Mod6-Obj4: Neurotransmitters Flashcards
Signal transmission across a synapse: Step 1
Action potential arrives at the presynaptic axon terminal and depolarises the membrane
Signal transmission across a synapse: Step 2
Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal
Signal transmission across a synapse: Step 3
Ca2+ surge triggers synaptic vesicles to release their stored neurotransmitters (ACh) into the synaptic cleft
Signal transmission across a synapse: Step 4
NT diffuse across synaptic cleft and binds receptors (chemically-gated ion channels) on the postsynaptic membrane
Signal transmission across a synapse: Step 5
Binding of NT opens chemically-gated ion channels, producing a graded potential
Signal transmission across a synapse: Step 6
If graded potential reaches threshold, action potential is generated and signal is transmitted
Signal transmission across a synapse: Step 7
Transmission of signal terminated if
- Degraded by enzymes
- Re-enters the axon terminal
- Diffuses away from synapse
Postsynaptic potentials
Are either excitatory or inhibitory
- Depends on membrane potential generated at postsynaptic neuron
- Postsynaptic potential=a graded potential
- EPSP and IPSP result from opening different gated ion channels
Excitatory postsynaptic potential (EPSP)
Depolarisation of postsynaptic membrane towards threshold
Inhibitory postsynaptic potential (IPSP)
Hyperpolarisation of postynaptic membrane away from threshold
EPSP
- NT binds receptors on the postsynaptic membrane
- Opens gated ion channels
- Na+ ions flow in > K+ out
- Graded potential = depolarisation
- Postsynaptic membrane is closer to threshold (facilitated)
IPSP
- NT binds receptors on the postsynaptic membrane
- Opens gated ion channels
- Cl- (chloride) ions flow in, K+
flow out - Graded potential = hyperpolarisation
- Postsynaptic membrane is further to threshold (inhibited)
The postsynaptic cell
Excitatory synapse
◦ Na+ K+ gated channels ◦ Towards threshold
Inhibitory synapse
◦ Cl- and K+ gated channels ◦ Away from threshold
The postsynaptic cell
Excitatory synapse
◦ Na+ K+ gated channels ◦ Towards threshold
Inhibitory synapse
◦ Cl- and K+ gated channels ◦ Away from threshold
ANS neurotransmitters: ACh
Any axon that releases acetylcholine is known as a cholinergic fibre
- All preganglionic axons
- Parasympathetic postganglionic axons
ANS neurotransmitters: NA
Any axon that releases noradrenalin (NA) is known as an adrenergic fibre
-Sympathetic postganglionic axon
Cholinergic receptors
Receptors that bind ACh
Two types:
1.) Nicotinic: always excitatory
2.) Muscuarinic: Excitatory or inhibitory
Cholinergic receptors: Nicotinic
Location: -All postganglionic neurons -Adrenal medullary cells -Skeletal muscles Effect of NT binding: -Excitation
Cholinergic receptors: Muscuarinic
Location: -All parasympathetic target organs Effect of NT binding: -Excitation of most targets -Inhibition of cardiac muscle
Adrenergic receptors
Receptors that bind NA and A
Two major types:
-Alpha (a1, a2)
-Beta (B1, B2, B3)
Adrenergic receptors: B1
Location:
-Heart
Effect of binding:
-Increases heart rate and force of contraction
Adrenergic receptors: B2
Location:
-Lungs, most sympathetic targets, blood vessels in heart, liver and skeletal muscle
Effect of binding:
-Most inhibitory: dilates blood vessels and bronchioles, relaxes smooth muscles in digestive and urinary organs
Adrenergic receptors: a1
Location:
-Blood vessels (except in heart, liver and skeletal muscles), and all sympathetic targets except the heart
Effect of binding:
-Mostly stimulatory: constricts blood vessels and visceral organ sphincters, dilates pupils
Effect of drugs
Drugs that bind NT receptors
- Agonist (mimic the action)
- Antagonist (block the action)
Sympathetic (adrenergic): α
Receptor location and normal response to NT binding
Smooth muscle of: • Most arterioles • Sphincters of bladder and GI tract • Iris dilator Effect: contraction
Sympathetic (adrenergic): α
Typical agonist drugs
- Noradrenalin
- Adrenalin
Sympathetic (adrenergic): α
Typical antagonist drugs
- Phenoxybenz- amine
- Phentolamine Prazosin
Sympathetic (adrenergic): B1
Receptor location and normal response to NT binding
Cardiac muscle
Kidneys
Effect: increased rate and force of contraction. Increased renin release from kidneys
Sympathetic (adrenergic): B1
Typical agonist drugs
-Noradrenalin
-Adrenalin
-Isoproteronol
0Dobutamine
Sympathetic (adrenergic): B1
Typical antagonist drugs
“Beta blockers” Propranolol (used to lower blood pressure)
Sympathetic (adrenergic): B2
Receptor location and normal response to NT binding
Smooth muscle of: • Bronchioles of lung • Arterioles of skeletal muscles, brain and lungs • Uterus Effect: relaxation
Sympathetic (adrenergic): B2
Typical agonist drugs
- Adrenalin (used for anaphylaxis)
- Albuterol (used in asthma puffers)
Sympathetic (adrenergic): B2
Typical antagonist drugs
“Beta blockers” Propranolol
Parasympathetic (cholinergic): Muscuarinic
Receptor location and normal response to NT binding
All parasympathetic effectors
Parasympathetic (cholinergic): Muscuarinic
Typical agonist drugs
- Acetylcholine
- Muscarine
- Carbachol
Parasympathetic (cholinergic): Muscuarinic
Typical antagonist drugs
- Atropine (pupil dilaiton)
- Scopolamine
