11. Peripheral Nervous System Flashcards
2 systems in the PNS
- afferent
- efferent
Afferent systems contain …
sensory nervous systems
Efferent systems contain …
- autonomic nervous system (controls smooth muscle/outside the influence of voluntary control)
- somatic nervous system (voluntary motor control of skeletal muscle)
Functions in the sympathetic autonomic nervous system
- pupils dilate
- lens of eye adjusts for far vision
- airways in lungs dilate
- heart rate increases
- respiratory rate increases
- salivary secretions reduced
- blood vessels to limb muscles dilate and those to visceral organs constrict
- brain activity alertness
Functions of parasym autonomic nervous system
- pupils constrict
- lens of eye readjust for closer
- airways in lungs constrict
- resp rate decreases
- heart rate decrease
- blood vessels to limb muscles constrict and blood vessels to visceral organs more dilated
- salivary secretions normalise and brain activity normalise
Main transmitters for preganglionic fibres in PNS for
- parasym
- sym
- somatic
- acetylcholine
- acetylcholine
- none
Main transmitters for postganglionic fibres in PNS for
- parasym
- sym
- somatic
- acetlycholine
- noradrenaline
- none
Main transmitters for NMJ fibres in PNS for
- parasym
- sym
- somatic
none
none
acetylcholine
What do we need to know about acetylcholine neurotransmission?
- fundamentals of neurotransmission
- synthesis, storage, release, receptor interaction, termination
ACh synthesises what?
Stored in …
Released by …
Receptor interactions?
Termination?
- choline/choline acetyl transferase
- vesicles
- exocytosis
- muscarinic/nicotinic
- in synapse by acetylcholine esterase
2 main classes of ACh receptors
- muscarinic
- nicotinic
There are at least … main mACh receptor subtypes
3
M1,2,3
Muscarinic receptors are located where …
postganglionic parasympathetic synapses
Muscarinic receptors are … receptors
- G-protein coupled or metabotropic
There are … main subtypes of nicotinic receptors
2
- neuronal type - brain and autonomic ganglia (excitatory)
- muscle-type - neuromuscular junction (excitatory)
What type of receptors are nicotinic receptors?
- ligand gated ion channels or ionotropic
How does ACh act in sympathetic system?
- acts on nicotinic receptor on preganglionic fibre
- goes along postganglionic to heart
How does ACh react in parasym system?
- long preganglionic fibre
- acts on nicotinic receptor at ganglion then postganglionic fibre
- acts on muscarinic receptor in heart
How does ACh work in somatic system?
- acts on nicotinic receptors at target
Muscarinic receptors mainly mediate …
parasympathetic effects
Parasympathetic activation
- pupils constrict
- lens of eye adjusts to closer vision
- airways in lungs constrict
- heart rate decreases
- blood vessels to limb muscles constrict
- blood vessels to visceral organs more dilated
- salivary secretions normalise
Effects of muscarinic agonists
- increased pupil constriction - contraction of constrictor pupillary muscles
- decreased focal length of lens (contraction of ciliary muscles)
- bronchoconstriction
- decreases cardiac output (rate and force)
- increased GI motility
- increased exocrine gland secretion (sweating, salivation, bronciol secretion)
Muscarinic agonsists are also known as …
parasympathomimetics
Effect of muscarinic antagonists
- pupils dilate (relaxation of constrictor pupillary muscle - blurred vision)
- increased focal length of lens (relaxation of ciliary muscle)
- bronchodilation
- increased cardiac output (rate and force)
- decreased GI motility
- decreased exocrine gland secretion (dry mouth, decreased sweating)
Muscarinic antagonists are also known as …
parasympatholytic
Give 2 muscarinic receptor agonists
- pilocarpine
- carbachol
Clinical uses of pilocarpine
- treat glaucoma
- local application causes ciliary muscle contraction - focus on near vision and increase drainage of aqueous humour
- contraction of sphincter muscle causes pupil constriction
OR - treat xerostomia
- dry mouth/reduced saliva secretion can happen due to radiation, antineoplastic drugs, side effect of other drugs
- take it to stimulate saliva secretions systemically - side effects of muscarinic-sweating, nausea, miminal cardiovascular side effect due to low dose
Explain clinical use of muscarinic receptor antagonist
- pupils dilation in eye surgery - causes pupil dilation (tropicamide 2-6 hrs compared to 6 days of atropine)
- decrease oral/respiratory secretions before oral procedures and adjunct to anaesthesia (atropine/belladonna, deadly nightshade or glycopyrronium)
- resuscitation in bradycardia to increase heart rate (atropine)
- asthma to cause bronchodilation - ipratropium by inhalation
- motion sickness - orally to decrease gastric motility - hyoscine
Use of deadly nightshade - atropa belladonna
- cosmetics over 2000 years ago
- pupils dilate and make you more fit
Neuronal nicotinic receptors are located where?
- sympathetic and parasympathetic ganglia
Agonists on neuronal nicotinic receptors activate what?
- both sympathetic (vasoconstriction, tachycardia, hypertension) and parasympathetic (bradycardia, hypotension, increases GIT motility and secretion) systems
Effect of agonists of neuronal nicotinic receptors
- autonomic confusion
- no clinically useful - ganglionic /neuronal nicotinic agonists
Antagonists of neuronal nicotinic receptors do what? Example?
- hexamethonium
- loss of sym and parasym reflexes especially cardiac
- not of great therapeutic use
Muscle type nicotinic receptors are located where?
NMJ
Stimulation of nicotinic receptors by ACh does what?
- depolarisation
- in muscle fibre known as end plate potential and contraction of skeletal muscle fibre
Agonist and antagonist of muscle type nicotinic receptors
- suxemethonium
- tubocurarine
Effects of nicotinic agonists at NMJ
- initial depolarisation/EPP and muscle fibre contraction (muscle twitch)
- because synthetic agonist is not metabolised rapidly by acetylcholinesterase, the fibre is persistently depolarised resulting in loss of further electrical excitability - known as depolarising block
- paralysis/muscle relaxation for surgery (depolarising block)
Effect of nicotinic antagonists at NMJ
- hyperpolarisation
- inhibition of EPPs
- muscle fibre relaxation
- paralysis for surgery
- non-depolarising surgery
ACh release is inhibited by …
Result?
- botulinum toxin and bungarotoxin
- causes autonomic and motor paralysis if injected
Role of botulinum toxin
- injected locally to treat muscle spasm
- and in plastic surgery/botox
What enzyme metabolises ACh?
acetylcholinesterase
How to inhibit ACh metabolism?
- anticholinesterases
- increases ACh transmission
Give 2 anticholinesterases
- neostigmine
- organophosphates
Effects of anticholinesterases on autonomic nervous system
- reflect increased transmission at parasympathetic postganglionic synapses
- increases secretions, bradycardia, hypotension, pupil constriction
Effects of anticholinesterases on NMJ
- increased muscle tension and twitching
- large doses cause depolarising block
- neostigmine can treat myasthenia gravis (autoimmune disease, circulating antibodies against nicotinic receptors)
