muscle)
effect of sympathetic on trachea and bronchioles
dilates
effect of parasympathetic on trachea and bronchioles
constriction
effect of sympathetic on liver
gluocgenolysis, glucogenesis
effect of sympathetic on adipose
lipolysis
effect of sympathetic on kidney
increased renin secretion
effect of sympathetic on ureters & bladder
relaxes detrusor, constricts trigone & sphincter
effect of parasympathetic on ureters & bladder
relaxes detrusor, relaxes trigone and sphincter
effect of sympathetic on salivary glands
thick, viscious secretion
effect of parasympathetic on salivary glands
watery copious secretion
effect of sympathetic on skin
piloerection, increased sweating
effect of sympathetic on heart
increase rate & contractility
effect of parasympathetic on heart
decrease rate & contractility
effect of sympathetic on gi
decrease motility and tone; sphincter contraction
effect of parasympathetic on gi
increase motility and tone, increase secretions
effect of sympathetic on blood vessels (skeletal muscle)
dilatation
effect of sympathetic on blood vessels (skin, mucuous membranes and splanchnic area)
constriction
There are two different types of acetylcholine receptors – nicotinic and muscarinic:
• Parasympathetic effects predominate at rest → blocking nicotinic receptors leads to an ________________ at rest
• Sympathetic effects predominate during exercise → blocking nicotinic receptors leads to a _______________ during exercise (hypothetical)
Nicotinic receptors
- Found in all _________________ (allows for propagation of signals onto the postganglionic neurone)
- Interference affects all autonomic functions
Muscarinic receptors
- Found within the ______________ (mediates parasympathetic effects)
- Interference affects only __________________
increase in heart rate;
decrease in heart rate;
autonomic ganglia;
effector tissues;
parasympathetic response
Nicotinic acetylcholine receptors (nAChR) are found in both sympathetic and parasympathetic ganglia, stimulated by _____________ and inhibited by _______________:
• ________________ receptor → acetylcholine binding causes ion channel opening, ion influx and excitation of postganglionic neurone
• Rapid response (_________ for whole process from release of ACh to signal propagation)
ACh and nicotine;
hexamethonium;
Type I (ionotropic);
few ms
what are the AchR subunit in muscle type (in skeletal muscles)
2 α, β, δ, ε
what are the AchR subunit in ganglion type (in CNS )
2 α, 3 β
why is there bronchodilation when nicotinic receptors in resting individuals are blocked?
Parasympathetic nervous system normally keeps the bronchi partially constricted (if not they will start to dilate)
why is there decreased sweat production when nicotinic receptors in resting individuals are blocked?
nAChR are involved in sweat production
why is there constipation when nicotinic receptors in resting individuals are blocked?
Parasympathetic system increases gut motility (blocking its effects causes the gut to slow down)
why is there decreased urinary frequency when nicotinic receptors in resting individuals are blocked?
Parasympathetic system contracts the bladder detrusor smooth muscle and relaxes the urethral sphincter (urinary frequency decreases and retention occurs if blocked)
why is there longsightedness when nicotinic receptors in resting individuals are blocked?
Parasympathetic system innervates the ciliary muscle (via CN III through ciliary ganglion) → allows it to contract to focus on near objects (i.e. acutely promotes short-sightedness):
• Lens becomes more spherical → focal length decreases
• Ciliaris paralysed if blocked → cannot focus near objects
Muscarinic receptors are found in all effector organs receiving _________________ and tissues receiving __________________:
• Stimulated by muscarine and ACh (actions correspond to parasympathetic effects) and inhibited by atropine (after atropine blockage of muscarinic actions, larger ACh doses can induce effects similar to those caused by nicotine)
• ___________________ receptor → ACh binding promotes G-protein activation and generation of second messenger which activates cell signalling pathways
• Fast response (_______________; slower than Type I receptors)
*M1, M3, M5 work via ________________ (influences intracellular Ca2+), while M2 and M4 work via _______________.
postganglionic parasympathetic fibres;
postganglionic sympathetic cholinergic fibres;
Type II (G-protein coupled);
seconds to minutes;
Gq stimulation of IP3 and DAG;
Gi reduction of cAMP
what are the functions of M1 receptors?
gastric parietal cells, salivary glands
What are the functions of M2 receptors?
cardiac cells and smooth muscles
What are the functions of M3 receptors?
bladder, exocrine glands (sweat, salivary), smooth muscles, eyes
What are the functions of M4 receptors?
regulate striatal DA release, modulate PPI, analgesia, keratinocyte migration
What are the functions of M5 receptors
cerebral arterial vasoconstriction
ADRENERGIC RECEPTORS Adrenoceptors are found at all effector organs innervated by postganglionic sympathetic fibres (except organs like sweat glands), stimulated by noradrenaline (NA) and adrenaline:
*α1-adrenoceptors work via _________________ while α2 and β adrenoceptors work via _______________ respectively (increased intracellular cAMP is stimulatory on the heart, and inhibitory in vascular smooth muscles and platelets)
• _____________ receptors → similar mechanism to muscarinic receptors
• 4 main subtypes (α1, α2, β1, β2) with different effects and locations in the body
o Mnemonic: AC-BD (alpha-constrict; beta-dilate), 1 heart, 2 lungs
Type II (G-protein coupled);
Gq stimulation of IP3 and DAG (increases intracellular Ca2+);
Gi and Gs effects on cAMP
what are the functions of α1 receptors?
Mainly on effector cells (e.g. smooth muscles) (causes contraction)
what are the functions of α2 receptors?
Mainly on presynaptic membranes (causes inhibition of NA release → negative feedback); some on smooth muscles of certain blood vessels (vasoconstriction)
what are the functions of β1 receptors?
Mainly in the heart (causes increased contractility and heart rate)
what are the functions of β2receptors?
Mainly in the lungs/bronchi (causes bronchodilation)
The synthesis, release, metabolism of amine transmitters (e.g. ACh) follow the same steps:
- Precursor molecule (acetyl-CoA and choline) is converted into the transmitter (ACh)
- Neurotransmitters are packaged into vesicles sitting near the cell surface membrane
- Wave of depolarisation promotes ____________, driving exocytosis of synaptic vesicles to release the neurotransmitters into the synaptic cleft
- Neurotransmitter molecules diffuse across the synaptic cleft
- Binding to receptors on the postsynaptic membrane
- Synaptic transmission is terminated via __________________
- Neurotransmitter molecules or breakdown products are removed from the synaptic cleft via uptake mechanisms into the presynaptic membrane
Ca2+ influx;
enzymatic metabolism (acetylcholinesterase breaking ACh back down into acetate and choline)
Noradrenaline is produced from the precursor tyrosine taken up into the neurone, via 3 enzyme-catalysed steps (first conversion is rate-limiting):
- Tyrosine → DOPA (via ____________); DOPA → dopamine (via ____________); dopamine → NA (via _____________ packaged in the same vesicle)
- Same process as ACh then occurs, with the release of NA to activate postsynaptic adrenoceptors
- No enzymatic breakdown of NA within the synapse occurs (removed from the synapse via uptake proteins)
tyrosine hydroxylase;
DOPA decarboxylase;
dopamine β hydroxylase
how is NA being uptaked?
- Uptake of NA back into the presynaptic neurone → broken down by monoamine oxidase A (MAO-A)
- Uptake of NA into other tissues (e.g. postsynaptic cell) → broken down by catechol-O-methyltransferase (COMT)
